June 7, 2005— Human-caused global warming is measurably heating up the oceans, say U.S. and U.K. researchers.

Calling on more than 7 million temperature readings from different depths in every ocean, the international team compared the realities over the last 40 years to predictions by computer climate models.

They discovered that the only way they could get the models to act like the real oceans was to add a human-induced greenhouse effect.

"I think this is the most compelling yet," said Tim Barnett of Scripps Institution of Oceanography at the University of California at San Diego. Barnett is lead author of the paper, published in the June 3 issue of Science.

To be as rigorous as possible, Barnett and his colleagues played devil's advocate with eight different high-quality computer climate models, tweaking them various ways to try and see if they could make the warming trend appear in the oceans without a greenhouse effect — but nothing worked.

"This present work is different in that it does not attempt to quantify this imbalance, but to attribute it to a specific cause," said Tim Boyer, oceanographer with the Ocean Climate Laboratory at the National Oceanographic Data Center. Previous studies have looked at ocean warming with more of an eye towards measuring it, says Boyer.

In fact, the oceans have been delaying the rapid increases in air temperatures from the greenhouse effect by absorbing a lot of heat, Boyer explains.

The greenhouse effect is what happens when the atmosphere becomes richer in carbon dioxide and other gases that hold on to heat instead of letting it dissipate into space.

The danger of warming oceans is their unpredictability. Oceans operate as a heat sink; water retains heat longer than air, and oceans can also move heat into the depths — only to bring it back up decades or centuries later.

In the deep ocean, water may not return to the surface for thousands of years, said Boyer. "If it returns to the surface in an area where the water is warmer than the air, heat will be released from the ocean to the atmosphere," said Boyer.

For this reason, and others, it's hard to predict the exact climatic consequences of ocean heating, says Boyer.

What is certain, said Barnett, is that there will be consequences.