In the heat of the night

CHENNAI: For most of us Chennaiites, the running joke pertaining to the city being beset only by three seasons — hot, hotter, and hottest, is something we have learned to swallow like a bitter pill. Unfortunately, what was once considered a malady limited to the waking hours of the day, seems to have crossed over into the night time as well. Earlier this week, the State Planning Commission released a report titled Urban Heat Island- Hotspot Analysis and Mitigation Strategies for Tamil Nadu 2024. The study said that over the past 20 years, a trend of increasing night-time temperatures was witnessed in Tamil Nadu, with a 59% rise in areas experiencing night temperatures of 24-26°C, while extreme daytime temperatures (46-50°C) have decreased by 28%.

The pattern suggests that dense urban areas, particularly Chennai, retain heat overnight, intensifying Urban Heat Island (UHI) effects. Key hotspots include Thoothukudi, which records the highest daytime temperatures, and Chennai, which is significantly impacted by increased night-time heat. The UHI intensity has risen by nearly 3°C in cities like Chennai and Thiruvallur, driven by dense urban development and reduced green cover. Urban Heat Island Effect (UHIE) is a phenomenon where urban areas experience significantly higher temperatures than surrounding rural regions due to human activity, extensive buildings, and limited green spaces. This effect intensifies heat levels, impacting public health, increasing energy demand, and even entails severe economic consequences .

The Intergovernmental Panel on Climate Change‟s Sixth Assessment Report (IPCC AR6) highlights the rising risks of severe heat stress, compounded by drought, water scarcity, forest fires, and tropical cyclones. These conditions are expected to lead to a 1,540% increase in health-related deaths among people over 65 by 2090, with India projected to face an additional 1 million heat-related deaths if warming continues unchecked. Additionally, the international Labour Organization (ILO) estimates that India lost 4.3% of working hours to heat stress in 1995, a figure expected to rise to 5.8% by 2030, equivalent to the loss of 34 million jobs.

Being bordered by the sea and the Western Ghats, Tamil Nadu regularly endures temperatures exceeding 40°C for significant portions of the year. The proximity to the coast results in high humidity which worsens the situation by trapping heat close to the surface, and reducing cooling through evaporation. The water vapour in the air tends to act like a blanket, which prevents the heat from escaping, especially during the night.

Between 2011 and 2021, Tamil Nadu averaged more than eight heat wave days annually, and in 2023, 12 heat-related deaths were reported by June alone. Studies predict that both temperatures and heat stress levels will continue to rise in coming years.

A major data point highlighted by the study was how concrete regions in the city of Chennai had grown by 70% between 1991 and 2016, essentially taking over 88% of the city’s land. During the same period, waterbodies shrank by 50%, wetlands declined by a whopping 70% and the vegetation cover plummeted by 34%.

Mitigating the fallout of such heat related hazards will require a coordinated effort, not just on part of the government, but the private sector and citizens on the whole. Increasing urban green cover through forestry and green corridors, adopting cool pavements and reflective

materials, and restoring natural heat sinks like water bodies could be a start point. At the building or construction level, we will need to gravitate towards climate-sensitive designs, green roofs, and energy-efficient materials.