Updated Sept. 7 with summer 2018 temperatures setting a record. 

Well after dinner time in Southern California, the thermometers read 100 degrees. In Santa Barbara County, fires lit up the skies and destroyed homes.

At breakfast time the next morning, in San Bernardino County, the temperature was “ridiculously above 100 degrees,” as the National Weather Service put it. People fled blazes there, too.

Not only is it unusually hot all over the world, to a remarkable degree, the heat lingers overnight. With a changing climate, overnight low temperatures are going up even faster than daytime highs. The diurnal anomaly, as scientists call it, is posing big risks to public health, safety, ecosystems and agriculture.

In June, a city in Oman went well over 24 hours without the temperature ever dropping below 108 degrees. That was the highest daily minimum temperature ever recorded anywhere on the planet—a new milestone of global warming.

People often talk about record high temperatures for a given day of the year as the most immediate evidence that climate change has arrived. Heat waves, when records fall for several days in a row across broad regions, are another. And this summer, that’s been the focus of news reports. Temperature records have been broken across the United States and all over the world. Other extreme events punctuated the narrative, such as the extreme flooding in Japan, where the death toll surpassed 100.

But it’s also important to consider the problem of higher temperatures during the coolest part of the day.

It may help explain why at least 50 people died in Quebec, Canada, in early July as temperatures reached into the 80s and 90s. Although it cooled somewhat at night, for most of the week in Montreal, the overnight temperature still stayed above 70 degrees. Homes and buildings without air conditioning didn’t have much chance to cool down, and neither did the people inside.

Night Temps Are Rising Faster Than in the Day

Scientists have been remarking on the nighttime anomalies for several years.

In 2015, the National Oceanic and Atmospheric Administration noted that, “As the world warms, nighttime temperatures are slightly outpacing daytime temperatures in the rate of warming.”

The following year, 2016 ranked as the third warmest year ever in the United States when looking at average temperatures. But when looking at the nation’s overnight minimums, 2016’s were the warmest ever. This summer beat that record again, with the nationally averaged minimum hitting 60.9 degrees Fahrenheit in the contiguous U.S.— 2.5 degrees Fahrenheit above average.

When the cooler part of the day tends to warm up more than the warmer part of the day, the result is a smaller daily temperature range—a noticeable change in one of the basic patterns of life.

And over time, the disappearance of what used to be the normal temperature swings each day can have far ranging consequences.

How Hotter Nights Can Threaten Human Health

When temperatures fail to drop at night—when the overnight lows are too high—the heat can become deadly, especially for the elderly and children.

In the hottest of cities, it’s becoming a crisis. In Phoenix last year, 155 people died of heat-related causes, according to National Public Radio. In recent decades, the average overnight low in the area has gone up several degrees, and there are significantly more days each year above 110 degrees.

Cooler nighttime temperatures allow bodies to “reset” and recover from scorching daytime highs as buildings and houses cool. But when external temperatures stay above 80 degrees, internal body temperatures don’t have a chance to cool.  

If humidity is also high—as was the case in Quebec—the body perspires more, but the humidity means sweat can’t evaporate, cranking up internal temperatures even more. Recent research has shown that higher nighttime temperatures can also mean less sleep, potentially adding more physical stress on the body.

In cities, that stress can be even greater. Asphalt and concrete trap heat during the day, then release it very slowly at night, meaning urban areas are often much hotter than rural ones.  

Crops Need Cool Nights for Variety of Reasons

Higher temperatures at night also harm crops and livestock. Like humans, livestock need lower nighttime temperatures to recuperate and offload excess heat built up during the day. Dairy cows produce less milk when nighttime temperatures rise.

“Nighttime temperatures matter a lot for comfort for humans, as well as other animals,” explained Kevin Trenberth, a senior scientist with the National Center for Atmospheric Research. “They are often a more critical factor in heat wave deaths.”

Warmer nighttime temperatures also mean earlier frost-free dates and later first-frost dates, so pests aren’t killed by cold temperatures and weeds have more time to grow. Warmer nighttime temperatures also increase transpiration from some crops, drying them out, introducing health problems and lowering yield.  In certain areas, this can also lead to increased risk of fire.

“The minimum temperatures are often important in keeping bugs and some diseases at bay,” Trenberth said. “In fact, a good frost can really help kill many undesirable bugs. Various molds, and rusts, on plants like cotton and wheat, are more likely to flourish in warmer conditions.”

Warmer nights also mean warmer mornings, exposing agricultural and other outdoor workers to more intense temperatures at a time of day when they used to work comfortably in the summer months.

What Hot Nights Mean for Wildfire Risk

As fires rage in the American West, predicted rising temperatures there could spark more fires or make the existing ones harder to extinguish.

Firefighters historically have counted on lower temperatures and higher humidity at night to bring “recovery” periods that help them tamp down blazes, but that’s been changing. Hot air holds more moisture, meaning lower relative humidity, so fires can continue to rage through the night.

Firefighters have seen intense night burning with several recent blazes, and U.S. Forest Service scientists have taken note that unusually warm nights followed by hot days could be predictors of when existing fires might blow up.

“Even without rain, fine fuels on the ground such as dead grass and pine needles take moisture in from the atmosphere when relative humidity is high and they give the moisture back to the atmosphere when the relative humidity goes down again in the day,” explained Park Williams, a researcher at Columbia University’s Lamont-Doherty Earth Observatory. “Warmer nights, especially when combined with dry conditions, can allow fires to continue spreading quite rapidly after sunset.”