2025’s warming comes even despite the start of La Niña, which typically has a cooling effect on global temperatures
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Even with the start of a new La Niña phase – which typically has a cooling influence on temperatures – 2025 ranked among the three warmest years ever recorded for the surface global ocean, according to Mercator Ocean International.
These results are based on Mercator Ocean’s operational ocean monitoring systems, which deliver detailed analyses of sea surface temperatures and the global ocean state.
81 per cent of the ocean was above average temperatures, with 2025 being the hottest year for many regions including the eastern North Atlantic, parts of the Southern Ocean and the western Pacific.
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In the North Atlantic, sea surface temperatures were the sixth warmest in 33 years, with 69 per cent of the basin in the warmest 10 years. In the Mediterranean, waters were in their second-warmest year, with 98 per cent of the basin above average.
89 per cent of the global ocean experienced marine heatwaves, while more than half of the global ocean (55 per cent) experienced intense marine heatwaves classified as strong, severe or extreme.
In 2025, sea ice extent was also impacted with Arctic and Antarctic sea ice well below long-term averages. For 11 out of.12 months of the year, global sea ice ranked within the top five lowest extents.
In Antarctica, sea ice formation was delayed across unusually large areas, particularly in the Indian Ocean sector. As a result, sea ice extent remained well below average for most of the winter.
Despite the natural cooling influence of La Niña, 2025 was warmer on average than the strong El Niño years of 2015 and 2016. This apparent paradox is largely explained by the long-term rise in global sea surface temperatures driven by human-induced climate change.
‘Despite being a La Niña year, 2025 ranks among the three warmest years ever recorded,’ said oceanographer at Mercator Ocean International, Romain Bourdalle-Badie. ‘In fact, what was considered ‘warm’ ten years ago has effectively become the new “cool” in today’s climate baseline – further evidence of the long-term rise in global sea surface temperatures driven by human-induced climate change.’
A warming planet
As well as 2025 being one of the warmest years for ocean temperatures, Copernicus revealed earlier this month that 2025 was the third-warmest year on record globally with average temperatures of almost 15C.
The period 2023–2025 marks the first time that a three-year span has exceeded 1.5C above pre-industrial levels, with extreme heat seen globally, particularly in the poles, and major heatwaves occurring across Europe.
Between 2023 and 2025, two main reasons combined to cause an exceptionally warm period. The first is the build-up of greenhouse gases in the atmsophere from continued emissions, as well as reduced uptake of carbon dioxide by natural sinks. Secondly, sea-surface temperatures reached high levels across the ocean, a phenomenon amplified by climate change. Additional factors include changes in the amount of aerosols and low cloud, and variations in atmospheric circulation.
