A New Heat Stress Index for Climate Change Assessment

The heat index (HI), based on Steadman's model of thermoregulation, estimates heat stress on the human body from ambient temperature and humidity. It has been used widely both in applications, such as the issuance of heat advisories by the National Weather Service (NWS) and for research on possible changes in the future due to climate change. However, temperature/ humidity combinations that exceed the applicable range of the model are becoming more common due to climate warming. Recent work by Lu and Romps has produced an extended heat index (EHI), which is valid for values outside the range of the original HI. For these values, the HI can underestimate the EHI by a considerable amount. This work utilizes observed data from 15 U.S. weather stations along with bias-adjusted output from a climate model to explore the spatial and temporal aspects of the disparity between the HI and the EHI from the recent past out to the end of the twenty-first century. The underestimate of human heat stress by the HI is found to be the largest for the most extreme cases (5 degrees-10 degrees C), which are also the most impactful. Conditions warranting NWS excessive heat warnings are found to increase dramatically from less than 5% of days historically at most stations to more than 90% in the future at some stations. Although, by design, the scope of this work is limited, it demonstrates the need for the adoption of the EHI for both applications and research.