Chapter 1 Earth and the Greenhouse Effect

Chapter 1

Earth and theGreenhouse Effect

“. . . if the carbonic acid content of the air [atmospheric CO2] rises to 2 [i.e.doubles] the average value of the temperature change will be. . . +5.7 degrees C”

Svante Arrhenius 1896

Introduction

The physics and chemistry of the Earth’satmosphere largely determines our climate(Lockwood 1979). Although the atmosphereseems like a huge reservoir capable of absorb-ing almost limitless quantities of our industrialemissions, it is really only a thin film. Indeed,if the Earth were shrunk to the size of a grape-fruit, its atmosphere would be thinner than theskin of the grapefruit. Our understanding ofhow the chemistry and physics of the atmo-sphere affect climate developed over manycenturies, but has greatly accelerated duringthe past few decades (Box 1.1).

The Earth’s atmosphere is layered. In thelower atmosphere, from the surface up toabout 11-km altitude (troposphere), temper-ature decreases with increasing altitude. Thislayer is only about 1/1,200 of the diameterof the globe, but its physics and chemistryare crucial to sustaining life on the planet.Because cold dense air on top of warm lessdense air is unstable, the layer is fairly turbu-lent and well mixed. It contains 99% of the

atmospheric mass. From 15 to 50 km, the tem-perature increases with altitude, resulting ina stable upper atmosphere (stratosphere) withalmost 1% of the atmospheric mass. Above50 km are the mesosphere and the thermo-sphere, which have little effect on climate(Figure 1.1).

During the past 100 years we humans, as aresult of burning coal, oil, and gas and clear-ing forests, have greatly changed the chemicalcomposition of this thin atmospheric layer.These changes in chemistry, as described insubsequent chapters, have far-reaching con-sequences for the climate of the Earth, theecosystems that are sustained by our climate,and our own human health and economy.

The Greenhouse Effect

Three primary gases make up 99.9% byvolume of the Earth’s atmosphere – nitrogen(78.09%), oxygen (20.95%), and argon(0.93%). However, it is the rare trace gases,that is, carbon dioxide (CO2), methane(CH4), carbon monoxide (CO), nitrogen

Climate Change: Causes, Effects, and Solutions John T. Hardy 2003 John Wiley & Sons, Ltd ISBNs: 0-470-85018-3 (HB); 0-470-85019-1 (PB)

3

4 CLIMATE CHANGE

Box 1.1 The history of atmospheric science

Our current understanding of the chemistry and physics of the atmosphere has a long andfascinating history (Crutzen and Ramanathan 2000). Some highlights of this history includethe following:

340 B.C. – The Greek philosopher Aristotle publishes Meteorologica; its theoriesremain unchallenged for nearly 2000 years.

1686 – Edmond Halley shows that low latitudes receive more solar radiation thanhigher ones and proposes that this heat gradient drives the majoratmospheric circulation.

1750s – Joseph Black identifies CO2 in the air.1781 – Henry Cavendish measures the percentage composition of nitrogen and

oxygen in air.1859 – John Tyndall suggests that water vapor, CO2, and other radiatively active

ingredients could contribute to keeping the Earth warm.1896 – Svante Arrhenius publishes a climate model demonstrating the sensitivity

of surface temperature to atmospheric CO2 levels.1938 – GS Callendar calculates that 150 billion tons of CO2 was added to the

atmosphere during the past half century, increasing the Earth’stemperature by 0.005 ◦C per year during that period.

1920 – Milutin Milankovitch publishes his theory of ice ages based on variationsin the Earth’s orbit.

1957 – Roger Revelle and Hans Suess, renowned oceanographers, proclaim that“human beings are now carrying out a large-scale geophysicalexperiment,” that is, altering the chemistry of the atmosphere withoutknowing the result.

1959 – Explorer Satellites provide images of cloud cover. Verner Suomi estimatesthe global radiation heat budget.

1967 – Syukuro Manabe and Richard Wetherald develop the one-dimensionalradiative-convective atmospheric model and show that a doubling ofatmospheric CO2 can warm the planet about 3 ◦C.

1970–1974 – Destruction of stratospheric ozone by man-made chlorofluorocarbons isdescribed through the work of several researchers.

1985 – The British Antarctic Survey reports a 40% drop in springtimestratospheric ozone between 1956 and 1985.

1986 – Many countries sign the Montreal Protocol on Substances that Deplete theOzone Layer.

1990s – Researchers discover the cooling effect of atmospheric aerosols and theirimportance in offsetting the greenhouse effect. The global warmingtrend continues and record temperatures are repeatedly set.

EARTH AND THE GREENHOUSE EFFECT 5

10−13

10−8

10−3

10−1

1−100 −50 0

Temperature (°C)

Pre

ssur

e (a

tm)

Alti

tude

(km

)

50 100

1000

500

200

100

50

20

10

5

2

1

0

Thermosphere

Mesopause

Stratopause

Tropopause

Troposphere

Mount Everest

Altocumulus cloud

Cumulus cloud

Stratus cloud

Cirrus cloud

Mesosphere

StratosphereOzoneregion

Fig. 1.1 Vertical temperature and pressure structure of the Earth’s atmosphere (From Graedel TE andCrutzen PJ 1997. Atmosphere, Climate, and Change. Scientific American Library, New York:W.H. Freeman and Co, p. 3. Henry Holt & Co.).

oxides (NOx), chlorofluorocarbons (CFCs),and ozone (O3) that have the greatest effecton our climate. Water vapor, with highlyvariable abundance (0.5–4%), also has astrong influence on climate. These trace gasesare known as greenhouse gases or radiativelyimportant trace species (RITS). They areradiatively important because they influencethe radiation balance or net heat balance ofthe Earth.

Thermonuclear reactions taking place on ournearest star, the Sun, produce huge quantities ofradiation that travel through space at the speedof light. This solar radiation includes energydistributed across a wide band of the elec-tromagnetic spectrum from short-wavelengthX rays to medium-wavelength visible light,

to longer-wavelength infrared. The greatestamount of energy (44%) is in the spectralregion, visible to the human eye from 0.4 (vio-let) to 0.7 µm (red) (Figure 1.2a).

As incoming solar radiation passes throughthe atmosphere, particles and gases absorbenergy. Owing to its physical or chemicalstructure, each particle or gas has specificwavelength regions that transmit energy andother regions that absorb energy. For example,ozone in the stratosphere absorbs short- andmiddle-wavelength ultraviolet radiation. Alarge percentage of incoming solar radiationis in the visible region. Atmospheric watervapor, carbon dioxide, and methane have lowabsorption in this region and allow most ofthe visible light to reach the Earth’s surface.

6 CLIMATE CHANGE

20,000

15,000

10,000

5,000

0.25 0.50 0.75 1.0

Wavelength (µm)

(a)

Rad

iatio

n (c

al, c

m−2

, min

, mm

−1)

1.25 1.50 1.75 2.00

Ultraviolet

Visiblelight

(7%) (44%) (37%)(11%)

Nearinfrared

Farinfrared

andlonger

150

100

50

06.2 7.1 8.3 10.0

Wavelength (µm)

(b)

Em

issi

on

12.5 16.7 25.0

CO2

H2O

CH4

O3

Fig. 1.2 (a) Spectral distribution of incoming solar radiation; 44% is visible light with a maximum at awavelength of 0.49 micrometers (µm) in the blue-green part of the spectrum (From Oliver JE andHidore JJ 2002. Climatology: An Atmospheric Science. Upper Saddle River, NJ: Prentice Hall, p. 410).(b) Spectral distribution of outgoing far-infrared radiation emitted by the Earth. Upper line indicatesenergy distribution emitted by warm Earth in the absence of an atmosphere. The light area is the actualenergy escaping through the top of the atmosphere (as measured by satellite). Water vapor and cloudsabsorb and reduce heat loss over a wide band of wavelengths (light shaded area). CO2 absorbs stronglyat wavelengths from 12 to 18 µm and methane (not shown) at about 3.5 µm (Adapted fromRamanathan V 1988. The greenhouse theory of climate change: a test by an inadvertent globalexperiment. Science 240: 293–299. Reproduced/modified by permission of the American GeophysicalUnion).


After absorption by the Earth’s surface,visible energy is transformed and radiatedback in the far-infrared (heat) region ofthe spectrum at wavelengths greater than1.5 µm (Figure 1.2b). The transparency of theatmosphere to outgoing far-infrared radiation(heat) determines how much heat can escapefrom the Earth back into space and how muchis trapped. The important feature of green-house gases is that they absorb (are opaqueto) certain infrared wavelengths. Water vapor,carbon dioxide, and methane, the same gasesthat transmit visible wavelengths, absorb

strongly in the far infrared (Figure 1.2b).Thus, they trap heat in the troposphere andstop it from escaping to space. Window glassused to trap heat in a greenhouse has similarabsorption and transmission properties; hence,the term “greenhouse gases.”

Of the total amount of solar energy reachingthe Earth’s atmosphere (342 W m−2), an aver-age 31% is reflected back to space by theupper surface of clouds, the particles in theatmosphere (dust and aerosols), or the surfaceof the Earth (Figure 1.3). While the overallaverage reflectivity (also called albedo) of the

Reflected solarradiation

107 W m−2 Incoming solarradiation

342 W m−2

Outgoing longwave radiation235 W m−2

Absorbed byatmosphere

67

Emitted byatmosphere

165 Atmosphericwindow

40

Reflected byclouds, aerosal,and atmosphere

77

78Latentheat

30

350

235

Greenhousegases

Backradiation

324Reflectedby surface

30

Absorbedby surface

168

Thermals24

Evapo-transpiration

78

Surfaceradiation

390

Fig. 1.3 The Earth’s radiation and heat balance. Greenhouse gases are transparent to visible andnear-infrared wavelengths of sunlight, but they absorb and reradiate downward a large fraction of thelonger far-infrared wavelengths (heat). The net incoming solar radiation of 342 W m−2 is partiallyreflected by clouds, the atmosphere, and the Earth’s surface, but 49% is absorbed by the surface. Someof that heat is returned to the atmosphere as sensible heat – most as evapotranspiration that is releasedas latent heat in precipitation. The rest is radiated as thermal infrared radiation. Most of that is absorbedby the atmosphere, which in turn, emits radiation both up and down, producing a greenhouse effect(From Kiehl JT and Trenberth KE 1997. Earth’s annual global mean energy budget. Bulletin of theAmerican Meteorological Society 78(2): 197–208. Reproduced by permission of the AmericanMeteorological Society).

8 CLIMATE CHANGE

Earth is 31%, albedo differs greatly betweensurfaces. Clouds, with an albedo of 40 to 90%,are by far the most important reflectors ofincoming solar radiation. The albedo of freshsnow is 75 to 90%, forests 5 to 15%, andthat of water, which depends on the angleof inclination of the Sun, ranges from 2 to>99%. Incoming energy that is not reflected(the remaining 69%) is absorbed by the tro-posphere and the Earth’s surface (Figure 1.3).Evaporation of water requires a considerableamount of energy. This energy is essentiallystored as “latent heat” in water vapor andreleased back into the air as heat when watervapor condenses (Figure 1.3).

Of the total far-infrared (heat) energy rera-diated from the Earth’s surface, 83% is back-radiated and does not directly escape theatmosphere. This back radiation is about dou-ble the amount of energy absorbed by thesurface directly from the Sun. This additionalatmospheric energy warms the Earth to itspresent temperature. Without the greenhousewarming effect of the atmosphere, the Earth’saverage surface temperature would be about−20 ◦C (−4 ◦F) instead of 15 ◦C (59 ◦F). Sim-ply put, greenhouse gases trap solar heat inthe lower atmosphere and keep the Earthwarm. As long as the amount of incomingsolar energy and the amount of greenhousegas in the atmosphere remain fairly constant,the Earth’s temperature remains in balance(Figure 1.3). However, the greater the con-centration of greenhouse gases, the greater theamount of long-wave (heat) radiation trappedin the lower atmosphere.

Large-Scale Heat RedistributionThe Earth’s temperature is not uniformbut differs greatly – geographically (horizon-tally), by elevation (vertically), and over time(seasons and decades). To understand climateand how it might change over decades to cen-turies in the future, we need to understand

Atmosphere

Earth'ssurface Sun

Arctic Circle

Tropic ofCancer

Tropic ofCapricornAntarcticCircle

(a)

(b)

(c)

Fig. 1.4 (a) The low-latitude tropics are warmbecause they receive greater amounts of heat perunit surface area than the high-latitude temperateregions. The amount of heat received at higherlatitudes is less than that at lower latitudes forthree reasons: (1) because of the greater angle ofincidence, a unit of solar energy striking asurface is spread over a greater area, (2) becauseit is absorbed through a greater thickness ofatmosphere before reaching the surface, and(3) because more of the energy is reflected owingto the low angle of incidence. (b) In winter in theNorthern Hemisphere, areas north of the ArcticCircle receive no direct solar radiation, whileareas south of the Antarctic Circle receivecontinuous radiation for months. (c) The reverseof the above is true during the NorthernHemisphere summer. Throughout the year, theSun is directly over some latitude between thetropics (From Thurman HV 1991. IntroductoryOceanography (6th Edition). New York:Macmillan Publishing Co., p. 169).

how heat is distributed over the Earth. Thetropics are warmer than temperate or polarregions because incoming solar radiationstrikes the Earth’s surface at a greater angle ofincidence, resulting in more radiation per unitsurface area (Figure 1.4). The same energy


Polar high

60Polarfront

30

0

Equatorial low

SE Trade Winds

High

NE Trade Winds

Westerlies

Polar easterlies

Fig. 1.5 Major atmospheric circulation patterns. The great quantity of heat received in the tropicsmoves poleward driving large-scale atmospheric currents (From Thurman HV 1991. IntroductoryOceanography (6th Edition). New York: Macmillan Publishing Co., p. 170).

striking higher-latitude areas is distributedover a greater surface area. Thus, the radiationbalance of the Earth is unevenly distributedand the excess heat from the tropics will(according to the laws of thermodynamics)tend to redistribute itself poleward to coolerareas. This occurs as direct heat transfer by thepoleward movement of warm air and watermasses. Also, significant heat energy, used

in evaporation of surface water in the trop-ics, is carried as latent heat and released athigher latitudes when water vapor cools andcondenses into rainfall.

The initial uneven distribution of heat anddensity in the atmosphere and ocean resultsin important large-scale circulation patterns(Figure 1.5). Warm air near the equator rises,and cools as it flows poleward at high altitude.

Box 1.2 The Coriolis force

Because of the Earth’s rotation, moving objects (including air and water masses) are divertedto the right or left in the Northern and Southern Hemispheres, respectively. This apparentforce is named after the French mathematician Gustave Gaspard Coriolis (1792–1843), whofirst described it. Imagine watching from outer space as an object moves from the NorthPole toward the equator. The object appears to move straight toward the south. Now, asobservers on the Earth’s surface, we watch the same object. However, since we rotate along

10 CLIMATE CHANGE

with the Earth (in an easterly direction), under the path of the object, it appears to usto veer in a curve toward the right (in a westerly direction), with respect to its directionof movement.

The Coriolis force manifests itself in a number of ways, from riverbanks that erode deeperon one side than the other to winds that rotate counterclockwise around low-pressure areasin the Northern Hemisphere and clockwise in the Southern Hemisphere. The large-scalecirculation of the atmosphere and ocean is strongly affected by the Coriolis force. Usefulgraphical animations of the Coriolis force have been presented by a number of authors (e.g.http://www.windpower.dk or http://satftp.soest.hawaii.edu/ocn620/coriolis/ ).

At about 30◦ latitude, it sinks and flows south-ward again at lower altitudes. To replacethe sinking air mass, air is drawn frompolar latitudes, flows south at high altitudes,sinks, and then flows poleward near the sur-face. These patterns are modified by the

Coriolis force, which pushes the circulationclockwise in the Northern Hemisphere andcounterclockwise in the Southern Hemisphere(Box 1.2). This results in major surface windpatterns at lower altitudes including, forexample, in the Northern Hemisphere, the

Heat transferfrom sea to air

AtlanticOcean

IndianOcean

PacificOcean

Cold, salty, deep current Cold, salty, deep current

Warm, shallow current

Warm, shallow current

Warm

, shallowcurrent

Fig. 1.6 The oceanic conveyor belt transports huge quantities of stored heat. The Atlantic Ocean losesmore water by evaporation than it receives from runoff. The resulting saline dense water sinks in theNorth Atlantic and flows southward near the bottom. Carrying 20 times more water than all the world’srivers combined, it flows near the bottom, back to the Pacific, where it rises to the surface (upwells).The upwelled water then flows along the surface as a warm shallow current back to the Atlantic (FromHileman B 1989. Global warming. Chemical and Engineering News 19: 25–40).


temperate westerly and northeast trade winds(Figure 1.5).

In the ocean, wind patterns, along withdensity (salinity) differences drive the majorcirculation currents such as the Gulf Streamin the North Atlantic and the Koroshio Cur-rent in the North Pacific. Huge quantitiesof heat are transported with the surfacecurrents from south to north in the West-ern Atlantic and Pacific Oceans by whathas been termed the oceanic conveyor belt(Figure 1.6). Thus, major ocean and atmo-sphere circulation patterns are closely tied tothe Earth’s heat balance and any disruptionof these patterns could cause rapid changes inglobal climate.

Greenhouse Gases

Historically, greenhouse gas concentrations inthe Earth’s atmosphere have undergone naturalchanges over time and those changes havebeen closely followed by changes in climate.Warmer periods were associated with higheratmospheric greenhouse gas concentrationsand cooler periods with lower greenhouse gasconcentrations. However, those changes werepart of natural cycles and occurred over periodsof tens of thousands to millions of years(Chapter 2). Recent human-induced changesin atmospheric chemistry have occurred overdecades (Ramanathan 1988). When referring tothe postindustrial era, scientists generally usethe term climate change in the way definedby The UN Framework Convention on ClimateChange. Thus, “climate change” is a change ofclimate that is attributed directly or indirectlyto human activity that alters the composition ofthe global atmosphere and which is, in additionto natural climate variability, observed overcomparable time periods.

Human activities generate several differentgreenhouse gases that contribute to climaticchange. To determine the individual and

cumulative effects of these gases on theEarth’s climate, we need to examine theirtotal quantity, their natural and human sourcesto the atmosphere, their rates of loss tonatural sinks, their past and projected rates ofincrease, and their individual and cumulativeheating capacities (Table 1.1).

Water vapor traps heat in the atmosphereand makes the greatest contribution to thegreenhouse effect. Its level in the atmosphereis not directly the result of human activities.However, because warmer air can hold morewater vapor, an increase in the Earth’stemperature resulting from other greenhousegases produces a “positive feedback,” that is,more warming means more water vapor inthe atmosphere, which in turn contributes tofurther warming (Chapter 3).

Carbon dioxide is a natural componentof the atmosphere and is very biologicallyreactive. It can be reduced to organic car-bon biomass through photosynthetic uptakein plants and, through biological oxidation(respiration), converted back to gaseous CO2

and returned to the atmosphere. Major naturalsources to the atmosphere are animal respi-ration, microbial breakdown of dead organicmatter and soil carbon, and ocean to atmo-sphere exchange (flux). Sinks include photo-synthetic uptake by plants and atmosphere toocean flux. These natural cycles maintainedthe atmospheric concentration of CO2 at about280 ± 10 ppmv (parts per million by volume)for several thousand years prior to industrial-ization in the mid-nineteenth century.

During the past 150 years, and especiallyduring the last few decades, humans greatlyincreased the concentration of atmosphericCO2. Huge reservoirs of carbon, stored formillions of years as fossilized organic car-bon (coal, oil, and gas) in the Earth’s crust,have been removed and burned for fuel.When carbon fuels burn, they combine with

12 CLIMATE CHANGE

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atmospheric oxygen to produce carbon diox-ide, which enters the atmosphere (Figure 1.7).Globally, more than 80% of human CO2 emis-sions come from transportation and industrialsources. The remaining 20% comes primarilyfrom deforestation and biomass burning. Aforest stores about 100 tons of carbon peracre and about half the world’s forest wasdestroyed in the last half of the twentiethcentury. Carbonate minerals used in cementproduction also release CO2 to the atmo-sphere. These sources altogether contribute6.5 billion tons or gigatons of carbon (GtC)to the atmosphere each year (Figure 1.7).The largest emitters of CO2 are the UnitedStates, China, and the Russian Federation(Figure 1.8). Furthermore, the rate of addition

to the atmosphere from these sources exceedsthe rate of loss to major CO2 sinks by about3.3 GtC per year (Box 1.3, Figure 1.9). Thus,the atmospheric concentration of CO2 contin-ues to increase.

The GWP of atmospheric carbon diox-ide is not a new concept, but dates backto the nineteenth century. In 1896, theSwedish chemist Svante Arrhenius estimatedthat fossil-fuel burning would result in a dou-bling of the atmospheric CO2 concentration3,000 years in the future (Arrhenius 1896). Hecorrectly described the process, but severelyunderestimated the time frame in whichhumans could double the atmospheric CO2

concentration. By 1938 some scientists con-cluded that human combustion of fossil fuel

7

6

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1850 1900 1950 2000

Gig

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n (G

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Per

cap

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O2

emis

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s (m

tonn

es c

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n)

Total CO2 emissions from fossil fuels

Solid fuelLiquid fuelGasCement productionGas flaringPer capita CO2 emissions

Fig. 1.7 Total global CO2 emissions from fossil-fuel combustion and cement production and averageglobal per capita emissions (Adapted from Marland G, Boden TA and Andres RJ 2002. Global, regional,and national fossil fuel CO2 emissions. In Trends: A Compendium of Data on Global Change. CarbonDioxide Information Analysis Center, Oak Ridge National Laboratory, US Department of Energy, OakRidge, Tenn., USA).

14 CLIMATE CHANGE

1.6

1.4

1.2

1

0.8

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0.4

0.2

0

CO

2 em

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ons

1996

(G

tC)

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Rus

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Japa

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Ger

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Can

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Italy

Rep

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of K

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Fig. 1.8 The top 10 CO2-emitting countries account for 66% of the total global CO2 emissions. TheUS is number one accounting for 23% of the global total (Data from Marland G, Boden TA and AndresRJ 2002. Global, regional, and national fossil fuel CO2 emissions. In Trends: A Compendium of Data onGlobal Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, USDepartment of Energy, Oak Ridge, Tenn., USA).

Global net primary production and respiration60

61.3

1.6

92 90

0.5Changing land use

Atmosphere750

Fossil fuels andcement production

Net addition to atmosphere = +3.3 Gtc year−1

Vegetation 610Soils and detritus 1,580

2,190

−1.3

−2

+5.5+1.1

Marine biota 3

Dissolved organiccarbon <700

Surface sediment 150

Intermediate and deep

ocean 38,100

Surface ocean 1,020

0.2

6

40 91.6

100

4

50

Fig. 1.9 The global carbon cycle showing reservoirs (boxes) in gigatons of carbon (GtC) and fluxes(arrows) in GtC year−1. Anthropogenic emissions are adding about 6.6 GtC year−1 to the atmosphericreservoir of 750 GtC. Because of natural sinks, the net contribution to the atmosphere is about 3.3 GtCyear−2 (From Schimel D, Alves D, Enting I, Heimann M, Joos F, et al. 1996. Radiative forcing ofclimate change. In: Houghton JT, Filho LGM, Callander BA, Harris N, Kattenberg A, Maskell K, edsClimate Change 1995: The Science of Climate Change. Intergovernmental Panel on Climate Change.Cambridge: Cambridge University Press, p. 77. Reproduced by permission of the IntergovernmentalPanel on Climate Change).


Box 1.3 The global carbon cycle

The Earth’s carbon exists in a number of reservoirs. The largest is the intermediate anddeep water of the ocean that contains about 38,100 gigatons of carbon (GtC) (Figure 1.9).Terrestrial vegetation and soils represent the second largest reservoir, totaling 2,190 GtC.The atmosphere contains about 750 GtC. Carbon is the chemical building block of life.Through the process of photosynthesis, terrestrial plants remove carbon dioxide (CO2) gasfrom the atmosphere and marine phytoplankton take up dissolved carbon from seawater.Both reduce it to organic carbon, a temporary living carbon reservoir. Animals, and mostmicrobes, derive their energy from the breakdown of organic carbon and, through respiration,release CO2 back to the atmosphere. Because the solubility of CO2 in water is inverselyproportional to temperature, the cold high-latitude regions of the ocean absorb atmosphericCO2, while warm tropical waters release it back to the atmosphere. This movement of carbonbetween atmosphere, ocean, and land constitutes the natural carbon cycle. Undisturbed, thiscycle has maintained atmospheric CO2 levels at a relatively stable level for millennia.

However, there is another large carbon reservoir – one being exploited by humans. Denseforests and swamps in past eras left their stored carbon as deposits preserved in the Earth’scrust. The industrial era, beginning in the nineteenth century, was and continues to be drivenby the burning of huge quantities of this fossil carbon (coal, oil, and gas). As the carbonis burned (oxidized), it is converted to CO2, water, and energy. Thus, carbon that remainedundisturbed for millions of years in the Earth’s crust is being injected into the atmosphereas CO2 in what represents an instant in geologic time. In addition, carbon stored as biomassin trees and vegetation is being transferred to the atmosphere by biomass burning and landclearing. The land and the ocean store about half of the carbon emitted annually by fossil-fuel combustion and industrial activity; the other half is accumulating in the atmosphere.The net result is that human activities are increasing the concentration of heat-trapping CO2

in the atmosphere by 3.3 GtC each year (Figure 1.9).Although the quantities of carbon described here are generally accurate, some uncertainty

remains and research continues in an effort to refine carbon cycle models. For example,some research suggests that more carbon is taken up from the atmosphere than can beaccounted for, implicating a “missing sink.” Satellites and other techniques are being usedto map the global distribution of terrestrial and oceanic plant productivity (USGCRP 2002).Studies using NASA’s Sea-Viewing Wide Field-of-view Sensor (SeaWiFS) indicate thatglobal net photosynthetic carbon uptake may be slightly greater than previously thought(111 to 117 GtC per year) with about half being accounted for by marine phytoplankton(Behrenfeld et al. 2001).

was already leading to a significant increasein atmospheric CO2 and global average tem-perature (Callendar 1938).

By the end of the twentieth century, theatmospheric concentration of CO2 had risento over 367 ppmv – an increase of 31% above

its preindustrial level – and it continues toincrease exponentially at about 0.5% per year(Figure 1.10). The present atmospheric CO2

concentration has not been exceeded duringthe past 420,000 or perhaps even 20 millionyears (Houghton et al. 2001). Even if current

16 CLIMATE CHANGE

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Global atmospheric concentrationsof three greenhouse gases

Sulfate aerosols deposited in Greenland ice

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Fig. 1.10 (a) During the past 1,000 years or more, the atmospheric concentrations of greenhouse gasesremained relatively constant. However, since the beginning of the industrial era in the nineteenthcentury, human activities have led to an exponential increase in greenhouse gases. (b) Anthropogenicsulfate concentrations in Greenland ice cores. Sulfates cause negative radiative forcing (cooling). Forboth (a) and (b), earlier data are based on polar ice core and other paleoclimatological evidence(Chapter 2). Later, mostly twentieth-century, values are from actual chemical analysis (From IPCC2000. Summary for Policymakers: The Science of Climate Change. Intergovernmental Panel on ClimateChange. IPCC Working Group I, 26 Feb. Available from: http://www.ipcc.ch/pub/ Reproduced bypermission of Intergovernmental Panel on Climate Change).


CO2 emissions are reduced and maintainedat or near 1994 rates, the atmospheric con-centration will reach 500 ppmv (nearly dou-ble the preindustrial level) by the end ofthis century – far sooner than Arrhenius couldhave imagined. The major long-term reservoir(sink) for CO2 is the deep ocean. Carbon diox-ide produced today will take more than 100years to be absorbed by this reservoir. Thus,even if all emissions ceased today, atmo-spheric CO2 would remain above its prein-dustrial level for the next 100 to 300 years.

Fossil fuels are nonrenewable and theirsupply is finite. However, current supplies areabundant, relatively inexpensive, and couldlast for another 40 to 200 years (Table 1.2).If we continue to burn the carbon remainingin tropical rainforests, oil, gas, and coalreserves, we could more than quadruple theconcentration of atmospheric CO2 in the nextfew centuries.

Methane gas (CH4) is produced by themicrobial breakdown of organic matter in theabsence of oxygen. Natural wetland soils,swamps, and some coastal sediments releasesignificant quantities of CH4 to the atmo-sphere. In the atmosphere it can combinewith hydroxyl radicals (OH−) to form carbonmonoxide (CO). Its atmospheric concentra-tion has increased by 150% since 1750 andis increasing rapidly by about 1.1% per year(Figure 1.10). About half the current methane

Livestock30%

Solid waste16%

Oil & gasproduction

16%

Wet riceagriculture

25%

Coal mining13%

Fig. 1.11 Anthropogenic sources of methane.Total = 270 million tones per year (Based on1996–1997 data) (WRI 2002).

emissions are from anthropogenic (human-produced) sources. These sources includelivestock production (incomplete digestion offood), wetland rice cultivation, solid wastelandfills, and coal, oil, and gas production(Figure 1.11). However, global emission ratesappear variable and are difficult to quantifyexactly (Houghton et al. 2001).

Nitrous oxide (N2O) originates from themicrobial breakdown of agricultural fertilizers,fossil-fuel combustion, and biomass burning.Coal combustion is a major contributor ofN2O to the atmosphere. N2O has a long atmo-spheric lifetime (170 years). Its atmospheric

Table 1.2 World reserves of fossil fuel (Data from Dresselhaus MS and Thomas IL 1991. Alternativeenergy technologies. Nature 414: 332–337).

Source World reserves World consumptionrate

Approximate lifetime(years) at currentconsumption ratea

Oil 1.6 × 1014 L 1.2 × 1010 L day−1 37Natural gas 1.4 × 1014 m3 2.4 × 1012 m3 year−1 58Coal 9.1 × 1011 tonnes 4.5 × 109 tonnes year−1 202

aAssuming no new discoveries.

18 CLIMATE CHANGE

concentration has increased since the preindus-trial era by 16%, and it continues to increaseby about 0.25% per year. It makes a signifi-cant contribution to the overall global warming(Figure 1.10).

Chlorofluorocarbons (CFCs) and hydro-chlorofluorocarbons (HCFCs) are a rela-tively inert class of manufactured industrialcompounds containing carbon, fluorine, andchlorine atoms. They are used as coolantsin refrigerators and air conditioners, and infoam insulation, aerosol sprays, and industrialcleaning solvents. These compounds escape tothe atmosphere where they destroy the strato-spheric ozone layer that shields the Earth from

harmful ultraviolet radiation. Their role inozone depletion led to the first comprehen-sive international environmental treaty – theMontreal Protocol – to phase out the useof chlorofluorocarbons. However, CFCs andHCFCs are also greenhouse gases. The atmo-spheric concentration of CFCs has increasedrapidly since the 1960s. Although they areinvolved in the destruction of the strato-spheric ozone layer (Box 1.4), which leadsto some cooling, they still make an overallpositive contribution to greenhouse warming.The Montreal Protocol now restricts their use.However, because of their long lifetimes inthe atmosphere (60 to >100 years), they must

Box 1.4 Stratospheric ozone

The stratospheric ozone layer extends upward from about 10 to 30 miles and protects life onEarth from the Sun’s harmful ultraviolet-b radiation (UV-b, 280- to 320-nm wavelength).Ozone occurs naturally in the stratosphere and is produced and destroyed at a constantrate. But, man-made chemicals, CFCs, and halons (used in coolants, foaming agents, fireextinguishers, and solvents) are gradually destroying this “good” ozone. These ozone-depleting substances degrade slowly and can remain intact for many years as they movethrough the troposphere until they reach the stratosphere. There they are broken downby the intensity of the Sun’s ultraviolet rays and release chlorine and bromine molecules,which destroy “good” ozone. One chlorine or bromine molecule can destroy 100,000 ozonemolecules, causing ozone to disappear much faster than nature can replace it. It can takeyears for ozone-depleting chemicals to reach the stratosphere, and even though we havereduced the use of many CFCs, their impact from years past is just starting to affect theozone layer. Substances released into the air today will contribute to ozone destruction wellinto the future. Satellite observations indicate a worldwide thinning of the protective ozonelayer. The most noticeable losses occur over the North and South Poles because ozonedepletion accelerates in extremely cold weather conditions. As the stratospheric ozone layeris depleted, higher UV-b levels reach the Earth’s surface. Increased UV-b can lead to morecases of skin cancer, cataracts, and impaired immune systems. Damage to UV-b-sensitivecrops, such as soybeans, reduces yield. High-altitude ozone depletion is suspected to causedecreases in phytoplankton, a plant that grows in the ocean. Phytoplankton are an importantlink in the marine food chain and, therefore, food populations could decline. Because plants“breathe in” carbon dioxide and “breathe out” oxygen, carbon dioxide levels in the air couldalso increase. Increased UV-b radiation can be instrumental in forming more ground-level(tropospheric) or “bad” ozone pollution (Box 10.1).


be considered as significant greenhouse gases.Also, hydrofluorocarbons (HFCs), a CFC sub-stitute, and related chemicals [perfluorocarbon(PFC) and sulfur hexafluoride (SF6)] cur-rently contribute little to warming, but theirincreasing use could contribute several per-cent to warming during the twenty-first cen-tury (IPCC 2000).

Tropospheric ozone (O3) – motor vehicleemissions are the major source of this green-house gas. On clear warm days with a stableatmosphere, vehicle combustion hydrocarbonsand nitrogen oxides undergo a photochemicalreaction to produce a hazy air pollution con-dition (smog) with high concentrations ofO3 (Box 10.1). The atmospheric concentra-tion increased an estimated 20 to 50% dur-ing the twentieth century and continues toincrease at about 1% per year (Beardsley1992). In the atmosphere, chemical reactionwith hydroxyl radicals OH− results in lossof O3; however, as a result of other reac-tions, increasing atmospheric CO2 will prob-ably decrease this removal process. Globally,the degree of warming due to O3 is notwell known, but believed to be on theorder of 15% of the total warming. Tropo-spheric ozone (bad ozone) is not to be con-fused with the natural stratospheric ozonelayer (good ozone) that protects the Earthfrom excess damaging ultraviolet radiation(Box 1.4).

Aerosols are small microscopic particlesresulting from fossil-fuel and biomass com-bustion, and ore smelting. They are formedlargely from sulfur, a constituent of somefuels, particularly some high-sulfur coal andoil. Sulfate aerosols increase the acidity ofthe atmosphere and form acid rain. Theyalso reflect solar energy over a broadband,including the infrared, and thus have anegative radiative forcing or cooling effecton the atmosphere. Sulfate aerosols, unlike

the greenhouse gases discussed above, havea short lifetime in the atmosphere (daysto weeks). Globally, sulfate aerosols maybe responsible for counteracting 20 to 30%of human-induced greenhouse warming. Insome regions of the industrialized North-ern Hemisphere, the sulfate-induced coolingappears to be great enough to completely off-set the current warming effect of greenhousegases. Natural sources of aerosols such as vol-canic eruptions can also inject particles intothe atmosphere, resulting in temporary global-scale cooling events, lasting months to sev-eral years.

Other greenhouse gases in total accountfor approximately 9% the total net warming.These include carbon monoxide (CO) andnitrogen oxides (NOx) – both largely fromfossil-fuel and biomass combustion.

Black carbon (soot), from the incompletecombustion of fossil fuel, may contributesubstantially to greenhouse warming, at leaston a regional scale (Chameides and Bergin2002). It is not a gas, but the black particlesmaking up soot absorb solar radiation. Itslifetime in the atmosphere is short comparedto most greenhouse gases and its warmingpotential depends on the source and itsfate on the atmosphere. Recent studies areassessing the contribution of black carbon toglobal warming.

Warming Potentials

The postindustrial increases in greenhousegases have resulted in an increase in globalradiative forcing (warming) of 2.45 watts persquare meter (W m−2). This is only about1% of the net incoming solar radiation, butit amounts to the energy content of about1.8 billion tonnes of oil every minute ormore than 100 times the world’s current rateof commercial energy consumption (UNFCC2002). Each greenhouse gas contributes to

20 CLIMATE CHANGE

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Fig. 1.12 Relative contribution of anthropogenic increases in atmospheric greenhouse gasconcentrations to global radiative forcing (warming) (Data from IPCC 2000. Summary for Policymakers:The Science of Climate Change. Intergovernmental Panel on Climate Change. IPCC Working Group I,26 Feb. Available from: http://www.ipcc.ch/pub/).

this warming. Equal quantities of differentgreenhouse gases have widely different warm-ing potentials (Table 1.1). Also, the lifetimeof the gas in the atmosphere affects its resul-tant concentration and warming potential. Forexample, carbon dioxide, nitrous oxide, andCFCs have average lifetimes of 100 years ormore, whereas methane has a lifetime of 5 to10 years and carbon monoxide only 5 months.Each greenhouse gas has a characteristicper molecule greenhouse effect or warmingpotential. For example, one molecule of CFC-11 or CFC-12 traps 6 to 7 thousand timesmore heat than one molecule of CO2 andone molecule of methane traps 21 times moreheat than one molecule of CO2. However,because CO2 is much more abundant, about60% of the current human-induced green-house warming results from CO2, 15 to 20%from methane, and the remaining 20% or sofrom nitrous oxide, chlorofluorocarbons, andtropospheric ozone (Figure 1.12).

Summary

A thin layer of mixed gases surrounds theEarth. The greenhouse gases (CO2, CH4, N2O,CFCs, and O3), although less than 0.1% ofthe atmospheric volume, have a profoundinfluence on the Earth’s climate. These gases,most importantly carbon dioxide and methane,allow sunlight to penetrate, but trap outgo-ing heat. A large quantity of heat, received inthe tropics, is redistributed to higher latitudesby major atmospheric and oceanic currents.During the past 150 years, human activitieshave led to an exponential growth in green-house gas emissions. These activities includeextracting and burning fossilized carbon (coal,oil, and gas) for fuel, forest clearing and burn-ing, wetland rice cultivation, livestock rearing,solid waste landfilling, and nitrogen fertil-ization of agriculture. The result has been amajor increase in the concentrations of green-house gases, with a consequent increase in


the warming potential (heat-trapping ability)of the atmosphere.

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Documents

Chapter 1 Earth and the Greenhouse Effect