Global change

Natural short and long term changes

Anthropogenic changes

Natural cycles

• Glacial cycles

• Holocene climate variability– Orbital cycles– Milankovitch cycles – regular shifts in earth’s

climate

Glacial-interglacial• Relationship between glacial-interglacial cycles and CO2

• During glacial periods, older avg global temp (5-6 oC), lower sea level (~100 m), change in ocean circulation

• Causes– Changes in Earth’s orbit around the sun – also cause seasons

• Gravitational attraction between Earth and other bodies interact with orbital factors

• Tilt (obliquity) – changes solar lumination input (41 ky periodicity) result in seasonality; earth’s spin axis is tilted

• Eccentricity – the degree to which the Earth’s orbit is elliptical (100 ky periodicity); affects seasonality in N hemisphere; affects annual average solar input

• Precession of spin axis – spin axis moves do to gravitational forces between bodies (wobble)

• Interaction effects between tilt and eccentricity (positive or negative interaction)

• All lead to differences in solar input

Natural fluctuations

Figure 14.21 Geometry of the Earth's orbit and axial tilt. A. Precession. The Earth wobbles on its axis like a spinning top, making one revolution every 26,000 years. The axis of the Earth's elliptical orbit also rotates, though more slowly, in the opposite direction. These motions together cause a progressive shift, or precession, of the spring and autumn equinoxes, with each cycle lasting about 23,000 years. B. Tilt. The tilt of the Earth's axis, which now is about 23.5 degrees, ranges from 21.5 to 24.5 degrees. Each cycle lasts about 41,000 years. Increasing the tilt means a greater difference, for each hemisphere, between the amount of solar radiation received in summer and that received in winter. C. Eccentricity. The Earth's orbit is an ellipse with the Sun at one focus. Over 100,000 years, the shape of the orbit changes from almost circular (low eccentricity) to more elliptical (high eccentricity). The higher the eccentricity, the greater the seasonal variation in radiation received at any point on the Earth's surface.

Fig. 14-6 Obliquity, insolation, and seasons.

(but NH is tilted away from the sun)

(but NH is tilted toward from the sun)

Making N hemisphere wintersmilder.

Today, tilt and eccentricity (+precession) oppose one another in the N Hemisphere

Today tilt and eccentricity (+precession) reinforce one another in the S Hemisphere

Not equally scaled

Frequency and amplitude of variability

Astronimical theory of ice ages• Changes in seasonal contrasts over geological

time• Due to 3 dominant factors – precession

(wobble), tilt (obliquity), and eccentricity (shape of earth’s orbit around the sun)

• Driver of ice ages is Milankovitch cycles• Exact causes not well-understood• Needed some major change to initiate cycles

– India colliding with Asia (increased weathering)?– Initial cooling due to plate tectonics slowing down?– Feedback loops likely important

Fig. 14-8

Pleistocene glaciations• Pleistocene glaciation (~ 1mybp) – start of quaternary

– Why did they start? Some perturbation?

• Ice-albedo feedbacks – positive– What kept them going, but what reversed them?

• Timing of glacial cycles –– Initially glacials and interglacials equal in length (40-50 ky cycles)– Increase in length of glacials more recently, why?

Frequency of glaciationsincreasing

The Carbonate-Silicate Cycle and Long-Term

Controls on Atmospheric CO2

CO2

CO2

CO2

CaSiO3 + 2CO2 + H2O Ca2+ + 2HCO3- + SiO2

Weathering ofsilicate rocks

+ SiO2

CaCO3 + SiO2 CaSiO3 + CO2

Subduction(increased P and T)

CO 2

Ions (and silica) carriedby rivers to oceans

Ca2+ + 2HCO3-

(+ SiO2[aq])

CaCO3 + CO2 + H2O(+ SiO2(s)]

Organisms build calcareous(and siliceous) shells

Decrease in spreading rates also decreases subduction

Disturbance to carbonate-silicate cycle via decreased seafloor spreading rates

Fig. 8-17 Collision of India with Asia.

The Carbonate-Silicate Cycle and Long-Term

Controls on Atmospheric CO2

CO2

CO2

CO2

CaSiO3 + 2CO2 + H2O Ca2+ + 2HCO3- + SiO2

Weathering ofsilicate rocks

+ SiO2

CaCO3 + SiO2 CaSiO3 + CO2

Subduction(increased P and T)

CO 2

Ions (and silica) carriedby rivers to oceans

Ca2+ + 2HCO3-

(+ SiO2[aq])

CaCO3 + CO2 + H2O(+ SiO2(s)]

Organisms build calcareous(and siliceous) shells

increase in weathering

http://www.moraymo.us/uplift_overview.php

Continental collision creatingmonsoonal climate with lots of rain and weathering?

Lots of Buts….• Eccentricity cycle (100 ky) is weakest of 3 cycles

but sets frequency of glacial cycles?• Rate of change during glacial transitions is rapid

relative to astronomical changes?• N vs. S hemisphere have same schedules for

glaciation but should they?– Ice core data from both hemisphere similar

Role of the oceans?

• Deep water circulation altered or shut off during glacials

• CO2 changes – cause or effect/positive feedback? Unlikely that CO2 itself triggered glacial/interglacial transitions

Implications• Global climates and greenhouse gases change over

glacial/interglacial timescales

• Also shorter timescales – Heinrich events (due to FW inputs and feedback from ocean circulation) and Dansgaard-Oeschger events (rapid warming in N hemisphere) within glacial cycles

• Due to reorganization of ocean-atm system (multiple steady states? Glacial and interglacial?)

• Will there be a third, warmer

quasi-stable state

Fig. 3 During a Heinrich event, icebergs surge into the North Atlantic Ocean. The lower panel illustrates the entrainment of debris (black) by icebergs and the subsequent sedimentation of the debris in the deep North Atlantic.

Holocene climate variability

• Short-term variability– 1-2 ky sub-Milankovitch periodicity– Other cycles

• Important to understanding natural variability versus anthropogenic

• After last ice age• Medieval warm period (imp for Europe),

Little Ice age (Greenland)

Causes

• Astronomical• Early anthropogenic hypothesis – human’s staved

off next glacial; greenhouse gases behaved differently in initial stages of this interglacial

• Episodic factors – changes in solar activity, tectonic activity

• Changes in ocean circulation – FW inputs• Changes in sunspot activity• Volcanic activity

Fig. 14-1 (Ruddiman)

Holocene climate optimum

Little Ice Age

Medieval Warm Period

LGMYounger Dryas- Sudden cooling

maximum seasonal contrast- tilt and eccentricity reinforce another- CO2 at a max.- “start of interglacial’

Correlation = Causality (?)

Sunspot activity

Maunder Minimum (1645-1715)Low sunspot activity

Wolf Minimum (1282-1342)Low sunspot activity

Spörer Minimum (1450-1534)Low sunspot activity

High sunspot activity

Cooling due to volcanic eruption: The global mean temperature changes for 5 years preceding and following a large volcanic eruption (at year zero). The temperatures are the average changes noted for five major eruptions: Krakatau, August 1883; Santa Maria, October 1902; Katmai, June 1919; Agung, March 1963; and El Chichón, April 1982. The effects of ENSO on temperatures have been removed. (After A. Robock and J. Mao, 1995. The Volcanic Signal in Surface Temperature Observations. Journal of Climate, 8:1086–1103.)

Fig. 15-6

Present-day climate variability

Present day

• ENSO events

• Sea ice atm-ocean interactions at high latitudes

High volcanic activity

Low volcanic activity

Eruption of Mt. Tambora (1815)

• Timing and amplitude of forcing

• Stochastic resonance – superimpose random forcing on low amplitude periodic forcing

Global warming• Recent climate change

– What controls climate and what’s changed?• Present day forcings

– Changes in solar input (luminosity)

– Changes in albedo (volcanoes, land cover, ice)

– Changes in greenhouse gases

– Changes in feedbacks

– Planetary forcings

– Stochastic events

– Humans – the new element• Can’t explain current T trends without it – radiative forcing (changes in

balance of incoming and outgoing radiation)• Radiative forcing affected by: increases and decreases in solar input,

planetary albedo, and concentrations of greenhouse gases

Archer - Fig. 11.8 (Hadley Centre results)

Albedo• Aerosols, clouds, ice/water/land distribution

• Feedbacks between temp and changes in things affecting albedo

• Aerosols – cooling effect (reflect incoming radiation)– Fine particles

– Also form cloud condensing nuclei

– Produced by natural (volcanoes) and unnatural (fossil fuel burning) processes

• Relative albedos of ice/water/land– Ice (0.8) > deserts & unvegetated land (~0.5) > water & vegetated

land (< 0.1)

Ice - ~0.8

Oceans - < ~0.1

Non-vegetated land - ~0.5Vegetation - < ~0.25

Earth Surface Albedo

Greenhouse effect• Radiative balance

– Temp controlled by balance between incoming solar flux, amount of outgoing IR radiated from Earth, redistribution of radiation before it is reradiated to space (e.g., outgoing IR retained by greenhouse gases)

• Natural greenhouse

• Unnatural greenhouse– CO2 – excursions greater than glacial/interglacial

• Correlations with human activities (fossil fuel burning), ocean uptake, land use changes (deforestation)

• Where does this stuff go? Oceans and atm

• Effects on earth system (ecological, climatological, etc)

– Other greenhouse gases• Same as above (S, methane, water vapor)

(also see Fig. 16.4 in your book)

Uncertainties

• Clouds– warming or cooling– Albedo versus greenhouse

Fig. 3-18 The different effects of high and low clouds on the atmospheric radiation budget.

Low albedo (rel. to low clouds) low temp. (low outgoing IR flux)Greenhouse effect dominates

High albedo and high sfc. temp. (large outgoing IR flux)Albedo effect dominates

Evidence of Climate Change

• Temperature records (ground thermometers, proxy records)• Atmospheric temperature records• Ocean warming• Glacier melting• Ecosystem changes• Changes in the hydrologic cycle

Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global mean sea level.

The IPCC also finds that it is “very likely” that emissions of heat-trapping gases from human activities have caused “most of the observed increase in globally averaged temperatures since the mid-20th century.”

IPCC Fourth Assessment Report (2007)

IPCC History: Evolution of IPCC History: Evolution of Our KnowledgeOur Knowledge

• FAR (1990): “FAR (1990): “The size of the warming is broadly consistent The size of the warming is broadly consistent with predictions of climate models, . . . but the unequivocal with predictions of climate models, . . . but the unequivocal detection of the enhanced greenhouse effect from observations detection of the enhanced greenhouse effect from observations is not likely for a decade or more.”is not likely for a decade or more.”

• SAR (1996):SAR (1996): “The balance of evidence suggests a discernible “The balance of evidence suggests a discernible human influence on climate.”human influence on climate.”

• TAR (2001):TAR (2001): “There is new and stronger evidence that most of “There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to the warming observed over the last 50 years is attributable to human activities.”human activities.”

CO2 removal

• Oceans

• Atm accumulation

• Reforestation

• Problem of not understanding feedbacks or their direction – how far can the ocean go? How will changes in temperature affect the direction of changes, etc.

Fig. 11.12

(Fig. 16-2)

Long term projection

Fossil fuel reserves exhausted

Projections equally depressing

• Stabilizing total emissions or stabilizing rates of emissions

• Models are simply C cycle models – what about everything else and their feedbacks?

• Emissions – radiative forcing calculations are not linear

Figs. 16-3 and 6

Fig. 6-3

If nothing is done to slow If nothing is done to slow greenhouse gas emissions. . .greenhouse gas emissions. . .

• COCO22 concentrations will likely concentrations will likely

be more than 700 ppm by 2100be more than 700 ppm by 2100

• Global average temperatures Global average temperatures projected to increase between projected to increase between 2.5 - 10.4°F2.5 - 10.4°F

2100

Source: OSTP

• 1000 to 1861, N. 1000 to 1861, N. Hemisphere, proxy Hemisphere, proxy data data

• 1861 to 2000, 1861 to 2000, Global, instrumentalGlobal, instrumental

• 2000 to 2100, SRES 2000 to 2100, SRES projectionsprojections

Source: IPCC TAR 2001

Variations of the Earth’s SurfaceVariations of the Earth’s SurfaceTemperature - 1000 to 2100Temperature - 1000 to 2100

Main Findings of WG I Main Findings of WG I • Extensive and wide-spread evidence that the earth is Extensive and wide-spread evidence that the earth is

warming; we are already seeing the first clear signals of a warming; we are already seeing the first clear signals of a changing climate.changing climate.

• Human activities are changing the atmospheric Human activities are changing the atmospheric concentrations of greenhouse gases.concentrations of greenhouse gases.

• New and stronger evidence of a human influence on New and stronger evidence of a human influence on climate.climate.

• Global temperature will rise from 2.5 to 10.4°F over this Global temperature will rise from 2.5 to 10.4°F over this century. century. Precipitation patterns will change, sea level will Precipitation patterns will change, sea level will rise and extreme weather events will increase.rise and extreme weather events will increase.

• Human influence will continue to grow during the next Human influence will continue to grow during the next century unless measures are taken to reduce GHG century unless measures are taken to reduce GHG emissions.emissions.

Uncertainties in feedbacks

• Clouds

• Water vapor

• Snow/ice albedo

• Ocean circulation

Summary

• The greenhouse effect exists, is natural and we are perturbing it

• Past climate changes can occur rapidly but not of the same magnitude

• Rate of increase/change is unprecedented• Major climate and ecological changes in store• How do we deal with the change

– Change behavior to stem the rate of increase– Adapt to what is already in the cards