In recent years, the air quality in the coastal city has been declining—often more polluted than the notoriously smoggy Delhi. We look at why this is happening—and how climate change is affecting wind patterns around the world, not just in Mumbai.
Researched by: Nirmal Bhansali & Aarthi Ramnath
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How bad is it in Mumbai?
Things are much better now but the city got a rude shock back in October—when the Air Quality Index shot past 300—which is classified as ‘very poor’. And the concentration of PM2.5 in early November was 17X the amount considered safe by the WHO. FYI: The most toxic location in the city was Colaba!
A long-term trend: Mumbai’s winter pollution has been steadily increasing in recent years. The city experienced the longest spell of poor air quality last year—from November all the way to January. The PM2.5 numbers have more than doubled since 2019. A comparison of PM2.5 concentrations across major Indian cities—between 2019 and 2023—offers surprising results. Usual suspects like Delhi and Lucknow are indeed more polluted—but their numbers are improving. But Mumbai, Bangalore and Kolkata are getting worse.
Defining the terms: The Air Quality Index measures the level of pollution on an hourly, daily and annual basis. According to global measures—on a range from 0 to 500—an AQI above 300 is ‘hazardous’. There are no other designations for the insane heights that are routinely charted in our part of the world. But in India, we have categories for higher levels—where anything between 301 and 400 is ‘very poor’, and 401 and 500 is ‘severe’.
And PM stands for Particulate Matter. The measurements—typically 2.5 or 10—refer to the size, i.e. less than 2.5 micrometres or less than 10 micrometres. Smaller the particles, and more prolonged the exposure, the more hazardous they are to human health.
So why is Mumbai getting worse?
For two basic reasons. One, there is simply more pollution in the air. And two, the city’s air-cleaning system—the sea breeze—is becoming weaker. Let’s start with reason #1.
An emissions problem: Unsurprisingly, power plants and factories contribute 35.82% of all emissions. But Mumbai slums have a particularly nasty problem—caused by poor households that burn kerosene, coal and wood. These account for around 27% of the air pollution in Mumbai. In fact, Delhi has done a far better job of reducing dependence on such fuels than Mumbai.
Those ‘kaccha’ roads: There are 6,000 construction sites in the city—including buildings and for the metro. But experts say that construction dust—“contained by green curtains around under-construction buildings”—is not the chief culprit. The real problem: unpaved roads—which contribute 21.2% of dust-related pollution: “As a vehicle speeds on this road, the dust rises,” he said. “It might settle soon, but the road is never free of vehicles.” And that brings us to every Indian’s greatest bane: traffic!
[T]he linear, island city has no way around traffic jams. Traffic, said transportation expert Ashok Datar, is terrible for the environment. “When you’re forced to stop and start or drive slowly in first gear, you keep the engine idling,” he said. “At that time, exhaust from the engine goes up seven-eight times and the pollution naturally goes up several times. There is a great surge in the exhaust irrespective of the type or quality of the vehicle. When hundreds of vehicles move like that, it creates localised high particulate pollution, much higher than the dust.”
There are 1.2 million private vehicles on Mumbai’s streets—but “roads have been halved, vehicles crawl as a result and take double the time to reach their destination.”
And what’s wrong with the sea breeze?
For the longest time, Mumbai’s coastal location has been its secret anti-pollution weapon. The pollution has always been as high as other major metros—but the breeze could be relied on to clean the air.
A temperature gradient: Sea breezes are a function of a difference in temperature. During the day, land heats up faster than the sea. Warmer and therefore lighter air over the city starts to rise. The cooler air over the sea flows in to replace the rising warm air. At night, the pattern reverses itself—when the surface on land cools faster than the water surface.
As for Mumbai: The sea breeze is especially effective since the city is surrounded by water on three sides. During winter, the breeze reversal repeats itself over the course of three to four days—as opposed to every day during summer. That hasn’t been happening for the last couple of winters. One reason: Mumbai winter was a lot cooler last year—so the clean sea breeze blowing into the city was much milder. Another reason: sea surfaces could be much warmer due to climate change.
Normally, a coastal city like Mumbai would experience a wind reversal every three to four days, with wind speeds reaching up to 12-13 kmph. This helps to disperse air pollutants and particulate matter. However, in recent years, wind speeds have dropped to just below 4-5 kmph, resulting in pollutants getting trapped in the air.
The bigger picture: Wind speeds have been slowing down around the world due to climate change—precisely because the differences in temperature have been narrowing:
[Ecohydrologist Michael] Roderick takes a more telescopic view: air movements are powered by differences in temperature at different places. The bigger the difference between warm and cold air, the stronger the wind. One effect of global warming is to flatten those differences. The poles are warming faster than the equator, winters are warming faster than summers, and nights warming faster than days. “Everything becomes more uniform,” Roderick says.
Data point to note: A 2022 study found that the Arabian Sea is “experiencing a rapid increase in heatwave days with sea surface temperature extremes in the last decade sometimes stretching out through a season.” In the long term, differences between sea and land temperatures are likely to narrow for Mumbai, as well.
The El Niño effect: We are in the process of shifting to a weather pattern called El Niño (explained in detail in this Big Story). Researchers say that there is an 89% chance that we will experience a ‘Super El Niño’. And it is going to make the planet feel hotter than ever—including its oceans. In August, around 27% of the global ocean experienced a marine heatwave due to El Niño. And sea surface temperatures since April show a far sharper spike than the warming over land. We don’t know what this specifically means for sea breezes in Mumbai.
The bottomline: We tend to take a narrow and short-term view of air pollution. Mumbai teaches us the importance of looking at the big picture offered by climate change. We can cut down on vehicle emissions in the city—or curb construction. But it will take a lot more to restore the all-important sea breeze. And that requires a collective global effort. Something to think about as we head into a critical climate change summit in Dubai this week.
Scroll is best on the reasons for Mumbai’s increasing air pollution problem—for a more technical explanation, read Dr Gufran Beig in Indian Express. Also in Scroll: the 2022 study on heatwaves in the Arabian Sea. Down to Earth has an eye-opening interview with environmental scientist Abhishek Chakraborty. Citizen Matters explains what’s happening to the sea breeze in Chennai. Cosmos explains why winds around the world are slowing down. Also read Yale Environment 360 on ‘wind stilling’.