Sea-level rise

1. Meaning, measurement and essential distinctions

Sea level is not a fixed, uniformly flat surface. Tides, atmospheric pressure, winds, currents, temperature and salinity produce variations over hours to decades. Sea-level rise refers to a persistent increase after these shorter-term fluctuations are accounted for. Global mean sea level is the spatial average of ocean-surface height; regional and local trends may differ substantially from this average.

Absolute, or geocentric, sea level describes ocean height relative to an Earth-centred reference frame. Relative sea level describes the sea surface relative to the land at a particular coast. Tide gauges primarily record relative sea level, whereas satellite radar altimeters measure sea-surface height in a geocentric reference frame. Satellite altimetry has provided near-global observations since 1993.

Measurements are combined to identify causes. Satellite gravimetry, including GRACE and GRACE Follow-On, tracks changes in ice and water mass. Argo profiling floats measure ocean temperature and salinity, helping estimate expansion-related changes. GNSS observations near tide gauges help distinguish ocean rise from vertical land movement.

• Eustatic change traditionally refers to global sea-level change associated with changes in ocean-water volume or ocean-basin capacity.
• Steric change results from changes in seawater density: thermosteric change concerns temperature, while halosteric change concerns salinity.
• A storm surge is a short-lived abnormal rise caused mainly by storm winds and atmospheric pressure; it is not the same as long-term sea-level rise.

2. Physical causes and uneven regional patterns

Thermal expansion occurs because seawater generally occupies more volume as it warms. The ocean absorbs most of the excess heat accumulating in the climate system, and warming can penetrate progressively into deeper layers. Expansion therefore continues beyond the initial atmospheric warming. IPCC AR6 attributed approximately half of the observed global mean rise during 1971–2018 to thermal expansion.

The second major mechanism is transfer of water from land to ocean. Mountain glaciers and the Greenland and Antarctic ice sheets lose mass through surface melting, runoff and ice discharge. Floating sea ice already displaces approximately its own weight of seawater, so its melting makes a negligible direct contribution. However, sea-ice loss reduces reflectivity, while thinning or collapse of floating ice shelves can weaken their buttressing effect and accelerate the discharge of grounded ice.

Terrestrial water storage also matters. Groundwater extraction can transfer previously stored water into the ocean through runoff and the water cycle, whereas reservoir impoundment stores water on land. Over geological timescales, tectonic changes in ocean-basin capacity also influence sea level.

Rise is geographically uneven because winds, ocean currents, density changes and gravitational effects redistribute water. Ice-sheet mass loss changes the gravitational attraction exerted on nearby seawater and deforms the solid Earth, producing distinct sea-level fingerprints. Meanwhile, groundwater withdrawal, sediment compaction and reduced sediment replenishment can lower coastal land. Tectonic uplift and post-glacial rebound can instead reduce or reverse local relative sea-level rise.

• Subsidence and sea-level rise are distinct processes, but both increase relative sea level.
• Glacial isostatic adjustment is the continuing response of the Earth to past loading and unloading by ice sheets.
• No single global rate should be applied mechanically to every Indian coastal settlement.

3. Observed trends, projections and long-term commitment

IPCC AR6 reported a global mean rise of about 0.20 metre during 1901–2018. The average rate increased from 1.3 mm per year during 1901–1971 to 1.9 mm during 1971–2006 and 3.7 mm during 2006–2018. These figures show acceleration rather than a constant linear trend. Individual years may depart from the long-term trend because phenomena such as El Niño and La Niña alter ocean heat and terrestrial water storage.

Relative to 1995–2014, the likely rise by 2100 is 0.28–0.55 metre under the very low-emissions scenario SSP1-1.9 and 0.63–1.01 metres under the very high-emissions scenario SSP5-8.5. These are conditional projections, not fixed predictions. Uncertain ice-sheet processes create additional risks beyond the likely ranges.

Sea level responds slowly to changes in climate. Deep-ocean warming and ice-sheet adjustment mean that rise will continue for centuries to millennia even after temperatures stabilise. Strong mitigation reduces its eventual magnitude and rate but does not immediately stop it. Coastal planning must therefore consider both near-term observations and longer-term commitments.

• Always identify the projection’s baseline period, target year, emissions scenario and uncertainty range.
• Global mean projections must be combined with local land motion and regional ocean conditions for adaptation planning.

4. Coastal impacts and Indian vulnerability

A higher mean sea level raises the starting point for tides, storm surges and wave action. Consequently, an extreme water level that was formerly rare may occur more frequently even without stronger storms. Impacts include permanent inundation of some lowlands, more frequent coastal flooding, shoreline retreat, impaired drainage and saltwater intrusion into rivers, soils and aquifers.

Vulnerability depends on exposure and adaptive capacity, not elevation alone. Deltas are particularly sensitive because they combine low relief, compressible sediments and dense settlement. Upstream dams, embankments and sediment extraction may reduce sediment delivery, limiting the ability of delta surfaces to build upward. Mangroves and tidal wetlands can accumulate sediment or migrate inland, but rapid rise and development barriers may prevent adjustment.

In India, the Sundarbans and the Mahanadi, Godavari, Krishna and Cauvery delta regions warrant attention, alongside low-lying urban coasts and island territories. Mumbai and Chennai illustrate the importance of considering coastal water levels together with heavy rainfall and drainage constraints. Lakshadweep’s low-lying coral islands face risks to freshwater lenses, infrastructure and beaches. Nevertheless, shoreline erosion is not automatically proof of sea-level rise: ports, sand mining, waves and sediment deficits may also be responsible.

• Coastal squeeze occurs when intertidal habitats are trapped between rising seas and fixed landward barriers.
• Compound flooding combines interacting drivers such as heavy rainfall, river discharge, high tides and storm surge.
• Salinisation can affect drinking-water security and agriculture before land becomes permanently submerged.

5. Adaptation, mitigation and governance

Adaptation is commonly organised into protection, accommodation and retreat. Protection includes seawalls, surge barriers and ecosystem-based measures. Accommodation includes elevated buildings, flood-resilient infrastructure, improved drainage and suitable farming practices. Planned retreat moves people or assets away from persistently hazardous locations. Avoiding new development in exposed areas often prevents costly future dependence on defences.

Each approach involves trade-offs. Hard structures may protect valuable assets but disrupt sediment transport, intensify erosion elsewhere and require maintenance. Mangrove restoration and dune conservation provide ecological benefits, but their success depends on suitable hydrology, sediment supply and space. Nature-based measures complement rather than universally replace engineered protection.

India’s Coastal Regulation Zone framework, including the CRZ Notification, 2019 under the Environment (Protection) Act, 1986, regulates coastal activities and recognises sensitive areas. Integrated Coastal Zone Management promotes coordination across sectors. INCOIS supplies ocean information and coastal hazard services, while the National Centre for Coastal Research studies shoreline change and coastal processes. Effective planning combines such information with local surveys, community participation and periodic reassessment.

• Mitigation limits future warming; adaptation manages unavoidable impacts.
• Use flexible adaptation pathways with clear thresholds for upgrading defences or relocating assets.
• Prioritise vulnerable communities, livelihood continuity and fair compensation in relocation decisions.

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