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Web Extra Friday, November 5, 2004

Past warming for the future

As the Bush administration prepares for a second term, speculations abound on the next step for climate change policy. In the meantime, discussions of the science behind climate changes abound in the journals and within the scientific community.

Scientists are notoriously reluctant to state a position with complete certainty, but for most of the climate science community, the results are sure: Humans are putting huge amounts of carbon dioxide into the atmosphere, through consumption of fossil fuels. Just how this will play out in the climate system is still under debate. Richard Alley, a glaciologist from Pennsylvania State University, says that in his experience, the majority of climate scientists agree that the excess carbon dioxide contributes to global warming.

Alley and Dan Schrag, a geologist at Harvard University, are looking to the past for clues on what will happen from these current changes. Just as "kids ought to learn history in high school because you don't know what's going on today without knowing history," Alley says, "knowledge of climate history should improve [climate scientists'] ability to project the future accurately."

Reporting in the Oct. 29 Science, Schrag and Alley give a synopsis of what scientists have learned, pointing out that the past warming episodes on Earth have varied according to carbon dioxide levels in the planet's atmosphere. They also note that modeling and data have provided a broad idea as to just how much carbon dioxide may have been around for each of the major warmings shown in "deep" geologic time — such as during the Eocene 50 million years ago. The point of their paper, Schrag says, is that "the paleoclimate record shows that Earth is sensitive to small forcings, and the models are slightly less sensitive — making it more likely that the models will underestimate the effects of [a] future rise in greenhouse gases."

Earth's current configuration is less conducive to abrupt climate changes today, as have happened in the geologic past. Nonetheless, Alley says, abrupt changes might still be possible in the North Atlantic. The big permanent ice sheets on land in Greenland and the Antarctic — which could greatly affect sea level were they to melt quickly — are not changing much in total volumes at the moment, Alley says, but "things are happening on the margins that are big and happening pretty fast. To some of us, those changes locally are pretty surprising." And, he emphasizes, the community is highly aware of rapid changes occurring to Arctic sea ice.

An upcoming report from the Arctic Climate Impact Assessment, sponsored by eight countries, including the United States, Canada and Russia, was leaked to the press at the end of October, before its expected presentation at an international conference in Reykjavik, Iceland, on Nov. 9. The product of several hundred scientists' work, it reportedly states a consensus that the Arctic ocean could be seasonally ice-less in the next half century. The loss of that ice would be a substantial change to the albedo — or reflectivity — of Earth, with potentially immense consequences for global warming.

"Clearly sea ice is very important," says Alley, who cites the third Intergovernmental Panel on Climate Change (IPCC) report. "It's one of the very big amplifiers of the climate system, and it looks like sea ice had huge effect on the Younger Dryas" warming event, around 12,000 years ago.

In their article, Schrag and Alley note that the refining of cloud cover and ice cover in current global climate models will improve models' forecasting capacity. Alley calls current models "very good," but "the view from the past is that if the models are in error, they are changing a little less than the real world, not more than the real world," something to take into account for policy decisions informed by the science.

Also writing in the Oct. 29 Science, Eileen Claussen, president of the Pew Center on Global Climate Change, is critical of the Bush administration's policies on global climate change. Her column is a direct response to an essay published in Science on July 30 by Secretary of Energy Spencer Abraham (see Geotimes, October 2004). Claussen says that "the United States has fallen behind" in a world where the international community is working to reduce emissions and other approaches to mitigating human impacts on climate. She lays out measures that should be taken now to reduce greenhouse gases, while reducing the uncertainty in climate forecasts.

Only time will tell how climate policies will change in the next four years. Regardless, Alley and other scientists say, people are responsible for adding carbon dioxide to the atmosphere at potentially climate-changing levels, considering the story that the past has to tell.

Naomi Lubick

Links:
Arctic Climate Impact Assessment, available Nov. 9
"Climate tipping point," Geotimes, October 2004

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