Fossil fuel combustion, land use change and other human activities have increased the atmospheric carbon dioxide (CO₂) abundance by almost 50% since the beginning of the industrial age. These changes would have been much larger if natural sinks in the land biosphere and ocean had not removed over half of this anthropogenic CO₂. Here, we review the current state of knowledge of the ocean, land and atmospheric carbon cycles, identify emerging measurement and modeling capabilities, and gaps that must be addressed to diagnose the processes driving the carbon cycle and predict their response to human activities and a changing climate. The anthropogenic CO₂ uptake by the ocean has increased over this period, as the atmospheric CO₂ partial pressure (pCO₂) has increased. For the land carbon cycle, the emerging picture is more complicated. Over the past three decades, the uptake of CO₂ by intact tropical humid forests appears to be declining, but these effects are offset by a net greening across mid- and high-latitudes associated with afforestation, agricultural, and longer growing seasons. These studies have also revealed measurement gaps and other limitations in our understanding of the evolving carbon cycle. They show that continued ship-based observations combined with expanded deployments of autonomous platforms are needed to quantify ocean-atmosphere fluxes on policy relevant spatial and temporal scales. They also reinforce the urgent need for more comprehensive measurements of stocks, fluxes and atmospheric CO₂ in humid tropical forests and across the Arctic and boreal regions, which appear to be experiencing rapid change.