Why Megacities Are Not Cooling Down

The heat index in major metro cities in the country has majorly risen since 2010 compared to a decade earlier.

India's megacities are not cooling down even during nights in the summer months, according to a report. These cities also face significant heat stress during the Monsoon.

In fact, the heat index (HI) in major metro cities in the country has majorly risen since 2010 compared to a decade earlier, a latest report by the Centre for Science and Environment pointed out.

Concretisation, rising humidity along with global warming have played a role in increasing the heat index of highly populated and urban cities.

The annual mean land surface air temperature averaged over India during 2023 was 0.65 degree Celsius above the long-term average (1981 TO 2003), the CSE report said.

The report has studied the HI of Delhi, Mumbai, Hyderabad, Bengaluru, Kolkata and Chennai.

HI is a measure of how hot it really feels when humidity is factored in with the actual temperature and anything above 41 degree Celsius H1 is considered dangerous to human health.

According to the report, Decoding the Urban Heat Stress Among Indian Cities, cities are not cooling down at night at the rate they used to during 2001 to 2010.

Citing decadal data, the report said the land surface temperature used to come down by 6.2-13.2 degree Celsius from the day-time peak to nighttime peak.

In the last 10 summers, the night-time cooling has reduced to 6.2 degree Celsius to 11.5 degree Celsius. Mumbai nights have faced the most reduction in night-time cooling by 24 per cent.

Reduction in night-time cooling has far reaching impact on human health.

Citing a study published in the Lancet Planetary Health by a group of scientists from China, South Korea, Japan, Germany and the US, the report noted that the risk of death from excessively hot nights could increase nearly six-fold.

Apart from health, high HI round the clock puts pressure on the power supply load.

High use of air conditioners, coolers and refrigerators spikes load even during what used to be low demand hours such as late evening and early morning.

High usage of HFC based cooling equipment increases the ambient heat in any area. Cities are becoming hotter with rising use of ACs, several research reports have concluded.

Several states have already crossed their record historic high power demand -- Delhi (8Gw), Uttar Pradesh (27 Gw), Maharashtra (28 Gw) among others.

When it rains, it doesn't pour

The CSE report highlighted another worrying trend -- Monsoon is more thermally uncomfortable in Delhi, Mumbai, Kolkata and Chennai with their HI being higher than the pre-Monsoon period.

During 2001 to 2010, the HI used to rise during the monsoon in Delhi, Mumbai and Kolkata while it used to decline in southern megacities of Hyderabad, Bengaluru and Chennai, the report noted.

This trend has changed and in the last 10 summers. The monsoon has turned hotter in Delhi, Mumbai and Kolkata, while in Chennai the marginal cooling noted with the monsoon has disappeared, it explained.

Anumita Roychowdhury, executive director, research and advocacy at CSE and the research direction for the report, said 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.

"This is needed to implement emergency measures during heatwaves to protect public health and also to develop longer term strategies to mitigate heat by increasing green areas and waterbodies, improving thermal comfort in buildings, and reducing waste heat from vehicles, air conditioners and industries," she said.

Last month, the World Meteorological Organisation in its state of climate report in Asia said this continent is warming faster than the global average and the warming trend in Asia in 1991-2023 was almost double the warming trend during the 1961-1990 period.

On India, it said severe heat waves in April and June resulted in about 110 reported fatalities due to heatstroke in 2023.

Feature Presentation: Ashish Narsale/Rediff.com