3.3 4.a, 4.b - Ocean warming, acidification and island adaptation

Acidification and marine livelihoods

Ocean acidification is a reduction in seawater pH caused mainly by uptake of additional atmospheric carbon dioxide. A falling pH means increasing acidity; it does not require the ocean to become acidic below pH 7. Surface seawater generally remains alkaline while becoming less alkaline than before. Warming and acidification share an important cause in emissions, but heating water is not the chemical definition of acidification.

Dissolved carbon dioxide participates in reactions that increase hydrogen ions and reduce the availability of carbonate ions. A simplified net relationship is:

This helps explain why organisms using calcium carbonate to build shells or skeletons can face greater difficulty maintaining them. Sensitivity varies among species, life stages and local conditions, so acidification does not dissolve every shell at the same rate. NOAA's acidification overview explains the changing chemistry and biological consequences.

Effects on fish stocks can be indirect as well as direct. Changes in shell-forming plankton or other prey can alter food supply, while weakened coral habitats can reduce shelter and nursery functions. Some fish are also affected through physiological or behavioural responses, but outcomes depend on the species and exposure. Stock depletion cannot automatically be attributed to acidification alone: fishing pressure, habitat change, warming and pollution can interact. The IPCC's 2022 ocean and coastal assessment examines these combined pressures.

People may face reduced or less predictable catches, altered species distributions and additional costs for fishing or aquaculture. Communities dependent on seafood for income and nutrition have less room to absorb those changes than diversified economies. Monitoring and local measures can reduce some stresses, while reducing carbon dioxide emissions addresses the main global driver. Protecting one local site from overfishing cannot by itself stabilise global ocean chemistry, but it can improve resilience to interacting pressures.

Ocean warming and coral ecosystems

Many tropical reef-building corals live in partnership with photosynthetic microorganisms within their tissues. The coral provides a sheltered setting; the symbionts supply much of the coral's energy and contribute colour. Temperature stress can disrupt this relationship, causing loss of symbionts or their pigments and making the coral appear pale or white: coral bleaching.

A bleached coral is stressed but not necessarily dead. If conditions improve soon enough, it may recover its symbionts and energy supply. Severe or prolonged stress can lead to starvation, disease and mortality; repeated events can reduce recovery time. NOAA's bleaching explanation makes this distinction. Other stresses, including freshwater exposure and pollution, can also contribute, although warming is a major driver of widespread heat-related events.

In a 2 June 2026 assessment, NOAA reported that bleaching-level heat stress affected about 84% of the world's coral reef area from early 2023 to mid-2025. That is an exposure measure, not a statement that 84% of corals died or that all exposed reefs bleached equally. The account judged that the fourth global event likely ended in mid-2025; its historical window should not be described as an uninterrupted ongoing event in every place.

When living coral cover and reef structure deteriorate, habitat for fish and invertebrates can decline. Species dependent on specialised coral habitats may be particularly vulnerable, changing biodiversity and food-web relationships. Loss of a three-dimensional reef can also reduce its ability to dissipate wave energy, with effects depending on reef shape, water depth and later erosion.

Local communities can consequently face reduced fisheries, damaged tourism attractions and increased coastal exposure. Benefits and burdens vary: a household reliant on reef fishing may have fewer alternatives than a mobile tourism company. Improved wastewater treatment, careful fishing management and protection of remaining habitats can reduce local stress, but cannot remove global heat stress. Restoration also requires appropriate conditions; planting corals into repeated severe warming is not a complete substitute for climate mitigation.

Thermal expansion and land ice

Global mean sea level changes when the volume of ocean water or the amount stored in ocean basins changes. Contemporary climate-related rise has two major contributions: warming seawater expands, and melting land ice transfers additional water into the ocean. The mechanisms should be explained separately before their effects are combined.

Thermal expansion increases the volume occupied by a given mass of seawater as it warms. The precise response depends on its initial temperature, salinity and pressure. Because the ocean is deep and has a large heat capacity, warming below the surface also matters. Sea level can therefore respond over long periods rather than instantly matching each year's air temperature.

Melting mountain glaciers and ice sheets on land adds water that was previously stored outside the ocean. Greenland and Antarctica are important ice-sheet reservoirs. Floating sea ice already displaces water, so its melting has very little direct effect on sea level compared with melting land ice. Losing sea ice can nevertheless affect climate through albedo and other processes. A floating ice shelf can also influence land ice indirectly by providing restraint: reduced restraint may permit faster land-ice discharge.