The lifetime of excess atmospheric CO₂ (above preindustrial) x_CO2 governs the future consequences of present x_CO2 and those of future CO₂ emissions. Recent assessments of the decrease of x_CO2 following abrupt cessation of anthropogenic emissions (zero emissions commitment, ZEC) inferred from studies with carbon-cycle (CC) models (e.g., Joos et al., ACP, 2013; MacDougall et al., BG, 2020) vary substantially, with the fraction of x_CO2 remaining in the atmosphere 100 years after cessation, f_CO2(100) = x_CO2(100)/x_CO2(0), ranging from 0.55 to 0.85 (Figure 1a; Schwartz, JGR, 2018; Schwartz, in review). In this study prior atmospheric and oceanic CO₂ and future x_CO2 for ZEC were calculated with a 5-compartment global model. Model compartments are the atmosphere, upper and deep ocean, and labile and obdurate terrestrial biosphere (TB). Model parameters are obtained mainly from observation (e.g., rate of uptake of heat by the deep ocean) and theory (e.g., CO₂-dependent solubility of CO₂ in seawater); uptake of CO₂ by the two TB compartments is apportioned by parameterization, with parameters rather narrowly constrained by observations of CO₂ and radiocarbon. CO₂ is found to decay much more rapidly than in CC models; f_CO2(100) = 0.41 ± 0.8 (1 σ), Figure 1a. These results indicate that cessation of anthropogenic CO₂ emissions would result in discernible decrease in atmospheric CO₂ on a time scale as short as a human lifetime, much faster than in current CC models. Shown in Figure 1b is a quantity denoted τ_E(t), the equivalent 1/e lifetime of x_CO2, as a function of time subsequent to cessation of emissions t, evaluated as τ_E(t) = -1/ln f_CO2(t). τ_E(t) is a generalization of the relation between half-life of a decaying quantity and its 1/e lifetime and is an integral measure of decay over time t. The present model yields τ_E(t) of excess atmospheric CO₂ about 100 years, much shorter than obtained with current CC models. Figure 1. a, Fractional excess CO₂ f_CO2(t) as function of time t following abrupt cessation of anthropogenic CO₂ emissions as calculated in a recent model intercomparison (MacDougall et al., 2020) and with present model (best estimate, thick red, and uncertainty range); dotted black lines denote exponential decay with lifetime indicated at right. b, Equivalent 1/e lifetimes as function of t.