Data dispatch #2: the most important climate concept you’ve never heard about
Heat is dead, long live humidity
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Last summer was one of the hottest on record. In France, for instance, tens of millions of people suffered under a late-August heat wave reaching up to 42 degrees Celsius or 108 Fahrenheit (Le Monde). People living in the “urban heat island” of Paris were more at risk of dying from heat than those in any other European capital (France24).
This raises the question how much heat is too much for us. Can Parisians deal with 43 degrees? What about 44?
When I visited Las Vegas in 2017 and felt fine in 45° Celsius (113°F), I concluded there must be no limit. But a new study, published in the journal PNAS last week, shows that that was naïve of me.
The desert of Vegas sees mostly “dry heat”—high temperatures combined with low humidity—in which the human body can easily cool down by sweating. If the air is wetter, your sweat has nowhere to go and you start to overheat at much lower absolute temperatures.
Scientists have captured this heat-humidity interaction in a quantity known as wet-bulb temperature. If you cover a thermometer in a damp cloth and swing it around to create air flow, it will mimic wet skin and indicate what temperature it feels like for us sweaty humans. (This is the “feels like” temperature reported on U.S. weather channels.) Wet-bulb temperature is much lower than absolute temperature in dry conditions, but in high humidity the difference between the two disappears.
Scientists have long thought that if the wet-bulb temperature is at most 35° Celsius—just under our core body temperature of 37°—we can survive. But according to the new PNAS study, the true “critical wet-bulb temperature” for human survival actually depends on a bunch of factors, including your age and activity level. Even for young, healthy, slothful humans, the wet-bulb temp we can stomach is closer to 31° Celcius or even less.
This has huge implications for estimating what parts of the world will remain livable as climate change marches on. The researchers drew on the most advanced climate model from the World Climate Research Programme. They found that even with just 1.5 degrees Celsius global temperature increase relative to pre-industrial levels—the optimistic goal from the Paris climate agreement—several densely populated swaths of the world will exceed the manageable wet-bulb temperature limit for multiple weeks each year. Areas of Eastern China, the Indus river valley, the Middle East, and sub-Saharan Africa will be hardest hit.
See the image below for the outcomes in four different climate scenarios: 1.5°, 2°, 3°, and 4° of warming. And in inspecting the picture, consider that we’ve already reached 1° of global warming.
I always thought sea level rise was going to be the biggest driver of climate-mediated migration. But this report shows that heat itself, when combined with humidity, is enough to push hundreds of millions of people away from their homes.
The worst wet-bulb temperature increases occur in some of the most densely populated regions of the world. Coastlines and rivers give rise to life—but also to humidity.
In reporting on climate change, absolute temperature is out and feels-like temperature is in. (Tell your friends to put their thermometer in a wet sock.)
Next week: More on what we need to adapt to climate change.