Stabilization at about 500 ppm requires holding the current emissions of 7 Gt C y"1 constant for the next 50 years, rather than letting them rise to around 14 Gt C y“!, which is roughly the rate expected if we allow the situation to follow the current trend. Assuming the same
constant fraction of close to a half is absorbed by the Earth during this period, as has been the case over the last 50 years, then we would estimate the level of CO2 to rise to around 475 ppm. After 2055, Pacala and Socolov assume that the amount of CO2 would fall linearly until there is no net gain per year after 2105. In our simple model the effect of this would be that the level of C02 would stabilize at around 525 ppm.
A fall in the amount emitted each year would be possible if by 2055 we had developed carbon capture and sequestration, expanded the use of nuclear power, and were using a much greater fraction of renewable energy sources, in particular, solar and wind. By 2055 it is just feasible that the first commercial fusion reactors could have been built, which could be used to supply both power and hydrogen.
Pacala and Socolov propose stabilizing emissions at 7 Gt С у"1 by means of seven different methods, each of which is already established. Each method is capable of reducing emissions by 1 Gt C y“l by 2055 and they are illustrated in Fig. 11.4. Each method grows linearly with time, producing a wedge. Pacala and Socolov identified reductions through energy efficiency and conservation, the reduction and capture of carbon emissions, the use of more carbon-free and carbon-neutral energy, and the enhancement of biological storage.