Observed large-scale and deep-reaching compound ocean state changes over the past 60 years

nature climate change Article https://doi.org/10.1038/s41558-025-02484-x Observed large-scale and deep-reaching compound ocean state changes over the past 60 years Received: 9 June 2025 Accepted: 10 October 2025 Published online: xx xx xxxx Check for updates Zhetao Tan 1,2, Karina von Schuckmann3, Sabrina Speich Jiang Zhu 4 & Lijing Cheng 1,5 , Laurent Bopp2, 2 Multiple climate-related stressors affect the ocean, including warming, acidification, deoxygenation and variations in salinity, with profound effects on Earth system cycles, marine ecosystems and human well-being. Nevertheless, a global perspective on the combined impacts of these changes on both surface and subsurface ocean conditions remains unclear. Here, applying a time-of-emergence methodology to observed physical and biogeochemical variables, collectively referred to as compound climatic impact-drivers, we show individual and compound ocean state changes have become increasingly prominent globally over the past 60 years. In particular, observations show the simultaneous emergence of compound climatic impact-drivers in regions spanning the subtropical and tropical Atlantic, the subtropical Pacific, the Arabian Sea and the Mediterranean Sea. We highlight extensive exposure of different ocean layers to compound emergence, characterized by significant intensity, duration and magnitude. These results provide a comprehensive framework and perspective to illustrate the ocean’s vulnerability to pervasive and interconnected changes in a warming climate. The ocean is vulnerable to a wide range of environmental stressors in a warming climate1,2, commonly referred to as ‘climatic impact-drivers’ (CIDs)3,4, which include phenomena such as surface and subsurface ocean warming, salinity variations, acidification, deoxygenation and other changes in relevant biogeochemical variables5. The evolving impacts of these CIDs on marine species, habitats and ecosystems, and the resulting biological responses6, pose prominent threats to the ocean’s overall health and resilience7. Previous studies have examined the emergence of persistent shifts in several individual CIDs in the context of increasing anthropogenic greenhouse gas (GHG) emissions8–11. Simultaneous changes in these CIDs potentially amplify persistent pressures on marine life. However, previous efforts have been limited to a subset of individual CIDs9,10,12, focused on compound extreme events13,14, limited their scope to sea surface conditions or specific ocean layers11,13,15,16, or relied exclusively on model-derived data11,16. There is thus an urgent need for a comprehensive global investigation of simultaneous changes based on direct observations in multiple CIDs, hereafter referred to as ‘compound CIDs’. Of particular relevance is which regions have already experienced substantial impacts from prolonged compound CIDs from the surface to the deep ocean. The temporal and spatial dynamics (that is, when and where), as well as the mechanisms underlying such changes (that is, how), are also poorly understood. Here we focus on the concurrent State key Laboratory of Earth System Numerical Modeling and Application, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China. 2Laboratoire de Météorologie Dynamique, Institut Pierre Simon Laplace, Ecole Normale Supérieure–Université PSL, CNRS, Ecole Polytechnique, Sorbonne Université, Paris, France. 3Mercator Ocean international, Toulouse, France. 4State Key Laboratory of Atmospheric Environment and Extreme Meteorology, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China. 5University of Chinese Academic of Sciences, Beijing, China. e-mail: 1 Nature Climate Change Article https://doi.org/10.1038/s41558-025-02484-x BOX 1 Definitions for long-term compound CIDs, their emergence and resulting exposure CIDs have been well defined and studied4. They can provide the climate change information of the physical science basis (IPCC Working Group I) with its impacts, adaptation and vulnerability (IPCC Working Group II), helping assess climate change risk across various sectors and spheres4. Following the IPCC Sixth Assessment Report, we define CIDs as “physical climate system conditions (for example, means, trends, extremes) that affect an element of society or ecosystems and their changes can be detrimental, beneficial, neutral or a mixture of each across interacting system elements and regions”89. However, a comprehensive definition of compound CIDs with respect to long-term changes remains elusive. In this assessment framework, ‘compound CID’ refers to multiple CIDs occurring simultaneously, which may have complex relationships and interactions, such as through joint relationship90,91, causal relationship92,93 and composite relationship94. They may exhibit complex interactions that can potentially affect the ocean by exacerbating or sometimes reducing the overall effects95, thus posing challenges to the ocean27,96,97 (see Fig. 4 for an example of the composite relationship to the multiple effects). Although similar terminologies have been used in previous climate change studies92,93,98, precise definitions are still lacking. In this framework, the joint relationship refers to the case where the change of two or more independent CIDs simultaneously influences the change of a dependent CID (for example, temperature change together with salinity change can lead to changes in ocean density and ocean stratification40). The causal relationship here refers to a change in one CID that causes a change in another CID (for example, warming can lead to deoxygenation due to reduced solubility35). The composite relationship refers to the combined effect of multiple causal and joint relationships. That is, changes in multiple CIDs may collectively affect the ocean and, in turn, induce potential positive or negative feedback on a CID. For example, the joint relationship between ocean warming and salinization and the causal relationship between warming and deoxygenation may indicate a ToE and emergence metrics 2 itu gn Ma ) nd tre ( de |SNR| O2 Duration (triple) Noise Single emergence Baseline: 1960–1989 0 1960 1970 1980 1990 Year b Low (short) Normalization probability Double Triple emergence emergence Medium High (long) 2000 2010 2020 c High exposure Medium exposure Low exposure High Medium Median Zero Median + 1 median absolute deviation Low Low Medium Low Duration/intensity/magnitude (normalized) Intensity 1 Intensity (SNR) Mag Medium nitud e High High n io at r Du Box Fig. 1 | The definition and assessment of CIDs for long-term change using the ToE since 1960. a, Definitions of single, double and triple emergence of individual or compound CIDs for temperature (black), salinity (red) and dissolved oxygen (blue). The signal emergence metrics (magnitude, intensity and duration) are defined (see Methods for detail). b, Normalized probability d (...truncated)