Science Summary: The heavy precipitation event in Chennai, India, 1 December 2015

Introduction

A relentless downpour pounded Tamil Nadu’s capital city of Chennai on 1 December 2015, flooding and submerging one of India’s largest cities. The heaviest one-day rainfall in the region in more than a century – as much as 494 mm (19.45 inches) – left more than three million people without basic services, collapsed roads and bridges, and halted air and train travel in and out of the city, stranding hundreds of passengers. “Chennai has become a small island,” said India Home Minister Rajnath Singh.

The catastrophic rains not only isolated the city, but forced tens of thousands from their homes. The military dropped food supplies to residents stranded on rooftops. For the first time in its 137-year history, the local paper went unpublished because staff could not reach the press. On 2 December, authorities declared Chennai a disaster zone. Costs to India’s economy due to the flood waters in Chennai are estimated at US$3 bn.

In order to be able to act to reduce disaster risk and plan for the impacts of climate change, decision-makers need to understand the extent to which extreme events such as this are the result of human emissions of greenhouse gases in the atmosphere and related global warming. In response to this need a scientific team comprising Climate Central, the Royal Netherlands Meteorological Institute (KNMI), the University of Oxford and the Indian Institute of Technology Delhi (IITD) – as part of the World Weather Attribution (WWA) partnership, which also includes the Red Cross/Red Crescent Climate Centre and the University of Melbourne – assessed whether the downpour which affected Chennai on 1 December 2015 could in part be attributed to human emissions of greenhouse gases in the atmosphere and related climate change.

This science summary, outlines the team’s overall methodology and conclusions. The original manuscript was published in the special supplement to the Bulletin of the American Meteorological SocietyVol. 97, No. 12, December 2016.

This publication is an output from the Raising Risk Awareness project. Excerpts from the publication are provided below – download the full text from the right-hand column for more detail.

Quick key messages

• On the 1 December 2015 a record amount of rainfall fell in a 24-hour period, leading to catastrophic flooding in Chennai, India. The city was declared a disaster zone.
• The downpour occurred during the 2015 rainy season, which coincided with a strong El Niño, record warm global surface temperatures and high Indian Ocean temperatures.
• Very high levels of aerosol pollution (see below) exist over much of India, including the area around Chennai. Aerosols intercept solar radiation and either reflect it back or absorb it high in the atmosphere, resulting in a cooling effect.
• For the extreme event attribution analysis, scientists used a multi-method approach that included climate models and observational weather data.
• No measurable effect of human-caused emissions was detected in the extreme one-day rainfall in Chennai on 1 December 2015, likely because of the masking effects of aerosol pollution.

Methods and Tools

To assess the potential link between the heavy rainfall in Chennai on 1 December 2015 and human-caused greenhouse gases in the atmosphere, the science team conducted independent assessments using multiple peer-reviewed approaches of extreme event attribution methodologies. These approaches involve statistical analyses of the historical precipitation record, the trend in global climate models, and the results of thousands of simulations of possible weather with a regional climate model. Applying multiple methods provides scientists with a means to assess confidence in the results.

For the observational analysis, scientists analysed two datasets of daily precipitation in the region. The public GHCN-D dataset has 50 stations with at least 40 years of data in the coastal area 10°–15°N, 79.5°–81°E for a total of 3,504 station-years. However, all but two of the series end in 1970. The India Meteorological Department (IMD) provided 19 non-public series from the region for 1969-2013.

Note on aerosols:

In climate science, the word ‘aerosol’ is generally used to mean “atmospheric particulate”, particles like dust and sea salt that occur naturally and others that come from the burn- ing of types of fossil fuels and biomass. This is not to be confused with ozone-damaging chlorofluorocarbons (CFCs) released from spray cans. Like greenhouse gases, aerosols affect the Earth’s radiative balance, intercepting solar radiation and either reflecting it back or absorbing it high in the atmosphere. Overall, aerosols have had a cooling effect on the surface, countering global warming. However, aerosols come with a host of negative im- pacts, specifically on climate and weather patterns, and on human health. The World Health Organization estimates that these pollutants are one of the leading causes of worldwide mortality, contributing to as many as seven million premature deaths each year.

Conclusions

The highest observed daily rainfall, 494 mm, was an extreme occurrence meteorologically, with a probability of occurrence in the current climate of one in 600 to one in 2,500 per year (95% level of confidence). This would have overwhelmed flood prevention measures in any city.

No effect of human-induced climate change was detected in the extreme one-day rainfall that caused widespread flooding in Chennai, India on 1 December 2015, neither from 1900-1970, nor from 1970-2014.

Coupled models show more extreme one-day precipitation events from 1970 to 2015, but a large ensemble of observed SST-forced models again shows no increase in the probability of extreme one-day precipitation due to human-caused emissions. (An ‘SST-forced model’ is a climate model without an ocean, i.e. it does not calculate the ocean temperatures itself but it is provided with the observed sea surface temperatures at every time step.) A plausible factor is the lack of increase in SSTs in the western Bay of Bengal over the last 40 years, which is not reproduced correctly by the coupled models but is realistic in the SST-forced models. The team therefore discarded the results from the coupled global models as unrealistic. The other results, from observations and the SST-forced regional models, 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.

There is a small but clear increase in probability of extremes in the SST-forced regional models, associated with El Niño and other non-anthropogenic SST anomalies. In the observations, the El Niño–Southern Oscillation signal is also present but not statistically significant.