Stabilisationof carbon dioxideconcentrations 255sinks. The main known such sink is due to the dissolution of calcium
carbonate from the oceans into ocean sediments that, for high levels of
carbon dioxide concentration, is probably less than 0.1 Gt per year.^15
In the work presented in Figure 10.1, many different pathways to
stabilisation could have been chosen. The particular emission profiles
illustrated in Figure 10.1 begin by following the current average rate of
increase of emissions and then provide a smooth transition to the time of
stabilisation. To a first approximation, the stabilised concentration level
depends more on the accumulated amount of carbon emitted up to the
time of stabilisation than on the exact concentration path followed en
route to stabilisation. This means that alternative pathways that might
assume higher emissions in earlier years would require steeper reduc-
tions in later years. Table 10.3 lists the accumulated emissions for the
period 2001–2100 for the different stabilisationprofiles and also those
for the SRES scenarios. It shows that if the atmospheric concentration of
carbon dioxide is to remain below about 500 ppm, the future global an-
nual emissions averaged over the twenty-firstcentury cannot exceed the
current level of global annual emissions. Figure 10.1(c) shows the pro-
jected global mean surface temperature response to the carbon dioxide
concentration profiles shown in Figure 10.1(a).
The main results shown in Table 10.3 do not include the effect of
climate feedbacks on the carbon cycle (see box in Chapter 3 on page 40).
Two of the feedbacks are important in the context of the consideration
of stabilisation scenarios; namely, increased respiration from the soil as
the temperature rises and die-back especially from forests as the climate
changes. As we saw in Chapter 3, the effect of these feedbacks could
lead to the biosphere becoming a substantial source of carbon dioxide
during the twenty-first century. The size of that source will depend on
the amount of climate change. To take it into account, the accumulated
amount of that source has to be subtracted from the figures in Table 10.3
to arrive at the emissions from fossil fuel burning that would lead to
different stabilisation levels. Some of the estimates of what would need
to be subtracted are large – for instance, for the 450 ppm and 550 ppm
stabilisation scenarios, they are as large as 200 Gt and 300 Gt respectively
during the twenty-first century.^16 In which case, if these estimates are
confirmed, emissions scenarios that are aiming at 450 ppm stabilisation
(but not allowing for the feedbacks), when the feedbacks are included,
would in fact achieve around 550 ppm, and aiming at 550 ppm would in
fact achieve around 750 ppm.
It is instructive also to look at annual emissions of carbon dioxide
expressed per capita. Averaged over the world in 2000 they were just
over one tonne (t) (as carbon) per capita but they varied very much from