Increasing concretization and humidity levels are exacerbating heat stress in India's megacities, which are not cooling down at night at the same rate as a decade ago, according to a new report by the Centre for Science and Environment (CSE).

The CSE analysed summer air temperature, land surface temperature, and relative humidity data for six megacities -- Delhi, Mumbai, Calcutta, Hyderabad, Bengaluru and Chennai -- from January 2001 to April 2024.

The think tank found that increased humidity is worsening heat stress across all climate zones, even nullifying the marginal drop in air temperatures in Delhi and Hyderabad.

Barring Bengaluru, summertime average relative humidity has increased by 5-10 per cent in the other five megacities from 2014-2023 compared to the 2001-2010 average.

The CSE report comes at a time when blistering heatwaves are affecting health and livelihoods in large parts of India.

“Assessing the changing trend in heat, relative humidity, and land surface temperature along with day and night-time temperatures is necessary to develop a comprehensive heat management plan for urban centres,” said Anumita Roychowdhury, executive director of research and advocacy at CSE.

Avikal Somvanshi, senior programme manager at CSE's Urban Lab, said addressing the combination of high heat and humidity is particularly important as it can compromise the human body’s main cooling mechanism, sweating.

“The evaporation of sweat from the skin cools our bodies, but higher humidity levels limit this natural cooling. As a result, people can suffer heat stress and illness, and the consequences can even be fatal at much lower ambient temperatures,” he explained.

The study showed that megacities are not cooling down at night.

The land surface temperature during the summers of 2001-2010 would drop by 6.20-13.20 degrees Celsius from the daytime peak to the nighttime low. In the last 10 summers (2014-2023), night-time cooling has reduced to 6.20-11.50 degrees Celsius, the CSE said.

“Hot nights are as dangerous as midday peak temperatures. People get little chance to recover from daytime heat if temperatures remain high overnight,” Somvanshi said.

“A study published in 'The Lancet Planetary Health' noted that the risk of death from excessively hot nights could increase nearly six-fold in the future. This prediction is much higher than the mortality risk from daily average warming suggested by climate change models,” he said.

The study said that rising humidity levels have also made monsoons in Delhi, Mumbai, Calcutta, and Chennai hotter than pre-monsoon periods.

It noted that all megacities have become more concretized in the last two decades, increasing heat stress. The CSE highlighted that an increase in green cover is not effective in mitigating nighttime heat.

The study showed that Kolkata has the highest percentage of land under concrete and the lowest green cover among the megacities, while Delhi has the least area under concrete and the maximum green cover.

Green cover has declined in Mumbai, Kolkata, and Chennai. The built-up area in Chennai has doubled in the last two decades, while its green cover shrank by almost 14 percentage points.

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