Exchanges of Atmospheric CO2 and 13CO2 with the Terrestrial Biosphere and Oceans from 1978 to 2000. IV. Critical Overview
CO2 are inferred by a three-dimensional tracer inversion model from simultaneous measurements of the concentration and 13C/12C isotopic ratio of atmospheric CO2. Terrestrial exchange of CO2 with the atmosphere range from −0.9 to +1.6 PgC yr−1 in the northern polar zone, −3.1 to +0.2 PgC yr−1 in the north temperate zone, −2.3 to +6.7 PgC yr−1 in the tropics, and −2.1 to +0.8 PgC yr−1 in the southern temperate zone. Oceanic exchange shows relatively little interannual variability, except in the tropics where a weak sink, up to −0.6 PgC yr−1 during El Nin˜o events in 1983, 1987, and 1998, alternated with a source, as great as 1.7 PgC yr−1. If isotopic data are disregarded as an indicator of interannual variability, inferred fluxes are altered only slightly in the polar and temperate zones, but substantially reduced in the tropical zone to a minimum sink of − 0.2 PgC yr−1 and a maximum source of 4.1 PgC yr−1. For three broader zones, divided at 30° N and S, average fluxes that we deduce from 1980-1989 and 1990-1996 agree within about 1 PgC yr−1 with decadal averages of 7 other inversion studies, except in the tropics where we find a larger source of CO2 than most other studies in the 1980’s and all other studies from 1990-1996. The interannual variability that we find exceeds that of most other studies.