Are humans warming the world? Confident answers depend in large part on the credibility of climate models. Now a new, independent reality check from the ocean has strengthened the case that these models are pointing to a human cause for recent global warming. Two research groups report that their models accurately simulate the warming of deep oceans during the past half-century.
The oceans are an important part of climate models because they can hold an enormous amount of heat. And like the rest of the world, the oceans appear to be warming. Last year, oceanographer Sydney Levitus of the National Oceanic and Atmospheric Administration (NOAA) in Silver Spring, Maryland, and his colleagues reported that the top 3000 meters of oceans worldwide had gained 18.2 x 1022 joules of heat--enough to warm parts of the ocean 0.1 degree C or more--between 1955 and 1996 (Science, 24 March 2000, p. 2126). This is about 10 times as much heat as went into warming the global atmosphere and melting sea ice and glaciers. Their conclusion: If you're keeping track of the heat trapped by the strengthening greenhouse, the ocean is almost all that matters.
With that pivotal role in mind, Levitus's group and another team led by climate modeler Tim P. Barnett of the Scripps Institution of Oceanography in La Jolla, California, used climate models to see what effect carbon dioxide and other greenhouse gases from human activities would have had on ocean temperature over the past century. Both groups found that the models' world oceans warmed by just about as much as observed, as they report in the 13 April issue of Science (pp. 267 and 270). The warming in the model ocean so closely matched the strength and geographical distribution of the actual warming that Barnett calculated with confidence exceeding 95% that human-produced greenhouse gases are behind real-world warming.
The models' success at predicting ocean warming "provides stronger evidence climate is changing," says climate modeler Simon Tett of the Hadley Centre for Climate Prediction and Research in Bracknell, U.K., "and it's likely due to human influence."