Greenhouse gas emissions are likely the cause of significant ocean warming over the past 50 years, two new modeling studies show.

Last year, researchers described how the temperature of the upper Atlantic, Pacific, and Indian Oceans had increased 0.06 °C since 1955 [Science, 287, 2225 (2000)]. But the authors of that study couldn't determine whether the change was a natural variation or the result of a rise in greenhouse gases. The new studies--led by Sydney Levitus of
the National Oceanic & Atmospheric Administration's National Oceanographic Data Center and Tim P. Barnett of Scripps Institution of Oceanography--show that greenhouse gases are very likely responsible.

HEATING UP Pacific Ocean has warmed 0.06 °C since 1955 and greenhouse gases are likely culprits, studies show.

Barnett says his model "reproduces the warming seen in the ocean over the past 50 years. This will make it much harder for naysayers to dismiss predictions from climate models," [Science, 292, 270 (2001)].

From temperature measurements, Levitus calculated the change in heat content of the upper 3,000 meters of the world's oceans from 1957 to 1994. He then used an atmosphere-ocean model that incorporates measurements of greenhouse gases, sulfate and volcanic aerosols, and solar irradiance, to simulate the change in oceanic heat content. The simulated change agrees well with the observed change.

Barnett used a different ocean-atmosphere model to simulate the change in ocean heat content that would be caused by the extra greenhouse gases and sulfate aerosols produced by human activity. Without human inputs, the ocean did not warm much. "The odds are exceedingly good that the model did not trick us with this signal; it couldn't have done it by itself," he says.