September 9, 1997

Experts on Climate Change Ponder: How Urgent Is It?

By WILLIAM K. STEVENS

ith the nations of the world counting down to a Dec. 10 deadline for negotiating cuts in emissions of heat-trapping greenhouse gases that many scientists say are altering the earth's climate, a fundamental question continues to pervade the debate:

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Just how urgent is the problem of climate change?

The problem has often been perceived as lying off in the dim future - a century away, say, since that is the time scale on which most analysis of the question has been based. It can be very hard to get excited about something that seems so remote. "We see the train coming," President Clinton said not long ago, "but most ordinary Americans in their day-to-day lives can't hear the whistle blowing."

Mainstream scientists say that the whistle is getting louder all the time, that many signs of a changing climate are already evident. On the night of Aug. 20 and 21, for instance, a cloudburst poured up to nine inches of rain on New Jersey in three hours. No conclusions about climate change can be drawn from a single downpour. But it provided one more data point in what government scientists say is a clear trend in the United States: heavy rainstorms have become more common, making damaging floods more likely, and over all it is raining more than it used to.

Other parts of the world have also experienced increases in precipitation in this century. And while the trend's association with global warming is not yet firmly nailed down, mainstream scientists say it is precisely, and unsurprisingly, what would be expected as the earth's atmosphere heats up.

The average surface temperature of the globe has risen by about 1 degree Fahrenheit in the 20th century, and about half a degree in the last 40 years - with temperatures generally rising more the farther away one gets from the Equator. A warmer atmosphere sucks up more water from the oceans and in general makes more moisture available to developing storms. Observations show that atmospheric water vapor has increased. Therefore, the scientists say, global warming may well be revving up the planet's rainmaking machine.

A rainier world is just one of several climatic changes already observed that are associated with the warming, scientists say. Temperatures have risen more at night than in the day, portending milder winter evenings but also more deadly heat waves. Experts, for instance, attributed the record death toll in Chicago's 1995 heat wave mainly to a combination of unusually high nighttime temperature and humidity.

The area of the Northern Hemisphere covered by snow has generally declined in the last 25 years. Mountain glaciers have receded in many areas. Sea level has risen as warmth has caused the water to expand. Spring comes earlier and fall arrives later in Northern latitudes, which have become about 10 percent greener.

"The climate reality is that if you look out your window, part of what you see in terms of the weather is produced by ourselves," said Thomas R. Karl, senior scientist at the National Climatic Data Center in Asheville, N.C. "If you look out the window 50 years from now, we're going to be responsible for more of it."

Karl has not been known as a doomsayer on the question of global warming. It was he and his colleagues who analyzed 20th-century climatic data for the United States and southern Canada and found that both overall precipitation and the frequency of intense rainstorms and snowstorms had, on average, increased by 10 percent over the century. Intense storms were defined as those in the top 10 percent, in terms of the amount of precipitation delivered, at the start of the century.

The trend, said Karl, is consistent with what would be expected in a world made warmer by greenhouse gases - principally carbon dioxide, which is produced by the burning of coal, oil and natural gas.

This is only the beginning, if a panel of more than 2,000 scientists advising the world's governments is right. While no one is predicting apocalypse any time soon, the scientists' best estimate is that the world will warm by more than another half degree in the next 20 years if greenhouse emissions are not reduced (although the underlying trend would surely be modified from time to time by natural climatic variations). With another half degree of warming, the average global temperature would approach the highest experienced in the 10,000 years since the last ice age, said Dr. Thomas J. Crowley, a paleoclimatologist at Texas A&M University.

A century from now, according to the scientific panel's best estimate, the earth's surface will be some 3.5 degrees warmer than in 1990. "You have to go back millions of years to find global temperatures like that," said Dr. Crowley. By way of comparison, it was 5 to 9 degrees colder in the depths of the last ice age.

The panel predicted that the warming would bring widespread climatic, environmental and economic dislocation. Some benefits are expected to result, including longer growing seasons in Northern Hemisphere breadbaskets and more robust plant growth stimulated by increased carbon dioxide. But many effects would be adverse, including intensified cycling of water through the ocean-atmosphere system that would result in both more frequent and severe floods and droughts; it would rain harder but things would dry out more and faster between storms. Sea level is expected to rise by about a foot and a half, inundating many low-lying coastlines. Climatic zones would shift away from the Equator, upsetting natural ecosystems. Entire forest types might disappear. Agriculture in some parts of the world, especially Africa, might be devastated. The geographical range of tropical diseases would expand.

How much to do now is the crux of the discussions that will culminate in Kyoto. The decision is complicated by the fact that while the intergovernmental panel estimated a 3.5-degree increase in average global temperature by 2100, it also said the increase could be as low as 2 degrees and as high as 6.

This poses a dilemma, in the view of Dr. Michael E. Schlesinger, a climatologist at the University of Illinois at Urbana-Champaign: If the warming is on the low end of the panel's estimate and the Kyoto delegates decide to do too much, it will be too costly to the world economy. But if it is on the high end and they decide to do too little, climatic damage will be too high and it will still be too costly.

What should governments do? "We should not do nothing and hope for the best; that's crazy," Schlesinger said. "On the other hand, we shouldn't do everything, either; we don't want to squander resources on something that might turn out to be a nonproblem." He and others call for an adaptive strategy in which at least some action is taken now, and then both the climatic effects and the economic cost of acting are periodically reassessed. Mid-course corrections would be made as needed. This principle is, in fact, built into the 1992 Rio treaty on climate change, under which the present negotiations are proceeding.

But then a further difficulty arises, in Schlesinger's view. Since the effects of any emissions reductions undertaken now would not be apparent in the next two decades, it would be impossible to tell for some time whether the right thing was done. And if the reductions did subsequently succeed in mitigating the warming, that very fact would make it harder to know how serious the warming would otherwise have been, and therefore what further action might or might not be needed.

"That," said Schlesinger, "is what's so difficult about this cockamamie problem."