Some parts of the world could actually benefit from climatechange, while others could suffer tremendously. But for the foreseeable fu­turethe effects will he uncer­tain. No nation can plan on benefiting, and so, says Schneider,we must all "hedge our global bets," by reducing emissions of green­housegases. "The longer we wait to take action," he says, "and theweaker the action, the larger the effect and the more likely that it will benegative." Says meteorolo­gist Howard Ferguson, assis­tant deputy ministerof the Canadian Atmospheric Envi­ronment Service, "All the greenhousescenarios are consistent. These numbers are real. We have to start be­having asif this is going to happen. Those who advo­cate a program consisting only ofadditional research are missing the boat."

While the greenhouse ef­fect threatens to make life on Earthmiserable, it is also part of the reason life is livable in the first place.For at least the last 100,000 years atmospheric carbon dioxide, naturallygenerated and con­sumed by animals and plants, was in rough equilib­rium, at acouple of hundred parts per million. Without this minute but critical trace tohold in heat, the globe's mean temperature would be in the forties instead of acomfortable 59 degrees. The amount of carbon dioxide has risen and fallen abit, co­inciding with the spread and retreat of glaciers as ice ages have comeand gone. But until the Industrial Revolu­tion, atmospheric carbon di­oxidelevels never rose above a manageable 280 parts per million.

Then, beginning early in the nineteenth century, the burningof fossil fuels, espe­cially coal, took off. By 1900, carbon dioxide levels inthe atmosphere had begun to rise steadily, reaching 340 parts per million lastyear.

Levels of the other green­house gases have also risen. Methane,for example, is generated primarily by bac­terial decomposition of or­ganicmatter—particularly in such places as landfills, flooded rice paddies, and theguts of cattle and termites— and by the burning of wood. Methane concentrationin the atmosphere has grown steadily as Earth's human population has grown,rising one percent a year over the last decade. Levels of chlo­rofluorocarbons,which are used as refrigerants, as cleaning solvents, and as raw materials formaking plastic foam, have climbed 5 percent annually.

The amount of nitrous ox­ide in the atmosphere has quicklyincreased as well, with about a third of the total added by human activity—much of that emitted by ni­trogen-based fertilizers, and half of that from justthree nations: China, theSoviet Union, and the United States. This gas is also re­leased by theburning of coal and other fossil fuels, includ­ing gasoline. And ozone, whichforms a beneficial shield against ultraviolet ra­diation when high in thestratosphere, is an efficient greenhouse gas when it ap­pears at airlineraltitudes— as it increasingly does, since it too is a by-product of fossil fuelburning.

All these gases are far more efficient at absorbing infraredenergy (the invis­ible radiation that ordinarily carries Earth's excess heatinto space) than is carbon dioxide. Indeed, atmo­spheric chemists have esti­matedthat the combined warming effect of these trace gases will soon equal or ex­ceedthe effect from carbon dioxide. And even as growth has slowed in the industrial­izednations, the Third World is rushing full tiltinto development. All told, bil­lions of tons of greenhouse gases enter theatmosphere each year.

The big question is, given the inexorable buildup of thesegases—a growth that even the most spirited optimists concede can only beslowed, not stopped— what will the specific effects be? It's hard to say,because the relationship between worldwide climate and local weather is such acomplex phenomenon to begin with. The chaotic patterns of jet streams and vorticesand ocean currents swirling it around the globe and gov­erning the weatherstill con­found meteorologists; in fact, weather more than two weeks in thefuture is thought by some to he inher­ently unpredictable.

So far, the best answers have come from computer models thatsimulate the workings of the atmosphere. Most divide the atmosphere intohundreds of boxes, each of which is represented by mathematical equations forwind, temperature, mois­ture, incoming radiation, outgoing radiation, and thelike. Each mathematical box is linked to its neighbors, so it can respond tochanging conditions with appropriate changes of its own. Thus, the modelbehaves the way the world does—albeit at a very rough scale. A typical modeldivides the atmosphere verti­cally into nine layers and horizontally into boxesthat are several hundred miles on a side.

Climate modelers can play with "what if" scenariosto see how the world would re­spond to an arbitrary set of conditions. Severalyears ago, for example, computer models were used to holster the theory ofnuclear winter, which concluded that smoke and dust lofted into the at­mospherein a nuclear war would block sunlight and dangerously chili the planet. Tostudy the greenhouse ef­fect, climatologists first used models to simulatecurrent conditions, then instantly doubled the amount of car­bon dioxide in theatmo­sphere. The computer was allowed to run until condi­tions stabilized at anew equilibrium, and a map could be drawn showing changes in temperature, pre­cipitation,and other factors.

But Hansen's latest simu­lations—the ones he used in hisstartling congressional testimony—are more sophis­ticated. In them he addedcarbon dioxide to the atmo­sphere stepwise, just as is happening in the realworld. The simulations, begun in 1983, took so much com­puter time that theywere not completed and pub­lished until this summer.

Even the best climate model, however, has to over­simplifythe enormous com­plexity of the real atmo­sphere. One problem is the size ofthe boxes. The model used at the National Center for Atmospheric Research, forexample, typically uses boxes 4.5 degrees of latitude by 7 degrees oflongitude— about the size of the center's home state of Colorado— and treatsthem as uniform masses of air. While that's inherently inaccurate—the real Colorado contains such fundamentally different fea­turesas the Rocky Moun­tains and the Great Plains—using smaller boxes would take too much computing power.

Another problem is that modelers must estimate the influenceof vegetation, ice and snow, soil moisture, ter­rain, and especially clouds,which reflect lots of sun­light back into space and also hold in surface heat."Clouds are an important factor about which little is known," saysSchneider. "When I first started looking at this in 1972, we didn't knowmuch about the feed­back from clouds. We don't know any more now than we didthen."

So it is not surprising that while the inure than a dozenmajor global climate models in use around the world tend to agree on thebroadest phenomena, they differ wildly when it comes to regional effects. And,says Robert Cess, a climate mod­eler at the State University of New York atStony Brook, "The smaller the scale, the bigger the disagreement."

That makes it extremely hard to get national and localgovernments to take action. Says Stephen Leatherman, director of the Laboratoryfor Coastal Research at the Uni­versity of Maryland, "Unlessyou can put something down on paper and show the effects on actual locations—even actual buildings—then it's just pie in the sky."

There are, however, some consequences of a warming Earththat will be universal. Perhaps the most obvious is a rise in sea level."If we went all out to slow the warming trend, we might stall sea levelrise at three to six feet," says Robert Buddemeier of Lawrence LivermoreNa­tional Laboratory, who is studying the impact of sea- level rise on coralreefs, "But that's the very best you could hope for." And a six- footrise, Buddemeier pre­dicts, would be devastating.

It would, for one thing, render almost all low cor­alislands uninhabitable. "Eventually," Buddemeier says, "a lot ofthis real estate is going to go underwater," For places like the MarshallIslands in the Pacific, the Maldives off the west coast of India, and someCarib­bean nations, this could mean nothing less than na­tional extinction."You're re­ally looking at a potential refugee problem of unprece­denteddimensions," says Buddemeier. "In the past, people have run away fromfamine or oppression. But they've never been physi­cally displaced from a coun­trybecause a large part of it has disappeared."

Coastal regions of conti­nents or larger islands will alsobe in harm's way, par­ticularly towns or cities built on barrier islands andthe fertile flat plains that typi­cally surround river deltas. Bangladesh,dominated by the Ganges-Brahmaputra-­Meghna Delta, is the classic case, saysBuddemeier. "It's massively populated, ach­ingly poor, and something likea sixth of the country is going to go away."

Egypt will be in similar trouble, according to a study byeconomist James Broadus and several colleagues at Woods Hole OceanographicInstitution. Like the Ganges-­Brahmaputra-Meghna, the soft sediments of theNile Delta are subsiding. Given even an intermediate sce­nario for sea-levelrise by the year 2050, Egyptcould lose 15 percent of its arable land, land that currently houses 14 percentof its population and produces 14 percent of its gross domestic product.

One mitigating factor for some coastal nations that arestill developing, such as Be­lize and Indonesia, is that they generally havecommit­ted fewer resources to the coastline than their devel­opedcounterparts—Austra­lia, for example, or the United States, with such vul­nerablecities as Galveston and Miami. "Developed countries have billions in­vestedin a very precarious, no-win situation," Budde­meier says. "The lessdevel­oped countries will have an easier time adapting."

Indeed, the impact on coastal cities in developed countriesmay be enormous. The Urban Institute, a non­partisan think tank, is com­pletinga study for the En­vironmental Protection Agency on what a three-foot sea levelrise would do to Mi­ami.Miami isparticularly vulnerable. Not only is it a coastal city, but it is nearlysurrounded by water, with the Atlantic to the east, the Everglades to the west,and porous limestone beneath— "one of the most permeable aquifers in theworld," says William Hyman, a senior re­ search associate at the insti­tute."The aquifer in Miamiis so porous that you'd actu­ally have to build a dike down one hundred fiftyfeet beneath the surface to keep water from welling up." In an unusuallysevere storm nearby Miami Beachwould be swept by a wall of water up to 16 feet above the cur­rent sea level.

Storms are an even greater danger to Galveston, whichLeatherman has studied ex­tensively. Given just a cou­ple of feet in sea-levelrise, a moderately bad hurricane, of the type that occurs about once every tenyears, would have the destructive impact of the type of storm that oc­curs oncea century. And Galveston is typical of a whole range of resort areas on theeastern and Gulf coasts, such as Atlantic City, New Jersey ("almost thewhole New Jersey coast, re­ally," says Leatherman); Ocean City, Maryland;and Myrtle Beach, South Caro­lina. "The point is, all these cities havebeen built on low-lying sandy barrier is­lands, mostly with eleva­tions nohigher than ten feet above sea level," Leather­man says. "Just asmall rise in sea level will result in a lot of complications."

Even as cities become more vulnerable to moderate storms,the intensity of hur­ricanes may increase dra­matically, says Kerry Eman­uel, ameteorologist at MIT. Hurricane intensity is linked to the temperature of thesea surface, Emanuel explains. According to his models, if the sea warms topredicted levels, the most intense hurricanes will be 40 to 50 per­cent moresevere than the most intense hurricanes of the past 50 years.

James Titus, director of the Environmental Protec­tionAgency's Sea Level Rise Project, says communities will have two choices: buildwalls or get out of the way. For cities such as New Yorkor Boston theanswer may well he to build walls. But for most other coastal regions, pickingup and moving may work out better. One of the first examples of a regionalgovernment making a regu­lation based on the green­house effect took place in Maine last year. Thestate approved regulations allow­ing coastal development with the understandingthat if sea level rises enough to inundate a property, the property will revertto na­ture, with the owner footing the bill for dismantling or movingstructures.

Another worldwide con­sequence of global warming isincreased precipitation: warmer air will mean more evaporation of ocean water,more clouds, and an overall rise in rain and snow of be­tween 5 and 7 percent.But it won't be evenly distrib­uted. One climate model at PrincetonUniversity's Geo­ physical Fluid Dynamics Laboratory predicts that cen­tralIndia will have doubled precipitation, while the cen­ters of continents atmiddle latitudes--the midwestern United States, for example— will actually havemuch drier summers than they have now (this summer's drought could, in other words,be a foretaste). Some and areas, including south­ern Californiaand Morocco,will have drier winters; and winters are when such areas get most of theirprecipita­tion. Moreover, the effect may be self-perpetuating: drier soil, saysSyukuro Manabe, the climatologist who developed the model, leads to even hotterair.

The changes could be po­litical dynamite for nations thatalready argue over water resources. A prime ex­ample is Egypt and Sudan,both of which draw their lifeblood from the north- flowing Nile.Sudan has been trying to divert a bigger share of the river's water; butdownstream, Egypt is experiencing one of Africa's fastest population explo­sionsand will need every drop of water it can get. A string of droughts in the Su­dan couldmake the conflict far worse. The same situ­ation occurs in many other parts ofthe world.

Not all the tensions will be international. Within na­tions,local effects of global warming will cause interne­cine fights for increasinglyscarce water. In the United States,for example, western states have long argued over who owns what fraction of thewater in such rivers as the Colorado.In California 42 percent of the water comesfrom the Sacramento and San Joaquin river ba­sins,which are fed by runoff from the Sierra Nevadaand other mountain ranges. Most of the water falls as snow in the winter, whichmelts in the spring to feed the rivers, reservoirs, and subterranean aquifers.The state's normal strategy for water management calls for keeping thereservoirs low in winter, to provide protection against floods, and keepingthem as high as possible in summer, to ensure an ade­quate supply for the giantfarming operations in the Central Valley (one of the most productiveagricultural regions in the world) and for arid southern California.

Peter Gleick of the Pacific Institute for Studies in De­velopment,Environment and Security, in Berkeley, California, has devised a widelypraised model that predicts a dramatic disrup­tion of the state's water sup­plyin the event of global warming, even if total precipitation remains un­changed.It focuses on the Sacramento River basin, which alone provides 30 per­cent of thestate's water and almost all the water for agri­culture in the Central Valley.

According to the model, higher temperatures will mean thatwhat falls in win­ter will increasingly be rain, not snow, and that more of itwill run off right away. Californiamay get the same amount of total annual run­off, but the water-distribu­tionsystem won't be able to deal with it. "California will get the worst of all possibleworlds—more flooding in the winter, less available water in the summer,"Gleick says. "This will re­verberate throughout the state." San Francisco Bay will feel a secondary effect. Asfreshwater supplies shrink in the summer, seawater, which has alreadyinfiltrated freshwater aqui­fers beneath the low-lying Sacramento Delta, willcon­tinue its push inland. Rising sea level will just compound the effect.

Food is another crucial re­source that will be affected bythe global green­house. Taken by itself, a rise in atmospheric carbon diox­idemight not be so bad. For many crops more carbon di­oxide means a rise in therate of photosynthesis and, there­fore, in growth; and with in­creased carbondioxide some plants' use of water is more efficient, according to stud­ies donein conventional glass greenhouses. Also, as the planet gets warmer, crops mightbe cultivated farther north. But as usual, things are not so simple. Atemperature rise of only 3.5 degrees in the tropics could reduce riceproduction by more than 10 percent.

In temperate regions also, the picture is mixed. CynthiaRosenzweig, a researcher based at Goddard, has been using crop-growth computermodels to predict effects of carbon dioxide buildup and climate change onwheat, the most widely cultivated crop in the world. Plugging in temperaturechanges de­rived from the Goddard cli­mate model, Rosenzweig tested a worldwith doubled carbon dioxide levels. Be­cause the Goddard model is had atpredicting precipita­tion, she did separate runs for normal and dry condi­tions.She found that in nor­mal years the wheat grew better, thanks to the extracarbon dioxide. But in dry years there was a marked in­crease in crop failures,be­cause of excessive heat. Given the likelihood that heat waves and droughtsare increasing, she says, no one should count on better yields in years tocome.

The nations most likely to reap the benefits of warmerclimate are Canada and the Soviet Union, much of whose vast land area is too coldfor large-scale crop cul­tivation. There has even been speculation that thesecountries might go slowly on controlling the greenhouse effect, or even opposesuch control; anyone who has spent the winter in Mos­cowor Saskatoonwould be sorely tempted by the prospect of better weather.

But again, atmospheric scientists stress that no na­tion cancount on benefits. "The models suggest that ecological zones will shiftnorthward," says planetary scientist Michael McElroy of Harvard. "Thesouthwestern desert to the Grain Belt; the Grain Belt to Canada. There might he winners andlosers if this shift occurs slowly. But suppose it shifts so fast thatecosystems are unable to keep up?" For example, he says, there is a limitto the distance that a forest can propagate in a year. "If it is unable topropagate fast enough, then either we have to come in and plant trees, or elsewe'll see total devasta­tion and the collapse of the ecosystem."

According to Irving Mintzer, a senior associate with theEnergy and Climate Project of the World Re­sources Institute in Wash­ington,there is another rea­son to be leery of projections for regional agriculturalbenefits. Just because cli­matic conditions conducive to grain cultivation movenorth, that doesn't mean that other conditions neces­sary for agriculturalsuper­powerdom will be present. Much of Canada, for exam­ple, does not havethe opti­mum type of soil for growing wheat and corn.

Wildlife will suffer, too. In much of the world, wildernessareas are increas­ingly hemmed in by devel­opment, and when climate shifts,these fragile ecosys­tems won't be able to shift with it. Plants will suddenly beunable to propagate their seeds, and animals will have no place to go. Speciesin the Arctic, such as caribou, may lose vitalmigratory routes as ice bridges between islands melt.

In the United States the greatest impact will likely be oncoastal wetlands: the salt marshes, swamps, and bay­ous that are among theworld's most diverse and productive natural habitats. James Titus of theEnviron­mental Protection Agency estimates that a five-foot rise in sealevel—not even the worst-case scenario—would destroy between 50 and 90 percentof America'swet- lands. Under natural condi­tions marshes would slowly shift inland. Butwith levees, condominiums, and other man-made structures in the way, theycan't. The situ­ation is worst in Louisiana,says Titus, which has 40 per­cent of U.S.wetlands (ex­cluding those in Alaska); much ofthe verdant Missis­sippi River delta may wellvanish.

In many parts of the trop­ics, low forests of mangrove treesthrive in the shallow waters along coastlines. Their dense networks of rootsand runners are natu­ral island-building systems, trapping sediment and cush­ioningthe damaging effects of tropical storms. But rising sea levels will flood theman­groves; the natural response would be for them to shift with the tide,spreading their roots farther inland. But in places where development hasencroached on the shore, the mangrove forests will feel the same squeeze thatwill threaten marshes.

The only way to eliminate the greenhouse problem completelywould be to return the world to its pre­industrial state. No one pro­posesthat. But researchers agree that there is plenty that can be done to at leastslow down the warming. Energy conservation comes first: us­ing less coal,finding more efficient ways to use cleaner- burning fossil fuels, and tak­ing anew look at nonfossil alternatives, everything from solar and geothermal energyto—yes, even some environ­mentalists are admitting it— nuclear power.

Getting the world's frac­tious nations to agree to a programof remedial mea­sures sounds extremely diffi­cult, but Stephen Schneider seessigns that it may not be impossible. Schneider was one of more than 300 dele­gatesfrom 48 countries who attended the International Conference on the ChangingAtmosphere, which took place in Toronto,coinciden­tally, just a week after Hansen's congressional testi­mony. It was,says Schnei­der, the "Woodstock of CO2" (an obvious reference to the"Woodstock of Phys­ics" meeting held last year, during which news ofthe high-temperature supercon­ductors exploded into the public consciousness).

The meeting was the first large-scale attempt to bridge thegap between scientists and policymakers on a wide range of atmospheric prob­lems,including not just the greenhouse effect but also acid rain and the depletionof the protective layer of ozone in the stratosphere. Four days of floordebates, panel discussions, and closed-door sessions pro­duced an ambitiousmani­festo calling for, among other things, the following:

• A 20percent reduction in carbon dioxide emissions by industrialized nations by theyear 2005, using a com­bination of conservation ef­forts and reduced consump­tionof fossil fuels. A 50 percent cut would eventu­ally be needed to stabilize at­mosphericcarbon dioxide.
• A switchfrom coal or oil to other fuels. Burning natu­ral gas, for example, pro­duceshalf as much carbon dioxide per unit of energy as burning coal.
• Much morefunding for development of solar power, wind power, geothermal power, and thelike, and ef­forts to develop safe nuclear power.
• Drasticreductions in de­forestation, and encourage­ment of forest replanting and restoration.<
• Thelabeling of products whose manufacture does not harm the environment.
• Nearlycomplete elimi­nation of the use of chloro­fluorocarbons, or CFCs, by the year2000.

Of all the anti-greenhouse measures, the last should proveeasiest to achieve. Although CFCs are extremely persistent, re­maining in theupper atmo­sphere for decades, and al­though they are 10,000 times moreefficient than carbon dioxide at trapping heat, the process of controlling themhas been under way for years, for reasons having nothing to do with thegreenhouse effect. Since the early 1970s atmospheric sci­entists have knownthat CFCs could have destructive effects on ozone. CFCs were banned from spraycans in the United Statesand Can­ada in the late1970s, and the appearance of a "hole" in the ozone layer over Antarc­tica in the early 1980s cre­ated an internationalcon­sensus that CFCs must go. Last year 53 nations crafted an agreement thatwill cut CFC production by 50 per­cent over the next decade; the chemicals maywell be banned altogether by the turn of the century.

CFCs are a special case, however. Since they are en­tirelyman-made, and since substitutes are available or under development, control isstraightforward. "There are only thirty-eight compa­nies worldwide thatproduce CFCs," says Pieter Win­semius, former minister of the environmentof the Netherlands."You can put them all in one room; you can talk to them. But you can't dothat with the pro­ducers of carbon dioxide— all the world's utilities andindustries."

Also, there is a lack of ba­sic information on the flow ofcarbon dioxide and the other greenhouse gases into and out of the atmosphereand biosphere. Just as one example, there is no good es­timate of how muchcarbon dioxide, methane, and ni­trous oxide are produced by fires, both man-madeand naturally occurring. "We need to better assess global biomass burningas a source of greenhouse gases," says Joel Levine of the NASA LangleyResearch Centerin Hampton, Virginia. "We have to understand whatwe're actually doing when we burn tropical forests and when we burnagricultural stubble after harvest. We don't know on a global basis what thecontribution is."

Remarkably, the confer­ence spurred some specific promisesfrom political lead­ers rather than just vague platitudes. Standing before a40-foot-wide photorealist painting of a cloud-studded skyscape, prime ministersBrian Mulroney of Canada andGro Harlem Brundtland of Norway pledged that their countries will slow fossilfuel use and forgive some Third World debt,allowing devel­oping countries to grow in a sustainable way. Says Schneider,"In the fifteen years that I've been trying to convince people of theseriousness of the green­house effect, this is the first time I've seen a broadcon­sensus: First, there is a con­sensus that action is not pre­mature. Second, that so­lutions have to occur on a globalas well as a national scale."

In the end, the greatest ob­stacle facing those who aretrying to slow the output of greenhouse gases is the fun­damental and pervasivena­ture of the human activities that are causing the prob­lem: deforestation,in­dustrialization, energy pro­duction. As populations boom, productivity mustkeep up. And even as the de­veloped nations of the world cut back on fossilfuel use, there will be no justifiable way to prevent the Third World from expanding its use of coal and oil. How can thedeveloped countries ex­pect that China, for example, which has plansto double its coal production in the next 15 years in order to spur de­velopment,will be willing or even able to change course?

And then there is poverty, which contributes to the greenhouseeffect by encour­aging destruction of forests. "Approximately seventy-fivepercent of the deforesta­tion occurring in the world today is accounted for bylandless people in a desper­ate search for food," says Jose Lutzenberger,director of the Gala Foundation, an influential Brazilian environ­mental group.Commercial logging accounts for just 15 percent of tropical forest lossworldwide. Unfortunately for the atmosphere and the forests themselves, workingout an agreement with the tropical timber industry will be far easier thaneliminat­ing rural poverty.

Industrialized nations, which created most of the greenhouseproblem, should lead the way to finding solu­tions, says State Departmentofficial Richard Benedick, who represented the United States during negotiationsfor cuts in CFCs and who was a conference attendee. The first priority, hesays, should be strong conserva­tion efforts—an area in which the United States lags far behind such countries as Japan. Theeffect of such measures, Benedick feels, can only be positive and the cost isnot great. "Certain things make sense on their own merits," he says.Tech­nology can be transferred to developing countries. In some Third World nations a partial solution can be as simpleas modernizing en­ergy production and distri­bution. Upgrading India'selectric-power distribution system, Benedick says, could double the effectiveenergy output of existing coal-fired power plants.

Addressing the confer­ence, Canadian minister of energyMarcel Masse noted that there is cause for opti­mism. One need look no fur­therthan the energy crisis of a decade ago. From 1979 to 1985, thanks primarily toconservation, substantial cuts were made in the use of fossil fuels byindustrialized nations. Only since 1986 and the current oil glut, said Masse,has there been a re­surgence in oil use and coal burning.

Michael McElroy con­cluded, "If we choose to take onthis challenge, it appears that we can slow the rate of change substantially,giving us time to develop mecha­nisms so that the cost to society and thedamage to eco­systems can be minimized. We could alternatively close our eyes,hope for the best, and pay the cost when the bill comes due."