Extreme heat is covering more of Earth, study says
New York Times Published 11:17 p.m., Monday, August 6, 2012
The percentage of Earth's land surface covered by extreme heat in summer has soared in recent decades, from less than 1 percent before 1980 to as much as 13 percent in recent years, a new scientific paper says.
The change is so drastic, the paper says, scientists can claim with near certainty that events like the Texas heat wave last year, the Russian heat wave of 2010 and the European heat wave of 2003 wouldn't have occurred without planetary warming caused by humans' release of greenhouse gases.
Those claims, which go beyond the established scientific consensus about the role of climate change in causing weather extremes, were advanced by James Hansen, a prominent NASA climate scientist, and two co-authors in a scientific paper published online Monday in the Proceedings of the National Academy of Sciences.
“The main thing is just to look at the statistics and see that the change is too large to be natural,” Hansen said in an interview.
The findings provoked an immediate split among his scientific colleagues, however.
Some experts said he had come up with a smart new way of understanding the magnitude of the heat extremes that people around the world are noticing.
Others suggested he had presented a weak statistical case for his boldest claims and the rest of the paper contained little that had not been observed in the scientific literature for years.
The divide is characteristic of the strong reactions Hansen has elicited playing dual roles in the debate over climate change and how to combat it.
As the head of the Goddard Institute for Space Studies in Manhattan, he is one of NASA's principal climate scientists and the primary custodian of its records of Earth's temperature.
Yet he has also become an activist who marches in protests to demand new government policies on energy and climate.
Climate-change skeptics routinely accuse Hansen of manipulating the temperature record to make global warming seem more serious, though there is no proof he's done so and the warming trend has repeatedly been confirmed by other researchers.
Scientists have long believed that the warming — roughly 2.5 degrees Fahrenheit over land in the past century, with most of that occurring since 1980 — was caused largely by the human release of greenhouse gases from burning fossil fuels.
Such emissions have increased the likelihood of heat waves and some other types of weather extremes such as heavy rains and snowstorms, they say.
But researchers have struggled with the question of whether any particular heat wave or storm can be definitively linked to human-induced climate change.
In the new paper, Hansen and his co-authors compared the global climate of 1951 to 1980, before the bulk of global warming had occurred, with the climate of the years 1981 to 2011.
They computed how much of Earth's land surface in each period was subjected in June, July and August to heat that would have been considered particularly extreme in the period from 1951 to 1980. In that era, they found, only 0.2 percent of the land surface was subjected to extreme summer heat. But from 2006 to 2011, extreme heat covered 4 percent to 13 percent of the world, they found.
“It confirms people's suspicions that things are happening” to the climate, Hansen said. “It's just going to get worse.”
Martin Hoerling, a researcher with the National Oceanic and Atmospheric Administration who studies the causes of weather extremes, said he shared Hansen's general concern about global warming. But he has in the past criticized Hansen for, in his view, exaggerating the connection between global warming and specific weather extremes. In an interview, he said he felt Hansen had done so again.
Hoerling contended Hansen's new paper confuses drought, caused primarily by a lack of rainfall, with heat waves.
“This isn't a serious science paper,” Hoerling said. “It's mainly about perception, as indicated by the paper's title. Perception is not a science.”
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