1390. Unprecedented Complex Changes in the Deep Ocean

The ocean, Earth's life-support system, is experiencing rapid and widespread changes that extend to its deep layers due to multiple climate-related stressors, including warming, acidification, deoxygenation, and salinity fluctuations. However,  assessing the combined impacts of these changes on ocean conditions from a global perspective has been difficult.

A paper published in Nature Climate Change found evidence that climate change is pushing marine environments into uncharted territory, with vast areas of the world's oceans experiencing simultaneous warming, salinity changes (increase or freshening), oxygen loss, and acidification.

The study standardized and integrated multiple key ocean variables to demonstrate that over the past 60 years, ocean conditions influenced by physical and biogeochemical variables (collectively known as complex climate influences) have become more pronounced globally. In particular, 30% to 40% of the surface ocean showed significant and alarming trends, with simultaneous changes in temperature, salinity, and oxygen levels compared to 60 years ago. The most dramatic complex changes occurred in the tropical and subtropical Atlantic, the North Pacific, the Arabian Sea, and the Mediterranean. This suggests that even the deep ocean, once thought to be stable, is responding more rapidly than previously thought.

Complex ocean changes are transforming marine ecosystems and threatening the organisms that depend on them. Marine organisms face greater stress when exposed to multiple stressors simultaneously, forcing them to migrate or decline. These disruptions could destabilize global fisheries, threaten food security, and impact human livelihoods. In addition to biodiversity, these changes could weaken the ocean's carbon and heat absorption capacity, potentially undermining its role as a global climate regulator. These findings highlight the urgent need for sustained, high-quality ocean monitoring to support global climate change countermeasures.

(Reference)
Tan, Z., von Schuckmann, K., Speich, S. et al. Observed large-scale and deep-reaching compound ocean state changes over the past 60 years. Nat. Clim. Chang. (2025).
https://www.nature.com/articles/s41558-025-02484-x

Contributor: IIYAMA Miyuki, Information Program