Is global warming behind the recent heat waves?

It's no surprise to those who follow climate science that temperature patterns have shifted as the world has warmed up. But in a new study, outspoken climate scientist James Hansen goes a step further, saying devastating heat waves in recent years are the result of global warming.

Since natural dynamics — such as fluctuations in sea-surface temperatures in the equatorial Pacific Ocean — contribute to extreme events like heat waves, this connection can be controversial.

Scientists disagree on the degree to which global warming can be blamed for extreme events, such as heat waves, and Hansen, of NASA's Goddard Institute for Space Studies and no stranger to controversy, is among those most aggressive about making this connection.

In the last three decades, both average and extreme temperatures have warmed up and heat waves hit much large areas, Hansen and colleagues write in a study published online today (Aug. 6) in the journal Proceedings of the National Academy of Sciences.

"It follows that we can state, with a high degree of confidence, that extreme anomalies such as those in Texas and Oklahoma in 2011 and Moscow in 2010 were a consequence of global warming because their likelihood in the absence of global warming was exceedingly small," they write. [Extreme Weather Facts: Quiz Yourself]

Martin Hoerling, a research meteorologist at the National Oceanographic and Atmospheric Administration's Earth Systems Research Laboratory, called this interpretation "flawed scientifically."

"The weather patterns responsible for most of today's heat waves would have happened regardless of human-induced climate change," Hoerling told LiveScience. Bringing carbon dioxide down to the level Hansen sees as safe "would not eradicate heat waves."

Hansen's team acknowledges that dynamics other than global warming are at play when a heat wave hits, saying that the abnormally warm temperatures of late are the result of a combination of specific weather patterns and global warming.

This description is closer to the truth, Hoerling said. "In some cases that combination is 95 percent natural and 5 percent climate change, sometimes the mix is a little bit different."

Historically, climate scientists have shied from making a connection between an extreme event and global warming, but more and more so-called attributional studies examine these events for human fingerprints.

For example, a 2004 study concluded that human alterations to climate more than doubled the odds of heat waves on the scale of the 2003 scorcher that devastated western Europe.

The record-setting 2010 Russian heat wave to which Hansen refers appears to have generated some conflicting analyses. A study in which Hoerling participated concluded this heat wave was due mainly to natural atmospheric variability. Meanwhile, a more recent study led by Kevin Trenberth, a senior scientist the National Center for Atmospheric Research in Boulder, Colo., linked record high sea-surface temperatures in the Indian Ocean and tropical Atlantic Ocean — which were, in turn, influenced by global warming — with the heat wave.

With regard to the Russian heat wave, Trenberth told LiveScience he sees his study and Hansen's as taking complementary approaches.  

Both he and Hansen are at the aggressive end of the spectrum when it comes to attributing weather events to climate change, Trenberth said. "James Hansen and I are pushing to get scientists to think about and do statistics on this rather differently, and now we are not part of mainstream in this regard."

Trenberth questions why an attribution study is necessary for every single event: "Human influence is changing the odds, it is pushing things in a direction where we have greater extremes," he said.

"Could these things have actually happened without global warming? In most cases, they probably wouldn't. There are very small odds they could have occurred," Trenberth said.

Hansen could not be reached by deadline.

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