February 17, 2011

Extreme Storms Linked To Greenhouse Gas Emissions

Two studies published Wednesday in the journal Nature suggest that extreme rainstorms and snowfalls have grown considerably stronger over the last half of the 20th century, with scientists linking man-made global warming to the torrential downpours.

The studies are among the first to identify the telltale signs of climate change and its impact on deadly and damaging extreme weather events.

Australia, Brazil, Pakistan and Sri Lanka have all been devastated recently by massive flooding, raising questions about whether global warming was at least partially to blame.

Indeed, computer models have long predicted that growing levels of atmospheric greenhouse gases would amplify instances of diluvian rainfall.

However, until now that link has been largely theoretical.

"This paper provides the first specific evidence that this is indeed the case," Francis Zwiers, a researcher at the University of Victoria in Canada and a co-author of one of the studies, told the AFP news agency.

"Humans influence the intensity of precipitation extremes," he said.

Data collected between 1951 and 2000 from Europe, Asia and North America showed that, on average, the most extreme 24-hour precipitation event in a given year increased in intensity over the last half of the 20th century.

Zwiers said that when these events were compared with changes simulated by climate models, the fingerprint of human influence on the planet's weather patterns was unmistakable.

"The observed change cannot be explained by natural, internal fluctuations of the climate system alone."

The primary driver was more water in the air.

"In a warmer world the atmosphere has greater moisture-holding capacity," Zwiers said.

However, that doesn't necessarily mean that places with scarce precipitation will see more rain, he added. In fact, some places are likely to be drier. 

What it does mean is that when a storm does occur, there will be more water available.

One reason it took so long for scientists to begin finding definitive connections between global warming and extreme weather events is that only in recent years has the accumulated influx of greenhouse gases become more clear.

"We are finding it easier and easier to detect that signal in observations," Zwiers said.

A lack of reliable, long-term data has also hindered progress, along with the massive computer power required to test increasingly complex computer models against reality.

The second study sought to assess the impact of global warming in England in 2000, when the nation experienced its wettest fall on record. 

Scientists led by Myles Allen of the University of Oxford compared two climate models -- one based on historical weather data and another on a "parallel" autumn 2000 simulating conditions in which no greenhouse gases had been emitted in the 20th century.

The researchers found that global warming likely doubled the chances that such an event would occur.

"To really pin down the difference between these two worlds, we needed to repeat the simulation thousands of times," said Pardeep Pall, the study's lead author, who began the project in 2003 while a graduate student.

"We asked members of the public across the world to run the simulations for us on their own personal computers using their idle time," Pall told the AFP.

Britain's national climate and weather office is developing tools based on the study's results that will measure the human impact on future extreme weather events.

"This kind of study is going to allow us to quantify how climate change is affecting people now so it ceases to be some hypothetical projection of the future," Allen said.

People interested in providing computing power to the study can find information at www.climateprediction.net.


On the Net: