By Sarah Trent, High Country News

At 5 a.m. on Aug. 18, the National Weather Service in Seattle tweeted that the temperature that night still hadn’t dropped below 71 degrees, a whopping 14 degrees warmer than average this time of year. In an update later that day, the agency said that just before 8 a.m., the air finally cooled a little, bottoming out at 68 degrees but still breaking the daily record. The reprieve was only momentary: A few minutes later, the day began to warm again.

“On the west side of the Cascades, if the minimum temperature doesn’t dip below 60, that’s relatively unusual,” said Nick Bond, Washington’s state climatologist. “But we’re seeing a lot more of those kinds of nights in recent years.” In the past, Bond said, there were maybe 10 nights that warm in any given year. In 2019, there were 38. This year, there have already been almost 20, and since summers are getting longer as temperatures rise, there will be more and warmer nights in the future.

Climate change is causing overnight lows to rise at a faster clip than daytime highs. June through August highs — which usually peak in the late afternoon — have gone up at a rate of 3.5 degrees per century since 1970 on average across the U.S., according to data from the National Oceanic and Atmospheric Administration. Daily summer lows, which typically occur overnight, have risen by 4.8 degrees per century. In July, the U.S. set a record for the warmest nights in 128 years of national recordkeeping. And winter temperatures are going up even faster, with the lows from December through February rising 6.1 degrees every century, compared to 5.6 degrees for daytime highs.

This time-of-day asymmetry is a significant effect of climate change: Warmer air holds more moisture, as both humidity and cloud cover, and acts like a sort of blanket over the landscape. “So when the sun sets at night, not as much of the heat that’s been collected during the day is allowed to escape, and the nighttime doesn’t cool off as much,” said Karin Gleason, a climatologist at NOAA. The consequences are far-reaching.

“When you set a new daytime high temperature, it seems to get all of the attention,” Gleason said. “But in reality, when it comes to human health, animal health, crop health, plant health, what really seems to have the biggest impact is not so much that it got so warm during the day, it’s that the temperatures didn’t cool off enough at night.”

Here’s just some of what’s at stake: