Abstract

Using δ13C measurements in atmospheric CO2 from a cooperative global air sampling network, we determined the partitioning of the net uptake of CO2 between ocean and land as a function of latitude and time. The majority of δ13C measurements were made at the Institute of Arctic and Alpine Research (INSTAAR) of the University of Colorado. The network included 40 sites in 1992 and constitutes the most extensive data set available. We perform an inverse deconvolution of both CO2 and δ13C observations, using a two-dimensional model of atmospheric transport. New features of the method include a detailed calculation of the isotopic disequilibrium of the terrestrial biosphere from global runs of the CENTURY soil model. Also, the discrimination against 13C by plant photosynthesis, as a function of latitude and time, is calculated from global runs of the SiB biosphere model. Uncertainty due to the longitudinal structure of the data, which is not represented by the model, is studied through a bootstrap analysis by adding and omitting measurement sites. The resulting error estimates for our inferred sources and sinks are of the order of 1 GTC (1 GTC = 1015 gC). Such error bars do not reflect potential systematic errors arising from our estimates of the isotopic disequilibria between the atmosphere and the oceans and biosphere, which are estimated in a separate sensitivity analysis. With respect to global totals for 1992 we found that 3.1 GTC of carbon dissolved into the ocean and that 1.5 GTC were sequestered by land ecosystems. Northern hemisphere ocean gyres north of 15°N absorbed 2.7 GTC. The equatorial oceans between 10°S and 10°N were a net source to the atmosphere of 0.9 GTC. We obtained a sink of 1.6 GTC in southern ocean gyres south of 20°S, although the deconvolution is poorly constrained by sparse data coverage at high southern latitudes. The seasonal uptake of CO2 in northern gyres appears to be correlated with a bloom of phytoplankton in surface waters. On land, northern temperate and boreal ecosystems between 35°N and 65°N were found to be a major sink of CO2 in 1992, as large as 3.5 GTC. Northern tropical ecosystems (equator-30°N) appear to be a net source to the atmosphere of 2 GTC which could reflect biomass burning. A small sink, 0.3 GTC, was inferred for southern tropical ecosystems (30°S-equator).