27 January 2009

A Millennia-Long Greenhouse Disaster

By Richard A. Kerr, ScienceNOW Daily News, 26 January 2009
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Dry so long. Models call for Dust Bowl-like dryness (purples and reds) around the globe and to the end of the millennium. CREDIT: S. SOLOMON ET AL., PNAS EARLY EDITION (JANUARY 2009)

Climate scientists have painted an unpleasant picture of the end of this century if humankind keeps spewing climate-changing gases into the atmosphere. Now they are pointing out that the ill effects won't be going away for a long, long time. The carbon dioxide we're emitting this century is so slow to disappear and climate so slow to respond, they say, that the effects felt in a century or two will be almost as strong 1000 years from now.

The discouraging word comes in a paper published online today in theProceedings of the National Academy of Sciences. Climate researcher Susan Solomon of the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colorado, and her colleagues report how they used results from two kinds of climate models--century-scale and millennial-scale--to calculate the climate out to the year 3000. The end results showed how two climate processes work against each other to sustain the peak warming to be seen a century from now until the year 3000: The slow escape of atmospheric carbon dioxide into the deep sea tends to ease the warming in models, the researchers note, but the equally slow drawdown of atmospheric heat into the deep sea tends to compensate for the carbon dioxide loss.

As a result of the counteracting processes, almost all of the warming that could be seen by 2100--1.5°C to 4°C, depending on how much carbon dioxide ends up being emitted--is still there in 3000. "The time constants [of the climate system] are so slow, people have a hard time appreciating how long the climate change persists," says Solomon. A bevy of climate changes would accompany the persistent warming, Solomon and colleagues point out, but they highlight one: drought. Even given a modest 2°C global warming, southwestern North America, eastern South America, and southern Africa would suffer added summer dryness comparable to the dryness of the disastrous American Dust Bowl of the 1930s, but for centuries, not for just a decade or two. Northern Africa, southern Europe, and western Australia would have it twice as bad.

Essentially, irreversible climate change "is not a new idea," notes climate researcher David Archer of the University of Chicago in Illinois, "but it's widely misunderstood." Much of scientists' presentations to the public and policymakers "had made it seem like climate change was a century-scale issue," he says, but this latest work should help correct the misimpression.

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