How Climate Change Is Changing the Ocean

The ocean absorbs most of the extra heat trapped by greenhouse gases, which is why the sea has changed more measurably than the air above it.

Heat

In 2024 the heat content of the upper 2,000 metres of ocean reached a record high, the highest in the WMO's 65-year observational record. Regional records fell alongside it across the Mediterranean, Indian Ocean, North Pacific, tropical and North Atlantic, and the Southern Ocean.

The rate matters more than the level. Ocean warming over 2005 to 2024 ran at more than twice the rate of the period from 1960 to 2005.

Heat in water behaves differently from heat in air. It mixes slowly, it persists, and it drives thermal expansion, which is a substantial component of sea level rise independent of any ice melting.

Marine heatwaves

Rising average temperature raises the frequency and intensity of extremes. A marine heatwave is a prolonged period of anomalously warm water, and the last several years have produced them at a scale that broke the records used to describe them.

NOAA confirmed the onset of the fourth global coral bleaching event on 15 April 2024. From early 2023 through 2025, bleaching-level heat stress affected roughly 84.4 percent of the world's coral reef area, with mass bleaching documented in at least 83 countries and territories. Taking the longer window from 2018 to 2025, an unprecedented 87 percent of reef area has experienced heat stress.

Bleaching is not death. A bleached coral has expelled the symbiotic algae it depends on for most of its energy, and it can recover if conditions ease quickly enough. What is changing is the interval between events. Recovery takes years. Near-annual bleaching does not leave years.

Acidification

The ocean absorbs roughly a quarter of atmospheric CO2, which reacts with seawater to form carbonic acid. Seawater pH fell from 8.11 in 1985 to 8.04 in 2024, and acidity has risen about 30 percent since pre-industrial times, a decline of roughly 0.1 pH units.

A tenth of a unit sounds trivial and is not, because the scale is logarithmic. In 2025 ocean acidification was formally assessed as having crossed its planetary boundary, the seventh to be judged outside a safe operating space.

The mechanism that matters biologically is carbonate availability. Organisms building shells and skeletons from calcium carbonate find it harder as the water acidifies, and at low enough saturation existing structures begin to dissolve. Pteropods, corals, oysters and coralline algae are all directly exposed, and several of them sit near the base of food webs.

Oxygen

Warm water holds less dissolved gas, and warming also strengthens stratification, which slows the mixing that carries oxygen from the surface into deeper layers.

Oxygen minimum zones have expanded measurably. Fish and mobile animals move out. Anything that cannot move does not.

Circulation

The large overturning circulation that moves heat around the planet is driven by density differences created by temperature and salinity. Freshwater from melting ice and increased precipitation reduces surface salinity in the North Atlantic, which weakens the sinking that drives the whole system.

Whether it is weakening now, and how fast, is genuinely contested. Direct observation runs to about two decades, which is short against a system that varies on multi-decadal timescales. The disagreement in the literature is real rather than manufactured.

Why measurement is the constraint

Almost everything above rests on sustained observation: the Argo float array profiling temperature and salinity, moored buoys, repeat hydrographic sections, and satellite altimetry.

Argo is why ocean heat content is knowable at all. Several thousand autonomous floats drifting at depth, surfacing on a cycle to report a profile, produced the first genuinely global picture of the upper ocean. Before it, heat content estimates leaned on ship tracks that followed shipping lanes and left most of the ocean unsampled.

The gaps that remain are the deep ocean below 2,000 metres, the polar regions under ice, and the coastal margins where the physics is most complicated and the observations are most fragmented.