The Heat Index as a Measure of Future Heat Stress with Climate Change

Abstract

The wet-bulb temperature and wet-bulb globe temperature (WBGT) have been widely used to forecast the physiological consequences of extreme heat associated with climate change. Unfortunately, those metrics are not based in any physiological model of thermoregulation, which means their values do not map one-to-one onto physiological states. In contrast, the widely used heat index is based on a model of human thermoregulation: each value of the heat index maps onto a unique state of the human body. This mapping is obscured in practical applications of the heat index, which use the National Weather Service's empirical fit to values tabulated in Robert Steadman's seminal 1979 paper. In addition, that empirical fit is used, without scientific justification, to extrapolate the heat index to high heat and humidity, where Steadman's paper gave no values because of an apparent failure in the underlying physiological model. In recent work, Steadman's model has been extended, allowing the heat index to be calculated for extreme heat and humidity while retaining full backward compatibility. This reveals that the extrapolation used by the National Weather Service underestimates the heat index during severe US heat waves by as much as 10 degrees Celsius. For example, the peak heat index at Chicago's Midway International Airport was underreported by 9 degrees Celsius during the July 1995 heat wave, which killed hundreds of people. This correction has important implications for physiology: these corrected values reveal that even walking in the shade would have brought a healthy person close to the point of hyperthermia. This highlights how close global warming is to making the most severe US heat waves a fatality risk for even young and healthy adults. Department of Energy's Atmospheric System Research Program This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.