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Antarctic ice record warns of greater warming than today’s climate models predict


If Earth’s past cycles of warming and cooling are any indication, temperatures by the end of the century will be even hotter than current climate models predict, according to a report by researchers in Berkeley, California.

The scientists studied Antarctic ice cores containing a 360,000-year record of global temperature and levels of carbon dioxide and methane--two of the major greenhouse gases implicated in global warming. They found that during periods of warming, greenhouse gas levels rose and created significantly higher temperatures than would be expected solely from the increased intensity of sunlight that triggered these warm periods.

Though the ice core data do not point to specific processes that amplify the warming, the researchers suspect that it is due to warmer soils and oceans giving off more carbon dioxide and methane, which add to the greenhouse effect of carbon dioxide from fossil fuel burning and other human activities.

Thus, while current models predict temperature increases of 1.5 to 4.5 degrees Celsius [2.7 to 8.1 degrees Fahrenheit] from a doubling of atmospheric carbon dioxide levels, the natural processes injecting additional carbon dioxide into the atmosphere will lead to temperature increases of 1.6 to 6 degrees Celsius [2.9 to 11 degrees Fahrenheit], with the higher temperatures more likely, the researchers say. The report is scheduled for publication on 26 May in Geophysical Research Letters.

"The warming caused by our release of carbon dioxide triggers changes in the Earth system that lead to release of more carbon dioxide to the atmosphere," says lead author Margaret Torn, head of the Climate Change and Carbon Management program at the Lawrence Berkeley National Laboratory. "If that is the case, then every bit of carbon dioxide released now is actually committing us to a larger carbon dioxide change in the atmosphere."

"We are underestimating the magnitude of warming, because we are ignoring the extra carbon dioxide dumped into the atmosphere because of warming," says John Harte, professor of energy and resources at the University of California in Berkeley. "Warming gets an extra kick from carbon dioxide feedback."

The result, Torn and Harte conclude in their paper, is "that the upper value of warming that is projected for the end of the 21st century, 5.8 degrees C [10 degrees F], could be increased to 7.7 degrees C [14 degrees F], or nearly 2 degrees C [4 degrees F] additional warming."

Current climate models, called General Circulation Models, start from fundamental physical processes to calculate a probable temperature increase based on likely atmospheric carbon dioxide levels, typically a doubling of today’s carbon dioxide concentration. But models are only now beginning to take into account the extra carbon dioxide and methane injected into the atmosphere as global temperatures increase.

This effect is expected, because warmer soils decompose faster, releasing more carbon dioxide, and warmer oceans also release more carbon dioxide, but scientists have yet to quantify the full impact of these processes. "Without a mechanism, people feel uncomfortable putting it in a model. I think that’s a big mistake," Harte says.

It is possible to estimate the effect of carbon dioxide feedback by looking at how Earth responded to past cycles of warming and cooling, which were caused by natural variations in the strength of sunlight hitting Earth, rather than by human production of greenhouse gases. Ice cores drilled in Antarctica’s Vostok ice sheet in 1998 and 1999 span nearly 420,000 years and carry information about four major climate cycles and many smaller temperature swings. Climate scientists had pointed out that the ice core data imply a strong positive feedback to global carbon dioxide and methane levels, but how much this impacted warming trends was unclear.

Torn and Harte devised a way to use these data and current global climate models to estimate the effect of increased carbon dioxide entering the atmosphere as a result of warming, called the "gain." From previously published data, they were able to extract the effect of temperature on carbon dioxide and methane levels. They then calculated the reverse--the effect of carbon dioxide and methane levels on temperature, or the so-called climate sensitivity--from climate models.

The researchers added the resultant gains from carbon dioxide and methane to the gain already known for other climate feedbacks, in particular the largest source, increased atmospheric water vapor, to arrive at a total gain. They used this figure to calculate the temperature increase that would result from a doubling of current carbon dioxide levels.

Both researchers emphasize that the wide temperature range they predict--1.6 to 6 degrees Celsius [2.9 to 11 degrees Fahrenheit]-- does not mean that Earth has an equal chance of ending up with less warming as with greater warming, in other words, that the uncertainties are symmetric about an average increase of 3.8 degrees Celsius [6.8 degrees Fahrenheit].

"People see this uncertainty and think that we have an equal probability of dodging a bullet as catching it. That is a fallacy," Torn says.

"By giving the appearance of symmetric feedback, people have an excuse to say maybe we don’t have to worry so much," Harte says. "But while there are uncertainties in the feedbacks, all the major feedbacks are positive, meaning they would increase warming, and we know of no significant negative feedbacks that would slow warming."

"Whatever the mechanisms that cause temperature to create a change in carbon dioxide and methane, they are repeatable again and again and again over many cooling and warming cycles. So, although the world is different today than it was then, we don’t have a basis for ignoring them," Torn says.

"We need to know the effect of warmer temperatures in all different habitats "says Harte, "not just temperate Rocky Mountain forests," where he has conducted experiments, "but also the tropics and European boreal forests and Eastern U.S. deciduous forests and savanna and prairie. There are huge data gaps."

Torn notes, however, that humans are the biggest unknown. "To predict the future, you have to guess how much carbon dioxide levels will go up. That depends on the biggest uncertainty of all: what humans decide to do. Do we get smart and prevent carbon dioxide emissions? Do we continue with business as usual? Or will we end up somewhere in between?"

The research was supported by the U.S. Department of Energy’s Climate Change Research Division and by the National Science Foundation.

[Note: See also AGU/CEH/WU joint press release 06-17 of 22 May on a related topic.]

Harvey Leifert | American Geophysical Union
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