Climate Change

IPCC report warning: Mumbai will be worst hit among Indian metros

The city is set to experience the highest increase in total annual rainfall among other metropolitan cities in the 21st century; mean temperature and sea-level rise will increase too

By Jayanta Basu
Published: Friday 03 September 2021

Mumbai is set to experience the highest increase in total annual rainfall among Indian metropolitan cities in the 21st century, the recently released Intergovernmental Panel on Climate Change (IPCC) report warned.

The report observed that while total rainfall would gradually increase throughout the current century, the maximum single-day rainfall is likely to increase substantially in the city.

It noted that the city is likely to suffer from high mean temperature and sea-level rise towards the end of the century.

Experts Down to Earth spoke to said “the predictions are broadly consistent with the observed trend”.

Analysis of the interactive map released by IPCC showed that Mumbai’s total precipitation is set to increase consistently over the coming years under various greenhouse gas emission scenarios.

Under Representative Concentration Pathway (RCP) 8.5 — worst emission scenario possible that may lead to 3-4 degree Celsius global temperature rise — the total rainfall may increase a substantial 20.5 per cent during 2021-2040; 32.7 per cent during 2041-2060; and 61.2 per cent during 2081-2100.

Mumbai’s total rainfall rise is the highest in the country, followed by Delhi with about 50 per cent among metro cities.

The rainfall during 1850-1900 was considered a benchmark for these assessments as anthropogenic emission started to influence global temperature beyond the period.

If recent 1995-2014 benchmark is considered, the total rainfall is likely to increase 45 per cent in Mumbai in 2081-2100 under the worst possible emission scenario. This indicates that nearly three-fourth of the total rise in 200 years leading to 2100 will occur only in the last 85 years.  

Under RCP 2.6 — the best possible global emission scenario — the comparative rainfall rise from now to end-century is likely to remain restricted in the range of 23.2 to 25 per cent compared to preindustrial period.

Under RCP 4.5 — the scenario that considers 2 °C temperature rise compared to pre-industrial period — the rise ranges from 18.3 per cent during 2021-2040 to 36.7 per cent during 2081-2100, under various greenhouse emission scenarios.

When it comes to rainfall within short duration — as expressed through maximum one-day rainfall — Mumbai is set to experience an increase of 53 per cent compared to pre-industrial period under the worst emission scenario. Around 102 millimetres rainfall is expected on average on such days.

Temperature, sea level set to rise

The analysis showed that under the worst possible emission scenario, Mumbai may have 4.6°C rise in mean temperature rise during end of current century compared to pre-industrial period; with close to 90 per cent of the rise likely to happen from 2015 to 2100.

Under best possible emission scenario – RCP 2.6 – the rise in mean temperature by century-end is expected to be 1.8°C, while the moderate RCP 4.5 emission scenario will lead to 2.6°C rise.

The interactive map analysis showed that during end of the century, under worst emission scenario, annually the temperature may shoot over 35°C for about 117 days, that is, roughly one out of every three days.

Mumbai coast is likely to experience a 60 centimetres rise by century-end compared to 1995-2014 benchmarks under the worst emission scenario; while the rise is likely to be 40 cm under the best possible emission scenario.

Arabian Sea temperature playing duet with urban heat island factor

Experts Down to Earth spoke to were unanimous the prediction is broadly in sync with the observed trends. A few expressed reservation about the robustness of the prediction especially on total rainfall dynamics over the long term.

“Extreme rainfall has increased in Mumbai and is likely to increase even more. The prediction is in sync with the observed trend as well as the studies that we have been carrying out regarding the future pattern of rainfall,” said Subimal Ghosh of Indian Institute of Technology, Mumbai. He is one of the lead authors of the IPCC report.

Ghosh pointed out that high warming rate in Arabian Sea caters to extremely high rainfall bouts in Mumbai region, Western Ghats as well as the entire central India.

“Urbanisation has led to the development of urban heat island effect in Mumbai that often plays a duet and catapult huge amount of rainfall, often within short period, on the city and surrounding areas,” Ghosh said.

He pointed out that urban heat island and related weather factors often lead to huge regional variability in rainfall pattern over Mumbai.

“Our studies show the patterns completely change within a distance of 10 km within Mumbai. Even during the 2005 deluge, Santacruz received 900 mm rain in a day while Colaba, hardly 25 km away, received 81 mm,” he said.

Roxy Mathew Koll, a climate scientist from Indian Institute of Tropical Meteorology, agreed that short duration very heavy rainfall is on the rise in Mumbai and Western Ghats.

He, however, pointed out that the ‘confidence’ level of total rainfall prediction in the IPCC report was ‘not high’.

Harjeet Singh, a senior advisor in Climate Action Network International, pointed out that “Indian cities, including Mumbai, are unprepared to deal with frequent and intense climatic disasters even at the current levels of global warming, and hence will be more in trouble if IPCC predictions are found true”.

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