Fossil fuel combustion, land use change and other human activities have
increased the atmospheric carbon dioxide (CO2) abundance by about 50%
since the beginning of the industrial age. The atmospheric CO2 growth
rates would have been much larger if natural sinks in the land biosphere
and ocean had not removed over half of this anthropogenic CO2. As these
CO2 emissions grew, uptake by the ocean increased in response to
increases in atmospheric CO2 partial pressure (pCO2). On land, gross
primary production (GPP) also increased, but the dynamics of other key
aspects of the land carbon cycle varied regionally. Over the past three
decades, CO2 uptake by intact tropical humid forests declined, but these
changes are offset by increased uptake across mid- and high-latitudes.
While there have been substantial improvements in our ability to study
the carbon cycle, measurement and modeling gaps still limit our
understanding of the processes driving its evolution. Continued
ship-based observations combined with expanded deployments of autonomous
platforms are needed to quantify ocean-atmosphere fluxes and interior
ocean carbon storage on policy-relevant spatial and temporal scales.
There is also an urgent need for more comprehensive measurements of
stocks, fluxes and atmospheric CO2 in humid tropical forests and across
the Arctic and boreal regions, which are experiencing rapid change.
Here, we review our understanding of the atmosphere, ocean, and land
carbon cycles and their interactions, identify emerging measurement and
modeling capabilities and gaps and the need for a sustainable,
operational framework to ensure a scientific basis for carbon