Study projects how much worse global warming will make heat, humidity

The heat and humidity continue to pile onto the D.C. area, and a new study finds that if nothing is done about global warming, we and our children had better get used to it.

The Union of Concerned Scientists’ Killer Heat in the United States study puts a number on the rise in extreme heat that will ensue if nothing is done about global warming.

Astrid Caldas, a senior climate scientist at the Union of Concerned Scientists, a nonprofit group of scientists dedicated to environmental problems, said that without action to stop the release of heat-trapping gases, heat indexes — the combination of heat and humidity that determines how miserable the hot weather can get — “will increase across the whole nation, including areas that never felt a 90-degree heat index before.”

The D.C. area has historically averaged about 35 days per year with a heat index over 90 degrees in the years 1970 to 2000. If nothing is done, that number will rise to about 80 days per year by the midcentury period (2036-2065) and over 100 by the late century (2070-2099).

As for heat indexes of 105 — “like we’re going to see this weekend” — the number will go from 1 per year historically, to 20 in the midcentury period and over 40 by end of the century.

A man takes a swig of water in Washington, D.C., on Sunday, July 1, 2018. (WTOP/Dave Dildine)

And that number could be conservative in certain parts of the region, Caldas added — it doesn’t take into account several factors, including the “urban heat island” phenomenon that occurs when cities trap heat.

Nationwide, the report finds that the average number of days per year with a heat index over 100 will quadruple in the late century; the number of days over 105 will go up eightfold.

And there will be days when the heat index will be what the UCS calls “off the charts” — when the scale developed by the National Weather Service isn’t adequate to measure the misery.

Limiting the rise in global temperatures to 3.6 degrees above preindustrial levels — the target specified in the Paris Agreement — would “spare millions of people in the United States from the threat of relentless summer heat,” the report said.

Without any action, the UCS said temperatures by century’s end will be 8 degrees higher.

Caldas had sharp words for people who don’t believe the warming is real.

“Feel the heat. That’s all you need to do — you can’t deny it’s getting hotter. The 20 warmest years on record have happened since [1998]. The last five years have been the warmest on record. You cannot deny facts and data,” Caldas said.

WTOP’s Kristi King contributed to this report.

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