A significant challenge in understanding, and therefore modeling, the response of terrestrial carbon cycling to climate and environmental drivers is that vegetation varies on spatial scales of order a few kilometers whereas Earth system models (ESMs) are run with characteristic length scales of order 100 km. Atmospheric CO2 provides a constraint on carbon fluxes at spatial scales compatible with the resolution of ESMs due to the fact that atmospheric mixing renders a single site representative of fluxes within a large spatial footprint. The variations in atmospheric CO2 at both seasonal and interannual timescales largely reflect terrestrial influence.

I discuss the use of atmospheric CO2 observations to benchmark model carbon fluxes over a range of spatial scales. I also discuss how simple models can be used to test functional relationships between the CO2 growth rate and climate variations. In particular, I show how atmospheric CO2 provides constraints on ecosystem sensitivity to climate drivers in the tropics, where tropical forests and semi-arid ecosystems are thought to account for much of the variability in the contemporary carbon sink.