Studies link man-made causes to rise in humidity

By Doyle Rice, USA TODAY

Two new studies have uncovered the first links between man-made global warming and an increase in humidity throughout all levels of Earth's atmosphere.

One study, published in today's edition of the journal Nature, found that the overall increase in worldwide surface humidity from 1973-99 was 2.2%, which is due "primarily to human-caused global warming," according to study co-author Nathan Gillett of the University of East Anglia, in Norwich, U.K.

The burning of fossil fuels (oil, coal, natural gas) is considered the chief way humans contribute to climate change because it releases heat-trapping greenhouse gases into the atmosphere.

Along with the added heat stress on people from the increased humidity, Gillett says the additional moisture in the atmosphere could lead to heavier rains and more volatile tropical storms. "If the humidity is increasing, then hurricane intensity will increase too," he says.

Scientists had observed significant increases in humidity at the Earth's surface over the last few decades, but it had been unclear whether these changes were from a natural or human influence on climate.

The earlier September study — which looked at the amount of water vapor in the full depth of the atmosphere over the oceans — was the first one to determine that human-induced warming is having a significant effect on the atmosphere's total moisture content.

"When you heat the planet, you increase the ability of the atmosphere to hold moisture," says physicist Benjamin Santer of the Lawrence Livermore National Laboratory in California and lead author of the earlier study, which was published in the Proceedings of the National Academy of Sciences. "We found that the total amount of atmospheric moisture has increased dramatically in the past 19 years," he says.

"Natural variability in climate just can't explain this moisture change. The most plausible explanation is that it's due to the human-caused increase in greenhouse gases," Santer says. His study also discounted influences from solar activity and the 1991 eruption of Mount Pinatubo.

Both studies relied primarily on computer models of the Earth's climate system to reach their conclusions. This use of models is troubling to some scientists.

Roy Spencer, principal research scientist at the University of Alabama-Huntsville, says, "The main thing they're trying to show is that the recent warming and moistening in the last 30 years is outside the range of natural variability, and that man is causing the warming. The use of climate models to do this is not convincing. … The idea that you can use climate models as a surrogate for reality is circular reasoning."

Spencer, who was not involved with the research, says, "I don't find either study terribly convincing."

But both Santer and Gillett say they've found the "human fingerprint" in the link between increased humidity and global warming. "Fingerprinting" involves searching for a computer model-predicted pattern of climate change (the fingerprint) in observed climate records.

"Both studies have impacts for human health, extreme precipitation and hurricanes. … They both show that there are signs of an emerging human signal in the moisture content of the atmosphere," Santer says.

What do you think of these studies? Does scientists' use of computer models to measure climate change convince you? Leave a comment below.

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