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Abstract This article investigates the effect of urban expansion and climate change impacts on heat stress (HS) for Arizona's (AZ; USA) two largest urban agglomerations, the Phoenix and Tucson metropolitan areas, under relatively dry and moist warm conditions with the Weather Research and Forecasting (WRF)‐urban modeling system. We dynamically downscale two contemporary summers, one dry and one moist, relatively to their respective seasonal‐mean specific humidity across AZ. Urban expansion impacts on HS are assessed by performing two identical simulations for each contemporary summer using different land use‐land cover representations: one simulation with the current urban landscape, and one simulation replaces the urban cover with the region's most representative MODIS vegetation type. Climate change impacts on HS are evaluated by performing four additional future simulations, two via dynamical downscaling of relatively dry conditions (one summer under the RCP8.5 and one summer under the RCP4.5 emissions pathways) and two of relatively moist conditions (one summer for each RCP pathway). The selection of future summers is based on their respective seasonal‐mean specific humidity across AZ from an end‐of‐century analysis of 2086–2100. We characterize impacts on HS by examining changes in near‐surface air temperature, Heat Index (HI), and the Universal Thermal Climate Index (UTCI) across urban areas under dry and moist warm conditions. Our results demonstrate that climate change impacts on HS are not well captured by examining only the projected changes in air temperature and are dependent on the bioclimate index considered. Additionally, we apply a new human heat balance (HHB) approach to evaluate the number of hours per day that an acclimatized and non‐acclimatized person would experience uncompensable HS and compare these results (with the number of hours per day) that we obtain when the HI and UTCI surpass commonly used thresholds considered “dangerous” and of “extreme heat stress”, respectively. The HI and UTCI overestimate the number of hours per day that a healthy, acclimatized person would experience uncompensable HS and underestimate dangerous HS for a non‐acclimatized person under both dry and moist conditions, emphasizing that standard metrics may not produce the most informative physiological estimates of HS. 

Abstract Most studies projecting human survivability limits to extreme heat with climate change use a 35 °C wet-bulb temperature (T_w) threshold without integrating variations in human physiology. This study applies physiological and biophysical principles for young and older adults, in sun or shade, to improve current estimates of survivability and introduce liveability (maximum safe, sustained activity) under current and future climates. Our physiology-based survival limits show a vast underestimation of risks by the 35 °C T_wmodel in hot-dry conditions. Updated survivability limits correspond to T_w~25.8–34.1 °C (young) and ~21.9–33.7 °C (old)—0.9–13.1 °C lower than T_w = 35 °C. For older female adults, estimates are ~7.2–13.1 °C lower than 35 °C in dry conditions. Liveability declines with sun exposure and humidity, yet most dramatically with age (2.5–3.0 METs lower for older adults). Reductions in safe activity for younger and older adults between the present and future indicate a stronger impact from aging than warming. 

Abstract Manual outdoor work is essential in many agricultural systems. Climate change will make such work more stressful in many regions due to heat exposure. The physical work capacity metric (PWC) is a physiologically based approach that estimates an individual's work capacity relative to an environment without any heat stress. We computed PWC under recent past and potential future climate conditions. Daily values were computed from five earth system models for three emission scenarios (SSP1‐2.6, SSP3‐7.0, and SSP5‐8.5) and three time periods: 1991–2010 (recent past), 2041–2060 (mid‐century) and 2081–2100 (end‐century). Average daily PWC values were aggregated for the entire year, the growing season, and the warmest 90‐day period of the year. Under recent past climate conditions, the growing season PWC was below 0.86 (86% of full work capacity) on half the current global cropland. With end‐century/SSP5‐8.5 thermal conditions this value was reduced to 0.7, with most affected crop‐growing regions in Southeast and South Asia, West and Central Africa, and northern South America. Average growing season PWC could falls below 0.4 in some important food production regions such as the Indo‐Gangetic plains in Pakistan and India. End‐century PWC reductions were substantially greater than mid‐century reductions. This paper assesses two potential adaptions—reducing direct solar radiation impacts with shade or working at night and reducing the need for hard physical labor with increased mechanization. Removing the effect of direct solar radiation impacts improved PWC values by 0.05 to 0.10 in the hottest periods and regions. Adding mechanization to increase horsepower (HP) per hectare to levels similar to those in some higher income countries would require a 22% increase in global HP availability with Sub‐Saharan Africa needing the most. There may be scope for shifting to less labor‐intensive crops or those with labor peaks in cooler periods or shift work to early morning.