Human-induced climate change worsened 2015 heatwave in India

The deadly heat waves that killed nearly 2,500 people in India and 2,000 people in Pakistan last year were exacerbated by human-induced climate change, scientists including those from IIT-Delhi have found.
Researchers examined observational and simulated temperature and heat indexes and found that the heat waves in the two countries "were exacerbated by anthropogenic climate change."
While the two countries typically experience severe heat in the summer, the 2015 heat waves - which occurred in late May/early June in India and in late June/early July in Pakistan - have been linked to the deaths of nearly 2,500 people in India and 2,000 in Pakistan.

Researchers used "factual" simulations of the world and compared them to "counterfactual" simulations of the world that might have been had humans not changed the composition of the atmosphere by emitting large amounts of carbon dioxide, said Daithi Stone from the Lawrence Berkeley National Laboratory (Berkeley Lab) in US.
"It is relatively common to run one or a few simulations of a climate model within a certain set of conditions, with each simulation differing just in the precise weather on the first day of the simulation; this difference in the first day propagates through time, providing different realisations of what the weather 'could have been,'" said Stone.

"The special thing about the simulations used here is that we ran a rather large number of them. This was important for studying a rare event; if it is rare, then you need a large amount of data in order to have it occurring frequently enough that you can understand it," Stone added.

Researchers, including those from Indian Institute of Technology (IIT) Delhi, examined both observational and simulated temperature alone as well as the heat index, a measure incorporating both temperature and humidity effects.
From a quality-controlled weather station observational dataset, they found the potential for a very large, human-induced increase in the likelihood of the magnitudes of the two heat waves.

"Observations suggested the human influence; simulations confirmed it," said Michael Wehner, climate researcher at Berkeley Lab.
The heat waves in Pakistan in late June/early July of 2015 were also similar killing around 2,000 people.
The research team also found that, despite being close in location and time, the two heat waves were "meteorologically independent.