Emergence of uncompensable heat stress during monsoon season in India

Abstract

Uncompensable heat stress (UHS)—characterized by the loss of homeostasis due to excessive environmental thermal loading—causes substantial mortality in India. However, the spatial and seasonal heterogeneity, as well as temporal changes of UHS in India remain poorly understood. Using observations, reanalysis data, and climate model projections, we highlight the surge of UHS during the monsoon season (July–October) as the climate warms. In the observed period (1979-2021), the frequency and area affected by UHS have increased significantly across India. The observed UHS is more prevalent and affects 8% of India in summer (March-June), whereas only 1% of the country is affected in the monsoon season. The summer UHS is also more strongly associated with annual heat-related mortality (R² = 0.38). However, monsoon-season UHS, predominantly characterized by hot-humid conditions, is projected to increase rapidly with climate warming, and affect nearly equivalent areas of the country with the summer season (60% in summer and 53% in the monsoon season) under 2°C warming relative to the preindustrial period. This will create long-lasting UHS across both seasons, posing critical challenges to public health, labour productivity, and climate resilience in densely populated and vulnerable regions.