A new study warns that India's extreme heat threat, long considered a summer crisis, could increasingly spill into the monsoon season as rising temperatures combine with humidity to create conditions where the human body struggles to cool itself. The research was conducted by scientists from the Indian Institute of Technology (IIT) Gandhinagar, Stanford University, and Purdue University.
Study Details
The study, published in AGU Advances, examined 'uncompensable heat stress' (UHS) across India using observations, reanalysis datasets, and future climate projections. UHS refers to conditions where the body can no longer maintain a stable core temperature through sweating and other natural cooling mechanisms, putting even healthy individuals at risk during routine activities.
Current and Future Impact
Researchers found that UHS is currently concentrated mainly in summer (March to June), affecting about 8% of India's land area, while only about 1% is affected during monsoon months (July to October). However, under a world that warms by 2°C above pre-industrial levels, the seasonal gap nearly disappears, with nearly 60% of India projected to face UHS during summer and 53% during monsoon.
'Our findings indicate that what has conventionally been considered a summer-season extreme will also become a concern during the monsoon season,' the authors said. They added that long-lasting UHS across both seasons would pose serious challenges to public health, labour productivity, and climate resilience in densely populated and vulnerable regions.
Historical Trends
The study analysed data from 1979 to 2021 and found that both the frequency and geographical spread of UHS have increased significantly over the past four decades. The Indo-Gangetic Plain, northwestern India, and parts of the eastern coast emerged as particularly vulnerable due to high population density, outdoor work exposure, and climatic conditions that can push heat stress to dangerous levels.
Heat-Related Mortality
Researchers found that present-day summer UHS is more strongly associated with heat-related mortality than monsoon UHS, largely because such conditions are still far more frequent in summer. The area affected by summer UHS explained 38% of the year-to-year variation in heat-related deaths reported by the National Disaster Management Authority and 32% of the variation in mortality data maintained by the India Meteorological Department.
Future Risks
The future risk changes sharply as warming advances. Unlike many summer heatwaves dominated by very high temperatures and relatively drier conditions, monsoon UHS is expected to be driven by humid heat, where moisture in the air slows sweat evaporation and weakens the body's ability to shed heat. Researchers said this combination can make moderately high temperatures dangerous, especially for outdoor workers, the elderly, children, and people with limited access to cooling.
Under a 4°C warming scenario, the frequency of UHS could rise more than tenfold compared with a 1.5°C world. In some regions, monsoon heat stress could eventually become even more frequent than summer heat stress, with parts of the Gangetic Plain projected to experience nearly 75% more UHS days during monsoon than during summer if global temperatures rise by 4°C.
Implications for India
The implications are significant for a country where millions work outdoors and many households lack reliable cooling. Researchers estimated that between 800 million and 1.2 billion people could be exposed to at least one day of uncompensable heat stress under future warming scenarios, while the most vulnerable regions together are home to more than 750 million people and have relatively low social and economic development indicators.
Need for Better Preparedness
The study also questioned whether India's current heat preparedness systems adequately capture the danger. Existing heat action plans often rely heavily on air temperature thresholds, even though humidity is central to physiological heat stress. The authors said future warning systems must account for both temperature and humidity to reflect the risk faced by communities during summer as well as monsoon.
A related public note by one of the researchers said India's dangerous heat could no longer be seen only through dry-bulb temperature, because humidity changes how the body experiences heat. 'Most heat studies in India rely on dry-bulb temperature alone, which ignores humidity,' the note said, adding that UHS marks the point where even a young and healthy adult cannot cope for long.
Recommendations
Researchers said the findings should push planners to treat heat as a longer seasonal hazard rather than a short summer emergency. They called for humidity-sensitive heat alerts, better occupational safety rules, cooling shelters, water access, urban heat reduction measures, and targeted protection for workers and communities in districts where heat and humidity are likely to converge during both summer and monsoon.
