The year 2023 was by far the hottest ever according to a recent World Meteorological Organization (WMO) report. Global average temperatures reached 1.45° C higher than pre-industrial levels, almost touching the 1.5° C limit set in the Paris Agreement. Scientists predict that 2024 could be similar. With global emissions still growing, climate impacts are worsening. Heatwaves are sweeping through the Indian sub-continent. And, more hotter and longer-lasting heat waves are being predicted in the years to come.
The reality of the urban heat island
In cities, this problem is exacerbated by a phenomenon termed the Urban Heat Island (UHI) effect. Temperatures in large, crowded urban settings can be several degrees higher than in surrounding rural areas, and even hotter at night. Concrete structures and tarmac roads retain heat which stays trapped inside this “urban bubble” along with air pollutants. A lack of green spaces and waste heat from air conditioners and other machinery add to the UHI.
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Chennai, a coastal city, is affected by yet another feature which is cause for worry. Humidity reduces the cooling effect of perspiration, leading to a person experiencing an elevated body temperature, debilitating heat stress, exhaustion, and even a potentially fatal heat stroke.
As shown by available heat maps, the UHI in Chennai adds between 2° to 4° C to temperatures in nearby rural areas. So, when the maximum temperature is 40° C elsewhere, parts of Chennai could register between 42° to 44° C. Under high humidity conditions, wet-bulb temperature (indicating the extent to which evaporation can take place and facilitate cooling) of around 38.5° C is considered by the World Health Organization to be “near the limits of human survivability”.
In India, a heatwave is officially declared in coastal areas when the maximum temperatures are over 37°C and 4.5°C above normal. Clearly, with an UHI, heatwave conditions are quite easily breached in Chennai. The effects could be much worse, even dangerous, when compared to inland, rural areas.
India has national, State and even some district-level Heat Action Plans (HAP) to reduce morbidity and mortality, especially among the vulnerable poor, infants and the elderly. The National Disaster Management Authority (NDMA) Guidelines, which are being upgraded, and those of several States, outline measures to deal with heatwaves including early warning bulletins, and staggered work hours at outdoor construction sites, with shaded areas and temporary shelters, and strategic provisioning of drinking water and oral rehydration salts. Besides such post facto responses to heatwaves, longer term measures are needed to deal with UHI and reduce urban heat. The Chennai Metropolitan Development Authority (CMDA) has recently commissioned the preparation of presumably more detailed heat maps than what are available now. While awaiting more localised planning and actions, several broad brush measures may be readily thought of and implemented. The Chennai Climate Action Plan (CCAP) offers several meaningful suggestions including those discussed here, albeit scattered under different sections. In our view, they underestimate causative factors and, therefore, remedial measures and targets.
A study and findings
Our study on Chennai and climate change (see Proposed Action Plan under www.inhaf.org/climact) looks at UHI as one among several inter-linked multi-dimensional issues. Our major findings and recommendations are discussed here.
Increasing green cover, obviously, tops the list. Green areas such as urban forests, large greens and parks, avenue and other trees, even lawns, release moisture which evaporates and cools the environs. Well-distributed green areas also influence local micro-climate, reduce air pollution, and promote health and well being. Tree-lined and shaded walkways and tracks provide pedestrians, cyclists and itinerant workers shelter from the blazing sun, and also encourage non-motorised transportation. With such multiple benefits, green areas are considered essential for sustainable urban development by UN Habitat, which recommends that green spaces be available for all citizens within 400 metres from their residence.
Regrettably, Chennai has among the lowest percentage of green cover of all the metros in India. The area under the Corporation is greener, with promising initiatives such as “miyawaki forests”, although questions remain about the species planted. However, the expansion of the city has heavily depleted green areas and waterbodies.
Varied figures are cited for green cover in the larger Chennai Metropolitan Area (CMA) depending on the assessment methodology. But an estimated 12% appears reasonable (subject to correction), compared to an estimated over 20% in Bengaluru, Kolkata, Mumbai and Delhi. The densely populated city-state of Singapore has an astounding 47% under green cover. Many European cities have green cover that is well over the EU norm of 30%.
Congested, poorly ventilated localities and informal settlements of the urban poor suffer the most from UHI and would benefit from green areas, parks and waterbodies that are nearby. The Master Plan III should provide for inviolable green areas and local parks with equitable access.
Rough estimates indicate that increasing green cover in the CMA to a well-distributed 25% could significantly reduce UHI by about 1.5° C or more. This could also absorb around 10% of its carbon dioxide emissions and assist moving towards a “net zero” future.
On the use of air-conditioners, energy saving
A less understood factor behind UHI is waste heat from air-conditioning. In Chennai, as in other Indian metros, roughly 50% of electricity consumption during summer is for air-conditioning alone, which vent heat out. The more the UHI, the greater the use of air-conditioning, generating even more heat in a nasty feedback loop. It is estimated that moving towards more energy-efficient (EE) air-conditioning, through a combination of mandates for the purchase of five-star or split EE air-conditioners and incentives for the exchange of older air-conditioners for new EE units (as offered by the electricity distributor in Delhi, to reduce peak load, a win-win for distributor and consumer), could reduce UHI by as much as 1.5° C.
Cities such as Shanghai and Seoul have reported a significant reduction in UHI through such strategies. Several east Asian cities have in addition mandated other energy-saving measures for air-conditioning such as having a thermostat setting of 25° C in offices and commercial buildings. Energy savings can also accrue from switching off air-conditioners (and other appliances) from the mains rather than by remote control (this leaves appliances on low power-consuming stand-by mode). Greater consciousness about climate change would undoubtedly help but savings of roughly 25% on electricity charges, would also act as a powerful driver of change.
Further, if buildings are better insulated and ventilated, and constructed using appropriate designs and materials according to “green” building codes, they would require less air-conditioning and generate less waste heat.
Total energy savings could then rise to roughly 40%-50% and reduce UHI by, say, around 3° C. There would also be a significant co-benefit — of emissions reduction from thermal power plants in Chennai.
Having permeable pavings and walkways using alternative materials, increased shrubbery along sidewalks, berms and dividers, and reflective paint on roofs, walls and streets, are other measures to reduce UHI. A sharp reduction in personal vehicles (most four-wheelers have powerful engines and airconditioners), through a rapid scaling-up of effective public transport with electric buses, would be another major contribution.
Chennai is one of a very few cities in India to have adopted a Climate Action Plan, but there is considerable scope for improvement. The city and its residents should utilise this opportunity to ensure long-term policies and measures to cool the city, improve liveability, and set an example for the rest of India.
D. Raghunandan is Climate Change Lead, Inhaf ClimACT-Chennai. Bindhu Bhuma was formerly Project Associate, Inhaf ClimACT-Chennai
