Climate's role in US droughts is under scrutiny

Peter Aldhouse and Michael Marshall iin New Scientist: Can the extreme drought that devastated crops across the US Midwest in 2012 be blamed on global warming? Probably not, according to a report from the US National Oceanic and Atmospheric Administration (NOAA). But the conclusion has been met with stinging criticism from another leading climate scientist.

The controversy highlights the need for better methods to assess if and how extreme weather events are linked to climate change.

Martin Hoerling of NOAA's Earth System Research Laboratory in Boulder, Colorado, and colleagues investigated why seasonal forecasts failed to predict last year's drought. They also ran computer simulations to see if a similar drought would have occurred in the absence of human greenhouse gas emissions.

The team did not find any clear warning signs that could have been used to predict the drought, and their simulations suggest it would have taken place irrespective of global warming. Instead, they blame the event on natural atmospheric variability, which prevented moist air from moving north of the Gulf of Mexico during the summer.

But Kevin Trenberth of the National Center for Atmospheric Research, also in Boulder, claims the models used by Hoerling's team were not good enough for the task. What's more, he says the team asked the wrong questions. "Why did we break all the records? That's the real question," he says. He argues that climate change could have made the drought more severe, even if the event was triggered by natural factors....

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