The record heat wave and drought tormenting much of the United States is part of a "new extreme category" of weather that is most likely the result of global warming, a top U.S. government climate scientist said Monday.
And if carbon emissions continue unchecked, such events will become routine, with more extremes common within 50 years, according to research published Monday.
"You would not have these extremes without global warming," James Hansen, NASA's top climate-change scientist and the main author of the paper, said in an interview.
Hansen was one of the first scientists to identify threats from climate change and to advocate for action to counter carbon-dioxide emissions. The paper by Hansen and two co-authors was published in the Proceedings of the National Academy of Sciences. It was written before the recent spate of record heat and drought across the nation, but Hansen said the weather fits the pattern identified in the paper.
This year, the January through June period was the warmest on record in the United States, with temperatures 4.5 degrees higher than the 20th century average.
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-based Trinnovim, which provides support for agencies such as NASA.
They looked for examples of "weather anomalies," defined as three standard deviations from a statistical history.
From 2006 to 2011, areas meeting the criteria covered about 4 percent to 13 percent of the Earth. Such extremes were basically nonexistent in the period studied from 1951 to 1980, they said.
If the researchers had picked a longer period that included the Dust Bowl of the 1930s and the Texas heat waves of the 1950s they would not have found such extreme differences, said John Christy, director of the Earth System Science Center at the University of Alabama in Huntsville.
Hansen countered that if they had compared recent years with the previous 10,000 years, the study would have found similar, and perhaps even stronger, results showing the effects of climate change.