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Rising heat stress in India


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Since 2016, the summer of 2022 has been the second hottest after the summer of 2010

Even as the monsoon has made its footfalls felt across the country, there seems to be little respite from the searing heat – aided by steep rises in humidity levels — in many pockets. The heat waves lashing India this summer are symptomatic of the anomalous temperature trend that is expected to worsen with growing climate change impacts.

Since 2016, the summer of 2022 has been the second hottest after the summer of 2010. Mega cities including Delhi, Mumbai, Kolkata and Hyderabad are much hotter than the larger region around them due to heat island effects caused by surface absorption of heat and local waste heat generated by traffic, industry and air conditioning (among other urban activities). A new country-wide analysis by Centre for Science and Environment (CSE) indicates while extreme heat waves in the states of the northwest — as per the classification of the India Meteorological Department (IMD) — draw maximum public attention, the increase in overall anomalous temperature in other regions of the country have been largely neglected.

Since 2016, the summer of 2022 has been the second hottest after the summer of 2010

“This is a very disturbing trend as policy preparedness to mitigate rising heat due to climate change is nearly absent in India. Without heat action plans, rising air temperature, radiating heat from land surfaces, concretisation, heat-trapping built structures, waste heat from industrial processes and air conditioners, and erosion of heat dousing forests, urban greens and waterbodies will worsen public health risks. This requires urgent time-bound mitigation,” says Anumita Roychowdhury, executive director, research and advocacy, CSE.

“Understanding the overall temperature anomaly, extreme heat conditions, and the mixed trends in heat patterns across different regions of India has become necessary to assess the emergent risk. Currently, the attention is largely on the maximum daily heat levels and extreme conditions of heat waves. But it is equally important to pay attention to the overall rising temperature and humidity trends in different regions to understand the gravity of the problem,” says Avikal Somvanshi, senior programme manager, Urban Lab, CSE.

The Urban Lab of CSE has analysed the temperature trends in India from January 2015 till May 2022. The effort has been to understand the warming trends by covering all three dimensions of heat stress — surface air temperature, land surface temperature, and relative humidity (heat index) — at the national, regional and local levels. The trends in the metropolises of Delhi, Mumbai, Kolkata and Hyderabad (one located in each of IMD’s four homogenous regions) have also been assessed.

The analysis has studied the temperature anomaly (which refers to a departure from a reference value or long-term average or a baseline). A positive anomaly indicates that the observed temperature was warmer than the baseline, while a negative anomaly says the observed temperature was cooler. It is generally used to report monthly, seasonal, annual, or decadal temperature changes. Heatwaves are also anomalous temperature events, but of much shorter duration and are defined in comparison to an absolute temperature threshold in addition to departure from the normal.

Key findings of the analysis

1. 2022 pre-monsoon summer heat trends overtake 2016 as the second hottest pre-monsoon season on record for India. 

2. Monsoon is hotter than the pre-monsoon period on an average, while winter and post-monsoon seasons are warming up faster.

3. High heat stress and heat waves in the northwestern states (Chandigarh, Delhi, Haryana, Himachal Pradesh, Jammu & Kashmir, Ladakh, Punjab, Rajasthan, Uttar Pradesh and Uttrakhand).

4. In absolute terms, most of India outside the northwestern parts was hotter, even if the number of its extreme heat wave days were lesser.

5. Most deaths due to heat-stroke reported from states outside India’s northwestern region.

6. Mega cities like Delhi, Mumbai, Kolkata and Hyderabad were much hotter than the larger region around them.

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