Most studies to determine the rate of global warming have concentrated on changes in atmospheric temperatures. The atmosphere, however, contains only a small fraction of the heat received from the sun. Oceans store far more of the sun's heat, and two recent studies have shown that the heat content of the upper 3000 meters of the ocean has increased during the past 50 years.
Researchers had previously reported that the Atlantic, Pacific, and Indian oceans have collectively warmed an average of .06 degrees celsius since 1955, but the new studies tied the ocean heating directly to 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.
Tim Barnett of Scripps Institution of Oceanography, lead author of the second study, said "The parallel climate model 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."
Covering 72 percent of the Earth's surface, the oceans are often called the "memory" of the earth's climate system. They can absorb and retain large amounts of heat and store it for thousands of years.
"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."
Earlier climate models didn't include an ocean component, and therefore frequently predicted that air temperatures would increase more than they actually have. This discrepancy has been useful fodder for skeptics, who have argued that global warming wouldn't be as severe as predicted.
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. 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 used different climate models to simulate how ocean temperature should respond to current levels of greenhouse gases and other modern day atmospheric conditions. Both models predicted an amount of warming quite similar to what has actually been measured.
