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Lifetime of Excess Atmospheric Carbon Dioxide
  • Stephen Schwartz
Stephen Schwartz
Brookhaven National Laboratory

Corresponding Author:[email protected]

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The lifetime of excess atmospheric CO2 (above preindustrial) xCO2 governs the future consequences of present xCO2 and those of future CO2 emissions. Recent assessments of the decrease of xCO2 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 xCO2 remaining in the atmosphere 100 years after cessation, fCO2(100) = xCO2(100)/xCO2(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 xCO2 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; fCO2(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 τE(t), the equivalent 1/e lifetime of xCO2, as a function of time subsequent to cessation of emissions t, evaluated as τE(t) = -1/ln fCO2(t). τE(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 τE(t) of excess atmospheric CO2 about 100 years, much shorter than obtained with current CC models. Figure 1. a, Fractional excess CO2 fCO2(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.