Ocean Issues and Threats: Warming, Acidification, Overfishing and Pollution

• The oceans cover about 71% of the Earth, supply food and livelihoods to hundreds of millions of people, take up about nine-tenths of the excess heat and roughly a quarter of the carbon dioxide that humans add to the atmosphere.
• That service now carries a cost. The ocean faces two sets of threats that reinforce each other:
  • climate-driven threats: warming, acidification, deoxygenation and sea-level rise, all caused by greenhouse-gas emissions;
  • direct human pressures: overfishing, pollution, destruction of coastal habitats, seabed mining and shipping.
• No part of the ocean is untouched. Microplastics have been found in the deepest trenches, and heat and carbon dioxide are reaching the deep water.

Climate-Driven Threats

Ocean warming and marine heatwaves

• Global mean sea surface temperature (SST) rose by about 0.88 °C between 1850–1900 and 2011–2020, according to the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report. 2024 was the warmest year on record for the ocean surface, followed by 2023 and 2025.
• The heat is not only at the surface. The ocean’s temperature is rising through its upper layers, and the upper 2,000 m set a record for heat content for the ninth year running in 2025.
• Marine heatwaves, spells of unusually warm water lasting at least five days, have roughly doubled in frequency since the 1980s.
• Effects:
  • Coral bleaching. The fourth global bleaching event (2023–25) exposed about 84% of the world’s reef area to bleaching-level heat.
  • Stronger cyclones that intensify rapidly over warm water.
  • Stratification. A warm, light surface layer mixes less with the water below, so fewer nutrients rise and plankton production falls.
  • Fish move poleward and deeper, away from traditional fishing grounds.
• India: the tropical Indian Ocean is warming fastest of the tropical oceans, about 1.2 °C per century over 1950–2020. INCOIS now issues marine-heatwave and coral-bleaching alerts. Cyclone Biparjoy (June 2023) became the longest-lived Arabian Sea cyclone on the India Meteorological Department’s record since 1977.

Sea-level rise

• Thermal expansion of warming water and melting land ice raised global sea level by about 20 cm between 1901 and 2018 (IPCC AR6). The rate has more than doubled since the 1990s.
• Low islands and deltas face flooding, salt intrusion and erosion. India’s Lakshadweep atolls, the Sundarbans and the Odisha coast are among the most exposed stretches of its coast.

Ocean acidification

• Ocean acidification is the fall in the pH of seawater as the ocean absorbs carbon dioxide. Dissolved CO₂ forms carbonic acid, which releases hydrogen ions and uses up carbonate ions.
  • Average surface pH has fallen from about 8.2 to 8.1 since the industrial era. On the logarithmic pH scale this means seawater is 30–40% more acidic.
  • Seawater is still alkaline. “Acidification” describes the direction of change, not an acid ocean.
• Effects: shell-building organisms such as corals, pteropods, oysters and coccolithophores find it harder to build calcium carbonate. Weaker reefs and plankton shells ripple up the food web.
• In September 2025 the Potsdam Institute for Climate Impact Research reported that the planetary boundary for ocean acidification has been crossed, making it the seventh of nine boundaries breached.

Deoxygenation and dead zones

• The open ocean has lost more than 2% of its oxygen since 1960, and the volume of water with no oxygen at all has roughly quadrupled.
• Two causes:
  • Warming. Warm water holds less dissolved oxygen, and stronger stratification cuts the supply from the surface.
  • Nutrient runoff (eutrophication). Fertiliser and sewage feed algal blooms. When the bloom dies and decays, bacteria consume the oxygen, leaving a hypoxic dead zone.
• More than 500 coastal sites have reported low oxygen since 1950; fewer than one in ten of them were known before then. The Gulf of Mexico dead zone, fed by the Mississippi, and the Baltic Sea are the best-known examples.
• India: the Arabian Sea holds one of the world’s thickest oxygen minimum zones, and the Bay of Bengal’s is close to anoxic. A seasonal low-oxygen zone forms on the shelf off Goa and Kerala after the monsoon.

Direct Human Pressures

Overfishing and IUU fishing

• Overfishing means catching fish faster than stocks can breed and replace themselves.
  • In 1974 about 10% of assessed marine stocks were overfished. The FAO’s State of World Fisheries and Aquaculture put the share at 37.7% in 2021 (SOFIA 2024) and 37.6% in 2023 (SOFIA 2026).
  • Better-managed stocks still provide about 72.6% of landings, so well-regulated fisheries can recover.
  • Aquaculture has overtaken wild capture: in 2024 farmed production of aquatic animals reached 103 million tonnes, against about 92 million tonnes caught.
• Illegal, unreported and unregulated (IUU) fishing may take up to one fish in five caught worldwide, worth an estimated US$10–23.5 billion a year.
• Destructive gear adds to the damage. Bottom trawling scrapes the sea floor, and bycatch kills turtles, dolphins and juvenile fish. Lost ghost nets keep catching for years.
• Top predators are hit hardest. Tens of millions of sharks are killed each year, many for their fins, and oceanic sharks and rays have declined by about 71% since 1970. They breed slowly, and their loss upsets the whole food web.
  • Commercial whaling has been banned under an International Whaling Commission moratorium since 1986. Japan left the commission in 2019 and resumed commercial whaling; Norway and Iceland also continue whaling.
• India: marine landings were about 3.47 million tonnes in 2024. Kerala’s oil sardine catch crashed to just 3,297 tonnes in 2021, a mix of overfishing of juveniles and changing sea temperature, before recovering. Trawler conflicts in the Palk Bay show the social side of the problem.
• Fisheries and marine resources are managed through the WTO Agreement on Fisheries Subsidies (in force 15 September 2025; India joined in July 2026), the FAO Port State Measures Agreement against IUU fishing, and India’s annual 61-day monsoon fishing ban.

Marine pollution

• About 80% of marine pollution comes from land, carried by rivers, sewage outfalls and the air.
• Main pollutants:
  • Plastics: about 11 million tonnes enter the ocean each year. They break into microplastics, which enter the food chain, and collect in the ocean gyres as “garbage patches”.
  • Nutrients from farms and sewage, which cause dead zones.
  • Oil from tanker accidents, offshore rigs and routine discharges.
  • Heavy metals and persistent chemicals (mercury, lead, DDT, PCBs). They bioaccumulate and are biomagnified up the food chain. Minamata disease in Japan, identified in 1956, was mercury poisoning from contaminated fish and shellfish. The Minamata Convention (2013) now limits mercury use.
• India: the Ennore oil spill (December 2023) followed flooding from Cyclone Michaung. The container ship MSC Elsa 3 sank off Kochi on 25 May 2025, spilling plastic pellets (nurdles) along the Kerala coast.

Loss of coastal and marine habitats

• Coral reefs occupy under 1% of the sea floor but shelter about a quarter of marine species. They face heat stress, acidification, sediment, pollution and destructive fishing. The world lost about 14% of its coral between 2009 and 2018.
• Mangroves protect coasts from cyclones and storm surges and store large amounts of carbon. The first global IUCN Red List of Ecosystems assessment of mangroves (2024) found half of the world’s mangrove ecosystems at risk of collapse.
• Seagrass meadows, which feed dugongs and nurse young fish, are being lost to dredging, pollution and coastal building.
• Coastal squeeze: ports, reclamation, sand mining and sea walls replace natural shores.
  • India: about one-third of India’s coastline is eroding (National Centre for Coastal Research, 1990–2018). Reefs in Lakshadweep, the Gulf of Mannar and the Andamans bleached again in 2024.

Deep-sea mining

• Deep-sea mining targets polymetallic nodules, cobalt-rich crusts and sulphide deposits on the abyssal floor, seamounts and ridges.
• Risks: collectors strip the sea floor, sediment plumes smother life far beyond the site, and noise and light disturb deep species. Deep ecosystems recover over decades to centuries.
• Governance is unsettled. The International Seabed Authority (ISA) failed again in July 2026 to adopt a mining code, and about 45 countries back a moratorium or precautionary pause. The United States opened a unilateral licensing route in 2025.
• India holds ISA contracts to explore nodules in the Central Indian Ocean Basin and sulphides on the Indian Ocean ridges, and its Deep Ocean Mission is building the crewed submersible Matsya-6000.

Shipping, noise and invasive species

• Underwater noise from ships, seismic air-gun surveys and sonar masks the calls that whales and dolphins use to navigate, feed and find mates.
• Ship strikes kill large whales on busy routes.
• Ballast water carries organisms across oceans. Invasive species, such as the comb jelly that collapsed Black Sea fisheries in the 1980s, can take over ecosystems. The IMO’s Ballast Water Management Convention came into force in 2017.
• Shipping also emits sulphur, black carbon and CO₂, and risks spills of fuel and cargo.

Responses

• Global:
  • UNCLOS (1982) sets the law of the sea. The High Seas Treaty (BBNJ Agreement) entered into force on 17 January 2026, allowing marine protected areas on the high seas. India signed it in 2024.
  • SDG 14 (Life below Water) and the 30×30 target of the Kunming–Montreal framework (2022) aim to protect 30% of the ocean by 2030. Under a tenth is protected today.
  • MARPOL controls pollution from ships. Talks on a global plastics treaty stalled at Geneva in August 2025 and continue in 2026.
• India:
  • Single-use plastic ban (2022), the Coastal Regulation Zone rules, marine protected areas such as the Gulf of Mannar Marine National Park, and the National Oil Spill Disaster Contingency Plan.
  • Monitoring by INCOIS and the National Centre for Coastal Research, and the Deep Ocean Mission for ocean science.