From the tropics to the poles, evidence is growing stronger than ever that Earth's climate is warming dangerously.

In the Arctic Ocean, floating masses of sea ice are shrinking and splitting apart, and the massive Greenland ice cap melted more this past summer than ever before. Meanwhile, warming ocean temperatures are endangering coral reefs in the tropics.

At the annual fall meeting of the American Geophysical Union in San Francisco earlier this month, a flurry of new reports examining evidence of global climate change all tell the same story.

If the trends continue unchecked, scientists say, rising sea levels will drown coastlines. Droughts in some regions -- and increased rainfall in others -- will alter harvests drastically. And other climate disruptions will destabilize regional ecologies and global economies.

Some of these alarming phenomena may be due to the natural climate variability that the planet has seen over millions of years. But most scientists agree, after years of debate, that humans and their addiction to fossil fuels are at least partly to blame.

"It is humans who are clearly forcing the abrupt climate change we see right now," said Richard B. Alley of Pennsylvania State University, who recently chaired a National Research Council committee looking specifically at climate change.

So-called greenhouse gases trap the sun's radiation much the way glass windows trap heat inside a home or a greenhouse. The most powerful of those gases is carbon dioxide, which comes primarily from burning fossil fuel, while other gases include methane, sulfur dioxide and ozone.

THE BUSH REPORT

A recent NRC report, which the Bush administration requested last year when scientists criticized the White House for its slow response to growing evidence of global warming, concluded that "human-induced warming" will continue through the 21st century.

While it conceded great uncertainties in the many models of climate trends that experts have produced, the report predicted that the planet's climate would warm by 2.5 to 10.4 degrees Fahrenheit by the century's end due to human activity.

Signs of the striking pace of that trend came in reports from many scientists who monitor the ice of the globe's far north.

The Arctic's sea ice -- large masses of snow-covered ice that float everywhere around the polar latitudes -- usually covers 2.4 million square miles of the ocean north of Canada, Greenland and Russia in September, the height of the ice season.

This past summer, however, measurements showed that the sea ice had decreased by nearly a half-million square miles. The flat ice floes left wider sections of open water between them and became extremely thin in many areas, reported Ted Scambos of the National Snow and Ice Data Center in Boulder, Colo.

It marked the most abrupt change in the ocean's ice cover that scientists monitoring the region have seen in 24 years, said Mark Serreze of the data center. Records kept by Icelandic fishermen indicate the cover may not have been so low for centuries.

"I was really surprised by the change," Serreze said. "This was the craziest summer season I've ever seen up there."

MELTING FASTER THAN EVER

Equally ominous was a report by Konrad Steffen, a glaciologist at the University of Colorado, on Greenland's vast ice cover, second only in size to Antarctica. It was melting faster this year across nearly 265,000 square miles than at any period in recorded history, Steffen said.

The ice sheet is a mile and a half thick in some places. As meltwater from the surface seeps through crevices in the ice, it loosens the edges of the sheet and causes the ice to flow more swiftly to the sea, where it breaks off into icebergs.

If the entire Greenland ice sheet were to completely melt -- admittedly an unlikely event, at least in the near future -- then scientists calculate that sea levels would rise by a globally disastrous 23 feet.

Steffen had a firsthand experience of the dangers of melting ice. He and his colleagues were camped on the normally hard-frozen Greenland ice last June when their camp and equipment were flooded under a foot of meltwater and they had to be rescued by helicopter.

The high Arctic is by no means the only part of the world where climate change is becoming more dramatic. Scientists are equally concerned about the impact of changes on tropical oceans.

KILLING CORAL

Coral reefs are living creatures. As they die, their calcite skeletons build up the reefs over millions of years. They are a crucial part of the world's marine ecosystems, vital to the productivity of many tropical fisheries.

Most reefs are in shallow waters near continental and island coasts, where human-caused destruction is widespread from coastal pollution, from tourists trampling the reef organisms, from fishermen ravaging them, and from the hulls of ships grinding over them.

But five years ago, the corals in many parts of the world were afflicted by a mysterious episode of bleaching that slowed their growth and in many regions killed them outright. Researchers note that the bleaching has coincided with increasing ocean temperatures.

"There is growing agreement that doubling of the carbon dioxide in the atmosphere means a 15 percent decline in the coral population," said Robert W. Buddemeier, a senior chemist with the Kansas Geological Survey, who has studied the impact of climate change on coral reefs.

"By the end of the century, with the effects of increasing levels of carbon dioxide on temperature and on ocean chemistry, the corals will be in the worst shape we've seen in the past 50 million years. Things are really dicey," he added.

NO TOUGH MEASURES

The growing evidence of damage from climate change has goaded the Bush administration to push its own research program, although the president does not support any tough measures to control greenhouse gas emissions, including the Kyoto Protocol agreed to by most industrialized nations.

Earlier this month, Assistant Secretary of Commerce James Mahoney, an atmospheric physicist who is Bush's point man on global warming, staged a "workshop" in Washington where 1,500 people from industry, government, academic and environmental organizations worked on plans for a "strategy for climate change research."

"There are still any number of science questions to be resolved," Mahoney told reporters at the AGU meeting in San Francisco. He conceded, however, that already "we will most likely need profound changes in greenhouse gas emissions. "

But to many analysts, time is wasting. Global warming will cause "major political instabilities in the developing world that could disrupt the global economy," said Lester R. Brown, founder of the Earth Policy Institute and a noted environmental analyst who spent 10 years as a policy adviser in the Department of Agriculture.

FOOD SUPPLY IMPACT

If measures aren't taken soon to curb greenhouse gas emissions, the changes in climate will force rapid changes in the way the world's food crops are grown. That has important implications for feeding the world's growing population, expected to increase to at least 9 billion by 2050.

"The vast corn belt of the Northern Hemisphere, for example, will become hotter and dryer, and that change can't be resolved merely by creating new corn belts further north, because the soils further north are not the same at all," Brown said.

"Each global increase of 1 degree Celsius (1.8 degrees Fahrenheit) around the world will reduce grain yields like rice and wheat, as well as corn, by at least 10 percent," he said.

And because aquifers are being tapped at an increasing pace throughout the world and water tables are falling, the outcome will soon mean a devastating blow to agriculture -- particularly in the developing world, he said.

"This disruption by a combination of climate change and water shortages has the potential for creating political instabilities on a scale that we can't even foresee," Brown declared.

©2002 San Francisco Chronicle

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