Climate change intensified cyclones in South Asia significantly: Report

This year, recent cyclones in Asia like Ditwah and Senyar, which claimed more than 1,600 lives, became more intense due to human-induced climate change. This was the central finding of an analysis by the World Weather Attribution. 

In late November, two intense storms hit simultaneously different parts of Asia: Tropical Cyclone Ditwah struck Sri Lanka and Tropical Cyclone Senyar made landfall in Sumatra and Peninsular Malaysia. The report found that these cyclones have become more intense over the decades, and have increased extreme accompanying rainfall as well. 

Other factors contributing to this are rapid urbanisation and widespread deforestation, exposing large populations and critical infrastructure concentrated in floodplains to the flooding. 

Increasing probability of extreme rainfall

The report found that the probability of rain falling over the Malacca Strait with power equivalent to the one triggered by Cyclone Senyar is about 1.4%. In other words, such events can happen once in 70 years. For Cyclone Ditwah, the chance of a similar rain event over Sri Lanka is about 3.3%, corresponding to a 1-in-30-year event. 

Now, across the entire south east Asia region, extreme rainfall is becoming more intense and common, as suggested by historical weather observations over recent decades. The report found an increasing trend in heavy rain over recent decades of 9-50% for Cyclone Senyar, and 28-160% for Cyclone Ditwah. 

“The combination of heavy monsoon rains and climate change is a deadly mix,” said Sarah Kew, Climate Researcher at the Royal Netherlands Meteorological Institute, and lead author of the study. “Monsoon rains are normal in this part of the world. What is not normal is the growing intensity of these storms and how they are affecting millions of people and claiming hundreds of lives.”

Sea surface temperatures over the North Indian Ocean were 0.2o°C higher than the historical 1991-2020 average, providing heat and moisture to the storms. This also contributed to increased intensity of the cyclonic rainfall. 

Without fossil fuel warming, temperatures would have been about 1o°C cooler than the historical average, found the report. 

However, the report faced certain limitations when estimating the precise contribution of climate change in the event due to limitations in climate models, which do not represent well some of the key features of the climate in the region, such as seasonal cycles and natural modes like El Niño and the Indian Ocean Dipole (a system with two opposing poles). 

Improved early warning systems and sustainable land-management are critical for reducing future risk, found the report. 

“Cyclones like Ditwah have become an alarming new reality for Sri Lanka and the wider South and Southeast Asian region, bringing unprecedented rainfall, widespread loss of life, massive disruption to economic activities, and unrecoverable damage to the environment. “Preliminary assessments place total economic losses in Sri Lanka alone between USD 6-7 billion, nearly 3-5% of the national GDP. This should be an unequivocal eye-opener to the scale of future climate-driven extremes the country and the region must prepare for,” said Lalith Rajapakse, Professor in the Department of Civil Engineering at the University of Moratuwa.