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Posted by James Hrynyshyn on May 11, 2010
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Most of the alarmism generated by climate predictions deals with sea level rise, drought, and biodiversity loss. But what happens to waterfront property, farms and polar bears could be the least of our worries if temperatures rise much more than a few degrees. A new paper in PNAS, “An adaptability limit to climate change due to heat stress,” paints a much more dire future for much of the larger mammals on the planet, including humans.


In the paper, Steven Sherwood of the University of New South Wales and Matthew Huber of Purdue University try to estimate how warm the Earth can get before species like us — those with low surface-area-to-mass ratios — overheat. Unlike estimate of global heat budgets, ice mass loss rate, ocean conductance, and other staples of climatological prognostication, the math here is pretty simple, thanks to the second law of thermodynamics, which prevents objects from losing heat in an environment in which the ambient temperature is warmer than the object itself. And skin temperature happens to be not to far away from a hot day.

What they come up with is this:

We conclude that a global-mean warming of roughly 7 °C would create small zones where metabolic heat dissipation would for the first time become impossible, calling into question their suitability for human habitation. A warming of 11-12 °C would expand these zones to encompass most of today’s human population.

…

If warmings of 10 °C were really to occur in next three centuries, the area of land likely rendered uninhabitable by heat stress would dwarf that affected by rising sea level.

The obvious criticism is that such warming scenarios are extreme, worst-case scenarios that no one believes will be realized. But the authors (and presumably their editor, Kerry Emanuel), don’t think it is unreasonable to explore such possibilities. Indeed, a warming of 7 °C is actually well within the realm of possibility by the end of this century if we keep doing what we’re doing, and burn all the coal and oil and gas we can get our hands on. “In the absence of strong mitigation measures” is how the paper puts it. There are plenty of solids references for this sort of thing. And we don’t have to stop at 2100. A warming of 11-12 °C is almost certain by 2300 if we burn all the unconventional petroleum.

Granted, the mid-range estimate of warming by 2100 is only 5 °C or so, but given the huge uncertainties, particularly at the upper end of the range of possibly warming values, it would be foolish not to consider some of the more extreme scenarios.

Also, Sherwood and Huber make a point of low-balling their predictions:

This likely overestimates what could practically be tolerated: Our limit applies to a person out of the sun, in gale-force winds, doused with water, wearing no clothing, and not working.

Plus, they deal with the argument that humans can adapt. That is what we do best, after all. The problem with that is that only wealthy humans can afford air conditioning. Much of the developing world don’t dare dream about about AC and couldn’t afford it if they did have such dreams. And of course, more AC only increases the GHG emission rates.

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Comments

  1. #1 the backpacker
    May 11, 2010

    It looks like they are just considering further burning of fossil fuels. Is there any consideration for what kind of warming might happen if say the Arctic Permafrost thaws or that layer of methane in the deep ocean floor fractures? I am from Minnesota and a swing of 7 degrees C does not seem like that much to me. I know weather and climate relate the same way the nightly news relates human history but 7 degrees is a scary small number when it means large swaths of the earth will become uninhabitable.

  2. #2 JBC
    May 11, 2010

    They do mention in one sentence that increased degassing of CH4 and CO2 may increase warming. It’s impossible to quantify how that may or may not happen so that’s all the attention it gets.

    The 7 degree measurement is a global average. Places with higher sensitivity, such as the poles, may possibly see a local temperature increase several times the average.

  3. #3 darwinsdog
    May 11, 2010

    ..humans can adapt. That is what we do best, after all.

    Adaptation is the result of natural selection. Individuals don’t adapt, populations do. The rate at which a population can adapt is a function of generation time. Humans have long generation times, hence, it takes a long time for human populations to adapt. So long, in fact, that little adaptation has taken place since the Pleistocene.

    For humans to adapt to mean temperature increases of this extent, homeostatic mechanisms would have to maintain a much elevated core body temperature, so that heat could still be lost to the environment. It would require tens or hundreds of millenia for such adaptation to occur in human populations. The gene which codes for virtually every metabolic enzyme would have to evolve to maintain a new and higher temperature optima for its polypeptide product. Mean global temperatures are rising MUCH too rapidly for this to be feasible. The bottom line is that humans CAN’T adapt to a 7^oC increase in mean annual temperature. Instead, extinction looms.

  4. #4 Eric Lund
    May 11, 2010

    Backpacker said: “I am from Minnesota and a swing of 7 degrees C does not seem like that much to me.”

    A swing of 7 degrees C in the other direction would be more than enough to bury your home under an ice sheet ~1 km thick. What we are discussing here is the shift in annual average temperatures. Of course a location like yours will see much larger temperature swings over the course of a year, and even between daily minimum and maximum.

    That’s why global warming is such a critical issue. Since the invention of agriculture the Earth’s climate has been relatively stable, with total variations in the annual averages of a couple of degrees C or so. Which is why everybody hopes to limit the change to within a couple of degrees–civilization as we know it could survive that much, but not much more than that (food scarcity becomes an issue around that point).

  5. #5 Steve Bloom
    May 11, 2010

    I suspect 7C is enough to trigger the “clathrate gun,” at which point we’re in for a repeat of the PETM, although probably with a delay of several centuries due to the lag in warming the deep oceans. Talk about going from bad to worse.

  6. #6 Steve Bloom
    May 12, 2010

    James, speaking of Kerry Emanuel, he’s presenting a new paper this week demonstrating that the model results projecting no increase in TC frequency over the next century are in error since they didn’t take into account lower stratospheric cooling. I have to say that seems like an obvious thing to have missed.

    It does resolve a serious contradiction with another Emanuel proposal (which Matt Huber is also working on) that the enhanced poleward heat transport required to keep the tropics relatively cool in the mid-Pliocene resulted from a substantial increase in TC activity (involving tropical storms as far north as Seattle and Svalbard). The most recent paper on that one made the cover of Nature a couple of months ago.

    A big question is how fast a transition to a mid-Pliocene-like climate state could happen given continued anthropogenic forcing. I’ve seen nothing on that subject, although it’s obviously something KE is thinking about, but it seems to me if TCs are key things could proceed rather quickly.

    Anyway, it seems like a good subject for a post. Maybe Kerry or Matt would be willing to comment on how some of these pieces fit together. Also, it would be very interesting to get Tom Knutson’s view of how the correct temperature values for the lower stratosphere would affect his model results.

    Thanks, Steve. Looks like an interesting paper. I’ll try to assemble something shortly. — jh

  7. #7 Luna_the_cat
    May 12, 2010

    …And that’s not even going into how many other species of mammals would be thoroughly duffed over by this, especially those in cut off “island” habitats which are unable to migrate to new areas because of human land use.

  8. #8 Steve Bloom
    May 12, 2010

    Thanks, James, I’ll look forward to it. Also, I just now noticed this paper:

    Sriver, R. L., Goes, M., Mann, M. E., and Keller, K. (In Press), Climate response to realistic tropical cyclone-induced ocean mixing in an Earth system model of intermediate complexity, Submitted to Journal of Geophysical Research-Oceans

    I don’t have access to it, but it looks as if it may be an attempt to model the transition.

    FYI Sriver was Huber’s grad student and is now Mann’s post-doc.

  9. #9 Steve Bloom
    May 13, 2010

    And there’s more: Jeff Masters blogged today on some other new research results bolstering Emanuel’s ideas.

  10. #10 CherryBomb
    May 14, 2010

    Humans could not possibly adapt fast enough to keep pace with a temperature increase like this, but they can move. If parts of the Earth become uninhabitable, the people living there will attempt to leave, won’t they? Presumably headed for colder regions that have warmed up enough to live in. I;m not saying this is a good solution, just saying nobody is gonna simply wait around to die of hyperthermia.

  11. #11 MPW
    May 15, 2010

    darwinsdog at #3 and CherryBomb at #10: “Adapt” is clearly being used here in the sense of “adapting our culture and technology to the situation,” not in the sense meant in biological evolution. Not that there aren’t good points otherwise in your comments (and not that such cultural and technological adaptation is likely to be terribly helpful, as far as I can see).