Representational image. (Photo | Ashwin Prasath, EPS)
Representational image. (Photo | Ashwin Prasath, EPS)

Wet heat hits threshold in Chennai, urgent action needed

Deadly combo of heat and humidity can prevent temperature regulation by our bodies, say researchers; Chennai & Mumbai face high risk

CHENNAI: Although Chennai’s daytime temperature breaches the 40 degree Celsius mark only for a few days in a year, the city is seen as one of the roughest places to live-in. The reason is humidity.  Researchers, who are decoding this phenomenon, are calling it ‘Wet Bulb Temperatures’ which is also called mugginess, or wet heat, in local parlance. In hot conditions, humans cool themselves by sweating; but if the humidity is too high, sweating no longer works, and we risk dangerous overheating. 

At wet bulb temperatures (WBT) of 32 degrees Celsius, even fit, acclimatised people can’t work; and at WBT of 35 degrees, even fit, acclimatised people who sit in the shade die within about 6 hours. Climate change is making these WBTs increasingly likely. In the 2015 Indian Heatwave, WBT in Andhra Pradesh reached 30 degrees.

Despite people being well acclimated to heat, this heatwave killed about 2,500 people. Currently, most of India experiences potentially deadly heat and humidity combinations, 12-66 days annually. Hotspots along the east coast such as Kolkata sees such combinations for 124 days, the Sundarbans 171 days, Cuttack 178 days, Brahmapur 173, Thiruvananthapuram 113 and Chennai 140 days, with 47 days in Mumbai and around 63 days in New Delhi. 

As per Intergovernmental Panel on Climate Change (IPCC) projections, by 2050, the number of hot and humid days will increase to 229 in Chennai and by 2100, the number will shoot up to a whopping 309 days. These projections were based on a warming scenario called “RCP 8.5”, where society does not make concerted efforts to cut greenhouse gas emissions. 

On Saturday noon, Chennai’s WBT was 27 degrees, which researchers say is the threshold limit. During this year’s heatwave in April, when the temperature in Chennai’s Nungambakkam weather station clocked a sizzling 41.6 degree Celsius, the WBT value breached 30 degrees. Based on research conducted by Fahad Saeed, lead author of the study titled ‘Deadly heat stress to become commonplace across South Asia already at 1.5°C of Global Warming’ published by Geophysical Research Letters, the worst is yet to come. Densely-populated coastal cities like Mumbai and Chennai will be particularly at risk. 

Saeed said almost all of India would see WBT of 33-35 degrees at least every 8 years with 2 degrees of global heating and witness WBT of over 32 degrees every two years. The Sindh region in Pakistan, and in India, Gujarat, Mumbai, Kolkata and Chennai are likely to see wet-bulb-temperature peaks. Explaining how high WBTs impact the human body,  Camilo Mora from University of Hawaii at Manoa says, “Your sweat doesn’t evaporate if it is very humid, so heat accumulates in your body. People can then suffer heat toxicity, which is like sunburn on the inside of your body.

The blood rushes to the skin to cool you down so there’s less blood going to the organs. A common killer is when the lining of your gut breaks down and leaks toxins into the rest of your body.” A study done by the Centre for Climate Change and Disaster Management, Anna University shows there is a steady rise in Chennai’s temperature levels, especially in summer. “The daily average temperature may go up by 2-3 degrees Celsius by 2100,” the study says. 

Sources in the TN Environment, Climate Change and Forests Department said the government was working on a major climate change mitigation and heat action plan, which will be announced shortly by Chief Minister MK Stalin.

What is ‘wet bulb’ temperature?

Like the heat index, which is used by the Met office to issue heatwave warnings, wet bulb temperatures take into account humidity and temperature but also factor in wind speed, cloud cover, sun angle & physical activity

A wet bulb temperature of 32 degrees is when outdoor labour becomes unsafe, and 35 degrees is the upper limit of human survivability, beyond which the body can no longer cool itself

The threshold is 27-28 degrees

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