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CSE study tracks heat wave; exposes dangerous trends in India’s biggest cities

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 India’s biggest cities.

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.

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.

Methodology and data: The Urban Lab at CSE has carried out a comprehensive assessment of all three factors during summer – air temperature, land surface temperature and relative humidity – that contribute to heat stress, in six megacities of India. These include Delhi, Mumbai, Kolkata, Hyderabad, Bengaluru and Chennai.

The time frame of the study is the summer of January 2001 till April 2024. These six cities are located in different climatic zones and provide insights into regional variations in heat stress.

The analysis has focused on the trends in day and night time temperature, humidity levels, seasonal variations, trend in land surface temperature and trend in built-up area. This summary report provides highlights and some key findings from all the cities.

The study is based on comparative statistical analysis of temperature and the humidity condition observed in Delhi since 2001. The study’s definition of summer is the period from March to August. It is further divided into pre-monsoon (March-May) and monsoon (June-August) as per IMD classification. This is based on publicly available datasets from various national and global agencies. Ambient temperature and humidity data have been sourced from the India Meteorological Department (IMD) weather stations at Palam and Safdarjung. An average of the findings from these two weather stations is used to represent Delhi in this study. Heat Index computation has been done using the US National Oceanic and Atmospheric Administration’s (NOAA) formula. Complex geospatial calculations have been done in python and ArcGIS.

Moreover, freely accessible MODIS Land Science data from NASA Earth Observations has been used for seasonal and long term analysis of land surface temperature. For more granular analysis of heat and land use conditions on extremely hot days, satellite imagery data from the United States Geological Survey (USGS) Earth Explorer website has been used. Landsat 7 Enhanced Thematic Mapper Plus (ETM+) and Landsat 8 operational land imager/thermal infrared sensor (OLI/TIRS) satellite imagery were downloaded and used to analyze the land surface temperature, land use, land cover and Normalized Difference Vegetation Index (Green cover).

This city-level assessment focuses on changes in heat patterns over the years for the summer season, urban expansion over the years, and land surface temperature variation during the summer of 2003, 2013, and 2022.

Key highlights

Decoding decadal trends: Heat trends that add to thermal discomfort and heat stress-related diseases are now being tracked more granularly by technical monitoring bodies. The IMD tracks air temperature at its weather stations: according to its Annual Climate Summary-2023, the year 2023 was the second warmest year on record since nationwide tracking commenced in 1901.

The annual mean land surface air temperature averaged over India during 2023 was +0.65°C above the long-term average (period 1981-2010). However, this is lower than the highest warming observed over India during 2016.

On the other hand, the recently released World Meteorological Organisation’s (WMO) State of the Climate in Asia 2023 has brought out the variations in land surface temperatures across India. This behaves differently from ambient air temperature trends. This states that the average land surface temperatures were below normal (1991-2020 reference) in parts of the inland Indian peninsula in 2023 even though the mean land surface temperature over Asia was the second highest on record. The WMO’s assessment is based on land surface temperature data derived from satellite observations.

The observations of the two agencies show that while air temperatures are higher than average in India, land temperature is below average. Yet, when relative humidity is combined with high heat, it worsens the heat stress and adversely impacts human thermal comfort and health.

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.

Kolkata

  • Normal decadal air temperature change, but higher humidity worsening heat stress: Kolkata’s summer-time has registered insignificant change in decadal average ambient air temperatures, but the relative humidity has increased by 5 per cent between 2001-10 and 2014-23.
  • High humidity responsible for adding an average 6.6°C of heat stress: Decadal heat index has risen by 3.5 per cent on an average.
  • Days with daily heat index exceeding 41°C (danger mark) have tripled compared to 2001-10.
  • Monsoons getting more thermally unconformable than pre-monsoons: Average heat index during monsoons is 3.5°C more than in pre-monsoons.
  • Unlike other metros, Kolkata cools down at night at the same rate as in 2001-10.
  • But diurnal cooling down of land surface temperatures between day- and night-time has a difference of 6°C on an average.
  • During day-time core of Kolkata is 1.8°C warmer than its peripheries and peri-urban areas; at night, the difference is of 1.2°C. This is evidence of urban heat island effect.
  • Direct co-relation between increase in built-up area and rise in urban heat stress: Built-up area has increased from 70 per cent in 2001 to 80.1 per cent in 2023. Green cover has decreased from 15.2 per cent in 2001 to 14.5 per cent in 2023.

Hyderabad

  • Air temperature has not seen appreciable increase, but humidity has gone up, worsening the heat stress: Hyderabad’s summers have registered a 0.9°C drop in decadal average ambient air temperatures, but relative humidity has increased by 10 per cent between 2001-10 and 2014-23.
  • High humidity responsible for adding an average 1.5°C of heat stress
  • High number of summer days with high temperatures: Hyderabad has 30-90 days in summer when the daily ambient temperature exceeded the 37°C mark.
  • Pre-monsoons thermally more unconformable than monsoons: Average heat index during monsoon is about 3°C less than pre-monsoon.
  • Not cooling down at night at the same rate as in 2001-10: The diurnal cooling down of land surface temperatures between day- and night-time is down by 13 per cent.
  • Urban heat island phenomena stronger at night: During day-time in summers, the core of the city is 0.7°C cooler than its peripheries and peri-urban areas. At night, the core gets 1.9°C warmer.
  • Direct co-relation between increase in built-up area and rise in urban heat stress: Built-up area has increased from 20.6 per cent in 2003 to 44 per cent in 2023. Green cover has also gone up from 8.9 per cent in 2003 to 26.5 per cent in 2023. This rise in green cover indicates an impact on day-time temperatures, but none on night-time temperatures and the increasing heat index in the city.

Bengaluru

  • Rising summer-time temperature, but stable relative humidity: Bengaluru has registered a 0.5°C increase in decadal average ambient air temperature in summers, while relative humidity has remained stable over the last two decades. March-April 2024 was significantly hotter (about 3°C) compared to the average of 2014-23.
  • Humidity responsible for adding an average 0.6°C of heat stress: Heat Index of the city has increased by 2 per cent.
  • Pre-monsoons more thermally unconformable than monsoons: Average heat index during pre-monsoon is 3°C more than during monsoons.
  • City not cooling down at night: Diurnal cooling down of land surface temperature between day- and night-time is down by 15 per cent.
  • Urban heat island phenomena stronger at night: During day-time in summers, core of Bengaluru is 0.6°C cooler than its peripheries and peri-urban areas. But at night, the core is 2.5°C warmer.
  • Direct co-relation between increase in built-up area and rise in urban heat stress: Built-up area has increased from 37.5 per cent in 2003 to 71.5 per cent in 2023. Green cover has also increased from 18.8 per cent in 2003 to 26.4 per cent in 2023. The rise in green cover has had an impact on day-time temperatures, but none on night-time temperatures and increasing heat index in the city.

Chennai

  • Both air temperature and relative humidity have increased, worsening the heat stress: A summer-time rise of 0.4°C in decadal average ambient air temperature; relative humidity has increased by 5 per cent between 2001-10 and 2014-23. March-April 2024 was 1°C hotter compared to average of 2014-23.
  • High humidity responsible for adding an average of 6.3°C heat stress: Heat Index of the city has increased by over 5 per cent.
  • More days with high heat index: Days with daily heat Index exceeding 41°C (danger mark) has tripled compared to 2001-10.
  • Both pre-monsoons and monsoons have become thermally more uncomfortable: With about 2°C rise in heat index, thermal distinction between pre-monsoon and monsoon has almost disappeared — both periods are now equally hot and muggy.
  • City not cooling down at night: Diurnal cooling down of land surface temperature between day- and night-time is down by 5 per cent.
  • Urban heat island phenomena is strong: During day-time in summers, core of Chennai is 0.8°C warmer than its peripheries and peri-urban areas. At night, the core is 0.9°C warmer.
  • Direct co-relation between increase in built-up area and rise in urban heat stress: Built-up area has increased from 30.7 per cent in 2003 to 73.5 per cent in 2023. Green cover has decreased from 34 per cent in 2003 to 20.3 per cent in 2023.

Trends in heat stress will worsen with climate change: 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 Says Roychowdhury: “Combining climate change projections with urban growth scenarios, it can be said with very high confidence that future urbanisation will amplify the projected increase in local air temperatures.”

With reference to urban centres, the IPCC notes with confidence in AR6 WG-II that hot extremes, including heat waves, have intensified in cities. It further notes that urban areas experience air temperatures that are several degrees warmer than surrounding areas, especially during the night. The urban heat island effect can add 2°C to local warming, reducing the adaptive capacity of cities and increasing the aforementioned 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.

What should be done? We need heat management plans in our cities

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.

(A CSE Report)