London – An increase in mean temperature of 0.5 degrees Celsius over half a century may not seem all that serious, but it’s enough to have more than doubled the probability of a heat wave killing in excess of 100 people in India, according to researchers at the University of California, Irvine and other institutions.

This could have grim implications for the future, because mean temperatures are projected to rise by 2.2 to 5.5 degrees Celsius by the end of this century in the low- and mid-latitude countries of the Asian subcontinent, the Middle East, Africa and South America.

“The impact of global climate change is not a specter on the horizon. It’s real, and it’s being felt now all over the planet,” said Amir AghaKouchak, UCI associate professor of civil & environmental engineering and co-author of the study, published today in Science Advances. “It’s particularly alarming that the adverse effects are pummeling the world’s most vulnerable populations.”

Using data gathered by the India Meteorological Department from 1960 to 2009, the UCI-led team analyzed changes in summer temperatures; the frequency, severity and duration of heat waves; and heat-related deaths.

They found that when mean summer temperatures in the South Asia nation went from 27 to 27.5 degrees Celsius, the probability of a heat wave killing more than 100 people grew from 13 percent to 32 percent – an increase of 146 percent.

In real terms, there were only 43 and 34 heat-related fatalities in 1975 and 1976, respectively, when the mean summer temperature was about 27.4 degrees Celsius. However, at least 1,600 people died from excessive heat in 1998, when the mean summer temperature was higher than 28 degrees Celsius.

The average number of heat wave days over the five-decade study period was 7.3 per year. The most heat wave days occurred in 1998 (18), when 1,655 people died, and 2003 (13), when 1,500 people died.