Global Warming Forecasts Tempered : Environment: Dire predictions are being toned down and, some scientists say, not all the changes will be bad.
The most dire predictions about global warming are being toned down by many experts, who now predict that temperatures and sea levels in the next century will not rise as dramatically as once feared.
While climate researchers tend to agree that our grandchildren will inherit a warmer world, many scientists are now betting on an average warming of only three or four degrees Fahrenheit by the middle of the next century, the low end of a range that prognosticators once put as high as nine degrees.
A rise in sea level, one of the most feared consequences of global warming, also is expected to be far less drastic than previously thought, largely because of the surprising observation that the polar ice sheets are not melting, but growing in size. According to Mark Meier of the Institute of Arctic and Alpine Research at the University of Colorado, the consensus now is that sea level will rise by about a foot in the next century, rather than the three feet that had been suggested in previous studies.
But even if the revised forecast is correct, there would be major effects, the scientists warn. A one-foot rise in sea level would inundate some shorelines, moving the water’s edge inland by hundreds or thousands of feet. And a global warming of only a few degrees might upset communities of plants and animals that have developed over millennia. Weather patterns also could change, meaning more frequent droughts and severe storms.
Yet along with the new forecasts are indications that not all changes will be bad. Some researchers have begun to stress that a warmer, wetter world might be welcome in some regions.
“It is an interesting point that people ought to think about more. By and large, there are positive effects. More wheat in the Ukraine. Thinner ice in the Arctic. Better winters in Moscow. But at the same time, you’re building up a bigger time bomb that is becoming harder and harder to dismantle,” said John Perry, staff director of the Board of Atmospheric Sciences and Climate at the National Research Council.
Perry points to a recent study by the climate office of National Oceanic and Atmospheric Administration that concluded that a slightly warmer, wetter world would increase food production, enhance forest growth and enlarge water supplies. An obvious drawback was that billions of dollars would have to be spent protecting coastal areas from rising seas.
The great uncertainty in predicting global warming and its effects is making life difficult for government policy-makers. The computer models upon which all forecasts are based are generated by a small group of scientists who acknowledge that they do not understand or even recognize all the variables that affect their predictions, thus compounding policy uncertainties.
“The things we can say with confidence, the policy-makers are not interested in. And the things the policy-makers are interested in, we don’t know with much confidence,” said Jerry Mahlman, director of NOAA’s Geophysical Fluid Dynamics Laboratory at Princeton University.
“Modelers have been thrust into the limelight way too soon,” said Richard Cooper, an economist at Harvard University. “They don’t have models today you can build public policy on.”
The frustration was evident at a meeting Cooper and Mahlman attended recently at the National Academy of Sciences, where an expert panel was grappling with the policy implications of global warming. After a long afternoon, during which the invited scientists displayed graphs with past and future temperatures bouncing up and down like rubber balls, they were finally begged to put aside the carefully hedged forecasts and simply make a “best guess.” While the ranges of guesses varied, the favorite seemed to be a four-degree increase by the middle of the next century, when concentrations of carbon dioxide gas--which contributes to the “greenhouse effect”--are expected to double.
One reason many researchers are shying away from the worst-case scenarios so popular a year ago has to do with the climate record itself. While researchers argue that the oldest records are highly suspect, in general they agree that temperatures have risen over the past 100 years by about a degree. But there are tremendous arguments over whether or not this increase is due to an accumulation of greenhouse gases such as carbon dioxide and methane, which trap heat close to Earth. Instead it may be the result of some other climatic cycle unrelated to human activity. But whatever its cause, the temperature record is more consistent with a rise of four degrees, rather than nine degrees, by the middle of the next century.
“The new runs do favor the lower estimate,” said Warren Washington of the National Center for Atmospheric Research, another top climate modeler.
Two of the most recent climate simulations, done by the National Center for Atmospheric Research in Boulder, Colo., and the Geophysical Fluid Dynamics Lab at Princeton, suggest slightly lower temperature increases, according to the scientists who run them.
Both models showed uneven warming in the Northern and Southern hemispheres. In one run, the region around the Southern Ocean that surrounds Antarctica did not warm at all. The model showed it cooling, thanks to improved simulations of how the ocean currents circulate and mix. Another world-class model, run by the United Kingdom Meteorological Office, cuts temperature rises in half when the researchers change the way raindrops are represented.
Of course, not all the climate modelers agree with the lower estimates. One is James Hansen of National Aeronautics and Space Administration’s Goddard Institute for Space Studies in New York. Hansen said he thinks his model and the observed temperature record are still in keeping with a greater future increase in temperatures. At the meeting at the National Academy of Sciences, Hansen stated again that he thinks warming associated with increased greenhouse gases has already begun and that the Earth will warm by more than four degrees on average in the next century.
All the modelers stress how uncertain the forecasts are. “People are hedging,” Washington said. “You get put into a defensive position, because everyone wants the lower estimates.”
Given all the uncertainties, what is known for sure? It is agreed that carbon dioxide has increased over the last 100 years. Since 1958, when accurate surveys were begun, carbon dioxide has increased from 316 parts per million parts of air to 353 parts per million today, an 11% increase. This accumulation is attributed primarily to the burning of fossil fuels, of which the United States is the largest user.
The basic phenomena of the greenhouse effect are also agreed upon. It has been operating for millions of years and is one reason Earth is as warm as it is. In the last few months, the greenhouse effect was actually measured for the first time by University of Chicago researchers who used satellites to measure radiation coming into the atmosphere and bouncing back out to space.
So it is almost universally agreed that Earth probably will warm as carbon dioxide and other greenhouse gases accumulate. How much it warms is the question.
Richard Lindzen, one of the most outspoken critics of current projections, conceded at the recent meeting that temperatures probably will rise, but not by much. The Massachusetts Institute of Technology professor said he thinks the planet will respond to the buildup of greenhouse gases by producing “negative feedbacks,” such as high, dry clouds, that will lift warmed air from the ground toward the upper atmosphere where its heat will radiate into space, counteracting a warming trend.
But researchers tend to think that while Hansen’s projections may be too hot, Lindzen’s are definitely too cold.
Researchers say it may take another decade or more to settle the issues. At present, the most common response by government officials and scientists has been a call for more research and a push to reduce emissions of carbon dioxide by increasing energy efficiency--a painless remedy that would provide benefits regardless of future temperatures.
