CHENNAI: Chennai is facing a growing heat stress crisis, driven by rising temperatures, soaring humidity, and rapid urban expansion, according to a recent analysis by the Centre for Science and Environment (CSE).
The Urban Heat Stress Tracker: Chennai report reveals a troubling combination of climate change and urban heat island effects is pushing the city into thermal discomfort.
The study shows a summertime decadal average ambient air temperature increase of 0.4°C, from 30.3°C in 2001-2010 to 30.7°C in 2014-2023.
Relative humidity has also climbed by 5 per cent over the same period, adding significant heat stress.
Last year, March- April period was 1°C hotter than the 2014-2023 average, signaling an accelerating trend.
This year is no different. The temperatures are above normal and there are early indications of the coming summer months being extremely harsh.
The Indian Meteorological Department (IMD) has forecasted above normal temperatures this summer.
High humidity has proven especially burdensome, contributing an average of 6.3°C to the city’s Heat Index (HI)—a measure of how hot it feels when humidity is factored in—resulting in a 5 per cent rise in HI overall.
The consequence are days with a daily Heat Index exceeding 41°C—the threshold considered dangerous to human health—have tripled since the 2001-2010 period.
This spike reflects the combined impact of temperature and humidity, making summers increasingly unbearable.
Both pre-monsoon (March- May) and monsoon (June- August) seasons have seen a roughly 2°C rise in HI, erasing the thermal distinction between these periods.
Chennai residents now endure equally hot and muggy conditions year-round, except during the north-east monsoon months from November to January.
Rajneesh Sareen, programme director, Sustainable Building and Habitat Programme, CSE said the problem is that nighttime offers little relief.
The city’s ability to cool down after sunset has weakened, with diurnal cooling of land surface temperatures dropping by 5%.
In the 2001-2010 period, nighttime temperatures fell by 10.2°C from daytime peaks, but this has shrunk to 9.5°C in recent years, leaving the urban core perpetually warm.
This is largely due to the urban heat island effect, with daytime summer temperatures in Chennai’s core averaging 0.8°C higher than its peripheries and peri-urban areas, and 0.9°C warmer at night.
Dr Anand Sharma, former Additional Director General at IMD, said more than climate change, it is the urbanisation that is turning cities into heat traps.
For instance, the built-up area in Chennai has surged from 30.7% in 2003 to 73.5% in 2023, a more than two-fold increase.
Meanwhile, green cover has dwindled from 34% to 20.3% over the same period.
"There is a direct correlation between this loss of vegetation and the rise in urban heat stress, as concrete-heavy landscapes absorb and retain heat more than natural surfaces," Sharma said.
He also said there is temperature variation even within city limits. Few years back, TNIE with the help of IIT Madras students, recorded temperatures at different locations in Chennai, and the data showed temperature variation as high as 4-5 degrees.
"This is a challenge for IMD. We need to increase the number of weather stations to get more granular data to improve the forecasting," he told TNIE.