The cost of rising sea

The United Nations (UN), in a new report, has warned that rising sea levels are no longer a distant environmental threat but an economic emergency. Global sea levels rose by 4.7 mm a year during 2014-23 and by a record 5.9 mm in 2024. A rise of at least 50 cm is already effectively locked in, making adaptation unavoidable. For India and others in the region, the implications are more serious than the loss of physical infrastructure. The UN estimates that more than 14 million people in Mumbai, Kolkata and Dhaka face the risk of losing their lives, homes, and livelihoods to permanent inundation. This points to an important distinction between vulnerable South Asian cities and richer coastal cities such as Miami and New York. In the latter, the principal concern is the destruction or loss of value of buildings, transport networks and other physical assets. In Mumbai and Kolkata, additionally, permanent inundation can trigger displacement, disrupt livelihoods, and place additional pressure on already constrained urban housing markets. 

The risks are amplified by land subsidence. Where coastal land is sinking at the same time as oceans are rising, the effective increase in relative sea level is considerably greater. Saltwater intrusion can contaminate groundwater and agricultural land, while flooding can damage sanitation, transport, energy and water systems. These effects can feed into each other. Damaged infrastructure disrupts economic activity, displacement raises housing demand elsewhere, and declining property values and higher insurance costs weaken household and business balance sheets. India’s own development choices have also increased coastal vulnerability. The country is estimated to have lost about 40 per cent of its mangroves over the past century owing to pressures including coastal development. While recent assessments show improvement in mangrove cover in parts of the country, mangroves are not merely environmental assets. They are natural coastal infrastructure, reducing exposure to flooding and erosion while supporting fisheries and livelihoods. Protecting and restoring them should, therefore, be treated as an economic investment in resilience, alongside seawalls, drainage and other engineered infrastructure. 

The financing challenge remains formidable. The UN estimates that developing countries require $310 billion-365 billion a year for adaptation. India needs to combine public investment with multilateral and concessional finance. Importantly, fiscal planning should recognise that repeated disaster relief can ultimately cost more than investing in resilience before assets and communities are exposed. This requires a change in how infrastructure and urban projects are evaluated. Coastal development should be assessed against long-term sea-level and subsidence projections rather than historical flood patterns. Master plans should identify areas where new construction is unsuitable, protect evacuation corridors and critical infrastructure, and reserve land for resilient housing and, where unavoidable, managed relocation. Early-warning systems and local sea-level monitoring also need strengthening. Some sea-level rise is unavoidable. The policy priority should be to reduce the avoidable damage. That means pricing climate risk into today’s land-use, housing, infrastructure and fiscal decisions. While the cost of doing so will be substantial, the cost of leaving millions without secure homes, while repeatedly rebuilding infrastructure in increasingly vulnerable locations, will be substantially greater.