Powerful evidence that greenhouse gas emissions are warming the planet also undermines the significance of a discrepancy often cited by global warming sceptics.
Although surface air temperatures have risen in past decades, temperatures higher in the atmosphere have not. This, say some, casts doubt on global warming. Previous computer models focused mainly on the atmosphere because heat transfer to the ocean is more difficult to model.
However, new models that account for the heat absorbed by the world鈥檚 oceans emphasise that the seas, not the atmosphere, are 鈥渂y far the dominant part of the Earth鈥檚 climate system for storing heat,鈥� says Sydney Levitus of the National Oceanic and Atmospheric Administration in Silver Spring, Maryland. He believes this adds up to 鈥渟trongest evidence to date鈥� that humans are causing global warming.
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His group compared the modelled heat transfer to oceans caused by greenhouse gases with the measured long-term trend and got good agreement. In other words, they showed that warming of this giant heat sink matches greenhouse predictions better than the smaller heat sink of the atmosphere.
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The group had already found that the top three kilometres of the global ocean warmed a fraction of a degree from 1955 to 1996, but could not pinpoint a cause. By ruling out other possible sources of the heat, they now believe greenhouse gas emissions are the only plausible cause.
The warming oceans absorbed 20 times more heat over the past four decades than either melting continental glaciers or the warming atmosphere. Melting of mountain glaciers and shrinkage of sea ice in the northern hemisphere and Antarctic accounted for even less heat.
A different computer model of greenhouse warming tested separately by Tim Barnett and colleagues at the Scripps Institution of Oceanography also matched observed ocean warming. Barnett鈥檚 group said the agreement was so good that they were 95% certain that human activities caused the warming.
Researchers concede a few uncertainties do remain. The models did not match year-by-year trends, but they had not attempted to simulate decadal climate oscillations.
More at: Science (vol 292 p 267, 270)