Accurate knowledge of the dependence of anthropogenic atmospheric CO2, the excess over preindustrial, on future emissions is essential to developing approaches to limit climate change. At present, the lifetime of excess CO2, as represented in current carbon cycle models, is uncertain by more than an order of magnitude, 70 to more than 700 years (Schwartz, JGR, 2018). Consequently observation-based top-down analysis provides an important alternative approach. The turnover time of excess CO2 (ratio of stock in the atmosphere and the mixed-layer ocean, which are in near equilibrium, to the net leaving flux into the terrestrial biosphere and deep ocean) is determined as 54 ± 10 years. A simple model for excess CO2, consisting of four compartments with three observationally determined global-mean parameters (deposition velocity of CO2 to the surface ocean, piston velocity describing the rate of exchange of water between the mixed-layer and deep ocean, and the transfer coefficient of CO2 kat from the atmosphere a to the terrestrial biosphere t), and one uncertain adjustable parameter kta, accurately reproduces CO2 mixing ratio over the Anthropocene. This model yields the adjustment time (inverse of fractional removal rate in the absence of emissions) as 65 ± 10 years over the first 100 years, depending on kta, over which time excess CO2 would decrease by 65 to 81%, depending on kta, Figure 1. The reduction of global emissions required to stabilize atmospheric CO2 over this time scale is 50 to 60%.