SAN DIEGO (KGTV) -- Last month was the hottest June ever recorded in North America, but climate scientists say the most dramatic temperature trend isn’t happening during the day; it’s happening at night.
June saw three times as many nighttime records as any June on record, according to data from the National Oceanic and Atmospheric Administration.
“Nighttime temperatures or minimum temperatures have been going up much faster than daytime temperatures over most of the world,” said Scripps Institution of Oceanography research meteorologist Alexander Gershunov.
Dr. Gershunov said it’s all a function of climate change. For more than a century, nighttime temperatures have been warming at nearly double the rate of days, according to the 2018 National Climate Assessment Report.
Since the U.S. started collecting national temperature records in 1895, daytime highs increased 0.7 degrees Fahrenheit per century. Overnight low temperatures, also called minimum temperatures, rose 1.4 degrees per century.
“Heat waves [in the western U.S.] are not only becoming more frequent, more intense and longer-lasting like they are over most of the world, but they’re also becoming more humid,” Gershunov said.
That increasing humidity is what’s driving our warmer nights in San Diego.
The moisture in the air acts like a blanket, trapping heat that would otherwise radiate back to space. It’s a feedback loop or a vicious circle: as temperatures rise, the air can hold more water, a vital feature of the greenhouse effect.
Hot, humid weather at night is especially harmful to humans, said UC San Diego epidemiologist Dr. Tarik Benmarhnia.
“When it’s humid, it’s particularly dangerous for our body because the body needs to cool down, and one of the main mechanisms for our body to cool down is through sweating,” he explained.
When it’s humid, sweat doesn’t evaporate as efficiently. Less evaporation means the body has trouble cooling itself off.
But the body dutifully tries harder and harder to lower its temperature, putting more stress on organs like the heart, Dr. Benmarhnia said.
That’s trouble for people with underlying conditions, all at a time when your body is supposed to be resting.
When it’s hot and humid at night, there’s no rest for the weary. Distress signals can be less noticeable when people are sleeping. Countermeasures like cooling zones are typically available during daylight hours only, Benmarhnia said.
A study by Dr. Gershunov found when temperatures rise, so do hospitalizations, increasing 14 percent along San Diego’s coastline where air conditioning is scarce.
A single heat wave can kill hundreds of Californians. Coroner’s reports directly linked nearly 150 deaths to the 2006 heat wave, but studies showed there were about 600 excess deaths, suggesting the actual number is much higher.
So why are nights and days heating up at different rates?
During the day, pollution acts like an umbrella and blocks some sunlight from reaching the surface, Dr. Gershunov said. But pollution doesn’t have the same effect on temperatures at night.