With global warming, the world isn't just getting hotter - it's getting stickier, due to humidity. And people are to blame, according to a study based on computer models published Thursday.
The amount of moisture in the air near Earth's surface rose 2.2 percent in less than three decades, the researchers report in a study appearing in the 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 the United Kingdom, 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. Higher 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, including the eastern United States, 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.
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."
The studies make sense, said University of Victoria climate scientist Andrew Weaver, who was not part of either team's research.
It will only feel worse in the future, Gillett said. Moisture in the air increases by about 6 percent with every degree Celsius (1.8 degrees Fahrenheit), he said. Using the Intergovernmental Panel on Climate Change's projections for temperature increases, that would mean a 12 to 24 percent increase in humidity by the year 2100.
"Although it might not be a lethal kind of thing, it's going to increase human discomfort," Willett said.