Environmental & Climate Sciences Department Seminar

Knowledge of the adjustment time of anthropogenic CO2, the e-folding time by which excess CO2 (above preindustrial) would decrease in the absence of anthropogenic emissions, is central to understanding the influence of anthropogenic CO2 on climate change and to prospective control of CO2 emissions to reach desired targets. Estimates of this adjustment time from current carbon-cycle models range from about 100 years to over 700 years. This talk examines the CO2 budget by a top-down, observationally based approach. Major stocks and fluxes are quantified. The net flux from the atmosphere and the ocean mixed layer, which are in near equilibrium, to the deep ocean and terrestrial biosphere is found to be proportional to the excess CO2 in these compartments throughout the Anthropocene. These observations, together with knowledge of the underlying physical and chemical processes, are used to develop a simple, transparent model that describes the transport of CO2 between major compartments — the atmosphere, the mixed-layer ocean, the deep ocean, and the terrestrial biosphere. This model compares well with observed atmospheric CO2 from 1750 to the present. The adjustment time of excess CO2, evaluated by multiple measures including the 1/e decay time and the negative inverse of the fractional annual transfer rate of excess CO2 into the terrestrial biosphere and the deep ocean, is found to be 54 ± 10 years. Such a short adjustment time, if correct, would mean that the atmospheric amount of CO2 would respond quickly and strongly to emission changes. For example, atmospheric CO2 could be immediately stabilized at its present value by decreasing anthropogenic emissions by about 50%.