Last year’s Chennai deluge not due to climate change

As Chennai cleans up after Cyclone Vardah, the inevitable question is being asked. Was it an effect of climate change? Climate science has not progressed to the point where it can give a quick answer. All that the Intergovernmental Panel on Climate Change said in a special report was that climate change makes storms more frequent and more severe.

But people and policymakers want to know if a particular storm, flood or drought was due to climate change, and climate scientists are looking for an answer. It may not be quick, but after a detailed study now they can tell you whether the heavy rainfall and subsequent floods in the same Chennai on December 1, 2015 could be attributed to climate change.

The answer is no.

Krishna Achuta Rao of the Indian Institute of Technology Delhi, Geert Jan van Oldenborgh of the Royal Dutch Meteorological Institute and Friederike E. L. Otto and Karsten Haustein of the Environmental Change Institute in the University of Oxford say, “No effect of human-induced climate change was detected in the extreme one-day rainfall that caused widespread flooding in Chennai, India on December 1, 2015.”

The scientists say the many models they studied show “no increase in the probability of extreme one-day precipitation due to human-caused emissions”, in a paper just published in the Bulletin of the American Meteorological Society.

Pollution versus warming

There is a specific reason for their conclusion. Chennai and its surroundings are among the most polluted parts of India, thanks to a concentration of factories and vehicles and widespread use of wood as cooking fuel. The result is a blanket of pollutants that scientists call aerosols. These aerosols block sunlight and act as a counter to the warming caused by greenhouse gases – mainly carbon dioxide – that are causing climate change.

After studying a plethora of models, the scientists said that over the last 40 years, sea surface temperature (SST) in the Bay of Bengal along India’s eastern coast has been lower than global climate models had forecast. Though there was an area of warmer water just off the Chennai coast around the time of the heavy rainfall, this part of the Bay remained cooler than the Indian Ocean overall.

The scientists conclude that this anomaly was due to aerosols. The results, they say, “Preclude an attribution of the heavy rains to human-caused factors, probably due to the two main pollutants, greenhouse gases and aerosols, having opposing effects.”

No El Niño effect

The heavy rainfall also came during an El Niño year, and there had been speculation that this may have had something to do with the freak weather. After their study, the scientists say they found the El Niño “signal”, but it was not “statistically significant”.

Aerosols no good

Aerosols intercept the sun’s radiation, either reflecting it back into space or absorbing it high in the atmosphere. But this does not mean it is a good idea to emit aerosols, the scientists hasten to add. “Aerosols come with a host of negative impacts, specifically on climate and weather patterns, and on human health,” they say. “The World Health Organisation estimates that these pollutants are one of the leading causes of worldwide mortality, contributing to as many as seven million premature deaths each year.”

City vulnerable

On December 1, 2015, it rained 494 mm (19.45 inches) within 24 hours in Chennai. Any city anywhere in the world would be flooded. Does that mean Chennai should prepare for such heavy rainfall in future? The scientists do not think so. They put the probability of a repeat in the present climate regime between one in 600 and one in 2,500 per year, within a 95% uncertainty range.

But, as the researchers also point out, Chennai remains vulnerable to flooding even if it does not rain as much. The metropolis of 8.2 million people – by the 2011 census – has an estimated 150,000 illegal buildings, many built over what used to be natural streams, ponds and even rivers. So now the water has nowhere to go. The researchers refer to a report of the Parliamentary Standing Committee on Home Affairs that acknowledged the lack of timely desilting, inadequate flood zone planning and large scale settlements in low-lying areas as major contributors to the impacts that occurred in the city.