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Gloomiest climate projections look closer to the mark

  • Article by: BRIAN VASTAG
  • Washington Post
  • November 10, 2012 - 3:36 PM

Climate scientists agree the Earth will be hotter by the end of the century, but their simulations don't agree on how much. Now a new study published in the journal Science suggests the gloomier predictions may be closer to the mark.

"Warming is likely to be on the high side of the projections," said John Fasullo of the National Center for Atmospheric Research in Boulder, Colo., a co-author of the report, which was based on satellite measurements of the atmosphere.

That means the world could be in for a devastating increase of some eight degrees Fahrenheit by 2100, resulting in drastically higher seas, disappearing coastlines and more severe droughts, floods and other destructive weather.

Such an increase would substantially overshoot what the world's leaders have identified as the threshold for triggering catastrophic consequences. In 2009, heads of state agreed to try to limit warming to 3.6 degrees, and many countries now want a tighter limit.

Climate scientists around the world use supercomputers to simulate the Earth's atmosphere and oceans. Sophisticated programs attempt to predict how climate will change as society continues burning coal, oil and gas, the main sources of heat-trapping gases such as carbon dioxide. But these simulations spit out a wide range of warming estimates. All foresee an overheated planet in 2100, but some predict just 3 degrees of warming while others estimate 8 or more. "As long as climate models have existed, there's been this spread in projections of the future," Fasullo said.

Clouding their predictions

One source of uncertainty involves the impact of cloud cover, especially clouds that form in the tropical and subtropical regions between about 30 degrees north and south of the equator. "Tropical clouds are so important to climate," Fasullo said. "Small changes in clouds near the equator have a big effect on where you end up" for temperature predictions.

That's because as sunlight pours onto the tropics, clouds bounce some of that heat back into space. Fewer clouds open up the atmosphere "like an iris," Fasullo said, allowing more heat to beam onto the Earth's surface.

No supercomputer is powerful enough to predict cloud cover decades into the future, so Fasullo and colleague Kevin Trenberth struck on another method to test which of the many climate simulations most accurately predicted clouds: They looked at relative humidity. When humidity rises, clouds form; drier air produces fewer clouds. That makes humidity a good proxy for cloud cover.

Looking back at 10 years of atmospheric humidity data from NASA satellites, the pair examined two dozen of the world's most sophisticated climate simulations. They found the simulations that most closely matched humidity measurements were also the ones that predicted the most extreme global warming.

In other words, by using real data, the scientists picked simulation winners and losers. "The models at the higher end of temperature predictions uniformly did a better job," Fasullo said. The simulations that fared worse -- the ones predicting smaller temperature rises -- "should be outright discounted," he added.

The most accurate climate simulations were run by the United Kingdom's Met Office, a consortium in Japan and a facility at the National Center for Atmospheric Research.

The study is part of a quickening trend to improve climate simulations. Over the past decade, these computer programs have become "tremendously more sophisticated," said Stephen Lord of the National Oceanic and Atmospheric Administration. International groups collaborate on simulations even as available computing power soars.

"As you make those improvements," Lord said, "the ability to simulate long-term climate gets better."

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