For three months last summer, temperatures in Texas soared higher than at any time in recorded history, and the state is still coping with the most expensive drought in its history. But can the 2011 Texas heat wave be attributed to global warming?
Most scientists are careful not to link specific weather events to climate change trends, but NASA's James Hansen and two colleagues from the NASA Goddard Institute for Space Studies and Columbia University have taken that plunge. They've gathered data they say shows that the 2011 Texas and Oklahoma heat wave—as well as a deadly Moscow heat in 2010—were "a consequence of global warming because their likelihood was negligible prior to the recent rapid global warming."
Their conclusions are based on more than 50 years of temperature data, Hansen told InsideClimate News. By comparing the recent shift toward extreme high summer temperatures with that data, he said his group was able to demonstrate that the record-breaking 2011 Texas heat wave wouldn't have occurred without global warming. This data also provides a broader context for the summer of 2011, which across the United States was the second warmest on record, with the National Oceanic and Atmospheric Administration's Climate Extremes Index twice the historical average.
Making a connection between the Texas heat wave and climate change could have profound practical and policy implications because, as Hansen and his colleagues write, the global warming trend "has been attributed with a high degree of confidence to human-made greenhouse gases."
Hansen has posted a draft of the new study, Perceptions of Climate Change: The New Climate Dice, on his website and is circulating it to colleagues for comment, a practice he has followed with other research. Meanwhile, NASA's Goddard Institute has posted to the institution's website the scientists' analysis of 2011 temperature data—an analysis that Hansen and his colleagues also used in the new paper.
Hansen, who directs the Goddard Institute, has become a target for climate change skeptics who say his activism undermines his science. But his scientific standing is so solid that his research continues to be published in respected scientific journals, including Environmental Science & Technology, Atmospheric Chemistry & Physics and the Review of Geophysics.
"Our paper deals with the frequency of hot seasons, mainly hot summers, because that is the most important season," Hansen said. "The times and places with hot summers tend to be where the weather is dominated ... by high pressure, so there is high correspondence between the [extremely hot outliers or extreme] heat waves and drought conditions." They also point out that summer, "when most biological productivity occurs, is the most important season for humanity and thus the season when climate change may have its biggest impact."
The Texas drought is now the most expensive on record, having caused more than $5 billion in damages. The latest outlook from the National Oceanic and Atmospheric Administration (NOAA) predicts that the drought will persist across much of Texas at least through end of April 2012.
Other climate scientists say that while the data Hansen and his colleagues have gathered is solid, they're not yet ready to draw such a definitive conclusion.
"The paper makes key arguments, but we haven't yet done the definitive analysis." said Don Wuebbles, professor of Atmospheric Sciences and Electrical and Computer Engineering at the University of Illinois. Wuebbles has also been a research scientist at Lawrence Livermore National Laboratory, and he shared the 2007 Nobel Peace Prize for his work with the Intergovernmental Panel on Climate Change (IPCC).
"It used to be that when an event like that [the Texas heat wave] would happen, a scientist would say 'We can’t say if that change is because of climate change,'" Wuebbles said. "But because of new analysis that looks at the probability of such an event occurring, we can say the likelihood of such an event occurring is greatly increased because of climate change." What's new in the Hansen paper, Wuebbles said, is "tying a specific event to climate change patterns."
Martin Hoerling, a research meteorologist with NOAA's Earth System Research Laboratory, thinks it's still too early to draw firm conclusions about the cause of the Texas heat wave. He also warned that the high Texas temperatures can't automatically be linked with the drought.
"The planet as a whole is warming as a result of greenhouse gases. That science is very solid," he said. "But we don't fully understand the variation from place to place."