When the sea comes calling Mumbai and Kolkata cannot look away!

Raju Korti
The question, therefore, is not whether Mumbai or Kolkata will suddenly go under water. The more pertinent question is how much flooding, disruption and displacement the two cities will experience in the years ahead, which areas will be affected first, and whether governments are preparing quickly enough.

The UN's latest report leaves little room for complacency. Global sea level rose by about 5.9 mm in 2024, the highest annual increase recorded, and the rate is expected to accelerate. Even under a very low emissions scenario, the Intergovernmental Panel on Climate Change projects a global mean sea-level rise of about 23 cm by 2050. The UN report also points out that around 770 million people live in coastal areas less than five metres above the high-tide line. Mumbai and Kolkata are singled out because the problem is compounded by their low-lying geography and, in places, by the sinking of the land itself.

That distinction is important. A rise of a few millimetres in a year does not mean that Mumbai will be underwater next year. Sea-level rise is a long-term process. What makes it dangerous is that every increment reduces the margin of safety available to a coastal city. When the higher sea level meets a high tide, an intense monsoon downpour or a storm surge, water that would previously have drained away can remain trapped. In other words, the immediate threat is not necessarily permanent submergence but increasingly frequent and severe episodes of flooding, followed eventually in the most vulnerable locations by chronic inundation.

The high tides foraying into human habitation 

For Mumbai, there is something particularly uncomfortable about the latest warning. It is not the first time the city has been told that its relationship with the sea is becoming dangerous. Mumbai's Climate Action Plan had already identified coastal flooding as one of the city's major climate risks. In 2021, then civic commissioner Iqbal Singh Chahal publicly warned that large portions of South Mumbai, including Cuffe Parade, Nariman Point and Mantralaya, could face severe inundation by 2050. The areas specifically mentioned included parts of Umarkhadi, Mohammed Ali Road, Marine Lines, Girgaon and Breach Candy.

Now the BMC is undertaking another climate risk assessment, its second in five years. The study will examine Mumbai's 149-kilometre coastline, changes in land use and climate patterns and the population living in vulnerable coastal stretches. It is understandable that the BMC wants updated information because Mumbai's physical landscape has changed substantially. The Coastal Road, reclamation and other developments have altered parts of the coastline, while salt-pan areas on the eastern side have also come under development pressure.

But there is an obvious question. Is Mumbai studying the problem again when it should already be acting on what it knows?

A fresh vulnerability assessment is useful, even essential. But a study should not become a substitute for action. Mumbai has had repeated warnings for years. It has experienced catastrophic flooding before. It knows that high tide combined with intense rainfall can overwhelm its drainage system. It knows that mangroves, wetlands, floodplains and other natural buffers are valuable. It knows that reclamation and indiscriminate concretisation can alter drainage patterns. What it now needs is not merely another map showing where the water might go. It needs a clear implementation strategy showing what must be done before the water gets there.

The most vulnerable areas are not necessarily those closest to the sea alone. They are low-lying areas where the sea, stormwater and inadequate drainage can interact.

South Mumbai is an obvious danger zone. Cuffe Parade, Nariman Point, Mantralaya, Colaba, parts of Marine Drive, Girgaon, Breach Candy, Haji Ali and Worli have repeatedly figured in assessments and warnings. A 2023 RMSI analysis identified several vulnerable locations, including areas around the Mithi River, parts of Colaba, Haji Ali, coastal stretches of Worli and settlements around Mahim Fort. It also identified risks around the airport and the Jawaharlal Nehru Port Trust area.

The vulnerability extends well beyond the postcard image of South Mumbai. Much of Mumbai's core and parts of its suburbs stand on reclaimed or very low-lying land. Research has pointed to the Bandra Kurla Complex, the airport and Worli as examples of major built-up areas located on reclaimed land and therefore exposed to high tides and storm surges. Dharavi too is particularly vulnerable because of its low-lying location and its proximity to the Mithi River and drainage channels.

The western coastal belt and the creek systems on the eastern side deserve equal attention. Low-lying areas around Malvani, Bhayander and the western suburbs have been identified as particularly susceptible to tidal flooding, while the Mithi, Mahim and other waterways can become conduits through which tidal water moves inland. The eastern suburbs face their own vulnerabilities around creeks, salt pans and low-lying terrain.

This is why the phrase "Mumbai will be submerged" can be misleading. Mumbai is not one uniform geographical entity. Its elevation varies, its coastline is irregular and its drainage systems are different from one locality to another. One neighbourhood may face regular tidal flooding while another may remain relatively safe. Some areas may experience temporary inundation during extreme events decades before they face any possibility of permanent inundation.

The year 2050 has acquired an almost ominous significance in discussions about Mumbai. But 2050 should be understood as a planning horizon, not a date stamped on a calendar when the sea is expected to arrive.

Climate models work with scenarios. Their projections depend on future emissions, ocean behaviour, ice-sheet dynamics, land movement and local geography. They therefore cannot tell us that a particular building will be underwater on a particular date. What they can tell us is that the probability and severity of coastal flooding will increase as sea levels rise.

The scientific evidence is already strong enough to act on. A 2025 study of major Indian coastal cities found Mumbai and Kolkata among the cities facing the highest future coastal flooding risks, particularly under higher-emissions scenarios. The study's modelling also showed that lower-emissions scenarios reduce, but do not eliminate, the threat.

There is consequently no justification for waiting until the threat becomes visually dramatic. By the time the sea regularly reaches homes, roads and electrical installations, adaptation will be considerably more expensive.

If Mumbai is a city built intimately around the Arabian Sea, Kolkata's vulnerability is inseparable from the Hooghly, the Bay of Bengal and the wider Ganga delta system.

The UN has placed Mumbai and Kolkata in the same category of concern, but their physical vulnerabilities are not identical. Kolkata is a low-lying delta city and is affected not only by rising sea levels but also by land subsidence, river flooding, storm surges and the loss of wetlands that historically absorbed excess water.

Research using satellite observations has found significant land subsidence in parts of Kolkata. Between 2003 and 2011, parts of eastern Kolkata experienced subsidence of roughly 4 to 12 mm a year, while newer subsidence zones emerged west of the Hooghly between 2017 and 2021. Excessive groundwater extraction is one of the factors associated with this sinking.

That makes the arithmetic particularly worrying. If the sea rises while the land sinks, the effective rise relative to the ground can be greater than the rise of the sea alone.

Kolkata's eastern and south-eastern fringes are particularly important because of their relationship with the East Kolkata Wetlands. Recent research has found that the wetlands have shrunk dramatically over the past three decades, while another study of flooding between 2016 and 2023 found south-eastern Kolkata, including the East Kolkata Wetlands and adjacent rapidly urbanising areas, to have the highest flood hazard. The study also concluded that land-use change and wetland loss can have a stronger influence on flood levels than elevation alone.

Kolkata's vulnerability is therefore also a story about what happens when a city gradually destroys the very ecological systems that once protected it.

It would be tempting to blame everything on global warming and leave it there. That would be convenient, but incomplete.

Climate change is the great multiplier. It raises the baseline sea level, warms the oceans and can intensify extreme weather. But cities determine how badly those changes affect their populations.

Mumbai's concretisation has reduced natural spaces through which water can be absorbed. Wetlands and mangroves have been degraded. Drainage channels have been narrowed or encroached upon. Reclamation has altered the relationship between land and sea. Kolkata has experienced wetland loss, urban expansion and groundwater-related subsidence.

The UN itself emphasises that sea-level rise is not simply an environmental problem. It threatens housing, drinking water, sanitation, health, infrastructure, livelihoods and economic stability.

The poorer sections of the population are likely to suffer first and most severely. A wealthy household may be able to move to a higher floor, reinforce a building, insure property or eventually relocate. A fishing community, slum dweller, street vendor or low-income household living close to a creek has considerably fewer options.

That is why the issue should not be reduced to a debate about whether Mumbai's luxury towers will be submerged. The real question is whether the city's poorest and most vulnerable citizens will be repeatedly displaced by floods long before the city's skyline is visibly threatened.

The first requirement is to stop treating climate adaptation as an occasional project. It has to become part of every major urban decision.

Mumbai needs a detailed ward-by-ward coastal risk map that is continuously updated rather than produced once every few years. The map must combine sea-level projections with tides, rainfall, storm surges, drainage capacity, land subsidence, land elevation and population vulnerability. Every major infrastructure project should then be tested against those projections.

The second requirement is to protect natural defences. Mangroves, wetlands, mudflats, salt pans and floodplains are not vacant land waiting to be developed. They are part of Mumbai's flood-control infrastructure. The same principle applies to Kolkata's East Kolkata Wetlands and the wider deltaic ecosystem.

Third, drainage must be treated as climate infrastructure rather than merely municipal housekeeping. Drains, pumping stations, stormwater channels and outfalls have to be designed for the rainfall and tidal conditions that cities are likely to experience in the future, not those they experienced decades ago.

Fourth, development permissions in vulnerable areas need to become much more stringent. It makes little sense for one arm of government to warn that a locality is at risk from inundation while another arm continues to encourage intensive construction there without adequate adaptation measures.

Mumbai also needs to ask whether every piece of coastal reclamation is compatible with the city's long-term survival. Infrastructure that solves a transport problem today but increases flood vulnerability tomorrow cannot be called successful development.

Kolkata faces an equally urgent task of protecting wetlands, regulating groundwater extraction, restoring drainage channels and ensuring that urban expansion does not eliminate the natural spaces that absorb floodwater.

The responsibility cannot be pushed entirely on to the BMC or the Kolkata Municipal Corporation. Municipal bodies are the first line of defence, but they do not control everything that determines coastal vulnerability.

The Maharashtra and West Bengal governments have crucial responsibilities in land-use regulation, coastal management, infrastructure and disaster preparedness. The Union government has a role in climate finance, scientific monitoring, coastal regulation and long-term adaptation policy. Metropolitan development authorities, ports, infrastructure agencies and planning bodies must also be brought into one integrated framework.

And there is a larger responsibility that belongs to all levels of government. Climate warnings cannot be treated as news events that generate anxiety for a few days and disappear from public discussion.

The BMC's new study should therefore not end as another report on a shelf. It should become the basis for a legally backed coastal resilience programme, with deadlines, funding, accountability and public disclosure. Kolkata needs an equivalent seriousness.

The UN warning should not frighten Mumbai and Kolkata into paralysis. It should frighten them out of complacency.

Neither city is going to vanish beneath the sea tomorrow. Nor does the UN report say that. What it says is considerably more important: sea-level rise is already happening, it is accelerating, and cities such as Mumbai and Kolkata are particularly exposed because geography, urbanisation and land subsidence magnify the consequences.

The rest of India's coastline faces varying degrees of risk too. Chennai, Kochi, Surat, Visakhapatnam, Mangaluru and other coastal cities are exposed to combinations of sea-level rise, storm surges, erosion and extreme rainfall. A recent comparative study of Indian coastal cities found Mumbai and Kolkata among the highest-risk cities, although the nature and intensity of vulnerability differ from place to place.

For Mumbai and Kolkata, however, the warning has a special resonance. Both cities have already received versions of it before. Mumbai has had its climate action plan. Kolkata has had studies on flooding, subsidence and wetlands. Scientists have repeatedly explained the danger. The evidence has accumulated.

What is required now is to move from prediction to preparation. The sea does not negotiate with municipal boundaries, political terms or development timetables. It advances in millimetres, almost imperceptibly, while cities advance in concrete and steel. The tragedy would not be that Mumbai and Kolkata were warned too late. The greater tragedy would be that they were warned early enough and still chose to wait.