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Extreme rain and snow events linked to global warming, study finds

Authors say a study that looked at the rise in extreme rain and snowfall in the Northern Hemisphere in the last half of the 20th century is the first to show a link to global warming.

By Staff writer / February 16, 2011

An increase in snow and rain levels in the second half of the 20th century is now being linked to global warming.

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Global warming helped drive a rise in the intensity of extreme rain and snowfall across much of the Northern Hemisphere during the last half of the 20th century, a new study has found.

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The work builds on previous research that established broad trends in extreme precipitation events over the 20th century. Those studies suggested a link between climate change and heavy precipitation, based on the atmosphere's ability to hold more moisture as it warms.

But, according to the research team that conducted the new study, those studies stopped short of presenting evidence for a direct link between human-triggered global warming and extreme precipitation.

This latest study, the team says, represents the first time scientists have formally identified a link between the warming during the last half of the 20th century and the trends in extreme precipitation.

Until now,"there has not been a study that formally identified this human effect in precipitation extremes," says Francis Zwiers, a climate scientist who heads the Pacific Climate Impacts Consortium at the University of Victoria in British Columbia. This new study "provides specific scientific evidence that that is indeed the case."

The team's results are based on precipitation records as well as on results from global climate models, which the team used to see which combination of factors affecting climate yielded precipitation patterns that most closely resembled the real-world drenching.

But the study also shows a gap between models and the real world that some researchers say warrants caution in claiming to have discovered the smoking gun linking warming and the more extreme precipitation.

The extreme-precipitation trends the models display when greenhouse gases from fossil fuels are included head in the same general direction as the real-world measurements. But the the size of the change is smaller in the models than in the real world. Resolving this mismatch will be vital to increasing the credibility of such attempts at detecting global warming's imprint on patterns of intense precipitation.

The study appears in Thursday's issue of the journal Nature.

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Precipitation measurements can be tricky to work with. Recording stations can move, for instance. Or one region may have fewer recording stations than another, some of which may not have been gathering data as long as others.

Indeed, the team notes that it selected the period it looked at – 1951 to 1999 – because after that period a significant number of the 6,000 recording stations whose data the team used shut down, particularly in Eurasia.

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