How hot is too hot for humans?

Coffel, Ethan D., R. M. Horton, and A. de Sherbinin. “Temperature and humidity based predictions of a rapid rise in global heat stress exposure during the 21st century.” Environmental Research Letters, vol. 13, no. 1, 2017,

As my fellow Floridians know, the only way to survive a humid summer day is to find air conditioning, fast. Humidity makes us uncomfortable because we sweat in order to cool off, but this cooling is less efficient when it’s humid. Past a certain threshold of heat and humidity, our bodies can no longer cool off by sweating, and the results can be lethal. A recent study by Coffel et al. suggests that, by the end of the century, climate change may expose millions of people to life-threatening heat stress.

What is heat stress?

Heat stress is a term used to describe a range of heat-related illnesses, including heat exhaustion and heat stroke. Heat stress ensues when we lose our ability to thermoregulate, or maintain a steady body temperature. When our bodies overheat, biological processes get disrupted and this may result in death if left untreated. Heat stress occurs for a variety of reasons, including over-exertion, exposure to very hot conditions, and a reduction in our ability to cool off.

What might stop us from cooling off?

Humans cool off by sweating. When sweat evaporates from our skin, it changes phase from a liquid (water droplets) to a gas (water vapor). This phase change requires energy. Water molecules draw this energy from the environment, so the environment – in this case, our skin – loses energy and cools off.

Hippos can’t sweat, so they keep cool by hanging out in water or mud. Image credit: Brian Snelson / Wikimedia Commons.

Unfortunately, there’s a limit to how much water can evaporate. If the amount of water vapor in the air has already reached this limit, known as saturation, then it’s practically impossible for any more water to evaporate. Meteorologists use the concept of humidity to tell us how close the air is to saturation. For instance, if the humidity is 20%, then the air is pretty far from saturation; it’s dry, and more water can evaporate. However, if the humidity is 95%, then the air has nearly reached saturation; now it’s much harder for more water to evaporate. We can’t cool off as efficiently by sweating, which is why humid heat feels worse than dry heat!

What happens when air reaches saturation? Water vapor condenses to form tiny liquid droplets, and we see clouds! Photo taken by Rohini Shivamoggi in NY (2017).

Meteorologists have several strategies for quantifying the effect of humidity. You might’ve seen your local forecaster mention the heat index, which combines temperature and humidity in order to give you an idea of how hot it actually feels. A more scientific version of the heat index is a quantity known as the wet-bulb temperature, which essentially tells us how much sweat can cool us off, given the current air temperature and humidity. If the wet-bulb temperature exceeds the temperature of our skin – roughly 95 F – that means that sweating can’t cool us back down to a temperature that’s normal for human bodies. Our bodies heat up and illnesses such as heat stroke might ensue. Sustained exposure to such conditions is probably fatal, even for healthy people who aren’t doing any physical work.

Melting asphalt during a heat wave in India (2015). Image Credit: The Telegraph.

To put this in context, a 2015 heat wave in India resulted in a wet-bulb temperature of 86 F. This is well below the 95 F threshold, but even so, the heat wave caused over 2,500 deaths (read more here). Fortunately, it is extremely rare for wet-bulb temperatures to exceed 85 or 86 F anywhere on the planet today. Unfortunately, a recent study suggests that this might change by the end of the century.

What do climate models predict?

Ethan Coffel, Radley Horton, and Alex de Sherbinin used a collection of global circulation models (GCMs) to predict how climate change will affect global wet-bulb temperatures over the next century. Their results suggest that wet-bulb temperatures that were once record-breaking will become commonplace by 2080. When the authors included regional projections for population growth, they found that by 2080, they expect a million person-hours, each year, of global exposure to a wet bulb temperature of 95 F. This has potentially serious consequences for the (mostly developing) nations in the tropics and subtropics, whose inhabitants might not be able to survive if they don’t have access to air conditioning.

The region in between the red lines is expected to experience the greatest increases in wet bulb temperature, according to Coffel et al.’s study. Image credit: CIA World Factbook / Wikimedia Commons, slightly modified.

Some regions are expected to experience more severe impacts than others. This study suggests that the most extreme wet-bulb temperatures will occur in China, India, and the Amazon, all of which are densely populated regions. In another study, Jeremy Pal and Elfatih Eltahir used a regional climate model and found that the Arabian Gulf may also become uninhabitable by the end of the century. The evolution of global wet-bulb temperature will have serious consequences for human lives, economies, and geopolitical conflicts, and the details of these scenarios will be important to ascertain in the coming years.

Is there any good news?

If we reduce our use of fossil fuels, yes!

Since the climate is sensitive to how much greenhouse gas is in the atmosphere, scientists usually run predictive climate models under multiple scenarios. This allows them to test how the climate might respond differently if we emit more or less carbon dioxide in the coming years (more details here). The results described above are all for the RCP 8.5 scenario, also known as “business as usual” because it assumes that we’ll continue to increase our greenhouse gas emissions through the end of this century. Coffel et al. also conducted their analysis using the RCP 4.5 scenario, which predicts that our greenhouse gas emissions will max out in 2040 and then decrease. In this case, exposure to dangerous wet-bulb temperatures is dramatically reduced as compared to the RCP 8.5 scenario.

We Floridians joke that we can’t survive without AC, but based on Coffel et al.’s study, that might become literally true for millions of people as soon as 2080. Fortunately, it is still possible to avert this disaster, as long we act quickly to reduce our dependence on fossil fuels.

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Rohini Shivamoggi

I'm a PhD student studying atmospheric sciences at MIT. I study the formation of secondary eyewalls in hurricanes, which hopefully will help us improve our forecasts of hurricane intensity. Before I got to MIT, I grew up in Florida and studied Chemistry and Physics at Harvard University. My other interests include weather forecasting, photography, and encouraging diversity in STEM! You can find me on Twitter @RShivamoggi.

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