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An increase in extreme heat events since the mid-20th century has been linked to overall climate warming, U.S. researchers say.

A statistical analysis by NASA scientists suggests the recent bouts of extremely warm summers, including the intense heat wave afflicting the U.S. Midwest this year, are very likely the consequence of global warming, the space agency's Goddard Institute for Space Studies in New York said in a release Monday.

"This summer people are seeing extreme heat and agricultural impacts," study lead author James Hansen said. "We're asserting that this is causally connected to global warming, and in this paper we present the scientific evidence for that."

The analysis of mean summer temperatures since 1951 showed the odds have increased in recent decades for what the researchers define as "hot," "very hot" and "extremely hot" summers.

"Extremely hot" is defined as a mean summer temperature experienced by less than 1 percent of Earth's land area.

Since 2006, about 10 percent of the land area across the Northern Hemisphere has experienced these temperatures each summer, researchers said.

The analysis suggests more hot events will be the new normal, they said.