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.