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Coastal Systems: Coastal lands are particularly vulnerable; sealevel rise is the most obvious climate-related impact. Densely settled and intensively used low-lying coastal plains, islands and deltas are especially vulnerable to coastal erosion and land loss, inundation and sea flooding, upstream movement of the saline/freshwater front and seawater intrusion into freshwater lenses. Especially at risk are large delta regions of Bangladesh, Myanmar, Viet Nam and Thailand, and the low-lying areas of Indonesia, the Philippines and Malaysia. Socio-economic impacts could be felt in major cities and ports, tourist resorts, artisinal and commercial fishing, coastal agriculture and infrastructure development. International studies have projected the displacement of several millions of people from the region's coastal zone, assuming a 1-m rise in sea level. The costs of response measures to reduce the impact of sea-level rise in the region could be immense.

Human Health: The incidence and extent of some vector-bome diseases are expected to increase with global warming. Malaria, schistosomiasis and dengue-which are significant causes of mortality and morbidity in Tropical Asia-are very sensitive to climate and are likely to spread into new regions on the margins of presently endemic areas as a consequence of climate change. Newly affected populations initially would experience higher fatality rates. According to one study that specifically focused on climate influences on infectious disease in presently vulnerable regions, an increase in epidemic potential of 12-27 per cent for malaria and 31-47 per cent for dengue and a decrease of schistosomiasis of 11-17 per cent are anticipated under a range of GCM scenarios as a consequence of climate change. Waterborne and water-related infectious diseases, which already account for the majority of epidemic emergencies in the region, also are expected to increase when higher temperatures and higher humidity are superimposed on existing conditions and projected increases in population, urbanization, declining water quality and other trends.

Conclusions: The potential direct effects of climate change assessed here, such as changes in water availability, crop yields and inundation of coastal areas, all will have further indirect effects on food security and human health. The suitability of adaptation strategies to different climatic environments will

vary across the diverse subregions and land uses of the region. Adaptive options include new temperature- and pest-resistant crop varieties; new technologies to reduce crop yield loss; improvements in irrigation efficiency, and integrated approaches to river basin and coastal zone management that take account of current and longer-term issues, including climate change.

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Better baseline data, both climatic and socio-economic Better scenarios, especially of precipitation, extreme events, sulfate aerosol effects and regional-scale changes Better understanding of the ecological and physiological effects of increasing CO2 concentrations, taking account of species competition and migrations, soil and nutrients, acclimation, and partitioning between crop yields, roots, stems and leaves Dynamic models of climate, biospheric processes and other socio-economic factors to take account of the developing, time-varying nature of global change Impact assessments across a range of scenarios and assumptions to enable the assessment of risk, particularly in regions comprised primarily of developing countries and small island states, where resources for research and assessment have been inadequate to date Analysis of adaptation options, including the need for development of new technologies and opportunities for adapting existing technologies in new settings Integrated assessments across sectors, from climate change to economic or other costs, across countries and regions, including adaptations, and including other socio-economic changes.

Authors/Contributors

Robert T. Watson (USA), Marufu C. Zinyowera (Zimbabwe), Richard H. Moss (USA), Reid E. Basher (New Zealand), Martin Beniston (Switzerland), Osvaldo F. Canziani (Argentina), Sandra M. Diaz (Argentina), David J. Dokken (USA), John T. Everett (USA), B. Blair Fitzharris (New Zealand), Habiba Gitay (Australia), Bubu P. Jallow (The Gambia), Murari Lal (India), R. Shakespeare Maya (Zimbabwe), Roger F. McLean (Australia), M.Q. Mirza (Bangladesh), Ron Neilson (USA), Ian R. Noble (Australia), Leonard A. Nurse (Barbados), H.W.O. Okoth-Ogendo (Kenya), A. Barrie Pittock (Australia), David S. Shriner (USA), S.K. Sinha (India), Roger B. Street (Canada), Su Jilan

Greenhouse Forecasting Still Cloudy

An international panel has suggested that global warming has arrived, but many scientists say it will be a decade before computer models can confidently link the warming to human activities

The headlines a year and a half ago positively brimmed with assurance: "Global Warming: No Longer in Doubt," "Man Adversely Affecting Climate, Experts Conclude," "Experts Agree Humans Have 'Discernible Effect on Climate," "Climate Panel Is Confident of Man's Link to Warming." The official summary statement of the UNsponsored Intergovernmental Panel on Climate Change (IPCC) report that had prompted the headlines seemed reasonably confident, too: "... the balance of evidence suggests that there is a discernible human influence on global climate." But as negotiators prepare to gather in Bonn in July to discuss a climate treaty that could require nations to adopt expensive policies for limiting their emissions of carbon dioxide and other greenhouse gases, many climate experts caution that it is not at all clear yet that human activities have begun to warm the planet-or how bad greenhouse warming will be when it arrives.

What had inspired the media excitement was the IPCC's conclusion

computer climate modeling. The models are key to detecting the arrival of global warming, because they enable researchers to predict how the planet's climate should respond to increasing levels of greenhouse gases. And while predicting climate has always been an uncertain business, some scientists assert that developments since the IPCC completed its report have, if anything, magnified the uncertainties. "Global warming is definitely a threat as greenhouse-gas levels increase," says climate modeler David Rind of NASA's Goddard Institute for Space Studies (GISS) in New York City, "but I

"In the climate system, there are 14 orders of magnitude of scale. from the planetary scale-which is 40 million meters down to the scale of one of the little aerosol particles on which water vapor can change phase to a liquid [cloud particle)-which is a fraction of a millionth of a millimeter."

Of these 1+ orders of magnitude, notes Schlesinger. researchers are able to include in their models only the two largest, the planetary scale and the scale of weather disturbances. "To go to the third scale-which is [that of thunderstorms] down around 50kilometers resolution-we need a computer

Rough approximation. Models can't reproduce clouds, but they incorporate some cloud
effects, including those of water (white) in the atmosphere, seen in the above model output.

that the half-degree rise in global temperature since the late 19th century may bear a "fingerprint" of human activity. The patchy distribution of the warming around the globe looks much like the distinctive pattern expected if the heat-trapping gases being poured into the atmosphere were beginning to warm the planet, the report said. But IPCC scientists now say that neither the public nor many scientists appreciate how many if's, and's, and but's peppered the report. "It's unfortunate that many people read the media hype before they read the (IPCC) chapter" on the detection of greenhouse warming, says climate modeler Benjamin Santer of Lawrence Livermore National Laboratory in Livermore, California, the lead author of the chapter. "I think the caveats are there. We say quite clearly that few scientists would say the attribution issue was a done deal."

Santer and his IPCC colleagues' overriding reason for stressing the caveats is their understanding of the uncertainty inherent in

myself am not convinced that we have gained] greater confidence" in recent years in our predictions of greenhouse warming. Says one senior climate modeler who prefers not to enter the fray publicly: "The more you learn, the more you understand that you don't understand very much." Indeed. most modelers now agree that the climate models will not be able to link greenhouse warming unambiguously to human actions for a decade or more.

The effort to simulate climate in a computer faces two kinds of obstacles: lack of computer power and a still very incomplete picture of how real-world climate works. The climate forecasters' basic strategy is to build a mathematical model that recreates global climate processes as closely as possible, let the model run, and then test it by comparing the results to the historical climate record. But even with today's powerful supercomputers, that is a daunting challenge, says modeler Michael Schlesinger of the University of Illinois, Urbana-Champaign:

a thousand times faster, a teraflops machine that maybe we'll have in 5 vears." And including the smallest scales, he says. would require 10 to 1037 more computer power. "So we're kind of stuck."

To get unstuck, modelers "parameterize" smaller scale processes known to affect climate, from the formation of clouds to the movement of ocean eddies. Because they can't model, say, every last cloud over North America, modelers specify the temperatures and humidities that will spawn different types of clouds. If those conditions hold within a single grid box-the horizontal square that represents the model's finest level of detail-the computer counts the entire area as cloudy. But as modelers point out, the grid used in today's models-typically a 300-kilometer square-is still very coarse One over the state of Oregon, for instance, would take in the coastal ocean, the low coast ranges, the Willamette Valley, the high Cascades. and the desert of the Great Basin.

Having the computer power to incorporate into the models a more detailed picture of clouds wouldn't eliminate uncertainties, however, because researchers are still hotly debating the overall impact of clouds on future climate. In today's climate, the net effect of clouds is to cool the planet-although they trap some heat. they block even more by reflecting sunlight back into space. How that balance would change under greenhouse warming, no one knows. A few years ago, a

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Model Gets It Right-Without Fudge Factors

Climate modelers have been "cheating" for so long it's almost become respectable. The problem has been that no computer model could reliably simulate the present climate. Even the best simulations of the behavior of the atmosphere, ocean, sea ice, and land surface drift off into a climate quite unlike today's as they run for centuries. So climate modelers have gotten in the habit of fiddling with fudge factors, so-called “flux adjustments," until the model gets it right.

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No one liked this practice (Science, 9 September 1994, p. 1528). "If you can't simulate the present without arbitrary adjustments, you have to worry," says meteorologist and modeler David Randall of Colorado State University (CSU) in Fort Collins. But now there's a promising alternative. Thirty researchers at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, have developed the first complete model that can simulate the present climate as well as other models do, but without flux adjustments. The new NCAR model, says Randall, is an important step toward removing some of the uneasiness people have about trusting these models to make predictions of future climate” (see main text).

Surface Temperature (K)

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the model's sensitivity to greenhouse gases near the low end of current estimates. Based on an array of different models and other considerations, the UN-sponsored Intergovernmental Panel on Climate Change estimated in 1995 that a carbon dioxide doubling could raise global temperatures by as much as 4.5°C; their best guess was 2.5°C.

A 300-year run without any increase in greenhouse gases produced slow, natural variations in global temperature of about 0.5°C. If the real climate behaves the same way, "two-thirds to three-quarters of the [temperature variations of the] last 130 years

Constant Co, (355 ppmv)
Increasing Co, (1% year)

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The NCAR modelers built a host of refinements into their new Climate System Model (CSM). But the key development, says CSM co-chair Byron Boville, was finding a better way to incorporate the effects of ocean eddies, swirling pools of water up to a couple of hundred kilometers across that spin off strong currents. Climate researchers have long known that the eddies, like atmospheric storms, help shape climate by moving heat around the planet. But modelers have had a tough time incorporating them into their simulations because they are too small to show up on the current models' coarse geographic grid. The CSM doesn't have a finer mesh, but it does include a new "parameterization" that passes the effects of these unseen eddies onto larger model scales, using a more realistic means of mixing heat through the ocean than any earlier model did, says Boville.

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50 60 70 80 90 100 110 120 130 Model Years

Drift-free. The NCAR model, which suggests that Earth will warm moderately (red), can reliably simulate present climate (blue).

Even when run for 300 model "years," the CSM doesn't drift away from a reasonably realistic climate, says NCAR's Climate and Global Dynamics director Maurice Blackmon. "Being able to do this without flux corrections gives you more credibility," he says. "For better or worse, we're not biasing the results as was necessary before."

The first results from this still vastly simplified model imply that future greenhouse warming may be milder than some other models have suggested-and could take decades to reveal itself. Doubling atmospheric carbon dioxide concentrations in the model raised the global temperature 2 degrees Celsius, which puts

leading climate model-developed at the British Meteorological Office's Hadley Center for Climate Prediction and Research, in Bracknell-predicted that an Earth with twice the preindustrial level of carbon dioxide would warm by a devastating 5.2 degrees Celsius. Then Hadley Center modelers. led by John Mitchell, made two improvements to the model's clouds-how fast precipitation fell out of different cloud types and how sunlight and radiant heat interacted with

can be explained as natural variation," says Blackmon. That would make the detection of a modest-size greenhouse warming all the more difficult.

The CSM is available on the Internet, but Blackmon warns that if you want to check out future climate scenarios, you'll "need the biggest supercomputer you can get." Indeed, even NCAR researchers haven't been able to experiment with the model on as large a computer as they would like. While their purchase of an NEC SX4 computer is tied up in a trade dispute with Japan (Science, 30 August 1996, p. 1177), they are making do with a leased Cray C-90 with perhaps 20% of the speed of the SX4. That worries some modelers. Americans have "been among the leaders of the field from the beginning," says CSU's Randall, but “if we can't get access to the most powerful machines, we are going to be left behind." -R.A.K.

clouds. The model's response to a carbon
dioxide doubling dropped from 5.2°C to a
more modest 1.9°C.

Other models of the time also had a wide
range of sensitivities to carbon dioxide,
largely due to differences in the way their
clouds behaved. That range of sensitivity has
since narrowed, says modeler and cloud spe
cialist Robert Cess of the State University of
New York. Stony Brook, but "the [models]
may be agreeing now simply because they're

SOURCE NCAR

all tending to do the same thing wrong. It's not clear to me that we have clouds right by any stretch of the imagination."

Nor are clouds the only question mark in researchers' picture of how climate works. Modelers saw for the first time the fingerprint of global warming when they folded an additional process into the models: the effect of pollutant hazes on climate. Windblown soil and dust, particles from the combustion of fossil fuels, and ash and soot from

agricultural burning all reflect sunlightshading and cooling the surface beneath them. Including this aerosol effect in four independent climate models at three centers-Livermore, the Hadley Center, and the Max Planck Institute for Meteorology (MPI) in Hamburg, Germany-produced geographic patterns of temperature changes that resembled those observed in the real world over the past few decades, such as the greater warming of land than ocean.

Fingerprinting work since then has only strengthened the confidence of IPCC's more confident scientists that greenhouse warming has arrived. "I've worked with the models enough to know they're not perfect, but we keep getting the same answer," says Tim P. Barnett, a climatologist at the Scripps Institution of Oceanography in La Jolla, California, and a co-author of the IPCC chapter. Another climatologist and IPCC contributor, Gerald North of Texas A&M University in College Station, is similarly heartened. "I'm pretty optimistic about climate modeling.... I don't know anybody doing [fingerprinting] who is not finding the same result."

ing to Christopher Folland of the Hadley Center. Folland and his colleagues have been trying to sort out what was behind the intermittent warming of recent decades, which in the third quarter of the century was more rapid in the Southern than Northern Hemisphere. Earlier work by Santer and a dozen other colleagues showed an increasing resemblance between the observed pattern of warming through 1987, the end of their temperature record, and the results of a model run that incorporated aerosol effects. The researchers suggested that the North's more abundant pollutant aerosols could have been moderating the warming there from greenhouse gases. But when Folland

Crucial component. Thunderstorms like the one above help to shape climate by lofting heat and moisture.

But the assumptions about how hazes affect climate may have taken a hit recently from climatologist and modeler James Hansen of NASA's GISS-the man who told Congress in 1988 that he believed "with a high degree of confidence" that greenhouse warming had arrived. In a recent paper, Hansen and his GISS colleagues pointed out that recent measurements suggest that aerosols don't just cool; they also warm the atmosphere by absorbing sunlight. The net effect of this reflection and absorption, Hansen estimates, would be small-too small to have much effect on temperature.

Hansen and his colleagues conclude that aerosols probably do have a large effect on climate, but indirectly, through clouds. By increasing the number of droplets in a cloud, aerosols can amplify the reflectivity of clouds, and thus may have an overall cooling effect on the atmosphere. If true, this would greatly complicate the modelers' work, because meteorologists are only just starting to understand how efficiently particles of different sizes and compositions modify clouds. "I used to think of clouds as the Gordian knot of the problem." says cloud specialist V. Ramanathan of Scripps. "Now I think it's the aerosols. We are arguing about everything."

And the complications don't stop with the multiplication of aerosol effects, accord

compared the results of his model run with a longer, more recent temperature record, the resemblance that had been building into the 1980s faded by the early 1990s. Contrarian Patrick Michaels of the University of Virginia, Charlottesville, also has pointed out this trend.

The Hadley model suggests that "there appears to be more than one reason" for the variations, says Folland. The waning of aerosols as pollution controls took effect probably helped the North catch up, he says, but so did natural shifts in atmospheric circulation that tended to warm the continents (Science, 7 February, p. 754). Most provocatively, Folland and his colleagues are suggesting that a shift in North Atlantic Ocean circulation that has tended to warm the region also has contributed. "There's no doubt," says Santer, "that multiple natural and anthropogenic factors can contribute, and probably have, to the interhemispheric temperature contrast.... We've learned something about detection."

All of which only adds to the skepticism of scientists who might be called the "silent doubters": meteorologists and climate modelers who rarely give voice to their concerns and may not have participated even periph erally in the IPCC. "There really isn't a per

suasive case being made" for detection of greenhouse warming, argues Brian Farrell of Harvard University, who runs models to understand climate change in the geologic past. Farrell has no quarrel with the IPCC chapter on detecting greenhouse warming, but he says the executive summary did not "convey the real uncertainties the science has." He further contends that if IPCC scientists had had real confidence in their assertion that global warming had arrived, they would have stated with more precision how sensitive the climate system is to greenhouse gases. But the IPCC left the estimate of the warming from a doubling of carbon dioxide at 1.5°C to 4.5°C. where it has been for 20 years. "That's an admission that the error bars are as big as the signal," says Farrell.

Climate modeler Max Suarez of NASA's Goddard Space Flight Center in Greenbelt, Maryland, agrees that it's "iffy" whether the match between models and temperature records is close enough to justify saying that greenhouse warming is already under way. "Especially if you're trying to explain the very small [temperature] change we've seen already," he says, "I certainly wouldn't trust the models to that level of detail yet."

Rather than dwelling on model imperfections, IPCC co-author Barnett emphasizes some of the things that current models are doing fairly well-simulating present and past climates and the changing seasons, predicting El Niño a year ahead, and producing good simulations of decades-long climate variations. But he agrees that too much confidence has been read into the IPCC summary statement. "The next 10 years will tell; we're going to have to wait that long to really see." he says. Klaus Hasselmann of the MPI also sees a need to wait. He and his colleagues "think we can see the [greenhouse warming] signal now with 97% confidence." But, as North notes, "all that assumes you knew what you were doing to start with in building the models. Hasselmann has faith in his model but recognizes that his faith is not universally shared. "The signal is not so much above the noise that you can convince skeptics," he observes. "It will take another decade or so to work up out of the noise."

That's no excuse for complacency, many climate scientists say. Basic theory, this century's warming, and geologic climate records all suggest that increasing carbon dioxide will warm the planet. "I'd be surprised if that went away," says Suarez, as would most climate researchers. North suggests that while researchers are firming up the science, policy-makers could inaugurate "some cautious things" to moderate any warming. The last thing he and his colleagues want is a rash of headlines saying the threat is over.

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-Richard A. Kerr

I NEWS & COMMENT.

Apocalypse Not

Predictions that global warming will spark epidemics have little basis, say infectious-disease specialists, who argue that public health measures will inevitably outweigh effects of climate

The story of global climate change and disease is what a newspaper reporter would call great copy. It has dire predictions of pestilence and death, with the imprimatur of topnotch science. The plague is coming, and it's coming home to the developed world. The idea, as proposed by a handful of public health researchers, is that global warming and the attendant climatic extremes of floods and droughts, storms and heat waves, may play havoc with public health. As the heat rises, hundreds of thousands may die yearly from heat-related ailments, while disease vec tors and their pathogens may be redistributed far and wide with apocalyptic results. "To the laymen," wrote Harvard physician Paul Epstein in The Washington Post, "it means a global spread of infections."

Epstein and others have predicted that deaths from malaria may increase by a million a year, that malaria, dengue, and yellow fever may move north into the United States and Europe; that cholera epidemics may intensify; and that emerging diseases such as hantavirus and Ebola may run rampant Already, they say, bursts of warming from she:t-lived climate shifts like the Pacific warming called El Niño may have triggered disease outbreaks that offer an ominous preview of what is to come. If it is not the beginning of the end, it has certainly read like it: "Global Fever" was just one of the headlines, from the 8 July 1996 Time. It continued: "Climate change threatens more than megastorms, floods and droughts. The real peril may be disease." Even Science was suitably concemed: "If the Mercury Soars, So May Health Hazards" (17 February 1995, p. 957).

The salient word in all these stories, how ever, was "may." These predictions are getting renewed attention with the approach of the December climate change summit in Kyoto, Japan, and leading infectious-disease experts have taken to criticizing them sharply. Duane Gubler, for instance, director of the division of vector-bome infectious diseases at the Centers for Disease Control and Prevention (CDC), calls the prognostications "gloom and doom" speculations based on "soft data." Johns Hopkins epidemiologist D. A. Henderson, who led the international smallpox eradication program from 1966 to 1977, says they are based on "a lot of simplistic thinking, which seems to ignore the fact that as climate changes, man changes as well." Henderson, Gubler, and oth

ers argue that breakdowns in public health rather than climate shifts are to blame for the recent disease outbreaks-and that public health measures will be far more important than climate in future disease patterns.

Many of the researchers behind the dire predictions concede that the scenarios are speculative. But they say their projections play a useful role in consciousness raising. "What it does is serve notice on us; we need to be aware we're tinkering with fundamentals, and there could be a range of consequences for human health," says Anthony McMichael of the London School of Hygiene and Tropical Medicine.

Report Says Global Warming
Poses Threat to Public Health

GLOBAL FEVER

Climate change threatens more than megastorms, floods and droughts. The real peril may be disease

Shope, then director of the Yale Arbovirus Research Unit, pointed out in Environmental Health Perspectives that with rising heat, the Aedes aegypa mosquito, which transmits dengue fever and yellow fever, might move northward, while the life cycles of the mosquito and the virus might accelerate, which "could lead to epidemics in North America." Cholera could also become epidemic in North America, Shope said, as changes in marine ecology favor the growth and transmission of the pathogen, which is "harbored persistently in the estuaries of the U.S. Gulf Coast." In 1992, microbiologist Rita Colwell of the University of Maryland, College Park, with Epstein and Harvard biologist Timothy Ford, took that idea further. They suggested in a Lancet article that an El Niño warming of the tropical Pacific was at least partially responsible for a 1991 cholera epidemic in Latin America that affected a half-million people and killed nearly 5000. Over the next 3 years, the tide of concern rose inexorably. In January 1996, the predictions erupted into the press when The Journal of the American Medical Association published a paper by Epstein, Jonathan Patz, an expert in occupational and environmental medicine at Johns Hopkins, and collaborators, speculating about the effects of a 4degree warming over the next century on a range of public health threats from malaria and arboviral encephalitis to cholera and toxic algae.

Should We Fear a Global Plague?

Yes-Disease is the Deadliest Thomat of Rising Temperatures

The increasingly heated tone of the debate has prompted the CDC and the National Research Council (NRC) to begin putting together an expert panel that will try to set the discussion on a footing of solid science. Says Henderson, who has been chosen as a co-chair, "What's worrying is the question of credibility when sweeping predictions like these are being made." The panel will also set an agenda for further research, he says: "Fundamentally, we would like to have a better understanding of the transmission of disease under different circumstances of temperature and climate, not necessarily because of global warming, but because it can be of value to us whenever we have climate fluctuations."

Heat or light?

The current controversy has been building for at least 6 years, since climatologists began agreeing that the planet's temperature is rising (although they still do not agree on the cause of the warming to date, or on how much warmer the planet will get). In 1991, virologist Robert

Global

The last step toward turning these speculations into what Gubler calls "gospel" came last year, when the United Nations' Intergovermental Panel on Climate Change (IPCC), which is meant to offer scientists' consensus voice on climate change and its effects, included a chapter on public health impacts in an update of its landmark 1990 assessment. The public health chapter, written by a team led by McMichael and including Patz and Epstein, concluded that "climate change is likely to have wide-ranging and mostly adverse impacts on human health, with significant loss of life."

The growing official acceptance of these predictions has irritated some other public health experts. "What I find astounding," says epidemiologist Mark L. Wilson of the University of Michigan, Ann Arbor, "is how little

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