Humans have made the skies more moist

Study models rises in atmospheric water vapour 

Nature, Oct. 10, 2007

Human activity is behind the rising levels of water vapour in the lower atmosphere over the past few decades, climatologists have concluded. The rises in humidity could affect patterns of extreme storms, they warn.

Although water vapour is an important greenhouse gas in the upper atmosphere, the rising levels closer to Earth's surface will not greatly affect the overall warming of the planet, says Nathan Gillett of the University of East Anglia in Norwich, UK, and an author of the study. But the results provide yet more evidence that we are changing the climate.

According to the researchers' computer models, the best explanation for the rising humidity is that human-produced greenhouse gases have warmed the planet and boosted the amount of water vapour produced by evaporation as part of the global water cycle. "To put it crudely it's because warmer air can hold more moisture," Gillett says.

Previous tools used to forecast world precipitation patterns may have underestimated the effect of this extra moisture in the lower atmosphere, the researchers say. Higher humidity is likely to exacerbate problems with extreme precipitation and tropical cyclones, they conclude.

Getting wetter

Gillett and his team studied existing measurements of atmospheric humidity from 1973 to 2002, recorded by a network of land-based stations as well as ships, buoys and marine platforms. Over this period the humidity in the lower atmosphere rose by an average of 0.07 grams per kilogram of air worldwide. This is in line with what would be expected in a warming world  air can absorb roughly 6-7% more moisture for every 1 °C rise in temperature.

The researchers simulated atmospheric moisture levels using a computer model that evaluates the effects of various natural factors, such as changes in solar activity and greenhouse-gas emissions. They then compared the trends seen in the different simulations with the real-world results.

The simulations that incorporated the effects of human activities most accurately reproduced the real-world trend, Gillett and his colleagues report in this week's Nature ¹. The best-fit simulation was so good that it even mimicked variations in the real data, such as the spike seen in 199798, when a strong El Niño event warmed Caribbean waters and led to increased humidity.

The work follows research published last month², in which researchers led by Benjamin Santer of the Lawrence Livermore National Laboratory in California used a similar method to rule out other natural effects as a cause of rising humidity. They showed that the clearing of skies that happened after the 1991 eruption of Mount Pinatubo, though it let more sunlight in, wasn't wholly responsible for increased water in the air as some people had thought.

"As the world is warming up, it is changing as we expected," says Peter Stott, a climatologist at the Hadley Centre for Climate Prediction and Research in Exeter, UK. Work like this shows how mankind is having an effect on complex climate issues, he says - beyond just the temperature.

The research shows that climate models are performing well, Stott adds. But predicting the incidence of highly localized, extreme events such as tropical storms will require new models with finer detail, he adds. "Basic climate models don't have the high resolution needed."

1. Willett, K.M., Gillett, N.P., Jones, P.D. & Thome, P.W. Nature 449 710-712 (2007)

2. Santer, B.D. et al. Proc. Natl. Acad. Sci, USA 104 15248-15253 (2007)

That Sticky Weather? Blame Global Warming

The Associated Press, Oct. 10, 2007

The world isn't just getting hotter from man-made global warming, it's getting stickier. It really is the humidity. 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 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. 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, 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.