If you get the feeling lately that whenever it rains, it pours, blame greenhouse gases. Episodes of intense rainfall are happening more frequently, thanks to human emissions of carbon dioxide, a new study suggests. A companion article marks the first time that researchers have pinned the blame for an extended bout of flooding—the inundations that swamped parts of the United Kingdom in autumn 2000—on those planet-warming gases.
In October and November 2000, floods soaked large swaths of England and Wales, damaging almost 10,000 homes and businesses and causing losses estimated to exceed $2 billion. Lacking hard data, scientists are loath to assign blame for this flood or any weather event to climate change. Now, however, research reported  in online today in Nature suggests that human-caused climate change, brought about by past emissions of carbon dioxide, almost certainly boosted the risk of those floods.
To determine how rising temperatures have altered flood risk, atmospheric scientist Pardeep Pall of the University of Oxford in the United Kingdom and his colleagues ran thousands of climate simulations. In roughly half of them, they reduced atmospheric concentrations of carbon dioxide to levels measured in the year 1900, and they adjusted ocean temperatures and the amount of Arctic sea ice—which affects high-latitude weather patterns—accordingly. In the other simulations, they modeled modern conditions. Then they compared the rainfall amounts generated in both types of simulations. Finally, they fed the rainfall values into a model that assesses the potential for flooding.
In 90% of the simulations, results suggested that the flood risk in England and Wales in autumn 2000 was at least 20% higher than it would have been in 1900. In two-thirds of the cases, the flood risk was at least 90% higher—nearly double the risk at the beginning of the 20th century.
Such extremes in precipitation are evident everywhere, says Francis Zwiers, a climate modeler with the government agency Environment Canada in Toronto. In large part, it’s simple physics. Warm air can hold more water vapor than cold air. For each 1°C rise in temperature, air can contain about 7% more moisture, which can condense to fall as rain.
In another analysis, also reported  online today in Nature , Zwiers and colleagues analyzed meteorological data collected at more than 6000 weather stations in the Northern Hemisphere from 1951 through 1999. By comparing those data with the output of several climate models, the researchers found that warming caused by human emissions of carbon dioxide has boosted the numbers of 1- and 5-day-long episodes of intense precipitation across North America, Eurasia, and India.
Both studies improved on previous work. Zwiers and colleagues compared observational data with results from simulations rather than running a model-to-model comparison. Pall’s team performed a large number of simulations and analyzed them statistically.
“Since we first recognized the threat of global warming, scientists have consistently stated that no single extreme weather event can be definitively link to anthropogenic climate change,” says Mark Maslin, a climate scientist at the Environment Institute at University College London. The paper by Pall and colleagues “fundamentally changes this,” he notes.
Recent floods in Pakistan, Brazil, and Australia were larger and did more damage than the U.K. floods of 2000, Maslin notes. Together, the Nature papers “provide a very strong scientific message to politicians that the occurrence and magnitude of huge floods around the world have been made worse by [human-caused] climate change,” he adds. “And there we were thinking all the effects of climate change would be in the far future.”