When denier ideologues make the transition to accepting the reality of anthropogenic global warming, one of the arguments they start to use tends to go something along the following lines: “Sure, the polar bears might get screwed over, but otherwise things will be just great. Crop yields will increase and northerners will get to have their own sun-drenched beaches”. You wish. New research* indicates that beyond temperature rises of 7C, “zones of uninhabitability” will begin to overspread much of the world (“An adaptability limit to climate change due to heat stress” by Sherwood & Huber 2010). Not a Mediterranean world, more like Mad Max in Waterworld.
Of late climate models have been leaning to the upper range of the IPCC’s projections for global warming, e.g. the median forecast from a recent MIT study gives a rise of 5C by 2100 (with a 10% chance it will exceed 7C). According to the Sherwood paper, “peak heat stress” (quantified by the wet-bulb temperature) never climbs above 31C across today’s climes, which is safely below the body’s normal temperature of 37C. But with a global temperature rise of 7C possible by as early as the late 21st century – even without accounting for predictable tripwires such as accelerated release of Siberian and Arctic methane – some regions of the world will be subjected to peak wet-bulb temperatures of 35C, inducing “hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible“. With a global temperature rise of 11-12C, a belt of uninhabitability will come to encompass the bulk of today’s densely populated areas.
“Periods of net heat storage can be endured, though only for a few hours… and with ample time needed for recovery”, but since “adjacent night time minima of [wet-bulb temperatures] are typically within 2-3C of the daytime peak, and adjacent daily maxima are typically within 1C”, conditions would prove intolerable “if the peak wet-bulb temperature exceeded, by more than 1-2C, the highest value that could be sustained for at least a full day”. Thus, e ven healthy individuals cannot sustain heat stress levels of above 35C for prolonged periods, because the skin must be at least 2-3C cooler than the body temperature, whose normal level is 37C. If the wet-bulb temperature rises to 38C, for instance, then the result will be a rise in body temperature to above 40C and death from hyperthermia.
Which areas will be effected? Just look at the maps below (click to enlarge, source).
[Map of max rates of global heat stress during 1999-2008. Graph represents incidences of various temperatures during this period at 60S-60N - Black is average temperature, Blue is max temperature, Red is max wet-bulb temperature. Note how maximum wet-bulb temperature takes a nose-dive after 30C, such that there are practically no instances of this measure exceeding 32C (in practice this means that today no areas are blocked off to human habitation because of excessive heat stress levels).]
[Same as above, except this time it's a high-CO2 world model in which global average temperatures are 12C higher than today. The white and purple areas are completely uninhabitable, while the yellow areas are only marginally habitable at best. Note how much of Siberia experiences a higher max wet-bulb temperature than today's tropics!]
For a preview of things to come, look no further than the 20,000 Parisians who died in the 2003 European heatwave**. True, most most were elderly folks with no air conditioning or climate control. But these things require a lot of energy. With dammed reservoirs drying up throughout Europe and hydrocarbon supplies peaking by 2030-50, a reliable electricity supply should not be expected (least of all during heatwaves). As the authors put it, “the power requirements of air conditioning would soar; it would surely remain unaffordable for billions in the third world and for protection of most livestock; it would not help the biosphere or protect outside workers; it would regularly imprison people in their homes; and power failures would become life threatening”. Not only people will begin to die away. So will plants and animals – indeed, the main catalyst for migration may become failed harvests and famine. Entire nations will have to pack up their bags and move north towards the peoples-teeming Mediterranean shores of the Arctic Ocean.
Finally, the authors point out that even these dark conclusions may be too optimistic, since “our limit applies to a person out of the sun, in gale-force winds, doused with water, wearing no clothing, and not working” (i.e. quite a lot of leeway for enhancing chances of survival!). In other words, it is quite possible that only polar, sub-polar, and mountainous areas will remain comfortably habitable (at least by the new standards).
One more thing I’d like to note is that the map seems to indicate that at least along coastlines, the moderating effects of the oceans will keep those areas livable. That is highly significant… even with the loss of continental interiors, some tropical and mid-latitude nations may continue eking out a bare bones sustenance existence by intensive use of permaculture, organic farming, and hydroponics (the oceans will be too acidic to support fisheries). But don’t forget about sea level rise and the ocean anoxic event.
Waves slowly lap on the quiet shore, slow-motion waves with the consistency of gelatin. Most of the shoreline is encrusted with rotting organic matter, silk-like swathes of bacterial slick now putrefying under the blazing sun… [W]e look out on the surface of the great sea itself, and as far as the eye can see there is a mirrored flatness, an ocean without whitecaps. Yet that is not the biggest surprise. From shore to the horizon, there is but an unending purple color — a vast, flat, oily purple. No fish break its surface, no birds or any other kind of flying creatures dip down looking for food. The purple color comes from vast concentrations of floating bacteria, for the oceans of Earth have all become covered with a hundred-foot thick veneer of purple and green bacterial soup. …There is one final surprise. We look upward, to the sky. … We are under a pale green sky, and it has the smell of death and poison. We have gone to Nevada of 200 million years ago only to arrive under the transparent atmospheric glass of a greenhouse extinction event, and it is poison, heat and mass death that are found in this greenhouse.
** Incidentally, I happened to be in Paris during the 2003 heatwave. Overall, it was a pleasant visit, though the heat and multiple cold showers every day stand out in my memory.