In a recent episode of the Mint Climate Change Tracker podcast, I’d asked climatologist Chirag Dhara about some of the effects of the climate crisis on India that he’s personally concerned about. Speaking of the Indo-Gangetic plains and coastal India, he said, “The area is very humid, because of the massive rivers that flow through. So you have very hot conditions there during the summer, along with very humid conditions, and this combined effect of temperature and humidity really affects human health.”
A lethal combination of heat and humidity, what’s referred to as “wet-bulb temperature”, has emerged as a major source of worry around the world, and nowhere more so than South Asia. A paper published by the McKinsey Global Institute on 12 August, Climate Risk And Response In Asia: Research Preview, which looks at a nearer time-frame of 2030-2050, says, “…large cities in parts of India, Bangladesh, and Pakistan could be among the first places in the world to experience heat waves that exceed the survivability threshold.” This threshold, it says, is that of a human being resting in shade over a three-day period, during a wet-bulb (WB) heatwave of temperatures in excess of 34 degrees Celsius. The paper is based on climate modelling done by scientists at the US-based climate think tank, Woods Hole Research Center.
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A study published in Science Advances in May corroborates this in greater detail. Titled The Emergence of Heat Humidity Too Severe For Human Tolerance, the paper states that a WB temperature of 35 degrees Celsius marks the upper physiological limit for human beings. The study establishes that extreme humid heat has doubled in frequency worldwide since 1979. One of these global heat hotspots comprises north-west India, the Indo-Gangetic plains and eastern coastal India, where WB temperatures of over 31 degrees Celsius are already common.
The McKinsey paper further says that by 2050, India, Bangladesh and Pakistan could have 500 to 700 million people living in regions that have a 20% probability of lethal WB heatwaves every year. Since high WB temperatures severely limit people’s ability to work, the paper estimates an up to 13% hit to the GDP of these three countries. Dhara explained in the podcast how a 35 degree Celsius heat in cities with more humidity is far worse than in cities with lower humidity. “Wet-bulb temperature is much more relevant to human health than dry-bulb. The latter is what is reported more in the newspapers,” he said.
The McKinsey paper is one in a long line of heat stress warnings issued by the climate scientists in recent months. In May, a study showed how the climate niche that humans have occupied for the past 6,000 years is now shrinking. Another study, published in July, found that heatwaves across the world have increased, both in frequency and length, since the 1950s. India’s first official assessment of climate change in the country, released in June, says that even in a mid-emissions scenario, heatwaves in India will become more frequent and last longer.
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