State of the Climate 2025 - Global Oceans

This chapter synthesizes changes in the ocean’s physical state and key biogeochemical indicators during 2025, highlighting the persistence of exceptional warmth following the record-breaking years of 2023 and 2024 and the continued influence of large-scale climate variability, particularly the El Niño–Southern Oscillation (ENSO). The strong El Niño that developed in 2023 and weakened through the spring of 2024, transitioned towards La Niña conditions by the second half of 2024 (see section 4b). Weak La Niña was prevalent for most of 2025 except later spring/summer when neutral conditions dominated. This transition influenced ocean–atmosphere exchanges, the redistribution of heat, freshwater, and momentum across ocean basins while the long-term warming trend continued.

Air–sea fluxes in 2025 reflected ongoing ocean heat uptake and ENSO-related variability. Compared to 2024, heat gain increased notably in the tropics and subtropics. Freshwater flux patterns were shaped by the developing La Niña conditions, with increased precipitation and freshening near the Maritime Continent and western tropical Pacific and enhanced freshwater loss and salinification across the central and eastern equatorial Pacific. These changes were
driven mainly by precipitation anomalies rather than evaporation. Wind stress anomalies were generally weak in the tropics but showed strengthened westerlies in the Southern Ocean and North Atlantic and weaker winds over parts of the North Pacific, influencing regional circulation
and wind-driven vertical velocity.

Global sea surface temperatures (SSTs) remained exceptionally high in 2025, marking the third consecutive year of extreme ocean warmth. Long-term warming has accelerated, increasing from approximately 0.12°C per decade over 1950–2025 to about 0.21°C per decade since 2000.
Although the shift toward neutral and weak La Niña conditions produced some cooling in the tropical Pacific compared to the peak warmth of 2023/24, global SSTs remained elevated due to the continued accumulation of heat in the climate system. Marine heatwaves remained widespread, affecting about 87% of the ocean surface. While heatwave occurrence declined slightly relative to 2024, 2025 remained among the highest occurrence years on record. In contrast,
marine cold spells became even less frequent, reaching their lowest occurrence in the observational record and highlighting the continued shift toward warmer ocean conditions.

Ocean heat content also remained at record highs, with continued warming in the upper 2000 m and detectable warming extending into deeper layers. The transition from El Niño to
weak La Niña conditions redistributed heat within the tropical Pacific, reducing heat content in the eastern equatorial Pacific and increasing it in the western and central equatorial Pacific, but did not offset the long-term warming trend. Most ocean basins remained warmer than average. The Atlantic Ocean continued to exhibit widespread warm anomalies, though slightly cooler than the extreme conditions of 2024. The Indian Ocean remained warmer than the climatological average despite localized cooling in the southwestern Indian Ocean compared to 2024. The sustained heat uptake by the oceans contributes to accelerated sea level rise, enhanced tropical cyclone activity, amplified stratification, and increased marine heatwaves.

Ocean salinity patterns reflected both interannual variability and long-term hydrological changes. The equatorial Pacific generally became saltier under La Niña-related precipitation shifts, while high-latitude regions of the Atlantic experienced freshening that may influence
deep water formation and overturning circulation. The Indian Ocean exhibited mixed salinity changes, with both freshening and salinification depending on location, and shifted from anomalously fresh to slightly salty relative to climatology. Subsurface salinity changes were most pronounced in the upper few hundred meters and varied by basin. Despite these interannual fluctuations, long-term trends remain consistent with an intensifying global hydrological cycle, in which salty regions become saltier and fresh regions fresher.

Global mean sea level reached a new record high in 2025, rising to 111.2 mm above the 1993 baseline and continuing a long-term increase of about 3.5 mm per year, with clear acceleration over recent decades. Although ocean warming drove a substantial thermosteric increase, the total annual rise from 2024 to 2025 was relatively modest, likely due to La Niña-related redistribution of water between the ocean and land. Regional patterns reflected ENSO-driven
circulation changes, with lower sea levels in the central and eastern tropical Pacific and higher levels in the Indo–western Pacific. Long-term sea level rise continues to increase the frequency and severity of coastal flooding and extreme sea level events.

Ocean circulation patterns in 2025 showed strong regional variability linked to ENSO and atmospheric forcing. Surface currents in the equatorial Pacific exhibited enhanced westward flow associated with developing La Niña conditions, while the Kuroshio Extension remained displaced northward of its long-term mean position. Seasonal monsoon forcing drove pronounced circulation changes in the Indian Ocean, and variability in the Atlantic reflected regional climate
modes. The Atlantic meridional overturning circulation continued to exhibit strong interannual variability, with some evidence of localized weakening in the subtropical North Atlantic and strengthening in the subtropical South Atlantic, but limited evidence of a long-term basin-wide
decline.

The global ocean continues to play a major role in the global carbon cycle by absorbing approximately 29% of anthropogenic carbon dioxide emissions and helping slow its atmospheric accumulation, while contributing to ocean acidification. The oceanic uptake of carbon dioxide
in 2025 increased compared to the prior decade. Despite regional variability and observational uncertainties, the ocean carbon sink remains strong and continues to increase. Satellite observations showed that global phytoplankton biomass and distribution in 2025 remained largely
within historical ranges, with modest productivity increases in the tropical Pacific associated with weak La Niña conditions.

This year’s report features a new section on dissolved oxygen, which is an essential climate variable, especially for marine ecosystems and biogeochemical cycling. In 2025, global ocean dissolved oxygen continued to decline, with inventories decreasing by about 0.45% per decade since 1965 due to warming, reduced ventilation, ocean circulation changes, and enhanced biological respiration. An ensemble of statistical and machine-learning analyses confirms
widespread deoxygenation, especially in the North Pacific and Southern Oceans, with broadly negative oxygen anomalies in 2025 consistent with elevated ocean heat content and ENSO-driven variability.

Overall, the ocean in 2025 remained in a state of persistent change. Record-high global sea level, high SSTs and heat content, widespread marine heatwaves, and continued carbon uptake and deoxygenation highlight the ocean’s central role in regulating Earth’s climate.