John Fialka: Global Warming Report May Understate Effects

WASHINGTON -- U.S. government scientists Friday said the long-term outlook for global warming may be more dire than suggested by this week's United Nations' report, which they say doesn't fully address the impact of clouds and melting glaciers.

Recent evidence of accelerated melting of glaciers in Greenland and the Antarctic ice cap came too late to be included in the report released Thursday by the U.N.-sponsored Intergovernmental Panel on Climate Change.

Glaciers are among the largest sources of fresh water in the world and are contributing to rising ocean levels. Rising sea levels could expose population centers bordering the ocean to more storm damage and could require evacuation in some areas. But the computer models used for the IPCC report based their predictions only on the results of heating of the existing water in the world's oceans, causing the oceans to expand and sea levels to rise, said Tom Delworth, a climate modeler for the National Oceanic and Atmospheric Administration, the government agency in charge of climate science and weather service.

The IPCC report predicts sea levels will rise by between one to two feet over the next 100 years. Mr. Delworth said there remains "much more uncertainty" over how much accelerated melting of glaciers might add to that.

A second area of continuing uncertainty has to do with the impact of clouds on climate change. Warming the ocean sends more water vapor into the air, and the resulting clouds accelerate global warming by trapping more of the sun's heat in the atmosphere and further warm the ocean. Jim Butler, deputy director of NOAA's global monitoring division, called this "a very scary feedback mechanism."

But, so far, the supercomputers the agency uses to model the effect on the earth's climate -- which were also used for the IPCC report -- aren't detailed or fast enough to predict how much clouds are accelerating the problem. Mr. Delworth said computer models divide the earth's oceans and atmosphere into four million boxes, each about 150 square miles, and that these boxes are too large to model the effects of clouds.

"We could use computers that are one million times faster than they are today and still not be satisfied," Mr. Delworth said.

Further complicating the issue are layers of haze containing pollutants from human activity. Such pollutants, including sulfates, soot, dust and nitrates, tend to make the atmosphere brighter, reflecting more of the sun's heat back into space. The IPCC has found that the net effect of the added pollution is to cool the atmosphere.

A.R. Ravishankara, an atmospheric chemist for NOAA, said this raises a problem for governments attempting to clean the air by removing pollutants. "If you take away this cooling effect, then the heating effect would be exacerbated. It's a highly complex problem."