Hot and humid conditions prevailing during India's monsoon season could extend the duration of uncompensable heat stress of the summer season under a global warming of 2 degrees Celsius, a study has found.

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".

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

Uncompensable heat stress occurs when one's 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.

The study shows that over the period of 1979-2021, uncompensable heat stress has become more frequent and is affecting significantly more areas across India -- from less than 0.01 million square kilometres during the 1980s to 0.04 million square kilometres by 2020.

Uncompensable heat stress was found to be more prevalent during the summer months of March-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 the monsoon season.

"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.

The total population affected by uncompensable heat stress 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 the Indian states during 1980-2019 was obtained from the National Disaster Management Authority (NDMA) and the India Meteorological Department (IMD).

Spatially, the uncompensable heat stress during summer primarily occurs in the Indo-Gangetic plain and coastal regions of India, 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 uncompensable heat stress within a narrower air temperature range between 35 degrees Celsius and 38 degrees Celsius.

The monsoon-season uncompensable heat stress has occurred in the Punjab state of northwestern parts of the country, the team said.

The presence of high humidity during the monsoon season 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 uncompensable heat stress primarily during the summer season.

"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.