Jude Coleman in Nature: Human-induced climate change made the deadly heatwave that gripped India and Pakistan in March and April 30 times more likely, according to a rapid analysis of the event. Temperatures began rising earlier than usual in March, shattering records and claiming at least 90 lives. The prolonged heat has yet to subside. “High temperatures are common in India and Pakistan, but what made this unusual was that it started so early and lasted so long,” said co-author Krishna AchutaRao, a climate scientist at the Indian Institute of Technology in New Delhi, in a press release. “We know this will happen more often as temperatures rise and we need to be better prepared for it,” AchutaRao said.
To quantify the role of climate change in the extreme heatwave, a global team of researchers from the World Weather Attribution (WWA) initiative used the average daily maximum temperatures across northwestern India and southeastern Pakistan between March and April to characterize the heatwave. They then compared the possibility of such an event occurring in today’s climate compared with the climate in pre-industrial times, using a combination of climate models and observation data going back to 1979 in Pakistan and 1951 in India.
The team found that climate change increased the probability of the heatwave occurring to once in every 100 years; the odds of such an event would have been once every 3,000 years in pre-industrial times, says Zeke Hausfather, a climate scientist and researcher at Berkeley Earth, a non-profit organization in California that focuses on climate change and analysis of global temperatures. The researchers also show that the event was around 1 ºC warmer than it would have been in a pre-industrial climate.
“This study is really well done. It follows in the footsteps of a lot of excellent work that the WWA has done,” says Hausfather. “It shows an unambiguous role of climate change in making extreme heat events like this worse.”
In India, March’s temperatures were consistently 3–8 ºC above average, reaching highs of 44 ºC — the highest they’ve been since records began 122 years ago. Pakistan reported temperatures that exceeded 49 ºC in some regions. The heatwave was coupled with below-average rainfall in the region. Pakistan received 62% less rainfall than usual for March, and India 71% less. Although the lack of rain added to heating from the land’s surface, it also reduced the humidity of the heatwave — potentially decreasing health impacts, says Hausfather.