Decadal climate modes and ocean carbon sink variability from 1960 to 2020

The ocean has absorbed approximately 30% of global carbon dioxide emissions since the pre-industrial era, thereby mitigating climate change. The uptake of atmospheric carbon depends on coupled changes in ocean circulation, atmospheric CO₂ forcing, and ocean biogeochemical processes, and exhibits pronounced variability on interannual to decadal timescales. However, the scarcity of in situ observations makes it difficult to robustly quantify the magnitude, temporal variability, and drivers of oceanic carbon uptake over the period of 1960-2020.

Using multiple observation-based data products together with numerical experiments based on the Community Earth System Model version 2 (CESM2), we examine the variability of air-sea carbon fluxes on interannual to decadal timescales. Consistent with previous studies, we find that externally forced variability driven by anthropogenic emissions, along with natural variability associated with climate modes such as the El Niño–Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO), are key contributors to carbon-flux variability over the past decades. Earth System Models including the CESM2 tend to underestimate the magnitude of this variability because they struggle to capture complex physical and biogeochemical processes as well as inter-decadal climate variability. Assimilating realistic anomalies in observed surface winds improves the representation of variability in the equatorial ocean, enhancing the model’s ability to reproduce observed changes in ocean carbon uptake since the 1960s. Additional assimilation of observed ocean temperature and salinity anomalies further improves extra-tropical variability, although model performance degrades in the equatorial regions.

Based on these assimilated simulations, we demonstrate how a large-scale climate mode in the Pacific led to a redistribution of both natural and anthropogenic dissolved inorganic carbon in the global ocean, accompanied by a slowdown of ocean carbon sink during the 1990s (the so-called carbon-sink hiatus). Finally, we discuss the inferred decadal variability of the land carbon sink, estimated by incorporating the newly constrained ocean carbon sink into the global carbon budget.