New evidence found by teams of climate researchers leaves no doubt that industrial emissions of greenhouse gases are responsible for increasing global temperatures -- an ominous trend that has speeded up in the past 50 years and threatens to continue for centuries, according to a report by two of the nation's leading atmospheric scientists.

The two government experts said climate change "may prove to be humanity's greatest challenge" and warned that "it is very unlikely to be adequately addressed without greatly improved international cooperation and action."

The two government experts said climate change "may prove to be humanity's greatest challenge" and warned that "it is very unlikely to be adequately addressed without greatly improved international cooperation and action."...Their published comments reflected the growing concerns of most climate experts over the White House stance.

Thomas Karl, a meteorologist at the National Climatic Data Center in Asheville, N.C., and Kevin Trenberth, chief of the climate analysis section at the National Center for Atmospheric Research in Boulder, Colo., are publishing their analysis in Friday's edition of the journal Science.

Neither scientist criticized the Bush administration's refusal to ratify the Kyoto treaty, which is designed to regulate emissions of heat-trapping greenhouse gases worldwide. But their published comments reflected the growing concerns of most climate experts over the White House stance.

The two disagreed with assertions by some scientists that swings in worldwide temperatures over the years are normal and natural. "Modern climate change is dominated by human influences, which are now large enough to exceed the bounds of natural variability," they said.

Karl and Trenberth also agreed that many uncertainties remained about how swiftly global temperatures are rising, how much they are likely to rise and how long ago the problem began.

However, "there is no doubt," they say, "that the composition of the atmosphere is changing because of human activities, and today greenhouse gases are the largest human influence on global climate.''

They estimate that by the end of this century there is a 90 percent chance that the world's climate will heat up between 3.1 and 8.9 degrees Fahrenheit because of those human influences.

Among the consequences, they predict, are more frequent heat waves, more widespread droughts in some parts of the world and "extreme precipitation events" in others.

They also predict more wildfires, abrupt changes in vegetation and continued melting of glaciers and of the great Greenland Ice Sheet, causing floods along many continental coastlines.

Additionally, as snow cover melts on land and icebergs shrink at sea, both the darker ground and the darker ocean surfaces will be exposed to solar radiation, increasing temperatures even more, the climate forecasters say.

While some climate analysts have noted that the vast quantities of soot emitted by many industries and volcanic eruptions can actually cool the atmosphere, that kind of cooling can last only a few years, with little or no effect on the long-range trend, the scientists say.

The report by Karl and Trenberth adds new data to scores of previous international studies and computer models of future climate changes as well as analyses by the Intergovernmental Panel on Climate Change.

The new conclusions met with some disagreement Wednesday from James Mahoney, a noted meteorologist and the Bush administration's assistant secretary of commerce for oceans and atmosphere, who directs all the government's weather and climate research and forecasting agencies.

In a telephone interview, Mahoney agreed that climate change is indeed a global problem that "has no political boundaries." He noted that the United States has a large delegation of experts attending an international conference on climate change in Milan right now.

Mahoney also insisted that the United States under President Bush had developed a "substantial involvement" in international activities aimed at researching the problems of global warming and at resolving their uncertainties.

But he took issue with Karl's and Trenberth's insistence that there's clear evidence that human activity far outstrips natural variation as the main cause of global warming.

"That's their assertion," Mahoney said. "They are extremely competent, and there are many in the climate community who would agree with them. That's not surprising, but there are many others who would disagree with them. My own view is somewhat more open-minded, and from my perspective we don't really understand these things as well as we might.''

No one disputes that there has been a sharp rise in carbon dioxide levels in the atmosphere during the past decades, Mahoney said, "but there remains disagreement about just how severe its impact has been."

As to the grim future that Karl and Trenberth see as a result of global warming, "I do challenge them on that," Mahoney said, "because all future projections are based on many, many models of how the atmosphere behaves, and I think a number of skeptical scientists would also challenge them."

©2003 San Francisco Chronicle

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