Annual to decadal variations in air-sea fluxes of carbon dioxide (CO 2 ) impact the global carbon cycle and climate system, and previous studies suggest that these variations may be predictable in the near-term. Here, we quantify and understand the sources of near-term (annual to decadal) predictability and predictive skill in air -sea CO 2 flux on global and regional scales by analyzing output from a novel set of retrospective decadal forecasts of the Earth system. These initialized forecasts exhibit the potential to predict year-to-year variations in the globally-integrated air-sea CO 2 flux up to ∼7 years in advance. This 5 initialized predictability exceeds the predictability obtained solely from foreknowledge of variations in external forcing or a simple persistence forecast. The near-term CO 2 flux predictability is largely driven by predictability in the surface ocean partial pressure of CO 2 , which itself is a function of predictability in surface ocean dissolved inorganic carbon and alkalinity.Comparison with an observationally-based product suggests that the initialized forecasts exhibit moderate predictive skill in the tropics and subtropics, but low skill elsewhere. In the subantarctic Southern Ocean and northern North Atlantic, we find 10 long-lasting initialized predictability that beats that derived from uninitialized and persistence forecasts. Our results suggest that year-to-year variations in ocean carbon uptake may be predictable well in advance, and establish a precedent for forecasting air-sea CO 2 flux in the near future.
IntroductionObservations collected over the past few decades indicate that the ocean has absorbed 160 Pg of excess carbon from the at-15 mosphere since the beginning of the industrial revolution (Le Quéré et al., 2018); projections from climate models suggest that ∼540 Pg of excess carbon will reside in the ocean by the end of the century (under the RCP8.5 emission scenario;Ciais and Sabine, 2013). Accurate projections of past and future air-sea CO 2 flux are important for quantifying and understanding the changing global carbon cycle and for estimating future global climate change (Le Quéré et al., 2018).Superimposed on the background of long-term changes in ocean carbon uptake is substantial variability on global and 20 regional scales (McKinley et al., 2017). The recent literature highlights ocean carbon uptake variability that manifests on timescales of years to decades. Interannual variability in globally-integrated air-sea CO 2 flux has been estimated to have a standard deviation of 0.31 Pg C yr −1 and 0.2 Pg C yr −1 from observationally-based products and ocean biogeochemical models (Wanninkhof et al., 2013), respectively, which is on the order of 10% of the global-mean CO 2 flux (2.3 Pg C yr −1 ). A global extrapolation of sparse pCO 2 observations suggests that there is large variability on decadal 25