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W W W . S K E P T I C . C O M

ATMOSPHERIC CARBON DIOXIDEconcentrations are higher today than at any timein at least the past 650,000 years. They are about35% higher than before the industrial revolution,and this increase is caused by human activities,primarily the burning of fossil fuels.

Carbon dioxide is a greenhouse gas, as aremethane, nitrous oxide, water vapor, and a hostof other trace gases. They occur naturally in theatmosphere. Greenhouse gases act like a blanketfor infrared radiation, retaining radiative energynear the surface that would otherwise escapedirectly to space. An increase in atmosphericconcentrations of carbon dioxide and of othergreenhouse gases augments the natural green-house effect; it increases the radiative energyavailable to Earth’s surface and to the loweratmosphere. Unless compensated for by otherprocesses, the increase in radiative energy avail-able to the surface and the lower atmosphereleads to warming. This we know. How do weknow it?

How do we know

carbon dioxide concentrations have increased?

The concentrations of carbon dioxide and othergreenhouse gases in atmospheric samples havebeen measured continuously since the late 1950s.Since then, carbon dioxide concentrations haveincreased steadily from about 315 parts per mil-lion (ppm, or molecules of carbon dioxide permillion molecules of dry air) in the late 1950s toabout 385 ppm now, with small spatial variationsaway from major sources of emissions. For themore distant past, we can measure atmosphericconcentrations of greenhouse gases in bubbles of

ancient air preserved in ice (e.g., in Greenlandand Antarctica). Ice core records currently goback 650,000 years; over this period we knowthat carbon dioxide concentrations have neverbeen higher than they are now. Before theindustrial revolution, they were about 280 ppm,and they have varied naturally between about180 ppm during ice ages and 300 ppm duringwarm periods (Fig. 1). Concentrations ofmethane and nitrous oxide have likewiseincreased since the industrial revolution (Fig. 2)and, for methane, are higher now than they havebeen in the 650,000 years before the industrialrevolution.

How do we know the increase in carbon dioxide

concentrations is caused by human activities?

There are several lines of evidence. We knowapproximately how much carbon dioxide isemitted as a result of human activities. Addingup the human sources of carbon dioxide—primarily from fossil fuel burning, cement pro-duction, and land use changes (e.g., deforesta-tion)—one finds that only about half the carbondioxide emitted as a result of human activitieshas led to an increase in atmospheric concentra-tions. The other half of the emitted carbon diox-ide has been taken up by oceans and the bios-phere—where and how exactly is not complete-ly understood: there is a “missing carbon sink.”

Human activities thus can account for theincrease in carbon dioxide concentrations.Changes in the isotopic composition of carbondioxide show that the carbon in the added car-bon dioxide derives largely from plant materi-als, that is, from processes such as burning of

How We Know Global Warming is RealThe Science Behind Human-induced Climate Change

T A P I O S C H N E I D E R

biomass or fossil fuels, which are derived fromfossil plant materials. Minute changes in theatmospheric concentration of oxygen showthat the added carbon dioxide derives fromburning of the plant materials. And concentra-tions of carbon dioxide in the ocean haveincreased along with the atmospheric concen-trations, showing that the increase in atmos-pheric carbon dioxide concentrations cannotbe a result of release from the oceans. All linesof evidence taken together make it unambigu-ous that the increase in atmospheric carbondioxide concentrations is human induced andis primarily a result of fossil fuel burning.(Similar reasoning can be evoked for othergreenhouse gases, but for some of those, suchas methane and nitrous oxide, their sourcesare not as clear as those of carbon dioxide.)

How can such a minute amount of carbon dioxide

affect Earth’s radiative energy balance?

Concentrations of carbon dioxide are meas-ured in parts per million, those of methaneand nitrous oxide in parts per billion.These are trace constituents of the atmos-phere. Together with water vapor, theyaccount for less than 1% of the volume ofthe atmosphere. And yet they are cruciallyimportant for Earth’s climate.

Earth’s surface is heated by absorption ofsolar (shortwave) radiation; it emits infrared(longwave) radiation, which would escapealmost directly to space if it were not for

water vapor and the other greenhouse gases.Nitrogen and oxygen, which account forabout 99% of the volume of the atmosphere,are essentially transparent to infrared radia-tion. But greenhouse gases absorb infraredradiation and re-emit it in all directions.Some of the infrared radiation that wouldotherwise directly escape to space is emittedback toward the surface. Without this naturalgreenhouse effect, primarily owing to watervapor and carbon dioxide, Earth’s mean sur-face temperature would be a freezing -1°F,instead of the habitable 59°F we currentlyenjoy. Despite their small amounts, then, thegreenhouse gases strongly affect Earth’s tem-perature. Increasing their concentration aug-ments the natural greenhouse effect.

How do increases

in greenhouse gas concentrations

lead to surface temperature increases?

Increasing the concentration of greenhouse gasesincreases the atmosphere’s “optical thickness” forinfrared radiation, which means that more of theradiation that eventually does escape to spacecomes from higher levels in the atmosphere. Themean temperature at the level from which theinfrared radiation effectively escapes to space (theemission level) is determined by the total amountof solar radiation absorbed by Earth. The sameamount of energy Earth receives as solar radia-tion, in a steady state, must be returned asinfrared radiation; the energy of radiation

32

V O L U M E 1 4 N U M B E R 1 2 0 0 8

YEARS AGO

400,000 300,000 200,000 100,000 0

Carb

on d

ioxi

de [ppm

v]

350

300

250

200

Today

1850

FIgure 1:

Carbon dioxide

concentrations in

Antarctica over

400,000 years.

“The graph combines

ice core data with recent

samples of Antarctic air.

The 100,000-year ice age

cycle is clearly recognizable.”

(Data sources: Petit et al.

1999; Keeling and Whorf

2004; GLOBALVIEW-CO2

2007.)

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W W W . S K E P T I C . C O M

depends on the temperature at which it is emittedand thus determines the mean temperature at theemission level. For Earth, this temperature is-1°F—the mean temperature of the surface if theatmosphere would not absorb infrared radiation.

Now, increasing greenhouse gas concentrationsimplies raising the emission level at which, in themean, this temperature is attained. If the tempera-ture decreases between the surface and this leveland its rate of decrease with height does not

400

350

300

250

2000

1800

1600

1400

1200

1000

800

600

0 500 1000 1500 2000

C0

2 (

ppm

). N

2)

(ppb)

CH

. (ppb)

Carbon Dioxide (CO2)

Methane (CH4)

Nitrous Oxide (N20)

Year

Concentrations of Greenhouse Gases from 0 to 2000 C.E.

0.4

0.2

0

-0.2

-0.4

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-0.8

1840 1860 1880 1900 1920 1940 1960 1980 2000

Year

Degre

es C

Köppen 1881

Callendar 1938

Willett 1950

Callendar 1961

Mitchell 1963

Budyko 1969

Jones et al. 1986

Hansen and Lebedeff 1987

Brohan et al. 2006

Figure 3—How We Know the Globe is Warming

Evidence from multiple sources corroborate the fact

that the Earth is warming. The IPCC report caption for

this graph reads: “Published records of surface tem-

perature change over large regions. Köppen (1881)

tropics and temperate latitudes using land air tempera-

ture. Callendar (1938) global using land stations.

Willett (1950) global using land stations. Callendar

(1961) 60°N to 60°S using land stations. Mitchell

(1963) global using land stations. Budyko (1969)

Northern Hemisphere using land stations and ship

reports. Jones et al. (1986a,b) global using land sta-

tions. Hansen and Lebedeff (1987) global using land

stations. Brohan et al. (2006) global using land air

temperature and sea surface temperature data is the

longest of the currently updated global temperature

time series (Section 3.2). All time series were

smoothed using a 13-point filter. The Brohan et al.

(2006) time series are anomalies from the 1961 to

1990 mean (°C). Each of the other time series was

originally presented as anomalies from the mean tem-

perature of a specific and differing base period. To

make them comparable, the other time series have

been adjusted to have the mean of their last 30 years

identical to that same period in the Brohan et al.

(2006) anomaly time series.” (Graph is Figure 1.3

from the IPCC Working Group 1 report.

Figure 2—Greenhouse Gases Then and Now

The IPCC report caption for this graph reads:

“Atmospheric concentrations of important long-

lived greenhouse gases over the last 2,000

years. Increases since about 1750 are attrib-

uted to human activities in the industrial era.

Concentration units are parts per million (ppm)

or parts per billion (ppb), indicating the number

of molecules of the greenhouse gas per million

or billion air molecules, respectively, in an

atmospheric sample.” (Graph is FAQ 2.1, Figure

1 from the IPCC Working Group 1 report. Data

combined and simplified from Chapters 6 and 2

of report.)

Global Temperature Time Series

change substantially, then the surfacetemperature must increase as the emis-sion level is raised. This is the green-house effect. It is also the reason thatclear summer nights in deserts, under adry atmosphere, are colder than cloudysummer nights on the U.S. east coast,under a relatively moist atmosphere(Figs. 4 and 5).

In fact, Earth surface temperatureshave increased by about 1.3°F overthe past century (Fig. 3). The temper-ature increase has been particularlypronounced in the past 20 years (foran illustration, see the animations oftemperature changes at www.gps.cal-tech.edu/~tapio/discriminants/anima-tions.html). The scientific consensusabout the cause of the recent warm-ing was summarized by theIntergovernmental Panel on ClimateChange (IPCC) in 2007: “Most of theobserved increase in global averagetemperatures since the mid-20th cen-tury is very likely due to the observedincrease in anthropogenic greenhousegas concentrations. … The observedwidespread warming of the atmos-phere and ocean, together with icemass loss, support the conclusion thatit is extremely unlikely that global cli-mate change of the past 50 years canbe explained without external forcing,and very likely that it is not due toknown natural causes alone.”

The IPCC conclusions rely on cli-mate simulations with computer mod-els (Fig. 6). Based on spectroscopicmeasurements of the optical proper-ties of greenhouse gases, we can cal-culate relatively accurately the impactincreasing concentrations of green-house gases have on Earth’s radiativeenergy balance. For example, theradiative forcing owing to increases inthe concentrations of carbon dioxide,methane, and nitrous oxide in theindustrial era is about 2.3 Watts persquare meter. (This is the change inradiative energy fluxes in the lowertroposphere before temperatures haveadjusted.) We need computer models

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Figure 4 and 5—Two Cheers for the Greenhouse Effect

Some global warming is necessary in order to make the Earth habitable for

creatures like us. These two graphics show how it works. The IPCC caption

reads: “Estimate of the Earth’s annual and global mean energy balance. Over

the long term, the amount of incoming solar radiation absorbed by the Earth

and atmosphere is balanced by the Earth and atmosphere releasing the same

amount of outgoing longwave radiation. About half of the incoming solar radia-

tion is absorbed by the Earth’s surface. This energy is transferred to the

atmosphere by warming the air in contact with the surface (thermals), by evap-

otranspiration and by longwave radiation that is absorbed by clouds and green-

house gases. The atmosphere in turn radiates longwave energy back to Earth

as well as out to space.” Source: Kiehl and Trenberth (1997). (Graphics are

FAQ 1.1, 1.3, Figure 1 from the IPCC Report.)

to translate changes in the radiative energy bal-ance into changes in temperature and other cli-mate variables because feedbacks in the cli-mate system render the climate response tochanges in the atmospheric composition com-plex, and because other human emissions(smog) also affect climate in complex ways. Forexample, as the surface and lower atmospherewarm in response to increases in carbon diox-ide concentrations, the atmospheric concentra-tion of water vapor near the surface increasesas well. That this has to happen is well estab-lished on the basis of the energy balance of thesurface and relations between evaporation ratesand the relative humidity of the atmosphere (itis not directly, as is sometimes stated, a conse-quence of higher evaporation rates).

Water vapor, however, is a greenhouse gas initself, and so it amplifies the temperatureresponse to increases in carbon dioxide concen-trations and leads to greater surface warmingthan would occur in the absence of water vaporfeedback. Other feedbacks that must be takeninto account in simulating the climate responseto changes in atmospheric composition involve,for example, changes in cloud cover, dynamicalchanges that affect the rate at which temperaturedecreases with height and hence affect thestrength of the greenhouse effect, and surfacechanges (e.g., loss of sea ice). Current climatemodels, with Newton’s laws of motion and thelaws of thermodynamics and radiative transfer attheir core, take such processes into account.They are able to reproduce, for example, Earth’sseasonal cycle if all such processes are taken intoaccount but not, for example, if water vaporfeedback is neglected. The IPCC’s conclusion isbased on the fact that these models can onlymatch the observed climate record of the past 50years if they take human-induced changes inatmospheric composition into account. They failto match the observed record if they only modelnatural variability, which may include, for exam-ple, climate responses to fluctuations in solarradiation (Fig. 7).

Climate feedbacks are the central source ofscientific (as opposed to socio-economic) uncer-tainty in climate projections. The dominantsource of uncertainty are cloud feedbacks, whichare incompletely understood. The area coveredby low stratus clouds may increase or decreaseas the climate warms. Because stratus clouds are

low, they do not have a strong greenhouse effect(the strength of the greenhouse effect dependson the temperature difference between the sur-face and the level from which infrared radiationis emitted, and this is small for low clouds); how-ever, they reflect sunlight, and so exert a coolingeffect on the surface, as anyone knows who has

W W W . S K E P T I C . C O M

Chemistry

Rain Clouds

Land Surface

Prescribed IcePrescribed Ice

CO2

Prescribed Ice

Mid-1970s Mid-1980s

1990

"Swamp" Ocean"Swamp" Ocean"Swamp" Ocean

Sulfates

Volcanic Activity

1995

Ocean

2001

AerosolsCarbon Cycle

2007

RiversRiversOverturning Overturning CirculationCirculation

InteractiveInteractiveVegetationVegetation

RiversOverturning Circulation

InteractiveVegetation

Figure 6—The History of Climate Models. The IPCC caption reads: “The

complexity of climate models has increased over the last few decades.

The additional physics incorporated in the models are shown pictorially by

the different features of the modelled world.” (Graphic is Figure 1.2 from

the IPCC Report.)

35

The World In Global Climate Models

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V O L U M E 1 4 N U M B E R 1 2 0 0 8

Figure 7—Global and Continental Temperature Change.The IPCC caption reads: “Comparison of observed continental- and glob-

al-scale changes in surface temperature with results simulated by climate models using either natural or both natural and anthro-

pogenic forcings. Decadal averages of observations are shown for the period 1906-2005 (black line) plotted against the center of

the decade and relative to the corresponding average for the period 1901-1950. Lines are dashed where spatial coverage is less

than 50%. Darker shaded bands show the 5 to 95% range for 19 simulations from five climate models using only the natural

forcings due to solar activity and volcanoes. Lighter shaded bands show the 5 to 95% range for 58 simulations from 14 climate

models using both natural and anthropogenic forcings.” (Graphic is Figure SPM.4 from the IPCC Report.)

Global and Continental Temperature Change

37

W W W . S K E P T I C . C O M

been near southern California’s coast on an over-cast spring morning. If their area coverageincreases as greenhouse gas concentrationsincrease, the surface temperature response willbe muted; if their area coverage decreases, thesurface temperature response will be amplified. Itis currently unclear how these clouds respond toclimate change, and climate models simulatewidely varying responses. Other major uncertain-ties include the effects of aerosols (smog) onclouds and the radiative balance and, ontimescales longer than a few decades, theresponse of ice sheets to changes in temperature.

Uncertainties notwithstanding, it is clear thatincreases in greenhouse gas concentrations, inthe global mean, will lead to warming. Althoughclimate models differ in the amount of warmingthey project, in its spatial distribution, and inother more detailed aspects of the climateresponse, all climate models that can reproduceobserved characteristics such as the seasonalcycle project warming in response to the increas-es in greenhouse gas concentrations that areexpected in the coming decades as a result ofcontinued burning of fossil fuels and otherhuman activities such as tropical deforestation.The projected consequences of the increasedconcentrations of greenhouse gases have beenwidely publicized. Global-mean surface tempera-tures are likely to increase by 2.0 to 11.5°F bythe year 2100, with the uncertainty range reflect-ing scientific uncertainties (primarily aboutclouds) as well as socio-economic uncertainties(primarily about the rate of emission of green-house gases over the 21st century). Land areasare projected to warm faster than ocean areas.The risk of summer droughts in mid-continentalregions is likely to increase. Sea level is projectedto rise, both by thermal expansion of the warm-ing oceans and by melting of land ice.

Less widely publicized but important for poli-cy considerations are projected very long-termclimate changes, of which some already now areunavoidable. Even if we were able to keep theatmospheric greenhouse gas concentration fixedat its present level—this would require an imme-diate and unrealistically drastic reduction in emis-sions—the Earth surface would likely warm byanother 0.9–2.5°F over the next centuries. Theoceans with their large thermal and dynamicinertia provide a buffer that delays the responseof the surface climate to changes in greenhouse

gas concentrations. The oceans will continue towarm over about 500 years. Their waters willexpand as they warm, causing sea level rise. Icesheets are thought to respond over timescales ofcenturies, though this is challenged by recentdata from Greenland and Antarctica, which showevidence of a more rapid, though possibly tran-sient, response. Their full contribution to sealevel rise will take centuries to manifest. Studiesof climate change abatement policies typicallyend in the year 2100 and thus do not take intoaccount that most of the sea level rise due to theemission of greenhouse gases in the next 100years will occur decades and centuries later. Sealevel is projected to rise 0.2–0.6 meters by theyear 2100, primarily as a result of thermal expan-sion of the oceans; however, it may eventuallyreach values up to several meters higher thantoday when the disintegration of glaciers and icesheets contributes more strongly to sea level rise.(A sea level rise of 4 meters would submergemuch of southern Florida.)

Certainties and Uncertainties

While there are uncertainties in climate projec-tions, it is important to realize that the climate pro-jections are based on sound scientific principles,such as the laws of thermodynamics and radiativetransfer, with measurements of optical propertiesof gases. The record of past climate changes thatcan be inferred, for example, with geochemicalmethods from ice cores and ocean sedimentcores, provides tantalizing hints of large climatechanges that occurred over Earth’s history, and itposes challenges to our understanding of climate(for example, there is no complete and commonlyaccepted explanation for the cycle of ice ages andwarm periods). However, climate models are notempirical, based on correlations in such records,but incorporate our best understanding of thephysical, chemical, and biological processes beingmodeled. Hence, evidence that temperaturechanges precede changes in carbon dioxide con-centrations in some climate changes on thetimescales of ice ages, for example, only showsthat temperature changes can affect the atmos-pheric carbon dioxide concentrations, which inturn feed back on temperature changes. Such evi-dence does not invalidate the laws of thermody-namics and radiative transfer, or the conclusionthat the increase in greenhouse gas concentrationsin the past decades is human induced. �