Sea Level Rise

Sea level rise means the average height of the world's oceans is going up over time. It is happening mainly because the Earth is warming. Warm water takes up more space, and melting ice on land pours extra water into the sea. Even a rise of a few centimetres matters, because it makes storm floods reach further inland and lets salt water creep into farms and drinking water.

Why does it matter so much?

Hundreds of millions of people live in low-lying coastal areas around the world, and many of the biggest cities, ports and farmlands are near the coast. A higher sea makes every storm surge and high tide more dangerous. For small island nations, it threatens their very existence. Sea level rise is also very slow to reverse: once the oceans warm and ice sheets lose mass, the sea keeps rising for centuries.

What makes the sea rise?

There are four main causes. According to the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6), during 1971–2018:

1. Thermal expansion: about 50%. Water expands when it warms. The oceans absorb about 90% of the extra heat trapped by greenhouse gases, so this is the biggest single cause.
2. Melting glaciers: about 22%. Mountain glaciers, such as those in the Himalaya, the Alps and Alaska, are losing ice.
3. Ice sheets: about 20%. The giant ice sheets of Greenland and Antarctica are losing ice by melting and by sliding faster into the sea.
4. Changes in land water storage: about 8%. For example, pumping out groundwater that later flows to the sea.

Note that melting sea ice (ice already floating on the ocean, like Arctic sea ice) barely raises the sea level, just as melting ice cubes do not overflow a glass. It is land ice that adds new water.

How is sea level measured?

• Tide gauges: instruments fixed at ports and harbours that record the height of the water. Some records go back more than 100 years. They measure sea level relative to the land they stand on.
• Satellite altimetry: since 1993, satellites (starting with TOPEX/Poseidon, followed by the Jason series and Sentinel-6 Michael Freilich, launched in 2020) bounce radar pulses off the ocean surface to measure its height across the globe.
• Gravity satellites: missions like GRACE measure tiny changes in Earth's gravity to track how much ice and water is being lost from land.

How much has it risen?

IPCC AR6 found that the global mean sea level rose by about 20 cm between 1901 and 2018. The speed has been increasing:

• about 1.3 mm a year during 1901–1971
• about 1.9 mm a year during 1971–2006
• about 3.7 mm a year during 2006–2018

Satellite data compiled by the WMO show the rate has more than doubled within the satellite era, from about 2.1 mm a year in 1993–2002 to about 4.7 mm a year in 2014–2023.

What lies ahead?

IPCC AR6 projects that by 2100 (compared with 1995–2014), sea level will rise by about:

• 0.28 to 0.55 metres in a very low emissions path (SSP1-1.9)
• 0.63 to 1.01 metres in a very high emissions path (SSP5-8.5)

A much larger rise, approaching 2 metres by 2100, cannot be ruled out if ice sheets break down faster than expected, though scientists consider this less likely. Even if warming stops, the sea will keep rising for centuries because the deep ocean and ice sheets react slowly.

Global vs local sea level

The sea does not rise equally everywhere. Winds, currents and gravity shift water around. On top of that, the land itself may be sinking (for example, from groundwater pumping or soft river delta soil) or rising. What a coastal city feels is the relative sea level: the sea's rise plus the land's sinking. This is why the rate differs widely along India's coast.

Tide gauge records reported to Parliament show about 0.33 mm a year at Chennai (1916–2005) but about 5.16 mm a year at Diamond Harbour (1948–2005) in the sinking Hooghly delta near Kolkata.

India's position and Indian examples

• India's coastline was re-measured as 11,098.81 km (earlier 7,516.6 km) by the National Hydrographic Office with the Survey of India, using better maps; the revised figure was notified in April 2025.
• The Ministry of Earth Sciences has told Parliament that the Indian Ocean rose about 1.7 mm a year during 1900–2000, and the north Indian Ocean about 3.3 mm a year during 1993–2015, faster than before.
• Hotspots include the Sundarbans delta in West Bengal, low islands of Lakshadweep, and big coastal cities such as Mumbai, Chennai and Kolkata. Kerala and Odisha face heavy coastal erosion.
• India's tools include the Coastal Regulation Zone (CRZ) Notification, 2019, which controls building near the shore; INCOIS (Indian National Centre for Ocean Information Services, Hyderabad), which runs ocean monitoring and storm surge warnings; the National Centre for Coastal Research in Chennai, which maps shoreline change; and mangrove programmes such as MISHTI, since mangroves act as natural sea walls.

Commonly confused concepts

• Eustatic vs isostatic change: eustatic change is a worldwide change in sea level because of more water or warmer water. Isostatic change is when the land rises or sinks, for example as land rebounds after a heavy ice sheet melts.
• Global mean vs relative sea level: global mean is the average for all oceans. Relative sea level is what a particular coast experiences, including land sinking or rising.
• Sea ice vs land ice: melting sea ice adds almost nothing to sea level; melting glaciers and ice sheets on land do.
• Sea level rise vs storm surge: sea level rise is a slow, long-term change. A storm surge is a short, sudden rise in water pushed ashore by a cyclone. A higher sea makes each surge worse.
• Thermal expansion vs melting: expansion makes the same water take more space; melting adds new water.

Issues, criticism and the way forward

• Saltwater intrusion: salty water enters rivers, groundwater and farmland, harming crops and drinking water, as already seen in the Sundarbans.
• Displacement and loss: coastal erosion and flooding can force people to move, creating climate-displaced communities. Small island states fear losing their land, and even raise legal questions about keeping their maritime zones.
• Uncertainty about ice sheets: the biggest unknown is how fast Antarctica and Greenland will lose ice. This makes long-term planning hard.
• Hard vs soft defences: sea walls protect cities but can shift erosion to nearby beaches. Experts favour mixing them with natural defences like mangroves, dunes and coral reefs, and with careful planning that keeps new buildings out of high-risk zones.
• Way forward: deep cuts in greenhouse gases to slow the rise; better local sea level data and early warnings; coastal zone plans based on future, not past, sea levels; and money for adaptation and loss and damage for the most exposed countries.

Concepts to Know

• Thermal expansion: the increase in volume of a substance when it gets warmer. Warm ocean water occupies more space, so the sea rises.
• Ice sheet: a huge mass of land ice covering more than 50,000 square kilometres. Only two exist today: Greenland and Antarctica.
• Altimetry: measuring height. A satellite altimeter sends radar pulses down and times how long they take to bounce back from the sea surface.
• Storm surge: a temporary wall of seawater pushed onto the coast by a cyclone's strong winds and low pressure.
• SSP (Shared Socioeconomic Pathway): a scenario used by the IPCC to describe how the world's emissions might develop. SSP1-1.9 is a very low emissions path; SSP5-8.5 is a very high one.
• Mangroves: salt-tolerant trees that grow along coasts and river mouths. Their thick roots slow waves and hold soil in place.