Earth's oceans are undergoing rapid, widespread transformations that extend far below their surface. A new study reveals that vast regions of the global ocean are experiencing compound state change: simultaneously warming, growing saltier or fresher, losing oxygen, and acidifying. These shifts are indicators that climate change is pushing marine environments into uncharted territory.

The study was published in Nature Climate Change on Nov.25.

A research team from the Institute of Atmospheric Physics of the Chinese Academy of Sciences, along with collaborators from Mercator Ocean International (MOI, France) and the Laboratoire de Météorologie Dynamique at the École Normale Supérieure (ENS–PSL, France), has developed an assessment and monitoring framework. The tool standardizes and integrates multiple "essential ocean variables" to pinpoint when and where the ocean is clearly affected by compound state change amid a warming climate.

Using this framework, the study documents a growing impact of these compound changes across much of the ocean's upper 1,000 meters, while identifying the most severely affected areas. The researchers found that 30% to 40% of the ocean's upper layers have already undergone significant shifts in at least two critical properties compared to 60 years ago.

"In some regions, up to a quarter of the ocean shows simultaneous changes in temperature, salinity, and oxygen—a striking and alarming trend," said Dr. TAN Zhetao, the study's lead author.

The most intense compound changes are occurring in the tropical and subtropical Atlantic, North Pacific, Arabian Sea, and Mediterranean Sea. Their combined impact is particularly concerning: while each variable affects marine life independently, their simultaneous alteration can push ecosystems beyond their adaptive limits.

"The ocean is experiencing strong, multidimensional compound change," said Prof. CHENG Lijing. "Ocean conditions are transforming across multiple dimensions at once, and even the deep ocean, once considered stable, is responding more rapidly than we anticipated."

The new framework enables scientists to identify when climate change signals surpass short-term variability. It also shifts the focus from analyzing individual variables in isolation to combining them into a multivariate composite index. This approach allows researchers to determine when the ocean has transitioned into a new state and how deeply these changes penetrate—critical insights for monitoring and mitigating climate-related risks to marine systems.

Compound ocean changes are reshaping marine ecosystems and threatening the communities that depend on them. "Marine species face heightened stress when exposed to multiple stressors simultaneously, which forces them to migrate or leads to population decline," noted Dr. Laurent Bopp from ENS–PSL. "This disruption can destabilize global fisheries, compromise food security, and jeopardize livelihoods."

Beyond biodiversity loss, these shifts may reduce the ocean's capacity to absorb carbon and heat, undermining its role as Earth's climate regulator.

See Compound Climatic Impact-drivers (CIDs) dynamic evolution since 1985: http://www.ocean.iap.ac.cn/pages/dataV/dataV.html?navAnchor=dataV .

Compound change in the global ocean. (Image by TAN Zhetao)