The weather in India and Pakistan this spring has been unusually hot. Unusually large heat waves have followed since March, continuing into May. This presents a difficult problem for rapidly studying the role of climate change in this event, as we can’t yet be sure when it will end. However, two studies have already explored the effect of climate on high temperatures in March and April. Single-day and monthly temperature records have been broken in many regions. The thermometer was reading as high as 49°C and the weather was unusually dry. Record-breaking heatwaves often coincide with droughts, as drier ground heats up more without the cooling effect of evaporation. Working outdoors is extremely challenging, and as the heat continues, so too does the impact on the economic slowdown. The impact on agriculture is significant, for example, an estimated 10% to 35% loss in wheat yields in northern India. India had previously planned to increase exports due to a slump in Ukrainian exports due to the war, but this month imposed an export ban. In Pakistan, the heat also caused flooding of a glacial lake, destroying an important bridge and many buildings, including a power plant.
The World Weather Attribution team’s routine analysis of high temperatures through the end of April tells us how it relates to climate change. These rapid studies were possible because the team used the same (peer-reviewed) method every time. The goal is not to make a judgment about whether such weather events are “caused” by climate change — that’s not the right question. Instead the focus of these studies is on whether we will see more/less such weather patterns in a warmer climate. If the answer is “more,” we can ask how much climate change affects the probability of occurrence.
The study was limited by the fact that there was only daily temperature data for the region going back to 1979, with data within India going back to 1951—not ideal for trend analysis. But to assess the impact of climate change, the researchers pulled data from a large collection of climate models collected daily, including simulations with and without anthropogenic greenhouse gas emissions (and sunlight-reflecting aerosols). The study analyzed statistics on event rarity. Historical data shows that the annual probability of high temperature from March to April this year is about 1%, which is commonly referred to as “once in a century”. But in model simulations of pre-industrial climates (temperatures 1.2°C cooler than today), such extreme heat would be “once in 3,000 years.” In other words, climate change has increased the probability of such heat waves by about 30 times .
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