• Steven C. Sherwood and Matthew Huber, An adaptability limit to climate change due to heat stress, Proceedings of the National Academy of Sciences, early edition 2010.

Abstract: Despite the uncertainty in future climate-change impacts, it is often assumed that humans would be able to adapt to any possible warming. Here we argue that heat stress imposes a robust upper limit to such adaptation. Peak heat stress, quantified by the wetbulb temperature T_W, is surprisingly similar across diverse climates today. T_W never exceeds 31 °C. Any exceedence of 35 °C for extended periods should induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible. While this never happens now, it would begin to occur with global-mean warming of about 7 °C, calling the habitability of some regions into question. With 11–12 °C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12 °C are possible from fossil fuel burning. One implication is that recent estimates of the costs of unmitigated climate change are too low unless the range of possible warming can somehow be narrowed. Heat stress also may help explain trends in the mammalian fossil record.

Huh? Temperatures going up by 12 degrees Celsius??? Well, this is a worst-case scenario — the sort of thing that’s only likely to kick in if we keep up ‘business as usual’ for a long, long time:

Recent studies have highlighted the possibility of large global warmings in the absence of strong mitigation measures, for example the possibility of over 7 °C of warming this century alone. Warming will not stop in 2100 if emissions continue. Each doubling of carbon dioxide is expected to produce 1.9–4.5 °C of warming at equilibrium, but this is poorly constrained on the high side and according to one new estimate has a 5% chance of exceeding 7.1 °C per doubling. Because combustion of all available fossil fuels could produce 2.75 doublings of CO₂ by 2300, even a 4.5 °C sensitivity could eventually produce 12 °C of warming. Degassing of various natural stores of methane and/or CO₂ in a warmer climate could increase warming further. Thus while central estimates of business-as-usual warming by 2100 are 3–4 °C, eventual warmings of 10 °C are quite feasible and even 20 °C is theoretically possible.

A key notion in Sherwood and Huber’s paper is the concept of wet-bulb temperature. Apparently this term has several meanings, but Sherwood and Huber use it to mean “the temperature as measured by covering a standard thermometer bulb with a wetted cloth and fully ventilating it”.

This can be lower than the ‘dry-bulb temperature’, thanks to evaporative cooling. And that’s important, because we sweat to stay cool.

Indeed, this is the big difference between Riverside California (my permanent home) and Singapore (where I’m living now). It’s dry there, and humid here, so my sweat doesn’t evaporate so nicely here — so the wet-bulb temperature tends to be higher. In Riverside air conditioning seems like a bit of an indulgence much of the time, though it’s quite common for shops to let it run blasting until the air is downright frigid. In Singapore I’m afraid I really like it, though when I’m in control, I keep it set at 28 °C — perhaps more for dehumidification than cooling?

Sherwood and Huber write:

A resting human body generates ∼100 W of metabolic heat that (in addition to any absorbed solar heating) must be carried away via a combination of heat conduction, evaporative cooling, and net infrared radiative cooling. Net conductive and evaporative cooling can occur only if an object is warmer than the environmental wet-bulb temperature T_W, measured by covering a standard thermometer bulb with a wetted cloth and fully ventilating it. The second law of thermodynamics does not allow an object to lose heat to an environment whose T_W exceeds the object’s temperature, no matter how wet or well-ventilated. Infrared radiation under conditions of interest here will usually produce a small additional heating.

[…]

Humans maintain a core body temperature near 37 °C that varies slightly among individuals but does not adapt to local climate. Human skin temperature is strongly regulated at 35 °C or below under normal conditions, because the skin must be cooler than body core in order for metabolic heat to be conducted to the skin. Sustained skin temperatures above 35 °C imply elevated core body temperatures (hyperthermia), which reach lethal values (42–43 °C) for skin temperatures of 37–38 °C even for acclimated and fit individuals. We would thus expect sufficiently long periods of T_W > 35 °C to be intolerable.

Now, temperatures of 35 °C (we say 95 degrees Fahrenheit) are entirely routine during the day in Riverside. Of course, it’s much cooler in my un-air-conditioned home because we leave open the windows when it gets cool at night, and the concrete slab under the floor stays cool, and the house has great insulation. Still, after a few years of getting acclimated, walking around in 35 °C weather seems like no big deal. We only think it’s seriously hot when it reaches 40 °C.

But these are not wet-bulb temperatures: the humidity is usually really low! So what’s the wet-bulb temperature when it’s 35 °C and the relative humidity is, say, 20%? I should look it up… but maybe you know where to look?

If you look on page 2 of Sherwood and Huber’s paper you’ll see three graphs. The top graph is the world today. You’ll see histograms of the average temperature (in black), the average annual maximum temperature (in blue), and the average annual maximum wet-bulb temperature (in red). The interesting thing is how the red curve is sharply peaked between 15 °C and 30 °C, dropping off sharply above 31 °C.

The bottom graph shows an imagined world that’s about 12 °C warmer. It’s too hot.

As the authors note:

The highest instantaneous T_W anywhere on Earth today is about 30 °C (with a tiny fraction of values reaching 31 °C). The most-common T_{W, max} is 26–27 °C, only a few degrees lower. Thus, peak potential heat stress is surprisingly similar across many regions on Earth. Even though the hottest temperatures occur in subtropical deserts, relative humidity there is so low that T_{W, max} is no higher than in the deep tropics. Likewise, humid mid-latitude regions such as the Eastern United States, China, southern Brazil, and Argentina experience T_{W, max} during summer heat waves comparable to tropical ones, even though annual mean temperatures are significantly lower. The highest values of T in any given region also tend to coincide with low relative humidity.

But what if it gets a lot hotter?

Could humans survive > 35 °C? Periods of net heat storage can be endured, though only for a few hours, and with ample time needed for recovery. Unfortunately, observed extreme-T_W events (T_W 26 °C) are long-lived: Adjacent nighttime minima of T_W are typically within 2–3 °C of the daytime peak, and adjacent daily maxima are typically within 1 °C. Conditions would thus prove intolerable if the peak T_W exceeded, by more than 1–2 °C, the highest value that could be sustained for at least a full day. Furthermore, heat dissipation would be very inefficient unless T_W were at least 1–2 °C below skin temperature, so to sustain heat loss without dangerously elevated body temperature would require T_W of 34 °C or lower. Taking both of these factors into account, we estimate that the survivability limit for peak six-hourly T_W is probably close to 35 °C for humans, though this could be a degree or two off. Similar limits would apply to other mammals but at various thresholds depending on their core body temperature and mass.

I find the statement “Adjacent nighttime minima of T_W are typically within 2–3 °C of the daytime peak” quite puzzling. Maybe it’s true in extremely humid climates, but in dry climates it tends to cool down significantly at night. Even here in Singapore there seems to be typically a 5 °C difference between day and night. But maybe it’s less during a heat wave.

The paper does not discuss behavioral adaptations, and that makes it a bit misleading. Even without fossil fuels people can do things like living underground during the day and using windcatchers to bring cool underground air into the house. Here’s a windcatcher that my friend Greg Egan photographed in Yazd during his trip to Iran:

But, of course, this sort of world would support far fewer people than live here now!

Another obvious doubt concerns the distant past, when it was a lot warmer than now. I’m talking about the Paleogene, which ended 23 million years ago. If you haven’t heard of the Paleogene — which is term that came into play after I learned my geological time periods back in grade school — maybe you’ll be interested to hear that it’s the beginning of the Cenozoic, consisting of the Paleocene, Eocene, and Oligocene. Since then the Earth has been in a cooling phase:

How did mammals manage back then?

Mammals have survived past warm climates; does this contradict our conclusions? The last time temperatures approached values considered here is the Paleogene, when global-mean temperature was perhaps 10 °C and tropical temperature perhaps 5–6 °C warmer than modern, implying T_W of up to 36 °C with a most-common T_{W, max} of 32–33 °C. This would still leave room for the survival of mammals in most locations, especially if their core body temperatures were near the high end of those of today’s mammals (near 39 °C). Transient temperature spikes, such as during the PETM or Paleocene-Eocene Thermal Maximum, might imply intolerable conditions over much broader areas, but tropical terrestrial mammalian records are too sparse to directly test this. We thus find no inconsistency with our conclusions, but this should be revisited when more evidence is available.