Climate change is making extreme heat more common and more severe. Humans have a constant body temperature, around 98 °F (37 °C). And our bodies are designed to work pretty much right at that temperature, so there’s a constant balance between heat loss and heat gain.
The sweat we produce evaporates into the air, sucking heat from the skin and thus cooling us down.
Humidity cripples this cooling method—if it’s so humid that there’s already a lot of water vapor in the air, then sweat can’t evaporate as quickly, and sweating won’t cool you down as much.
Problems start when our bodies can’t lose heat fast enough (or lose it too fast in the cold, but let’s focus on heat for now).
When our core temperature gets too hot, everything from organs to enzymes can shut down. Extreme heat can lead to major kidney and heart problems, and even brain damage.
A wet-bulb temperature of 35 °C, or around 95 °F, is pretty much the absolute limit of human tolerance. That doesn’t mean the heat will kill you right away, but if you can’t cool down quickly, brain and organ damage will start.
In a study done in 2017, the research team analyzed hundreds of extreme heat events around the world to determine what combinations of heat and humidity were most likely to be deadly, and where those conditions were likely to occur in the future.
They found that while today around 30% of the world’s population is exposed to a deadly combination of heat and humidity for at least 20 days each year, that percentage will increase to nearly half by 2100, even with the most drastic reductions in greenhouse-gas emissions.
Reference- Nature Climate Change Study, MIT Technology Review, Journal Nature, BBC Earth, Forbes, NOAA Weather Service