Can we reconcile atmospheric estimates of the Northern terrestrial carbon sink with land-based accounting?

We estimate the northern hemisphere (NH) terrestrial carbon sink by comparing four recent atmospheric inversions with land-based C accounting data for six large northern regions. The mean NH terrestrial CO2 sink from the inversion models is 1.7 Pg C year(-1) over the period 2000-2004. The uncertainty of this estimate is based on the typical individual (1-sigma) precision of one inversion (0.9 Pg C year(-1)) and is consistent with the min-max range of the four inversion mean estimates (0.8 Pg C year(-1)). Inversions agree within their uncertainty for the distribution of the NH sink of CO2 in longitude, with Russia being the largest sink. The land-based accounting estimate of NH carbon sink is 1.7 Pg C year(-1) for the sum of the six regions studied. The 1-sigma uncertainty of the land-based estimate (0.3 Pg C year(-1)) is smaller than that of atmospheric inversions, but no independent land-based flux estimate is available to derive a 'between accounting model' uncertainty. Encouragingly, the top-down atmospheric and the bottom-up land-based methods converge to consistent mean estimates within their respective errors, increasing the confidence in the overall budget. These results also confirm the continued critical role of NH terrestrial ecosystems in slowing down the atmospheric accumulation of anthropogenic CO2.