研究突發性氣候變遷趨勢　IMPACTS科學團隊成軍

Abrupt and rapid climate change is a threat that the federal government has just decided to take seriously. Scientists from six national laboratories have been assigned to a new project that will undertake to define possible mechanisms of abrupt climate change well enough to build comprehensive computer models and make accurate predictions before the climate changes abruptly.

Most people think of climate change as something that occurs gradually, with average temperatures rising two or three degrees Celsius over a century or longer due to forcing mechanisms such as the accumulation of carbon dioxide in the atmosphere from the burning of fossil fuels or widespread changes in land use. But a 10 degree Celsius rise in global temperatures is not unlikely, the scientists warn.

"There are lots of names for abrupt climate change - nasty surprises, the jokers in the deck, the tipping point," says William Collins of the Lawrence Berkeley National Lab Earth Sciences Division, who will head the project.

Collins will coordinate the work of scientists from the six labs under the name IMPACTS, which stands for Investigation of the Magnitudes and Probabilities of Abrupt Climate Transitions. The program will bring together the work of experts in physical, chemical, and biogeochemical climate processes and in computer simulations of the whole Earth system.

"When the national lab participants first met to decide on the most significant potential sources of abrupt climate change in future, the first thing we had to do was define what we meant," Collins said.

The scientists agreed that abrupt climate change means "a large-scale change that happens more quickly than that brought on by forcing mechanisms - on a scale of years to decades, not centuries - and that persists for a very long time."

Collins, who heads the Climate Science Department in the Berkeley Lab's Earth Sciences Division, coordinated the development of the most recent version of the Community Climate System Model, one of the leading computer models underpinning the physical science basis of the 2007 climate change assessment conducted by the Intergovernental Panel on Climate Change, which shared the Nobel Peace Prize with former U.S. Vice President Al Gore.

The IMPACTS program will be based on this computer model and will add new capabilities and new diagnostics for processes that could initiate abrupt climate change.

Argonne, Los Alamos, Lawrence Berkeley, Lawrence Livermore, Oak Ridge, and Pacific Northwest are the participating national laboratories.

"IMPACTS is one part of a two-pronged approach to studying abrupt climate change, one based in the universities and the other in the national labs," says Collins. "Both elements will share a central web-based portal, with all the participants meeting annually."

The IMPACTS team will initially focus on four types of abrupt climate change. Only half joking, Collins refers to these as "The Four Horsemen of the Apocalypse."

1. Instability among marine ice sheets, particularly the West Antarctic ice sheet

For 40 years earth scientists have worried about what would happen if global warming eventually caused the West Antarctic ice sheet, some 3.8 million cubic kilometers of ice, to break up and slide into the ocean. In January 2006, in a report commissioned by the UK government, the head of the British Antarctic Survey, Chris Rapley, warned that this huge west Antarctic ice sheet may be starting to disintegrate.

James Hansen, a senior NASA scientist, said the results of Rapley's study were deeply worrying. "Once a sheet starts to disintegrate, it can reach a tipping point beyond which break-up is explosively rapid," he said.

Sea level would rise four to six meters - 13 to 20 feet. Port facilities worldwide would be submerged; atolls and island chains would vanish; parts of Bangladesh, Brazil, Burma, America's Gulf States, and other low-lying areas would flood; Venice, New Orleans, and many other cities would sink.

It is now apparent that these events may not be "eventual." Abrupt climate change could cause rapid melting and the subsequent rise of sea level not by centimeters but by meters per century.

2. Positive feedback mechanisms in subarctic forests and arctic ecosystems, leading to rapid methane release or large-scale changes in the surface energy balance

More than a third of Earth's terrestrial organic carbon is concentrated in the ecosystems north of the 45th parallel, much of it in soil, peatland basins, and permafrost. Positive feedback within ecosystems and among terrestrial ecosystems, climate, and ocean currents could rapidly release much of this stored carbon into the atmosphere.

Positive feedback involving ice-melt has already accelerated the pace of global warming in the far north. It now seems likely that changes in terrestrial ecosystems, which could occur over only 20-30 years, may amplify currently predicted global warming by two or three times, in the Arctic and possibly globally.

3. Destabilization of methane hydrates - vast deposits of methane gas caged in water ice - particularly in the Arctic Ocean

A vast quantity of carbon - possibly more than all the recoverable fossil fuels on Earth - is trapped in frozen methane hydrates under the oceans. Methane gas molecules are locked inside cages of water ice in a form so concentrated that when the ice melts the gas expands to 164 times its frozen volume.

High pressure and low temperature insure that most deep-water methane hydrate deposits would be stable even with considerable warming of the atmosphere.

But in the Arctic, methane hydrate deposits exist near the edge of the safe temperature-pressure zone; in these locales, methane release could be abrupt. The resultant rapid warming would trigger yet more releases of methane: permafrost would melt, the deep sea would become a dead zone, the hole in the Arctic ozone would grow bigger and occur more frequently.

4 Feedback between biosphere and atmosphere that could lead to megadroughts in North America

Ordinary greenhouse warming as forecast by the IPCC will result in warmer and dryer conditions in the subtropics, including Mexico and the southwestern U.S. More than warming of the air and sea surface are involved. Storm tracks are likely to shift north, and the jet stream will probably stabilize in a new configuration. Dried-out soil and hot, dry atmosphere could interact to start abrupt climate change.

If dried-out soil and hot, dry atmosphere interact to trigger abrupt climate change in the Southwest, megadroughts like those that plagued North America a millemium ago could quickly return.

Conditions as severe as the Dust Bowl of the 1930s will return and could persist for decades: a megadrought. The Dust Bowl of the thirties turned many once productive fields and rangelands into shifting sand dunes. The past was even worse, with vast sheets of sand on the move all across the High Plains.

Better science, more sophisticated models, and more powerful computers like those at the Department of Energy's National Energy Research Scientific Computing Center have only recently made it possible to build comprehensive climate models.

One of the great benefits of IMPACTS, in Collins's view, will be to bring together parts of the climate community whose communication has traditionally been poor.

For scientists concerned with the effects of climate change and those concerned with modeling it, "the working relationship hasn't been there," he says. "For example, of the major threats to human health from global warming, the worst is malnutrition - but the second worst is extreme weather!"

Collins says modeling a specific region with a specific set of circumstances in high resolution and making a solid prediction for the next few decades is much tougher than modeling what will happen to the whole planet a century from now.

"Those concerned with the effects of climate change on humans have never asked modelers to do this before," he says. "We hope that IMPACTS will demonstrate that it can be done."