A joint atmosphere‐ocean inversion for the estimation of seasonal carbon sources and sinks

We have estimated global surface fluxes of carbon dioxide for the period 1992–1996 using an inverse approach that sequentially considers four constraints: (1) atmospheric CO 2 , (2) ocean interior DIC (dissolved inorganic carbon) interpreted through an ocean interior inversion and surface ocean p CO 2 (partial pressure of CO 2 ), (3) annual prior fluxes for selected land regions, and (4) atmospheric model selection based on vertical transport skill. Estimated fluxes are monthly resolved for each of the 22 Transcom regions over land and ocean. The ocean constraint is particularly valuable, as it does not only add prior information about air‐sea fluxes to the inversion problem but also preserves the regional variance‐covariance structure from the underlying ocean interior inversion. It allows to constrain annual oceanic uptake of 1.8 PgCyr −1 to within 0.2 PgCyr −1 , which implies a net annual land uptake of 1.3 (±0.3) PgCyr −1 . Furthermore, it leads to a pronounced asymmetry in the seasonal pattern of global land uptake, which was not seen in previous atmosphere‐only inversions. Tropical land is consistently estimated to be a source of carbon, though the source magnitude is reduced as more constraints are applied. With all four constraints, the inversion suggests a net tropical source of 1.1 (±0.9) PgCyr −1 , which is comparable to global estimates of deforestation rates in tropical forests and therefore implies an annually balanced tropical land biosphere flux. This balance is not found, however, at the regional level: For the Amazonian region and after accounting for deforestation, we find a biospheric source of 0.6 (±0.5) PgCyr −1 . This is at the upper range of estimates from bottom‐up methods, which tend to identify the region as a sink.