Sea level rise

Also called: Rising sea level · Topic: Climate Change: Causes, Evidence and the Global Response · NCERT: Class 7, Ch 2 "India and Her Neighbours"; Class 9, Ch 3 "Atmosphere and Climate"; Class 11, Ch 4 "Climate"; Class 11, Ch 11 "World Climate and Climate Change"

Meaning

Sea-level rise is the long-term increase in the average height of the ocean surface. It happens because melting land ice adds water to the ocean, and because seawater expands as it warms. It matters because it floods low coasts, deltas and small islands, pushes salt water into farmland, and can force millions of people to move. India's coastal cities and the Sundarbans are among the places at risk.

Explanation

How it happens: the two main causes

• Melting land ice adds new water to the ocean.
• Global warming melts mountain glaciers and the Greenland and Antarctic ice sheets.
• This meltwater runs into the sea.

• The total amount of water in the ocean goes up, so the sea level rises.

• Thermal expansion makes the same water take up more space.

• Thermal expansion means water takes up more space as it warms.
• The oceans are absorbing heat. Ocean heat content (the heat stored in the top 2,000 m of the ocean) reached a record high in 2025, and each of the past nine years set a new record [2].

• Warmer water takes up more room, so the sea surface rises even when no water is added.

• Melting sea ice adds very little. Sea ice is already floating on the ocean, so when it melts, the sea level barely changes. Only ice on land adds much.

Feedbacks that speed it up

• Arctic amplification (the Arctic warming much faster than the rest of the world): since 1979 the Arctic has warmed nearly four times faster than the global average.
• Ice-albedo feedback drives this:
• Albedo is the share of sunlight a surface reflects. Snow and ice reflect most sunlight.
• When the ice melts, darker land and sea underneath absorb more heat.

• This causes more warming and more melting. It is a positive feedback (a loop in which a change makes itself stronger).

• As a result, Greenland's ice sheet melts faster and adds more water to the sea.

• Tipping points (thresholds beyond which a system changes suddenly and often cannot recover):
• Greenland ice sheet: if it keeps melting, sea level could rise by about 7 m over many centuries.
• West Antarctic ice sheet, including the Thwaites Glacier (Amundsen Sea), nicknamed the "Doomsday Glacier": warm ocean water melts it from below, which can make the ice sheet unstable.

How much and how fast

• Global mean sea level has risen about 20 cm since 1901.
• Satellites measured a rise of 11.1 cm between 1993 and 2023 [3].
• The rise is speeding up. The rate has more than doubled, from about 2.1 mm a year in 1993 to 4.5 mm a year by 2024 [3].
• Projections for 2100 (IPCC AR6):

Emissions pathway	Projected rise by 2100
Low emissions	about 0.3–0.6 m
Very high emissions	about 0.6–1.0 m
Very high emissions with ice-sheet collapse	more than 1 m is possible (low likelihood)

• NCERT gives an older single estimate of 48 cm by 2100. This falls inside the AR6 range.

What it does

• Coastal flooding of low land and islands.
• Salt water entering farmland and groundwater, which spoils crops and drinking water.
• Larger storm surges during cyclones, because the surge starts from a higher sea level.
• People forced to move from coasts and islands.

Where it occurs

Type	Example	Location
Mega-delta	Ganga-Brahmaputra (Sundarbans)	India and Bangladesh, Bay of Bengal
Mega-delta	Mekong	Vietnam, South China Sea
Mega-delta	Nile	Egypt, Mediterranean Sea
Small island state	Maldives	Indian Ocean; most land is less than 1–2 m above sea level
Small island states	Tuvalu, Kiribati	Coral atolls in the Pacific Ocean
Indian coastal city	Mumbai	Arabian Sea coast, Maharashtra
Indian coastal city	Kolkata	On the Hooghly river, close to the delta, West Bengal
Indian coastal city	Chennai	Coromandel coast, Bay of Bengal, Tamil Nadu

• Sources of the extra water:
• Mountain glaciers.
• The Greenland ice sheet (North Atlantic).
• The West Antarctic ice sheet, including the Thwaites Glacier in the Amundsen Sea.

Don't confuse with

• Melting sea ice vs melting land ice: melting sea ice (for example, Arctic sea ice) adds very little to sea level because it is already floating. Melting land ice (glaciers, and the Greenland and Antarctic ice sheets) does raise sea level.
• Storm surge: a short-lived rise of seawater pushed onto the coast by a cyclone's winds. Sea-level rise is a slow, long-term change in the average level. A higher sea level makes storm surges larger.
• Tides: the daily rise and fall of the sea caused by the pull of the Moon and the Sun. Sea-level rise is a change in the average level over decades, not a daily cycle.
• Thermal expansion vs meltwater: thermal expansion raises sea level without adding any water, because warm water takes up more space. Meltwater raises it by adding new water.

Prelims Hooks

• The two main causes of sea-level rise are melting land ice and thermal expansion. Trap: melting floating sea ice does not add much.
• The rate of sea-level rise more than doubled, from 2.1 mm a year (1993) to 4.5 mm a year (2024). Satellites recorded a total rise of 11.1 cm over 1993–2023 [3].
• Global mean sea level has risen about 20 cm since 1901. NCERT projects 48 cm by 2100. IPCC AR6 projects about 0.3–0.6 m (low emissions) to 0.6–1.0 m (very high emissions).
• Thwaites Glacier ("Doomsday Glacier") – West Antarctica, Amundsen Sea. The full loss of the Greenland ice sheet could raise sea level by about 7 m over many centuries.
• Matches: Maldives – Indian Ocean. Tuvalu, Kiribati – Pacific coral atolls. Mekong delta – Vietnam. Nile delta – Egypt. Sundarbans – Ganga-Brahmaputra delta (India and Bangladesh).
• Arctic amplification (nearly 4 times the global rate since 1979, driven by the ice-albedo feedback) speeds up Greenland melt and adds to sea-level rise.

Mains Points

• India's coasts are exposed (GS-I, GS-III Disaster Management).
• Rising seas threaten Mumbai, Kolkata, Chennai and the Sundarbans.
• Risks include coastal flooding, salt water in farmland and groundwater, and larger cyclone storm surges along the Bay of Bengal.

• Fishers, farmers and the urban poor are hit hardest, and many may have to move.

• The rise is speeding up, and some of it cannot be undone (GS-III Environment).

• The rate doubled from 2.1 to 4.5 mm a year between 1993 and 2024 [3], and ocean heat content set a new record in 2025 [2].

• Ice-sheet tipping points (Greenland, West Antarctica) could lock in metres of rise over centuries. This makes a strong case for meeting Paris Agreement targets early.

• Small island states and mega-deltas face the biggest threat (GS-I, GS-II).

• The Maldives, Tuvalu and Kiribati sit only 1–2 m above the sea, so their survival as nations is at stake.
• Crowded deltas (Ganga-Brahmaputra, Mekong, Nile) put food security at risk.
• Responses include coastal protection, early warning for storm surges, mangrove restoration and planned relocation.

Related concepts

• Global warming
• Global temperature record
• Arctic amplification
• Glacier retreat in the Himalaya
• Climate tipping points

Read more

• Global warming: the record, the evidence and the risks