The interactions between the atmosphere, the land biosphere and the ocean surface layer take place relatively rapidly, and are predicted to play a dominant role in the changes in atmospheric CO2 concentration over the 21st century (see Section 6.2.4). By contrast, the exchanges of CO2 with the deep ocean are much slower, taking place on timescales from centuries to millennia. Consider, for instance, a strongly idealised scenario in which CO2 emissions follow a pathway that would lead to a long-term stabilisation at a level of 750 ppm but, before reaching this level, the emissions were abruptly reduced to zero in 2100. The goal here is not to provide a realistic projection but to analyse the long-term changes in the system after all emissions cease. Figure 6.7 includes an estimate of the warming during the 21st century if the CO2 concentrations were stabilised at the 2000 level; here, Figure 6.16 shows the changes in CO2 and surface temperature which will still take place even if there are no additional emissions after 2100.
In all the models driven by this scenario, atmospheric CO2 concentration decreases after 2100. The deep ocean is not in equilibrium with the surface in 2100, and so carbon uptake by the deep ocean continues during the whole of the third millennium. Depending on the model, the concentrations reached by the year 3000 are between 400 and 500 ppm, i.e. much higher than the pre-industrial level.
Despite this decrease in the CO2 concentration, the global mean surface temperature is more or less stable during the third millennium, with the majority of models predicting only a slight cooling. The radiative forcing due to CO2 decreases after 2100 but the heat uptake by the ocean also decreases (see Section 4.1.4) as the ocean warms. The two effects nearly balance each other, leading to the simulated stabilisation of temperature.
|
The results displayed in Figure 6.16 mainly deal with the long-term adjustment between the ocean and the atmosphere. However, on long timescales, the changes in acidity caused by the oceanic uptake of CO2 induce dissolution of some of the CaCO3 in the sediments (carbonate compensation, see Section 4.3.1), modifying the ocean alkalinity and allowing an additional uptake of atmospheric CO2. Those processes are neglected in the models used in Figure 6.16. If they are included, the interaction with CaCO3 in the sediments increases the ability of the ocean to store CO2, producing a further reduction of the atmospheric concentration. However, this process is very slow and after 10,000 years, the atmospheric CO2 concentration is still predicted to be significantly higher than in pre-industrial times (Fig. 6.17). Even after several tens of thousands of years, the atmospheric CO2 will not return to pre-industrial levels through this mechanism. On even longer timescales, this will be achieved by the reactions of CO2 with some rocks, and in particular by the negative feedback caused by weathering (see Section 4.3.2). Because of this long term perturbation of the carbon cycle, the temperature remains significantly higher than in pre-industrial times during the whole period investigated in Figure 6.17, the amplitude of the temperature rise over several millennia being related to the release of carbon at the end of the second and the beginning of the third millennia.
This section illustrates that, because of the wide variety of processes involved, we cannot reliably estimate the timescale for the response of atmospheric CO2 concentration to fossil fuel burning, as we could for other anthropogenic forcings (Figure 4.2). To give an accurate representation of the time changes of atmospheric CO2 concentration, several different timescales, corresponding to the dominant mechanisms, are required
|