Climate Change & the Oceans

Ocean Acidification

The ocean absorbs approximately 25–30% of all CO₂ emitted by human activity. When CO₂ dissolves in seawater, it forms carbonic acid, which releases hydrogen ions that lower the pH of the ocean — a process known as ocean acidification. Since the Industrial Revolution, average ocean surface pH has fallen from 8.2 to approximately 8.1, representing a 26% increase in acidity.

Acidification is especially damaging to organisms that build shells or skeletons from calcium carbonate — including corals, oysters, mussels, sea urchins and some species of plankton. As pH falls, the mineral aragonite — used by many organisms — becomes increasingly undersaturated, making shell formation harder and dissolving existing structures. This threatens both the biodiversity of coral reefs and the economic livelihoods of shellfish aquaculture industries worldwide.

Ocean Warming

The ocean has absorbed over 90% of the excess heat trapped by greenhouse gases since 1971. This has caused widespread ocean warming, with average sea surface temperatures rising by approximately 0.13°C per decade since the 1950s. While this may seem small, marine organisms have evolved within narrow temperature ranges and are highly sensitive to even modest changes.

• Warmer waters expand in volume, contributing directly to sea level rise.
• Changes in ocean circulation patterns affect the distribution of nutrients, productivity and fisheries.
• Species are migrating poleward as warm water expands, disrupting established food webs.
• Species are migrating poleward as warm water expands, disrupting established food webs.

Sea Level Rise

Global mean sea level has risen by approximately 20 cm since 1900, and the rate of rise is accelerating. The two main causes are thermal expansion of warming seawater and the melting of land-based ice sheets and glaciers. Current projections suggest sea levels could rise by between 0.3 and 1 metre by 2100 under a moderate emissions scenario, with higher-end projections exceeding 2 metres if ice sheet instability occurs.

Coastal communities worldwide face increased risks from flooding, storm surge and saltwater intrusion into freshwater supplies. Low-lying island nations — including Tuvalu, Kiribati and the Maldives — face the prospect of complete inundation. Sea level rise also threatens coastal ecosystems including mangroves, salt marshes and beaches that provide critical habitat and natural flood protection.

Deoxygenation

Warming oceans hold less dissolved oxygen than cooler ones — a phenomenon known as deoxygenation. The ocean has lost approximately 2% of its oxygen since 1960, and oxygen minimum zones (OMZs) — regions of very low oxygen at depth — are expanding. This is compounding the effects of nutrient pollution-driven dead zones in coastal waters.

• Deoxygenation forces fish and other mobile animals into shallower, warmer waters — concentrating them and making them more vulnerable to fishing.
• Animals with high oxygen requirements, such as large tuna and billfish, are particularly affected.
• Expanding OMZs reduce the habitable volume of the ocean, compressing biodiversity into increasingly crowded surface waters.
• Deoxygenation can also trigger the release of toxic hydrogen sulphide gas from seafloor sediments.

Addressing these interconnected climate threats requires urgent global action to reduce greenhouse gas emissions. Ocean health and climate stability are inseparable — protecting one means protecting the other.