The influence of soil moisture on wet-bulb temperature extremes and excess mortality in South America

The impact of extreme heat stress on mortality has received growing attention in recent years. Historically, South America has been characterized by a relatively high number of days per year with combined extreme temperature and humidity posing a risk to human health. Future projections suggest that, under global warming, the continent will be one of the regions worldwide where humid-heat extremes are expected to intensify the most¹. Temperature (T) and specific humidity (q) are key variables for determining heat stress, as human thermoregulation relies on heat dissipation through cutaneous vasodilation, sweating and evaporative cooling². Wet-bulb temperature (WBT), which integrates the effects of T and q, has been widely used as a proxy to quantify human exposure to heat and the physiological capacity to cool down through sweat evaporation.

In water-limited conditions, soil moisture plays a critical role in the land surface energy partitioning, influencing sensible and latent heat fluxes, cloud cover, downward long-wave radiation and boundary layer height, thus modulating T, q and, ultimately, WBT³. Decreased soil moisture typically enhances T via sensible heat increases, while potentially reducing q by constraining latent heat. Hence, the overall impact of soil moisture on WBT—and potentially on heat-related mortality—is not straightforward and depends on the relative contributions of these competing processes.

Using daily mortality records for the 2000–2023 period, this study aims to unravel the dual effect of soil moisture on WBT extremes for several metropolitan regions in South America. Preliminary results show that the sensitivity of mortality to summer WBT is stronger in subtropical urban areas that are more water-limited, with the local influence of soil moisture varying significantly in nature and intensity. In São Paulo and Rio de Janeiro, high mortality rates linked to WBT extremes are mainly explained by increasing values of T associated with substantial reductions in the evaporative fraction and enhanced sensible heat flux. On the other hand, in Porto Alegre, the local impact of soil moisture manifests through exceptional values of both sensible and latent heat fluxes leading to WBT and mortality extremes leveraged by enhanced T and q. These results highlight the role of soil moisture as a key modulator of heat stress, shaping wet-bulb temperature extremes through competing thermodynamic pathways.   

 

1. IPCC, 2023: Summary for Policymakers. In: Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, H. Lee and J. Romero (eds.)]. IPCC, Geneva, Switzerland, pp. 1-34, doi: 10.59327/IPCC/AR6-9789291691647.001

2. Armstrong B., Sera F., Vicedo-Cabrera A. M., et al. (2019). The role of humidity in associations of high temperature with mortality: a multicountry, multicity study. Environ. Health Perspect. 127, 097007. https://doi.org/10.1289/EHP5430.

3. Chagnaud G., Taylor CM., Jackson, L. S., Birch, C. E., Marsham, J. H., & Klein, C. (2025). Wet-bulb temperature extremes locally amplified by wet soils. Geophysical Research Letters, 52, e2024GL112467. https://doi.org/10.1029/2024GL112467