Communicating the deadly consequences of global warming for human heat stress

In December of 2015, the international community pledged to limit global warming to below 2 °C above preindustrial (PI) to prevent dangerous climate change. However, to what extent, and for whom, is danger avoided if this ambitious target is realized? We address these questions by scrutinizing heat stre ss, because the frequency of ex- tremely hot weather is expected to continue to rise in the approach tothe2°Climit.Weuseanalogsand the extreme South Asian heat of 2015 as a focusing event to help inter pret the increasing frequency of deadly heat under specified amount s of global warming. Using a large ensemble of climate models, our res ults confirm that global mean air temperature is nonlin early related to heat stress, meaning that the same future warming as realized to da te could trigger larger increases in societal impacts than historicall y experienced. This nonlinearity is higher for heat stress metrics that integrate the effect of rising humid- ity. We show that, even in a clima te held to 2 °C above PI, Karachi (Pakistan) and Kolkata (India) coul d expect condition s equivalent to their deadly 2015 heatwaves every year. With only 1.5 °C of global warming, twice as many megacitie s (such as Lagos, Nigeria, and Shanghai, China) could become h eat stressed, exposing more than 350 million more people to dead ly heat by 2050 under a midrange population growth scenario. The results underscore that, even if the Paris targets are realized, there could still be a significant adaptation imperative for vulnerable urban populations