Man's Role in 2015 India, Pakistan Heat Wave Confirmed by Supercomputer Simulations

The India/Pakistan Heat Wave of 2015 affected more than the people who lost members of their communities. Scientists from the Lawrence Berkeley National Laboratory were greatly distressed by the climate disaster that they worked together on a paper that presented evidence of human influence on extreme weather events.

The research titled "The Deadly Combination of Heat and Humidity in India and Pakistan in Summer 2015" studied observational and simulated temperature and heat indexes. This led to the conclusion that the heat waves in India and Pakistan "were exacerbated by anthropogenic climate change." An estimated 2,500 people died of the 2015 heat wave in India while Pakistan's death toll reached 2,000.

"I was deeply moved by television coverage of the human tragedy, particularly parents who lost young children," shared Michael Wehner. A climate researcher at Berkeley Lab and the lead author on the paper, Wehner had studied extreme weather events and anthropogenic climate change extensively. He and his collaborators from Berkeley Lab, the Indian Institute of Technology Delhi and UC Berkeley decided to look more closely into the cause of the 2015 heat waves to determine if the two separate meteorological events were somehow linked.

Using simulations from the Community Atmospheric Model version 5 (CAM5) that takes into account human influence on extreme weather in the context of long-term climate change, the researchers were able to run "factual" simulations of the world and contrast them to "counterfactual" simulations that might have occurred if humans had not changed the composition of the atmosphere by producing large amounts of carbon dioxide.

"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 realizations of what the weather 'could have been,'" explained Dáithí Stone, the second author of the study and a research scientist in Berkeley Lab's Computational Research Division. "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."

The research team also discovered that even though the heat waves' proximity in terms of location and time, the two heat waves were "meteorologically independent." In spite of this discovery, Wehner emphasized, "the India/Pakistan paper confirms that the chances of deadly heat waves have been substantially increased by human-induced climate change, and these changes will certainly increase as the planet continues to warm."