The world’s oceans absorbed a record 23 zettajoules of heat in 2025 an amount nearly 40 times greater than the total energy used by humanity during the year according to the latest Copernicus Ocean State Report.
The report highlights continuing changes in the oceans, including rising sea levels, increasing acidity and growing stress on marine ecosystems.
Oceans absorb most excess heat
The oceans have absorbed around 90% of the additional heat trapped in the Earth’s climate system by greenhouse gas emissions. They have also taken up about one third of the carbon dioxide released by human activities.
According to the report, global sea levels have risen by an average of about 3.8 millimetres per year since 1999. The rate increased to approximately 4.2 millimetres per year between 2012 and 2025.
Ocean acidity has also increased, with the global ocean becoming about 17% more acidic over the past 40 years.
Mediterranean faces rising temperatures and sea levels
The Mediterranean and Black seas have experienced repeated marine heatwaves, with unusually high temperatures extending deep below the surface.
The Mediterranean as a whole has warmed by about 0.32°C per decade. Scientists also recorded an acceleration in sea level rise in the western Mediterranean although changes in the eastern part have been more uneven.
Warmer coastal waters can intensify heatwaves on land because they release additional heat into the atmosphere particularly at night.
Marine ecosystems under pressure
The increasing ocean heat is already affecting marine ecosystems. Between January 2023 and September 2025, 84% of the world’s coral reefs experienced heat stress severe enough to cause bleaching according to the report.
The study also found that nearly 5% of the global ocean has experienced persistent changes in phytoplankton growth patterns over the past three decades. Phytoplankton are microscopic organisms that form the foundation of many marine food webs and play an important role in the carbon cycle.
In Antarctica four emperor penguin colonies experienced complete breeding failures in 2022 during periods of exceptionally low sea ice.
Climate change and air pollution linked
The Copernicus findings were released alongside separate research on air pollution. That analysis found that more than one-third of the world’s population was exposed to dangerous levels of ground-level ozone during the first eight months of 2026.
Scientists described this as an “ozone climate penalty”, in which higher temperatures and sunlight accelerate chemical reactions involving pollutants and increase ozone formation.
Only about 5% of the global population experienced ozone levels during the period that remained within World Health Organization guideline levels according to the report.
Researchers said the findings demonstrate the close links between climate change, ocean warming, air pollution and public health. They also highlighted the ability of wildfire smoke, ozone and dust to travel thousands of kilometres, meaning pollution generated in one region can affect communities far away.
The latest ocean data underline the scale of changes taking place in the marine environment, from increasing heat and acidity to rising seas and growing pressure on coral reefs and other ecosystems.