Millions of people living in low-lying parts of Kolkata and Mumbai face an “immediate risk” of losing their homes to permanent inundation as global sea levels continue to rise, according to a new United Nations report. The report warns that the threat is no longer confined to distant island nations but is increasingly affecting major coastal cities and densely populated communities.
The UN Secretary-General’s report, titled Challenges related to sea level rise and ways and approaches to address it, said more than 14 million people across the low-lying deltas of South Asia, including Kolkata, Mumbai and Dhaka, could face displacement as rising seas permanently inundate vulnerable areas.
“Rising seas are no longer a distant threat: they are happening now, faster than at any point in recorded history, and the pace is accelerating,” the report said, describing sea-level rise as one of the most serious threats facing populations in both developed and developing countries.
Kolkata and Mumbai may face human displacement crisis?
The UN highlighted a key difference between the risks faced by wealthy coastal cities and those in South Asia. Cities such as Miami, New York and Guangzhou face enormous financial exposure, with vulnerable coastal assets estimated at between $2 trillion and $3.5 trillion.
For low-lying South Asian cities, however, the principal concern is the displacement of people. “In the low-lying deltas of South Asia, in cities such as Kolkata and Mumbai in India, and Dhaka, the defining impact is human displacement,” the report said.
The report warned that rising seas are already damaging coastal infrastructure, water and sanitation systems, transport networks, energy grids, buildings and groundwater resources. The consequences could extend to human rights, economic stability and long-term development.
Flooding, saltwater intrusion add to the threat
The danger is being compounded by land subsidence in some coastal areas. The combination of sinking land and rising seas can intensify flooding and accelerate the degradation of coastal ecosystems.
Saltwater is also moving further inland, threatening freshwater wells and coastal aquifers. The UN warned that salinisation of water sources and soils could affect drinking-water supplies, agriculture and irrigation, while storm-driven seawater can leave surface and groundwater contaminated for prolonged periods.
The health consequences are also significant. Damage to sanitation and healthcare infrastructure, increased exposure to water-borne diseases and higher risks of injury and mortality could place additional pressure on vulnerable communities. The report also flagged growing mental-health stress associated with climate-related disruption.
Sea levels rising at unprecedented pace
The UN attributed accelerating sea-level rise primarily to melting glaciers and the thermal expansion of warming oceans, both linked to human-induced global warming.
Between 2014 and 2023, global sea levels rose at an average rate of 4.7 mm per year. In 2024, the rate reached 5.9 mm, marking a record annual increase driven largely by exceptional ocean warming. The report said sea-level rise is expected to continue accelerating through 2050 because of warming already locked into the climate system.
Nearly 770 million people — around 10% of the world’s population — live in coastal areas less than five metres above the high-tide line, putting them at acute risk from rising seas, according to the UN.
The report described sea-level rise as a “threat multiplier”, capable of damaging critical infrastructure, forcing communities to relocate and, in extreme cases, permanently destroying land.
UN calls for urgent adaptation measures
With sea-level rise largely irreversible and its impacts expected to intensify over time, the UN has called for early and long-term adaptation planning. It stressed the need for predictable financing, stronger scientific and data capabilities, and robust technical, institutional and legal frameworks to protect communities most exposed to the threat.
“Accelerating at an unprecedented level, sea level rise driven by melting glaciers and ocean warming is redefining coastlines and posing major threats to low-lying islands, coastal cities and communities, tropical deltas and Arctic regions,” the report said.
