A Texas State Park police officer walked across the lake bed of O.C. Fisher Lake in San Angelo, Texas. A new scientific paper says that the drought and other recent extreme weather events have been caused by global warming.
By JUSTIN GILLIS
The percentage of the earth’s land surface covered by extreme heat in the summer has soared in recent decades, from less than 1 percent in the years before 1980 to as much as 13 percent in recent years, according to a new scientific paper.
The change is so drastic, the paper says, that 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 would not have happened without the planetary warming caused by the human 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 E. Hansen, a prominent NASA climate scientist, and two co-authors in a scientific paper published online on 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,” Dr. 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 that he had presented a weak statistical case for his boldest claims and that 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 that Dr. 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 the earth’s temperature. Yet he has also become an activist who marches in protests to demand new government policies on energy and climate.
The latter role — he has been arrested four times at demonstrations, always while on leave from his government job — has made him a hero to the political left, and particularly to college students involved in climate activism. But it has discomfited some of his fellow researchers, who fear that his political activities may be sowing unnecessary doubts about his scientific findings and climate science in general.
Climate-change skeptics routinely accuse Dr. Hansen of manipulating the temperature record to make global warming seem more serious, although there is no proof that he has 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, like 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, titled “Perception of Climate Change,” Dr. 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 the 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 from 4 to 13 percent of the world, they found.
“It confirms people’s suspicions that things are happening” to the climate, Dr. Hansen said in the interview. “It’s just going to get worse.”
The findings led his team to assert that the big heat waves and droughts of recent years were a direct consequence of climate change. The authors did not offer formal proof of the sort favored by many climate scientists, instead presenting what amounted to a circumstantial case that the background warming was the only plausible cause of those individual heat extremes.
Dr. Hansen said the heat wave and drought afflicting the country this year were also a likely consequence of climate change.
Some experts said they found the arguments persuasive. Andrew J. Weaver, a climate scientist at the University of Victoria in British Columbia who reviewed the paper before publication, compared the warming of recent years to a measles outbreak popping up in different places. As with a measles epidemic, he said, it makes sense to suspect a common cause.
“You can actually start to see these patterns emerging whereby in any given year more and more of the globe is covered by anomalously warm events,” Dr. Weaver said.
But some other scientists described the Hansen paper as a muddle. Claudia Tebaldi, a scientist with an organization called Climate Central that seeks to make climate research accessible to the public, said she felt that the paper was on solid ground in asserting a greater overall likelihood of heat waves as a consequence of global warming, but that the finding was not new. The paper’s attribution of specific heat waves to climate change was not backed by persuasive evidence, she said.
Martin P. Hoerling, a researcher with the National Oceanic and Atmospheric Administration who studies the causes of weather extremes, said he shared Dr. Hansen’s general concern about global warming. But he has in the past criticized Dr. Hansen for, in his view, exaggerating the connection between global warming and specific weather extremes. In an interview, he said he felt that Dr. Hansen had done so again.
Dr. Hoerling has published research suggesting that the 2010 Russian heat wave was largely a consequence of natural climate variability, and a forthcoming study he carried out on the Texas drought of 2011 also says natural factors were the main cause.
Dr. Hoerling contended that Dr. Hansen’s new paper confuses drought, caused primarily by a lack of rainfall, with heat waves.
“This isn’t a serious science paper,” Dr. Hoerling said. “It’s mainly about perception, as indicated by the paper’s title. Perception is not a science.”
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