The researchers then compared the observations to the changes in rainfall that multiple climate models predict should be attributable to human activity.
"Because we have this large number of simulations, we can average them all together in essence and filter out the effects of internal variability ... to obtain a best estimate," said study co-author Francis Zwiers, a climate scientist with Environment Canada.
The comparison, which appears in this week's journal Nature, shows that most of the changes in precipitation are due to human actions.
"You have this large-scale engine that moves moisture around the planet, and under greenhouse gas forcing, this engine essentially become more intense," Zwiers said.
However, the reasoning fails to fully explain why the region just south of the Equator is wetter. All things being equal, it should be drier, Zwiers noted.
The researchers suspect the increase is the result of a southward shift in the zone where trade winds of the Northern and Southern Hemispheres meet, but why the zone may have shifted remains unresolved.
Looking to the future, the study increases confidence in the prediction of climate models.
The models indicated a change should have occurred in the 20th-century rainfall, and the pattern shows up in the observations, Zwiers said.
However, the change predicted by the models was less than observed, which indicates the models, as a group, are not responding to greenhouse gases as much as they should.
Climate models' under-prediction of 20th-century change makes their accuracy for the 21st century a bit worrisome, Zwiers said. (Related: "Global Warming Models Underpredict Increase in Rainfall, Study Says" [May 31, 2007].)
"Maybe the projections for the future are not projecting a big enough change," he said.
The models predict the 20th-century trend should continue in the Northern Hemisphere, but most suggest the wet region just south of the Equator will revert to a drier pattern.
Any specific event cannot be pinned on human emissions of greenhouse gases, Zwiers said. Further studies of the climate models should reveal if increased greenhouse gases raise the risk of extreme rainfall events.
Others are not waiting for these scientific studies.
The findings were released as heavy flooding in the United Kingdom and China make headlines around the world.
"Extreme events such as we have seen in recent weeks herald the specter of climate change," Nick Reeves, the executive director of the Chartered Institution of Water and Environmental Management in London, England, told the Reuters news agency today.
"And it would be irresponsible to imagine that they won't become more frequent."
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