It's getting hotter. Now a new study says it's also getting stickier -- and we should sweat the details.

Humidity worldwide has risen 2.2 percent the past 30 years, and the only explanation is global warming from the burning of fossil fuels.

That's the conclusion of a study published today in Nature that says rising humidity levels represent the latest human fingerprint on global warming.

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

The trend of rising humidity, while not lethal, already has raised levels of discomfort, he said. Those levels will worsen throughout the century if the trend continues.

Dr. Gillett studied changes in specific humidity -- the measurement of total moisture in the air -- from 1973 to 2002. He concluded that rises in humidity already are significant. Computer simulations of climate could not replicate real-world results until the effects of human activities -- including the burning of fossil fuels -- were added.

The study said humidity levels are higher in the eastern United States, but the local effect was not immediately known. AccuWeather in State College, Centre County, and the National Climatic Data Center in Asheville, N.C., said coming up with humidity trends for Western Pennsylvania would take time to compute.

Dian Seidel, a meteorologist with the National Oceanic and Atmospheric Administration's Air Resources Laboratory in Silver Spring, Md., also declined comment until she reads the study.

Ken Reeves, senior meteorologist and director of forecasting operations at AccuWeather.com, cautioned against accepting the study results wholesale. He said computer models often fail to incorporate all pertinent climatic factors. He also said the study fails to include a measurement of humidity over the oceans, which he said is a serious flaw.

"The research could be 100-percent correct, but there is good reason to approach it with more caution," he said. "We need more research and study."

What's clear is that temperatures have been edging up, and when temperatures rise, so does humidity.

Meteorologist Josh Nagelberg, also of AccuWeather.com, said the average temperature in Pittsburgh the past 10 months has been up 1 degree, and the average temperature since Sept. 1 is up 5 degrees. But those short-term numbers cannot be extrapolated to make conclusions about global warming.

Rising humidity would create problems for people.

"The more moisture that's tied to heat, the slower the human body can evaporate the water," Mr. Nagelberg said. "When the body is holding more water, the tougher time it has dealing with the heat.

"The worst heat of summer will feel even worse if this keeps up."

While human discomfort will rise with higher humidity levels, so will environmental effects.

The study said rising levels of water vapor in the lower atmosphere "could affect patterns of extreme storms."

"Higher humidity is likely to exacerbate problems with extreme precipitation and tropical cyclones," it states.

The study provides some new insights, but climatologists long have expected humidity levels to increase. Physics dictates that warmer air holds more moisture.

Jay Lawrimore, chief of the climate monitoring branch at the NOAA's National Climatic Data Center, said water vapor "is a powerful greenhouse gas." That's why climatologists give humidity levels a lot of attention and consider Dr. Gillett's findings to be a significant piece in the global-warming puzzle.

As temperatures rise, Mr. Lawrimore said, the air holds more water and "water vapor traps a lot of [the sun's] radiation."

"As amounts of water vapor rise, if they do rise, they add even more warming to the atmosphere that we get with gases like [carbon dioxide]," he said.

Only time will reveal the precise effects, with humidity levels representing an important piece in a very large puzzle.

"Only when taken as a whole ... will we be able to understand how the climate is changing," Mr. Lawrimore said.

Nature's article about the study, "Humans have made the skies more moist," is available at www.nature.com/news.