Extending the Heat Index to Quantify the Physiological Response to Future Warming: A Modelling Study

Abstract

Backgrounds: The Heat Index is a widely used measure of apparent temperature that accounts for the effects of humidity using Steadman's model of human thermoregulation. Steadman's model, however, gives unphysical results in sufficiently hot and humid conditions, leading to an undefined Heat Index. In a business-as-usual climate scenario, an undefined Heat Index will become increasingly frequent, eventually occurring across a third of the planet at any given moment. Hence, an extension of the Heat Index to all conditions is needed, and the direct health impacts of an elevated Heat Index should be quantified. Methods: We extend the Heat Index to all conditions by allowing sweat to drip off the skin in very hot and humid conditions and, in severe conditions, allowing the body's core temperature to rise. The index is mapped onto measurable quantities, including the elevated core temperature in severe conditions and, in fatal conditions, the time it takes for the core to exceed a survivable temperature. The Heat Index is then calculated everywhere on Earth in a business-as-usual climate simulation up to the year 2300. Findings: By 2300, heat exhaustion, heat stroke, and even heat death are frequently the unavoidable consequences of being outdoors, even for healthy adults, over large swaths of Africa, Australia, India, South America, Eastern China, and the Midwestern and Eastern United States. Interpretation:During the preindustrial, all locations and times on Earth were survivable by a healthy adult with sufficient access to clothing, shade, and water. This will cease to be true with a continuation of business-as-usual warming. Funding Statement: This work was supported by the U.S. Department of Energy’s Atmospheric System Research program through the Office of Science’s Biological and Environmental Research program under Contract DE-AC02-05CH11231. Declaration of Interests: We declare no competing interests.