Will the Human Body Be Able to Adapt to Rising Temperatures?

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You thought that last heat wave was bad? Climatologist Matthew Huber discusses our broiling, miserable, wish-it-were-sci-fi future.

That fan is not going to help. Olaf Speier/Shutterstock

Purdue University climatologist Matthew Huber gets plenty of death threats, but that hasn't stopped him from exploring the outer limits of just how much global warming human beings can tolerate. Whatever our recent Great American Heat Wave may or may not portend, most credible climate scientists agree that human-caused global warming is real -- oh yes they do! -- and most of the research out there, Huber says, predicts dire consequences for people (and other mammals) if average global temperatures rise by 6° Celsius or more.

That could well happen this century: By 2100, Huber points out, the mid-range estimates predict a rise of 3°C to 4°C in average global temperatures based on current economic activities, but those studies ignore accelerating factors like the release of vast quantities of methane -- a potent greenhouse gas -- now trapped beneath permafrost and sea ice that's becoming less and less permanent. Other models foresee rises in the 10°C range this century; at the outer fringe, predictions range as high as 20°C. Truth is, we simply don't know exactly when we'll reach these milestones or what they will cost us. And thanks to the uncertainty, it's been hard to get nations to agree on limits.

All of this got Huber and Steven Sherwood, his colleague at Australia's University of New South Wales, to thinking: Economic considerations aside, they asked, how much warming can we physiologically tolerate? At what point does it get so bad that our bodies can no longer keep cool, so bad that we can no longer work or play sports or even survive for long out of doors? Will we flee for colder climes? Live underground like hobbits, surviving on cold fungus? Okay, I'm projecting -- they didn't actually ponder that last bit that I'm aware of.

The most direct way for humans to respond physiologically is to get small and skinny, to decrease our volume and maximize our surface area so we can lose heat more effectively.

In any case, the pair crunched the numbers and published the results in a May 2010 paper in the Proceedings of the National Academy of Sciences. Using a measurement called "wet-bulb temperature," which Huber explains below, they modeled what might happen in several warming scenarios. At the point where the average global temperature rise hits 10°C, "even Siberia reaches values exceeding anything in the present-day tropics" and many populated parts of the globe might become, if habitable at all, places where the relatively affluent would likely find themselves "imprisoned" in air-conditioned spaces and where "power failures would become life-threatening." Lacking access to AC, the world's poor would have little choice but to flee. Even "modest" global warming, Huber and Sherwood conclude, could "expose large fractions of the population to unprecedented heat stress."

Their paper makes for a good wish-it-were-sci-fi read for the scientifically inclined. For everyone else, the recent heat wave provided the perfect excuse to grill Huber (via email) on his underlying assumptions, the hate mail he gets, and whether humans can evolve or air-condition our way out of this prawndiddity -- that's a word my kids came up with to describe this sort of situation, and I'm rolling with it, since our fiasco is theirs to inherit.

First of all, is there anything you'd like to say about the recent heat wave?

It just goes to show you how random weather can be. It tells us about as much by itself as the occasional unseasonable cold snap. It is useful, however as an analogy for what the future climate might look like. When climate modelers say that spring might start a month earlier on average this sounds abstract to most people, but the recent weather provides a good tangible example of what statements like this mean. 

Are there currently places on Earth where average temperatures are beyond the ability of our bodies to stay cool?

In the shade, with plenty of water and ventilation, acclimated healthy adults can survive just about everywhere currently, assuming that they aren't exerting themselves. On the other hand, when physical exertion, sunlight, improper hydration, poor ventilation, lack of acclimatization, and other health conditions (including being very young or old) are a factor, many regions can experience severe enough heat stress that serious consequences arise. Every time someone gets heat stroke, that's someone who pushed themselves or were pushed by circumstance outside of their zone for regulating their temperatures. There is a wide zone over which people can adjust their behavior to withstand very warm conditions. Our paper asked the question: Is there a limit to that adaptability, and, if so, how hot does the world have to get before we reach that limit?

How would you characterize your underlying assumptions? 

We intentionally were trying to explore the upper limit of what humans can possibly stand. Essentially we were assuming a perfectly acclimated person, in perfect health, not performing physical labor, and out of the sun, and were then asking, "What would it take to kill them quickly?" A real person would be profoundly uncomfortable, miserable, and/or sick long before we reach the limit discussed in our paper. Infants, pregnant women, and the elderly would be especially vulnerable long before we hit the limit discussed. Thus the global mean temperature increase of about >10°C that causes widespread heat death in our paper probably is a significant overestimate of the threshold at which substantial harm [would come] to societies and individuals would suffer harm and/or reduced productivity. Put in more prosaic terms, large parts of the world would be violating OSHA and international health standards for work long before we approach this >10°C threshold. But we wanted to be sure we had a limit set by physical and thermodynamic laws and not by human ones (since those are mutable).

Your conclusions are based on a measurement known as "wet-bulb temperature," which refers to a thermometer bulb, wrapped in wet cloth and ventilated. What does it mean in human terms?

In practice, the wet-bulb temperature we calculated would correspond to a naked, healthy adult standing in the shade with gale force winds blowing on them while they were drinking gallons of water. Any deviation from that perfect scenario would increase the heat stress on an individual.

What kind of feedback has your paper received?

I get hate mail and death threats on a regular basis. I'm used to that. I didn't notice much of an uptick with publication of this paper. (I just delete those emails anyway.) Within the scientific community, the response has been initial caution and skepticism (which is warranted) from many of my colleagues followed by the rather interesting and disturbing conclusion I have heard repeated many times: "We sat down to figure out what was wrong in the paper -- because there had to be something wrong -- and we haven't been able to find any errors, so it appears to be correct." The paper is getting heavily cited, and not because it's wrong.

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