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Is global warming behind the recent heat waves? (+video)

The unusual heat waves felt in Texas, Oklahoma, Moscow, and elsewhere in recent years are almost certainly a result of global warming, according to a study led by NASA scientist and climate activist James Hansen. 

By Wynne ParryLiveScience Senior Writer / August 7, 2012

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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.

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The Northern Hemisphere over the past 30 years has seen an increase in the amount of land area experiencing what NASA scientists define as "extremely hot" summer temperatures, according to a new analysis led by James Hansen at the NASA Goddard Institute for Space Studies. These regions of 'extremely hot' temperatures are shown on the map as brown.

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.

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