Monsoon no longer a respite: Study flags rising heat stress in rainy season

Traditionally, monsoon means a respite from the gruelling Indian summer. But not anymore. Under a global warming of 2 degrees Celsius, the hot and humid conditions of monsoon could actually extend the duration of uncompensable heat stress (UHS) of summer, a study has found.

The findings, published in the journal American Geophysical Union (AGU) Advances, highlight a “surge of UHS (uncompensable heat stress) during the monsoon season (July-October) as the climate warms”.

What UHS means

Researchers from the Indian Institute of Technology (IIT) Gandhinagar and the US’s Stanford and Purdue universities said long-lasting UHS across both summer and monsoon could pose critical challenges to public health, labour productivity, and climate resilience in densely populated and vulnerable regions.

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UHS occurs when the body is unable to cool down through sweating or other mechanisms due to extreme heat and humidity. A sustained accumulation of heat can endanger human health, including causing heat-related illness, organ failure and death.

Monsoon UHS set to increase

The study shows that between 1979 and 2021, UHS became more frequent and affected significantly more areas across India—from less than 0.01 million square kilometres during the 1980s to 0.04 million square kilometres by 2020.

UHS was found to be more prevalent during the summer months of March to June, affecting eight per cent of India and more strongly associated with yearly heat-related mortality. Only one per cent was found to be impacted during monsoon.

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“However, the monsoon season (July-October) UHS, predominantly characterised by hot-humid conditions, is projected to increase rapidly with climate warming and affect nearly equivalent areas of the country as the summer season (60 per cent in summer and 53 per cent in the monsoon season) under 2 degrees Celsius warming relative to the preindustrial period,” the authors wrote.

Millions affected

The total population affected by UHS could be between 0.8 and 1.2 billion, based on the projected population, under different warming levels, the team estimated.

Population and density data for the study period came from the Gridded Population of the World dataset, while heat-related mortality data for Indian states during 1980–2019 was obtained from the National Disaster Management Authority (NDMA) and the India Meteorological Department (IMD).

Vulnerable regions

Spatially, the summer UHS primarily occurs in the Indo-Gangetic plain and coastal regions, potentially due to high temperatures and an inland moisture transport by pre-monsoon winds from the Bay of Bengal, among other factors.

In contrast, the researchers found that the monsoon season primarily experiences humid UHS within a narrower air temperature range between 35 degrees Celsius and 38 degrees Celsius. The monsoon-season UHS has occurred in Punjab, the team said.

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The presence of high humidity during monsoon, combined with moderately high air temperatures, especially during the monsoon breaks, makes favourable conditions for extreme heat stress occurrence, they said.

The Gangetic Plain, northwestern India, and eastern coastal regions were identified as hotspots that experience UHS primarily in summer. “However, northwestern and Gangetic plain regions will face more frequent occurrences of UHS during the monsoon than in summer under the warming climate,” the authors said.

(With agency inputs)