The oceans hit a new, alarming benchmark in 2023. A global analysis combining satellite records and high-resolution ocean reanalyses, including ECCO2 (Estimating the Circulation and Climate of the Ocean-Phase II), shows that marine heatwaves that year were unprecedented in strength, length and reach lasting roughly four times longer than historical averages and affecting about 96% of the planet’s surface waters.

The warming wasn’t uniform. The North Atlantic, Tropical Eastern Pacific, North Pacific and Southwest Pacific bore the brunt of the anomalies, together accounting for about 90% of the excess ocean heat. Some regional extremes were extraordinary: a North Atlantic event that began in mid-2022 persisted for roughly 525 days, while surface temperatures in the Tropical Eastern Pacific spiked as much as 1.63°C during El Niño onset. The Southwest Pacific marine heatwave set new records for both spatial scale and duration.

A mixed-layer heat-budget analysis revealed why the events were so intense and persistent: reduced cloudiness let more sunlight reach the sea, weakened winds cut heat loss and vertical mixing, and unusual ocean currents parked warm water where it could accumulate. In other words, slightly different drivers in different basins combined to produce a near-global pulse of marine warmth.

These heatwaves matter. Prolonged warming drives coral bleaching, seagrass die-offs, and mass marine life mortalities; it also disrupts fisheries and aquaculture that coastal communities depend on. The authors warn that the 2023 pattern may signal a more fundamental shift in ocean-atmosphere dynamics, presenting a possible early warning of a climate tipping point.