Drew Shindell .

Modeling Global Climate Change

The industrial and scientific revolutions of the past two centuries havebrought much of the physical world under our control. Yet our lives can stillbe upended by what insurance companies call" acts. of God" - floods, fires,

hurricanes, droughts, and so on. Does this mean that Earth's climate andweather remain forces of nature, entirely beyond our influence? Over thepast few decades, a consensus has gradually emerged among scientists thathuman activities can indeed affect climate and that they are already doing so.One way humans might affect climate is by causing "global warming" (anincrease in global average surface temperature). With observed surface tem~peratures risinCand governments considering large~scale, costly efforts tocounteract this trend, it is increasingly important to understand how and whyour climate is changing.

particles (aerosols, most often pro,duced from pollution), the sun'sbrightness, and changes in naturalpatterns of climate variability such asthe El Nifio oscillation. We have tobe concerned with anything thatmight respond to these changes, aswell as with whatever processes con,trol the "natural" climate state.Important physical processes includeabsorption and reemission of solarenergy by Earth and the atmosphere,evaporation, rainfall and cloud for,mation, atmospheric winds andstorms, and ocean currents, amongmany others. Chemical processesinclude the formation and destruc,tion of ozone and aerosol particles.Biological processes include carbondioxide intake and oxygen emissionby plants, and methane emissionsfrom wetlands and livestock. Thoughsome individual processes, such as

Earth's climate system is anextremely complex web of inter,connected physical, chemical, andbiological processes. Understand,ing the cause of a change in asingle parameter, such as surfacetemperature, is quite involved. Itrequires first a knowledge of whichprocesses govern the value, secondan understanding of how each ofthose processes functions, and thirdan understanding of how theyinteract. The interactions are fre-quently non-linear, meaning thatthe net change from variations inseveral processes is not equal to thesum of all independent changes.

What sorts of processes areinvolved? There are many potentialcauses of global climate change.They include changes in emissions ofcertain gases into the atmosphere,the amount of ozone, atmospheric

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physical equations governingwinds, precipitation, chemicalinteractions, and other phenomenaall have to be solved for each gridbox over the three-dimensionalglobe at every time step, timebecomes a serious concern. Ofcourse, the size of the grid boxes,

greenhouse, which allows higher-energy light to come in, but blockslower-energy heat from leaving.More greenhouse gases in theatmosphere lead to an increasedtrapping of energy and thus warmEarth's surface. While this effectoccurs in all GCMs, there are differ-

generation of wind by atmosphericheating, can be expressed throughrelatively simple mathematics, theentire system of equations is far toocomplex to be solved by hand.Rather, the only feasible solution isto create a computer' model of Earth'sclimate system.

Rotating Supercell Storm

ences in the amount of globalwarming induced. The differing cli-mate sensitivity of the various mod-els is thought to arise largely fromuncertainties in simulating theresponse of clouds and the oceans toatmospheric changes. Nevertheless,the consistency of predicted globalwarming has led to a general accep-tance of this result.

T hese models, called generalcirculation models

(GCMs), attempt to include therelevant physical principles govern-ing the behavior of the atmosphereand the oceans. Those principlesare formulated into mathematicalequations, which determine how agiven set of initial conditions willchange over time. Some processesobey fairly simple laws, such as thepassage of Earth around the sun,which gives us seasons. Others,such as cloud formation, areextremely complex processes thatare very difficult to understand andincorporate into models. Yet theonly way to approach a real under-standing of the climate system is tocreate a realistic model.

GCMs divide the atmosphereinto a three-dimensional grid ofboxes around Earth. By usingobservations, scientists can specifyinitial values for each box for quan-tities such as temperature, windspeed, pressure, and composition.Emissions at Earth's surface are alsospecified over time. For example,urban regions emit large amountsof pollutants. Based on physicallaws, the computer program thencalculates changes over an intervalof time called the "time step." Ingeneral, a smaller time step allowsinteractions to take place morenearly simultaneously, as they do inthe real atmosphere, and so pro-duces more realistic results.However, the simulations takelonger as more steps are required tocover the same length of "modeltime." Keeping in mind that the

that is, the mode~spatial resolu-tion, brings up tne same issue.Finer resolution is more realistic,but requires more time. CurrentGCMs typically have a resolutionof roughly 250-1000 km horizon-tally, and 1-4 km vertically, andcalculate changes hourly. Thisprocess requires some of the world'sfastest supercomputers. Even so,several months of actual time areneeded to simulate climate changesover a few decades.

GCMs have been developed byresearch groups throughout theworld. All find that the buildup ofgreenhouse gases in the atmosphereleads to warming at Earth's surfacethrough the "greenhouse effect."Greenhouse gases do not absorb thehigh-energy radiation emitted bythe sun, so this radiation passesthrough the atmosphere and isabsorbed by Earth. Earth reemitsthis radiation at a much lower ener-gy (because Earth is much colderthan the sun), and the reemittedradiation is absorbed by the green-house gases. This situation, ofcourse, is analogous to the glass in a

W ith observations showing

that Earth has steadily

wanned during the twentieth centu-ry, and indications that increasinggreenhouse-gas emissions cause glob-al wanning, the next question is howmuch of the observed wanning canbe attributed to greenhouse-gasincreases, and more generally, howmuch can be attributed to humanactivities? While greenhouse-gasemissions have certainly increased inthe twentieth century, concurrentwith global wanning, solar radiation

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considering the large role dynamicsplay in current climate. For exam,pIe, Miami is warmer than NewYork, largely because Miami is clos,er to the equator, and gets moresunlight. Yet Spain is also typicallywarmer than New York, althoughthey are at the same latitude,because oceanic and atmosphericcurrents carry extra heat to Spainrelative to New York.

Most climate models have beenprimarily concerned with Earth'slower atmosphere. To save compu,tational time, these models haveincluded only partial representa,tions of the srratosphere (theregion from roughly 17, to 50,kmaltitude). Our_recent research atthe NASA Goddard Institute forSpace Studies (GISS) indicatesthat changes in stratosphericdynamics can have a significanteffect on the flow of energy in thelower atmosphere, af{(;cting climateat Earth's surface. These changes instratospheric circulation have been

has also increased, pollution hasincreased, ozone has decreased, andthere have doubtless been naturalvariations as well. GCMs indicatethat greenhouse gases probably haveplayed the dominant role in theobserved warming.

While the broad phenomenon ofglobal warming is accepted by mostscientists, many details remain uncer-tain. Though sea level is expected torise, the amount remains faitlyunclear, largely because no one issure how fast ice sheets might meltin a warmer climate. Changes inrainfall and soil-moisture amounts,and resultant changes in crop yields,are also difficult to quantify becauseof their complexity. While the over-all warming of Earth will lead tomore very hot days, changes in thefrequency of such natural weatherdisasters as hurricanes, tropicalstorms, cyclones, and typhoons havenot yet been reliably simulated,though an increase in storms overallseems likely.

Though many people confuse ozone depletionwith the greenhouse effect, the two are quitedifferent, independent phenomena caused byentirely different gases. Ozone is chemicallydestroyed by chlorine from CFCs, while thelargely non-reactive greenhouse gases thatcause global warming have a negligiblechemical effect on ozone.

induced in our GCM simulationsby changes in greenhouse gases,solar irradiance, and ozone.

Current models reproduce theglobally averaged observed trendsfairly well, but the causes of region-al (continental-scale and smaller)changes are another area fraughtwith uncertainty. Changes indynamics, atmospheric winds, andcirculation may have a large effecton regional temperature trends.Dynamical changes occur inresponse to radiation changes thatalter temperatures, because temper-atures control atmospheric motions(for example, hot air tends to rise).It is perhaps not surprising thatsuch changes may be important,

A n interesting example of

stratospheric-circulationchanges is revealed in recentresearch into their effect on theformation of an ozone hole abovethe Arctic. Severe depletion of thestratospheric ozone layer takesplace in the polar regions, especial-

ly over Antarctica, because of theuniquely cold conditions presentthere. During the winter, a vortexof air forms over the polar region,confining the air within it to thedarkness of the polar winter night.This isolated air, receiving no heatfrom the sun, becomes steadilycolder, until the sparse gas mole-cules actually begin to freeze intopolar stratospheric clouds (PSCs), atype of cloud found nowhere elseand at no other time. Chlorine,released into the stratosphere fromchlorofluorocarbons (CFCs) pro-duced by humans over the past fewdecades, reacts on the surfaces ofthese clouds. These chemical reac-tions convert chlorine into mole-cules that are capable of destroyinglarge amounts of ozone.

Though many people confuseozone depletion with the greenhouseeffect, the two are quite different,independent phenomena caused byentirely different gases. Ozone ischemically destroyed by chlorinefrom CFCs, while the largely non-reactive greenhouse gases that causeglobal warming have a negligiblechemical effect on ozone. Increasedcarbon dioxide in the atmospheremay warm the planet, but it does noteat ozone. Nor does the ozone holecause global warming. Because ozoneis a greenhouse gas, ozone depletionactually leads to global cooling.Ironically, after years of pointing outthe distinct nature of these twoproblems, we have recently discov-ered an indirect connection betweenthe two.

Using the GISS GCM, wefound that increasing greenhouse-gas emissions lead to a colderstratosphere (in contrast to theirwarming effect at the surface, butconsistent with the "greenhouse"trapping of heat at lower altitudes).Furthermore, the polar vortex thatconfines air to polar latitudesbecomes stronger as a result ofstratospheric-circulation changes.These interactions have a largeeffect in the Arctic, where the vor-

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unexpected changes like this havehappened before.

The occurrence of the ozonehole in the mid-1980s is a goodexample of an unexpected change inclimate. Models in use at the timecould account reasonably well for thedecline in ozone amounts of a fewpercent observed up until then. Withthe models "validated" by their abili-ty to reproduce observations, no onehad any reason to suspect that wewould suddenly lose more than halfthe ozone layer by a process that wasnot in any of the models! Yet that isjust what happened. The nonlinearreactions on PSC surfaces were notimportant in the atmosphere until acritical threshold of chlorine wasreached. Their importance was dis-covered quickly after the ozone holeappeared, but there was no forewarn-ing, and therefore no chance to avertthe ozone hole. Could somethingsimilar be missing in current climatemodels that would lead to a precipi-tous climate change? We simply donot know. But given the complexityof the climate system, it is quite pos-sible. We can only hope that climateresearch will give us answers beforeclimate itself gives us any nastysurprises.

tex has typically been weaker thanover the Antarctic.

In our model simulations, theArctic begins to look more andmore like the Antarctic, with alarge ozone hole forming eachspringtime. The potential conSe-quences are much greater in theArctic, however. Because thestratospheric ozone layer absorbsbiologically harmful ultraviolet(UV) radiation, it protects livingthings on Earth's surface. Theaffected food chain is much morecomplex in the Arctic, where peo-ple live and where large forestsgrow, than in the Antarctic.Additionally, when an Arcticozone hole breaks up, its remainswill drift over heavily populatedareas of North America andEurope, potentially exposing manypeople to extra doses of UV. Themodel also predicts that theexpected recovery of the ozonehole, resulting from the interna-tionally agreed-upon limitations onCFC production, will be delayed bythe buildup of greenhouse gases inthe atmosphere. This result wasrecently supported by a GCM sim-ulation in Germany.

Siberia have increased by up to tendegrees Celsius (180 Fahrenheit)!Because wind speeds vary naturally,it has been difficult to separatevariations caused by humans fromnatural ones. .

However, our model simula-tions suggest that much of theincrease in surface winds and conti-nental surface temperatures isinduced by greenhouse-gas increas-es through their enhancement bfthe stratospheric vortex. Thestronger polar vortex alters the flowof energy near the surface as well,enhancing ground-level winds.Despite appearing as part of a natu-rally variable phenomenon, thevery large increases in wintertimesurface temperatures over the con-tinents may therefore result in largepart from human activities.Though it takes place through avery convoluted chain of cause andeffect, this indirect effect of green-house gases on climate via windchanges may be as large, in someareas, as the more direct effect ofgreenhouse-gas trapping of heat.

l am often asked, "Why shouldI worry about global warm,

ing?" While the effects on agricul,ture are likely to be positive insome areas, notably cold northernclimes, many areas will suffer lossesin food production. Current distrib,ution systems are optimized for cur,rent crop patterns, so there willalso be economic costs fromrestructuring. Sea levels will rise,and with so much of Earth's popu,lation living on coasts, many couldsuffer from flooding and stormdamage. But perhaps the most omi,nous possibility is a rapid, unfore,seen change that cannot bestopped. If the West Antarctic icesheet were to slide off Antarctica,ocean levels could rise dramatically.If the North Atlantic ocean cur'rent changed course, Europeantemperatures could plunge. Drastic,

1m

I ncluding the response ofstratospheric and tropospheric

winds to greenhouse-gas increasesmay lead to improvement in simu-lating observed regional surface-temperatures trends. During thewinter, the ocean retains heat betterthan the land. When the dominantwest-to-east winds increase, theycarry warmer air from over theoceans to the continents, and coldercontinental air to the oceans.Observations reflect this pattern.While the annual average globalmean surface temperature hasincreased by roughly half a degreeCelsius (10 Fahrenheit) over thetwentieth century, wintertime tem-peratures over North America and

Drew Shindell investigates inter~actions between atmosphericchemistry and climate change atNASA's Goddard Institute forSpace Studies and the Center forClimate Systems Research atColumbia University, where healso teaches. He received hisPh.D. in Physics from SUNYStony Brook in 1995, after sev~eral years of field research in theArctic and Antarctic, and nowlives happily among the crowdsof New York City. He hasrecently co~authored the chapteron the future of the ozone layerin the United Nations Assess~ment of Ozone.

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