It has been a sizzling April for Kolkata; Combination of climate change, urbanisation pushing city beyond tipping point

Kolkata has almost done a ‘Sergey Bubka’ when it comes to breaking its own heat records one day after another in the month just gone by, like the former Soviet athlete.

The eastern metropolis recorded a maximum temperature of 41.6 degree Celsius (°C) on April 25, the second-highest April temperature in the last 70 years. Subsequently on April 29, 2024, it recorded 41.7°C, the joint highest for the last seven decades. 

However, within 24 hours, that record got broken as the city recorded 43°C, 7.4°C above normal. It was also just 0.3°C less than the all-time highest April temperature in Kolkata — 43.3°C in 1954.

The temperatures recorded in areas like Dumdum and Salt Lake, adjacent to Kolkata, showed a similar trend, resulting in the warmest April in a decade.

“We feel that the continuous influx of northwesterly dry winds and the away movement of moisture generated in the Bay of Bengal are the chief causes of the sustained heatwave and high temperatures in the city,” Somnath Dutta, head of the India Meteorological Department (IMD)’s Regional Monitoring Centre in Alipore told Down To Earth (DTE).

“It has been definitely an unusual spell of heatwaves in Kolkata, though I won’t call it extremely unusual,” said another senior IMD official, explaining that Kolkata had experienced heatwaves in three phases this April, the first time in the ongoing century. 

“Global warming, coupled with the El Nino phenomenon, contributed to this upsurge,” said OP Sreejith, head of climate monitoring and prediction at IMD told DTE on the morning of May 2.

“El Nino, with its propensity to disrupt weather patterns worldwide, may well have contributed to Kolkata’s heatwave. However, it is not the sole culprit,” said Anjal Prakash, a professor in Bharti Institute of Public Policy, Indian School of Business and also an Intergovernmental Panel on Climate Change (IPCC) author.

Prakash added that human-induced climate change casts a long shadow over such extreme weather events, amplifying their intensity and frequency.

Ismail Mondal, a University of Calcutta professor working on Kolkata’s heat trends, observed that El Nino has been minimising the number of ‘norwesters’, further pushing up the Celsius quotient.

Incidentally, IMD sources show that the number of norwesters has minimised significantly in the last few years. While the average number during March-April is 6, Kolkata had none in 2022 and 2024, and only two in 2023.   

Worse to come

The ongoing record-breaking heatwave in Kolkata may just be a trailer, as the Assessment Report 6 (AR 6) — the UN report on climate change — projects that the highest temperature in the metropolis may touch 49.6°C between 2081 and 2100; under the worst possible global carbon emissions rise scenario.

So far, the highest temperature ever recorded in Kolkata is 43.9°C in June 1924.

The finding is part of the working group I report of IPCC, the UN body consisting of nearly 3,000 global scientists.

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The IPCC has also predicted that Kolkata’s mean temperature may rise 4.5°C around 2081-2100, compared to the pre-industrial era, at a rate higher than global average. The pre-industrial era is considered as the benchmark as human-made carbon emissions are considered to have been initiated after that period.

The report further points out that the rise in heat has been more pronounced in recent times. “The data shows that within a projected 4.5°C rise within the timescale of 1850-2081, nearly 91 per cent is set to happen in between 1995 and 2081,” explained Aniruddha Mukherjee, professor of environmental science at the University of Calcutta.

The report also states that the temperature of the city may cross 35°C for nearly 150 days in a year towards the end of the century, under the worst possible emissions scenario.

Urbanisation the cause

“Urbanisation has exacerbated the effects of global warming in cities (very high confidence),” the report says.

It further explains that “urban heat island effect results from… reduced ventilation and heat trapping due to close proximity of tall buildings, heat generated directly from human activities, the heat-absorbing properties of concrete and other urban building materials, and the limited amount of vegetation”.

The Regional Factsheet — Urban Areas part of the report shows that Kolkata, from 1950 to 2018, has recorded the highest rise in the surface air temperature — 2.7°C —among the global cities and regions studied, including New York City, Mexico City, Buenos Aires, Johannesburg, Moscow, Cairo and cities in China, Japan and Thailand. Experts point out that Kolkata has all the tell-tale attributes as reported in the report.

Subimal Ghosh, a scientist from the Indian Institute of Technology Bombay and a lead author of the IPCC report, said that “on basis of the report, we can say with high confidence that extreme heat and heatwave phenomena are going to increase in cities like Kolkata and hence, it definitely needs a heat action plan.”

Abhiyant Tiwari, a climate expert with the international advocacy platform Natural Resources Defense Council, pointed out that highly humid places like Kolkata should give priority to implementation of heat action plans.

Incidentally the report shows that urbanisation has contributed to a rise in the city’s temperature rise, with more than 80 per cent of the warming recorded in Kolkata being linked to the city area itself.

Experts pointed out that the city is suffering more because the blue-green infrastructure — water bodies and greenery required to combat heat rise — is fast depleting.

A 2022-23 pan-India study by Delhi-based environmental think-tank Centre for Science and Environment showed that Kolkata’s water bodies decreased by 39 per cent; and vegetation by 19 per cent, while built-up area increased by nine per cent during 1999-2021, triggering the rise in the heat.

“First and foremost, Kolkata must protect its numerous water bodies, as proactive measures are essential to mitigate future risks. From embracing renewable energy to implementing sustainable urban planning, collective action is paramount in safeguarding our cities and communities against the escalating impacts of climate change. Kolkata’s heat surge is a poignant reminder that the time for decisive climate action is now,” added Prakash.