What is causing severe heatwaves in Delhi, Mumbai, other big cities? New study explains factors causing heat stress

An analysis of six megacities in India — Delhi, Mumbai, Kolkata, Hyderabad, Bengaluru, and Chennai — revealed that nighttime temperatures are not cooling down at the rate they used to during 2001-10. The analysis also highlighted the factors causing increased heat stress in urban cities.

"Increasing relative humidity across all climate zones has worsened the heat index (combination of air temperature and relative humidity) and heat stress in every city," said a trend-analysis report released by the Centre for Science and Environment. The study was conducted by the CSE between the summers of January 2001 and April 2024.

What is heat stress?

The CSE explained in its analysis that the heat stress is not just about rising temperatures. "It is a deadly combination of air temperature, land surface temperature and relative humidity that leads to acute thermal discomfort and heat stress in cities," the report said.

What has made heat stress worse?

1. Rising Humidity: As per the report, the increase in relative humidity in all zones "has made heat stress worse in warm-humid and moderate climate zones". This increase has also "nullified the fall in air temperatures in composite and hot-dry climate zones, especially during monsoons".

The report highlighted the city-wise trends and added that the increased humidity in Delhi and Hyderabad, which are located in climate zones known for the driest and harshest summers, "somewhat nullifies their marginal drop in ambient air temperatures".

2. Spike in built-up area: An increase in the built-up area in cities is another factor contributing to the urban heat stress. "All megacities have become more concretised in the last two decades; this has contributed to the rise in heat stress," the report said.

It added that though an increase in green cover can moderate day-time heat, it is not that effective in arresting night-time heat.

"Built-up area has increased from 31.4 per cent in 2003 to 38.2 per cent in 2022. Green cover has increased from 32.6 per cent in 2003 to 44.2 per cent in 2022. An increase in green cover shows the impact on daytime temperatures, but it has had no impact on night-time temperatures and the increasing heat index," the CSE said in its analysis.

But in the other four cities, increased humidity intensifies the heat stress.

Rising temperatures in 6 cities

The analysis revealed that Mumbai, Kolkata, Bengaluru, and Chennai have seen an increase in air temperature, while Delhi and Hyderabad appear to be defying the trend.

Decadal summer-time average ambient temperature has risen by about 0.5°C in Mumbai, Bengaluru and Chennai compared to 2001-10. Kolkata's decadal average is also up by 0.2°C, the CSE study added.

Meanwhile, Delhi and Hyderabad registered "lower decadal average compared to 2001-10". It added that the decadal summer-time average for Delhi is down by 0.6°C and for Hyderabad, by 0.9°C, compared to 2001-10.

How does heat stress affect humans?

The combination of high heat and humidity may compromise the "human body's main cooling mechanism" – that is sweating. Avikal Somvanshi, senior programme manager, Urban Lab, CSE, explained that the evaporation of sweat from the skin cools 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 even at much lower ambient temperatures. Interestingly, nighttime temperature remains elevated in cities," said Somvanshi.

Are hotter nights ahead?

The analysis further showed that the six big cities are not cooling down at night at the rate they used to during 2001-10. Only Kolkata, among other metro cities, cools down at night at the same rate as in 2001-10.

Somvanshi, the expert from CSE, said hot nights are as dangerous as mid-day peak temperatures. She cited a study published in the Lancet Planetary Health to inform that the "risk of death from excessively hot nights would increase nearly six-fold in future".

"This prediction is much higher than the mortality risk from daily average warming suggested by climate change models," Somvanshi added.

The CSE study further revealed that the "urban heat island effect" is stronger at night. It said that at night, the peri-urban area cools down by 12.2°C, while the city core cools down by only 8.5°C. "Thus, the city core is cooling down 3.8°C less than its peri-urban areas," it added. The study noted that March-April of 2024 was 3°C cooler than the average of 2014-23.

What is the urban heat island effect?

The new analysis by the CSE showed that an unprecedented heat wave has been gripping Indian cities this summer, worsening the urban heat island effect.

The "urban heat island" effect happens when built-up areas in urban regions trap heat, reduce green cover, cause congestion, absorb heat, and generate additional heat from human activities.

It said the urban heat island effect can add 2°C to local warming, reducing the adaptive capacity of cities and increasing the risks.

"This is due to reduced ventilation, heat-trapping by closely spaced tall buildings, heat generated directly from human activities, heat-absorbing properties of concrete and urban building materials, and limited vegetation," the analysis explained.