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8.1: How Long Can We Wait to Act

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  • Carbon dioxide is a greenhouse gas of special concern because of its long residence time in the atmosphere. The left panel of Figure \(\PageIndex{1}\) shows estimates of the evolution of CO2 assuming that emissions abruptly stop when concentrations reach various values. Over the first 100 years or so, concentrations fall fairly rapidly, but then the rate of decay drops off and it will take many thousands of years for concentrations to return to preindustrial values. The right panel of Figure \(\PageIndex{1}\) shows projections of global mean temperature that correspond to the CO2 concentrations of the top panel. Curiously, the temperature hardly drops at all over the first thousand or so years after emissions cease, reflecting mostly the effects of heat storage in the oceans.

    Projections of CO2 and Temperature Assuming That Emissions Abruptly Stop After Certain CO2 Levels Are Reached

    Figure \(\PageIndex{1}\): Left: Evolution of atmospheric CO2 over time assuming that emissions abruptly cease when concentrations indicated by the numbers to the left of the curves are reached. Natural processes begin to relax concentrations back toward preindustrial values at the cessation of emissions. Right: Estimates of the evolution of global mean temperature (relative to its preindustrial value) corresponding to the CO2 concentrations in the top panel.Source: Solomon, S., G.-K. Plattner, R. Knutti, and P. Friedlingstein, 2009, PNAS 106: 1704–1709

    This is a crucial aspect of the challenge we face: absent technology for removing CO2 from the atmosphere, we will have to live with altered climate for many thousands of years. Thus we have a narrow time window within which to act.

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