The hottest part of a summer day may come in the afternoon, but one of the clearest signals of a warming climate is happening while most people are asleep.
Raleigh now experiences about 40 more warmer-than-normal summer nights than it did in the early 1970s, according to a new Climate Central analysis. About 71% of summer nights in Raleigh are now warmer than the historical average.
And this summer is following that trend.
Morning temperatures at Raleigh-Durham International Airport have averaged above normal every month this summer so far, running about 1 to 6 degrees warmer than normal, according to WRAL meteorologist Aimee Wilmoth. Overall, it’s the fifth-warmest summer on record at RDU so far.
Raleigh has gained 40 warm summer nights
The trend extends far beyond North Carolina.
Climate Central analyzed weather station data from 243 major U.S. cities, comparing the frequency of summer nights with minimum temperatures above the 1991-2020 normal. It found 95% of those cities now experience more unusually warm summer nights than they did in the early 1970s.
On average, cities have gained 24 warmer-than-normal nights each summer. The South and Southeast are among the regions experiencing the largest increases, averaging 33 to 35 additional warm nights. In Raleigh, Climate Central found about 40 additional warmer-than-normal nights each summer compared with the early 1970s.
“Nighttime temperatures are warming almost twice as fast as daytime temperatures are in North America and specifically in the United States,” Climate Central meteorologist Shel Winkley said. “We’re sleeping through one of the biggest signals of climate change.”
Winkley said humidity is an important part of the equation in the Southeast. During the day, the Earth’s surface absorbs energy from the sun. After sunset, that heat begins escaping. But humid air does not cool as efficiently as dry air, helping temperatures remain elevated overnight.
Climate change is leaving a growing fingerprint
Climate Central also used its Climate Shift Index to estimate how human-caused climate change has altered the likelihood of unusually warm nights.
A Climate Shift Index level of 2 means a particular temperature was made at least twice as likely because of human-caused climate change.
In the 1970s, Raleigh experienced an average of about four summer nights per year with that strong climate-change fingerprint. By the 2000s, that had climbed to roughly 14 nights. During the 2010s and 2020s, Raleigh has averaged roughly 25 to 30 such nights each summer.
“So about a month where we can confidently say, on average, yeah, climate change is playing a significant role in that warmth that you’re feeling during the overnight hours,” Winkley said.
The pattern is occurring nationwide. Climate Central found all 247 cities included in its climate attribution analysis have experienced an increase in warmer-than-normal summer nights strongly influenced by climate change since 1970.
Cities now average about 27 such nights each summer, compared with about one per summer during the 1970s.
Why a hot night can make the next hot day more dangerous
After hours of heat exposure during the day, cooler nights give the human body an opportunity to shed heat and recover. When temperatures remain elevated around the clock, that recovery can be interrupted.
“Our bodies are really master adapters, and we evolve to know how to respond to stress, like heat stress,” said Dr. Cathleen Colón-Emeric, a geriatric medicine specialist at Duke Health and chief of Duke University’s Division of Geriatrics. “But the prolonged exposure without an opportunity for our bodies to recover can often overwhelm those adaptive systems.”
Nishad Jayasundara, a professor at Duke University’s Nicholas School of the Environment and an assistant professor of medicine, said nighttime recovery appears to be an important factor in reducing the effects of daytime heat exposure.
That means a string of hot days can become more dangerous when the nights between them remain unusually warm.
Heat exposure can strain the cardiovascular system, Jayasundara said, and research is also examining its connection to longer-term kidney damage. He pointed to evidence from agricultural communities in tropical regions.
“We are seeing this potentially in North Carolina among farmworkers, and there seems to be a clear indication of heat contributing to long-term kidney injury among other potential disease outcomes,” Jayasundara said.
Heat doesn’t affect everyone equally
The ability to escape nighttime heat can depend heavily on where someone lives and whether they can afford to keep their home cool.
People without reliable air conditioning, outdoor workers, older adults, people with chronic health conditions and residents of urban heat islands can face greater risks from persistent heat.
For agricultural workers who spend their days outside, Colón-Emeric said access to a cool place at night is particularly important.
“The prolonged exposure without an opportunity for our bodies to recover can often overwhelm those adaptive systems,” she said. “That’s why it’s so important that we have policies in place that really mandate that there’s a cool place for particular high-risk folks like our agricultural workers to be able to recover at night.”
Cities can compound the problem.
Buildings, pavement and other developed surfaces absorb heat during the day and release it after sunset, producing the urban heat island effect. Cars and air-conditioning systems can add additional heat to the surrounding environment.
Winkley describes the relationship between development and global warming with an analogy.
“The plate is what you expect your nighttime and summer to be. Climate change is the cake, and then urban heat island is the frosting,” he said. “So it plays a role, but it’s just like that little extra.”
More hot nights can mean higher cooling costs
Warmer nights also mean air conditioners have to work longer.
Climate Central found cooling demand has increased since 1970 in 97% of the major U.S. cities it analyzed. That creates another vulnerability for households that have air conditioning but struggle to afford the electricity needed to run it continuously.
“For a lot of us, we’re lucky enough to have air conditioning, so you may not notice,” Winkley said. “But think about the next time you wake up in the middle of the night and you hear the air conditioner running maybe more than you used to.”
The result is that the thermometer’s daily high tells only part of the story of a warming North Carolina.
Increasingly, scientists say, what doesn’t happen overnight — a chance to cool down — matters, too.