Global Warming Debate Centers On Uncertainty

The Kyoto Accord to reduce "greenhouse" gas emissions faces a hazy future in the United States. A host of unresolved political and scientific issues swirls around the tentative plan to curb global warming-including the degree to which greenhouse gases actually affect the environment. Kyoto sets the stage for science, politics, and public opinion to interact on the largest possible scale, and, some would say, at the highest possible stakes. Science's pivotal role is to take on a formidable chall

Jan 5, 1998
Paul Smaglik

The Kyoto Accord to reduce "greenhouse" gas emissions faces a hazy future in the United States. A host of unresolved political and scientific issues swirls around the tentative plan to curb global warming-including the degree to which greenhouse gases actually affect the environment. Kyoto sets the stage for science, politics, and public opinion to interact on the largest possible scale, and, some would say, at the highest possible stakes. Science's pivotal role is to take on a formidable challenge-to convince both Congress and the public to reduce CO2 emissions, by providing a more precise measure of the impact increased amounts of CO2 will have on the planet and how, exactly, that increase will affect life.

The accord, hammered out by 38 industrialized nations at the United Nations climate conference in Kyoto, Japan, December 1-10, would allow the amount of CO2 in the atmosphere to double over preindustrial levels. Not passing it, or not adhering to it, would allow levels to increase much more quickly, to as high as four times preindustrial levels in less than 100 years, government climatologists say. The accord asks the U.S. to reduce emissions 5.2 percent below a 1990 benchmark; however, actual emissions have crept 10 percent above the 1990 level and are expected to climb even higher.


DOUBLE TROUBLE: A global climate model from NOAA's Geophysical Fluid Dynamics Lab shows that the effects of doubling CO2-the goal of the Kyoto Accord-still results in serious climate change.
Many scientists are sure of one thing-that increased CO2 will warm the Earth's climate. "We know that human activity has changed the environment-that's undeniable," asserts Benjamin D. Santer, an atmospheric scientist at Lawrence Livermore National Laboratory in California. Santer is the convening lead author of a chapter of the 1995 Report by the Intergovernmental Panel on Climate Change (IPCC) acknowledging that human activity contributes to global warming. "It's not whether, but how much," he says. Most models put the extent of doubling CO2 levels from preindustrial concentrations of 270 parts per million at warming the planet 3°F to 8°F.

Critics call that range the result of an imprecise science, while Santer and other climatologists say the range represents some of the inevitable uncertainties of modeling such a large system with many variables. Better science may produce a more precise range-something that Congress may demand before committing to the sacrifices adopting the Kyoto Accord represents. "There's some hope of narrowing it down," Santer says. Better science may also uncover new uncertainties that expand the range.


NESTED INTEREST: Penn State's Eric Barron says nested models-which provide more detail-may help make regional climate predictions possible.
The long-term solution demands better, more complex models. But short-term answers exist, too, Santer believes. For example, climatologists have begun comparing data and outcomes from different models. Another approach may be to better explain to Congress and the public that they must accept a range of possibilities rather than a specific outcome. "Uncertainty is a factor of life when it comes to climate modeling," Santer comments.

Global climate models don't provide enough detail, some critics say, since each grid point encompasses an area the size of West Virginia. Regional modelers are answering that criticism by dividing selected areas into data points whose boundaries can be measured in kilometers, then "nesting" that model back into the global one.

The smaller squares allow for more detail in factors that tend to vary over a large area, according to Eric J. Barron, director of Pennsylvania State University's Earth System Science Center. He heads Penn State's nested modeling project, the Susquehanna River Basin Experiment (SRBEX), which details the basin from central Pennsylvania to Washington, D.C. "Precipitation varies dramatically over a region," Barron says. Models that use a range of precipitation figures rather than one average number paint a more accurate regional picture, explains Barron, who with colleagues will discuss SRBEX at the American Association for the Advancement of Science's annual meeting in Philadelphia February 17.

Global modeling experts agree nested models look promising, but they also say the models are far from perfect. "In order to do regional modeling, you either have to have a lot of computing power or have the guts to make some fudges," contends Jerry D. Mahlman, a climate scientist at the National Oceanic and Atmospheric Administration's (NOAA's) Geophysical Fluid Dynamics Laboratory at Princeton University. Mahlman agrees that policymakers need nested models to help make predictions about how global warming will specifically affect people.

Benjamin D. Santer, a climatologist at Lawrence Livermore National Laboratory in California, doesn't think the models are sophisticated enough yet to do so. "We're quite some way from detailing regional predictions." However, he notes that developing nested models may someday provide the necessary detail.

The main problem with nested models is the way they do or don't interact with the global models of which they are a part, asserts David Rind, a climate research scientist with the National Aeronautics and Space Administration's (NASA's) Goddard Institute for Space Studies in New York. That problem becomes exacerbated when dealing with elements not contained within the nested area, like air flow. "The fine-grid model must always be limited by the coarse grids coming in," says Rind. "They're more precise, not more accurate."

-P.S.

Despite all the uncertainties that critics talk about, climate models do many things quite well, explains Jerry D. Mahlman, a climate scientist at the National Oceanic and Atmospheric Administration's (NOAA's) Geophysical Fluid Dynamics Laboratory at Princeton University. Mahlman evaluated the things models do and don't do well in a recent essay (J.D. Mahlman, Science, 278:1416-7, 1997). "Climate models do a reasonably good job of capturing the essence of large-scale aspects of the current climate and its considerable natural variability on time scales ranging from one day to decades," he wrote.

If one accepts his conclusions, then the Kyoto Accord represents, at best, "a sigh of resignation," Mahlman says. "We've accepted a double CO2 limit as the best we can achieve. A doubling of CO2 is not a small perturbation of the planet." Mahlman says most climate scientists do accept the models, with an understanding of their limitations, but the public sees a field filled with dissent, since the media tend to emphasize conflict. For example, Richard S. Linden, a physics professor at the Massachusetts Institute of Technology who did not return phone calls from The Scientist, has been widely quoted as saying that the Earth's temperature will only increase slightly from a doubling of CO2, since climate scientists overestimate the effects of increased water vapor as the Earth warms.

That opinion, Mahlman contends, represents a minority view with little evidence to support it, and that uncertainty is represented in the models' predictions. However, models aren't perfect. "If you want to fuzz up the problem, hang onto clouds," Mahlman says. Representing clouds' effects on global warming may prove to be one of climate scientists' biggest challenges. The problem boils down to a question of scale. The physics of clouds is small and diverse. However, each cell in a global warming model must, by necessity, be large and general. Those cells can contain broad measures like overall temperature, moisture, and CO2 concentration. Cloud activity, which is, in essence, diverse and fleeting, eludes easy representation.

Clouds are problematic, agrees David Rind, a climate research scientist with the National Aeronautics and Space Administration's (NASA's) Goddard Institute for Space Studies in New York. He also acknowledges that models don't yet include every contributing factor to climate change, but he thinks they do capture the main ones. Models can be dissected into their basic components, all of which are mathematical representations of physical properties. "All the models are doing is giving you quantifications of things that are physically evident anyway.

"We know CO2 absorbs energy and we know how much it absorbs," Rind says. The radiative effect of the doubled CO2 alone will force the global average up 1°C to 1.5°C. That warming results in the slow heating of the ocean, which eventually puts water vapor into the air. This effect, which has some critics, including Linden, accounts for about another 1°C of heating, Rind estimates. "Water vapor is a greenhouse gas. As air gets warmer, it can hold more water vapor." The cumulative effects of those processes will also melt snow and ice at the poles, where most climate experts say global warming will be most pronounced. Sea ice and snow cover act as giant mirrors, reflecting heat back into atmosphere. As the planet warms, there will be less of a mirror, meaning more heat will stay, resulting in about another 0.4°C of warming.

Finally, existing models tend to lose low-level clouds and gain high-level clouds. Low-level clouds are modeled like snow and ice, mathematically reflecting heat away from the planet's surface. "Could the clouds be wrong in either direction? Absolutely," Rind posits, adding that even if clouds help cool the planet by a degree rather than warm it, the net results of all the major processes point to a warmer planet.


MODEL CONFIDENCE: NASA's David Rind says climate models do a good job capturing the main factors contributing to global warming.
So far, models have matched up well with the history when scientists plug in earlier known values. How well they measure the future will remain a mystery until the future arrives. "We do not have any way of evaluating how good the model is once you force it strongly," Rind says. However, he has enough confidence in models to assert that even a doubling of CO2 could cause serious climatic problems. One thing that may be difficult to convey to Congress and the public is how much of a challenge reversing the warming trend poses. Rind again points to basic physics, comparing a slowly warming global system to a pot of water gradually heating on a burner. Simply turning off the heat doesn't immediately cool the water. The bigger the pot of water, the longer it holds heat. And the Earth is a pretty big pot of water, Rind says.

However, he acknowledges models aren't yet sophisticated enough to definitively answer important regional questions. For example, theoretically, the poles should warm more than equatorial regions, a crucial factor because most of the Earth's population resides 30° north and south of the equator. "How much the tropics warm up is a big deal," Rind says. History hasn't been much help here, either. Climatologists test the efficacy of models against recent history, but scholars have yet to agree on how the equatorial climate differed from the rest of the planet during the Ice Age, so climatologists lack a sound benchmark.


SOUND MODELS: NOAA's Jerry Mahlman says global climate models do an adequate job of predicting the effects of increased CO2.
Getting better regional information will help countries adapt to changes, says Michael MacCracken, director of the U.S. Global Change Research Program in Washington, D.C. "Impacts and consequences are felt regionally and locally." However, MacCracken agrees that current models aren't sensitive enough to regional factors. "We don't have detailed regional predictions," he acknowledges. "That doesn't mean we don't know that the regions will change." Some climatologists are developing regional models tied to, or "nested" within, global models that show promise but must also overcome some problems (see accompanying story).

MacCracken's office sponsored a national forum in November to address regional concerns and to promote better regional research. The two are inextricably linked, he says. For example, predicting how arid agricultural areas like Kansas and Oklahoma will be affected by warming or whether to rebuild Fargo, N.D., farther from the flood plain depends on an accurate understanding of how the climate will change in those areas over time. Right now, scientists and planners have only rough ideas. "The better evidence you can provide, the less caution you have to put into your decisions," MacCracken contends.

Some of the answers may come down to adding more components to global models and adding more details to existing components, which often paint pictures in broad strokes. Much recently published research seems to be heading in those directions. For example, a study by Bob H. Braswell, an ecologist at the University of New Hampshire, adds the effects of different kinds of vegetation into the global mix (B.H. Braswell et al., Science, 278:870-2, 1997). The study concludes that plant growth tends to increase in polar and temperate regions and decrease at lower latitudes after a "lag" of two or three years following an unusually warm year, Braswell says. This could be important to modelers, because plants play a key role in CO2 uptake. "The climate-modeling community seems to be moving toward terrestrial components, and our study seems to underscore their importance."

Another recent study takes a closer look at ocean currents, which are represented only in general terms in global models. Wallace S. Broecker, a climatologist at the Lamont-Doherty Earth Observatory at Columbia University, calls an interconnected system of ocean currents "the Achilles heel" of the global climate system (W.S. Broecker, Science, 278:1582-8, 1997). According to Broecker, that current governs regional temperatures throughout the globe. Although the effect hasn't been predicted in global models, Broecker forecasts that disrupting those currents could result in drastic climate change.

Rind and many colleagues agree that even the best models and the most dire predictions won't be enough to convince people to make sacrifices, like reducing their use of fossil fuels, to help solve a problem that seems distant. "People will not accept a reduced standard of living," Rind asserts. The problem with that approach, he says, is by the time uncertainty turns to certainty, the global warming will be difficult to halt, much less reverse.