Changing coasts and oceans

The first principle

The coast is where land, sea and air exchange energy and matter, so it changes faster than any other landscape. A warmer ocean expands and receives meltwater, so its level rises; it holds less oxygen and absorbs carbon dioxide that turns it more acidic; and its living buffers, coral reefs, mangroves and wetlands, are the first to suffer. When people reclaim those buffers for ports, farms and cities, they remove the very systems that absorb waves, store floodwater and breed fish, so human and climatic change reinforce each other.

The core

Why the sea is rising

• Sea level rises for two reasons: water expands as it warms (thermal expansion), and land ice adds new water. The IPCC Sixth Assessment Report (AR6, 2021) found global mean sea level rose about 0.20 m between 1901 and 2018, and that the rate reached about 3.7 mm a year in 2006 to 2018, nearly three times the rate of 1901 to 1971.
• Rise is not uniform: winds, currents and sinking land make it faster in places such as the western Pacific and the subsiding deltas of the Ganga and Brahmaputra.

Why island nations are threatened in their very existence

Atoll states such as Tuvalu, Kiribati, the Marshall Islands and the Maldives rise mostly 1 to 3 m above the sea, so even modest rise threatens them in several ways at once.

• Inundation and erosion. King tides and storm surges already flood parts of Funafuti in Tuvalu and Majuro in the Marshall Islands, and land is lost permanently as shorelines retreat.
• Freshwater. The only drinking water is a thin lens of rain water floating on seawater; salt intrusion spoils wells and the taro pits that feed families, so islands can become uninhabitable long before they go under.
• Statehood and maritime zones. An exclusive economic zone (EEZ) is measured from the coastline, so shrinking land could shrink fishing rights that pay much of a small state's budget; Pacific states declared in 2021 that their maritime zones stay fixed regardless of sea level rise.
• Migration. The Australia Tuvalu Falepili Union treaty (2023) offers up to 280 Tuvaluans a year a pathway to live in Australia, the first agreement of its kind tied to climate.
• Law. The International Tribunal for the Law of the Sea (ITLOS) advised in May 2024 that greenhouse gas emissions are marine pollution that states must control, and the International Court of Justice (ICJ), in July 2025, advised that states have binding obligations to protect the climate system and that breaching them can carry legal consequences.

Dimensions

Coral reefs and global warming

Reef building corals live in partnership with algae called zooxanthellae, which photosynthesise inside the coral tissue and supply most of its food. When water stays about 1°C above the normal summer maximum for weeks, this partnership breaks down and the coral expels its algae and turns white.

• Acidification. The ocean has absorbed about a quarter of human carbon dioxide emissions, which forms carbonic acid and lowers the carbonate ions that corals need to build skeletons.
• Global events. The National Oceanic and Atmospheric Administration (NOAA) confirmed the fourth global bleaching event in April 2024, after those of 1998, 2010 and 2014 to 2017; by 2025 bleaching level heat had touched over four fifths of the world's reef area.
• Examples. The Great Barrier Reef has bleached on a mass scale several times since 2016; in India, reefs of Lakshadweep, the Gulf of Mannar and the Andaman and Nicobar Islands bleached in 1998, 2010 and again in 2024.
• Why it matters. Reefs cover well under 1 per cent of the sea floor but shelter about a quarter of marine species, protect coasts from waves and support fisheries and tourism.

Mangroves and coastal ecology

Mangroves are salt tolerant trees of the intertidal zone, and their tangled prop and breathing roots slow water, trap mud and shelter young fish.

• Storm buffers. Studies after the 1999 Odisha super cyclone found fewer deaths in villages behind wide mangroves, and areas with mangroves suffered less damage in the 2004 tsunami.
• Nursery and livelihoods. Many commercial fish, prawns and crabs spend their young stages among the roots.
• Blue carbon. Their waterlogged soils hold carbon that does not decay, and they store several times more carbon per hectare than most dryland forests.
• Land building. Roots trap river sediment and help coasts keep pace with a rising sea, if sediment supply continues.
• Causes of depletion. Clearing for shrimp ponds and agriculture, ports and urban growth, pollution, dams that cut the freshwater and silt reaching deltas, and sea level rise where mangroves cannot move inland.

In practice

India's mangroves

The India State of Forest Report 2021 recorded about 4,992 square km of mangrove cover, nearly 40 per cent of it in the Sundarbans of West Bengal, followed by Gujarat and the Andaman and Nicobar Islands. The Mangrove Initiative for Shoreline Habitats and Tangible Incomes (MISHTI), announced in the Union Budget 2023 and run by the Ministry of Environment, Forest and Climate Change, aims to restore about 540 square km over five years.

Dead zones

• Where. The Gulf of Mexico at the Mississippi mouth and the Baltic Sea are the best known. The Arabian Sea has a natural oxygen minimum zone at mid depths that is among the world's largest and appears to be expanding.
• Consequences. Fish and shrimp that can swim away crowd into narrower waters, while crabs, clams and other bottom dwellers die, so food webs simplify and biodiversity falls. Fisheries lose catch and income, low oxygen water favours jellyfish and microbes, and bacteria in such water release nitrous oxide, a strong greenhouse gas. Warming worsens all of this, because warm water holds less oxygen and stratifies more.

Reclaiming water bodies for cities

Lakes, marshes and floodplains are flat and cheap, so cities fill them for housing, roads and dumps, but the water that used to spread there still comes.

• Environmental implications. Loss of flood storage, less groundwater recharge, loss of birds and fish breeding grounds, hotter neighbourhoods because water and vegetation cool the air, and loss of free services such as the East Kolkata Wetlands, a Ramsar site that treats much of Kolkata's sewage through fish ponds.

India's coastline: resources and preparedness

• Resource potential. The coast supports marine fisheries, ports and coastal shipping under Sagarmala, monazite and ilmenite placer sands in Kerala, Tamil Nadu and Odisha, offshore oil and gas at Mumbai High and the Krishna Godavari basin, salt in Gujarat, offshore wind off Gujarat and Tamil Nadu, and tourism; together these form the blue economy.
• Preparedness, the strengths. Better forecasts from the India Meteorological Department (IMD), cyclone shelters and evacuation cut deaths sharply: about 10,000 died in the 1999 Odisha super cyclone, but only a few dozen in Phailin (2013) and Fani (2019). The National Cyclone Risk Mitigation Project (NCRMP), led by the National Disaster Management Authority with World Bank support, built shelters and warning systems, and the Indian Tsunami Early Warning Centre at Hyderabad has warned the Indian Ocean since 2007.
• Preparedness, the gaps. The National Centre for Coastal Research found that about a third of the mainland coast eroded between 1990 and 2018. Heat waves, storm surge flooding in cities, the western coast's newer cyclone risk and poorly enforced Coastal Regulation Zone (CRZ) rules remain weak spots.

Issues and way forward

• Rising seas outpacing adaptation. Small island states lack money to raise land or build defences; the loss and damage fund agreed at the 2022 climate conference (COP27) needs real money, and the ICJ opinion strengthens their claim to it.
• Repeated bleaching. Reefs need about a decade between bleaching events to recover, which warming no longer allows; deep emission cuts are the only lasting fix, with local protection from pollution and overfishing buying time.
• Mangrove loss to shrimp ponds and ports. Regulation is weak at the local level; CRZ 2019 classes mangroves as ecologically sensitive, and community management, as practised in the Pichavaram mangroves of Tamil Nadu, has worked.
• Nutrient runoff. Overuse of urea feeds coastal blooms; soil health cards, neem coated urea and sewage treatment cut it.
• Wetlands treated as wasteland. The Wetlands (Conservation and Management) Rules, 2017 and courts have tried to protect them, but cities still need flood maps that keep lakes and drains free of building.

Common confusions

• Bleaching and death. Bleached coral is alive but starving; it dies only if the stress lasts.
• Thermal expansion and ice melt. Both raise the sea; melting floating sea ice does not.
• Dead zone and oxygen minimum zone. A dead zone is mostly caused by human nutrient runoff in coastal waters; an oxygen minimum zone is a natural mid depth layer, though warming is expanding it.

For the exam

• Keyword bank: thermal expansion, freshwater lens, saltwater intrusion, statehood and EEZ, Falepili Union, zooxanthellae, bleaching, acidification, blue carbon, MISHTI, hypoxia, eutrophication, stratification, flood storage, blue economy, NCRMP, shoreline erosion.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction (data): Global sea level rose at about 3.7 mm a year in 2006 to 2018, nearly three times the rate of the early twentieth century, according to IPCC AR6.
  • Introduction (judgement, quote or report): In July 2025 the International Court of Justice advised that states are legally obliged to protect the climate system, a landmark for island nations.
  • Conclusion: Reefs, mangroves and wetlands are free coastal infrastructure, and protecting them is cheaper than rebuilding what they protect.
  • Conclusion: India has proved with cyclones that preparedness saves lives; the same rigour must now extend to erosion, rising seas and urban wetlands.