As Tamil Nadu continues its breakneck urbanisation, a new decadal study commissioned by the State Planning Commission (SPC) has sounded alarm bells over rising land surface temperatures (LST) and worsening heat stress across the state.
Titled “Urban Growth and Thermal Stress: A Decadal Assessment of Built-Up Area and Climate Interactions in Tamil Nadu”, the report reveals how unregulated urban expansion has created dangerous heat islands, strained infrastructure, and intensified ecological degradation in the last two decades.
The study, which integrates high-resolution spatial data from MODIS LST, ERA5 air temperature records, and building footprint changes from 1985 to 2015, maps heat exposure at the block-level for all 389 blocks across Tamil Nadu. The results show 94 blocks have witnessed a steep rise in heat over the past 40 years, while 64 blocks currently suffer from temperatures well above the state average. Particularly vulnerable are 25 blocks—including those in Chennai, Karur, and Ramanathapuram—which fall into both categories, making them high-risk zones for future climate and health.
The heat driver
The report places rapid urbanisation at the heart of the problem. Between 1985 and 2015, Tamil Nadu witnessed unprecedented expansion in built-up areas, particularly in districts like Chennai, Coimbatore, Tiruppur, and Chengalpattu. For instance, S.S. Kulam block in Coimbatore more than doubled its built-up area from 62 sq km to 137 sq km—surpassing even Chennai, which expanded from 86 sq km to 131 sq km during the same period.
More worryingly, when built-up area is analysed relative to a block’s total geographical area, the percentage of land classified as “built” has jumped alarmingly. In Chennai, this rose from 48 per cent to 74 per cent in just three decades, while St. Thomas Malai in Chengalpattu posted a 44 pc rise, the highest relative growth recorded.
“Heat stress is becoming one of the most tangible impacts of climate change. As urban areas expand, integrating heat considerations into planning is essential to safeguard communities and protect ecosystems. Mainstreaming Heat Action Plans at the city level is now a necessity,” said Sudha Ramen, Member Secretary and Head of Division (Land Use), SPC.
Night-time heat: A silent killer
One of the most significant findings of the report relates to night-time land surface temperature (nLST), a key indicator of retained heat in urban zones. Using satellite data from MODIS, the study shows that between 2000–2005 and 2018–2023, average nLST increased by nearly 4°C in many urban and peri-urban zones. Even Tier-II cities like Madurai, Tiruchirappalli, and Erode are seeing worrying upward trends, which could have serious health impacts.
“We often ignore how hot nights affect human health, both physical and mental. Sustained heat exposure after sunset reduces the body’s ability to recover, increasing risks of dehydration, cardiovascular stress, and even mortality,” said Shweta Narayan, Campaign Lead, Global Climate and Health Alliance.
The Universal Thermal Comfort Index (UTCI) used in the study reinforces this concern, with large areas in and around Chennai, Coimbatore, and Madurai falling in high thermal discomfort zones. Roughly 27 pc of the population is now residing in blocks with above-average night-time LST, putting them at prolonged risk.
Hilly terrain no longer immune
Traditionally cooler regions like the Nilgiris, Kodaikanal, and parts of Theni are also showing heat stress. Expansion of settlements on hill slopes and shrinking forest buffers have altered the microclimate of these fragile ecosystems.
In hilly terrain blocks, the correlation between heat rise and increased built-up area is significant. For example, building footprint changes in 90 terrain blocks now mirror the same trends as those in urban plains, suggesting that highland zones are no longer climate havens.
Policy recommendations
The report offers detailed, block-wise categorisation of heat exposure based on both historic changes and current conditions, providing policymakers with a spatially disaggregated vulnerability map.
J Jeyaranjan, Executive Vice Chairman, State Planning Commission, said moving forward Tamil Nadu must integrate heat metrics into land use planning and zoning regulations, prioritise high-risk blocks for green infrastructure investments, enforce stricter construction codes in peri-urban and hill zones, revive wetlands and water bodies as natural coolants, and establish block-level heat action plans under State Action Plan for Climate Change (SAPCC 2.0).
The study contributes directly to Tamil Nadu’s Heat Mitigation Strategy (2024) and State Climate Change Mission. It also aligns with global Sustainable Development Goals (SDGs)—particularly SDG 11 (Sustainable Cities), SDG 13 (Climate Action), and SDG 15 (Life on Land).
With projections under the RCP 4.5 and 8.5 climate scenarios indicating a 0.9°C to 2.7°C increase in maximum air temperatures by 2050, Tamil Nadu is running out of time. The message from this decadal assessment is clear: either build sustainably — or burn.