"A new statistical analysis by NASA scientists has found that Earth's land areas have become much more likely to experience an extreme summer heat wave than they were in the middle of the 20th century. The research was published today in the journal Proceedings of the National Academy of Sciences.
"The statistics show that the recent bouts of extremely warm summers, including the intense heat wave afflicting the U.S. Midwest this year, very likely are the consequence of global warming, according to lead author James Hansen of NASA's Goddard Institute for Space Studies (GISS) in New York.
"'This summer people are seeing extreme heat and agricultural impacts,' Hansen says. 'We're asserting that this is causally connected to global warming, and in this paper we present the scientific evidence for that.'
"Hansen and colleagues analyzed mean summer temperatures since 1951 and showed that the odds have increased in recent decades for what they define as 'hot,' 'very hot' and 'extremely hot' summers.
"The researchers detailed how 'extremely hot' summers are becoming far more routine. 'Extremely hot' is defined as a mean summer temperature experienced by less than one percent of Earth's land area between 1951 and 1980, the base period for this study. But since 2006, about 10 percent of land area across the Northern Hemisphere has experienced these temperatures each summer."
Link to scientific paper: Perception of Climate Change (PNAS)