Indian Journal of Chemical Techno logy Vo l. 9, March 2002, pp. 165- 173

Svante Arrhenius and the Greenhouse Effect

Jaime Wisniak*

Department o f Che mical Engineering, Ben-Gurion Univers ity of the Negev, Beer-Sheva, Israe l 84 105

Svante Arrhenius ( 1859- 1927; 1903 Nobel Prize in Che mi stry) is one of the most famou s sc ienti sts of the last century. Hi s theories about electrolyte di ssoc iation, chem ical kine ti cs, physical chemistry, and immunoche mi stry set the direction for the development of modern e lec troche mi stry, theory of soluti ons, and catalytic processes. Very few are aware o f hi s sub­stantial contribut ion to the study o f the effect of atmospheric contamination upon climate and the possible reason s of the di f­ferent Ice Ages that the Earth has experie nced during different hi sto rical epochs. Here a general descripti on of the so-ca lled greenhouse effec t and Arrhe nius's contributi on to its interpreta ti on is be ing presented .

According to Trewartha 1 weather is a phenomenon that varies enormously over the face of the Earth , and hence it is of great geographical significance. By weather we understand the sum total of a ll atmos­pheric variab les at a g iven time in a g iven place. Cli­mate, on the other hand, is a generalization or com­posite of weather conditions for a long period of time within a given area; it refers to a more enduring re­gime of the atmosphere. Weather is an everyday expe­rience; climate is an abstract concept.

Climatology in the 18th century was essentially an empirical sc ience, the idea of a c limatic map had yet to be developed and upper-air data was lacking. All the efforts were dedicated to use hi storical weather observations to g ive the probabilities that the vario us meteorological pheno mena wou ld occur at given ph ases of the annu al and daily cyc les . Although more advanced than in early ages, climate prediction con­tinued to be vital for pl anning religious, military, navigation , and agricultura l activities. There was no central effort to develop a theory that cou ld ex plain the different pheno mena observed.

Synoptic charts, representing simultaneous weather conditions over a large area, began to be developed by the middle of the 19th century. With hi s tool it was possib le to re late pressure variations with other phe­nomena like winds, rains, and storms.

Today, it is assumed that the energy of the atmos­phere consists of four components : the kinetic energy of the major wind sys tems ; the potential energy of ~h e Earth's gravitati onal field; the latent energy o f water vapour; and the air's internal energy. The energy con­tent of the atmosphere at any instant refl ects the bal-

*For correspondence (E- mail: w is niak @hgurnai l.hgu.ac.i l; Fax (972 )-8-64729 16)

ance among these different components and their transformations2

. The contribution of each component can be calculated without difficulty , measuring the pressure and temperature at different altitudes.

Industrialization has been growing substantia ll y over the past century and reflected in c limate changes . The developing world is expec ted to grow even more rapidly in the future and also its influence on the c li­mate. Most model projections show future changes that are unprecedented re lative to past human experi ­ence, in terms both of the magnitude and the rap id ity of change. The consequences o f such changes in c li­mate for humans and the natural environment are po­tenti ally serious and dangerous.

It is a well-known and documented fact that the composition o f the atmosphere has changed markedly since pre-industrial times. For example, C02 concen­tration has ri sen from about 270 to 280 ppm, to o ver 360 ppm today. CH4 has ri sen from about 700 ppb to over 1700 ppb, and N20 has increased from about 270 ppb to over 3 10 ppb . Chemicals like halocarbo ns that do not exist naturally are now present in substanti al amounts and new gases are being added every year. The pre-industrial levels of these gases are known because the compositi on of ancient air trapped in ice cores fro m Antarctica can be measured direc tl i . These ice cores show that the changes since pre­industrial times far exceed any changes that occurred in the preceding 10,000 years.

For C02, analyses of radiocarbon changes prove that emiss ions fro m fossil fu el combustion have been a major contribution to the concentration increase. Land use changes, mainly associated with deforesta­tio n, particul arly o f the ra in forests, have a lso contrib­uted significantly . The main sources for methane in­crease appear to be agriculture, animal husbandry,

166 INDIAN J. CHEM. TECHNOL., MARCH 2002

landfill emissions, and leakage associated with fossil fuel production and distribution. The increase in ni­trous oxide, N20, is related to the increased use of nitrogen compounds in agriculture as fertilizers. For these gases, their total emissions are reasonably well defined. Their emission budgets (i.e., the breakdown into different source categories) are more uncertain.

The effect of halocarbons, particularly CFC II and CFC 12, on the ozone layer has been the subject of much scientific and political discuss ion . The source of all chlorofluorocarbons is anthropogenic and the Montreal Protocol controls the phase out of some of them. However, new substitute chemicals, which are not controlled because they do not cause depletion of stratospheric ozone, are being introduced . The future effect on the climate of these new chemicals is yet to be determined.

In addition to the gases mentioned above, there are other important atmospheric composition changes due to anthropogenic activities. Emissions of gases like CO, NOx, and volatile organic compounds (VOC) such as butane and propane, used in aerosol and paints, have led to large changes in tropospheric ozone. Tropospheric ozone is a powerful greenhouse gas.

Finally, emissions of S02 from foss il fuel burning, particularly coal, and other substances released by biomass burning act ivities, have increased the aerosol loading of the atmosphere. This increase is important because the presence of aerosols has a cooling effect that may partly offset the warming effect of green­house gases. Some action has also been taken by the automobile industry (catalytic converters) to decrease the emi ss ion of gases like CO, NOx, and S02.

Changes in atmospheric composition resulting from increased industria l act ivity have disturbed the overall energy budget of the planet, upsetting the delicate balance between incoming solar short-wave radiation and o utgoing long-waver radiation from the surface, the planet's radiative balance. Such a change is re­ferred as radiative forcing . The climate system re­sponds to positi ve radiative forcing by trying to re­store the radiative balance, which it does by warming the lower atmosphere. The larger the radiative forc ­ing, the larger the eventual surface temperature changes.

It is often hard to distinguish between ecological changes that can be directly attributed to human ac­tivities and those that were caused by natural proc­esses, but there is no doubt about the fact that the sur-

faces of large areas of the earth have a different heat and water balance than they use to have, before man­kind had a major influence on the vegetation cover4

•

Jones and Henderson-Sellers5 explain that the greenhouse theory is based upon the fact that while the gases in the Earth's atmosphere are transparent to incoming short-wave radiation, some of them are ca­pable of absorbing the outgoing thermal long-wave radiation emitted from the Earth's surface. This radia­tive interaction between selected gases (termed the greenhouse gases, GHGs) in the atmosphere and the outgoing heat radiation causes those gases to warm and consequently they reradiate heat in all directions. Part of this reradiated energy travels back down through the atmosphere and it is thi s additional heat­ing of the surface over and above the heating due to the absorption of solar radiation, wh ich is termed the greenhouse effect. The most signi ficant greenhouse agent in the Earth's atmosphere is water vapor, which contributes about 100 of the 148 watts of additional radiatively induced heating of each square meter of the surface. It must be understood that greenhouse effect is a natural phenomena that has existed from the moment the Earth and its atmosphere were created about four and a half billion years ago . Then natural greenhouse effect, without the participation of human activities, transformed the original planet with a

global mean temperature of -I goc into its present habitable state with a g lobal mean temperature of +!SOC: Venus, with a dense, almost pure C02 atmos­phere has temperatures near 500 K at its surface, while Mars, with a tenuous C02 atmosphere, has tem­perature of only 220 K. Mankind's ac tivities , has sim­ply provided a synergistic effect to the natural phe­nomena. All human activities, in particular those as ­sociated with industrialization, deforestation, large scale land clearance, and use of artificial fertilizers, have been adding increas ingly large amounts of the original radiatively active gases to the atmosphere, as well as new radiative gas components. As an example of the latter, we can mention meth ane, nitrous ox ide, ozone, and the chlorofluorocarbons (CFCs), used as propellants in spray cans and in refri gerators and air conditioners. Concentration of the artificial compo­nents are increasing more rapidly than carbon dioxide and some predict that in the 2 I st century their contri­bution to a global warming could nearly match that of carbon dioxide. The planet seems, therefore, to be committed to a g lobal scale warming, an intensifica­tio n of the hydrological cycle and many assoc iated climatic shifts. 5

EDUCATOR 167

The Greenhouse Effect One of the first scientists to propose that the gases

in the atmosphere could retard heat radiation from escaping to space and thereby warm the surface, was Jean-Baptiste-Joseph Fourier (1768-1830). In a mem­oir submitted to the Paris Academie des Sciences in 1824 Fourier6 compared the heating of the atmosphere to the action of a solar thermometer (heliothermome­ter). The solar thermometer was a wooden box insu­lated with black cork, and provided with an internal thermometer. Sunlight entered the box through a win­dow covered with three panes of glass separated by air spaces. The inter-posing of the glass and air spaces (and by analogy the atmosphere) meant that the heat was absorbed and trapped, resulting in a heating over and above that due to solar radiation . Thus, the air formed, relative to heat, a sort of souriciere (mouse­trap) . For Fourier the atmosphere was a giant helio­thermometer having radiative properties of its own.

Fourier's ideas were further elaborated by Claude­Servais-Mathias Pouillet ( 1790-1868)7

. According to Pouillet, the equilibrium temperature of the atmos­phere had to be lower than that of outer space and higher than the ground temperature because the at­mosphere exerted unequal absorbing actions on rays of heat derived from space. The atmosphere exercised a larger absorption upon the terrestrial than on solar rays, a phenomenon which Pouille t named effect of diathermanous envelopes.

Tyndaii8·9 conducted the first convincing experi­

ments on the radiative properties of gases de mon­strating that "perfectly colourless and invisible gases and vapours" were able to absorb and emit radiant heat. The elementary gases were almost transparent to radiant heat while other were opaque. Tyndall's meas­urements indicated that water vapour absorbed e ighty times thermal radiation than pure air and concluded that thi s property must exerci se the most important influence on climate. According to Tyndall, every variation of water vapour content must produce a change in climate and similar remarks would app ly to C02: "It is not necessary to assume alterati ons in the density and height of the atmosphere to account for different amounts of heat being preserved to the earth at different times; a s li ght change in its variable con­stituents would suffice for this. Such changes in fac t may have produced all the mutations of climate, which the researches of geologists reveal" . Tyndall was ap­parently the first to make an important additional de­duction, namely that g lacia l peri ods may have been caused by a decrease in atmospheric carbon dioxide.

It was soon realized that the concentration of car­bon dioxide was probably increasing, as a conse­quence of the increased burning of coal, petroleum, or natural gas. By the end of the 19th century Chamber­lin and Arrhenius published some important papers that greatly advanced the appreciation of the effects of C02 changes in climate 10

-14

• Arrhenius set on to cal­culate the effect on the Earth's average surface tem­perature of increasing or decreasing of the amount of carbon dioxide in the atmosphere. He did not perform this experimentally , he used Langley's measurements of radiation from the Moon under different humidity conditions to estimate the amount of radiation ab­sorbed by the atmosphere due to both carbon dioxide and water vapor. His results indicated that doubling the carbon dioxide would result in an increase of the average surface temperatures by 5 to 6 K, while a de­crease in carbon dioxide to two-thirds of the present value would cause a cooling of 3 to 3.4 K. To its credit it must be said that in his calculations he took into account that the total water content of the atmos­phere would increase with increasing temperature (feedback mechanism). His final conclusion was that "if the quantity of carbonic acid (carbon dioxide) in­creases in geometrical progression, the augmentation of the temperature will increase in nearly arithmetic progress ion 10

. " Between 1897 and 1899, Thomas C. Chamberlin ( 1843-1928) published three papers dealing with the geological implications of the carbon dioxide theory. In 1897 he reviewed the current hy­potheses of climatic changes 12 and suggested that ap­pearance and disappearance of glaciation periods could be accounted for by considering the action of the ocean as a C02 reservoir and the losses of organic matter under the influence of cold. C02 solubility in water decreases with temperature hence as the tem­perature decreased more C02 was he ld by the atmos­phere. Similarly, with increased cold the process of organic decay became less active and the loss of C02

through the organic cycle increased . The net result was hasteni ng of the atmosphere impoverishmen t and the consequent precipitation of a cold epoch. Cold resulted in the covering of rocks by ice and hence in the checking of rock alteration (formation of carbon­ates) . Since suppl y remained the same, enrichment of the atmosphere reinitiated and when it was suffi­ciently advanced the temperature increased. The oceans started releasing the accumulated C02, and the decay of the accumulated organic matter accelerated. The net resu lt was hastening of an interglacial period . In 1888 and 1889 he postul ated the effects of li me-

168 INDIAN J. CHEM. TECHNOL., MARCH 2002

stone forming periods might have had in contributing to subsequent glacial epochs 13

"14

• He proposed that the original atmosphere was a vast gaseous envelope that contained the entire C02 that is today contained in limestone and other carbonates, and in coal and other carbonaceous materials. The atmospheric history was simply a progressive depletion of this original supply . Chamberlin built on the arguments provided by Hog­born and Arrhenius to "frame and hypothesis of the cause of glacial epochs on at atmospheric bas is"13

.

Many of hi s arguments dealt with the changes in sea level and extension and elevati on of the land, and their effects on the weathering of silicate rocks and atmospheric depletion, a matter also di scussed by Hogbom.

According to Kellogg4 "many of the important pieces of the carbon di oxide/climate puzzle were in place by 1900, though of course there were serious gaps in the knowledge of rates of removal and re­pleni shment of carbon dioxide, and also many other factors that influence the climate system" . What was not known then was the significant increase that would take place in the use of foss il fuels and creati on of new atmosphere contaminants. In additi on, Kell ogg pointed out that it was not until the 1970s when the greenhouse effect acquired urgency in the context of global warming and a concerted effort was put to un­derstand the long-term effect and signi ficance of the problem.

In 1938 Callendar15 est imated that between 1890 and 1938 man had added about 150 mill ion tons of C02 to the air from the combustion of fuel gases and that approximately 75 % of thi s had remained in the atmosphere. Furthermore, Callendar calculated that if all other factors remained in equilibrium thi s arti ficial addition of C02 had increased the mean global atmos­phere temperature, above the one present in the 19th century, by 0.07°C between 19 10-1 930 and by 0. I6°C in the 20th century. The pertinent increases would be 0.39°C for the 2 1st century and 0.5TC for the 22 nd century. Callendar also predi cted that these tempera­ture increases would be accompani ed by polar di s­pl acement of climate zones of 36, 87, and 127 km, for the 20th , 21st, and 22nd centuries, respectively . In a later publication, Call endar 16 reiterated the role of human climate fo rcing, by calculating that the con­centration of atmospheric C0 2 had increased from about 290 ppm in 1900 to 325 ppm in 1956, repre­senting the addition of 280 million tons of fuel C02. Thi s increase could very well justify the observed sli ght ri se in the average northern latitude temperature

during the first four decades of the 20th century. This was fundamentally a re-examination of the hypothesis of Arrhenius and Chamberlin in the context of human activities.

In 1967, Manabe and Wetherald 17 developed a one­dimensional model for the co2~induced global warming, which considered that the change in C02 content not only affected the flux of net radiation but also the fluxes of sensible heat and latent heat from the Earth 's surface to the atmosphere. The fundamen­tal effect of the time dependent radiati ve heating was to push the climate system into a state of di sequilib­rium with the incoming and outgoing energy fluxes. The model predicted that the doubling of C02 in the air would increase the equilibrium temperature of the Earth's surface by about 2.3°C. Manabe and Wether­aid 18 improved their original model to a three­dimensional one having an idealized topography , no heat transport by ocean currents, and fi xed cloudiness . The new model showed that the surface and the tropo­sphere were strongly connected by convective heat transfer processes so that the surface temperature was governed not only by radi ati ve heating at the ground but by the heating of the joint surface-troposphere system. Their results showed that doubling the amount C0 2 would raise the temperature of the tropo­sphere, lower that of the stratosphere, and signi fi­cantly increase the hydrological cycle.

According to Schneider19, a simple one-

dimensionally globally averaged model of the Earth­atmosphere system that accounts for mutual adjust­ments among the stratosphere, troposphere, and sur­face, and assumes a constant tropospheric relati ve humidity profile is perhaps, the most reasonable available quantitati ve estimate of the sensiti vity of the mean global surface temperature to increases in C0 2 .

From a criti cal rev iew of different es ti mates of global surface temperature sensitive to a doubling of atmos­pheric C0 2 to 600 ppm, Schneider concluded that a reasonable estimate of the temperature increase was 1.5 to 3 K.

The recognition of the main sources of the green­house gases: energy production (COz and CH4) ; in­dustrial gases (C0 2 and CFCs); agricu lture (CH4 and N20 ); and land clearance (C0 2 and NzO), has prompted the use of simple equations such as the "the population multiplier" proposed by Erlich and Hol­dren20

EDUCATOR 169

Future greenhouse gase emissions

emission effluent 1 1

. . = . tota popu atJOn s1ze

technology capita

where the first term represents the level of engineer­ing skill, the second the standard of living, and the third demography5

.

Arrhenius's contribution At the end of the 19th century a series of discus­

sions took place at the Stockholm Physics Society regarding the causes that had produced the Ice Ages, and the possible reasons for their cyclical nature. Many possible arguments were given for the source of the phenomena: cosmic, astronomical, geodetic, physical, and geographical.

The geological evidence indicated that during the Tertiary Period the European flora and fauna occupied a more meridian habitat than today. It had also been estimated that the average temperature must have been go to 9°C higher than the present one. Afterwards came a very cold period, the Great Glacial Era, during which countries like Ireland, Britain, Holland, Scan­dinavia, Germany, Austria, and Russia (up to Kiev and Novgorod) were covered with ice. At the same time, an ice cap from the Alps covered Switzerland, parts of France, Bavaria, the northern part of Italy, and a large part of North America. Other parts of the world, like India, Australia, and Patagonia, also showed geological evidence to an Ice Age. Measure­ments of the displacement of the snow line showed that during the Great Glacial Period the temperature must have been 4° to soc lower than at present. Af­terwards came the so-called interglacial period, hav­ing a more benign climate that the present one. Thi s period gave place to another glacial one, less rigorous than the first one, but strong enough to cover with ice the Scandinavi an Peninsula and the surrounding

. 2 countnes .

Within the discussions Sven Otto Pettersson pre­sented his measurements of CO solubility and Arvid Hogbom his theories regarding the relation between glaci ation and the weathering of rocks2 1

• Pettersson made careful measurements of C02 in the water and along the Swed ish coast. The scattered data he pre­sented to the Society, however, did not permit any generali zations concerning the fac tors that m3y have made for seasonal or other vari ations in the observed C02. In 1893 Hogbom exposed to the Stockholm Phys ics Society hi s theory regarding the variations of C0 2 in the atmosphere. To do so, he developed a C0 2

budget for the Earth and the atmosphere, that is , a material balance accounting for all the possible sources producing and consuming C02. According to his estimates, industry was burning about 500 million tons of coal per year, (or one ton per km2 of the Earth's surface!) an amount that represented only one thousandth of the C02 existing in the atmosphere. This amount of C02 corresponded also to a layer of limestone of 0.003 mm thickness over the whole globe, or 1.5 km3 in cubic measure. It represented a layer of limestone of 0.003 mm thickness over the whole globe, or 1.5 km3 in cubic measure. Based on his studies on the formation and extension of lime­stone across the globe, Hogbom believed the quantity of C02 added to the atmosphere by the combustion of coal was essentially the same as the one consumed from the atmosphere in the process of weathering or decomposition of silicates:

C0 2 + CaSi0 3 <=> CaC0 3 + Si0 2

C0 2 + MgSi0 3 <=> MgC0 3 +Si0 2

These reactions going from left to right represent C02 removal by way of weathering and carbonate precipitation and going from right to left represent degassing due to the thermal decarbonation of the carbonates after burial to sufficient depths . According to Hbgbom from the analysis of the water of different rivers, in different countries and climates, as well as their flow , it was possible to estimate that these sources deposited an amount of carbonate equi valent to a volume of 3 km3

. In other words, the yearly vol­ume of limestone created by C02 originating from combustion was of the same order of magnitude as the volume of limestone deposited in the bottom of the sea.

Hogbom rai sed then the question of what would happen if an identical amount of C02, as that present in the atmosphere, would be suddenly added by vol­canic eruptions or other natural phenomena? To ac­count for the real effect one had to consider that the amount of C02 dissolved in the oceans was far from the saturation value. According to Hogbom the solu­bility effec t would act as a buffer to the results of the volcanic e;·uption since most of the C0 2 released would end di ssolved in the sea. In other words , any C02 budget had necessarily to take into account the oceans storage capability .

Other processes , like combustion and decay of or­ganic bodies, and decomposition of carbonates, did not have a significant effect in the material balance.

170 INDIAN J. CHEM. TECHNOL., MARCH 2002

They either contributed very little C02 or took place very rapidly . On the other hand, volcanic eruptions did not occur on a regular basis and could be of such magnitude as to represent a substantial sudden addi­tion of C02 into the atmosphere. Hogbom concluded that "an increase or decrease of the supply continued during geological periods must, although it may not be important, conduce to remarkable alterations of the quantity of carbonic acid in the air, and there is no conceivable hindrance to imagining that this might in a certain geological period have been several times oreater or on the other had, considerably less, than b ,

now"21. Arrhenius became interested in the subject after he

heard Hogbom's geochemical investigations of the variations of C02 in the atmosphere. This gave him the idea of placing the problem of the Ice Ages in a much broader, cosmic framework. What was the role of the atmosphere, and especially C02 in the atmos­phere, in absorbing and reflecting solar radiation, what was the influence on climate, and how was the mean temperature of the ground intluenced by the

. ?9 presence of C02 and water vapor tn the atmosphere. In another lecture given in 1894 to the Stockholm Physics Society, Arrhenius already suggested that it was perfectly poss ible to explain the variations in the Earth's temperature and the Ice Ages by changes in the amount of C02 present in the atmosphere: "would it not be possible that the Earth's temperature had de­creased during periods of low C02 and increased when the protective C02 had been present to a higher degree"?

The question of the origin of these temperature os­cillations was unanswered and many Academies of Sciences had offered a prize for its solution . In the beginning, the most popular theory was that of James Croll ( 1821-1890) by which cold periods in the north­ern hemisphere should be expected whenever there was an increase in the eccentricity of the elliptic. Eventually, Croll's theory was shown to be implausi ­ble2.

In 1894, a meteorologist, Luigi de Marchi , won the prize offered by the Lombardy Institute of Science with his essay Le Cause dell'Era Glaciale22

. In it, de Marchi rejected most of the current explanations for the Ice Ages and postulated that the actual reason was a reduction in the transparency of the atmosphere caused by a greater quantity of water vapor in the air produced by volcanic eruptions. After de Marchi's work was published, Arrhenius gave a lecture in which he reviewed de Marchi 's work and rejected his

basic hypothesis ; that water vapor did not cause a larger transparency because de Marchi had failed to take into account the condensation that would auto­matically follow a temperature decrease and offset its effect.

As explained above, before Arrhenius attacked the global problem, it had received only partial attention by physicists, notably Fourier, Tyndall and Samuel Pierpoint Langley ( 1834-1906). Although water vapor and carbon dioxide absorbed a small fraction of the solar radiation, Langley claimed that this absorption did not seem to depend on the humidity, provided the vapor pressure of the vapor did not exceed the limit of about 5 mmHg. A similar thing seemed to happen with C02. According to Langley, about 60% of solar radiation reached the ground when the skies were completely free of clouds. The action that these two gases exerted on the terrestrial radiation was com­pletely different. Terrestrial radiation was composed of longer wavelengths, the maximum intensity corre­sponding to a wavelength of about 10 p .. Upon radia­tion of thi s nature water vapor and C02 exerted an energy absorption that was proportional to their con­centration in the atmosphere. Langley calculated that about 30% of the radiation received by the Moon originated from the Earth. Since the radiation of a full Moon originated from a body that had a temperature not significantly different from that of the Earth, this meant that the heat radiations of the Moon and the Earth belonged to approximately to the same section of the spectrum and experienced the same absorption2. Langley's measurements justified the ideas of Fourier and Pouillet because the atmosphere let go through about 60% of the solar energy and about 30% of the Earth's radiation. In practice, the last figure had to be substantially higher because the terres trial radiation , before reaching the interplanetary spaces, crossed masses of water vapor and C02 considerably larger that those transversed by the Lunar radiation, in Lan­gley's experiments. Langley concluded that "the tem­perature of the Earth under direct sunshine, even though our atmosphere were present as now, would probably fall to -200°C, if the atmosphere did not possess the quality of selective absorption"23. Since the Moon was at about the same di stance as the Earth from the Sun and did not have an atmosphere, then, its surface temperature should be around -200oc. Langle/4 abandoned thi s view when he determined that the full moon had a mean effective temperature of about 45°C. Further determination made by Francis William Very put the value at least +100°C.

EDUCATOR 17 1

According to Arrhenius, analys is o f the probl em required hav ing an idea about the degree of the ab­sorption o f so lar heat and terrestrial heat by the a ir. To do so, it was necessary to consider three facto rs, the first one th at ex perimental ev idence indi cated that the three main compo nents of air (oxygen, nitrogen , and argon) did not influence in any sensible manner the solar or terrestria l radiatio n. The second factor was suspended dust: it exercised a notab le influence over the hi ghly refractive radiat ions that formed a large part o f the so lar spectrum, whil e these were to ta ll y mi ss ing in the terres trial spectrum . Finally, there were two atmosphe ri c constituents that a lthough they were present in very small amo un ts , they played a domi­nant role. These were wa ter vapor and carboni c ac id that were known to abso rb a small fraction of the so­lar radi ati on2

.

Arrhenius explained in hi s papers25 that a ir reta ined heat in two different ways ( I) the heat suffered a se­lect ive di ffusion as it passed through the air. This was greatest for the rays hav ing short wavelengths and insensible for those of long wavelengths that formed the chi ef part of the rad iati on of a body ,of the te m­perature of the Earth ; (2) the gases themselves had the power of absorbing se lect ive ly the li ght and heat of certain wavelengths. The se lecti ve absorptio n ability of C02 and wa ter vapor was not onl y s igni ficantly larger than oxygen, ni trogen, or argon; it a lso took pl ace in certain definite bands, whi ch were particu­larl y intensive in the infrared portion o f the spectrum. T hi s intensive band inc luded the rays w ith lo ng wavelengths such given off by bod ies at a low te m­perature.

As Tolman ind icates,25 Arrhen ius rea li zed that there were two ways to measure the amount of heat absorptio n by C0 2 and water vapor: (a) Experimen­tally, by direct measurement of the amount of heat absorbed by C02 and water vapo r in the concentration they were present in the atmospliere and at a tem­perature of !SOC, and (b) by measuring the amount of heat received from the full moon at different he ight above the hori zon. The amo unt of C0 2 through which the rays passed was ev idently a function of the a lti­tude of the moon above the ho ri zon, while that of the water vapor depended both upon the a ltitude and the humidity of the air.

The required experimenta l data was not avail able and Arrhenius was in no position to make them. Ar­rhenius selected the second procedure and set himself to ca lcul ate the effec t of change in atmospheric C0 2 on the temperature of the g lobe at every ten degrees

of latitude from 70°N to 60°S once the actual tem­perature and humidity of a g iven place was known. In the absence of firm ex perimenta l data Arrhenius made the assumption that water vapor contributed less than C0 2 because it was dependent on temperature in­crease, that is, it would increase when the temperature inc reased but not the reverse.

Hi s nex t step was to determine the absorptio n coef­fic ients of C0 2 and water vapor. Since re liable data were not at hand , Arrhenius used Lang ley 's data con­cerning the amount o f hea t received on the Earth from the full moon at di ffe rent he ights above the hori zon24

.

This was a lso the amo unt that was reflected back from the Earth into the atmosphere. Since the full moon had a surface temperature of I 00°C Arrhenius used Stephan's radiation law to correct Lang ley 's data to apply them to a body with the temperature 15°C.

Using Langley's data concerning the infrared por­ti on of the spectrum, where radiatio n from the Earth had the greatest intensity , he calcul ated absorpti on coeffici ents of a g iven atmosphere using as hi s po in t of departure the present average amount of C0 2 (K = I) and the average amount of wate r vapo r (W = 0 .3, that it , ten grams per cubic meter at the Earth's sur­face). He also introduced a factor p that corrected for the fact that the rays would be reflected from the Earth at an angle different from that at which they had ente red the atmosphere and hence traverse larger quantities of K and W .

Arrhenius used Langley's data to determine the fo llowing equati on describing the abso rption as a

function o f the wave-l e~gth A. after it had entered with the strength 1 and passed thro ugh the a ir-mass 1 (path of the ray, quantity of C02 trans versed in the a ir by a vertica l ray):

log a =- 0 .0463/'A-.008204//..3

where a represents the strength o f the a ray of wave­

length A (in J.l) . For example, for A = 0.358 J.l , a = 0.904.

Arrhenius ex pla ined that the selecti ve absorption of the atmosphere was of a different nature. It was not exerted by the main components of the a ir, but by water vapor and carbo nic ac id , which were present in the a ir in small quantities. The experimenta l evidence indicated that the absorption by C02 and water vapor was not continuous over the whole spectrum, it oc­curred mainl y in the long-wave region. Thus, radi a­tion coming fro m the Sun was very littl e absorbed.

172 INDI AN J. CI-IEM. TECH OL.. MARCH 2002

With the absorption coeffici ents thus ca lculated, Arrhen ius proceeded to calculate the fraction of the heat from a body at I SOC ( the Earth) which was ab­sorbed by an atmosphere th at contained specified amoun ts of water vapor (W = 0.3 to I 0.0) and C02,

(K = I to 40). A rrhen ius proceed to develop a formula that related

a gi ven change in C02 to changes in the heat rad iation

({3) of the Earth and the Earth's surface correcting for the influence of clouds and the heat-moderating ef­fec ts of snow and water. Thi s formula allowed him to ca lculate the changes in temperature that would ac­

company changes in {3.

Arrhenius conducted his ca lculati ons assuming that the amount of heat that was conclut:ted from the inte­rior of the Earth to its surface was neg! igi bl e, that the heat conducted to a g ive place as a consequence o f atmospheri c or oceanic current remained constant, and that the clouclecl part of the sky remained un­changed (es timated as 52.5 % for the part o f the Earth between 60°S and 60°N). A ll authors agreed that the Earth and its atmosphere were in thermal equilibrium. This condition required that the atmosphere radiate as much heat to the space as it gained partl y through the absorpti on of the Sun's rays, partl y through the radia­ti on from the hotter surface of the Earth , and by means the natural convec ti on of ascending currents o f ai r heated by contact with the ground. The cond ition of therm al equilibrium also required that the Earth lose as much heat by radiati on to space and to the at­mosphere as it gained by absorpti on of the Sun 's rays . With all thi s information A rrhen ius proceeded to es­timate how much a g iven vari at ion in C0 2 content would affect th e Earth's atmosphere. The first step in thi s es timation was to ca lculate th e absorpti on coeffi­cient for any given pl ace and parti cular va lues of its parameters K and W, using the auxili ary tab les he had prepared.

1 Arrhenius prepared then a tabl e th at gave the mean variati on in temperature that would occur i f the amount of C02 changed from its present value (K = I ) to K = 0.67, 1.5, 2, 2.5 , and 3. Inspecti on of hi s table showed that the influence was nearly the same all over the world. The influence was minimum near the equator and increased from there to a flat maximum th at li ed the further from the equator the higher the quantity of C02 in the air. For example, for K = 0.67 the max imum effec t li ed about the 40th parall el, for K = 2 on the 60th parallel, and for higher K values above the 70th parall el. The influence was, in general , greater in the winter than in the summer, except of the

case of the parts that lied between lhe maximum and the pole. On account of the nebulos ity o f the Southern hemisphere, the effect would be less there than in the Northern hemi sphere. An increase in C02 diminished the difference in temperature between clay and night. Use of the same tabl e showed that an Increase in the amount of C02 to 2.5 to 3 times the present va lue would increase the temperature of the A rctic regions by about 8-9°C. On the other hand. a decrease in C02

content to a va lue 0.62 to 0.55 of the present one would lower the temperature 4 to soc and lead to the Ice Age in the Earth surface located b tween the 40th and 50th parall els1

•

Arrhenius's model and tables had the additional advantage that it permitted to consider a wide range of C02- inducecl climate changes and their ef fec ts all over the globe. Hi s model could then be used to ex­plain phenomena that occurred both diachronicalll' (c limate variati ons such as glacial and geni al epochs) and synchronicolly, that is, the glac iations that af­fected simultaneously the Northern Hem isphere and other parts of the world.

A few years later, Arrhenius published a non tech­nical book, Worlds in tl1 e Making 26

, in whi ch he cle­scri bed for the f irst time the hot -house theory of the atmosphere, in the fo llowing words: "That the atmos­phere envelopes limit the heat losses fro m the planets had been suggested about 1800 by Fourier. Hi s ideas were further developed by Pouillet and T yndall . Their theory has been sty led the hot house theory because they thought th at the atmosphere acted after the man­ner o f the g lass panes of hot houses." Based on the results of the model he had deve loped Arrhenius stated th at the Earth's temperatur is about 30°C warmer th an it should be clue to the heat-protec ti on ac tion of the gases contained in the atmosphere. The eli ffcrence was clue to the "heat-protecting action of the gases contained in the atmosphere". He also showed that hi s model indi cated th at i f the atmos­phere had no carbon dioxide, the surface temperature o f the Earth would fall about 2 1 oc. Thi s coo ler at­mosphere wou ld contain less water vapor, resulting in an additi onal temperature decrease of about I ooc.

By 1904 Arrhen ius had already stated that the "s light percentage of carbonic acid in the atmosphere may, by the advances o f industry, be changed to a noti ceable degree in the course of a rew cen turi es", and that the doubling of C0 2 would occur some three thousand years hence. Such an increase he concluded, "w ill allow our descendants, even i f they only be those of a distant future, to li ve under a warmer sky

EDUCATOR 173

and in a less harsh env ironment that we were granted."

There was now an important ques tion that should be answered, namely: ls it probable that such great vari ations in the quantity of C02 had occurred in rela­ti vely short geo logical time? Wh at happens now if a very large mass of C02 (from volcanoes, for example) penetrated the atmosphere? This would mod ify the heat regime of the surface, because the di spersion of surface heat into the interplanetary spaces would be di mini shed, whil e the input of solar heat would not be influenced. Initi all y, the gain and the loss would co m­pensate because of the equilibrium, the ga in would then predomin ate and the surface and atmospheri c temperatures must increase. On the other side, thi s temperature increase woul d result in an increase in heat emission, a very rapid increase according to Stephan's radi ation law. Th us, for an increase in one degree of the ac tual average temperature ( I 5°C) the emi ss ion intensity would increase by about 1.4% and the temperature would increase until the eq uili brium is rees tab li shed2

. From Langley's measurements the influence that an increase in C02 content would have on the absorption power of the atmosphere could be estimated and, consequently, it would be possible to calcul ate the number of degrees that the Earth's tem­perature must increase to co mpensate the emi ssion increase.

Conclusion No so long ago, most sc ienti sts thought that indus­

tri al acti vities were unable to affect average global temperatures. Today , we know th at thi s is not so, new synthetic gases have been aclclecl to the atmosphere that seem to seriously affect the ozone layer, C02 lev­els have risen about 25 % (a rate much fas ter that Ar­rhenius first predicted) and average global tempera­tures have ri sen about O.SOC. Most papers that analyze the influence of increas ing composit ion of radiative gases quote Arrhenius's work because it spite of the limited information that he had available at hi s time, he was able to predict va lues that differ lilli e from thC?se predicted by the most sophisti cated models avail able today .

It is natural that we should try to predict what would take place in the future, if human activiti es will continue to burn fossil fuels and add more C02 (and other radi ative gases) to the atmosphere. The bas is for

the predi ction will certainly require guessing how much energy will mankind require, and its origin . Will it be fossil fuel , nuclear fuel , wi nd , or other "re­newable energy resources"? Although there are no certain ways to answer thi s question, scenari os have been built for va rious poss ibilities. A hi gh level sce­nario assumes that fuel consumption will continue to grow as it has clone since 1973 that is at a rate of 2% per yea r. A low level scenari o assumes a linear de­crease of the rate on increase of fossil fuel so that in fifty years it will return to the present annual con­sumpti on. In the high scenari o the atmospheric con­cen trati on of C02 wi ll reach twice its pre-industri al concentrati on in the firs t half of the 2 1st century; in the low scenario it will occur sometime after the 21 st centur/.

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