centuries (see Section 2.2). In a framework of cumulative carbon emissions, CO2 concentrations do not necessarily “stabilize” but rather change over time in response to a given CO2 emissions scenarios; in this case, it is the total cumulative carbon emitted over time, rather than the atmospheric CO2 concentration itself, that indicates the level of expected climate warming.

The clear advantage of the cumulative carbon framework is that a given level of cumulative emissions corresponds to a unique temperature change, which remains approximately constant for several centuries after the point of zero emissions (Matthews et al., 2008; Solomon et al., 2009). As can be seen in Figure 3.7, for this particular model, cumulative emissions of 1,000 GtC from 1750 to 2100 result in a year-2100 global temperature change of 1.8ºC over pre-industrial temperatures, which corresponds to a year-2100 CO2 concentration of 460 ppm; both the year-2100 temperature change and the year-2100 CO2 concentration are independent of the shape of the CO2 emissions scenario and depend only on the total cumulative carbon emitted. By contrast, the rate of temperature change, as well as the peak CO2 concentration in these simulations, varied as a results of differences in the rates of increase and decline of emissions in each scenario.

Figure 3.8 illustrates how the concept of CO2 stabilization can be reconciled with the cumulative emissions framework. This figure shows idealized CO2 concentration scenarios that reach between 350 and 1,000 ppm at the year 2100, along with the cumulative carbon emissions and temperature changes associated with each scenario.4 From this analysis, restricting global temperature change to 2ºC requires best-guess cumulative emissions of 1,150 billion tons of carbon between the years 1800 and 2100; this corresponds to a “stabilization” CO2 concentration of between 450 and 550 ppm at the year 2100, with the caveats that CO2 concentrations changes and warming after the year 2100 would depend on the level of additional post-2100 emissions. In general, at a given time (e.g., the year 2100) both the atmospheric CO2 concentration and the associated temperature change can be inferred from cumulative carbon emissions to date. If carbon emissions were subsequently eliminated, atmospheric concentrations would slowly decrease over time, whereas temperature would remain elevated for several


The temperature responses to cumulative emissions shown in Figure 3.8 include both the carbon cycle and climate sensitivity to emissions, but do not correspond directly to either the transient climate response or the equilibrium climate sensitivity associated with a given CO2 concentration. In general, for higher emissions scenarios where forcing is still increasing rapidly at 2100, this temperature change will more closely reflect the transient climate response. For lower emissions scenarios with stable or declining forcing during the latter half of the 21st century, the temperature change at 2100 will more closely reflect the equilibrium climate sensitivity.

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