Inverse modeling of annual atmospheric CO2sources and sinks: 1. Method and control inversion

A primary goal of developing the CO 2 atmospheric measurement network is to better characterize the sources and sinks of atmospheric CO 2 . Atmospheric transport models can be used to interpret atmospheric measurements in terms of surface fluxes using inverse methodology. In this paper we present a three‐dimensional (3‐D) inversion of CO 2 measurements in order to infer annual sources and sinks of CO 2 at a continental scale (continents and ocean basins) for a climatological year representing the 1985–1995 period. Solving this inverse problem requires (1) a data space representing monthly CO 2 measurements, here at 77 sites (surface, ships, planes), (2) a flux space describing a priori fluxes between carbon reservoirs, and (3) a 3‐D transport model linking the flux space to the data space. Knowledge of these three elements, together with their associated errors, allows one to reduce the uncertainties of the CO 2 sources and sinks. In the 1985–1995 period, for our control inversion, the global continental sink is found to be 2.7±1.5 Gt C yr 1 for an optimized deforestation source of 1.4±0.6 Gt C yr 1 , yielding a net land uptake of 1.3±1.6 Gt C yr 1 (fossil fuel removed). The continental partition of this budget is (in units of Gt C yr 1 ): Arctic +0.2±0.3, North America −0.5±0.6, Europe −0.3±0.8, north Asia −1.5±0.7, tropics (except Asia) +0.3±0.9, tropical Asia +0.8±0.4, and Southern Hemisphere −0.1±0.3. The inferred partition for the controversial Northern Hemisphere CO 2 sink reveals that a major sink is located over the north Asia continent. For oceans we find a net global sink of 1.5±0.5 Gt C yr 1 with the following partition (in units of Gt C yr 1 ): North Pacific −0.3±0.2, North Atlantic −0.8±0.3, equator +0.6±0.2, 20°S–50°S oceans −0.9±0.3, and austral ocean −0.1±0.1.