Global emergence of compound climatic impact-drivers

Changes in health and sustainability of the ocean that provide goods and services for human well-being are closely linked to climate change. However, the ocean is exposed to a range of climatic impact-drivers (CIDs, e.g., temperature increase, sea level rise, oxygen depletion, acidification etc.) from global to local scale concurrently. These multiple CIDs make the ocean environment shifting from the normal condition, posing either positive or detrimental effects to ocean ecosystems. Therefore, detecting and understanding the combined effect of different CIDs (named compound CIDs in this study) is critical to further unravel diverse and adverse impacts on the ocean ecosystems.

In this study, we analyzed compound CIDs from changes in the upper 2000m of the ocean temperature (T), salinity (S), and dissolved oxygen (DO) from 1960 to 2022, as well as surface pH changes from 1985 to 2021 by using several observation-based gridded products.

We used a time of emergence (ToE) approach to investigate the long-term change of  the compound CIDs. First, to quantify the ToE of each CID, we investigated when and where the long-term change (signal) is significantly larger than the background variability (noise). The long-term change is quantified by the 20 years of low-pass filtering of global time series, and the background variability is quantified by the magnitude of annual-interannual variability of the local time series. Additionally, the uncertainty of ToE is defined by using an ensemble approach. With the ToE of individual CID available, we defined the regions where ToE of the compound CIDs can be detected if the change of more than one CID has already emerged. The results we obtained provide a new insight on the 3D changing ocean properties. They differ from previous studies that were limited to a subset of individual CID (e.g., sea level rise, chlorophyll-a, net primary production) or to the ocean surface only (e.g., SST).

The analysis shows that, before 2021, for the upper 2000m, ~15% (±5%) of areas of global ocean has experienced the concurrent emergence of three CIDs (triple emergence), and ~30% (±8%) of global ocean has experienced concurrent emergence of two CIDs (double emergence) mainly in the Atlantic and northern Indian Ocean. Analyses at different depths reveal that ToE is stronger and starts earlier in deep layers (200-1300m) than in the upper ocean (0-200m) where the signal-to-noise ratio is lower (which may due to the strong interplay of the long-term change with natural variability).

As marine ecosystems rely on an environment determined by multiple drivers (T/S/oxygen etc.), the investigation of the compound CIDs provides a more complete description (and quantification) of their long-term exposure to the CIDs (multiple stressors).