Steamier Earth Likely, Due to Global Warming

John Roach
for National Geographic News
October 10, 2007
Human activity has long made the bedroom a hot and steamy place. Now, less sexy activities like burning coal and oil—major contributors to global warming—are making the whole planet steamier, a new study says.

Scientists expect the rising humidity to cause heavier rains, stronger hurricanes, and increased human heat stress. (Get the basics on global warming.)

Climate scientists have long predicted that a warmer world will allow more water to evaporate, thus making the planet more humid.

Indeed, several studies have shown trends of increasing surface humidity around the planet, but until now scientists were uncertain what was driving the trend.

The new study combined a fresh data set of surface humidity with climate models, "and actually attribute[s] those trends to human influence," said study co-author Nathan Gillett, a climate scientist at the University of East Anglia in Norwich, England. (Related news: "Global Warming "Very Likely" Caused by Humans, World Climate Experts Say" [February 2, 2007].)

He and his colleagues report the findings in tomorrow's issue of the journal Nature.

Stormy, Steamy Future

Climate models predict that increased water vapor in the atmosphere will lead to heavier rains and raise the maximum potential intensity for hurricanes, Gillett noted.

"In heat waves, if the humidity is higher, then that results in larger heat stress on humans," he said.

And increased water vapor will accelerate the warming, said Benjamin Santer, a climate modeler at the Department of Energy's Lawrence Livermore National Laboratory in California who was not involved in the study.

"Our best understanding is as we increase greenhouse gases and warm the atmosphere, we increase the atmosphere's capacity to hold moisture. And water vapor is in itself a potent greenhouse gas.

"Therefore you accelerate the warming," he said.

Steve Sherwood is a climate scientist at Yale University in New Haven, Connecticut, who was not involved in the study. He said the findings are uncontroversial, but the new data set is a boon.

"People haven't really been talking enough about what higher humidity is going to mean in the future," he said, "and part of the reason they haven't been talking about it is that we haven't had very good data."

"They've put out a data set that people can actually use to look at long-term changes," he added.

Global Warming Evidence

Santer of the U.S. Department of Energy led research published recently in the journal Proceedings of the National Academy of Sciences that linked human activity to an increase in atmospheric water vapor. (Gillett was a co-author of that paper.)

Gillett, co-author on the new Nature study, also contributed to a July paper in the journal Nature that linked human activity to changes in rainfall patterns over the past century.

This body of work should silence criticism that the only evidence of global warming comes from surface and ocean temperature records, Santer said. Human influence is prevalent throughout the climate system, he added.

The combined studies should also help scientists more accurately predict Earth's response to increasing levels of greenhouse gases, Gillett said.

For example, he said the models used for the July Nature study underestimated the change in rainfall due to global warming, but that the models do a "reasonably good" job of capturing the humidity changes.

"These results will help to pin down which parts of the hydrological cycle the models can simulate well and which not so well ... and help improve the models," he said.

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