Climate Change

What the new IPCC AR6 report means for Indian cities

The Sixth Assessment Report recognises compound extremes, multiple climate change drivers working together to maximise disaster impacts

By Jayanta Basu
Published: Thursday 12 August 2021

Burgeoning air pollution has reduced the intesity as well as frequency of monsoon rains in India and the rest of south Asia, the Sixth Assessment Report (AR 6) by the Intergovernmental Panel on Climate Change (IPCC) underlined.

The report,Climate Change 2021: the Physical Science Basis, for the first time gave cognisance to the role of compound extremes and multiple climate change drivers operating in tandem in maximising disaster impacts in India and elsewhere.

The IPCC report also spelt out with “moderate confidence”  how urbanisation has pushed up intense rainfall in cities across South Asia using several scientific evidences generated on India cities.

The political leadership in south Asian countries, including India, must keep these findings in mind and act proactively as they plan post-novel coronavirus disease (COVID-19) economic recovery, experts said.  

Heat can push up Indian monsoon

Rainfall has been on the decline and monsoon deficits on the rise in different regions in south Asia. Agreement among datasets invoke confidence about a decrease in mean rainfall over most parts of the eastern and central north regions of India,” the IPCC report said.

The report added that “concurrently the frequency of heavy precipitation events has increased over India, while the frequency of moderate rain events has decreased since 1950”.  

It said that the “the dominant cause of the observed decrease of south and southeast Asian monsoon precipitation since mid-20th century is anthropogenic aerosol forcing”.

Read more: IPCC shows what our planet will be like as climate change continues

Anthropogenic aerosol is a suspension of fine solid particles or liquid droplets in air created mostly by air pollutant like particulate and smoke emitted from vehicles, industries and other sources.

Incidentally, the level of air pollution, particularly toxic particulate like particulate matter 2.5, has been found to be the highest in Indian sub-continent with Bangladesh, Pakistan and India occupying top three positions respective at global benchmark.

“Air pollution has increased over the Indian subcontinent, and so have aerosol levels; it acts as a barrier and reduces the difference of temperature between sea surface and land. This provides a cooling effect and reduces monsoon intensity,” said Subimal Ghosh of Indian Institute of Technology (IIT), Mumbai. He is among the lead authors of the IPCC report.

Ghosh said while the AR6 report predicted stronger monsoon and fewer droughts in coming years, the finding has been earmarked “low confidence”.

The report predicted a turnaround in monsoon intensity in India and around linking it with rising temperature: “… models projected for the 21st century a significant increase in temperature over South Asia (high confidence with robust evidence) and in projections of increased summer monsoon precipitation (medium confidence)”.

Krishnan Raghaban, senior scientist at the Indian Institute of Tropical Meteorology (IITM) and a coordinating lead author of the latest IPCC report, said while the exact nature of monsoon in near future (2020-2040) looked uncertain due to inherent variability, there was some certainty that the monsoon rainfall would increase beyond 2040.

The report highlighted that throughout Asia, “intensity and frequency of hot extremes, such as warm days, warm nights, and heat waves; and decreases in the intensity and frequency of cold extremes, such as cold days and cold nights” is on the card.

But India and Pakistan are expected to suffer more: “More intense heat waves of longer durations and occurring at a higher frequency are projected over India and Pakistan”, the report said.

Cities will be affected by more rain, sea level rise and heat

Ghosh, however, pointed out that AR6 report has predicted with “medium confidence”  that the major Indian cities would experience more bursts of intense rainfalls, mainly influenced by urbanisation.

The report said:

“There has been new evidence of the effect of local land use and land cover change on heavy precipitation (with) a growing set of literature linking increases in heavy precipitation in urban centres to urbanisation.”

It added that “urbanisation intensifies extreme precipitation, especially in the afternoon and early evening, over the urban area and its down-wind region”. 

Ghosh pointed out that high intensity of heat released in the major cities, coupled with locally generated air pollution, triggers aerosol load that contributes in cloud formation. Urban structure often trap the wind and act together to bring bursts of intense rainfalls in Indian cities like Mumbai, Kolkata, Chennai and others.

The temperature of Kolkata during 1950-2018 increased 2.6 degree Celsius, the highest among all the cities mentioned in the IPCC regional factsheet.

“Aerosol acts in diverse ways — while at a macro scale it imparts a cooling effect and reduces monsoon rain, at a micro scale, especially in urban centres, it contributes to intense bursts of downpour,” said Ghosh.

Cities in the South Asia region would experience more intense rainfall, and in absence of adequate infrastructure, may face severe water logging in coming years, said Saleemul Huq, director, International Centre for Climate Change and Development (ICCCAD) in Bangladesh and a lead author of an earlier IPCC report, said

Huq added that the frequency of major floods in Ganga basin has now increased at least four times compared to earlier decades.

An analysis by the United States National Aeronautics and Space Administration predicted that several Asian cities located either on or near the coastline would have to withstand significant sea level rise in 2100.

It said Mumbai and Chennai have had 0.58 and 0.57 meters of sea level rise, the highest among the metros. This was followed by Kolkata (Khidderpur in report) with 0.15 meter rise.

Compounding climate drivers multiplying disaster

The report, for the first time, acknowledged the role of compounding extremes — several climate change drivers operating together — in maximising disaster impacts in India and elsewhere.

Cyclone Yaas a tell-tale example of compound extremes such as high intensity cyclonic wind, rising sea level, more intense rainfall,” said Roxy Mathew Koll, climate scientist at the IITM. 

“Increasing heat and forest fire is an ideal example of compound extremes,” said Raghaban from IITM.

Sunita Narain, director-general, think tank Center for Science and Environment, pointed out that though India’s contribution to emissions has been miniscule, it does absolve the country of its need to act. Narain observed that India is not “walking the talk on the issue”.

 The South Asian governments, including India, must keep the report findings in mind as they plan future courses of action for economic recovery, said Sanjay Vashist, director, Climate Action Network South Asia.

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