How Long Does Anthropogenic CO2 Stay in the Atmosphere? Presenter: Stephen E. Schwartz (Brookhaven National Laboratory) Carbon dioxide is the most important anthropogenic greenhouse gas, emitted mainly from fossil fuel combustion and also from deforestation. Rational planning of emissions reductions requires understanding how long anthropogenic CO2 stays in the atmosphere (the turnover time) or, alternatively, how rapidly excess CO2 (above preindustrial) would be removed from the atmosphere in the absence of emissions, the adjustment time of excess CO2, quantified as the inverse of the fractional removal rate. However the lifetime of excess CO2 is quite uncertain, with present estimates ranging more than an order of magnitude, from about 70 to over 700 years. This talk examines the budget of CO2, considering emissions and the rates of transfer from the atmosphere to the ocean and the terrestrial biosphere, yielding transfer coefficients coupling the several reservoirs. The turnover time can be determined directly from observations, whereas determination of the adjustment time requires a numerical model. The mixing ratio and removal rate of atmospheric CO2 over the Anthropocene are accurately represented in a four-compartment model with three independent observationally determined parameters, two of which are universal geophysical constants and only one of which is CO2-specific. The analysis yields a lifetime of excess atmospheric CO2 of about 60 years, implying that major reductions in CO2 emissions would yield substantial reduction of atmospheric CO2 over a time as short as a human lifetime.