m10 chri5988 07 se c10 - higher education | pearson · 278 part ii the water, weather, ... a...

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Palm–oak forest hammock (“shady place”), featuring a mixed hardwood forest. There are bromeliads(epiphytes) growing on the live oak trees and a few on the cabbage, or sabal, palms. This palm is Florida’sstate tree. A hammock is slightly higher ground, surrounded by wet prairies, mixed swamp, and cypressforest marshes. Such “islands” of vegetation result from having few fires, protected as they are by surroundingmarshes. These two photos are in Myakka River State Park, Sarasota County, about 15 miles inland from thecentral Gulf Coast. [Overhead and forest-floor photos by Bobbé Christopherson.]

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Global ClimateSystems� Key Learning Concepts

After reading the chapter, you should be able to:

� Define climate and climatology, and explain the difference between

climate and weather.

� Review the role of temperature, precipitation, air pressure, and air

mass patterns used to establish climatic regions.

� Review the development of climate classification systems, and

compare genetic and empirical systems as ways of classifying

climate.

� Describe the principal climate classification categories other than

deserts, and locate these regions on a world map.

� Explain the precipitation and moisture efficiency criteria used to

determine the arid and semiarid climates, and locate them on a

world map.

� Outline future climate patterns from forecasts presented, and

explain the causes and potential consequences of climate change.

Earth experiences an almost infinite variety of weather—conditions

of the atmosphere—at any given time and place. But if we consider

the weather over many years, including its variability and ex-

tremes, a pattern emerges that constitutes climate. Think of climatic pat-

terns as dynamic rather than static, owing to the fact that we are witnessing

climate change. Climate is more than a consideration of simple averages of

temperature and precipitation.

C H A P T E R

10

M10_CHRI5988_07_SE_C10.QXD 12/18/07 4:21 AM 77

278 Part II The Water, Weather, and Climate Systems

Today, climatologists know that intriguing global-scale linkages exist in the Earth–atmosphere–ocean sys-tem. For instance, strong monsoonal rains in West Africaare correlated with the development of intense Atlantichurricanes; or, one year an El Niño in the Pacific is tied torains in the American West, floods in Louisiana andNorthern Europe, and a weak Atlantic hurricane season.Yet, the persistent La Niña in 2007 strengtheneddrought’s six-year hold on the West. The El Niño/LaNiña phenomenon is the subject of Focus Study 10.1.

Climatologists, among other scientists, are analyzingglobal climate change—record-breaking global averagetemperatures, glacial ice melt, drying soil-moisture con-ditions, changing crop yields, spreading of infectiousdisease, changing distributions of plants and animals, de-clining coral reef health and fisheries, and the thawing ofhigh-latitude lands and seas. Climatologists are concernedabout observed changes occurring in the global climate,as these are at a pace not evidenced in the records ofthe past millennia. Climate and natural vegetation shiftsduring the next 50 years could exceed the total of allchanges since the peak of the last ice-age episode, some18,000 years ago. In Chapter 1, Geosystems began withthese words from scientist Jack Williams,

By the end of the 21st century, large portions of theEarth’s surface may experience climates not found at pre-sent, and some 20th-century climates may disappear. . . .Novel climates are projected to develop primarily in thetropics and subtropics. . . . Disappearing climatesincrease the likelihood of species extinctions and com-munity disruption for species endemic to particular cli-matic regimes, with the largest impacts projected forpoleward and tropical montane regions.*

We need to realize that the climate map and climate desig-nations we study in this chapter are not fixed but are on themove as temperature and precipitation relationships alter.

In this chapter: Climates are so diverse that no twoplaces on Earth’s surface experience exactly the sameclimatic conditions; in fact, Earth is a vast collection ofmicroclimates. However, broad similarities among localclimates permit their grouping into climatic regions.

Many of the physical elements of the environment,studied in the first nine chapters of this text, link togetherto explain climates. Here we survey the patterns of climateusing a series of sample cities and towns. Geosystems usesa simplified classification system based on physical factorsthat help uncover the “why” question—why climates arein certain locations. Though imperfect, this method iseasily understood and is based on a widely used classifica-tion system devised by climatologist Wladimir Köppen(pronounced KUR-pen). For reference, Appendix Bdetails the Köppen climate classification system and allits criteria.

Climatologists use powerful computer models tosimulate changing complex interactions in the atmo-sphere, hydrosphere, lithosphere, and biosphere. Thischapter concludes with a discussion of climate change andits vital implications for society. Climate patterns arechanging at an unprecedented rate. Especially significantare changes occurring in the polar regions of the Arcticand Antarctic.

Earth’s Climate System and Its ClassificationClimatology, the study of climate and its variability, ana-lyzes long-term weather patterns over time and space andthe controls that produce Earth’s diverse climatic condi-tions. One type of climatic analysis locates areas of similarweather statistics and groups them into climatic regions.Observed patterns grouped into regions are at the core ofclimate classification.

The climate where you live may be humid with dis-tinct seasons, or dry with consistent warmth, or moist andcool—almost any combination is possible. There areplaces where it rains more than 20 cm (8 in.) each month,with monthly average temperatures remaining above27°C (80°F) year-round. Other places may be rainless fora decade at a time. A climate may have temperatures thataverage above freezing every month yet still threatensevere frost problems for agriculture. Students readingGeosystems in Singapore experience precipitation everymonth, ranging from 13.1 to 30.6 cm (5.1 to 12.0 in.), or228.1 cm (89.8 in.) during an average year, whereas stu-dents at the university in Karachi, Pakistan, measure only20.4 cm (8 in.) of rain over an entire year.

Climates greatly influence ecosystems, the natural, self-regulating communities formed by plants and animals intheir nonliving environment. On land, the basic climaticregions determine to a large extent the location of theworld’s major ecosystems. These regions, called biomes,include forest, grassland, savanna, tundra, and desert.Plant, soil, and animal communities are associated withthese biomes. Because climate cycles through periodicchange, it is never really stable; therefore, ecosystemsshould be thought of as being in a constant state of adap-tation and response.

The present global climatic warming trend is produc-ing changes in plant and animal distributions. Figure 10.1presents a schematic view of Earth’s climate system, show-ing both internal and external processes and linkages thatinfluence climate and thus regulate such changes.

Climate Components: Insolation,Temperature, Pressure, Air Masses,and PrecipitationThe principal elements of climate are insolation, temper-ature, pressure, air masses, and precipitation. The firstnine chapters discussed each of these elements. We reviewthem briefly here. Insolation is the energy input for the

*Jack Williams et al., “Projected distributions of novel and disappearingclimates by A.D. 2100,” Proceedings of the National Academy of Sciences(April 3, 2007): 5739.


Chapter 10 Global Climate Systems 279

climate system, but it varies widely over Earth’s surface bylatitude (see Chapter 2 and Figures 2.9, 2.10, and 2.11).Daylength and temperature patterns vary diurnally (daily)and seasonally. The principal controls of temperature arelatitude, altitude, land-water heating differences, andcloud cover. The pattern of world temperatures and theirannual ranges are in Chapter 5 (see Figures 5.14, 5.15,5.17, and 5.18).

Temperature variations result from a coupling ofdynamic forces in the atmosphere to Earth’s pattern ofatmospheric pressure and resulting global wind systems(see Figures 6.10 and 6.12). Important, too, are the loca-tion and physical characteristics of air masses, those vastbodies of homogeneous air that form over oceanic andcontinental source regions.

Moisture is the remaining input to climate. Thehydrologic cycle transfers moisture, with its tremendouslatent heat energy, through Earth’s climate system (seeFigure 9.1). The moisture input to climate is precipitationin all its forms. Figure 10.2 shows the worldwide distribu-tion of precipitation, our moisture supply. Its patterns areimportant, for it is a key climate control factor. Averagetemperatures and daylength help us approximate POTET(potential evapotranspiration), a measure of naturalmoisture demand.

Most of Earth’s desert regions, areas of permanentwater deficit, are in lands dominated by subtropical high-pressure cells, with bordering lands grading to grasslandsand to forests as precipitation increases. The most consis-tently wet climates on Earth straddle the equator in theAmazon region of South America, the Congo region ofAfrica, and Indonesia and Southeast Asia, all of which areinfluenced by equatorial low pressure and the intertropi-cal convergence zone (ITCZ, see Figure 6.11).

Simply relating the two principal climatic components—temperature and precipitation—reveals general climatetypes (Figure 10.3). Temperature and precipitation pat-terns, plus other weather factors, provide the key to climateclassification.

Classification of Climatic RegionsThe ancient Greeks simplified their view of world climatesinto three zones: The “torrid zone” referred to warmerareas south of the Mediterranean; the “frigid zone” was tothe north; and the area where they lived was labeled the“temperate zone,” which they considered the optimum cli-mate. They believed that travel too close to the equator ortoo far north would surely end in death. But the world is adiverse place and Earth’s myriad climatic variations aremore complex than these simple views.

SPACE

CRYOSPHERE

Insolation

Terrestrialradiation

ATMOSPHEREComposition

N2, O2, CO2, H2O, O3,aerosols

Sea ice

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Atmosphere

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OceanOcean

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HYDROSPHERE

Changes in the land:elevation, vegetation,

albedo

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sea temperatureLITHOSPHERE

Externalprocesses

Internalprocesses

Land

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Clouds

FIGURE 10.1 A schematic of Earth’s climate system.Imagine you are hired to write a computer program that simulates Earth’s climates. Internal processes that influence climateinvolve the atmosphere, hydrosphere (streams and oceans), cryosphere (polar ice masses and glaciers), biosphere, andlithosphere (land)—all energized by insolation. External processes, principally from human activity, affect this climatic balanceand force climate change. [After J. Houghton, The Global Climate (Cambridge, UK: Cambridge University Press, 1984); and the GlobalAtmospheric Research Program.]



Cold

HotDry Wet

Increasing precipitation

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Tundra

Taiga – cool summer

Highland climatesat elevation

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Humid continental, warm-hot summer

Mediterranean,dry summer subtropics

Humid subtropics

Colddesert

Seasonal wet-and-drytropics – Savanna

Monsoontropics

Equatorial tropics –Rain forest

Cold steppe

Midlatitudearidhot

desert

Midlatitudesemiarid

and steppe

Hot steppe

Tropicalarid

Tropicalsemiarid

FIGURE 10.3 Climatic relationships.A temperature and precipitationschematic reveals climatic relationships.Based on general knowledge of yourcollege location, can you identify itsapproximate location on the schematicdiagram? Now locate the region of yourbirthplace.

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FIGURE 10.2 Worldwide average annual precipitation.The causes that produce these patterns should be recognizable to you: temperature and pressure patterns; air mass types;convergent, convectional, orographic, and frontal lifting mechanisms; and the general energy availability that decreases towardthe poles.

Classification is the process of grouping data or phe-nomena in related categories. Such generalizations areimportant tools in science and are especially useful for thespatial analysis of climatic regions. Just as there is noagreed-upon climate classification system, neither is therea single set of empirical (statistical) or genetic (causal) cri-teria on which everyone agrees. Any classification systemshould be viewed as developmental, because it is alwaysopen to change and improvement.

A climate classification based on causative factors—forexample, the interaction of air masses—is a genetic clas-sification. A climate classification determined by statistical

data of observed effects is an empirical classification.Climate classifications based on temperature and precipi-tation data are examples of empirical classifications.Genetic classifications explain climates in terms of netradiation, thermal regimes, or air mass dominance over aregion. This chapter uses a system that draws on both agenetic and an empirical approach, which allows descrip-tion of the climatic regions and also provides informationas to why such climates are found where they occur.

One empirical classification system, publishedby C. W. Thornthwaite in 1948, identified moistureregions using the water-budget approach (Chapter 9) and

(text continued on page 283)

Global Patterns ofPrecipitation



Focus Study 10.1

The El Niño Phenomenon—Global Linkages

Climate is the consistent behavior ofweather over time, but average weatherconditions also include extremes thatdepart from normal. The El Niño–Southern Oscillation (ENSO) in thePacific Ocean forces the greatest in-terannual variability of temperatureand precipitation on a global scale.The two strongest ENSO events in120 years hit in 1997–1998 and1982–1983. The spring wildflowerbloom in Death Valley in 1998 pro-vides visible evidence of the resultantheavy rains (Figure 10.1.1). Peruvianscoined the name El Niño (“the boychild”) because these episodes seem tooccur around the traditional Decem-ber celebration time of Christ’s birth.Actually, El Niños can occur as earlyas spring and summer and persistthrough the year.

Revisit Figure 6.21 and see thatthe northward-flowing Peru currentdominates the region off SouthAmerica’s west coast. These coldwaters move toward the equator andjoin the westward movement of thesouth equatorial current.

The Peru current is part of thenormal counterclockwise circulation ofwinds and surface ocean currentsaround the subtropical high-pressure

cell dominating the eastern Pacific inthe Southern Hemisphere. As a result,a location such as Guayaquil, Ecuador,normally receives 91.4 cm (36 in.) ofprecipitation each year under dominanthigh pressure, whereas islands in theIndonesian archipelago receive morethan 254 cm (100 in.) under dominantlow pressure. This normal alignment ofpressure is shown in Figure 10.1.2a.

What Is ENSO?Occasionally, for unexplained reasons,pressure patterns and surface oceantemperatures shift from their usuallocations. Higher pressure than nor-mal develops over the western Pacificand lower pressure develops over theeastern Pacific. Trade winds normallymoving from east to west weaken andcan be reduced or even replaced byan eastward (west-to-east) flow. Theshifting of atmospheric pressure andwind patterns across the Pacific isthe Southern Oscillation. Chapter 6 dis-cussed the Pacific Decadal Oscillation(PDO) and its interrelation withENSO.

Sea-surface temperatures in-crease, sometimes more than 8 C°(14 F°) above normal in the central

and eastern Pacific during an ENSO,replacing the normally cold, up-welling, nutrient-rich water alongPeru’s coastline. Such ocean-surfacewarming, the “warm pool,” mayextend to the International Date Line.This surface pool of warm wateris known as El Niño. Thus, the desig-nation ENSO is derived—El Niño–Southern Oscillation. This conditionis shown in Figure 10.1.2b in illustra-tion and satellite image.

The thermocline (boundary ofcolder, deep-ocean water) lowers indepth in the eastern Pacific Ocean.The change in wind direction andwarmer surface water slows the nor-mal upwelling currents that controlnutrient availability. This loss of nu-trients affects the phytoplankton andfood chain, depriving many fish, ma-rine mammals, and predator birds ofnourishment.

Scientists at the National Oceano-graphic and Atmospheric Admin-istration (NOAA) speculate thatENSO events occurred more than adozen times since the fourteenth cen-tury. More recently, there was anENSO in 1982–1983 (second-strongestevent), 1986–1987, 1991–1993 (oneof the longest), and the most intense

(a) 1998 (b) 2002

FIGURE 10.1.1 El Niño’s impact on the desert.Death Valley, southeastern California, in (a) full spring bloom following record rains triggered by the1997–1998 El Niño and (b) the same scene in spring 2002 in its stark desert grandeur. A dramatic effectcaused by changes in the distant tropics of the Pacific Ocean. [Photos by Bobbé Christopherson.] (continued)



Focus Study 10.1 (continued)

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El NiñoNovember 10, 1997

La NiñaOctober 12, 1998

SST 28°C

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(d) A persistent La NiñaMarch 11, 2000

(e) June 7, 2001 (f) July 12, 2004

(h) August 26, 2007(g) February 22, 2005

FIGURE 10.1.2 Normal, El Niño, and La Niña changes in the Pacific.(a) Normal patterns in the Pacific; (b) El Niño wind and weather patterns across the Pacific Ocean and TOPEX/Poseidonsatellite image for November 1997 (white and red colors indicate warmer surface water—a warm pool). (c) TOPEX/Poseidonimage of La Niña conditions in transition in the Pacific in October 1998 (purple and blue colors for cooler surface water—acool pool). (d) A persistent La Niña in a March 2000 satellite image. (e) Image from June 2001 showing no El Niño asequatorial waters slowly warm with sea-surface temperature near normal. (f) The warm pool of waters retreated westward,leaving neutral or a mild La Niña off South America, July 2004 image. (g) More neutral conditions February 22, 2005. (h) LaNiña growing in size, August 26, 2007. [(a) and (b) adapted and author corrected from C. S. Ramage, “El Niño.” © 1986 by ScientificAmerican, Inc.; (b)–(e) TOPEX/Poseidon and (f, g, and h) Jason–1 images courtesy of Jet Propulsion Laboratory, NASA.]

EI Nin~o/ EI Nin~a



episode in 1997–1998 that disruptedglobal weather. The PDO shifted intoits negative phase in 1999, whichseems to dampen ENSO cycles andbring drier conditions to the Ameri-can West.

The expected interval for recur-rence is 3 to 5 years, but the intervalmay range from 2 to 12 years. Thefrequency and intensity of ENSOevents increased through the twenti-eth century, a topic of much researchby scientists to see if there is a relationto global climate change. Recentstudies suggest ENSO might be moreresponsive to global change than pre-viously thought. Surface temperaturesin the central tropical Pacific returnedto near normal (neutral) by mid-2001(Figure 10.1.2e).

La Niña—El Niño’s CousinWhen surface waters in the centraland eastern Pacific cool to belownormal by 0.4 C° (0.7 F°) or more, thecondition is dubbed La Niña, Spanishfor “the girl.” This is a weaker condi-tion and less consistent than El Niño.There is no correlation in the strengthor weakness of each. For instance, fol-lowing the record 1997–1998 ENSOevent, the subsequent La Niña was notas strong as predicted and shared thePacific with lingering warm water.

Between 1900 and 1998 therewere 13 La Niñas of note, the latestin 1988, 1995, late 1998 to 2000(Figure 10.1.2c and d), and 2007(Fig. 10.1.2h). According to National

Center for Atmospheric Research(NCAR) scientist Kevin Trenberth,El Niños occurred 31% and La Niñas23% of the time since 1950, with theremaining 46% of the time the Pacificbeing in a more neutral condition.Don’t look for symmetry and oppositeeffects between the two events, forthere is great variability possible, ex-cept perhaps in Indonesia where re-markable drought (El Niño) and heavyrain (La Niña) correlations seemstrong.

Global Effects Related to ENSOand La NiñaEffects related to ENSO and La Niñaoccur worldwide: droughts in SouthAfrica, southern India, Australia, andthe Philippines; strong hurricanes inthe Pacific, including Tahiti andFrench Polynesia; and flooding inthe southwestern United States andmountain states, Bolivia, Cuba,Ecuador, and Peru. In India, everydrought for more than 400 yearsseems linked to ENSO events. TheAtlantic hurricane season weakensduring El Niño years and strengthensduring La Niñas.

Precipitation in the southwesternUnited States is greater in El Niñothan La Niña years. The PacificNorthwest is wetter with La Niñathan El Niño. El Niño–enhancedrains in 1998 produced the wildflowerbloom in Death Valley pictured inFigure 10.1.1. The Colorado Riverflooding shown in the Chapter 9

opening photo and discussed inChapter 15’s Focus Study 15.1 was inpart attributable to the 1982–1983El Niño. Yet, as conditions vary, otherEl Niños have produced drought inthe very regions that flooded during aprevious episode.

Since the 1982–1983 event, andwith the development of remote-sensing satellites and computingcapability, scientists now are able toidentify the complex global intercon-nections among surface temperatures,pressure patterns in the Pacific, oc-currences of drought in some places,excessive rainfall in others, and thedisruption of fisheries and wildlife.

Discovery of these truly Earth-wide relations and spatial impacts is atthe heart of physical geography. Theclimate of one location is related toclimates elsewhere, although it shouldbe no surprise that Earth operates as avast integrated system. “It is fascinat-ing that what happens in one area canaffect the whole world. As to why thishappens, that’s the question of thecentury. Scientists are trying to makeorder out of chaos,” says NOAA sci-entist Alan Strong. (For ENSO moni-toring and forecasts, see the ClimatePrediction Center at http://www.ncep.noaa.gov/ or the Jet PropulsionLaboratory at http://topex-www.jpl.nasa.gov/mission/jason-1.html/ orhttp://sealevel.jpl.nasa.gov/science/jason1 or NOAA’s El Niño ThemePage at http://www.pmel.noaa.gov/toga-tao/el-nino/nino-home.html.)

vegetation types. Another empirical classification system isthe Köppen classification system. Wladimir Köppen(1846–1940), a German climatologist and botanist, designedthe widely recognized system. His classification work beganwith an article on heat zones in 1884 and continued through-out his career. The first wall map showing world climates,coauthored with his student Rudolph Geiger, was intro-duced in 1928 and soon was widely adopted. Köppen contin-ued to refine it until his death. In Appendix B, you find adescription of his system and the detailed criteria he used todistinguish climatic regions. The Köppen system provides usan outline and general base map.

Classification Categories The basis of any classifica-tion system is the choice of criteria or causative factorsused to draw lines between categories. Some climate

elements used include average monthly temperatures,average monthly precipitation, total annual precipitation,air mass characteristics, ocean currents and sea-surfacetemperatures, moisture efficiency, insolation, and net ra-diation, among others. As we devise spatial categories andboundaries, we must remember that boundaries really aretransition zones of gradual change. The trends and overallpatterns of boundary lines are more important than theirprecise placement, especially with the small scales gener-ally used for world maps. For more on boundaries, seeNews Report 10.1.

In this chapter, we focus on temperature and precipi-tation measurements and, for the desert areas, moistureefficiency. Keep in mind these are measurable resultsproduced by the climatic elements listed in the last para-graph. Figure 10.4 portrays six basic climate categories



News Report 10.1

What’s in a Boundary?

The boundary between mesothermaland microthermal climates is some-times placed along the isothermwhere the coldest month is -3°C(26.6°F) or lower. That might be anaccurate criterion for Europe, but forconditions in North America, the 0°C(32°F) isotherm is considered moreappropriate. The difference betweenthe 0 and -3°C isotherms covers anarea about the width of the state ofOhio. Remember, these isothermlines are really transition zones and donot mean abrupt change from onetemperature to another.

A line denoting at least onemonth below freezing runs from

New York City roughly along theOhio River, trending westward untilit meets the dry climates in thesoutheastern corner of Colorado. InFigure 10.5 you see this boundaryused. A line marking -3°C as thecoldest month would run farthernorth along Lake Erie and the south-ern tip of Lake Michigan. In addition,remember that from year to year,the position of the 0°C isothermfor January can shift several hundredkilometers as weather conditions vary.

Climate change adds anotherdimension to this question of accurateplacement of statistical boundaries—

for they are shifting. The Inter-governmental Panel on ClimateChange (IPCC, discussed later in thischapter) predicts a 150- to 550-km(90- to 350-mi) range of possiblepoleward shift of climatic patterns inthe midlatitudes during this century.Such change in climate would placeOhio, Indiana, and Illinois withinclimate regimes now experienced inArkansas and Oklahoma. As you ex-amine North America in Figure 10.5,use the graphic scale to get an ideaof the magnitude of these potentialshifts. Boundaries are indeeddynamic.

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FIGURE 10.4 Climate regions generalized.Six general climate categories, keyed to the legend and coloration in the Figure 10.5 map. Relative to the question asked about your campus and birthplace in the caption toFigure 10.3, locate these two places on this climate map.

Global ClimateMaps, World MapReferences



and their regional types that provide us with a structurefor our discussion in this chapter.

• Tropical (equatorial and tropical latitudes)rain forest (rainy all year)monsoon (6 to 12 months rainy)savanna (less than 6 months rainy)

• Mesothermal (midlatitudes, mild winter)humid subtropical (hot summers)marine west coast (warm to cool summers)Mediterranean (dry summers)

• Microthermal (mid and high latitudes, cold winter)humid continental (hot to warm summers)subarctic (cool summers to very cold winters)

• Polar (high latitudes and polar regions)tundra (high latitude or high altitude)ice caps and ice sheets (perpetually frozen)polar marine

• Highland (compared to lowlands at the same lati-tude, highlands have lower temperatures—recall thenormal lapse rate)

Only one climate category is based on moisture efficiencyas well as temperature:

• Desert (permanent moisture deficits)arid deserts (tropical, subtropical hot and midlati-tude cold)

semiarid steppes (tropical, subtropical hot andmidlatitude cold)

Global Climate Patterns Adding detail to Figure 10.4,we develop the world climate map presented inFigure 10.5. The following sections describe specificclimates, organized around each of the main climate cate-gories listed previously. An opening box at the beginningof each climate section gives a simple description of theclimate category and causal elements that are in opera-tion. A world map showing distribution and the featuredrepresentative cities also is in the introductory box foreach climate. The names of the climates appear in italicsin the chapter.

Climographs exemplify particular climates for selectedcities. A climograph is a graph that shows monthly tem-perature and precipitation, location coordinates, averageannual temperature, total annual precipitation, elevation,the local population, annual temperature range, annualhours of sunshine (if available, as an indication of cloudi-ness), and a location map. Along the top of each climo-graph are the dominant weather features that areinfluential in that climate.

Discussions of soils, vegetation, and major terrestrialbiomes that fully integrate these global climate patterns arein Part IV. Table 20.1 synthesizes all this information andenhances your understanding of this chapter, so pleaseplace a tab on that page and refer to it as you read.

Tropical climates occupy about 36% of Earth’s surface, in-cluding both ocean and land areas—Earth’s most extensiveclimate category. The tropical climates straddle the equatorfrom about 20° N to 20° S, roughly between the Tropics ofCancer and Capricorn, thus the designation tropical. Tropicalclimates stretch northward to the tip of Florida and south-central Mexico, central India, and Southeast Asia. These cli-mates are truly winterless. Consistent daylength and almostperpendicular Sun angle throughout the year generate thiswarmth. Important causal elements include:

• Consistent daylength and insolation input, which pro-duce consistently warm temperatures;

• Intertropical convergence zone (ITCZ), which brings rainas it shifts seasonally with the high Sun;

• Warm ocean temperatures and unstable maritime airmasses.

Tropical climates have three distinct regimes: tropical rain for-est (ITCZ present all year), tropical monsoon (ITCZ present for

Tropical Climates (equatorial and tropical latitudes)

6 to 12 months), and tropical savanna (ITCZ present for lessthan 6 months).

Uaupés, Brazil

Yangon, Myanmar

Arusha,Tanzania

Tropical rain forest Tropical savannaTropical monsoon

Tropical Rain Forest ClimatesThe tropical rain forest climate is constantly moist andwarm. Convectional thunderstorms, triggered by localheating and trade-wind convergence, peak each day from

mid-afternoon to late evening inland and earlier in theday where marine influence is strong along coastlines.Precipitation follows the migrating intertropical conver-gence zone (ITCZ, Chapter 6). The ITCZ shifts north-ward and southward with the summer Sun throughout the



I.T.C.Z. July

I.T.C.Z. January

mT cT

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ppe

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Tropic of Cancer

Arid deserts Tropical, subtropical (hot) Midlatitude (cold)

Semiarid steppes Tropical, subtropical (hot) Midlatitude (cold)

DESERT CLIMATES

Tropical rain forest Rainy all yearTropical monsoon 6 to 12 months rainy

Tropical savanna Less than 6 months rainy

TROPICAL CLIMATES

Warm current

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ITCZ July

ITCZ January

Subtropical high

Low

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OCEAN CURRENTS (Fig. 6.21)

AIR PRESSURE SYSTEMS (Fig. 6.10)

INTERTROPICAL CONVERGENCEZONE

AIR MASSES (Fig. 8.2)

SHLH

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Continental arctic (very cold, dry)

Continental tropical (hot, dry summer only)

Maritime equatorial (warm, wet)

mP

cPmT

cAcT

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Figs. 6.10, 6.12

FIGURE 10.5 World climate classification.Annotated on this map are selected air masses, near-shore ocean currents, pressure systems, and theJanuary and July locations of the ITCZ. Use the colors in the legend to locate various climate types;some labels of the climate names appear in italics on the map to guide you.



I.T.C.Z. January

I.T.C.Z. January

I.T.C.Z. July

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BENGAL

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POLAR CLIMATESHIGHLANDS

Humid subtropical Moist all year, hot summer

Humid subtropical Winter-dry, hot to warm summers

Marine west coast Moist all year, warm to cool summers

Mediterranean Summer dry, hot to warm summers

MESOTHERMAL CLIMATES

Humid continental Hot summers: moist all year Asian winter-dry

Humid continental Mild summers: moist all year Asian winter-dry

Subarctic regions Cool summers

Subarctic regions Very cold winters

MICROTHERMAL CLIMATES

0

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High rainfall sustains lush evergreen broadleaf treegrowth, producing Earth’s equatorial and tropical rainforests (Figure 10.6). The leaf canopy is so dense that lit-tle light diffuses to the forest floor, leaving the groundsurface dim and sparse in plant cover. Dense surface vege-tation occurs along riverbanks, where light is abundant.(Widespread deforestation of Earth’s rain forest isdetailed in Chapter 20.)

High temperature promotes energetic bacterialaction in the soil so that organic material is quickly con-sumed. Heavy precipitation washes away certain mineralsand nutrients. The resulting soils are somewhat sterileand can support intensive agriculture only if supplementedby fertilizer.

Uaupés, Brazil (Figure 10.7), is characteristic oftropical rain forest. On the climograph you can see that thelowest-precipitation month receives nearly 15 cm (6 in.),and the annual temperature range is barely 2 C° (3.6 F°).In all such climates, the diurnal (day-to-night) temperaturerange exceeds the annual average minimum–maximum(coolest to warmest) range: Day–night temperatures canrange more than 11 C° (20 F°), more than 5 times theannual monthly average range.

The only interruption of tropical rain forestclimates across the equatorial region is in the highlandsof the South American Andes and in East Africa (see

FIGURE 10.6 The tropical rain forest.The lush equatorial rain forest and Sangha River, nearOuesso, Congo, Africa. [Photo by BIOS M. Gunther/PeterArnold, Inc.]

Uaupés

BRAZIL

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Station: Uaupés, Brazil Lat/long: 0°08' S 67°05' WAvg. Ann. Temp.: 25°C (77°F)Total Ann. Precip.: 291.7 cm (114.8 in.)

Elevation: 86 m (282.2 ft)Population: 10,000Ann. Temp. Range: 2 C° (3.6 F°)Ann. Hr of Sunshine: 2018

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⎧ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩

FIGURE 10.7 Tropical rain forest climate.(a) Climograph for Uaupés, Brazil (tropical rain forest).(b) The rain forest near Uaupés along a tributary of theRio Negro. [Photo by Will and Deni McIntyre/PhotoResearchers, Inc.]

year, but it influences tropical rain forest regions all yearlong. Not surprisingly, water surpluses are enormous,creating the world’s greatest stream discharges in theAmazon and Congo rivers.



Figure 10.5). There, higher elevations produce lowertemperatures; Mount Kilimanjaro is less than 4° south ofthe equator, but at 5895 m (19,340 ft) it has permanentglacial ice on its summit (although this ice is shrinkingdue to higher air temperatures). Such mountainous sitesfall within the highland climate category.

Tropical Monsoon ClimatesThe tropical monsoon climates feature a dry season thatlasts 1 or more months. Rainfall brought by the ITCZfalls in these areas from 6 to 12 months of the year.(Remember, the ITCZ affects the tropical rain forestclimate region throughout the year.) The dry season oc-curs when the convergence zone is not overhead. Yangon,Myanmar (formerly Rangoon, Burma), is an example of

this climate type, as illustrated by the climograph andphotograph in Figure 10.8. Mountains prevent cold airmasses from central Asia getting into Yangon, resulting inits high average annual temperatures.

About 480 km (300 mi) north in another coastal city,Sittwe (Akyab), Myanmar, on the bay of Bengal, annualprecipitation rises to 515 cm (203 in.) compared toYangon’s 269 cm (106 in.). Therefore, Yangon is a driertropical monsoon area than farther north along the coastbut still receives more than the 250-cm criteria.

Tropical monsoon climates lie principally alongcoastal areas within the tropical rain forest climaticrealm and experience seasonal variation of wind and precipitation. Evergreen trees grade into thornforests on the drier margins near the adjoining savannaclimates.

INDIAN

OCEAN

Yangon

Sittwe

MYANMAR

Equator

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Station: Yangon, Myanmar*Lat/long: 16°47' N 96°10' EAvg. Ann. Temp.: 27.3°C (81.1°F)Total Ann. Precip.: 268.8 cm (105.8 in.)

*(Formerly Rangoon, Burma)

Elevation: 23 m (76 ft)Population: 6,000,000Ann. Temp. Range: 5.5 C° (9.9 F°)

38(100)

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Subtropicalhigh⎧ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎩ ⎧ ⎪ ⎨ ⎪ ⎩

ITCZ⎧ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎩

FIGURE 10.8 Tropical monsoon climate.(a) Climograph for Yangon, Myanmar (formerly Rangoon, Burma) (tropical monsoon); city of Sittwe alsonoted on map. (b) The monsoonal forest near Malang, Java, at the Purwodadi Botanical Gardens.[Photo by Tom McHugh/Photo Researchers, Inc.]



Tropical Savanna ClimatesTropical savanna climates exist poleward of the tropical rainforest climates. The ITCZ reaches these climate regionsfor about six months or less of the year as it migrates withthe summer Sun. Summers are wetter than winters be-cause convectional rains accompany the shifting ITCZwhen it is overhead. This produces a notable dry condi-tion when the ITCZ is farthest away and high pressuredominates. Thus, POTET (natural moisture demand)exceeds PRECIP (natural moisture supply) in winter,causing water-budget deficits.

Temperatures vary more in tropical savanna climatesthan in tropical rain forest regions. The tropical savannaregime can have two temperature maximums during the yearbecause the Sun’s direct rays are overhead twice—beforeand after the summer solstice in each hemisphere as theSun moves between the equator and the tropic. Dominantgrasslands with scattered trees, drought resistant to copewith the highly variable precipitation, characterize thetropical savanna regions.

Arusha, Tanzania, is a characteristic tropical savannacity (Figure 10.9). This metropolitan area of more

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Station: Arusha, TanzaniaLat/long: 3°24' S 36°42' EAvg. Ann. Temp.: 26.5°C (79.7°F)Total Ann. Precip.: 119 cm (46.9 in.)

Elevation: 1387 m (4550 ft)Population: 1,368,000Ann. Temp. Range: 4.1 C° (7.4 F°)Ann. Hr of Sunshine: 2600

Nairobi

Mombasa

Tanga

Arusha

Mt. Kilimanjaro

MASAISTEPPE

K E N Y A

TANZANIA

5°

40°

0

50 KM0

50 MI

(b) (c)

FIGURE 10.9 Tropical savanna climate.(a) Climograph for Arusha, Tanzania (tropical savanna); note the intense dry period. (b) Characteristic landscape in Kenya, withplants adapted to seasonally dry water budgets. (c) Extreme northern Australia on the Cape York Peninsula; the tropical savannafeatures eucalyptus trees, grasses, and conical termite mounds, in an open savanna in Mungkan Kandju National Park. [Photosby (b) Stephen J. Krasemann/DRK Photo; (c) B. G. Thomson, courtesy of The Wilderness Society.]



Humid Subtropical ClimatesThe humid subtropical hot-summer climates are either moistall year or have a pronounced winter-dry period, as occursin eastern and southern Asia. Maritime tropical air massesgenerated over warm waters off eastern coasts influencehumid subtropical hot-summer climates during summer.This warm, moist, unstable air produces convectionalshowers over land. In fall, winter, and spring, maritimetropical and continental polar air masses interact, generat-ing frontal activity and frequent midlatitude cyclonicstorms. These two mechanisms produce year-roundprecipitation. Overall, precipitation averages 100–200 cm(40–80 in.) a year.

Nagasaki, Japan (Figure 10.10), is characteristic ofan Asian humid subtropical hot-summer station, whereasColumbia, South Carolina, is characteristic of the NorthAmerican climate region (Figure 10.11). Unlike theprecipitation of humid subtropical hot-summer cities inthe United States (Atlanta, Memphis, Norfolk, New

Orleans, and Columbia), Nagasaki’s winter precipitationis a bit less because of the effects of the Asian monsoon.However, the lower precipitation of winter is not quitedry enough to change its category to a winter-dry. Incomparison to higher rainfall amounts in Nagasaki(196 cm, 77 in.), Columbia’s precipitation totals 126.5 cm(49.8 in.); compare to Atlanta’s 122 cm (48 in.) annually(Figure 10.11b).

Humid subtropical winter-dry climates are related tothe winter-dry, seasonal pulse of the monsoons. Theyextend poleward from tropical savanna climates and havea summer month that receives 10 times more precipita-tion than their driest winter month. A representativestation is Chengdu, China. Figure 10.12 demonstratesthe strong correlation between precipitation and thehigh-summer Sun.

The habitability of the humid subtropical hot-summerand humid subtropical winter-dry climates and their abilityto sustain populations are borne out by the concentration

Mesothermal, meaning “middle temperature,” describes thesewarm and temperate climates where true seasonality beginsand seasonal contrasts in vegetation, soil, and human lifestyleadaptations are evident. Mesothermal climates occupy thesecond-largest percentage of Earth’s land and sea surface—more than half of Earth’s oceans and about one-third of itsland area. Approximately 55% of the world’s population re-sides in these climates.

The mesothermal climates, and nearby portions of the mi-crothermal (cold winter) climates, are regions of great weathervariability, for these are the latitudes of greatest air mass inter-action. Causal elements include:

• Shifting air masses of maritime and continental originare guided by upper-air westerly winds and undulatingRossby waves and jet streams.

• Migrating cyclonic (low-pressure) and anticyclonic (high-pressure) systems bring changeable weather conditionsand air mass conflicts.

• Sea-surface temperatures of offshore ocean currentsinfluence air mass strength: cooler water temperaturesalong west coasts (weaken) and warmer water along eastcoasts (strengthen).

• Summers transition from hot to warm to cool as youmove away from the tropics. Climates are humid, exceptwhere subtropical high pressure produces dry summerconditions.

Mesothermal Climates (midlatitudes, mild winters)

Mesothermal climates have four distinct regimes based onprecipitation variability: humid subtropical hot-summer(moist all year), humid subtropical winter-dry (hot to warmsummers, in Asia), marine west coast (warm to cool summers,moist all year), and Mediterranean (warm to hot summers, drysummers).

San Francisco,CA

Columbia, SC Sevilla,Spain

Chengdu,China

Nagasaki,Japan

Dunedin,New Zealand

Humid subtropicalMarine west coast

Mediterranean

Bluefield, WV

than 1,368,000 people is east of the famous SerengetiPlains savanna and Olduvai Gorge, site of humanorigins, and north of Tarangire National Park. Tempera-tures are consistent with tropical climates, despite the elevation (1387 m) of the station. Note the marked

dryness from June to October, which defines changingdominant pressure systems rather than annualchanges in temperature. This region is near the transi-tion to the dryer desert hot-steppe climates to thenortheast.



of people in north-central India, the bulk of China’s1.3 billion people, and the many who live in climaticallysimilar portions of the United States.

The intense summer rains of the Asian monsooncan cause problems, as they did each year between 2004and 2007, producing floods in India and Bangladesh. Occa-sional dramatic thunderstorms and tornadoes are notablein the southeastern United States, with tornado occur-rences seemingly breaking previous records each year.

The monsoonal winter-dry climates hold several pre-cipitation records. Cherrapunji, India, in the Assam Hillssouth of the Himalayas, is the all-time precipitation recordholder for a single year and for every other time intervalfrom 15 days to 2 years. Because of the summer monsoonsthat pour in from the Indian Ocean and the Bay of Bengal,Cherrapunji has received 930 cm (30.5 ft) of rainfall in1 month and 2647 cm (86.8 ft) in 1 year—both records.

Marine West Coast ClimatesMarine west coast climates, featuring mild winters andcool summers, dominate Europe and other middle-to-high-latitude west coasts (see Figure 10.5). In the UnitedStates, these climates with their cooler summers are incontrast to the hot-summer humid climate of the south-eastern United States.

Maritime polar air masses—cool, moist, unstable—dominate marine west coast climates. Weather systems form-ing along the polar front and maritime polar air massesmove into these regions throughout the year, makingweather quite unpredictable. Coastal fog, annually totaling30 to 60 days, is a part of the moderating marine influence.Frosts are possible and tend to shorten the growing season.

Marine west coast climates are unusually mild for theirlatitude. They extend along the coastal margins of theAleutian Islands in the North Pacific, cover the southernthird of Iceland in the North Atlantic and coastal Scandi-navia, and dominate the British Isles. It is hard to imaginethat such high-latitude locations can have average monthlytemperatures above freezing throughout the year.

Unlike the extensive influence of marine west coast re-gions in Europe, mountains restrict this climate to coastalenvirons in Canada, Alaska, Chile, and Australia. The tem-perate rain forest of Vancouver Island is representative ofthese moist and cool conditions (Figure 10.13). (A temper-ature graph for Vancouver, British Columbia, appears inFigure 5.12 with a photo of this marine west coast city.)

The climograph for Dunedin, New Zealand, demon-strates the moderate temperature patterns and the annualtemperature range for a marine west coast city in theSouthern Hemisphere (Figure 10.14).

Nagasaki

JAPAN

CHINA

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Station: Nagasaki, JapanLat/long: 32°44' N 129°52' EAvg. Ann. Temp.: 16°C (60.8°F)Total Ann. Precip.: 195.7 cm (77 in.)

Elevation: 27 m (88.6 ft)Population: 1,585,000Ann. Temp. Range: 21 C° (37.8 F°)Ann. Hr of Sunshine: 2131

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Asian monsooneffects⎧ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩

FIGURE 10.10 Humid subtropical hot-summer climate, rainy all year, Asianregion.(a) Climograph for Nagasaki, Japan (humidsubtropical). (b) Landscape near Nagasaki.[(b) Photo by Ken Straiton/First Light.]



An interesting anomaly occurs in the eastern UnitedStates. In portions of the Appalachian highlands, in-creased elevation lowers summer temperatures in the sur-rounding humid subtropical hot-summer climate, producinga marine west coast cooler summer. The climograph forBluefield, West Virginia (Figure 10.15), reveals marinewest coast temperature and precipitation patterns, despiteits location in the east. Vegetation similarities between the

Appalachians and the Pacific Northwest have enticedmany emigrants from the East to settle in these climati-cally familiar environments in the Northwest.

Mediterranean Dry-Summer ClimatesAcross the planet during summer months, shifting cells ofsubtropical high pressure block moisture-bearing windsfrom adjacent regions. This shifting of stable, warm to

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Cyclonicstormtracks

Cyclonicstormtracks ⎧ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎩⎧ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎩ ⎧ ⎪ ⎨ ⎪ ⎩

Station: Columbia, South CarolinaLat/long: 34° N 81° WAvg. Ann. Temp.: 17.3°C (63.1°F)Total Ann. Precip.: 126.5 cm (49.8 in.)

Elevation: 96 m (315 ft)Population: 116,000Ann. Temp. Range: 20.7 C° (37.3 F°)Ann. Hr of Sunshine: 2800

95

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Charlotte

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Augusta MyrtleBeach

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ATLANTICOCEAN

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(c)

FIGURE 10.11 Humid subtropical hot-summerclimate, rainy all year, American region.(a) Climograph for Columbia, South Carolina (humidsubtropical ). Note the more consistent precipitationpattern compared to Nagasaki, as Columbiareceives seasonal cyclonic storm activity andsummer convection showers within maritimetropical air. (b) The mixed deciduous and evergreenforest in southern Georgia, typical of the humidsubtropical southeastern United States. (c) Thisscene is from the humid subtropical portion ofArgentina along the Paraná River, northwest ofBuenos Aires. [(b) and (c) Photos by BobbéChristopherson.]



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Station: Chengdu, ChinaLat/long: 30°40' N 104°04' EAvg. Ann. Temp.: 17°C (62.6°F)Total Ann. Precip.: 114.6 cm (45.1 in.)

Elevation: 498 m (1633.9 ft)Population: 2,500,000Ann. Temp. Range: 20 C° (36 F°)Ann. Hr of Sunshine: 1058

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FIGURE 10.12 Humid subtropical winter-dry climate.(a) Climograph for Chengdu, China (humid subtropical ).Note the summer-wet monsoonal precipitation. (b)Landscape of southern interior China characteristic ofthis winter-dry climate. This valley is near Mount Daliangin Sichuan Province. [Photo by Jin Zuqi/Sovfoto/Eastfoto.]

FIGURE 10.13 A marine west coast climate.Temperate rain forest in the Macmillan Provincial Park, centralVancouver Island, British Columbia, Canada (marine westcoast), features a Douglas fir forest, with western red cedarand hemlock. [Photo by Bobbé Christopherson.]

hot, dry air over an area in summer and away from theseregions in the winter creates a pronounced dry-summerand wet-winter pattern. For example, the continentaltropical air mass over the Sahara in Africa shifts north-ward in summer over the Mediterranean region andblocks maritime air masses and cyclonic storm tracks. TheMediterranean climate designation specifies that at least70% of annual precipitation occurs during the wintermonths. This is in contrast to the majority of the worldthat experiences summer-maximum precipitation.

Worldwide, cool offshore ocean currents (the Cali-fornia current, Canary current, Peru current, Benguelacurrent, and West Australian current) produce stability inoverlying air masses along west coasts, poleward ofsubtropical high pressure. The world climate map (seeFigure 10.5) shows Mediterranean dry-summer climatesalong the western margins of North America, centralChile, and the southwestern tip of Africa, as well as acrosssouthern Australia and the Mediterranean Basin—the cli-mate’s namesake region. Examine the offshore currentsalong each of these regions on the world climate map.

Figure 10.16 compares the climographs of theMediterranean dry-summer cities of San Francisco and



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Station: Dunedin, New ZealandLat/long: 45°54' S 170°31' EAvg. Ann. Temp.: 10.2°C (50.3°F)Total Ann. Precip.: 78.7 cm (31.0 in.)

Elevation: 1.5 m (5 ft)Population: 120,000Ann. Temp. Range: 14.2 C° (25.5 F°)

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Cyclonic stormtracks⎧ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩

FIGURE 10.14 A Southern Hemisphere marine west coastclimate.(a) Climograph for Dunedin, New Zealand (marine west coast).(b) Meadow, forest, and mountains on South Island, NewZealand. [Photo by Brian Enting/Photo Researchers, Inc.]

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Station: Bluefield, West VirginiaLat/long: 37°16' N 81°13' WAvg. Ann. Temp.: 12°C (53.6°F)Total Ann. Precip.: 101.9 cm (40.1 in.)

Elevation: 780 m (2559 ft)Population: 11,000Ann. Temp. Range: 21 C° (37.8 F°)

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FIGURE 10.15 Marine west coast climate in theAppalachians of the East.(a) Climograph for Bluefield, West Virginia (marine westcoast). (b) Characteristic mixed forest of Dolly SodsWilderness in the Appalachian highlands. [Photo by DavidMuench Photography, Inc.]



SanFrancisco

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Station: Sevilla, SpainLat/long: 37°22' N 6°00' WAvg. Ann. Temp.: 18°C (64.4°F)Total Ann. Precip.: 55.9 cm (22 in.)


Station: San Francisco, CaliforniaLat/long: 37°37' N 122°23' WAvg. Ann. Temp.: 14°C (57.2°F)Total Ann. Precip.: 47.5 cm (18.7 in.)

Elevation: 5 m (16.4 ft)Population: 747,000Ann. Temp. Range: 9 C° (16.2 F°)Ann. Hr of Sunshine: 2975

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Cyclonicstorm tracks⎧ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎩

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Cyclonicstorm tracks

FIGURE 10.16 Mediterranean climates, California and Spain.Climographs for (a) San Francisco, California, with its cooler drysummer and (b) Sevilla, Spain, with its hotter dry summer.(c) Central California Mediterranean landscape of oak savanna.(d) The countryside around Olvera, Andalusia, Spain. [Photos by(c) Bobbé Christopherson; (d) Kaz Chiba/Liaison Agency, Inc.]



Sevilla (Seville), Spain. Coastal maritime effects moderateSan Francisco’s climate, producing a cooler summer. Thetransition to a hot summer occurs no more than 24–32 km(15–20 mi) inland from San Francisco. The photos inFigure 10.16 show oak-savanna landscapes near Olvera,Spain, and in central California.

The Mediterranean dry-summer climate brings summerwater-balance deficits. Winter precipitation recharges soil

moisture, but water use usually exhausts soil moisture bylate spring. Large-scale agriculture requires irrigation,although some subtropical fruits, nuts, and vegetablesare uniquely suited to these conditions. Natural vegeta-tion features a hard-leafed, drought-resistant varietyknown locally as chaparral in the western United States.(Chapter 20 discusses local names for this type of vegeta-tion in other parts of the world.)

Humid microthermal climates have longer winters thanmesothermal climates, with summer warmth. Here the termmicrothermal means cool temperate to cold. Approximately21% of Earth’s land surface is influenced by these climates,equaling about 7% of Earth’s total surface. These climatesoccur poleward of the mesothermal climates and experiencegreat temperature ranges related to continentality and air massconflicts.

Temperatures decrease with increasing latitude and towardthe interior of continental landmasses and result in intenselycold winters. Precipitation varies between moist-all-year re-gions (the northern tier across the United States and Canada,eastern Europe through the Ural Mountains) and winter-dryregions associated with the Asian monsoon.

In Figure 10.5, note the absence of microthermal climatesin the Southern Hemisphere. Because the Southern Hemi-sphere lacks substantial landmasses, microthermal climatesdevelop there only in highlands. Important causal elementsinclude:

• Increasing seasonality (daylength and Sun altitude) andgreater temperature ranges (daily and annually).

• Upper-air westerly winds and undulating Rossby waves,which bring warmer air northward and colder air south-ward for cyclonic activity, and convectional thunder-storms from mT air masses in summer.

• Asian winter-dry pattern for the microthermal climates,increasing east of the Ural Mountains to the PacificOcean and eastern Asia.

Microthermal Climates (mid- and high-latitudes, cold winters)

• Hot summers cooling northward from the mesothermalclimates, and short spring and fall seasons surroundingwinters that are cold to very cold.

• Continental interiors serving as source regions for intensecontinental polar (cP) air masses that dominate winter,blocking cyclonic storms.

Microthermal climates have four distinct regimes based onincreasing cold with latitude and precipitation variability:humid continental hot-summer (Chicago, New York); humidcontinental mild-summer (Duluth, Toronto, Moscow); andsubarctic climates featuring cool summers, such as Churchill,Manitoba, and the formidable extremes of frigid, very coldwinters in Verkhoyansk and northern Siberia.

Churchill, Manitoba

New York,NY

Moscow, Russia

Verkhoyansk,Russia

Dalian, China

Subarctic, cool summers

Subarctic, very cold winters

Hot, humid continental

Warm, humid continental

Humid Continental Hot-Summer ClimatesHumid continental hot-summer climates are differentiatedby their annual precipitation distribution. In the summer,maritime tropical air masses influence both humid conti-nental moist-all-year and winter-dry climates. In NorthAmerica, frequent weather activity is possible betweenconflicting air masses—maritime tropical and continentalpolar—especially in winter. The climograph for NewYork City and photo (Figure 10.17a, c) and Dalian, China(10.17b,d), illustrate these two hot-summer microthermalclimates.

Before European settlement, forests covered thehumid continental hot-summer climatic region of theUnited States as far west as the Indiana–Illinois border.Beyond that approximate line, tall-grass prairies extendedwestward to about the 98th meridian (98° W in central

Kansas) and the approximate location of the 51-cm (20-in.) isohyet (line of equal precipitation). Further west,the short-grass prairies reflected lower precipitationreceipts.

The dry winter associated with the vast Asianlandmass, specifically Siberia, results from a dry-winterhigh-pressure anticyclone. The dry monsoons of south-ern and eastern Asia are produced in the winter monthsby this system, as winds blow out of Siberia toward thePacific and Indian oceans. The Dalian, China, climo-graph demonstrates this dry-winter tendency. Theintruding cold of continental air is a significant winterfeature.

Deep sod made farming difficult for the first settlersof the American prairies, as did the climate. However,native grasses soon were replaced with domesticated



New YorkCity

Dalian

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Station: New York, New YorkLat/long: 40°46' N 74°01' WAvg. Ann. Temp.: 13°C (55.4°F)Total Ann. Precip.: 112.3 cm (44.2 in.)


35.0(14)

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Station: Dalian, ChinaLat/long: 38°54' N 121°54' EAvg. Ann. Temp.: 10°C (50°F)Total Ann. Precip.: 57.8 cm (22.8 in.)


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Cyclonic storm tracks(summer convection)⎧ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩


(c) (d)

FIGURE 10.17 Humid continental hot-summer climates,New York and China.Climographs for (a) New York City (humid continental hotsummer that is moist all year) and (b) Dalian, China (humidcontinental hot summer that is dry in winter). (c) The “LiteraryWalk” with a canopy of American elms in New York City’sCentral Park emerging from winter just as spring and the returnof leaves and warmth begin. (d) Dalian, China, cityscape andpark in summer. [Photos (c) by Bobbé Christopherson; and (d)courtesy of the Paul Louis collection.]



(a) (b)

FIGURE 10.18 Central Indiana and Buffalo, NY, landscapes.(a) Typical humid continental deciduous forest and ready-to-harvest soybean field in central Indiananear Zelma in the American Midwest. (b) This forest near Buffalo, NY, with its characteristic mix ofmaple–beech–birch, with some elm and ash, is in transition as temperatures increase this century.[Photos by Bobbé Christopherson.]

wheat and barley. Various inventions (barbed wire, theself-scouring steel plow, well-drilling techniques, wind-mills, railroads, and the six-shooter) aided nonnative peo-ples’ expansion into the region. In the United Statestoday, the humid continental hot-summer region is the loca-tion of corn, soybean, hog, feed crop, dairy, and cattleproduction (Figure 10.18a).

The region around Buffalo, New York, is in a climat-ic transition from milder to hotter summers. As the aver-age July temperature increases further above 22°C(71.6°F), Buffalo’s climatic designation will become morelike that of New York and Chicago. The surroundingforest in the latter half of this century will shift from thepresent maple–beech–birch, with some elm and ash, to adominant oak–hickory–hemlock mix (Figure 10.18b).Such shifting of climatic boundaries and their relatedecosystems is occurring worldwide.

Humid Continental Mild-Summer ClimatesSoils are thinner and less fertile in the cooler microther-mal climates, yet agricultural activity is important andincludes dairy cattle, poultry, flax, sunflowers, sugar beets,wheat, and potatoes. Frost-free periods range from fewerthan 90 days in the north to as many as 225 days inthe south. Overall, precipitation is less than in the hot-summer regions to the south; however, notably heaviersnowfall is important to soil-moisture recharge when itmelts. Various snow-capturing strategies are in use, in-cluding fences and tall stubble left standing after harvest

in fields to create snowdrifts and thus more moistureretention on the soil.

Characteristic cities are Duluth, Minnesota, and SaintPetersburg, Russia. Figure 10.19 presents a climographfor Moscow, which is at 55° N, or about the same latitudeas the southern shore of Hudson Bay in Canada. Thephotos of landscapes near Moscow and Sebago Lake, in-land from Portland, Maine, show summer and late winterscenes, respectively.

The dry-winter aspect of the mild-summer climateoccurs only in Asia, in a far-eastern area poleward of thewinter-dry mesothermal climates. A representative humidcontinental mild-summer climate along Russia’s east coast isVladivostok, usually one of only two ice-free ports in thatcountry.

Subarctic ClimatesFarther poleward, seasonal change becomes greater. Theshort growing season is more intense during long summerdays. The subarctic climates include vast stretches of Alaska,Canada, northern Scandinavia with their cool summers, andSiberian Russia with its very cold winters. Discoveries of min-erals and petroleum reserves and the Arctic Ocean sea-icelosses have led to new interest in portions of these regions.

Areas that receive 25 cm (10 in.) or more of precipita-tion a year on the northern continental margins and arecovered by the so-called snow forests of fir, spruce, larch,and birch are the boreal forests of Canada and the taiga ofRussia. These forests are in transition to the more opennorthern woodlands and to the tundra region of the farnorth. Forests thin out to the north when the warmest



month drops below an average temperature of 10°C(50°F). During the decades ahead, the boreal forests areshifting northward into the tundra in response to climatechange with higher temperatures (Figure 10.20b).

Soils are thin in these lands once scoured by glaciers.Precipitation and potential evapotranspiration both arelow, so soils are generally moist and either partially or to-tally frozen beneath the surface, a phenomenon known aspermafrost (discussed in Chapter 17).

The Churchill, Manitoba, climograph (Figure 10.20)shows average monthly temperatures below freezing for7 months of the year, during which time light snow coverand frozen ground persist. High pressure dominatesChurchill during its cold winter—this is the source regionfor the continental polar air mass. Churchill is representa-tive of the subarctic climate, with a cool summer: annualtemperature range of 40 C° (72 F°) and low precipitationof 44.3 cm (17.4 in.).

The subarctic climates that feature a dry and very coldwinter occur only within Russia. The intense cold ofSiberia and north-central and eastern Asia is difficult tocomprehend, for these areas experience an averagetemperature lower than freezing for 7 months; minimumtemperatures of below -68°C (–90°F) were recordedthere, as described in Chapter 5. Yet summer maximumtemperatures in these same areas can exceed �37°C(�98°F).

An example of this extreme subarctic climate withvery cold winters is Verkhoyansk, Siberia (Figure 10.21).For 4 months of the year, average temperaturesfall below -34°C (–30°F). Verkhoyansk has probablythe world’s greatest annual temperature range fromwinter to summer: a remarkable 63 C° (113.4 F°). Win-ters feature brittle metals and plastics, triple-thick win-dowpanes, and temperatures that render straightantifreeze a solid.

Moscow

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Station: Moscow, RussiaLat/long: 55°45' N 37°34' EAvg. Ann. Temp.: 4°C (39.2°F)Total Ann. Precip.: 57.5 cm (22.6 in.)


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FIGURE 10.19 Humid continental mild-summer climate.(a) Climograph for Moscow, Russia (humid continental mild summer). (b) Fields near Saratov,Russia, during the short summer season. (c) Late-winter scene of Sebago Lake and forests inlandfrom Portland, Maine. [(b) Photo by Wolfgang Kaehler/Liaison Agency, Inc.; (c) photo by BobbéChristopherson.]



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Station: Churchill, ManitobaLat/long: 58°45' N 94°04' WAvg. Ann. Temp.: –7°C (19.4°F)Total Ann. Precip.: 44.3 cm (17.4 in.)

Elevation: 35 m (114.8 ft)Population: 1400Ann. Temp. Range: 40 C° (72 F°)Ann. Hr of Sunshine: 1732

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(b) (c)

(d) (e)

FIGURE 10.20 Subarctic cool summer climate.(a) Climograph for Churchill, Manitoba (subarctic, cool summer).(b) Winter scene on the edge of the boreal forest; young pioneerspruce trees move into the tundra. (c) Lone polar bear hunkereddown in a protective dugout next to a frozen pond. (d) Mom and twocubs-of-the-year in the tundra near Hudson Bay, west of Churchill;characteristic willows in the background. (e) November street scenein Churchill. [All photos by Bobbé Christopherson.]

Polar Bear andHigh Latitude Animals PhotoGalleries



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Elevation: 137 m (449.5 ft)Population: 1500Ann. Temp. Range: 63 C° (113.4 F°)

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Continental air mass⎧ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩

FIGURE 10.21 Extreme subarctic cold winter climate.(a) Climograph for Verkhoyansk, Russia (subarctic, very cold winter). (b) Scene in the town ofVerkhoyansk during the short summer. [Photo by Dean Conger/National Geographic Society.]

The polar climates cover about 19% of Earth’s total surfaceand about 17% of its land area. These climates have no truesummer like that in lower latitudes. Poleward of the Arctic andAntarctic Circles, daylength in summer becomes continuous,yet average monthly temperatures never rise above 10°C(50°F). These temperature conditions are intolerant to treegrowth. Daylength, which in part determines the amount of in-solation received, and low Sun altitude in the sky are the prin-cipal climatic factors in these frozen and barren regions. Yet,in winter, the Sun drops below the horizon poleward of 66.5°latitude, producing continuous night. An extended dawn andtwilight period eases this seasonal shock a little and reducesthe time of true night.

Principal climatic factors in these frozen and barren regionsare the following:

• Extremes of daylength between winter and summerdetermine the amount of insolation received.

• Low Sun altitude even during the long summer days isthe principal climatic factor.

• Extremely low humidity produces low precipitationamounts—these regions are Earth’s frozen deserts.

• Light-colored surfaces of ice and snow reflect substantialenergy away from the ground surface, thus reducing netradiation.

Polar and Highland Climates

Polar climates have three regimes: tundra (high latitude, or el-evation); ice caps and ice sheet (perpetually frozen); and polarmarine (oceanic association, slight moderation of extremecold).

Also in this climate category we include highland climates,for even at low latitudes the effects of elevation can producetundra and polar conditions. Glaciers on tropical mountainsummits attest to the cooling effects of elevation. Highland cli-mates on the map follow the pattern of Earth’s mountainranges.

Tundra

Ice cap and ice sheetHighland

Antarctica



(a)

(b)

FIGURE 10.22 Greenland tundra and a small town.(a) Tundra is marked by an uneven, hummocky surface of mounds resulting from an active layer that freezesand thaws with the seasons, as it is here in east Greenland. Large trees are absent in the tundra; however,relatively lush vegetation for the harsh conditions includes willow, dwarf birch and shrubs, sedges, moss,lichen, and cotton grass (white tufts). (b) A town in the tundra, Scoresby Sund fjord in the distance. In theforeground, sledge dogs rest to get ready for the winter’s work ahead. [Photos by Bobbé Christopherson.]

Tundra ClimateIn a tundra climate, land is under continuous snow coverfor 8–10 months, with the warmest month above 0°C yetnever warming above 10°C (50°F). Because of elevation,the summit of Mount Washington in New Hampshire(1914 m, or 6280 ft) statistically qualifies as a highlandtundra climate on a small scale.

In spring when the snow melts, numerous plantsappear—stunted sedges, mosses, flowering plants, andlichens. Some of the little (7.5-cm-, 3-in.-tall) willows can ex-ceed 300 years in age. The September photo shows theemerging fall colors of these small plants (Figure 10.22a).Much of the area experiences permafrost and ground ice con-ditions; these are Earth’s periglacial regions (see Chapter 17).

Approximately 410,500 km2 (158,475 mi2) of Green-land are ice-free, an area of tundra and rock about the sizeof California. The rest of Greenland is ice sheet, covering1,756,00 km2 (677,900 mi2). Despite the severe climate, apermanent population of 56,500 lives in this province ofDenmark. There are only a couple of towns along Green-land’s east coast. Ittoqqortoormiit, or Scoresby Sund, has850 permanent residents (Figure 10.22b). Here is a villageon the tundra.

Global warming is bringing dramatic changes tothe tundra and its plants, animals, and permafrost groundconditions. In parts of Canada and Alaska, registered tem-peratures as much as 5 C° above average are a regularoccurrence, setting many records. As organic peat depositsin the tundra thaw, vast stores of carbon are released tothe atmosphere, further adding to the greenhouse gasproblem. Temperatures in the Arctic are warming at a ratetwice that of the global average increase.

Tundra climates are strictly in the Northern Hemi-sphere, except for elevated mountain locations in the South-ern Hemisphere and a portion of the Antarctic Peninsula.

Ice-Cap and Ice-Sheet ClimateMost of Antarctica and central Greenland fall within theice-sheet climate, as does the North Pole, with all monthsaveraging below freezing. Both regions are dominated bydry, frigid air masses, with vast expanses that never warmabove freezing. The area of the North Pole is actually asea covered by ice, whereas Antarctica is a substantial con-tinental landmass covered by Earth’s greatest ice sheet.For comparison, winter minimums in central Antarctica(July) frequently drop below the temperature of solidcarbon dioxide or “dry ice” (–78°C, or -109°F). Ice capsare smaller in extent than ice sheets, roughly less than50,000 km2 (19,300 mi2), yet they completely bury thelandscape like an ice sheet. The Vatnajökull Ice Cap insoutheastern Iceland is an example.

Antarctica is constantly snow-covered but receivesless than 8 cm (3 in.) of precipitation each year. However,Antarctic ice has accumulated to several kilometers deepand is the largest repository of freshwater on Earth.Earth’s two ice sheets cover the Antarctic continent andmost of the island of Greenland. Figure 10.23 shows twoscenes of these repositories of multiyear ice. The status ofthis ice is the focus of much attention during the presentInternational Polar Year of scientific research.

This ice contains a vast historical record of Earth’s at-mosphere. Within it, evidence of thousands of past volcaniceruptions from all over the world and ancient combinationsof atmospheric gases lie trapped in frozen bubbles.

East GreenlandPhotos



Dry climates are the world’s arid deserts and semiarid regions,where we consider moisture efficiency along with temperaturefor understanding the climate. These regions have uniqueplants, animals, and physical features. Arid and semiaridregions occupy more than 35% of Earth’s land area and clearlyare the most extensive climate over land.

The mountains, long vistas, and resilient struggle for life areall magnified by the dryness. Sparse vegetation leaves thelandscape exposed; moisture demand exceeds moisture sup-ply throughout, creating permanent water deficits (water bal-ance is discussed in Chapter 9). The extent of this drynessdistinguishes desert and steppe climatic regions. (In addition,refer to specific annual and daily desert temperature regimes,including the highest recorded temperatures, discussed inChapter 5; surface energy budgets covered in Chapter 4;desert landscapes in Chapter 15; and desert environments inChapter 20.)

Important causal elements in these drylands include

• Dry, subsiding air in subtropical high-pressure systemsdominates.

• Midlatitude deserts and steppes form in the rain shadowof mountains, those regions to the lee of precipitation-intercepting mountains.

Arid and Semiarid Climates (permanent moisture deficits)

• Continental interiors, particularly central Asia, are far frommoisture-bearing air masses.

• Shifting subtropical high-pressure systems produce semi-arid steppe lands around the periphery of arid deserts.

Dry climates are distributed by latitude and the amount ofmoisture deficits in four distinct regimes: arid deserts (tropical,subtropical hot, midlatitude cold) and semiarid steppes (tropi-cal, subtropical hot, midlatitude cold).

Lethbridge,Alberta

Albuquerque, NM

Semey, Kazakstan

Walgett,NSW

Australia

Ar Riyad,Saudi Arabia

Semiarid steppesArid deserts

Chapter 17 presents analysis of ice cores taken from Green-land and the latest one from Antarctica, which pushed theclimate record to 800,000 years before the present.

Polar Marine ClimatePolar marine stations are more moderate than otherpolar climates in winter, with no month below�7°C (20°F), yet they are not as warm as tundra climates.

Because of marine influences, annual temperature rangesare low. This climate exists along the Bering Sea, thesouthern tip of Greenland, northern Iceland, Norway,and in the Southern Hemisphere, generally over oceansbetween 50° S and 60° S. Macquarie Island at 54° S inthe Southern Ocean, south of New Zealand, is polar ma-rine. Precipitation, which frequently falls as sleet (icepellets), is greater in these regions than in continentalpolar climates.

(a) (b)

FIGURE 10.23 Earth’s ice sheets—Antarctica and Greenland.These are Earth’s frozen freshwater reservoirs. (a) On the Antarctic Peninsula, basaltic mountains and glacial multiyear ice along the Graham Coast and Flandres Bay, Cape Renard in the distance. (b) Three outlet glaciers drain the Greenland ice sheet into the North Atlantic from southeasternGreenland; the glacial front is in retreat. [Photos by Bobbé Christopherson.]

High LatitudeConnectionVideos



(a) (b)

FIGURE 10.24 Desert landscapes.Desert vegetation is typically xerophytic: drought-resistant, waxy, hard-leafed, and adapted to aridityand low transpiration loss. (a) Ocotillo (to left) and creosote (to right) in the Anza-Borrego Desert,southern California; such plants are particularly well adapted to the harsh environment. (b) Colorful rockformations and view along a winding desert highway in Valley of Fire State Park, southern Nevada.[Photos by Bobbé Christopherson.]

Desert CharacteristicsThe world climate map in Figure 10.5 reveals thepattern of Earth’s dry climates, which cover broadregions between 15° and 30° N and S latitudes. In theseareas, subtropical high-pressure cells predominate, withsubsiding, stable air and low relative humidity. Undergenerally cloudless skies, these subtropical desertsextend to western continental margins, where cool, sta-bilizing ocean currents operate offshore and summeradvection fog forms. The Atacama Desert of Chile, theNamib Desert of Namibia, the Western Sahara ofMorocco, and the Australian Desert each lie adjacent tosuch a coastline.

Orographic lifting intercepts moisture-bearingweather systems to create rain shadows along mountainranges that extend these dry regions into higher latitudes(Figure 10.24). Note these rain shadows in North andSouth America on the climate map. The isolatedinterior of Asia, far distant from any moisture-bearingair masses, falls within the dry arid and semiarid climatesas well.

Major subdivisions include: deserts (precipitationsupply roughly less than one-half of the natural moisturedemand) and semiarid steppes (precipitation supplyroughly more than one-half of natural moisture de-mand). Important is whether precipitation falls princi-pally in the winter with a dry summer, in the summerwith a dry winter, or is evenly distributed. Winter rainsare most effective because they fall at a time of lowermoisture demand. Relative to temperature, the lower-latitude deserts and steppes tend to be hotter with less

seasonal change than the midlatitude deserts andsteppes, where mean annual temperatures are below18°C (64.4°F) and freezing winter temperatures arepossible.

Tropical, Subtropical Hot Desert ClimatesTropical, subtropical hot desert climates are Earth’s true trop-ical and subtropical deserts and feature annual averagetemperatures above 18°C (64.4°F). They generally resideon the western sides of continents, although Egypt, So-malia, and Saudi Arabia also fall within this classification.Rainfall is from local summer convectional showers.Some regions receive almost no rainfall, whereas othersmay receive up to 35 cm (14 in.) of precipitation a year. Arepresentative subtropical hot desert city is Ar Riya-d (Riyadh),Saudi Arabia (Figure 10.25).

Along the Sahara’s southern margin is a drought-tortured region. Human populations suffered great hard-ship as desert conditions gradually expanded over theirhomelands. The sparse environment sets the stage for arugged lifestyle and subsistence economies, picturedhere near Timbuktu, Mali (Figure 10.25c). Chapter 15presents the process of desertification (expanding desertconditions).

Death Valley, California, features such a hot desertclimate with an average annual temperature of 24.4°C(76°F). July and August average temperatures are 46°Cand 45°C (115°F, 113°F), respectively. Temperatures over50°C (122°F) are not uncommon.

M10_CHRI5988_07_SE_C10.QXD 12/15/07 1:16 AM 5


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Station: Ar Riyad (Riyadh), Saudi ArabiaLat/long: 24°42'N 46°43' EAvg. Ann. Temp.: 26°C (78.8°F)Total Ann. Precip.: 8.2 cm (3.2 in.)

Elevation: 609 m (1998 ft)Population: 5,024,000Ann. Temp. Range: 24 C° (43.2 F°)

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Subtropicalhigh pressure⎧ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩

FIGURE 10.25 Tropical, subtropical hot desert climate.(a) Climograph for Ar Riyad (Riyadh), Saudi Arabia (tropical, subtropical hot desert). (b) The Arabiandesert sand dunes near Ar Riyad. (c) Herders bring a few cattle to market near Timbuktu, Mali, in westAfrica. Precipitation has been below normal in the region since 1966. [(b) Photos by Ray Ellis/PhotoResearchers, Inc.; and (c) by Betty Press/Woodfin Camp & Associates.]

During this decade there has been much reportingfrom Iraq on the war and related political events. Whatseemed overlooked in many reports was the fact theair temperatures in Baghdad (located on the map inFigure 10.25) were actually hotter than Death Valley

during some days in July and August. Soldiers and civil-ians experience temperatures of 50°C (122°F) and higherin the city. Such readings broke records for Baghdad in2007. In January, averages for Death Valley (11°C; 52°F)and Baghdad (9.4°C; 49°F) are comparable. Death Valley



is drier with 5.9 cm of precipitation compared to 14 cmin Baghdad (2.33 in.; 5.5 in.), both low amounts. Bagh-dad’s May to September period is remarkable, with zeroprecipitation, dominated as it is by an intense subtropicalhigh-pressure system. (Baghdad is 34 m elevation at33.3°N. Death Valley is at -54 m elevation at 36.5°N.)Keep these hot desert climates in mind as you followevents.

Midlatitude Cold Desert ClimatesMidlatitude cold desert climates cover only a small area: thecountries along the southern border of Russia, the GobiDesert, and Mongolia in Asia; the central third of Nevadaand areas of the American Southwest, particularly at highelevations; and Patagonia in Argentina. Because of lowertemperatures and lower moisture-demand values, rainfallmust be low for a station to qualify as a midlatitude colddesert climate; consequently, total annual average rainfallis only about 15 cm (6 in.).

A representative station is Albuquerque, New Mexi-co, with 20.7 cm (8.1 in.) of precipitation and an annualaverage temperature of 14°C (57.2°F) (Figure 10.26).Note the summer convectional showers on the climo-graph. Across central Nevada stretches a characteristicexpanse of midlatitude cold desert, greatly modified by acentury of livestock grazing (Figure 10.26b).

Tropical, Subtropical Hot Steppe ClimatesTropical, subtropical hot steppe climates generally existaround the periphery of hot deserts, where shifting sub-tropical high-pressure cells create a distinct summer-dryand winter-wet pattern. Average annual precipitation insubtropical hot steppe areas is usually below 60 cm (23.6 in.).Walgett, in interior New South Wales, Australia, providesa Southern Hemisphere example of this climate (Figure 10.27).This climate is seen around the Sahara’s periphery and in the Iran, Afghanistan, Turkmenistan, and Kazakstanregion.

Midlatitude Cold Steppe ClimatesThe midlatitude cold steppe climates occur poleward of about30° latitude and the midlatitude cold desert climates. Suchmidlatitude steppes are not generally found in the SouthernHemisphere. As with other dry climate regions, rainfall inthe steppes is widely variable and undependable, rangingfrom 20 to 40 cm (7.9 to 15.7 in.). Not all rainfall is convec-tional, for cyclonic storm tracks penetrate the continents;however, most storms produce little precipitation.

Figure 10.28 presents a comparison between Asianand North American midlatitude cold steppe. ConsiderSemey (Semipalatinsk) in Kazakstan (greater temperaturerange, precipitation evenly distributed) and Lethbridge,Alberta (lesser temperature range, summer maximumconventional precipitation).

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Station: Albuquerque, New MexicoLat/long: 35°03′ N 106°37′ WAvg. Ann. Temp.: 14°C (57.2°F)Total Ann. Precip.: 20.7 cm (8.1 in.)

Elevation: 1620 m (5315 ft)Population: 479,000Ann. Temp. Range: 24 C° (43.2 F°)Ann. Hr of Sunshine: 3420

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Subtropical high;(summer continental tropical)⎧ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩

FIGURE 10.26 Midlatitude cold desert climate.(a) Climograph for Albuquerque, New Mexico (midlatitudecold desert). (b) Cold, high desert landscape of the Basin andRange Province in Nevada, east of Ely along highway U.S. 50.This area is in the region shown in the chapter-openingphotos. [Photo by Bobbé Christopherson.]



*As a standard scientific reference on climate change, the IPCC uses thefollowing to indicate levels of confidence: Virtually certain �99% proba-bility of occurrence; Extremely likely �95%; Very likely �90%; Likely�66%; More likely than not �50%; Unlikely �33%; Very unlikely�10%; and Extremely unlikely �5%.

Global Climate ChangeSignificant climatic change has occurred on Earth in thepast and most certainly will occur in the future. There isnothing society can do about long-term influences thatcycle Earth through swings from ice ages to warmer peri-ods. However, our global society must address short-termchanges that are influencing global temperatures withinthe life span of present generations. This is especially truesince these changes are due to human activities—an an-thropogenic forcing of climate.

Record-high global temperatures have dominated thepast two decades—for both land and ocean and for bothday and night. The record year for warmth was 2005, andall the years since 1995 were near this record. Air temper-atures are the highest since recordings began in earnestmore than 140 years ago and higher than at any time inthe last 120,000 years, according to the ice-core record.This warming trend is very likely* due to a buildup ofgreenhouse gases. Understanding this warming and all itsrelated impacts is an important applied topic of Earth sys-tems science and an opportunity for spatial analysis inphysical geography.

Global WarmingTwenty years ago, climatologists Richard Houghton andGeorge Woodwell described the present climatic condition:

The world is warming. Climatic zones are shifting.Glaciers are melting. Sea level is rising. These are nothypothetical events from a science fiction movie; thesechanges and others are already taking place, and weexpect them to accelerate over the next years asthe amounts of carbon dioxide, methane, and othertrace gases accumulating in the atmosphere throughhuman activities increase.†

Your author knows this quote well because it has been inevery edition of Geosystems since the first edition in 1992.During the intervening years, we watched global temper-atures rise and climate-change science mature. There is astrong scientific consensus that global warming is occur-ring and that human activities are the cause, namely theburning of fossil fuels.

The 2007 Intergovernmental Panel on ClimateChange (IPCC) Fourth Assessment Report (AR4) concluded:

Warming of the climate system is unequivocal, as isnow evident from observations of increases in globalaverage air and ocean temperatures, widespread melt-ing of snow and ice, and rising global average sea

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Station: Walgett, New South Wales, AustraliaLat/long: 30°0′ S 148°07′ EAvg. Ann. Temp.: 20°C (68°F)Total Ann. Precip.: 45.0 cm (17.7 in.)

Elevation: 133 m (436 ft)Population: 8200Ann. Temp. Range: 17 C° (31 F°)

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FIGURE 10.27 Tropical, subtropical hot steppe climate.(a) Climograph for Walgett, New South Wales, Australia(tropical, subtropical hot steppe). (b) Vast plains characteristicof north-central New South Wales. [Photo by Otto Rogge/StockMarket.]

†R. Houghton and G. Woodwell, “Global climate change,” ScientificAmerican (April 1989): 36.



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Elevation: 206 m (675.9 ft)Population: 270,500Ann. Temp. Range: 39 C° (70.2 F°)

Station: Lethbridge, AlbertaLat/long: 49°42' N 110°50' WAvg. Ann. Temp.: 2.9°C (37.3°F)Total Ann. Precip.: 25.8 cm (10.2 in.)

Elevation: 910 m (2985 ft)Population: 73,000Ann. Temp. Range: 24.3 C° (43.7 F°)

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Continental air mass(summer convection)(winter cP air mass)⎧ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩

FIGURE 10.28 Midlatitude cold steppe climates, Kazakstan and Canada.Climograph for (a) Semey (Semipalatinsk) in Kazakstan. (b) Herders in the region near Semey.(c) Climograph for Lethbridge in Alberta. (d) Canadian prairies and grain elevators of southern Alberta.[(b) Photo by Sovfoto/Eastfoto; (d) photo by author.]



Peace Prize with former Vice-President Albert Gorefor their two decades of work raising understandingand awareness about global climate-change science.The Nobel Committee said that Gore is responsible“. . . for convincing world governments that climatechange was real, caused by human activity, and posed athreat to society.” Various organizations and agenciescoordinate and conduct global climate-change research.News Report 10.2 offers an overview and contact infor-mation for climate-change science.

The Arctic Climate Impact Assessment (ACIA)Symposium met in 2004 and released a lengthy scientificreport. The ACIA summarized:

Human activities, primarily the burning of fossil fuels(coal, oil, natural gas), and secondarily the clearing ofland, have increased the concentration of carbon diox-ide, methane, and other heat-trapping (“greenhouse”)gases in the atmosphere . . . this is projected to leadto significant and persistent changes in climate . . .these changes are projected to lead to wide-ranging

level. . . . Most of the observed increase in globallyaveraged temperatures since the mid-20th century isvery likely due to the observed increase in anthro-pogenic greenhouse gas concentrations. This is anadvance since the Third Assessment Report’s conclusionthat “most of the observed warming over the last50 years is likely to have been due to the increase ingreenhouse gas concentrations.” Discernible humaninfluences now extend to other aspects of climate,including ocean warming, continental-average temper-atures, temperature extremes and wind patterns.‡

The IPCC, formed in 1988, is an organization operatingunder the United Nations Environment Programme(UNEP) and the World Meteorological Organization(WMO) and is the scientific organization coordinatingglobal climate-change research, climate forecasts, andpolicy formulation. In 2007, the IPCC shared the Nobel

News Report 10.2

Coordinating Global Climate Change Research

A cooperative global network of UnitedNation members participate in theUnited Nations Environment Pro-gramme (UNEP, http://www.unep.org) and the World Meteorological Or-ganization (WMO, http://www.wmo.ch/). The World Climate ResearchProgramme (WCRP, http://wcrp.wmo.int/) and its network under thesupervision of the Global Climate Ob-serving System (GCOS, http://www.wmo.ch/web/gcos/gcoshome.html)coordinate data gathering and research.

The ongoing climate assessmentprocess within the UNEP is conductedby the Intergovernmental Panel onClimate Change (IPCC, http://www.ipcc.ch/), whose three working groupsissued completed reports in 1990, 1992(a supplementary report), 1995, and theThird Assessment Report in 2001. Thelatest is the Fourth Assessment Report(AR4), with all three working groupsreporting in 2007: Working Group I“Climate Change 2007: The PhysicalBasis,” WGII “Climate Change Im-pacts, Adaptation, and Vulnerability,”and WGIII “Mitigation of ClimateChange,” and a final Synthesis Report,

released at the end of the year. TheIPCC AR4 involved 2500 scientific ex-pert reviews, 800 contributing authors,450 lead authors from 130 countries,and 6 years of work. You can downloadtheir “Summary for Policy Makers” forfree from the www.ipcc.ch/ web site.

The Arctic Council (at http://www.arctic-council.org/) initiated theArctic Climate Impact Assessment(ACIA, http://www.acia.uaf.edu/) in2000; its three working groups com-pleted the research—Arctic Monitoringand Assessment Program (AMAP,http://www.amap.no/), the Conserva-tion of Flora and Fauna (CAFF,http://www.caff.is/), the InternationalArctic Science Committee (IASC,http://www.iasc.no/), comprised of na-tional scientific organizations, including18 national academies of science.

In the United States, coordinationis found at the U.S. Global ChangeResearch Program (http://www.usgcrp.gov/). An overall source for informationis http://globalchange.gov, which pub-lishes an on-line monthly summary ofall related developments. Also impor-tant are programs and services at

NASA agencies, such as Dr. JamesHansen’s work at Goddard Institutefor Space Studies (GISS, http://www.giss.nasa.gov/), Global Hydrologyand Climate Center (GHCC, http://www.ghcc.msfc.nasa.gov), and atNOAA agencies at the National Cli-mate Data Center (NCDC, http://www.ncdc.noaa.gov/) and the Na-tional Environmental Satellite, Data,and Information Service (NESDIS,http://www.nesdis.noaa.gov/), amongothers. The Pew Center on GlobalClimate Change offers credible analy-sis and overview and has issuedseveral policy reports at http://www.pewclimate.org/.

The multiagency National IceCenter is at http://www.natice.noaa.gov/. Important research is done at theNational Center for AtmosphericResearch (http://www.ncar.ucar.edu/).For Canada, information and researchis coordinated by Environment Canada(http://www.ec.gc.ca/climate/). Theeffect of global warming on permafrost,which involves half of Canadian land area, is found at http://www.socc.ca/.

‡IPCC AR4, Working Group I, Climate Change 2007: The Physical ScienceBasis (Geneva, Switzerland: IPCC Secretariat, 2007), pp. 5, 10.



FIGURE 10.29 A thousand years of record and thecovariance of carbon dioxide and temperature.Carbon emissions, CO2 concentrations, and temperaturechange correlate over the past 1000 years. This graph wasprepared before the record-setting temperatures of 2005. ByMay 2007, with CO2 levels at 387 ppm, carbon emissions fromfossil-fuel burning were approaching 8 Gt (gigatons, or 8billion tons of carbon) annually. Earth systems are in recordterritory. [Illustration from ACIA, Impacts of a Warming Arctic(London: Cambridge University Press, 2004), p. 2. Used bypermission of ACIA.]

consequences including significant impact on coastalcommunities, animal and plant species, waterresources, and human health and well being.‡‡

The American Association for the Advancement ofScience (AAAS) reported in “The scientific consensus onclimate change” (Science 306, December 3, 2004: 1686),the results of a survey of all 928 climate-change paperspublished in refereed scientific journals between 1993 and2003. The papers were divided into six categories andanalyzed. As study author Naomi Oreskes concluded,“Remarkably, none of the papers disagreed with the con-sensus position.” There is a consensus that human activi-ties are heating Earth’s surface and lower atmosphere.The author concluded:

This analysis shows that scientists publishing inthe peer-reviewed literature agree with IPCC, theNational Academy of Sciences, and public statementsof their professional societies. Politicians, economists,journalists, and others may have the impression ofconfusion, disagreement, or discord among climatescientists, but that impression is incorrect . . . there is ascientific consensus on the reality of anthropogenicclimate change.

In terms of paleoclimatology, the science that stud-ies past climates (discussed in Chapter 17), proxy indica-tors (ice-core data, sediments, coral reefs, ancient pollen,tree-ring density, among others) point to the present timeas the warmest in the last 120,000 years, and further, thatthe increase in temperature during the twentieth centuryis very likely the largest in any century over the past 1000years (Figure 10.29). Earth is less than 1 C° (1.8 F°) fromequaling the highest average temperature over this timespan. The latest Antarctic ice core at the Dome C locationpushes this climatic information back 800,000 years(described in Chapter 17).

The rate of warming in the past 30 years exceeds anycomparable period in the entire measured temperaturerecord, according to NASA scientists. Figure 10.30a plotsobserved annual temperatures and 5-year mean tempera-tures from 1880 through 2006: Worldwide, 2005 was thewarmest year; 2006 was fifth warmest but warmest everfor North America.

The map in 10.30b uses the same base period as thegraph (1951–1980) to give you an idea of temperatureanomalies across the globe in 2006. Note the Arcticregion, where new records for land and water tempera-tures and sea-ice melt were set in 2006 and again brokenin 2007. Anomalies exceeding 2.5 C° (4.5 F°) are por-trayed on the map in the Canadian Arctic and portions ofGreenland, Siberia, and the Antarctic Peninsula. The factthat 2007–2009 is the International Polar Year under-scores the global scientific concerns about the polarclimates and lands.

Scientists are working to determine the differencebetween forced fluctuations (human-caused) and unforcedfluctuations (natural) as a key to predicting future climatetrends. Figure 10.31 from the IPCC Fourth AssessmentReport shows a comparison of climate models of forcingfrom solar output changes and volcanoes (shaded bluearea) with anthropogenic (human) forcing (shaded redarea) for global, all land, and all ocean regions. The blackline on each graph plots observations from 1906 to 2005.Clearly, only those coupled climate models that includeanthropogenic factors accurately simulate increasing tem-peratures. Solar variability and volcanic output alone donot explain the increase.

Because the enhanced greenhouse gases are anthro-pogenic in origin, various management strategies are pos-sible to reduce human-forced climatic changes. Let’sbegin by examining the problem at its roots.

Carbon Dioxide and Global Warming Carbon diox-ide and water vapor are the principal radiatively activegases causing Earth’s natural greenhouse effect. Radia-tively active gases include atmospheric gases, such as car-bon dioxide (CO2), methane (CH4), nitrous oxide (N2O),chlorofluorocarbons (CFCs), and water vapor, which

‡‡ACIA, Impacts of a Warming Arctic (London: Cambridge UniversityPress, 2004), p. 2.


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FIGURE 10.30 Global temperature trends.(a) Global temperature trends from 1880 to 2006.The 0 baseline represents the 1951–1980 globalaverage. Comparing annual temperatures and 5-year mean temperatures gives a sense ofoverall trends. (b) Temperature map showstemperature anomalies during 2006, fifth-warmest year on record. The colorationrepresents C° departures from the base period1951–1980. On the CD-ROM that accompaniesthis text there is a movie of temperatureanomalies from 1881 to 2006 in which you cansee the warming patterns over this time span.[(a) and (b) Data courtesy of Dr. James Hansen,GISS/NASA, and NCDC/NOAA.]

FIGURE 10.31 Explaining global temperature changes.Computer models accurately track observed temperature change (black line) when they factor in human-forced influences onclimate (red shading). Solar activity and volcanoes do not explain the increases (blue shading). [Graphs from Climate Change2007: The Physical Science Basis, Working Group I, IPCC Fourth Assessment Report, February 2007: Fig. SPM-4, p. 11.]

Global Warming, Climate Change

GISS Surface Tem-perature AnalysisMovie, 1891 to 2006



absorb and radiate longwave energy. Figure 10.32 plotschanges in three of these greenhouse gases over the past10,000 years.

These gases are transparent to light but opaque to thelonger wavelengths radiated by Earth. Thus, they trans-mit light from the Sun to Earth but delay heat-energy lossto space. While detained, this heat energy is absorbed andemitted over and over, warming the lower atmosphere. Asconcentrations of these greenhouse gases increase, moreheat energy remains in the atmosphere and temperaturesincrease.

The present CO2 concentration tops anything overthe last 800,000 years in the Dome C ice core. In fact,over this ice-core record, changes of as much as 30 ppm inCO2 took at least 1000 years, yet, the concentration haschanged 30 ppm in just the last 17 years. With 4.5% ofthe world’s population, the United States continues toproduce 24% of global CO2 emissions. China, with 20%of global population, is responsible for 18% of CO2 emis-sions and is increasing its output. Clearly, per capita CO2emissions in the United States are far above what an aver-age person in China produces.

Methane and Global Warming Another radiativelyactive gas contributing to the overall greenhouse effect ismethane (CH4), which, at more than 1% per year, isincreasing in concentration even faster than carbon diox-ide. In ice cores, methane levels never topped 750 ppb inthe past 800,000 years, yet in Figure 10.32 we see presentlevels at 1780 ppb. We are at an atmospheric concentra-tion of methane that is higher than at any time in the past800 millennia.

Methane is generated by such organic processes asdigestion and rotting in the absence of oxygen (anaerobicprocesses). About 50% of the excess methane comes frombacterial action in the intestinal tracts of livestock andfrom organic activity in flooded rice fields. Burningof vegetation causes another 20% of the excess, and bacte-rial action inside the digestive systems of termite popula-tions also is a significant source. Methane is thoughtresponsible for at least 19% of the total atmosphericwarming.

Other Greenhouse Gases Nitrous oxide (N2O) is thethird most important greenhouse gas that is forced byhuman activity—up 17% in atmospheric concentrationsince 1750, higher than at any time in the past 10,000years (Figure 10.32). Fertilizer use increases the processesin soil that emit nitrous oxide, although more research isneeded to fully understand the relationships. Chlorofluo-rocarbons (CFCs) and other halocarbons also contributeto global warming. CFCs absorb longwave energy missedby carbon dioxide and water vapor in the lower tropo-sphere. As radiatively active gases, CFCs enhance thegreenhouse effect in the troposphere and are a cause ofozone depletion and slight cooling in the stratosphere.

Climate Models and Future TemperaturesThe scientific challenge in understanding climate changeis to sense climatic trends in what is essentially a nonlin-ear, chaotic natural system. Imagine the tremendous taskof building a computer model of all climatic componentsand programming these linkages (shown in Figure 10.1)over different time frames and at various scales.

Using mathematical models originally established forforecasting weather, scientists developed a complex com-puter climate model known as a general circulationmodel (GCM). There are at least a dozen establishedGCMs now operating around the world. Submodel

FIGURE 10.32 Greenhouse gas changes over the last10,000 years.Ice-core and modern data show the trends in carbon dioxide,methane, and nitrous oxide over the last 10,000 years. It isimmediately obvious that we are living in unique territory onthese graphs. During May 2007, CO2 passed the 387 ppmlevel, CH4 was at 1780 ppb, and N2O approached 330 ppb.[Graphs from Climate Change 2007: The Physical Science Basis,Working Group I, IPCC Fourth Assessment Report, February2007: Fig. SPM-1, p. 3.]



Atmosphere

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TemperaturePrecipitationAir pressure

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FIGURE 10.33 A general circulation model scheme.Temperature, precipitation, air pressure, relative humidity,wind, and sunlight intensity are sampled in myriad grid boxes.In the ocean, sampling is limited, but temperature, salinity,and ocean current data are considered. The interactionswithin a grid layer, and between layers on all six sides, aremodeled in a general circulation model program.

programs for the atmosphere, ocean, land surface, cryo-sphere, and biosphere operate within the GCM. The mostsophisticated models couple atmosphere and ocean sub-models and are known as Atmosphere-Ocean General Circu-lation Models (AOGCMs).

The first step in describing a climate is defining amanageable portion of Earth’s climatic system for study.Climatologists create dimensional “grid boxes” that extendfrom beneath the ocean to the tropopause, in multiple lay-ers (Figure 10.33). Resolution of these boxes in the atmo-sphere is about 250 km (155 mi) in the horizontal and 1 km(0.6 mi) in the vertical; in the ocean the boxes use the samehorizontal resolution and a vertical resolution of about 200to 400 m (650–1300 ft). Analysts deal not only with the cli-matic components within each grid layer but also with theinteraction among the layers on all sides.

A comparative benchmark among the operationalGCMs is climatic sensitivity to doubling of carbon dioxidelevels in the atmosphere. GCMs do not predict specifictemperatures, but they do offer various scenarios of globalwarming. GCM-generated maps correlate well with theobserved global warming patterns experienced since 1990.

The 2007 IPCC Fourth Assessment Report, using avariety of GCM forecast scenarios, predicted a range ofaverage surface warming for this century. Figure 10.34illustrates six of these scenarios, each with its own assump-tions of economics, population, degree of global coopera-tion, and greenhouse gas emission levels. The orange lineis the simulation experiment where greenhouse gas emis-sions are held at 2000 values with no increases. The graybars give you the best estimate and likely ranges of out-comes; for example, from a “low forecast” in B1 to a “highforecast” in A1Fl. Even the “B” scenario represents a sig-nificant increase in global land and ocean temperaturesand will produce consequences. Although regionally vari-able and subject to revision, the IPCC temperature changeforecasts for the twenty-first century are:

• High forecast: 6.4 C° (11.5 F°)• Middle forecast: 1.8 C°–4.0 C° (3.1 F°–7.2 F°)• Low forecast: 1.1 C° (2.0 F°)

Figure 10.35 offers us a look at the world of2020–2029 and 2090–2099 using three scenarios fromseveral different AOGCMs. Find the three scenarios usedfor these three pairs of maps on the graph in Figure 10.34.You can see from the maps why scientists are concernedabout temperature trends in the higher latitudes.

Consequences of Global WarmingThe consequences of uncontrolled atmospheric warmingare complex. Regional climate responses are expected astemperature, precipitation, soil-moisture, and air mass char-acteristics change. Although the ability to accurately fore-cast such regional changes is still evolving, someconsequences of warming have been forecasted and in sev-eral regions are already underway.The challenge for scienceis to analyze such effects on a global scale.

The following list is a brief overview from the IPCCFourth Assessment Report “Summary for Policy Makers” and

other sources that summarize global impacts emerging fromclimate change.

• The observed pattern of tropospheric warming andstratospheric cooling is very likely due to greenhousegas increases and stratospheric ozone depletion.

• Widespread changes in extreme temperatures havebeen observed over the last 50 years. Cold days,cold nights, and frost are less frequent, while hotdays, hot nights, and heat waves are more frequent.

• Observations since 1961 show the average globalocean temperature increased to depths of 3000 mand the ocean absorbed more than 80% of climatesystem heating. Such warming causes thermalexpansion of seawater, contributing to sea level rise.

• There is observational evidence of increasedintensity of tropical cyclones correlated withincreases of tropical sea-surface temperatures.Total “power dissipation” of these storms hasdoubled since 1970. Worldwide there are morecategory 3, 4, and 5 tropical storms than in theprevious record.

• Mountain glaciers and snow cover declined onaverage in both hemispheres, contributing to sea-level rise.

• Mount Kilimanjaro in Africa, portions of theSouth American Andes, and the Himalayas willvery likely lose most of their glacial ice within thenext two decades, affecting local water resources.Glacial ice continues its retreat in Alaska.



FIGURE 10.34 Model scenarios for surfacewarming.Ranging from business as usual, lack ofinternational cooperation, and continuation of theuse of fossil fuels (“A2Fl”) to a low-emissionscenario (“B1”), each scenario spells climaticcatastrophe of some degree. Holding emissions atthe 2000 concentration (orange line plot) results inthe least temperature impacts. [Graph from ClimateChange 2007: The Physical Science Basis, WorkingGroup I, IPCC Fourth Assessment Report, February2007: Fig. SPM-5, p. 14.]

FIGURE 10.35 Modelprojections of surfacetemperature for threescenarios.Three pairs of modelsimulations for 2020–2029and 2090–2099 for the B1,A1B, and A2 scenarios. Thecolors are defined in thetemperature scale along thebottom. Note the severity ofwarming in the worst case.Correlate these three caseswith the graph in Figure10.34. [Maps from ClimateChange 2007: The PhysicalScience Basis, Working GroupI, IPCC Fourth AssessmentReport, February 2007: Fig.SPM-6, p. 15.]

• The average atmospheric water vapor content hasincreased over land and ocean as well as in the uppertroposphere. The increase is consistent with the factthat warmer air can absorb more water vapor.

• Flow speed accelerated for some Greenland andAntarctic outlet glaciers as they drain ice from theinterior of the ice sheets. In Greenland this rate ofloss exceeds snowfall accumulation, with lossesper year more than doubling between 1996 and2005 (mass loses of 91 km3 compared to 224 km3).

• Average Arctic temperatures increased at almosttwice the global average rate in the past 100 years.Wintertime lower atmosphere temperatures overAntarctica are warming at nearly three times theglobal average, first reported in 2006.

• Temperatures in the permafrost active layerincreased overall since the 1980s in the Arctic upto 3 C°. The maximum area of seasonally frozenground decreased 7% in the Northern Hemi-sphere, with a decrease in spring of up to 15%.



• Since 1978, annual average Arctic sea ice extentshrunk in summer by 7.4% per decade, accordingto satellite measurements. September 2007 Arcticsea ice extent retreated to its lowest area coveragein the record.

• Changes in precipitation and evaporation over theoceans and the melting of ice are freshening mid-and high-latitude oceans and seas, together withincreased salinity in low-latitudes. Oceans areacidifying (lower pH) in response to absorption ofincreasing atmospheric CO2.

• Mid-latitude westerly winds strengthened in bothhemispheres since the 1960s.

• More intense and longer droughts have beenobserved over wider areas since the 1970s, partic-ularly in the tropics and subtropics. Increased dry-ing linked to higher temperatures and decreasedprecipitation observed in the Sahel, the Mediter-ranean, southern Africa, parts of southern Asia,Australia, and the American West.

• Higher spring and summer temperatures andearlier snowmelt are extending the wildfire seasonand increasing the intensity of wildfires in thewestern U.S. and elsewhere.

• The frequency of heavy precipitation events increasedover most land areas, consistent with warming andobserved increases of atmospheric water vapor. Sig-nificantly increased precipitation has been observed ineastern parts of North and South America, northernEurope, and northern and central Asia.

• Crop patterns, as well as natural habitats of plantsand animals, will shift to maintain preferredtemperatures. According to climate models, climaticregions in the midlatitudes could shift poleward by150 to 550 km (90 to 350 mi) during this century.

• Biosphere models predict that a global average of30% of the present forest cover (varying regional-ly from 15% to 65%) will undergo major speciesredistribution—greatest at high latitudes and leastin the tropics.

• Populations previously unaffected by malaria,dengue fever, lymphatic filariasis, and yellow fever(all mosquito vector), schistosomiasis (water snailvector), and sleeping sickness (tsetse fly vector) willbe at greater risk in subtropical and midlatitudeareas as temperatures increase the vector ranges.

Changes in Sea Level Sea-level rise must be ex-pressed as a range of values that are under constantreassessment. During the last century, sea level rose10–20 cm (4–8 in.), a rate 10 times higher than the aver-age rate during the last 3000 years.

The 2007 IPCC forecast scenarios for global meansea-level rise this century, given regional variations, are:

• Low forecast: 0.18 m (7.1 in.)• Middle forecast: 0.39 m (15.4 in.)• High forecast: 0.59 m (23.2 in.)

Unfortunately the new 2006–2007 measurements of Green-land’s ice-loss acceleration did not reach the IPCC in timefor its report. Scientists are considering at least a 1.2-m(3.94-ft) “high case” for estimates of sea-level rise this cen-tury as more realistic given Greenland’s present losses cou-pled with mountain glacial ice losses worldwide. Rememberthat a 0.3-m rise in sea level would produce a shorelineretreat of 30 m (98 ft) on average. Here is the concern:

The data now available raise concerns that the climatesystem, in particular sea level, may be responding morequickly than climate models indicate. . . . The rate ofsea-level rise for the past 20 years is 25% faster thanthe rate of rise in any 20-year period in the preceding115 years. . . . Since 1990 the observed sea-level hasbeen rising faster than the rise projected by models.*

These increases would continue beyond 2100 even ifgreenhouse gas concentrations were stabilized. InChapter 16, maps present what coastlines will experiencein the event of a 1-m rise in sea level.

A quick survey of world coastlines shows that even amoderate rise could bring change of unparalleled propor-tions. At stake are the river deltas, lowland coastal farmingvalleys, and low-lying mainland areas, all contending withhigh water, high tides, and higher storm surges. Particu-larly tragic social and economic consequences will affectsmall island states, which are unable to adjust within theirpresent country boundaries—disruption of biologicalsystems, loss of biodiversity, reduction in water resources,and evacuation of residents are among the impacts.

There could be both internal and international migra-tion of affected human populations, spread over decades,as people move away from coastal flooding caused by thesea-level rise—a 1-m rise will displace 130 million people.Presently, there is no body of world law that covers“environmental refugees.”

Political Action and “No Regrets” Reading through all this climate-change science mustseem pretty “heavy.” Instead, think of this information asempowering and as motivation to take action—personally,locally, regionally, nationally, and globally.

A product of the 1992 Earth Summit in Rio de Janeirowas the United Nations Framework Convention on Cli-mate Change (FCCC). The leading body of the Conven-tion is the Conference of the Parties (COP) operated bythe countries that ratified the FCCC, 186 countries by2000. Meetings were held in Berlin (COP-1, 1995) andGeneva (COP-2, 1996). These meetings set the stage forCOP-3 in Kyoto, Japan, in December 1997, where 10,000participants adopted the Kyoto Protocol by consensus. Sev-enteen national academies of science endorsed the KyotoProtocol. (For updates on the status of the Kyoto Proto-col, see http://unfccc.int/2860.php.) The latest 2007gathering, COP-13, was in Bali, Indonesia.

*S. Rahmstorf, et al., “Recent climate observations compared to projec-tions,” AAAS Science 316 (May 4, 2007): 709.



The Kyoto Protocol binds more-developed countriesto a collective 5.2% reduction in greenhouse gas emissionsas measured at 1990 levels for the period 2008 to 2012.Within this group goal, various countries promised cuts:Canada is to cut 6%, the European Union 8%, and Aus-tralia 8%, among many others. The Group of 77 countriesplus China favor a 15% reduction by 2010. With Russianratification in November 2004, the Kyoto Protocol is nowinternational law, with nearly 140 national signatories; thisis without United States participation.

The Intergovernmental Panel on Climate Change(IPCC) declared that “no regrets” opportunities to reducecarbon dioxide emissions are available in most countries.The IPCC Working Group III defines this as follows:

No regrets options are by definition greenhouse gasemissions reduction options that have negative netcosts, because they generate direct and indirect bene-fits that are large enough to offset the costs of imple-menting the options.

Benefits that equal or exceed their cost to society includereduced energy cost, improved air quality and health,reduction in tanker spills and oil imports, and deploymentof renewable and sustainable energy sources, among oth-ers. This holds true without even considering the benefitsof slowing the rate of climate change. For Europe, scien-tists determined that carbon emissions could be reducedto less than half the 1990 level by 2030, at a negative cost.One key to “no regrets” is the untapped energy-efficiencypotential.

In the United States, five Department of Energy na-tional laboratories (Oak Ridge, Lawrence Berkeley, Pacif-ic Northwest, National Renewable Energy, and Argonne)reported that the United States can meet the Kyoto car-bon emission reduction targets with negative overall costs(cash benefit savings) ranging from -$7 to -$34 billion.(For more, see Working Group III, Climate Change 2001,Mitigation, London: Cambridge University Press, 2001,pp. 21, 474–76 and 506–507.)

Summary and Review—Global Climate Systems

� Define climate and climatology, and explain the differ-ence between climate and weather.

Climate is dynamic, not static. Climate is a synthesis of weatherphenomena at many scales, from planetary to local, in contrast toweather, which is the condition of the atmosphere at any giventime and place. Earth experiences a wide variety of climatic con-ditions that can be grouped by general similarities into climaticregions. Climatology is the study of climate and attempts to dis-cern similar weather statistics and identify climatic regions.

climate (p. 277)climatology (p. 278)climatic regions (p. 278)

1. Define climate and compare it with weather. What isclimatology?

2. Explain how a climatic region synthesizes climate statistics.3. How does the El Niño phenomenon produce the largest

interannual variability in climate? What are some of thechanges and effects that occur worldwide?

� Review the role of temperature, precipitation, air pressure,and air mass patterns used to establish climatic regions.

Climatic inputs include insolation (pattern of solar energy in theEarth–atmosphere environment), temperature (sensible heat energycontent of the air), precipitation (rain, sleet, snow, and hail; the supplyof moisture), air pressure (varying patterns of atmospheric density),and air masses (regional-sized homogeneous units of air). Climate isthe basic element in ecosystems, the natural, self-regulating commu-nities of plants and animals that thrive in specific environments.

4. How do radiation receipts, temperature, air-pressureinputs, and precipitation patterns interact to produceclimate types? Give an example from a humid environmentand one from an arid environment.

5. Evaluate the relationships among a climatic region,ecosystem, and biome.

� Review the development of climate classification sys-tems, and compare genetic and empirical systems as waysof classifying climate.

Classification is the process of ordering or grouping data in re-lated categories. A genetic classification is based on causativefactors, such as the interaction of air masses. An empirical clas-sification is one based on statistical data, such as temperature orprecipitation. This text analyzes climate using aspects of bothapproaches, with a map based on climatological elements.

classification (p. 280)genetic classification (p. 280)empirical classification (p. 280)

6. What are the differences between a genetic and anempirical classification system?

7. What are some of the climatological elements used inclassifying climates? Why each of these?

� Describe the principal climate classification categoriesother than deserts, and locate these regions on a world map.

Here we focus on temperature and precipitation measures. Keepin mind these are measurable results produced by interactingelements of weather and climate. These data are plotted on aclimograph to display the characteristics of the climate.

There are six basic climate categories. Temperature andprecipitation considerations form the basis of five climatecategories and their regional types:

• Tropical (equatorial and tropical latitudes)rain forest (rainy all year)monsoon (6 to 12 months rainy)savanna (less than 6 months rainy)

• Mesothermal (midlatitudes, mild winters)humid subtropical (hot summers)



marine west coast (warm to cool summers)Mediterranean (dry summers)

• Microthermal (mid- and high latitudes, cold winters)humid continental (hot to warm summers)subarctic (cool summers to very cold winters)

• Polar (high latitudes and polar regions)tundra (high latitude or high altitude)ice caps and ice sheets (perpetually frozen)polar marine

• Highland (compared to lowlands at the same latitude, highlandshave lower temperatures—recall the normal lapse rate)

Only one climate category is based on moisture efficiencyas well as temperature:

• Desert (permanent moisture deficits)arid deserts (tropical, subtropical hot and midlatitude cold)semiarid steppes (tropical, subtropical hot and midlatitude cold)

climograph (p. 285)

8. List and discuss each of the principal climate categories.In which one of these general types do you live? Whichcategory is the only type associated with the annualdistribution and amount of precipitation?

9. What is a climograph, and how is it used to display climaticinformation?

10. Which of the major climate types occupies the most landand ocean area on Earth?

11. Characterize the tropical climates in terms of temperature,moisture, and location.

12. Using Africa’s tropical climates as an example, characterizethe climates produced by the seasonal shifting of the ITCZwith the high Sun.

13. Mesothermal (subtropical and midlatitude, mild winter)climates occupy the second-largest portion of Earth’sentire surface. Describe their temperature, moisture, andprecipitation characteristics.

14. Explain the distribution of the humid subtropical hot-summerand Mediterranean dry-summer climates at similar latitudesand the difference in precipitation patterns between the twotypes. Describe the difference in vegetation associated withthese two climate types.

15. Which climates are characteristic of the Asian monsoonregion?

16. Explain how a marine west coast climate type can occur in theAppalachian region of the eastern United States.

17. What role do offshore ocean currents play in thedistribution of the marine west coast climates? What type offog is formed in these regions?

18. Discuss the climatic conditions for the coldest places onEarth outside the poles.

� Explain the precipitation and moisture efficiency criteriaused to determine the arid and semiarid climates, andlocate them on a world map.

The dry and semiarid climates are described by precipitation ratherthan temperature. Dry climates are the world’s arid deserts and

semiarid regions, with their unique plants, animals, and physicalfeatures. The arid and semiarid climates occupy more than 35% ofEarth’s land area, clearly the most extensive climate over land.

Major subdivisions are arid deserts in tropical and mid-latitude areas (precipitation—natural water supply—less thanone-half of natural water demand) and semiarid steppes in tropicaland midlatitude areas (precipitation more than one-half ofnatural water demand).

19. In general terms, what are the differences among the fourdesert classifications? How are moisture and temperaturedistributions used to differentiate these subtypes?

20. Relative to the distribution of arid and semiarid climates,describe at least three locations where they occur across theglobe and the reasons for their presence in these locations.

� Outline future climate patterns from forecasts presented,and explain the causes and potential consequences ofclimate change.

Various activities of present-day society are producing climaticchanges, particularly a global warming trend. The highest averageannual temperatures experienced since the advent of instrumentalmeasurements have dominated the last 25 years. There is a scien-tific consensus building that global warming is related to theanthropogenic impacts on the natural greenhouse effect.

The 2007 Fourth Assessment Report from the Intergovern-mental Panel on Climate Change affirms this consensus. TheIPCC has predicted surface temperature response to a doublingof carbon dioxide ranging from an increase of 1.1 C° (2.0 F°) to6.4 C° (11.5 F°) between the present and 2100. Natural climaticvariability over the span of Earth’s history is the subject ofpaleoclimatology. A general circulation model (GCM)forecasts climate patterns and is evolving to greater capabilityand accuracy than in the past. People and their political institu-tions can use GCM forecasts to form policies aimed at reducingunwanted climate change.

paleoclimatology (p. 311)general circulation model (GCM) (p. 314)

21. Explain climate forecasts. How do general circulationmodels (GCMs) produce such forecasts?

22. Describe the potential climatic effects of global warming onpolar and high-latitude regions. What are the implicationsof these climatic changes for persons living at lowerlatitudes?

23. How is climatic change affecting agricultural and foodproduction? Natural environments? Forests? The possiblespread of disease?

24. What are the actions being taken at present to delay theeffects of global climate change? What is the KyotoProtocol? What is the current status of U.S. and Canadiangovernment action on the protocol?



NetWork

The Geosystems Student Learning Center provides on-line re-sources for this chapter on the World Wide Web. To begin:Once at the Center, click on the cover of this textbook, scrollthe Table of Contents menu, and select this chapter. You willfind self-tests that are graded, review exercises, specific updates

for items in the chapter, and in “Destinations” many links tointeresting related pathways on the Internet. Geosystems Stu-dent Learning Center is found at http://www.prenhall.com/christopherson/.

Critical Thinking

A. The text asked that you find the climate conditions for yourcampus and your birthplace and locate these two places onFigures 10.3, 10.4, and 10.5. Briefly describe the informa-tion sources you used: library, Internet, teacher, and phonecalls to state and provincial climatologists. Now, refer toAppendix B to refine your assessment of climate for thetwo locations. Briefly show how you worked through theKöppen climate criteria given in the appendix that estab-lished the climate classification for your two cities.

B. Many external factors force climate. The chart “Global andannual mean radiative forcing for the year 2000, relative to1750” is presented here (from IPCC Climate Change 2001,The Scientific Basis, Washington: Cambridge UniversityPress, 2001, Figure 3, p. 8, and Figure 6.6, p. 392).

The estimates of radiative forcing in Watts per squaremeter units are given on the y-axis (vertical axis). The levelof scientific understanding is noted along the x-axis

(horizontal axis), arranged from “high” to “very low.”Those columns above the “0” value (in red) indicate positiveforcing, such as the greenhouse gases grouped in the far-leftcolumn. Columns that fall below the “0” value (in blue) in-dicate negative forcing, such as the haze from sulfateaerosols, fourth column from the left. The vertical line be-tween the markers on each column is an estimate of the un-certainty range. Where no column appears but there isinstead a line denoting a range, there is no central estimategiven present uncertainties, such as for mineral dust.

Assume you are a policymaker with a goal of reducing therate of global warming, that is, reducing positive radiative forc-ing of the climate system. What strategies do you suggest toalter the height of the columns and adjust the mix of elementsthat cause warming? Assign priorities to each suggested strate-gy to denote most-to-least effective in moderating climatechange. Brainstorm and discuss your strategies with others.

3

2

1

0

–1

–2

High Medium

Stratosphericozone

Medium

Troposphericozone

Halocarbons

N2O

CH4

CO2

MineralDust Solar

Contrails Cirrus

Low VeryLow

VeryLow

VeryLow

VeryLow

VeryLow

VeryLow

VeryLow

VeryLow

Co

olin

gW

arm

ing

Rad

iati

ve f

orc

ing

(W

atts

per

sq

uar

e m

eter

)

Level of Scientific Understanding

Sulphate Biomassburning

Land-use

(albedo)ony

Organiccarbonfromfossilfuel

burning

Blackcarbon from

fossilfuel

burning

Aerosols⎧ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎩

Aviation-induced⎧ ⎪ ⎨ ⎪ ⎪ ⎩

The Global Mean Radiative Forcing of the Climate Systemfor the Year 2000, Relative to 1750

Aerosolindirecteffect


m10 chri5988 07 se c10 - higher education | pearson · 278 part ii the water, weather, ... a...

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