The past 10,000 years has been marked by a range of climate conditions in which human society is particularly well-adapted to thrive. But, as the planet warms, the range of weather conditions we can expect is changing—and it’s trending toward a combination of heat and humidity unprecedented in the three million years over which modern humans evolved.
It is often assumed that humans would be able to adapt to any possible warming. In 2010, Purdue Professor Matthew Huber, Department of Earth, Atmospheric and Planetary Sciences, and colleague Steven Sherwood, University of New South Wales in Sydney, Australia, were the first to demonstrate this is not so. They calculated the highest survivable combination of heat and humidity—an adaptability limit—and examined how likely these extremes are to occur in the future. The decade of research following this pioneering work has recently been reviewed by Dr. Jonathan Buzan (PhD 2018, EAPS) and Professor Huber in an article in Annual Reviews in Earth and Planetary Sciences (Buzan, J.R. and M. Huber, 2020).
Hot weather already limits our ability to work and stay healthy. In response to hot air temperatures, the human body relies on the evaporation of water, mainly through perspiration, to dissipate heat. While maximum temperatures may reach up to 50 °C in some parts of the world, these areas mostly have very low humidity which allows the evaporation of sweat to cool the skin and keep the body’s core temperature at a safe level. High humidity, however, prevents sweat from evaporating and this makes humid heat more dangerous than dry heat.
In their 2010 study, published in the Proceedings of the National Academy of Sciences, Sherwood and Huber determined the combined effects of heat plus humidity using “wet bulb” temperatures—a measurement of temperature which includes the cooling effect of evaporation on a thermometer. They estimated that once wet bulb temperature exceeds 35°C, the air is so hot and humid that sweating cannot lower your body temperature to a safe level, and with continued exposure, death by overheating can follow. Two important caveats were noted in the study. First, the proposed physiologic limit of 35°C considered a person in perfect health, at total rest, in the shade, with unlimited water—a best-case scenario that sets the upper bound of heat stress humans can withstand without technological assistance. Second, dangerously high wet bulb temperatures (>27°C) are currently rare and short-lived phenomena; however, the researchers note that heatwaves in Europe (2003) and Russia (2010) each killed tens of thousands of people with wet bulb temperatures at 28°C.
The results of this study show that if CO2 emissions continue to rise throughout the century, 50% of Earth’s surface where people currently live will become uninhabitable at some point during an average year. Huber and Sherwood warned that if climate change is not addressed, we will be facing a world in which heat stress is a vastly greater problem than it has been in all of human history.
In their review article, Buzan and Huber cover the fundamentals of moist heat stress, describe a theoretical and modeling framework for prediction of humid heat stress extremes and their spatial distribution in the future, and consider several important implications for human and natural systems. Humid heat stress has special thermodynamic and dynamic constraints associated with moist convective equilibrium that makes it uniquely predictable in a climatological sense. This robust finding allows scientists to determine the limit of habitability for humans (and other mammals) for a given amount of global temperature change—and include these values into a suite of simpler models such as integrated assessment models and economic damage models.
In a warmer world, humid heat stress will occur over large regions for months at a time, including in densely populated regions. This is a notable difference from the occurrence of dry heat stress that tends to come in discrete heat waves and in deserts where population densities are low. The tropics, already suffering from high heat stress summers today, will become permanently stressed year-round, even with relatively modest global warming.
Just as heat stress impacts individuals, widespread heat stress will also lead to negative aggregate effects that include reduced labor productivity and food security, diminished educational outcomes, and increased morbidity, crime, and conflicts. Communities will need to adapt mitigation measures to keep their citizens safe, and ensure their most vulnerable populations have access to air conditioning while also preparing for the added stress to energy and water production and distribution infrastructure—and for inevitable power outages.
Adaptation has its limits. The only way to avoid being carried further into more extreme, longer lasting and more widespread heat territory is to reduce greenhouse gas emissions to net zero. In the meanwhile, Buzan and Huber note, researchers will need to continue to study how irrigation and other land-use changes, clouds, wind, radiation, and other factors lead to humid heat stress to better predict when and where these events will occur.
Buzan, J.R. and M. Huber. 2020. Moist Heat Stress on a Hotter Earth. Annu. Rev. Earth Planet. Sci. 48:23.1-23.33