Biospheric CO2 emissions during the past 200 years reconstructed by deconvolution of ice core data

Abstract

Measurements on air trapped in old polar ice have revealed that the pre-industrial atmosphere contained 280 ppm of CO2 and that δ13C of atmospheric CO2 decreased by about 1.1”, until 1980. These measurements show that considerable amounts of non-fossil CO2 must have already been emitted into the atmosphere in the 19th century. Quantitative estimates of the emission rates were performed by deconvolving the CO2 and δ13C records, using models of the global carbon cycle (box-diffusion and outcrop-diffusion ocean, four-box biosphere). Depending on the structure of the ocean submodel, deconvolution of the CO2 record yields a cumulative non-fossil production of about 90 to 150 Gt C until 1980, of which more than 50% were released prior to 1900. According to the model results, the net non-fossil production rate was roughly constant in the 19th and the first part of the 20th century. In the past 30 years, smaller values are obtained (0-0.9 Gt C yr-1) which are at the lower limit or below current ecological estimates for deforestation and land use (1.6 ± 0.8 Gt C yr-l). The difference might possibly be due to other sinks, e.g., stimulation of plant productivity by the enhanced CO2 Concentration. Calculated 13C and 14C time histories agree well with the observed changes. While the change of the atmospheric CO2 concentration reflects more the cumulative carbon release, the isotope concentrations are more sensitive to short-term changes of the emission rate. The reason is that the oceanic uptake capacity is smaller for excess CO2 by the buffer factor of ~10 than for an isotopic perturbation.