Human-Induced Greenhouse Warming Pumps Heat Into Oceans, Two Science Studies Report

Researchers had previously reported in Science that the Atlantic, Pacific, and Indian oceans have collectively warmed an average of 0.06 degrees C since 1955.

Now, two new modeling studies have tied the ocean heating directly with global warming caused by human activity.

"I believe our results represent the strongest evidence to date that the Earth's climate system is responding to human-induced forcing," said Sydney Levitus of the National Oceanographic Data Center/NOAA, lead author of one of the new studies and the previous report.

"The parallel climate model, a cooperative product of the U.S. Department of Energy and the National Science Foundation, can reproduce the warming in the ocean seen over the last 50 years. This will make it much harder for naysayers to dismiss predictions from climate models," said Tim Barnett of Scripps Institution of Oceanography, lead author of the second Science study.

Covering 72 percent of the Earth's surface, the oceans are often called the "memory" of the earth's climate system. They can absorb large amounts of heat and sequester it at depth for up to thousands of years before circulating it back to the atmosphere.

"Warming in the oceans is bad news and good news," said Barnett. "It really does add strength to the claims that global warming is here. But, it also suggests that the immediate impact may not be as great, because the oceans may slow things down a little."

Measurements of past air temperatures are more numerous and reach farther back in time than those of ocean temperatures. Earlier climate models didn't include an ocean component, and therefore frequently predicted that modern air temperatures would increase more than they actually have, according to Levitus. This discrepancy has been useful fodder for skeptics, who have argued that global warming wouldn't be as severe as commonly thought.

Last year, Levitus and his colleagues determined an average for how much the oceans had warmed by compiling millions of deep ocean temperature measurements from 1948-1995. (Science, 24 March 2000, p. 2225.) But, it wasn't clear whether this heat came from greenhouse warming or just a natural swing in the climate cycle.

To investigate, Barnett and Levitus each used a different climate model to simulate how ocean temperature should respond to cu