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global warmingPrimary Contributors:Michael E. Mann,Henrik Selin

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global warming, Increase in the global average surface temperature resulting from enhancement ofthegreenhouse effect, primarily byair pollution.

In 2007 the UN Intergovernmental Panel on Climate Change forecasted that by 2100 global average surfacetemperatures would increase 3.27.2 F (1.84.0 C), depending on a range of scenarios for greenhouse gasemissions, and stated that it was now 90 percent certain that most of the warming observed over the previoushalf century could be attributed to greenhouse gas emissions produced by human activities (i.e., industrialprocesses and transportation). Many scientists predict that such an increase in temperature would cause polarice caps and mountain glaciers to melt rapidly, significantly raising the levels of coastal waters, and wouldproduce new patterns and extremes of drought and rainfall, seriously disrupting food production in certainregions. Other scientists maintain that such predictions are overstated. The 1992Earth Summitand the1997 Kyoto Protocol to the United Nations Framework Convention on Climate Change attempted to address theissue of global warming, but in both cases the efforts were hindered by conflicting national economic agendasand disputes between developed and developing nations over the cost and consequences of reducingemissions of greenhouse gases.

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global warming, the phenomenon of increasing averageairtemperaturesnear the surface ofEarthover the past one to two centuries. Since the mid-20th century,climate scientists have gathered detailed observations of variousweatherphenomena (such astemperature,precipitation, and storms) and of related influences onclimate(such asocean currentsand theatmospheres chemical composition). These data indicate that Earths climate has changed over almost every

conceivable timescale since the beginning of geologic time and that, since at least the beginning of th e IndustrialRevolution, the influence of human activities has been deeply woven into the very fabric o fclimate change.

Giving voice to a growing conviction of most of the scientific community,theIntergovernmental Panel on Climate Change(IPCC) reported that the 20th century saw an increase inglobal average surface temperature of approximately 0.6 C (1.1 F). The IPCC went on to state that most of thewarming observed over the second half of the 20th century could be attributed to human activities, and itpredicted that by the end of the 21st century the average surface temperature would increase by another 1.8 to

4.0 C (3.2 to 7.2 F), depending on a range of possible scenarios. Many climate scientists agree that significanteconomic and ecological damage would result if global average temperatures rose by more than 2 C [3.6 F] in
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such a short time. Such damage might include increased extinction of many plant and animal species, shifts inpatterns ofagriculture, and risingsea levels. The IPCC reported that the global average sea level rose by some17 cm (6.7 inches) during the 20th century, that sea levels rose faster in the second half of that century than inthe first half, and thatagain depending on a wide range of scenariosthe global average sea level could rise byanother 18 to 59 cm (7 to 23 inches) by the end of the 21st century. Furthermore, the IPCC reported thataveragesnowcover in the Northern Hemisphere declined by 4 percent, or 1.5 million square km (580,000square miles), between 1920 and 2005.

The scenarios referred to above depend mainly on future concentrations of certain trace gases,calledgreenhouse gases, that have been injected into the loweratmospherein increasing amounts through theburning offossil fuelsfor industry, transportation, and residential uses. Modern global warming is the result of anincrease in magnitude of the so-calledgreenhouse effect, a warming of Earths surface and lower atmospherecaused by the presence of water vapour,carbon dioxide,methane, and other greenhouse gases. Of all thesegases, carbon dioxide is the most important, both for its role in the greenhouse effect and for its role in the humaneconomy. It has been estimated that, at the beginning of the industrial age in the mid-18th century, carbondioxide concentrations in the atmosphere were roughly 280 parts per million (ppm). By the end of the 20thcentury, carbon dioxide concentrations had reached 369 ppm (possibly the highest concentrations in at least650,000 years), and, if fossil fuels continue to be burned at current rates, they are projected to reach 560 ppm bythe mid-21st centuryessentially, a doubling of carbon dioxide concentrations in 300 years. It has beencalculated that an increase of this magnitude alone (that is, not accounting for possible effects of othergreenhouse gases) would be responsible for adding 2 to 5 C (3.6 to 9 F) to the global average surface

temperatures that existed at the beginning of the industrial age.

A vigorous debate is in progress over the extent and seriousness of rising surface temperatures, the effects ofpast and future warming onhuman life, and the need for action to reduce future warming and deal with itsconsequences. This article provides an overview of the scientific background andpublic policydebate related tothe subject of global warming. It considers the causes of rising near-surface air temperatures, the influencingfactors, the process of climate research and forecasting, the possible ecological and social impacts of risingtemperatures, and the public policy developments since the mid-20th century. For a detailed description ofEarths climate, its processes, and the responses of living things to its changing nature, seeclimate. Foradditional background on how Earths climate has changed throughout geologic time,seeclimatic variationand change. For a full description of Earths gaseous envelope, within which climate change and global warmingoccur, seeatmosphere.

Climatic variation since the last glaciation

Global warming is related to the more general phenomenon of climate change, which refers tochanges in the totality of attributes that define climate. In addition to changes inair temperature, climate changeinvolves changes toprecipitationpatterns,winds,ocean currents, and other measures of Earths climate.Normally, climate change can be viewed as the combination of various natural forces occurring over diversetimescales. Since the advent of human civilization, climate change has involved an anthropogenic, orexclusively human-caused, element, and this anthropogenic element has become more important in the industrialperiod of the past two centuries. The term global warming is used specifically to refer to any warming of near-surface air during the past two centuries that can be traced to anthropogenic causes.

To define the concepts of global warming and climate change properly, it is first necessary to recognize thattheclimateofEarthhas varied across many timescales, ranging from an individual human life span to billions ofyears. This variable climate history is typically classified in terms of regimes or epochs. For instance,thePleistoceneglacial epoch (about 2,600,000 to 11,700 years ago) was marked by substantial variations in theglobal extent ofglaciersandicesheets. These variations took place on timescales of tens to hundreds ofmillennia and were driven by changes in the distribution of solar radiationacross Earths surface. Thedistribution of solar radiation is known as the insolation pattern, and it is strongly affected by the geometryof Earthsorbitaround theSunand by the orientation, or tilt, of Earths axis relative to the direct rays of the Sun.

Worldwide, the most recent glacial period, orice age, culminated about 21,000 years ago in what is often calledtheLast Glacial Maximum. During this time, continentalice sheetsextended well into the middle latituderegions of Europe andNorth America, reaching as far south as present-dayLondonandNew York City. Globalannualmean temperatureappears to have been about 45 C (79 F) colder than in the mid-20th century. It isimportant to remember that these figures are a global average. In fact, during the height of this last ice age,Earths climate was characterized by greater cooling at higher latitudes (that is, toward the poles) and relativelylittle cooling over large parts of the tropical oceans (near the Equator). This glacial interval terminated abruptlyabout 11,700 years ago and was followed by the subsequent relatively ice-free period known as the HoloceneEpoch. The modern period of Earths history is conventionally defined as residing within the Holocene. However,
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some scientists have argued that the Holocene Epoch terminated in the relatively recent past and that Earthcurrently resides in a climatic interval that could justly be called the Anthropocene Epoch that is, a period duringwhich humans have exerted a dominant influence over climate.

Though less dramatic than the climate changes that occurred during thePleistocene Epoch, significantvariations in global climate have nonetheless taken place over the course of the Holocene. During the earlyHolocene, roughly 9,000 years ago,atmospheric circulationandprecipitationpatterns appear to have beensubstantially different from those of today. For example, there is evidence for relatively wet conditions in what isnow theSaharaDesert. The change from one climatic regime to another was caused by only modest changes inthe pattern of insolation within the Holocene interval as well as the interaction of these patterns with large-scaleclimate phenomena such asmonsoonsandEl Nio/Southern Oscillation (ENSO).

During the middle Holocene, some 5,0007,000 years ago, conditions appear to have been relatively warmindeed, perhaps warmer than today in some parts of the world and during certain seasons. For this reason, thisinterval is sometimes referred to as theMid-Holocene Climatic Optimum. The relative warmth of average near-surface air temperatures at this time, however, is somewhat unclear. Changes in the pattern of insolationfavoured warmer summers at higher latitudes in the Northern Hemisphere, but these changes also producedcooler winters in the Northern Hemisphere and relatively cool conditions year-round in the tropics. Any overallhemispheric or global mean temperature changes thus reflected a balance between competing seasonal andregional changes. In fact, recent theoretical climate model studies suggest that global mean temperatures duringthe middle Holocene were probably 0.20.3 C (0.40.5 F) colder than average late-20th-century conditions.

Over subsequent millennia, conditions appear to have cooled relative to middle Holocene levels. This period hassometimes been referred to as the Neoglacial. In the middle latitudes this cooling trend was associated withintermittent periods of advancing and retreatingmountain glaciersreminiscent of (though far more modest than)the more substantial advance and retreat of the major continental ice sheets of the Pleistocene climate epoch.

Causes of global warming

Thegreenhouse effect

The average surface temperature of Earth is maintained by a balance of various formsofsolarand terrestrial radiation. Solar radiation is often called shortwave radiation because the frequencies ofthe radiation are relatively high and the wavelengths relatively shortclose to the visible portion oftheelectromagnetic spectrum. Terrestrial radiation, on the other hand, is often called longwave radiationbecause the frequencies are relatively low and the wavelengths relatively longsomewhere in theinfraredpartof the spectrum. Downward-movingsolar energyis typically measured inwattsper square metre. The energy ofthe total incomingsolar radiationat the top of Earthsatmosphere(the so-called solar constant) amountsroughly to 1,366 watts per square metre annually. Adjusting for the fact that only one- half of the planets surfacereceives solar radiation at any given time, the average surface insolation is 342 watts per square metre annually.

The amount of solar radiation absorbed by Earths surface is only a small fraction of the total solar radiationentering the atmosphere. For every 100 units of incoming solar radiation, roughly 30 units are reflected back to

space by eitherclouds, the atmosphere, or reflective regions of Earths surface. This reflective capacity isreferred to as Earths planetaryalbedo, and it need not remain fixed over time, since the spatial extent anddistribution of reflective formations, such as clouds andicecover, can change. The 70 units of solar radiation thatare not reflected may be absorbed by the atmosphere, clouds, or the surface. In the absence of furthercomplications, in order to maintainthermodynamic equilibrium, Earths surface and atmosphere must radiatethese same 70 units back to space. Earths surface temperature (and that of the lower layer of the atmosphereessentially in contact with the surface) is tied to the magnitude of this emission of outgoing radiation according totheStefan-Boltzmann law.

Earthsenergy budgetis further complicated by thegreenhouse effect. Tracegaseswith certain chemicalpropertiesthe so-calledgreenhouse gases, mainlycarbon dioxide(CO2),methane(CH4), andnitrousoxide(N2O)absorb some of theinfrared radiationproduced by Earths surface. Because of thisabsorption,some fraction of the original 70 units does not directly escape to space. Because greenhouse gases emit thesame amount of radiation they absorb and because this radiation is emitted equally in all directions (that is, as

much downward as upward), the net effect of absorption by greenhouse gases is to increase the total amount ofradiation emitted downward toward Earths surface and lower atmosphere. To maintain equilibrium, Earths
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surface and lower atmosphere must emit more radiation than the original 70 units. Consequently, the surfacetemperature must be higher. This process is not quite the same as that which governs a true greenhouse, but theend effect is similar. The presence of greenhouse gases in the atmosphere leads to a warming of the surface andlower part of the atmosphere (and a cooling higher up in the atmosphere) relative to what would be expected inthe absence of greenhouse gases.

It is essential to distinguish the natural, or background, greenhouse effect from the enhanced greenhouseeffect associated with human activity. The natural greenhouse effect is associated with surface warmingproperties of natural constituents of Earths atmosphere, especially water vapour, carbon dioxide, and methane.The existence of this effect is accepted by all scientists. Indeed, in its absence, Earths average temperaturewould be approximately 33 C (59 F) colder than today, and Earth would be a frozen and likely uninhabitableplanet. What has been subject to controversy is the so-called enhanced greenhouse effect, which is associatedwith increased concentrations of greenhouse gases caused by human activity. In particular, the burning of fossilfuelsraises the concentrations of the major greenhouse gases in the atmosphere, and these higherconcentrations have the potential to warm the atmosphere by several degrees.

Radiative forcing

In light of the discussion above of the greenhouse effect, it is apparent that the temperature ofEarths surface and lower atmosphere may be modified in three ways: (1) through a net increase in the solarradiation entering at the top of Earths atmosphere, (2) through a change in the fraction of the radiationreaching the surface, and (3) through a change in the concentration of greenhouse gases in the atmosphere. Ineach case the changes can be thought of in terms of radiative forcing. As defined by theIntergovernmentalPanel on Climate Change(IPCC), radiative forcing is a measure of the influence a given climatic factor has onthe amount of downward-directedradiant energyimpinging upon Earths surface. Climatic factors are dividedbetween those caused primarily by human activity (such asgreenhouse gasemissions and aerosol emissions)and those caused by natural forces (such as solar irradiance); then, for each factor, so-called forcing values arecalculated for the time period between 1750 and the present day. Positive forcing is exerted by climatic factorsthat contribute to the warming of Earths surface, whereas negative forcing is exerted by factors that coolEarths surface.

On average about 342 watts of solar radiation strike each square metre of Earths surface per year, and thisquantity can in turn be related to a rise or fall in Earths surface temperature. Temperatures at the surface mayalso rise or fall through a change in the distribution of terrestrial radiation (that is, radiation emitted by Earth)within the atmosphere. In some cases, radiative forcing has a natural origin, such as during explosive eruptionsfromvolcanoeswhere vented gases and ash block some portion of solar radiation from the surface. In othercases, radiative forcing has an anthropogenic, or exclusively human, origin. For example, anthropogenicincreases in carbon dioxide, methane, and nitrous oxide are estimated to account for 2.3 watts per square metreof positive radiative forcing. When all values of positive and negative radiative forcing are taken together and allinteractions between climatic factors are accounted for, the total net increase in surface radiation due to humanactivities since the beginning of theIndustrial Revolutionis 1.6 watts per square metre.

The influences of human activity on climate

Human activity has influenced global surfacetemperatures by changing the radiative balance governing the Earth on various timescales and at varying spatialscales. The most profound and well-known anthropogenic influence is the elevation of concentrations ofgreenhouse gases in the atmosphere. Humans also influence climate by changing the concentrations of aerosolsand ozone and by modifying the land cover of Earths surface.

GREENHOUSE GASES

As discussed above, greenhouse gases warm Earths surface by increasing the netdownward longwave radiation reaching the surface. The relationship between atmospheric concentration ofgreenhouse gases and the associated positive radiative forcing of the surface is different for each gas. A
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complicated relationship exists between the chemical properties of each greenhouse gas and the relative amountof longwave radiation that each can absorb. What follows is a discussion of the radiative behaviour of each majorgreenhouse gas.

Water vapour

Water vapour is the most potent of the greenhouse gases in Earths atmosphere, but itsbehaviour is fundamentally different from that of the other greenhouse gases. The primary role of water vapour isnot as a direct agent of radiative forcing but rather as a climate feedback that is, as a response within theclimate system that influences the system s continued activity (see belowWater vapour feedback). Thisdistinction arises from the fact that the amount of water vapour in the atmosphere cannot, in general, be directlymodified byhuman behaviourbut is instead set by air temperatures. The warmer the surface, the greatertheevaporationrate of water from the surface. As a result, increased evaporation leads to a greaterconcentration of water vapour in the lower atmosphere capable of absorbing longwave radiation and emitting itdownward.

Carbon dioxide

Of the greenhouse gases, carbon dioxide (CO2) is most significant. Natural sources ofatmospheric CO2 include outgassing fromvolcanoes, thecombustionand natural decay of organic matter,andrespirationby aerobic (oxygen-using) organisms. These sources are balanced, on average, by a set ofphysical, chemical, or biological processes, called sinks, that tend to remove CO2 from the atmosphere.Significant natural sinks include terrestrial vegetation, which takes up CO 2 during the processofphotosynthesis.

A number of oceanic processes also act ascarbonsinks. One such process, called thesolubility pump, involves the descent of surfacesea watercontaining dissolved CO2. Another process, thebiological pump, involves the uptake of dissolved CO2 by marine vegetation andphytoplankton(small, free-floating,photosynthetic organisms) living in the upper ocean or by other marine organisms that use CO 2 tobuildskeletonsand other structures made of calciumcarbonate(CaCO3). As these organisms expire and fall totheocean floor, the carbon they contain is transported downward and eventually buried at depth. A long-termbalance between these natural sources and sinks leads to the background, or natural, level of CO 2 in theatmosphere.

In contrast, human activities increase atmospheric CO2 levels primarily through the burning offossil fuels (principallyoilandcoal, and secondarilynatural gas, for use in transportation,heating, and thegeneration ofelectrical power) and through the production ofcement. Other anthropogenic sources include theburning offorestsand the clearing of land. Anthropogenic emissions currently account for the annual release ofabout 7 gigatons (7 billion tons) ofcarboninto the atmosphere. Anthropogenic emissions are equal toapproximately 3 percent of the total emissions of CO2 by natural sources, and this amplified carbon load fromhuman activities far exceeds the offsetting capacity of natural sinks (by perhaps as much as 23 gigatons peryear). CO2 has consequently accumulated in the atmosphere at an average rate of 1.4 parts per million (ppm) byvolume per year between 1959 and 2006, and this rate of accumulation has been linear (that is, uniform overtime). However, certain current sinks, such as the oceans, could become sources in the future (seeCarbon cyclefeedbacks). This may lead to a situation in which the concentration of atmospheric CO 2 builds at an exponentialrate.

The natural background level of carbon dioxide varies on timescales of millions of years due to slow changes inoutgassing throughvolcanic activity. For example, roughly 100 million years ago, during theCretaceousPeriod(145.5 million to 65.5 million years ago), CO2 concentrations appear to have been several times higher
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than today (perhaps close to 2,000 ppm). Over the past 700,000 years, CO 2 concentrations have varied over afar smaller range (between roughly 180 and 300 ppm) in association with the same Earth orbital effects linked tothe coming and going of thePleistoceneice ages (see belowNatural influences on climate). By the early 21stcentury, CO2 levels reached 384 ppm, which is approximately 37 percent above the natural background level ofroughly 280 ppm that existed at the beginning of the Industrial Revolution. According to ice coremeasurements,this level (384 ppm) is believed to be the highest in at least 650,000 years.

Radiative forcingcaused by carbon dioxide varies in an approximately logarithmic fashion with theconcentration of that gas in the atmosphere. The logarithmic relationship occurs as the result ofasaturationeffect wherein it becomes increasingly difficult, as CO2 concentrations increase, for additionalCO2moleculesto further influence the infrared window (a certain narrow band of wavelengths in the infraredregion that is not absorbed by atmospheric gases). The logarithmic relationship predicts that the surface warmingpotential will rise by roughly the same amount for each doubling ofCO 2 concentration. At current rates of fossil-fuel use, a doubling of CO2concentrations over preindustrial levels is expected to take place by the middle of the21st century (when CO2 concentrations are projected to reach 560 ppm). A doubling of CO2 concentrations wouldrepresent an increase of roughly 4 watts per square metre of radiative forcing. Given typical estimates of climatesensitivity in the absence of any offsetting factors, this energy increase would lead to a warming of 2 to 5 C (3.6to 9 F) over preindustrial times (seeFeedback mechanisms and climate sensitivity). The total radiative forcingby anthropogenic CO2 emissions since the beginning of the industrial age is approximately 1.66 watts per squaremetre.

MethaneMethane (CH4) is the second most important greenhouse gas. CH4 is more potent than CO2 because theradiative forcing produced per molecule is greater. In addition, the infrared window is less saturated in the rangeof wavelengths of radiation absorbed by CH4, so more molecules may fill in the region. However, CH4 exists in farlower concentrations than CO2 in the atmosphere, and its concentrations by volume in the atmosphere aregenerally measured in parts per billion (ppb) rather than ppm. CH4 also has a considerably shorter residence timein the atmosphere than CO2 (the residence time for CH4 is roughly 10 years, compared with hundreds of yearsfor CO2).

Natural sources of methane include tropical and northernwetlands, methane-oxidizingbacteriathat feed on organic material consumed bytermites, volcanoes, seepage vents of the seafloorin regions rich with organic sediment, and methane hydrates trapped along the continental shelves of the oceansand in polarpermafrost. The primary natural sink for methane is the atmosphere itself, as methane reactsreadily with thehydroxyl radical(OH-) within thetroposphereto form CO2 and water vapour (H2O). WhenCH4reaches thestratosphere, it is destroyed. Another natural sink issoil, where methane isoxidizedbybacteria.

As with CO2, human activity is increasing the CH4 concentration faster than it can be offset by natural sinks.Anthropogenic sources currently account for approximately 70 percent of total annual emissions, leading tosubstantial increases in concentration over time. The major anthropogenic sources of atmospheric CH 4 are ricecultivation,livestock farming, the burning of coal and natural gas, the combustion of biomass, and thedecomposition of organic matter in landfills. Future trends are particularly difficult to anticipate. This is in part dueto an incomplete understanding of the climate feedbacks associated with CH 4 emissions. In addition, as humanpopulations grow, it is difficult to predict how possible changes in livestock raising, rice cultivation, and energyutilization will influence CH4 emissions.

It is believed that a sudden increase in the concentration of methane in the atmosphere was responsible for awarming event that raised average global temperatures by 48 C (7.214.4 F) over a few thousand yearsduring the so-calledPaleocene-Eocene Thermal Maximum, or PETM. This episode took place roughly 55million years ago, and the rise in CH4 appears to have been related to a massivevolcanic eruptionthatinteracted with methane-containing flood deposits. As a result, large amounts of gaseous CH 4 were injected intothe atmosphere. It is difficult to know precisely how high these concentrations were or how long they persisted. Atvery high concentrations, residence times of CH4 in the atmosphere can become much greater than the nominal10-year residence time that applies today. Nevertheless, it is likely that these concentrations reached severalppm during the PETM.

Methane concentrations have also varied over a smaller range (between roughly 350 and 800 ppb) in association

with the Pleistocene ice age cycles (seeNatural influences on climate). Preindustrial levels of CH4 in theatmosphere were approximately 700 ppb, whereas early 21st-century levels exceeded 1,770 ppb. (Theseconcentrations are well above the natural levels observed for at least the past 650,000 years.) The net radiative
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forcing by anthropogenic CH4 emissions is approximately 0.5 watt per square metreor roughly one-third theradiative forcing of CO2.

Surface-levelozoneand other compoundsThe next most significant greenhouse gas is surface, or low-level, ozone(O3). Surface O3 is a result of airpollution; it must be distinguished from naturally occurring stratospheric O3, which has a very different role in theplanetary radiation balance. The primary natural source of surface O3 is the subsidence of stratospheric O3 fromthe upper atmosphere (see belowStratospheric ozone depletion). In contrast, the primary anthropogenic sourceof surface O3 isphotochemical reactionsinvolving the atmosphericpollutantcarbon monoxide(CO). Thebest estimates of the concentration of surface O3 are 50 ppb, and the net radiative forcing due to anthropogenicemissions of surface O3 is approximately 0.35 watt per square metre.

Nitrous oxidesand fluorinated gasesAdditional trace gases produced by industrial activity that have greenhouse properties includenitrousoxide(N2O) and fluorinated gases (halocarbons), the latter includingsulfur hexafluoride, hydrofluorocarbons(HFCs), and perfluorocarbons (PFCs). Nitrous oxide is responsible for 0.16 watt per square metre radiativeforcing, while fluorinated gases are collectively responsible for 0.34 watt per square metre. Nitrous oxides havesmall background concentrations due to natural biological reactions in soil and water, whereas the fluorinatedgases owe their existence almost entirely to industrial sources.

AEROSOLSThe production ofaerosolsrepresents an important anthropogenicradiative forcingof climate. Collectively,aerosols blockthat is, reflect and absorba portion of incoming solar radiation, and this creates a negativeradiative forcing. Aerosols are second only to greenhouse gases in relative importance in their impact on near-surface air temperatures. Unlike the decade-long residence times of the well-mixed greenhouse gases, such asCO2 and CH4, aerosols are readily flushed out of the atmosphere within days, either by rain or snow (wetdeposition) or by settling out of the air (dry deposition). They must therefore be continually generated in order toproduce a steady effect on radiative forcing. Aerosols have the ability to influence climate directly by absorbing orreflecting incoming solar radiation, but they can also produce indirect effects on climate by modifying cloudformationor cloud properties. Most aerosols serve ascondensation nuclei(surfaces upon which water vapourcan condense to form clouds); however, darker-coloured aerosols may hinder cloud formation by absorbingsunlight and heating up the surrounding air. Aerosols can be transported thousands of kilometres from theirsources of origin by winds andupper-level circulationin the atmosphere.

Perhaps the most important type of anthropogenic aerosol in radiative forcing is sulfateaerosol. It is producedfromsulfur dioxide(SO2) emissions associated with the burning of coal and oil. Since the late 1980s, globalemissions of SO2 have decreased from about 73 million tons to about 54 million tons of sulfur per year.

Nitrateaerosol is not as important as sulfate aerosol, but it has the potential to become a significant source ofnegative forcing. One major source of nitrate aerosol issmog(the combination of ozone with oxidesofnitrogenin the lower atmosphere) released from the incomplete burning of fuel in internal-combustionengines. Another source isammonia(NH3), which is often used infertilizersor released by the burning ofplants and other organic materials. If greater amounts of atmospheric nitrogen are converted to ammonia andagricultural ammonia emissions continue to increase asprojected, the influence of nitrate aerosols on radiativeforcing is expected to grow.

Both sulfate and nitrate aerosols act primarily by reflecting incoming solar radiation, thereby reducing the amountof sunlight reaching the surface. Most aerosols, unlike greenhouse gases, impart a cooling rather than warminginfluence on Earths surface. One prominent exception is carbonaceous aerosols such ascarbon blackor soot,which are produced by the burning of fossil fuels and biomass. Carbon black tends to absorb rather than reflectincident solar radiation, and so it has a warming impact on the lower atmosphere, where it resides. Because of itsabsorptive properties, carbon black is also capable of having an additional indirect effect on climate. Through itsdeposition in snowfall, it can decrease the albedo of snowcover. This reduction in the amount of solar radiationreflected back to space by snow surfaces creates a minor positive radiative forcing.

Natural forms of aerosol include windblown mineral dust generated in arid and semiarid regions and seasaltproduced by the action of waves breaking in the ocean. Changes towindpatterns as a result of climatemodification could alter the emissions of these aerosols. The influence of climate change on regional patterns ofaridity could shift both the sources and the destinations of dust clouds. In addition, since the concentration of seasalt aerosol, or sea aerosol, increases with the strength of the winds near the ocean surface, changes in windspeed due to global warming and cl imate change could influence the concentration of sea salt aerosol. Forexample, some studies suggest that climate change might lead to stronger winds over parts of the NorthAtlanticOcean. Areas with stronger winds may experience an increase in the concentration of sea salt aerosol.
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Other natural sources of aerosols include volcanic eruptions, which produce sulfate aerosol, and biogenicsources (e.g., phytoplankton), which producedimethyl sulfide(DMS). Other important biogenic aerosols, suchasterpenes, are produced naturally by certain kinds oftreesor otherplants. For example, the dense forests oftheBlue Ridge Mountainsof Virginia in theUnited Statesemit terpenes during the summer months, which inturn interact with the high humidity and warm temperatures to produce a natural photochemical smog.Anthropogenic pollutants such asnitrateand ozone, both of which serve as precursor molecules for thegeneration of biogenic aerosol, appear to have increased the rate of production of these aerosols severalfold.

This process appears to be responsible for some of the increased aerosol pollution in regions undergoing rapidurbanization.

Human activity has greatly increased the amount of aerosol in the atmosphere compared with the backgroundlevels of preindustrial times. In contrast to the global effects of greenhouse gases, the impact of anthropogenicaerosols is confined primarily to the Northern Hemisphere, where most of the worlds industrial activity occurs.The pattern of increases in anthropogenic aerosol over time is also somewhat different from that of greenhousegases. During the middle of the 20th century, there was a substantial increase in aerosol emissions. This appearsto have been at least partially responsible for a cessation of surface warming that took place in the NorthernHemisphere from the 1940s through the 1970s. Since that t ime, aerosol emissions have leveled off due toantipollution measures undertaken in the industrialized countries since the 1960s. Aerosol emissions may rise inthe future, however, as a result of the rapid emergence of coal-fired electric powergeneration in China andIndia.

The total radiative forcing of all anthropogenic aerosols is approximately 1.2 watts per square metre. Of thistotal,0.5 watt per square metre comes from direct effects (such as the reflection of solar energy back intospace), and0.7 watt per square metre comes from indirect effects (such as the influence of aerosols on cloudformation). This negative radiative forcing represents an offset of roughly 40 percent from the positive radiativeforcing caused by human activity. However, the relative uncertainty in aerosol radiative forcing (approximately 90percent) is much greater than that of greenhouse gases. In addition, future emissions of aerosols from humanactivities, and the influence of these emissions on future climate change, are not known with any certainty.Nevertheless, it can be said that, if concentrations of anthropogenic aerosols continue to decrease as they havesince the 1970s, a significant offset to the effects of greenhouse gases will be reduced, opening future climate tofurther warming.

LAND-USECHANGE

There are a number of ways in which changes inland usecan influence climate. The mostdirect influence is through the alteration of Earthsalbedo, or surface reflectance. For example, the replacementofforestby cropland and pasture in the middle latitudes over the past several centuries has led to an increase inalbedo, which in turn has led to greater reflection of incoming solar radiation in those regions. This replacementof forest byagriculturehas been associated with a change in global average radiative forcing ofapproximately 0.2 watt per square metre since 1750. In Europe and other major agricultural regions, such land-use conversionbegan more than 1,000 years ago and has proceeded nearly to completion. For Europe, the negative radiativeforcing due to land-use change has probably been substantial, perhaps approaching 5 watts per square metre.The influence of early land use on radiative forcing may help to explain a long period of cooling in Europe thatfollowed a period of relatively mild conditions roughly 1,000 years ago. It is generally believed that the mild

temperatures of this medieval warm period, which was followed by a long period of cooling, rivaled those of20th-century Europe.

Land-use changes can also influence climate through their influence on the exchange of heat between Earthssurface and the atmosphere. For example,vegetationhelps to facilitate the evaporation ofwaterinto theatmosphere throughevapotranspiration. In this process, plants take up liquid water from the soil throughtheirrootsystems. Eventually this water is released throughtranspirationinto the atmosphere, as water vapourthrough thestomatain leaves. While deforestation generally leads to surface cooling due to the albedo factordiscussed above, the land surface may also be warmed as a result of the release of latent heatby theevapotranspiration process. The relative importance of these two factors, one exerting a cooling effect and theother a warming effect, varies by both season and region. While the albedo effect is likely to dominate in middlelatitudes, especially during the period from autumn through spring, the evapotranspiration effect may dominateduring the summer in the midlatitudes and year-round in the tropics. The latter case is particularly important inassessing the potential impacts of continued tropical deforestation.
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