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By Climate Desk
30 March 2012

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.

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? […]

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

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