What can be learned about carbon cycle climate feedbacks from CO₂ airborne fraction?

Abstract

Abstract. The ratio of CO2 accumulating in the atmosphere to the CO2 flux into the atmosphere due to human activity, the airborne fraction (AF), is central to predict changes in earth's surface temperature due to greenhouse gas induced warming. This ratio has remained remarkably constant in the past five decades, but recent studies have reported an apparent increasing trend and interpreted it as an indication for a decrease in the efficiency of the combined sinks by the ocean and terrestrial biosphere. We investigate here whether this interpretation is correct by analyzing the processes that control long-term trends and decadal-scale variations in AF. To this end, we use simplified linear models for describing the time evolution of an atmospheric CO2 perturbation. We find firstly that the spin-up time of the system for the AF to converge to a constant value is on the order of 200–300 years and differs depending on whether exponentially increasing fossil fuel emissions only or the sum of fossil fuel and land use emissions are used. We find secondly that the primary control on the decadal time-scale variations of the AF is variations in the relative growth rate of the total anthropogenic CO2 emissions. Changes in sink efficiencies tend to leave a smaller imprint. Before interpreting trends in the AF as indication of weakening carbon sink efficiency, it is therefore necessary to account for these trends and variations, which can be achieved based on a predictive equation for the AF implied by the simple models. Using atmospheric CO2 data and emission estimates for the period 1959 through 2006 we find that those controls on the AF, omissions in land use emissions and extrinsic forcing events can explain the observed trend, so that claims for a decreasing trend in the carbon sink efficiency over the last few decades are unsupported by atmospheric CO2 data and anthropogenic emissions estimates.