There is no shortage of ways to die, and everyone has to do it, but as climate change becomes the “new normal,” the list has grown in some disturbing ways. Powerful superstorms are leveling our communities with growing frequency. Wildfires are raging through forest areas, turning entire towns to cinders. Floods are eroding and swallowing our buildings.
And now we can add to the list: Deadly heat is cooking us alive.
Late June’s deadly heat wave across the western half of North America killed hundreds in the Pacific Northwest and brought Canada its highest temperatures ever recorded. And that was hardly a fluke—India and Pakistan were struck by record-breaking heat at the same time, and another heat wave will bring temperatures over 100 degrees to much of the U.S. this week.
So, what gives? It gets hot every summer. Why are these new heat waves different, and so deadly?
If we continue to burn oil in our SUVs, coal and gas in our power plants, limestone in our cement kilns, and tropical forests in our bellies (in the form of hamburgers from Brazil and palm oil from Indonesia), temperatures around the globe will exceed the limits of human habitability multiple times per year—every year.
Imagine Portland, Oregon, routinely experiencing Death Valley temperatures in the summer.
But, as they say in Death Valley, “it’s a dry heat.” Who wouldn’t enjoy a few balmy, spring days in Palm Springs, California, after a long, cold, wet winter?
Palm Springs, just a few hours south of Death Valley, is in a desert. Given enough water and access to shade to avoid sunstroke, humans can survive for several hours in dry heat, up to temperatures in the 120s. (This is not doctor-recommended.)
But skip across the continent to Palm Beach, Florida, and it’s a very different story. With its position in the tropics, in one of the most humid regions of the Western Hemisphere, a single day of 120-degree temperatures in Palm Beach would be a mass casualty event. Dead bodies would pile up in the morgues, victims of hyperthermia, or heatstroke—cooked, alive, in their own bodies.
What gives? Why is 120 degrees in Palm Beach not the same as 120 degrees in Palm Springs?
It turns out there’s some truth to the old cliché “it’s not the heat, it’s the humidity.” In scientific terms, it’s called a “wet-bulb temperature,” and understanding it is crucial to surviving—and mitigating—the climate crisis.
In reasonable heat, the human body is very good at maintaining a constant internal temperature of 97 to 99 degrees. When it gets hot outside, our bodies produce sweat; when the sweat evaporates, its transformation from liquid water on your skin to water vapor in the air requires energy. That energy comes from your body’s heat, so as the sweat evaporates, your body cools down.
A dry heat feels comfortable because the evaporation happens so fast that you don’t even notice the sweat on your skin. (This is also why dehydration is a huge risk in desert climates—while you feel the dry air is helping you tolerate the heat, you’re also losing water from your body the whole time. “Hydrate or die” is not just a clever slogan; it’s good science.)
Now suppose you’re in the same amount of heat, but in Palm Beach, where the air is incredibly humid. The air is already holding all the water vapor it can hold. So your sweat stays on your skin, and the heat that the sweat is supposed to remove from your body … stays in your body, and accumulates.
Your body has lost its ability to shed heat, and so your core temperature starts creeping up to approach the temperature of the air around you. Let the process go on long enough, and body temperature rises from comfortable 98 to deadly 108.
That is why the temperature isn’t enough to know if the human body can survive. You need to know the “wet-bulb temperature.” The term comes from the bulb of a typical mercury thermometer. If you wrap a thermometer bulb with a piece of wet cloth and put it in a hot room, evaporation off the cloth will lower the temperature reading of the thermometer to a point that is cooler than the room; the bulb is functionally “sweating.” But if you increase the humidity of the room enough, the temperature of the water vapor in the air will reach equilibrium with the water on the wet cloth, at which point evaporation no longer results in heat loss. So the bulb is sweating, but the mercury will continue to rise.
Wet-bulb temperature is similar to heat index in that it captures the combined hit of heat and humidity. But it’s not exactly the same. Heat index attempts to capture what a given combination of heat and humidity “feels like,” and it’s imprecise: A “feels like 113 degrees” could be 90 degrees with a dew point of 72 degrees, or it could be 113 degrees and zero humidity. You’d have to know all of those figures to be able to unpack it—and, importantly, to gauge risk. Wet-bulb temperature, on the other hand, communicates risk precisely. And that risk is growing.
According to the best climate models, large swaths of the United States will experience several weeks of hot wet-bulb temperatures by the middle of this century—that’s in 30 years. “By 2050, parts of the Midwest and Louisiana could see conditions that make it difficult for the human body to cool itself for nearly one out of every 20 days in the year,” ProPublica reported in September. During these periods of deadly heat, shade and hydration won’t save you. Any human without access to reliable air conditioning risks death.
Provisions against deadly wet-bulb temperatures need to be a part of fighting—and preparing for—climate change.
What we know for sure: Growing our cities up, rather than sprawling our suburbs out, is one of the keys to the puzzle. Not only do denser areas have dramatically lower carbon emissions than suburban and rural developments, but it’s less expensive and more efficient to deliver energy and cooling services to people who are close together, as opposed to spread apart.
While NIMBYs rail against shadows from tall buildings, our cities are actually in desperate need of more shadows from trees and tall buildings to reduce urban heat island effects and provide respite from the midday sun—along with, of course, more housing.
The No. 1 intervention, though, is still to reduce climate pollution, and that means cars and SUVs. Reducing car use and replacing land used by cars with land used by housing, trees, and transit will make cities cooler in the short and long terms. Regardless of what we achieve with clean technology, we have to end our 75-year, failed experiment with energy-intensive sprawl and car culture.
Our ability to survive the heat waves of the future—and present—depends on it.
Future Tense is a partnership of Slate, New America, and Arizona State University that examines emerging technologies, public policy, and society.