An unprecedented heat wave has been enveloping Indian cities, worsening the urban heat island effect, this summer. A new analysis by Centre for Science and Environment (CSE) says there are far deeper and longer term evidences on the nature of this changing trend that is impacting the biggest cities of the country.
 
The analysis shows 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, CSE said in a release on Monday.
 
Even if there is a variation in air temperatures across climatic zones with some parts recording even a decline, the other two factors – relative humidity and land surface temperature – combine to enhance discomfort and heat-related disease burden. According to the US National Weather Service, the heat index is a measure of how hot it really feels when humidity is factored in with the actual temperature. It is considered that a heat index of 41°C is dangerous for human health.
 
“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,” says Anumita Roychowdhury, executive director, research and advocacy at CSE.
 
“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,” says Avikal Somvanshi, senior programme manager, Urban Lab, CSE.
 
The full CSE analysis as well as individual study reports on the six cities can be accessed here: https://www.cseindia.org/decoding-the-urban-heat-stress-among-indian-cities-12191
 
Key highlights
 
Overall trends in urban heat stress in mega cities
 
Variations across climatic zones: 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.
 
Ambient air in Mumbai, Kolkata, Bengaluru and Chennai has gotten hotter while Delhi and Hyderabad seem to be bucking 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. 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 to 2001-10. Decadal summer-time average for Delhi is down by 0.6°C and for Hyderabad, by 0.9°C, compared to 2001-10.
 
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 to the 2001-10 average. Barring Bengaluru, decadal summer-time average RH has increased by 5-10 per cent in the other five mega cities. The last 10 summers of Hyderabad have been on an average 10 per cent more humid compared to 2001-10. Similarly, Delhi’s last 10 summers have been 8 per cent more humid. Mumbai’s relative humidity is up by 7 per cent, while summers in Kolkata and Chennai are 5 per cent more humid on an average. Bengaluru has seen no change in humidity levels during summers.
 
Both Delhi and Hyderabad might have registered the biggest rise in RH levels, though they are located in one of the driest climatic zones. This jump in decadal RH still does not bring their overall humidity levels at par with that of other mega cities which are located in more humid climates. Mumbai, Kolkata and Chennai are still over 25 per cent more humid than Delhi and Hyderabad. The increased humidity in Delhi and Hyderabad somewhat nullifies their marginal drop in ambient air temperatures; in the other four cities, it intensifies the heat stress.
 
Heat index rising faster than ambient temperature in all the cities: 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).
 
Monsoons more thermally uncomfortable in Delhi, Mumbai, Kolkata and Chennai with their heat index being higher than during pre-monsoon period.
 
During 2001-10, the HI used to rise between pre-monsoon and monsoon in Delhi, Mumbai and Kolkata, while it used to drop for the southern megacities of Hyderabad, Bengaluru and Chennai. This trend has changed and in the last 10 summers, monsoon has become even hotter in Delhi, Mumbai and Kolkata, while in Chennai the marginal cooling noted with monsoon has disappeared. Monsoon is still a bit cooler than pre-monsoon for Bengaluru and Hyderabad, but the magnitude of cooling has reduced. Overall, monsoons on average have become 1°C hotter while pre-monsoons are hotter by 0.5°C on heat index compared to 2001-10.  
 
Cities not cooling down at night at the rate they used to during 2001-10: This phenomena is 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. In the current decade, Hyderabad on an average is relatively cooler by just 11.5°C -- 13 per cent down from 2001-10 levels. Delhi nights are a little cooler by just 11.2°C: 9 per cent down from 2001-10. Bengaluru nights are cooling down by just 10.1°C (15 per cent down). Chennai nights are cooler by just 9.7°C (5 per cent down from 2001-10), and Mumbai nights by 7.8°C (24 per cent down from 2001-10 level). Kolkata is the only megacity where night-time temperature is cooling down at the same rate as it used to during 2001-10 -- but it cools down the least among the megacities with land surface temperature dropping only 6.2°C at night. It must be noted that night-time cooling is getting lesser in the last few years for all megacities compared to the mid-2010s.
 
Says Somvanshi: “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.”
 
All cities have registered significant increase in their built-up area that contribute to urban heat island effect: There is 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. Over the last 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 concretization is concerned. Hyderabad has doubled its green cover in the last two decades -- fastest among the megacities. Green cover has declined in Mumbai, Kolkata and Chennai. Most decline was noted in Chennai whose green cover shrank by almost 14 percentage points.
 
City-wise highlights
 
Delhi
•    Cooler than previous averages, but high humidity worsening heat stress: March-April of 2024 was 3°C cooler than the average of 2014-23. Delhi’s summer-time has registered a 0.6°C lower decadal average ambient air temperature, but relative humidity has increased by 8 per cent between 2001-10 and 2014-23.
•    High humidity responsible for adding to heat stress: An average 3.3°C of heat stress is being added.
•    City not cooling down at night: Diurnal cooling down of land surface temperature between day- and night-time is down by 9 per cent.
•    Urban heat island phenomena stronger at night: 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.
•    Direct co-relation between increase in built-up area and rise in urban heat stress: 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. Increase in green cover shows impact on daytime temperatures, but it has had no impact on night-time temperatures and the increasing heat index.
•    Monsoons getting more thermally unconformable than pre-monsoons: Average heat index during monsoons is 9.4°C more than pre-monsoons.
 
Mumbai
•    Both air temperature and humidity have increased, worsening the heat stress: Mumbai’s summer-time has registered a 0.6°C increase in decadal average ambient air temperature; relative humidity has gone up by 7 per cent between 2001-10 and 2014-23.
•    High humidity responsible for adding to heat stress: This is adding an average of 5°C of heat stress to the city. The heat index of the city has increased by 7 per cent. March-April 2024 was similar thermally to the average of 2014-23.
•    Both pre-monsoons and monsoons more thermally uncomfortable by over 2°C: Thermal distinction between monsoon and pre-monsoon has disappeared.
•    Not cooling down at night at the same rate as in 2001-10: Diurnal cooling down of land surface temperature between day- and night-time is down by 24 per cent.
•    Urban heat island phenomena stronger at night: During day-time, the core of Mumbai is 3.5°C cooler than its peripheries and peri-urban areas. But at night, the core is 0.4°C warmer.
•    Direct co-relation between increase in built-up area and rise in urban heat stress: Mumbai’s built-up area has increased from 38.4 per cent in 2003 to 52.1 per cent in 2023. Green cover has decreased from 35.8 per cent in 2003 to 30.2 per cent in 2023.
 
What should be done?
 
Implement city-specific heat management plans: These plans need to go beyond immediate emergency responses to help cope with specific heat events during summer and prevent heat lock-in. Says Somvanshi: “This is not only about summer action for public health protection, but more sustained action throughout the year to heat-proof the city and undertake heat mitigation, along with monitoring, to improve the overall adaptive thermal comfort of built structures and reduce energy and carbon intensity of the built environment.”
 
Target all heat generators: 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. Adopt guidelines and action plans to reduce thermal load on buildings and enhance thermal comfort; manage waste heat.
 
Modify land use to mitigate heat: Ensure reversal land-use changes to expand the green areas and waterbodies for stronger cooling effect. Increase shaded areas.    
 
Develop a tracking mechanism: This is for tracking annual and diurnal trends in temperature, humidity and overall heat index to inform planning and implementation.
 
Consider both day- and night-time temperatures for planning: Often, health emergency action considers only the high day-time temperatures, and not the night-time temperatures and relative humidity. This trend poses risks to public health and energy security of the city. The heat problem is not just about focusing on daily maximum temperatures crossing the 45°C benchmark – the standard focus during summers – but involves a much more complex set of indices.
 
Strengthen scientific tracking of key factors impacting heat: Develop and combine a robust database on ambient heat and temperature, surface heat absorption and land surface temperatures, and changing land use, including vegetative cover and waterbodies that are determinants of heat island effect and relative humidity. This requires effective leveraging of available satellite technology. Given advancements in technology, such data is available but needs policy integration.  
 
Put in place emergency action plans for heat wave episodes to protect public health: 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.