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