Ocean Warming Studies Bolster Evidence of Human Hand in Climate Change. (Update).

Two new independent studies on the warming of the Earth's oceans add some of the most convincing evidence yet that human actions are playing a significant role in the Earth's rising temperatures. The studies, published in the April, 13 2001 issue of Science, compared different computer projections of climate change with new records showing the Earth's oceans have warmed 0.06 degrees C (0.11 degrees F) in the past 40 years, and found the computer simulations and the actual rate of ocean warming closely match up.

"Our results support climate modeling predictions that show increasing atmospheric greenhouse gases will have a relatively large warming influence on the earth's atmosphere," said the lead author of one study, Sydney Levitus of the U.S. National Oceanic and Atmospheric Administration. Until now, oceans have been a sort of missing link in our understanding about climate change. Computer models have predicted more warming of the atmosphere than has actually occurred, leading scientists to speculate that the "missing warming" is being absorbed by the oceans. The new studies strongly support this explanation.

"What we found is that the signal is so bold and big that you don't have to do any fancy statistics to beat it out of the data," said the lead author of the second study, Tim Barnett of the Scripps Institute of Oceanography in California. "It's just there, bang." Barnett said that the warming in his model so closely matched the rates and location of the actual warming that the "results are certainly compatible at the 95 percent confidence level with the hypothesis that the warming observed in the global oceans has been caused by [human] sources."

The research bolsters the growing body of research finding that human activities have been the dominant influence in the recent rise of the Earth's global temperature, including a new report by the Intergovernmental Panel on Climate Change (IPCC), a group of more than 100 of the world's top climate scientists. The IPCC report projects the Earth's average temperature will increase by 1.4 to 5.8 degrees C by 2100 (see "Climate change report released," March/April 2001).

The ocean temperature increase of 0.06 degrees C may seem small, but spread throughout the oceans, which cover more than 70 percent of the Earth's surface, the amount of energy being absorbed is enormous. Barnett estimates that the energy added to the oceans in the past 40 years could meet California's energy demands for the next 200,000 years. The ocean warming data, released last year, are based on 2.3 million ocean temperature readings painstakingly collected from 50 years of ship records (see "Oceanic findings confirm warming," July/August 2000, p. 11). While the ocean warming is likely to help delay the effects of climate change in the atmosphere, the energy will eventually be released back into the air, warn scientists.

The ocean's large mass and its ability to retain heat more easily than air means that it acts as the Earth's climate "memory." Changes in ocean temperatures may help scientists anticipate climate shifts in the atmosphere, said Levitus. "For example, we found that the increase in subsurface ocean temperatures preceded the observed warming of surface air and sea surface temperatures, which began in the 1970s."

While supporting the findings of the studies, Kevin Trenberth of the National Center for Atmospheric Research in Colorado warned that some details and questions remain to be resolved. For instance, Barnett's study, which relied on the Parallel Climate Model, does not take into account in its calculations the cooling effects of solar radiation and gases released by volcanoes. (The second model used by Levitus, the Geophysical Fluid Dynamics Laboratory model, does take into account these factors.)

Levitus and other scientists reiterate that the studies demonstrate a convincing human fingerprint on climate change. "To me the question of whether global warming is happening is receding as the central question," Texas A&M climate researcher Tom Crowley, told the Los Angeles Times. "In my view, it's already here--and I didn't believe that two years ago. Now the question is: How will it affect us?"

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