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What Happens When the Earth Can’t Cool Off Overnight?


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.

a graph from NOAA showing the continuous US temperature minimums

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:

Human mortality rates will rise, because people don’t sleep as well in heat and their bodies are less able to recover from daytime heat stress. A study this year in The Lancet Planetary Health showed that the mortality risk from hotter nights is higher than that from hotter days; in 28 cities in Asia, mortality rates could rise sixfold, as excessively hot nights double from 2016 to 2090. “Ambient heat during the night may interrupt the normal physiology of sleep,” the authors wrote. “Less sleep can then lead to immune system damage and a higher risk of cardiovascular disease, chronic illnesses, inflammation and mental health conditions.” People without air conditioners or homes will be disproportionately impacted by hot nights: When heat waves hit Seattle in July, for example, half the city’s 12 promoted cooling centers closed by 5 p.m., and none were open past 9.

Staple crop yields will shrink, because plants need something like sleep, too. This will have profound implications for global food security: Research has shown that ricecorn and wheat yields will all drop by 5% to 40% because plants, like people and animals, require a state of rest at night. In warmer nighttime temperatures, they respire more, processing the sugars made by photosynthesis and losing carbon and water in the process, both of which are vital to plant health and crop yields.


Fruit, vegetables, and wine will change, because many require cool nights to develop the qualities and flavors people like. Potatoes will get smaller, impacting farmers across the Northwest, who produce 60% of the U.S. supply. Tomatoes will have a harder time developing fruit. And wine will get weird: Cool night temperatures affect everything from grape acidity and ripeness to aroma and color. Warmer nights make for jammier, higher-alcohol reds and less acidic, lower-quality whites. They also make winemaking more expensive and harvest work more dangerous: Most grapes are picked at night, because cooler conditions are safer for workers and harvesting fruit cold preserves qualities that aid fermentation and keep energy costs low.

Changes in plant hardiness zones over time

Invasive species will continue to expand northward, because the coldest winter temperatures won’t kill as many of them. From the emerald ash borer’s recent arrival in Oregon to cogongrass found thriving in Boise, many invasive species once limited by cold winters are already spreading farther and faster. The temperature on the coldest night of the year — and the number of cold nights in a season — are important predictors of the survival rates of overwintering species. As winter nights get warmer, opportunistic invaders will be able to migrate. In addition, “sleeper species” — already-present nonnatives currently limited by local climate conditions — will become a problem, and some native species will, too.

Even more salmon and trout will die, because night temperatures play an outsized role in keeping rivers cool. During the day, river water warms primarily through radiative heating. Researchers say that won’t change much in a warmer climate. But at night, rivers cool because of the big difference between air and water temperature, which is shrinking as nights warm. Water temperatures over 68 degrees are becoming more common, making salmon and other cold-water fish extremely vulnerable to disease and other disruptions. Warmer water in rivers, lakes and ponds also holds less oxygen and is more prone to harmful algae blooms, both of which can cause large-scale fish die-offs and significant ecological problems.


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Wildfires will get harder to fight, because nighttime fire activity is on the rise. Wildfires typically slow at night, as temperatures and winds die down and relative humidity goes up, making vegetation — i.e., fuel — wetter. This allows firefighters to build breaks overnight to hamper a blaze’s progress. But that is happening less often now, especially in drought-stricken areas where fuel loads are unusually high. Research on fires from 2003-2020 showed that while fire activity overall has gone up, the rise is even more pronounced at night.

More solutions and adaptation will be needed, said Bond, the Washington climatologist, because these changes are already happening and will continue to ramp up. He believes that acknowledging the problem and taking appropriate steps could mitigate the worst consequences — planting different crops, for example, and restoring shady riverside habitats, as well as working harder to protect vulnerable people. “Hopefully, we can continue to have healthy agriculture, salmon in our streams, and keep people healthy, too,” he said. “It’s not like (the Pacific Northwest) will be unfit for human habitation. It’s just going to be a different place.”

Sarah Trent is an editorial intern for High Country News based in southwest Washington.

The above article originally appeared on High Country News and has been republished with permission. The Auburn Examiner has not independently verified its content and encourages our readers to independently confirm any information they feel is overly biased or questionable. 

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