Heatwaves are roasting the entire northern hemisphere this week.
The United Kingdom recorded its hottest average temperature ever, 40 degrees Celsius (104 Fahrenheit), and other countries in Europe are struggling through sweltering heat as well. As of Tuesday evening, more than 100 million people across the U.S. are under excessive heat warnings or advisories, especially the southern plains of Texas and Oklahoma, CNN reported. Parts of the Northeast are being hit by high temperatures and sticky humidity. Summers aren’t just getting hotter, heatwaves are breaking records, and they’re making it riskier for people to be outside for extended periods of time.
Signs of the coming scorchers first began with intense conditions earlier this year. A massive heatwave swept through India and Pakistan during April and May, bringing temperatures of over 110 degrees Fahrenheit. Official tallies pegged the number of people killed by the heatwaves at 90, but experts believed the actual death toll to be far higher. In addition, humidity made it impossible to escape the heat, as measured by “wet bulb temperatures.” It’s a term Americans are about to become uncomfortably familiar with.
A wet bulb temperature is a measure of stress in heat. It considers both the temperature and the humidity in the air. It’s measured with a thermometer covered in a water-soaked cloth, it can also be categorized as how effectively a person can cool off by sweating. Now it’s proliferating farther North due to the climate crisis.
Tapio Schneider, an environmental science professor at the California Institute of Technology, explained that a high wet bulb is even more dangerous than normal heat. “The way humans cool themselves is by [perspiring], or sweating, and evaporating,” he told Earther. “That’s absolutely essential for human survival.” When sweat evaporates from our skin, we cool us off. If the sweat doesn’t evaporate because there’s already too much humidity in the air, it’ll be harder for us to naturally cool off, Schneider explained.
This is why the combination of heat and humidity is a lot more dangerous than just dry heat alone. In a dry heat environment, a person can withstand temperatures at almost 115 degrees Fahrenheit, roughly 45 degrees Celsius, without completely overheating, according to a 2014 study. When there’s a lot of humidity, however, a lower temperature becomes dangerous for the human body—35 degrees Celsius could have the same effect, Schneider said.
The climate crisis is increasing the instance of extreme weather events, including heatwaves, which can worsen other events like wildfires and droughts. Warmer air can hold more water vapor, which leads to more extreme rainfall and more thunderstorms a la the tropics. Think rainforest weather. And so as temperatures rise globally, so will also mean more uncomfortable humidity. Areas that used to have bearably hot summers will now become dangerous, especially for people struggling to afford air conditioning and people who are experiencing homelessness.
“Human survival becomes impossible, especially in places that are already humid,” Schneider said. “India and Pakistan were the prime examples, it’s already humid in summer…it gets where the threshold can be exceeded… people can die just sitting in that heat and humidity.”
Major U.S. cities like New York and Washington D.C. are known for summer humidity, and Schneider predicted that it will become increasingly uncomfortable for residents in the years to come.