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  Yahoo News 10 Oct 07
Global warming driving up humidity levels, says study

Yahoo News 10 Oct 07
Earth getting wetter and stickier
By Michael Kahn

Yahoo News 10 Oct 07
Study: Rise in humidity caused by humans
By Seth Borenstein, AP Science Writer

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.

Yahoo News 10 Oct 07
Global warming driving up humidity levels, says study

Man-made global warming is driving up humidity levels, with the risk that rainfall patterns will shift or strengthen, tropical storms intensify and human health may suffer from heat stress, a study released on Wednesday said.

From 1976 to 2004, when the world's average surface temperature rose 0.49 degrees Celsius (0.9 degrees Fahrenheit), global levels of atmospheric water vapour rose 2.2 percent, according to the paper by British scientists.

By 2100, humidity levels could increase by another 10 percent, lead researcher Nathan Gillett of the Climate Research Unit at the University of East Anglia, eastern England, told AFP.

Previously, scientists had noted an increase in humidity over the past few decades as higher temperatures sucked more water from the land and ocean surface.

But it was unclear whether these changes were the result of a natural or a human impact on the climate, as the data was regional rather than global and different methods were used to make the calculations.

The new paper is based on a new set of observations of humidity levels. This data was then crunched through a powerful computer model of Earth's climate system in the late 20th century.

Gillett said water vapour was a "positive feedback" -- a vicious circle, in scientific parlance -- in the global warming equation.

Steam is a greenhouse gas, meaning that like carbon pollution that results from burning fossil fuels, it traps solar heat in the atmosphere, thus stoking the warming effect and so worsening humidity.

The ramifications could be wide-ranging, he said. The distribution and intensity of rainfall could be affected, and tropical cyclones could be beefed up, as humidity is one of the fuels for these storms.

The study is published on Thursday in Nature, the weekly British science journal.

Yahoo News 10 Oct 07
Earth getting wetter and stickier
By Michael Kahn

Greenhouse gases are making the earth's atmosphere wetter and stickier, which may lead to more powerful hurricanes, hotter temperatures and heavier rainfall in tropical regions, British researchers reported on Wednesday.

The findings, published in the journal Nature, are some of the first to show how human-produced greenhouse gases have affected global humidity levels in recent decades and could offer clues on future climate change, the researchers said.

"It is another piece of the puzzle that climate change is happening and we are influencing it," said Nathan Gillet, a climate researcher at the University of East Anglia.

Human emissions of gases such as methane and carbon dioxide that trap heat in the atmosphere are widely blamed for changes in the climate. Scientists say average global temperatures will rise by 2 to 6 degrees Celsius (4 to 11 Fahrenheit) by the end of the century, causing droughts, floods and violent storms.

Warmer air can hold more water vapor.

"It has been predicted for a long time that humidity would increase with greenhouse gas increases," said Gillet, who led the study.

"But this is the first study that shows a significant human impact on surface humidity," he said in a telephone interview.

The British team collected data from weather stations, buoys and ships across the world to measure the effect of rising greenhouse gases on humidity between 1973 and 1999.

A computer simulation showed that natural events such as volcanoes and variations in the sun's brightness could not alone have produced the increase in humidity, and pointed to greenhouse gases generated by humans, Gillet said.

"It is getting moister at the surface, so humidity is increasing," Gillet said. "You only see that in the model with the human effect."

The findings are especially important for tropical regions, which will see the largest increase in humidity because they are warm already, he said.

The research also provides a better understanding of potential changes in the earth's water cycle, which could result in floods and droughts that have an even bigger impact on people than rising temperatures, he added.
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