There’s nothing like the cool relief of nightfall after a sweltering summer day.

As the world warms, this natural balance may be shifting. New research suggests that a new trend is on the rise—extreme heat during the day followed by extreme heat overnight.

study published this week in Nature Communications finds that these day/night extremes have become both hotter and more frequent since 1960. Across the Northern Hemisphere, they’ve intensified by about 2 ½ degrees Fahrenheit, and they’re occurring about five days more frequently per year.

Continued warming will make it even worse, the research suggests. In a scenario involving around 3 degrees Celsius throughout the rest of the century (or about 5.5 F), these compound extremes would happen about four times more frequently than they do today—increasing from about eight days each summer to a whopping 32.

That’s approximately the amount of warming the world is on track for today, experts say, if greenhouse gas emissions don’t start falling at a faster pace.

In the future, “a hot day accompanied by a hot night without relief for human[s] might be a ’new norm,’” said study co-author Yang Chen of the Chinese Academy of Sciences in an email to E&E News. “These rapid increases are largely overlooked in previous studies.”

Day and night extremes are more than just a matter of comfort, the authors say. They’re also a potential threat to human health and natural landscapes.

The study didn’t directly examine human health impacts. But other research has suggested that prolonged exposure to extreme temperatures can worsen the odds of heat stroke or other heat-related illnesses, and can affect plant growth. Without the reprieve of a cool evening, living things may have less opportunity to recover from the effects of the daytime heat, the authors suggest.

While there’s no single definition of “extreme heat,” the authors of the new study—led by Jun Wang of the Chinese Academy of Sciences—focused on temperatures that exceed the 90th percentile for that location on any given day of the year.

After examining the historical climate data, they made their future projections using ensembles of climate models.

Unsurprisingly, more warming is associated with more extremes. In an unlikely worst-case scenario—involving about 5 C of warming through the end of the century—day and night heat extremes would increase more than eightfold, compared with the fourfold increase suggested by the more moderate trajectory.

Certain regions seem to be hit harder than others. Since the 1960s, for instance, the greatest changes in frequency have occurred in parts of the southern U.S., northwest and southeast Canada, Western and southern Europe, Mongolia, and southeast China.

The authors have some theories about why. They suspect that changes in certain atmospheric circulation patterns may increase the likelihood of compound heat events, and some regions may be more prone to them than others.

In any case, Yang noted, “these regions need to pay close attention to possibly larger increases in the number of compound hot extremes in the future.”

The new study is not the first to examine day and night heat extremes—but it’s one of the most comprehensive analyses to date, the authors say.

Still, there’s a steadily growing body of research on compound climate events in general, not just limited to day and night heat events. These are multiple climate-related extremes occurring at once, whether related to heat, precipitation or other factors.

Recent research increasingly suggests that climate change is likely to cause an increase in compound extremes across much of the world. Scientists have examined the likelihood of extreme hot and dry conditions occurring at once, of simultaneous storm surge and river flooding in coastal areas, of back-to-back extreme weather events, and so on.

It’s a growing topic in climate research, largely because of its implications for climate resilience and adaptation. With multiple extreme events occurring at the same time, both human societies and natural ecosystems may experience more damage at once, or have less time to recover between events.

More focus on these kinds of connected extremes could help communities prepare for the changing future and inform better adaptation strategies.

“[C]urrent knowledge about combined daytime-nighttime hot extremes remains too sparse to inform development of type-specific adaptation and mitigation strategies,” the new study says. “Overlooking this compounding effect may lead to serious underestimate of heat-induced consequences.”

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.