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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.

Gillett's study followed another last month that used the same technique to show that moisture above the world's oceans increased and that it bore the "fingerprint" of being caused by man-made global warming.

Climate scientists have now seen the man-made fingerprint of global warming on 10 different aspects of Earth's environment: surface temperatures, humidity, water vapor over the oceans, barometric pressure, total precipitation, wildfires, change in species of plants in animals, water run-off, temperatures in the upper atmosphere, and heat content in the world's oceans.

"This story does now fit together; there are now no loose ends," said Ben Santer, a scientist at Lawrence Livermore National Lab and author of the September study on moisture above the oceans. "The message is pretty compelling that natural causes alone just can't cut it."