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Abstract

The ocean and land absorb anthropogenic CO2 from industrial fossil-fuel emissions and land-use changes, helping to buffer climate change. Here, we compare decadal variability of ocean CO2 uptake using three independent methods and find that the ocean could be responsible for as much as 40% of the observed decadal variability of CO2 accumulation in the atmosphere. The remaining variability is due to variability in the accumulation of carbon in the terrestrial biosphere. Models capture these variations, but not as strongly as the observations, implying that CO2 uptake by the land and ocean is more sensitive to climate variability than currently thought. Models must capture this sensitivity to provide accurate climate predictions.Measurements show large decadal variability in the rate of CO2 accumulation in the atmosphere that is not driven by CO2 emissions. The decade of the 1990s experienced enhanced carbon accumulation in the atmosphere relative to emissions, while in the 2000s, the atmospheric growth rate slowed, even though emissions grew rapidly. These variations are driven by natural sources and sinks of CO2 due to the ocean and the terrestrial biosphere. In this study, we compare three independent methods for estimating oceanic CO2 uptake and find that the ocean carbon sink could be responsible for up to 40% of the observed decadal variability in atmospheric CO2 accumulation. Data-based estimates of the ocean carbon sink from pCO2 mapping methods and decadal ocean inverse models generally agree on the magnitude and sign of decadal variability in the ocean CO2 sink at both global and regional scales. Simulations with ocean biogeochemical models confirm that climate variability drove the observed decadal trends in ocean CO2 uptake, but also demonstrate that the sensitivity of ocean CO2 uptake to climate variability may be too weak in models. Furthermore, all estimates point toward coherent decadal variability in the oceanic and terrestrial CO2 sinks, and this variability is not well-matched by current global vegetation models. Reconciling these differences will help to constrain the sensitivity of oceanic and terrestrial CO2 uptake to climate variability and lead to improved climate projections and decadal climate predictions.