Inverse modeling of annual atmospheric CO2 sources and sinks 1. Method andcontrol inversion

A primary goal of developing the CO2 atmospheric measurement network is to better characterize the sources and sinks of atmospheric CO2. Atmospheric t ransport models can be used to interpret atmospheric measurements in terms of surface fluxes using inverse methodology, In this paper we present a thr ee-dimensional (3-D) inversion of CO2 measurements in order to infer annual sources and sinks of CO2 at a continental scale (continents and ocean basi ns) for a climatological year representing the 1985-1995 period. Solving th is inverse problem requires (1) a data space representing monthly CO2 measu rements, here at 77 sites (surface, ships, planes), (2) a flux space descri bing a priori fluxes between carbon reservoirs, and (3) a 3-D transport mod el linking the flux space to the data space. Knowledge of these three eleme nts, together with their associated errors, allows one to reduce the uncert ainties of the CO2 sources and sinks. In the 1985-1995 period, for our cont rol 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 conti nental 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, tr opics (except Asia) +0.3+/-0.9, tropical Asia +0.8+/-0.4, and Southern Hemi sphere -0.1+/-0.3. The inferred partition for the controversial Northern He misphere CO2 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) wit h 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 degrees S-50 degrees S oceans -0.9+/-0.3, and austral ocean -0.1+/-0.1.