The probability of heat wave-related deaths increased from 46% to 82% — a 78% increase between 1960 and 2009. – Photo: NOAA

The mean temperature across India has risen by 0.5 degree C during the period 1960 and 2009 and this has led to a significant increase in heat waves in the country. Based on modelling studies, researchers from Indian Institute of Technology (IIT) Bombay, Indian Institute of Technology (IIT) Delhi and the University of California, Irvine have found that when the summer mean temperature during this period increased from 27 to 27.5 degree C, the probability of a heat wave killing in excess of 100 people shot up from 13% to 32% — an increase of 146%.

For instance, there were only 43 and 34 heat-related fatalities in 1975 and 1976 respectively when the mean summer temperature was about 27.4 degrees C. But in 1998, at least 1,600 people died due to heat wave when the mean summer temperature was more than 28 degrees C.

Similarly, when the average number of heat wave days in the country increased from six to eight, the probability of heat wave-related deaths increased from 46% to 82% — a 78% increase. The average number of heat wave days between 1960 and 2009 was 7.3 per year.

“A 0.5 degree C increase in mean temperature does not mean that the extreme temperature will also increase by just 0.5 degree C. The extreme temperature will increase at a much higher rate,” says Prof. Subimal Ghosh from the Department of Civil Engineering, IIT Bombay and one of the authors of the paper.

As the Earth gets even warmer, there can be substantial increase in deaths and heat waves will become more frequent in the country, says a study published a few days ago in the journal Science Advances. When the mean temperature increases even further by the end of the Century, the northern, central and western India will witness increased spatial warming.

“The temperature in these regions is already high and so the chances of the mean temperature crossing the threshold are higher,” says Prof. Ghosh. Between 1960 and 2009, the intensity, number of heat wave events taking place each year and the duration in days have increased across the country, particularly in the northern, southern and western parts of India, says the paper.

Between 1985 and 2009, southern and western India experienced 50% more heat waves events compared with the period 1960 to 1984. But in most parts of the country, the number of heat wave days and mean duration of heat waves have increased by 25%, the study says.

Heat waves killed more than 1,300 people in Ahmedabad in 2010 and in 2013 the number of people who died due to heat wave shot up to 1,500. It reached a new peak in 2015 when more than 2,500 people died. But 2016 was even worse — the most intense heat wave swept the country in the month of May. The highest temperature recorded in 2016 was 52.4 degree C in Jaisalmer.

People who have no access to electricity will be particularly susceptible to deaths caused by heat waves. “The Indian Meteorological Department is doing extremely well in forecasting a heat wave. The only problem is that people are not aware of the adverse impact of heat waves,” says Prof. Ghosh.  Besides increasing awareness about the harmful effects of heat waves, steps should be taken to mitigate the impact of heat waves. Planting more trees is one way of reducing the impact of heat waves, especially in cities where the urban heat island effect compounds the problem.

Published in The Hindu on June 10, 2017