Centre for Atmospheric Sciences, IIT Delhi
On May 19 of 2016, India’s all-time temperature record was smashed when temperatures soared to 51°C in Phalodi, Rajasthan. The previous record of 50.6°C dating back to May 10, 1956 was set in Alwar, also in Rajasthan. In late May and early June of 2015 Andhra Pradesh, Telangana and nearby states suffered a deadly heat wave when daily high temperatures exceeded 45°C for several days in a row leading to nearly 2500 deaths. This year, the month of September was one of the busiest in recent history for hurricanes – tropical cyclones that occur in the Atlantic Ocean. Hurricane Harvey came onto the U.S state of Texas from the Gulf of Mexico and stalled over the city of Houston dumping so much rain that much of the city was flooded. Then came hurricane Irma, which inflicted heavy damage and loss of life all along its path over some of the Leeward Islands and Cuba in the Caribbean Sea and on to Florida and its neighbouring states on the West coast of the U.S. And then came the utter devastation of the islands of Dominica and Puerto Rico in the Caribbean by hurricane Maria. Are we seeing more extreme temperatures and heat waves and cyclones of late because of climate change or as people have asked – are we witnessing Global Weirding?
One usually tends to think of climate change as a gradual increase in average temperatures over many decades. But this average is of the weather we experience on a daily basis over many years. Just like the class average only increases when more students get higher marks, an increased average temperature means hot days and heat waves are more likely. A warming atmosphere also causes more evaporation, – for every 1°C increase in temperature, the atmosphere can hold around 7% more water vapour – which leads to heavier rain and increases the risk of flooding of rivers and lakes. This increased humidity also makes it harder for the human body to cool off – causing more heat stress. Climate change has also warmed ocean waters around the world by 0.5°C over the past century, including in regions where tropical cyclones develop. That allows cyclones to grow stronger, potentially increasing their maximum wind speed. Warming oceans and melting land ice have also caused about 20 centimetres of global sea level rise over the past century which means storm surge waters higher and push further inland than they used to and can cause more damage to physical property and lives. As destructive cyclones go, climate change certainly puts millions of people in India and around the world at higher risk.
High temperatures during summer are not new to India and heat waves are fairly commonplace – as are hurricanes in the Atlantic Ocean. So how can we answer the question that people ask after such events – Was this caused by climate change? Actually, that question is ill posed. The correct question would be “Did climate change make the event stronger or more frequent?” When we have an extreme heat event, observed temperatures sometimes are in the top 1 (or 0.1) percentile of past data – rare numbers that occur once in many decades. This means a 100-year event is something that has a probability of 0.01 of occurring in any given year! The question we answer is – would it have even more rare or perhaps less rare if human beings were not altering the climate? In our recent analysis of the 2015 heat wave, we found that Hyderabad experiences high temperatures that used to occur once in 10-years now once every 3 years. So there is about a three-fold increase in the risk of such events.
We at the Centre for Atmospheric Sciences in IITD in association with Oxford University and Climate Central are using event attribution to find out which extreme events are being affected by global warming. What we do is try and simulate the weather (which is different every day because of its chaotic nature) multiple times (in the thousands) for the current conditions using a computer model to capture the types of events we are interested in – some of which are of low probability – like throwing a pair of dice many times to estimate the probabilities of different pairs. This exercise can be repeated under the “counterfactual climate” – climatic conditions that would have been without us human beings affecting the climate. Note that the probability of some events such as cold waves could decrease in a warming climate.
However, the thousands of simulations required are very expensive to run on a supercomputer. This is where you can help through the Weather@Home program. Under this, you can volunteer your desktop or laptop computer to run a climate model – using only the idle time when you are not doing any other work on it – and send the results to a server here in IITD! This is a very nice way of crowdsourcing computing power and it costs you nothing but a few minutes to set up (go to http://www.climateprediction.net/weatherathome/) and in the bargain we get to understand how climate change is affecting extreme weather here in India and more importantly, how it will likely affect us in the future.