The Northern Hemisphere over the past 30 years has seen more hot (orange), very hot (red) and extremely hot (brown) summers, compared to a base period defined in this study from 1951 to 1980.
This visualisation shows how the area experiencing "extremely hot" summers grows from nearly nonexistent during the base period to cover 12 percent of land in the Northern Hemisphere by 2011, the journal Proceedings of the National Academy of Sciences reports.
The statistics show that the recent bouts of extremely warm summers, including the intense heat wave afflicting the US Midwest this year, very likely are the consequence of global warming, according to James Hansen of NASA's Goddard Institute for Space Studies (GISS), New York, who led the study.
"This summer people are seeing extreme heat and agricultural impacts. We're asserting that this is causally connected to global warming, and in this paper, we present the scientific evidence for that," says Hansen, according to a GISS statement.
Hansen and colleagues analysed 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.
Washington: Land areas are now much more likely to experience an extreme summer heat wave than they were in the middle of the 20th century, according to a new NASA statistical analysis.
First Published: Tuesday, August 07, 2012, 16:32