Construction, decimation of green cover turning Kol into urban heat island: Study

Time of India

22-04-2025

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Kolkata: The soaring summer heat is linked to shrinking green cover and rising built-up areas, according to a report by the Centre for Science and Environment (CSE).
Summers in Kolkata are becoming increasingly unbearable, not just because of rising mercury but due to the city's rapidly transforming landscape. The report highlights that unchecked urban expansion, shrinking green space and diminishing water bodies are altering the city's natural ability to stay cool, turning it into a heat trap.
CSE, a Delhi-based environmental think tank, conducted a study from 2001 to 2023, analysing land surface temperatures and humidity trends across six megacities — Delhi, Mumbai, Kolkata, Hyderabad, Bengaluru, Chennai — and four smaller cities — Chandigarh, Jaipur, Lucknow and Pune. The findings reveal a disturbing trend: urban areas are becoming heat zones, with heatwaves growing more intense.
In Kolkata, the study found that the built-up area, which includes concrete infrastructure and roads, rose sharply from 70% in 2001 to 80.1% in 2023. In contrast, green cover, crucial for temperature regulation, slightly declined from 15.2% to 14.5%. Alarmingly, scrubland and barren land, which earlier acted as buffer zones, plummeted from 9.8% to 3.4%, and water bodies reduced by more than half — from 5.1% to 1.9%.
These changes are key contributors to the Urban Heat Island (UHI) effect, where densely built-up urban areas retain more heat than their surroundings. With less vegetation and water to moderate temperatures, heat lingers well into the night, increasing public health risks, particularly during summer and monsoon months.
What makes Kolkata's heat even more punishing is its high humidity. The CSE report underscores that humidity has become a major driver of urban heat stress, particularly in coastal cities like Kolkata and Mumbai. The Heat Index — a measure of how hot it feels when relative humidity is factored in — has skyrocketed. In cities like Chennai, humidity added nearly 7°C to the perceived temperature. Kolkata, with its already high humidity levels, now frequently crosses thresholds that cause extreme heat stress.
CSE's analysis utilised satellite data from Landsat 7 and 8, focusing on the Normalised Difference Vegetation Index (NDVI) to assess green cover during India's hottest months — April to June. NDVI readings showed a consistent decline in vegetation, particularly during heatwave periods, correlating directly with the city's rising thermal stress. The reduced green cover means the city has lost its natural heat-mitigation shield, accelerating the impact of climate change at a hyperlocal level.
The CSE report makes it clear: India's cities are becoming hotter, wetter, and more dangerous, particularly for vulnerable populations. Without a clear and immediate shift in urban planning — focused on restoring green cover, conserving water bodies, and limiting unplanned construction — the situation is set to worsen.