Unveiling the influences of soil moisture on moist heat stress extremes: a global assessment using CMIP6 data

Soil moisture is a key variable in land-atmosphere interactions, as it affects the partitioning of near-surface energy fluxes and thereby temperature and humidity of the lower atmosphere. Both ambient temperature and humidity play a crucial role in the removal of heat from the human body through direct heat transfer and sweat evaporation, therefore these two factors are commonly used in measuring moist heat stress. As moist heat stress describes the combined effects of temperature and humidity on human health and well-being, understanding the intricate relationship between soil moisture and moist heat stress is crucial for accurately assessing and mitigating moist heat extremes. Whereas the impact of soil moisture on temperature is well understood, previous research has found non-trivial and complex relations between soil moisture and moist heat stress due to humidity feedbacks. We selected two metrics among four widely used metrics which involve both temperature and humidity, indoor and open-air wet-bulb globe temperature, heat index, and humidex, to represent the heat stress in our study. We use different levels to describe the significance of the heat stress and tolerance level among the population.

In this study, we aim to investigate the impacts of soil moisture on moist heat stress at the global scale using the Land Surface, Snow and Soil moisture Model Intercomparison Project (LS3MIP) dataset within the sixth phase of the Coupled Model Intercomparison Project (CMIP6). We use the historical and future simulations from LS3MIP to analyze the spatial and temporal variations of soil moisture-heat stress coupling, and to identify the regions that are most susceptible to moist heat stress. Interactions between soil moisture and moist heat stress tend to be particularly pronounced in hot and humid regions,. These regions are likely to experience more frequent events with higher moist heat stress, posing serious challenges for human health and adaptation.

To our best knowledge, this study is the first to show a global picture of the interactions between soil moisture and moist heat stress using CMIP6 dataset. The pattern of heat stress in relation to soil moisture in perspectives of the time of day, season, and soil moisture regime will be investigated. Our study provides a novel insight into the role of soil moisture in modulating moist heat stress, and highlights the need for more accurate representation of land surface processes and feedbacks in climate models. The findings are crucial for developing effective strategies in managing moist heat stress risks and protecting vulnerable populations.