Anticorrelation of Net Uptake of Atmospheric CO ₂ by the World Ocean and Terrestrial Biosphere in Current Carbon Cycle Models

The rate at which atmospheric carbon dioxide (CO ₂) would decrease in response to decrease of anthropogenic emissions or cessation (net zero emissions) is of great scientific and societal interest. Such decrease in atmospheric CO ₂ on the centennial scale would be due essentially entirely to transfer of carbon into the world ocean (WO) and the terrestrial biosphere (TB), which are sink compartments on this time scale. The rate of decrease of excess atmospheric CO ₂ and the apportionment of this decrease into the two sink compartments has been examined in two prior model intercomparison studies, subsequent either to a pulse emission of CO ₂ or to abrupt cessation of anthropogenic CO ₂ emissions. The present study examines and quantifies inter-model anticorrelation in those studies in the net rate and extent of uptake of CO ₂ into the two sink compartments. Specifically, in each study the time-dependent coefficients characterizing the net transfer rate into the two sink compartments, (evaluated as the net transfer rate normalized to excess atmospheric CO ₂ above pre-pulse amount, for the pulse experiment; or as the net transfer rate divided by excess atmospheric CO ₂ above preindustrial amount, for the abrupt cessation experiment) was found to exhibit strong anticorrelation across the participating models. That is, models for which the normalized rate of uptake into the WO was high exhibited low uptake rate into the TB, and vice versa. This anticorrelation in net transfer rate results in anticorrelation in net uptake extent into the two compartments that is substantially greater than would be expected simply from competition for excess CO ₂ between the two sink compartments. This anticorrelation, which is manifested in diminished inter-model diversity, can lead to artificially enhanced confidence in current understanding of the consequences of potential future reductions of CO ₂ emissions and in the global warming potentials of non-CO ₂ greenhouse gases relative to that of CO ₂.