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 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 three degrees of warming while others estimate eight or more degrees of extra heat.
"This problem has been around for 30 years," Mr. Fasullo said. "As long as climate models have existed, there's been this spread in projections of the future."
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," Mr. 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," Mr. 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 Mr. 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," Mr. 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 biggest benefit of this study is really just a reminder to go back" and see how well climate models match reality, said Jimmy Booth, a post-doctoral fellow at the NASA Goddard Institute for Space Studies in New York, who was not involved in the study. Booth works on a climate model called E2, and he said his team can now re-examine how well it simulates humidity in the tropics.
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.
The first climate models, some 30 years old, simulated only the Earth's atmosphere. The latest generation add the effects of ocean currents, the dwindling planetary ice cover, and even how plants and animals take up and release carbon.
"As you make those improvements," Mr. Lord said, "the ability to simulate long-term climate gets better."
Scientists not involved in the research said the new report, funded by NASA and scheduled for publication today in the journal Science, could improve the predictions made by the Intergovernmental Panel on Climate Change in its next comprehensive report, due in 2013. The IPCC is a world body organized by the United Nations to guide policymakers as they struggle to curb and adapt to climate change. The world has heated up some 1.4 degrees Fahrenheit since 1880, a rise scientists nearly uniformly attribute to carbon pollution from fossil fuels.weather - nation - science