By Brian Vastag
The Washington Post, 16 February 2011
Human emissions of carbon dioxide and other greenhouse gases helped trigger the increase in extreme rain events seen in North America over the second half of the 20th century, a group of climate scientists reported Wednesday in the journal Nature.
In a second Nature paper, another group reported that human greenhouse gas emissions likely contributed to the horrendous floods that inundated England and Wales in the fall of 2000. Those scientists ran sophisticated climate simulations across a network of tens of thousands of home computers that volunteers loaded with climate-modeling software.
"Human influence on the climate system has the effect of intensifying precipitation extremes," said Francis Zwiers, a climate researcher at Environment Canada in Toronto and lead researcher on the first study.
Zwiers and his team gathered 50 years of rainfall statistics, and compared those observations to predictions made by computer simulations of the 20th century climate.
Those simulations included the warming impact of the billions of tons of carbon dioxide human society has pumped into the atmosphere.
The study found that observed increase in deluges "cannot be explained by natural internal fluctuations of the climate system alone," said Zwiers. In other words, only the addition of greenhouse gases to the atmosphere explains why the United States and Canada have experienced a dramatic increase in heavy downpours.
"Large [rainfall] events are becoming larger," Zwiers said. His work found that from 1951 to 1999, the probability of heavy downpours becoming even more extreme grew by about 7 percent, a figure he characterized as "really substantial."
Richard Allan, a climate scientist at the University of Reading in England who was not part of the study, called the method employed by Zwiers "very rigorous."
He added, "There's already been quite a bit of evidence showing that there has been an intensification of rainfall" events across the globe.
But until now "there had not been a study that formally identified this human effect on precipitation extremes," Zwiers said. "This paper provides specific scientific evidence that this is indeed the case."
The explanation is simple physics: Warmer air holds more water vapor. That means when rainfall gets triggered, the air contributing to the storm is holding more water than it did in the cooler pre-industrial world.
In the second study, Pardeep Pall from the University of Oxford led an international team that found "human [greenhouse gas] emissions substantially increased the odds of floods occurring in what was the record wet autumn of 2000."
The floods that inundated the United Kingdom that year were the worst since at least 1766.
Pall's conclusion springs from two sets of many thousands of computer weather simulations. The first set simulated the atmosphere in its real state - loaded with all the extra carbon dioxide humans have added to it. The second set simulated a parallel world where no extra carbon dioxide had been added to the atmosphere.
The odds of the massive floods occurring in the no-extra-greenhouse-gases parallel world were about half the odds of the floods happening in the real world, Pall said.
Tens of thousands of volunteers loaded climate-predicting software onto their home computers via the Web site climateprediction.net, Pall said, providing a vital boost in computer power needed to run the many thousands of climate simulations.
The climateprediction.net project has been running for several years. As of Wednesday morning, some 54,000 computers around the world were helping climate scientists crunch data. The donated computer time has completed 118 million years of climate simulations, according to the climateprediction.net website.
The research could not absolutely determine that the floods had been triggered by greenhouse gases, said the University of Oxford's Myles Allen, who contributed to the study. "It's important to stress there is uncertainty in this work."
With the number and intensity of extreme deluges expected to climb, climate scientists and meteorologist are rushing to build better flood prediction systems, particularly for the developing world. On Monday,The Post reported on one scientist's assertion that last summer's floods in Pakistan could have been predicted - and the populace warned - if available data had been heeded.