World Ocean Assessment Report Highlights Changing Chemistry
The third World Ocean Assessment, released on June 8, offers scientific insights into the health of the Earth’s oceans based on the collective work of approximately 600 experts from 86 countries.
The Assessment reveals that the ocean continues to be under severe and accelerating anthropogenic pressure, from the surface to the deep sea, driven by climate change, pollution and increased human activities.
Approximately 16% of the total increase in ocean heat content since 1955 has occurred since 2018. The greatest relative warming has been observed in the Atlantic Ocean and the southern parts of the Indian and Pacific Oceans.
The sea level continues to rise at increasing rates, from less than 2.0 mm per year prior to 2015 to 4.3 mm per year in 2023.
The extent of Arctic sea ice continues to decrease, and the Southern Ocean has recently shown declines in sea ice extent. The Arctic Ocean could become entirely ice-free in September by the middle of the 21st century.
Ocean CO2 uptake and ocean acidification continue to increase, although these trends exhibit high spatial and interannual variability due to weather and climate conditions.
Global ocean deoxygenation persists due to rising water temperature (which reduces oxygen solubility and increases stratification) and intensified microbial activity caused by nutrient run-off from land. Ocean hypoxic zones continue to expand (covering an additional 4.5 million km2 in the past 50 years).
Each year, 52.1 million tonnes of plastic waste enter the ocean, contributing to an estimated 24.4 trillion microplastic particles, which are now known to affect more than 4,000 marine species.
Large gaps persist in ocean knowledge, with only 27.3% of the seafloor mapped as of 2025, leaving deep sea ecosystems, biological processes, and cumulative impacts poorly understood.
However, the Assessment notes the rapid development of technologies and methodologies for ocean observation. Together with advances in modelling, they are greatly increasing the scientific understanding of the ocean and the development of management tools.
The use and development of automated or semi-automated ocean observation platforms, such as Argo floats (which are now being expanded to include deep Argo floats that observe depths below 2,000 meters), gliders, submarine cables, unmanned autonomous vehicles, remote observing vehicles and drones, continue to expand.
Newer observation platforms are equipped with higher-resolution and more precise sensors, as well as sensors that can measure new variables.
Examples include the use of higher-resolution satellite sensors, the incorporation of biogeochemical variables into Argo floats, advances in image acquisition systems and the use of passive and active acoustic sensors in biological studies.
For now, the acquisition of real-time or near-real-time data is skewed towards physical variables rather than biological variables, with biochemical variables falling somewhere in between. This imbalance is due to the complexity of the biological components of marine ecosystems, the diverse nature of their variables (e.g. taxonomic, ecological and physiological), the lower number of sensors developed for biological variables and the energy consumption required for image acquisition systems and active acoustic sensors.
Long-term monitoring and observation systems are needed to understand long-term ecosystem dynamics, including the effects of climate change.
International initiatives such as the Argo program and the Global Ocean Acidification Observing Network, which are coordinated under the Global Ocean Observing System, as well as more recent programs under the Ocean Decade, continue to reinforce international cooperation and assist with the implementation and standardization of methodologies and ocean indicators. However, long-term monitoring programs are often hindered by a lack of adequate long-term funding.
The third World Ocean Assessment stresses the strong links between ocean health and food security, livelihoods, economic prosperity, and cultural identity.