The limits of human tolerance to climate change

A sobering new study in the Proceedings of the National Academy of Sciences emphasizes the incredibly high risks of a business as usual scenario in terms of fossil fuel use and greenhouse gas emissions, Sherwood & Huber, An Adaptability Limit to Climate Change Due to Heat Stress, PNAS, Early Edition (2010) (subscription required).

Among the findings in the study:

  1. While the middle-range estimate of temperature increases associated with a doubling of carbon dioxide is 1.9-4.5 degrees C of warming at equilibrium, a new study concludes that there is a 5% change of exceeding 7.1C. Moreover, because combustion of all available fossil fuel supplies could produce 2.75 doublings of carbon dioxide, temperatures could soar 12 degrees C, even with a 4.5 degrees C sensitivity;
    • If one factors in potential releases of natural stores of methane and carbon dioxide in a warmer climate, temperature increases by 2100 could be between 10-20 degrees C
  2. While many studies assessing climatic impacts have focused on ecosystem, social and economic costs, the direct impact on humans and other mammals in the form of heat stress may also be extremely serious. A key consideration is peak heat stress, including humidity. While humans can tolerate, and adapt, a wide range of climates, peak heat stress may reach “intolerable levels;”
    • Humans maintain a core body temperature at 37 degrees C; sustained temperatures above 35 degrees C can cause hyperthermia, reaching lethal values (42-43 degrees C) for skin temperatures of 37-38 degrees C
    • Under future warming scenarios, a large majority of locations reach 30 degrees C at some point during a typical year, with a few regions reaching closer to 50 degrees C, with peak heat stress surprisingly similar across many regions;
    • A global-mean warming of 7 degrees C would create small zones where metabolic heat dissipation would provide “impossible;” at 11-12 degrees C warming, these zones would expand to encompass most human populations. While air conditioning is a potential compensatory response, it would remain cost-prohibitive for billions, and wouldn’t help other species, e.g. livestock.;
    • If warmings of 10 degrees C occurred in three centuries, the area of land that would be habitable would be reduced by half, dwarfing humans affected by rising sea level

Some interesting class discussion could ensue from this piece, including the levels of risk that we’re willing to tolerate as a society (does a 5% risk of extremely high temperatures mandate massive interventions?) and the implications of severe impacts that would no longer be restricted to certain regions (could this mobilize more support for addressing climate change in developed countries?).

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