Scorching days and sweltering nights have baked the South Korean capital of Seoul over the month of July, with the heat reaching levels not seen in 117 years of record keeping, Al Jazeera reported. At least 16 people have died in South Korea already this year from heat-related causes.

"Normally, if it's hot for just one day, temperatures spike and then quickly return to normal," said Youn Ki-Han, the director of Seoul's meteorology forecast division, per the Guardian. "But when warm conditions persist for several days, the heat doesn't fully dissipate, it lingers and accumulates with each day." 

What's happening?

As daytime temperatures in Seoul have soared, nighttime temps have not dropped as much as usual, resulting in persistent heat around the clock.

Over the course of July, South Korea experienced its highest number of tropical nights for the month since record keeping began in 1908. A "tropical night" is defined as a night in which the temperature never drops below 77 degrees Fahrenheit, according to Al Jazeera. 

In July, Seoul experienced 22 tropical nights, topping the previous record set in 1994, per Al Jazeera, with records kept since 1908.

The hottest night came on the evening of Wednesday, July 30, when temperatures in Seoul never dropped below 84.7 degrees Fahrenheit, making it the hottest July night ever recorded in the South Korean capital, according to the Guardian.   

Why does nighttime heat matter?

When people think of heat waves, they tend to imagine sweltering daytime temperatures fed by an intense, midday sun. However, experts have warned that nighttime temperatures have been rising faster than daytime temps in many areas, with potentially deadly consequences. 

"Most people don't realize that hot nighttime temperatures have been outpacing daytime temperature increases across most populated regions worldwide in recent decades," Kelton Minor, a postdoctoral research scientist at Columbia University, told CNN. 

When heat persists at night, our bodies do not have a chance to recover from the day's high temps. With no nighttime relief, the health problems associated with dangerous levels of heat continue to accumulate over the course of multiple days.

A study published in Lancet Planetary Health found that hot nights following hot days led to a 50% increase in mortality risk as compared to a cooler night following a hot day. The same study also found that heat-related deaths could increase sixfold by 2100 purely as a result of hotter nights. 

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Scientists hypothesize that hotter nights are a result of more moisture being drawn into the air as daytime temperatures rise. 

"We think it's because as days grow warmer, there is more moisture in the air that traps the heat," said Lisa Patel, executive director of the Medical Society Consortium on Climate and Health, per CNN. "During the day, that moisture reflects the heat, but at night, it traps the heat in." 

This can be a cyclical effect, because hotter temperatures in the first place can cause more moisture to evaporate into the air — one of the key reasons that gradually increasing worldwide temperatures can lead to drought in some places and bigger rain storms and flooding in others. 

"A warmer atmosphere increases the rate at which water evaporates during dry periods," the Associated Press summarized in a report on scientists analyzing recent weather patterns. "It also holds more water vapor, which fuels heavy rainfall events."

What's being done about nighttime heat?

For decades, scientists have warned that releasing large amounts of heat-trapping pollution into the atmosphere would cause global temperatures to rise, supercharging severe weather events and increasing their magnitude. 

While no one specific weather event can be attributed directly to human causes, the South Korea heat wave is precisely the type of extreme weather event experts predicted.

While high nighttime temperatures have made South Korea's heat wave more dangerous, the silver lining is that both rising daytime and nighttime temperatures have the same root cause: planet-warming pollution.  

In the near term, the impact of rising temperatures can be mitigated, particularly in urban areas, by planting more trees and finding other ways to increase the levels of shade. 

However, to address rising temperatures over the long term, it is necessary to significantly reduce the amount of heat-trapping pollution entering the atmosphere. 

This can be accomplished in ways large and small. For example, taking public transit and driving an EV can help to reduce transportation pollution, and installing solar panels can help to decrease pollution from energy, heating, and cooling usage. All add up to make a difference. Check out the TCD Guide for more. 

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