Communicating the deadly consequences of global warming for human heat stress

Abstract

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 stress, because the frequency of extremely hot weather is expected to continue to rise in the approach to the 2°C limit. We use analogs and the extreme South Asian heat of 2015 as a focusing event to help interpret the increasing frequency of deadly heat under specified amounts of global warming. Using a large ensemble of climate models, our results confirm that global mean air temperature is nonlinearly related to heat stress, meaning that the same future warming as realized to date could trigger larger increases in societal impacts than historically experienced. This nonlinearity is higher for heat stress metrics that integrate the effect of rising humidity. We show that, even in a climate held to 2°C above PI, Karachi (Pakistan) and Kolkata (India) could expect conditions equivalent to their deadly 2015 heatwaves every year. With only 1.5°C of global warming, twice as many megacities (such as Lagos, Nigeria, and Shanghai, China) could become heat stressed, exposing more than 350 million more people to deadly 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.climate change | heat stress | megacities | extreme heat | CMIP5

Detectable Impacts of the Past Half‐Degree Global Warming on Summertime Hot Extremes in China

“…Note that these observed evidences should only be considered as conservative inferences regarding differential influences of 1.5 and 2°C global warming, accounting for potential presence of strong nonlinear responses at higher warming levels (Dosio & Fischer, 2018;Fischer & Knutti, 2015;Matthews et al, 2017). Therefore, model-based efforts, involving both dedicated experiments for equilibrium warming levels and existing transient scenarios passing through 1.5 and 2°C, are still warranted to quantify added risks due to the future 0.5°C difference, at both regional and global scales.…”

Projections of Human Exposure to Dangerous Heat in African Cities Under Multiple Socioeconomic and Climate Scenarios

“…Although some recent studies have considered only maximum temperature to define the heat index ( HI ) without accounting for humidity, for example (Dong et al, ; Harrington & Otto, ; Liu et al, ), evidence suggests that humidity plays an important role in temperature discomfort and dangerous heat and thus must be integrated into the construction of the HI (Coffel et al, ; Davis et al, ; Matthews et al, ; Mora et al, ), particularly in South America, Africa, and South Asia (Russo et al, ). Various heat metrics that include both temperature and humidity have been developed over the past few years, all performing well and rather similarly (Anderson et al, ; Matthews et al, ). Here we employed the apparent temperature ( AT ) and defined the annual HI as being the number of days for which the daily maximum AT exceeds a given threshold, being set at 105 °F (i.e., 40.6 °C).…”

“…Here we employed the apparent temperature ( AT ) and defined the annual HI as being the number of days for which the daily maximum AT exceeds a given threshold, being set at 105 °F (i.e., 40.6 °C). The latter is based on the U.S. National Weather Services (NWS) threshold of dangerous heat, widely used in the literature (e.g., Matthews et al, ; Russo et al, ). Although a fixed threshold was preferred to a relative threshold for the main exposure analysis—to ensure the consistency with past studies—we also explored future extreme heat using relative thresholds, set as the 95th, 97.5th, and 99th percentiles of the historical local maximum AT .…”

“…Driven by both high population growth and significant changes in climatic conditions, future exposure to dangerous heat in Africa is projected to show the highest increase worldwide during the 21st century, with South Asia being close behind (Coffel et al, 2018;Dong et al, 2015;Jones et al, 2018;Liu et al, 2017;Matthews et al, 2017). Recent regional studies conducted in Eastern Africa (Harrington & Otto, 2018) and across the Great African Lakes region (Asefi-Najafabady et al, 2018) have provided a closer look at the effects of changes in socioeconomic and climatic conditions on future exposure to dangerous heat in some parts of Africa.…”

A Region at Risk:

“…Such an assessment is even more urgent considering that recent empirical estimates show that the global mean air temperature is nonlinearly related to heat stress. In other words, the same future warming as realized to date could trigger larger increases in human heat stress and associated health impacts than historically experienced (Matthews, Wilby, and Murphy 2017).…”