Ocean Water Vapor Found to Boost Global Warming

Scientists at the University of Chicago reported Wednesday they have confirmed for the first time that water vapor from the oceans can amplify global warming caused by man-made gases and perhaps could trigger a “super-greenhouse” effect above the warmest part of the Earth’s oceans.

The scientists, who used temperature and radiation measurements from satellites, buoys and ocean-going ships, said they confirmed computer models indicating that as more water vapor accumulates, more energy is trapped on Earth. The study, published today in the journal Nature, also suggests that above the warmest oceans--where the water vapor would be greatest--the trapping of gases is increased at a much higher rate than models had predicted.

“I think this study is important because it offers strong support to greenhouse effect theories,” said Veorabhadran Ramanathan, one of the nation’s leading climate experts, who did the study with his graduate student, Ameet Raval. “Frankly, I didn’t expect that our work would support the theories so strongly.”

The study, coming at a time of increasing debate over the accuracy of global warming predictions, represents the most extensive testing yet of computer models.

The confirmation of what scientists describe as an essential feature of the greenhouse theory is likely to instill more public confidence in such computer modeling, but it is not expected to quell debate among scientists over the amount of warming that will occur and the precise role of industrial gases.

Scientists believe that carbon dioxide and other gases trap heat from the sun in the Earth’s atmosphere, much like the panes of a greenhouse. Eventually, such trapping could produce major changes in climate, triggering more droughts, hurricanes or other cataclysmic events.

Ramanathan, interviewed by telephone, noted that relatively little warming is caused by such industrial gases as carbon dioxide. But theories developed over many decades show that atmospheric warming caused by such gases would be exaggerated by water vapor, “the most powerful greenhouse gas,” coming primarily from evaporation of the Earth’s oceans.

“The basis of the theory was simply pointing out that we all know from common experience that when it gets hotter, it gets more humid, and when it gets colder, it gets dry,” Ramanathan said. “What is new in this study is that we went beyond that and computed the actual trapping” of the sun’s infrared energy in the Earth’s atmosphere.

He said he and Raval found that the trapping of such energy varies from the models by only 10% to 20%.

If the researchers had found that warm ocean temperatures had little effect on the rate of trapping of infrared energy, “that would have been sufficient cause to dismiss the entire greenhouse effect,” Ramanathan said. “On the contrary, we found the theory confirmed, or one aspect of the theory, an extremely critical aspect.”

The University of Chicago scientists computed the difference between the energy emitted from the ocean and the amount that escapes into space.

“So we know what is leaving at the surface and what is leaving at the top, and the difference is always positive, that is, the atmosphere is always trapping,” Ramanathan said.

The major departure from models detected by the researchers occurred over the western tropical Pacific Ocean, where the rate at which energy was trapped was much higher than models have predicted. The researchers noted, however, that they could not be certain that the increased trapping was a result of the higher ocean temperature.

“We need to understand this super-greenhouse effect because it could mean that greenhouse warming will become exponentially greater after we reach a certain threshold of temperature,” Ramanathan said.

He said the threshold appears to be about 82 degrees Fahrenheit, the temperature of slightly less than 5% of the world’s oceans. But if global warming increases as predicted, about three times as much of the Earth’s oceans would exceed that temperature and “lead to a much stronger greenhouse effect than we have predicted,” he said.

Ramanathan, a professor of geophysical sciences, said his measurements are consistent with predictions that the planet’s temperature will rise 2 to 3 degrees Fahrenheit in the next 50 years. He cautioned, however, that the role of clouds is not yet well enough understood. Some scientists believe that clouds could diminish the effect of greenhouse gases.

Richard S. Lindzen of the Massachusetts Institute of Technology, a leading skeptic of many greenhouse predictions, said the Chicago study does not change his reservations about temperature predictions. “That picture does not tell you how the system would respond if you added more gases,” he said. Although the research verified that water vapor is an important greenhouse gas, it failed to show that “water vapor will act to magnify the warming,” he said.

However, John Firor, director of advance studies at the National Center for Atmospheric Research in Boulder, Colo., called the study a “a very interesting piece of work.”

“I’m sure it’s going to keep all of us busy trying to see see what impact it will have on the model calculations,” Firor said. “One needs to keep an eye on the speculation that the process he found in the Western Pacific could accelerate the greenhouse effect after large parts of the ocean reach a certain temperature. I have not heard that suggested before.”

In terms of scientific advancement, the study “puts two new bricks” on top of a wall scientists are building on the greenhouse effect. It is significant largely because it is “probably the most extensive and solidly based survey in existence” and instills credibility in the models, he said.