Death, doom and disaster coming soon to a planet in your neighborhood

Just when the denialists have convinced themselves that there is no problem here, keep moving, comes Steven Sherwood and Matthew Huber in the Proceedings of the National Academy of Sciences, to point out that yes Virginia (GMU) and Roger, death, doom and disaster are saddling up and while they might not arrive before a couple of hundred years, the time horizon for climate change issues extends beyond 2100 and what is out there is seriously worrying.

Despite the uncertainty in future climate-change impacts, it is often assumed that humans would be able to adapt to any possible warming. Here we argue that heat stress imposes a robust upper limit to such adaptation. Peak heat stress, quantified by the wet-bulb temperature T_W, is surprisingly similar across diverse climates today. T_W never exceeds 31 °C. Any exceedence of 35 °C for extended periods should induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible. While this never happens now, it would begin to occur with global-mean warming of about 7 °C, calling the habitability of some regions into question. With 11–12 °C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12 °C are possible from fossil fuel burning.

Humans, and many animals upon which the world depends, must maintain core temperatures of ~ 37  °C by shedding energy via conductive, radiative and evaporative cooling. When the ambient wet bulb temperature exceeds 35  °C, core temperatures sore soar and people die fairly rapidly, in a few hours. The figure below gives you an idea of how problematic human existence would be on such a brave new world.

The consensus estimate for climate sensitivity is ~2-4.5  °C for doubling of CO2. With a bit of bad luck warmings of 8 °C are possible (Tw trails the global temperature by a bit) at 4x CO2, which is certainly within reach in a bit more than 100 years.

UPDATE: Anonymouse in the comments below points out that

It's the creeping statistical hints between the lines of this paper that really bother me. Long before or even if we never see broad areas permanently enter a existentially threatening torrid regime, what about excursions? For instance, Pakistan this year has seen record temperatures approaching 54 degrees C in places where many people live, fortunately with lower humidity and only for handful of days but what about when/if such aberrations extend to a handful of weeks and are accompanied by inexorably increasing humidity? The resulting disaster would cause migrations. The worst-case scenario in Sherwood and Huber would not have to happen before we effectively lose major swathes of territory for year-round habitability.

And if you follow the press, there have been lots of deaths in Pakistan and Northern India from these heat waves.

Sherwood and Huber are pushing the envelope here, but remind Eli again about the proper discount rate for starting to deal with climate change.

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. 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. A global-mean warming of only 3–4 °C would in some locations halve the margin of safety (difference between T_{W max} and 35 °C) that now leaves room for additional burdens or limitations to cooling. Considering the impacts of heat stress that occur already, this would certainly be unpleasant and costly if not debilitating. More detailed heat stress studies incorporating physiological response characteristics and adaptations would be necessary to investigate this.

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. Heat stress thus deserves more attention as a climate-change impact.

The onset of T_{W max} > 35 °C represents a well-defined reference point where devastating impacts on society seem assured even with adaptation efforts. This reference point constrasts with assumptions now used in integrated assessment models. Warmings of 10 °C and above already occur in these models for some realizations of the future (33). The damages caused by 10 °C of warming are typically reckoned at 10–30% of world GDP (33, 34), roughly equivalent to a recession to economic conditions of roughly two decades earlier in time. While undesirable, this is hardly on par with a likely near-halving of habitable land, indicating that current assessments are underestimating the seriousness of climate change.