An adaptability limit to climate change due to heat stress

Abstract

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 degrees C. Any exceedence of 35 degrees 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 degrees C, calling the habitability of some regions into question. With 11-12 degrees C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12 degrees C are possible from fossil fuel burning. One implication is that recent estimates of the costs of unmitigated climate change are too low unless the range of possible warming can somehow be narrowed. Heat stress also may help explain trends in the mammalian fossil record.

Fig. 1.

(A) Histograms of 2-meter T (Black), T_max (Blue), and T_W(max ) (Red) on land from 60S–60N during the last decade (1999–2008). “Max” histograms are annual maxima accumulated over location and year, while the T histogram is accumulated over location and reanalysis time. Data are from the ERA-Interim reanalysis 4xdaily product (similar results are found for the 50m level from the NCEP reanalysis, see SI Text). (B) Map of T_W(max ). (C and D) Same as A and B but from a slab-ocean version of the CAM3 climate model that produces global-mean surface temperature close to modern values. (E and F) Same as C and D but from a high-CO₂ model run that produces a global-mean T 12 °C warmer; accounting for GCM bias, the T_W(max ) distributions are roughly what would be expected with 10 °C of global-mean warming relative to the last decade (see text). Dashed line in E is T_W(max ) reproduced from C. White land areas in F exceed 35 °C.

Fig. 2.

The 75th percentile value of T_W(max ) (a measure of the peak occurrence value) at two or 75 meters above ground vs. global or tropical mean 75-m temperature in CAM3 simulations. Solid symbols are for a simulation representing possible Eocene conditions. Dashed lines show best linear fits, with slopes given (Eocene run not included in fit).