The ocean is overheating! Each year, we witness a surge of new heat records, a trend confirmed once again in the latest report from the European Earth observatory Copernicus, published in September 2025.
It reveals an acceleration in ocean warming, with average surface temperatures peaking at 21°C in the spring of 2024—a threshold never before reached. The years 2023 and 2024 were marked by marine heatwaves of unprecedented intensity and duration, exceeding previous sea surface temperature records from 2015 and 2016 by 0.25°C. Some areas of the Atlantic experienced over 300 days of marine heatwaves in 2023. The Mediterranean was also a hotspot, with temperatures recorded in June 2025 being the highest ever documented (23.86°C). A new study also reveals that in 2024, warming reached deeper waters down to 2000 meters. Thanks to its involvement in Mercator International, Ifremer actively contributes to enriching the data of the Copernicus observatory (Copernicus Marine Services) to better understand the scale of the phenomenon.
This warming has a chain reaction on the entire ecosystem and on the ocean’s ability to regulate the climate. The ocean could indeed be defined as the Earth’s « thermostat. » It captures heat from the sun, stores it, and redistributes it through ocean circulation, from the equator to the poles, thereby helping to moderate extreme climates. Beyond this thermostat role, the ocean is also a particularly effective carbon sink, even more efficient than a forest. It has the ability to dilute CO2 released into the atmosphere, trapping it in the depths and thus limiting the greenhouse effect. This carbon is also necessary for phytoplankton, the first link in the ocean food chain, to produce oxygen using sunlight.
Unfortunately, this complex climate machinery is now going haywire, as the ocean is gradually losing its capacity to absorb CO2 and play its role as a climate moderator. The IPCC indicates that the level of CO2 in the atmosphere is the highest in two million years, reaching 419 ppm (parts per million) in 2024 (source: NOAA). The result: the ocean is acidifying, its pH is decreasing, and its carbon absorption capacity is weakening, especially in warm waters. By 2100, predictions indicate an ocean four times more acidic than in the pre-industrial era. Elements like carbonate, essential for the formation of shells and calcareous skeletons of many marine organisms, are becoming scarcer and could become insufficient in the future. A warmer ocean is also an ocean that expands and occupies a greater volume of water, a phenomenon further exacerbated by melting ice. This combination of factors leads to a general rise in sea level, with an increase of 22.8 cm between 1901 and 2024, and a trend continuing at a rate of 3.7 mm per year. All these ongoing changes in the ocean will also bring their share of turbulence in the form of increasingly intense and frequent extreme weather events: storms, cyclones, floods, and marine heatwaves will feature prominently on the list.
The impact of climate warming necessarily influences the daily work of Ifremer’s ocean science researchers, serving as a cross-cutting subject relevant to all disciplines.
Source: ifremer
