Understanding spatiotemporal variation of heatwave projections across US cities.

Heatwaves are one of the deadliest meteorological hazards, posing a substantial risk to human health, environment, and the economy. The frequency and intensity of heatwaves have substantially escalated throughout the United States (US), as evidenced by a noticeable contrast between the average occurrence of two heatwaves per year during the 1960s and the surge to six per year during the 2010s. The primary objective of this study is to assess the impacts of past heatwave events in major US cities, project future scenarios, and analyze the relationship between heatwaves and their associated health and environmental consequences. First, we investigate the spatio-temporal variations in the intensity, frequency, and duration of past heatwaves, along with anticipated changes under various shared socioeconomic pathways till the end of 2100. Secondly, we examine the adverse heatwave impacts on human health and well-being, considering both current demographics and future projections. Our results highlight significant projected increases in heatwave frequency, intensity, and duration across all US regions, with the most dramatic escalations under high-emission scenarios. By the late twenty-first century, cities could experience up to 2-4 times the frequency and duration of heatwave days annually compared to the baseline period (1985-2014), with night-time heatwave durations potentially covering over half the year in some regions. Furthermore, analysis using composite heatwave indices reveals severe heat stresses, particularly in southwestern cities like Las Vegas and Yuma, and southern cities like Miami, indicating a rising trend of heatwave susceptibility due to both climatic and demographic shifts. This study contributes to the growing body of research advocating proactive measures to address the escalating threat of heatwaves. By integrating projected climate indices and demographic shift, it provides a nuanced assessment of urban heatwave vulnerability, with a specific focus on densely populated cities and high-risk regions.