Last week, the journal Nature made a big splash in the press with the publication of two studies which found that manmade climate change has contributed to the intensification of heavy rains and increased the likelihood of floods that have, collectively, affected millions of people.
After years of hearing the scientific refrain that no single weather event can be attributed to climate change, those papers—as well as a spate of abnormal events in the last year, including a heat wave in Russia, floods in Pakistan, drought in China, and intense snowfall in the United States—have many reporters wondering if they now have a green light to report at will that severe weather bears the hallmarks of global warming.
In column for the Seattle PostGlobe, for example, former Post-Intelligencer reporter Jake Ellison argued that The New York Times was remiss for not mentioning climate change in a February 8 article about one of China’s worst droughts in decades. “Reporters should include #climatechange angle in all weather-tragedy stories,” the PostGlobe wrote in a subsequent tweet. “What say u, @cjr?”
We say, “not necessarily.” When talking about a heavy storm, many scientists—seemingly more every day—will say, “This is what we expect to see more of in a warmer world.” It is still impossible to attribute any single weather event to manmade climate change, however, and the connections between severe weather and climate change remain highly complex, nuanced, and uncertain. Andrew Freedman, the managing editor for online content at Climate Central, summed up the situation perfectly in an excellent post for The Washington Post’s Capital Weather Gang blog on Wednesday:
Although the ties between climate change and extreme weather events often elicit absolute statements from advocates on various sides of the climate change issue, the reality is that while much can be said, much remains unknown. The challenge for reporters as well as scientists is to accurately convey both the scientific findings and the uncertainties surrounding them.
Many journalists, politicians, and climate scientists have run into trouble by portraying the links between climate change and extreme weather in stark terms, rather than shades of gray.
In one of the two studies published in Nature last week, researchers compared records of precipitation in the Northern Hemisphere from 1951 to 1999—which show an intensification of the heaviest rain and snow events—to dozens of model simulations that either accounted for human greenhouse-gas emissions or didn’t. They found that only when the gases were included did the models match what actually happened. In the other study, researchers harnessed the power of myriad personal computers through the Climateprediction.net project in order to run thousands of model simulations of a major flood in England and Wales in 2000. Again, some simulations included human greenhouse-gas emissions and some didn’t, and the researchers found that including the gases significantly increased the odds that the flood would occur.
The physical explanation for the connection between heavy precipitation and climate change is fairly straightforward—warmer air can hold more water vapor. It is important to note, however, that while the first study found that climate change had contributed to the intensification of heavy rains, it did not find that warming had caused any particular storm. Likewise, while the second study found that climate change had increased the likelihood of a particular flood, it did not find that warming has caused the flood. Moreover, both papers pointed out that, like all scientific studies, they are subject to a range of uncertainties. This doesn’t mean the studies aren’t robust and important, it just means that they cannot be used to generalize about weather-climate connections. (In a companion piece for Nature, climatologist Richard Allen, who wasn’t involved with either study, has a great rundown of their limitations and promise.)