If sunny February days in the mid-70s seem strange to you, even in Los Angeles, maybe you ain't seen nothin' yet.

Researchers at UCLA say cities around the world have been experiencing “significant increases in the frequency of extremely hot nights” during the last 40 years, according to a statement from the university.

UCLA geography professor Dennis Lettenmaier collaborated with researchers at the Indian Institute of Technology Gandhinagar, Northeastern University and the University of Washington, with the results published recently in the journal Environmental Research Letters.

The profs looked at daily rain, temperature and wind observations from the National Oceanic and Atmospheric Administration for 217 global cities with populations of more than 250,000 people. UCLA says they “found that prolonged periods of extreme heat increased significantly in 48 percent of them between 1973 and 2012.”

Heat waves of six or more days increased by more than 50 percent during the 40-year window, the academics found. Only about 2 percent of those cities saw a decline in heatwaves, the school says.

But the night time is the right time for this four-decade warm-up, apparently. Nearly two-thirds of the cities examined experienced “significant increases” in the frequency of extreme heat at night, UCLA says. Those included North American urban areas.

Credit: Robert Larson/L.A. Weekly Flickr pool

Credit: Robert Larson/L.A. Weekly Flickr pool

This is the result of our concrete jungles trapping in daytime warmth, Lettenmaier said:

The fact that the trend was so much stronger at night underscores the role of the heat island effect in urban areas. You have heat being stored in buildings and in asphalt, concrete and other building materials, and they don’t cool down as quickly as they would outside of the urban area. This effect was likely exacerbated by decreasing wind in most of the urban areas.

Four of the five years in that four-decade window that saw the largest number of heat waves were recent, says the university: 2009, 2010, 2011 and 2012.

UCLA says the results comprise “striking examples of climate change” — just in case you had any doubt. So be prepared to grab your lover and not bundle up at night in the future.


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