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.