The ocean keeps most of the extra heat trapped by greenhouse gases, and 2025 set a new mark. A study summarized by The Guardian put last year's ocean heat uptake near 23 zettajoules, plus or minus 8. A related Nature Reviews assessment cited about 24 zettajoules, plus or minus 6, with large shares in the Southern Ocean and Pacific.

A zettajoule is a billion billion joules — an abstract unit that becomes concrete in marine heat waves, heavier rainfall, and expanding seawater that lifts global sea level. Hotter upper oceans also help fuel stronger tropical cyclones when other ingredients line up.

Researchers treat annual rankings with care because different methods span slightly different depths and baselines. The direction of travel is clearer than any single error bar: the ocean has taken up the bulk of planetary warming for decades, and recent years keep landing near the top of the chart.

Coastal planners watch the knock-ons: warmer water stress on fisheries, coral bleaching risk, and higher baseline water levels that make storm surge worse. Inland readers feel the moisture side when hotter seas feed heavier downpours.

The papers are part of an annual accounting tradition that updates as late data arrive. Readers should prefer the scientific summaries over viral graphics that drop the uncertainty ranges.

Fisheries managers watch marine heat waves that can push species toward new grounds and stress aquaculture. Coral reef tourism towns feel bleaching risk when hot water lingers for weeks. Those local hits are how abstract zettajoules show up in paychecks.

What we know: Multiple 2025 ocean-heat estimates cluster near 23–24 zettajoules, described as record annual uptake.
What we don't: Exactly how 2026 year-to-date heat will compare once full data are in.