tion about the long-term evolution of climate. As discussed in Chapter 2, if the emissions are reduced to zero after some fixed time, the CO2 peaks at the time of cessation of emissions, and very gradually relaxes back to smaller values over the subsequent millennia. Over the first few centuries, the warming in any given year will fall short of the equilibrium value expected from the CO2 concentration prevailing in that year, but during the long, slow decline of CO2, the temperature has time to catch up to the equilibrium curve. This form of approach to equilibrium is illustrated in the simulation shown in Figure 3.2. On time scales longer than about a thousand years, the equilibrium sensitivity applied to the instantaneous CO2 value provides a good estimate of the warming.

The approach to equilibrium takes long enough that slow feedback processes can intervene and alter the long-term climate evolution. This will be taken up in Chapter 6, where it will be shown that the persistent warming computed on the basis of equilibrium climate sensitivity provides a valuable guide as to whether the human imprint on climate is likely to be

FIGURE 3.2 Comparison of equilibrium warming based on instantaneous CO2 values with actual modeled temperatures from Eby et al., 2009. The simulation shown is based on cumulative emission of 3840 Gt carbon in the form of CO2. Results are shown both for a pulse emission and for an exponentially increasing ramp lasting 350 years. Instantaneous equilibrium warming at any given moment in time is defined as the warming that would ultimately be reached in equilibrium if the CO2 prevailing at that moment were held fixed indefinitely. It provides an accurate estimate of the actual warming corresponding to the time varying CO2 when the CO2 concentrations are varying sufficiently slowly. In computing the instantaneous equilibrium warming, the climate sensitivity is held fixed at the value appropriate to the model used in the simulation. The equilibrium curves are proportional to the logarithm of the CO2 time series produced by the carbon cycle model used in this simulation. For this model ” T2X is approximately 3.5 C.

FIGURE 3.2 Comparison of equilibrium warming based on instantaneous CO2 values with actual modeled temperatures from Eby et al., 2009. The simulation shown is based on cumulative emission of 3840 Gt carbon in the form of CO2. Results are shown both for a pulse emission and for an exponentially increasing ramp lasting 350 years. “Instantaneous equilibrium warming” at any given moment in time is defined as the warming that would ultimately be reached in equilibrium if the CO2 prevailing at that moment were held fixed indefinitely. It provides an accurate estimate of the actual warming corresponding to the time varying CO2 when the CO2 concentrations are varying sufficiently slowly. In computing the instantaneous equilibrium warming, the climate sensitivity is held fixed at the value appropriate to the model used in the simulation. The equilibrium curves are proportional to the logarithm of the CO2 time series produced by the carbon cycle model used in this simulation. For this model Δ T2X is approximately 3.5 C.



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