Heat Stress Exposes Dangerous Trends in India’s Biggest Cities

Patna: Amid ongoing scorching summer, six mega cities across India are facing an unprecedented heat wave that is worsening the urban heat island effect. This has been revealed by a latest study done by New Delhi-based Centre for Science and Environment (CSE).

The CSE study highlighted that 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 CSE has tracked heat in six megacities – Bengaluru, Chennai, Delhi, Hyderabad, Kolkata and Mumbai. The time frame of the study is the summer of January 2001 till April 2024.

“An unprecedented heat wave has been enveloping Indian cities, worsening the urban heat
island effect, this summer. There are far deeper and longer term evidences on the nature
of this changing trend that is impacting India’s biggest cities” the study said.

Anumita Roychowdhury, executive director, research and advocacy at CSE, said in a statement that: “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 the 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”.

According to the study, cities in the warm-humid and moderate climate zones show an increase, while cities in composite and hot-dry climate zones indicate a decline. Ambient air temperature has changed by less than 0.5°C between 2001-10 and 2014-23. Decadal summer-time average ambient temperature has risen by about 0.5°C in Mumbai, Bengaluru and Chennai compared with 2001-10. Kolkata’s decadal average is also up by 0.2°C. Delhi and Hyderabad, two metros which are located in composite climate zones known for the driest and harshest summers, have registered lower decadal average compared with 2001-10, the study adds.

Decadal summer-time average for Delhi is down by 0.6°C and for Hyderabad, by 0.9°C, compared with 2001-10.

“Addressing the combination of high heat and humidity is particularly important as this
can compromise the human body’s main cooling mechanism: sweating. The evaporation of
sweat from 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 even at much lower ambient temperatures. Interestingly, night time temperature is
remaining elevated in cities,” Avikal Somvanshi, senior programme manager, Urban Lab, CSE
said in a statement related to the study’s release.

Relative humidity has increased in all zones. This increase has made heat stress worse in warm-humid and moderate climate zones, while it has nullified the fall in air temperatures in composite and hot-dry climate zones, especially during monsoons. Average relative humidity (RH) has significantly increased in the last 10 summers compared with the 2001-10 average, the study noted.

Barring Bengaluru, decadal summer-time average RH has increased by 5-10% in the other five mega cities, it added.

The CSE study said that given the rise of relative humidity during summers, the heat index
(HI) has risen among mega cities. Chennai´s summer average heat index stood at 37.4°C
(impact of humidity: 6.9°C) making it the hottest among the mega cities. Kolkata with a
summer HI average of 36.5°C (impact of humidity: 6.4°C) and Mumbai with 34.3°C (impact of
humidity: 5°C) were almost equally hot. Delhi’s summer HI average stood at 32.2°C (impact
of humidity: 3.3°C) and Hyderabad’s at 29.3°C (impact of humidity: 1.2°C). Bengaluru was
the least hot among the mega cities with a summer HI average of 26.9°C (impact of
humidity: 0.8°C).

The study noted that monsoons were more thermally uncomfortable in Delhi, Mumbai, Kolkata and Chennai with their heat index being higher than during pre-monsoon period.

Besides, cities are not cooling down at night at the rate they used to during 2001-10: This phenomena has been observed across all climatic zones. During the summers of 2001-10, land surface
temperatures used to drop by 6.2°C-13.2°C from the day-time peak. Hyderabad used to cool
down at night the most, while Kolkata the least. Now, in the last 10 summers (2014-23),
night-time cooling has reduced to 6.2°C-11.5°C.

The study noted that night-time cooling
was getting lesser in the past few years for all megacities compared with the mid-2010s.
 

“Hot nights are as dangerous as mid-day peak temperatures. People get little chance to
recover from day-time heat if temperatures remain high overnight. A study published in the Lancet Planetary Health has noted 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,” said
Somvanshi.

The study also noted a direct co-relation between increase in built-up area and increase in urban heat stress. All megacities have become more concretised in the last two decades; this has contributed to the rise in heat stress; increase in green cover can moderate day-time heat, but is not that ineffective in arresting night-time heat.

In 2023, Kolkata had the highest percentage of its land under concrete and the lowest green
cover among the megacities; Delhi has comparatively the least area under concrete and the maximum green cover, it said.

Over the past two decades, built-up area in Chennai has doubled. Kolkata has registered an only 10 percentage points increase in its built-up area, making it the slowest as far as concretisation is concerned.

Hyderabad has doubled its green cover in the past two decades -- fastest among the megacities. However, green cover has declined in Mumbai, Kolkata and Chennai. The maximum decline was noted in Chennai, whose green cover shrank by almost 14 percentage points, the CSE study said.

The technical summary of the Intergovernmental Panel on Climate Change (IPCC), Working
Group-I, Sixth Assessment Report (AR6 WG-I) notes that the frequency and intensity of
heat extremes and duration of heat waves have almost certainly increased since 1950, and
will keep rising even if global warming is stabilised at 1.5°C.

The CSE study recommended implementation of city-specific heat management plans to go beyond
immediate emergency responses to help cope with specific heat events during summer and
prevent heat lock-in.

Key heat generators, such as concrete built surfaces, barren land and waste heat generators like vehicles, industries, and cooling devices should be brought under the ambit of the plan. Guidelines and action plans to reduce thermal
load on buildings and enhance thermal comfort; manage waste heat must be adopted. It also called for ensuring reversal of land-use changes to expand green areas and waterbodies for stronger cooling effect.

Among the other recommendations are and increase in shaded areas.

“There should be tracking of annual and diurnal trends in temperature, humidity and overall heat index to inform planning and implementation.
It is critical to develop emergency healthcare systems for heat-related disease burden,
expand the shaded areas in cities, ensure availability of drinking water in public
spaces, and reduce heat exposure for vulnerable and occupationally exposed groups in
cities,” the study added.