Global warming: It's the humidity

he world isn't just getting hotter from man-made global warming, it's getting stickier.

The amount of moisture in the air near the surface - the stuff that makes hot weather unbearable - increased 2.2 percent in just under three decades. And computer models show that the only explanation is man-made global warming, according to a study published in Thursday's journal Nature.

"This humidity change is an important contribution to heat stress in humans as a result of global warming," said Nathan Gillett of the University of East Anglia in Britain, a co-author of the study.

Gillett studied changes in specific humidity, which is a measurement of total moisture in the air, between 1973-2002. Increases in humidity can be dangerous to people because it makes the body less efficient at cooling itself, said University of Miami health and climate researcher Laurence Kalkstein. He was not connected with the research.

Humidity increased over most of the globe, said study co-author Katharine Willett, a climate researcher at Yale University. However, a few regions, including the U.S. West, South Africa and parts of Australia were drier.

The finding isn't surprising to climate scientists. Physics dictates that warmer air can hold more moisture. But Gillett's study shows that the increase in humidity already is significant and can be attributed to gas emissions from the burning of fossil fuels.

To show that this is man-made, Gillett ran computer models to simulate past climate conditions and studied what would happen to humidity if there were no man-made greenhouse gases. It didn't match reality.

He looked at what would happen from just man-made greenhouse gases. That didn't match either. Then he looked at the combination of natural conditions and greenhouse gases. The results were nearly identical to the year-by-year increases in humidity.

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