Humid heat and climate change

Abstract

Extreme heat events cause significant societal impacts, prompting much concern and research about possible changes to their frequency and intensity as the climate warms. However, to date, extremes in air temperature have been emphasised at the expense of ‘heat-humidity’ indices, measures which incorporate the effect of atmospheric latent heat content on heat stress and provide a more complete picture of the thermal environment for human thermoregulation. This progress report restores balance by reviewing recent developments in the understanding of how heat-humidity indices have changed, and may continue to, as the climate warms further. The literature indicates that a concurrent rise in temperature and absolute humidity has already increased the frequency of potentially deadly conditions, and has reduced labour potential worldwide. More serious consequences may result if mitigation efforts are unsuccessful. The energetic basis of a heat-humidity perspective has permitted researchers to identify, for example, that by the end of the century, substantial parts of the Earth’s surface may be too hot and humid for human thermoregulation. Such consequences are avoided for less pessimistic scenarios of climate warming, but the societal impacts may still be very severe, as densely-populated low-latitude environments emerge as particularly at risk when a humid heat perspective is adopted. Counter to air temperature, changes in mean heat-humidity indices are actually amongst the largest worldwide at lower latitudes, where only small increases in the mean may be required to substantially enhance the frequency of dangerous conditions. The report concludes by outlining areas requiring improved process understanding, and it highlights the urgent role for societal adaptation if the worst impacts from rising humid heat are to be avoided.

Escalating environmental summer heat exposure—a future threat for the European workforce

“…Also, future heat waves are very likely to be more frequent and longer-lasting (IPCC 2013), mainly as a direct consequence of the increase in mean temperatures (Schär, et al 2004;Fischer and Schär 2010). Those changes relate to increasing environmental heat exposure throughout the twenty-first century (Willett and Sherwood 2010;Zhao et al 2015;Knutson and Ploshay 2016;Coffel et al 2018;Li et al 2018;Matthews 2018) which in turn might have an effect on mortality, well-being and labour productivity (Dunne et al 2013;Kjellstrom et al 2018;Mora et al 2017;Flouris et al 2018;Levi et al 2018;Moda et al 2019). The combination of environmental heat exposure and internal heat production generated from metabolic processes results in heat stress (Xiang et al 2014).…”

“…The combination of environmental heat exposure and internal heat production generated from metabolic processes results in heat stress (Xiang et al 2014). Air temperature is an important aspect, but all relevant environmental factors should be considered in order to provide improved early warning systems and future projections of heat exposure (Li et al 2018;Matthews 2018). Under high-airtemperature situations, the only means for the body to remain within healthy temperature limits is through loss of heat via sweat evaporation (Kenny and Flouris 2014).…”

An emerging tropical cyclone–deadly heat compound hazard

“…Low-latitude regions are projected to have rapid population growth over the 21st century, adding many more people to the regions with largest increases in deadly humid heat 26 .…”

“…Understanding could be improved by assessing TC climatologies projected by next-generation ensembles of highresolution, coupled physical models 27 or through downscaling 28 . TCs and humid heat are physically connected through moist enthalpy in the lower atmosphere 26,29 , so future work focussing on this diagnostic under climate change could improve understanding of evolving risks in low latitudes. Future studies could also add more depth to the understanding of TCheat impacts by explicitly modelling excess mortality as a function of humid heat, including the impact of increased vulnerability stemming from assumed AC loss 30 .…”

Changes in statistical distributions of sub-daily surface temperatures and wind speed

“…Some studies using climate models suggest that changes in extremes in a warming world (from anthropogenic forcings) come from changes in the shape as well as location of the temperature distributions (Clark et al, 2006;Hegerl et al, 2004). More recent work has looked at the effect of moisture on extreme temperatures in the soil (Whan et al, 2015) or the atmosphere (Sherwood and Huber, 2010;Matthews, 2018), where separation into latent and sensible heating when moisture is present can reduce the peak temperatures.…”