<jats:p>Measurements show large decadal variability in the rate of <jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi mathvariant="normal">C</m:mi><m:mi mathvariant="normal">O</m:mi></m:mrow><m:mrow><m:mn>2</m:mn></m:mrow></m:msub></m:math></jats:inline-formula> accumulation in the atmosphere that is not driven by <jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi mathvariant="normal">C</m:mi><m:mi mathvariant="normal">O</m:mi></m:mrow><m:mrow><m:mn>2</m:mn></m:mrow></m:msub></m:math></jats:inline-formula> 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 <jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi mathvariant="normal">C</m:mi><m:mi mathvariant="normal">O</m:mi></m:mrow><m:mrow><m:mn>2</m:mn></m:mrow></m:msub></m:math></jats:inline-formula> due to the ocean and the terrestrial biosphere. In this study, we compare three independent methods for estimating oceanic <jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi mathvariant="normal">C</m:mi><m:mi mathvariant="normal">O</m:mi></m:mrow><m:mrow><m:mn>2</m:mn></m:mrow></m:msub></m:math></jats:inline-formula> uptake and find that the ocean carbon sink could be responsible for up to 40% of the observed decadal variability in atmospheric <jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi mathvariant="normal">C</m:mi><m:mi mathvariant="normal">O</m:mi></m:mrow><m:mrow><m:mn>2</m:mn></m:mrow></m:msub></m:math></jats:inline-formula> accumulation. Data-based estimates of the ocean carbon sink from <jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi mathvariant="normal">p</m:mi><m:mi mathvariant="normal">C</m:mi><m:mi mathvariant="normal">O</m:mi></m:mrow><m:mrow><m:mn>2</m:mn></m:mrow></m:msub></m:math></jats:inline-formula> mapping methods and decadal ocean inverse models generally agree on the magnitude and sign of decadal variability in the ocean <jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi mathvariant="normal">C</m:mi><m:mi mathvariant="normal">O</m:mi></m:mrow><m:mrow><m:mn>2</m:mn></m:mrow></m:msub></m:math></jats:inline-formula> sink at both global and regional scales. Simulations with ocean biogeochemical models confirm that climate variability drove the observed decadal trends in ocean <jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi mathvariant="normal">C</m:mi><m:mi mathvariant="normal">O</m:mi></m:mrow><m:mrow><m:mn>2</m:mn></m:mrow></m:msub></m:math></jats:inline-formula> uptake, but also demonstrate that the sensitivity of ocean <jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi mathvariant="normal">C</m:mi><m:mi mathvariant="normal">O</m:mi></m:mrow><m:mrow><m:mn>2</m:mn></m:mrow></m:msub></m:math></jats:inline-formula> uptake to climate variability may be too weak in models. Furthermore, all estimates point toward coherent decadal variability in the oceanic and terrestrial <jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi mathvariant="normal">C</m:mi><m:mi mathvariant="normal">O</m:mi></m:mrow><m:mrow><m:mn>2</m:mn></m:mrow></m:msub></m:math></jats:inline-formula> 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 <jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi mathvariant="normal">C</m:mi><m:mi mathvariant="normal">O</m:mi></m:mrow><m:mrow><m:mn>2</m:mn></m:mrow></m:msub></m:math></jats:inline-formula> uptake to climate variability and lead to improved climate projections and decadal climate predictions.</jats:p>