The drought in Texas last year, the heat wave in Russia in 2010 are among weather extremes that "certainly would not have occurred" without global warming, according to a paper by a U.S. government climate scientist.
Abnormal weather episodes were so rare from 1951 to 1980 that the recent events can only be explained by the effects of the increasing accumulation of carbon dioxide in the atmosphere, according to research being published today. If carbon emissions continue unchecked, such events will become routine, with more extremes common within 50 years.
"We now know that the chances these extreme weather events would have happened naturally -- without climate change -- is negligible," James Hansen, the top climate-change scientist at the U.S. National Aeronautics and Space Administration and the main writer of the paper, said in a statement. "The natural changes in weather from day-to-day and year-to-year" don't explain those events.
The paper by Hansen and two co-authors, to be published in the peer-reviewed Proceedings of the National Academy of Sciences, was written before the recent record heat waves and drought across the U.S. Once that data is analyzed, it probably will show links to global warming as well, Hansen wrote in the Washington Post.
Hansen was one of the first scientists to identify threats from climate change, and to advocate for action to counter carbon-dioxide emissions. He testified to Congress in 1988 that global warming had begun, and last year was arrested during a protest in front of the White House against the proposed Keystone XL pipeline. In 2008, a government watchdog agency, responding to a complaint from Hansen, said political appointees in NASA's public-affairs office from 2004 through early 2006 "reduced, marginalized or mischaracterized" information on climate-change science for political advantage.
Hansen wrote the paper on extreme weather with Makiko Sato, also of NASA and the Columbia University's Earth Institute, and Reto Ruedy, of Arlington, Virginia-based Trinnovim LLC, which provides support for U.S. agencies such as NASA.
They looked for examples of "weather anomalies," defined as three standard deviations away from normal based on a statistical history. From 2006 to 2011, areas meeting the criteria covered about 4 percent to 13 percent of the Earth. Those types of extremes were basically nonexistent in the three decades prior to 1980, it said.
"Probably the most important change is the emergence of a new category of 'extremely hot' summers," the researchers concluded.
Drought and heat waves struck at least six states last year, and in Texas and Oklahoma are blamed for losses as high as $10 billion in crops, livestock and timber, according to the U.S. National Oceanic and Atmospheric Administration in a report.
The 2010 drought and heat wave in Russia destroyed 13 million hectares (32.1 million acres) of crops, Prime Minister Vladimir Putin said in March 2011.