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.
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| The high tides foraying into human habitation |
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.








