Figure 3.11: Projected CO2
concentrations resulting from the IS92a emissions scenario. For a strict comparison
with previous work, IS92a-based projections were made with two fast carbon cycle
models, Bern-CC and ISAM (see Box 3.7), based on CO2
changes only, and on CO2 changes plus land and ocean climate feedbacks.
Panel (a) shows the CO2 emisisons prescribed by IS92a; the panels (b)
and (c) show projected CO2 concentrations for the Bern-CC and ISAM
models, respectively. Results obtained for the SAR, using earlier versions of
the same models, are also shown. The model ranges for ISAM were obtained by tuning
the model to approximate the range of responses to CO2 and climate
shown by the models in Figure 3.10, combined with
a range of climate sensitivities from 1.5 to 4.5°C rise for a doubling of
CO2. This approach yields a lower bound on uncertainties in the carbon
cycle and climate. The model ranges for Bern-CC were obtained by combining different
bounding assumptions about the behaviour of the CO2 fertilisation effect,
the response of heterotrophic respiration to temperature and the turnover time
of the ocean, thus approaching an upper bound on uncertainties in the carbon cycle.
The effect of varying climate sensitivity from 1.5 to 4.5°C is shown separately
for Bern-CC. Both models adopted a “reference case” with mid-range behaviour
of the carbon cycle and climate sensitivity of 2.5°C.