7.3.5.2 Coupled Climate-Carbon Cycle Projections

The TAR reported two initial climate projections using AOGCMs with interactive carbon cycles. Both indicated positive feedback due largely to the impacts of climate warming on land carbon storage (Cox et al., 2000; Friedlingstein et al., 2001), but the magnitude of the feedback varied markedly between the models (Friedlingstein et al., 2003). Since the TAR a number of other climate modelling groups have completed climate-carbon cycle projections (Brovkin et al., 2004; Thompson et al., 2004; N. Zeng et al., 2004, Fung et al., 2005; Kawamiya et al., 2005; Matthews et al., 2005; Sitch et al., 2005) as part of C⁴MIP. The 11 models involved in C⁴MIP differ in the complexity of their components (Friedlingstein et al., 2006), including both Earth System Models of Intermediate Complexity and AOGCMs.

The models were forced by historical and Special Report on Emission Scenarios (SRES; IPCC, 2000) A2 anthropogenic CO₂ emissions for the 1850 to 2100 time period. Each modelling group carried out at least two simulations: one ‘coupled’ in which climate change affects the carbon cycle, and one ‘uncoupled’ in which atmospheric CO₂ increases do not influence climate (so that the carbon cycle experiences no CO₂-induced climate change). A comparison of the runs defines the climate-carbon cycle feedback, quantified by the feedback factor: F = ∆C_A^c / ∆C_A^u ,where ∆C_A^c is the change in CO₂ in the coupled run, and ∆C_A^u is the change in CO₂ in the uncoupled run. All of the eleven C⁴MIP models produce a positive climate-carbon cycle feedback, but with feedback factors varying from 1.04 (Model E) to 1.44 (Model A). This translates into an additional CO₂ concentration of between 20 and 224 ppm by 2100, with a mean of 87 ppm (Table 7.4).

All C⁴MIP models predict that an increasing fraction of total anthropogenic CO₂ emissions will remain airborne through the 21st century. Figure 7.13 shows the simulated partitioning of anthropogenic CO₂ for the entire simulation period to 2100 from each of the coupled models, and compares this with the partitioning simulated by the same models over the historical period to 1999. The dashed box shows observational constraints on the historical CO₂ partitioning, based on estimates of changes in ocean carbon storage (Sabine et al., 2004a) and total anthropogenic CO₂ emissions. The area of this box is largely due to uncertainties in the net land use emissions. The majority of the models sit within or very close to the historical constraints, but they differ in the magnitude of the changes projected for the 21st century. However, all models produce an increase in the fraction of total emissions that remain in the atmosphere, and most also indicate a decline in the fraction of emissions absorbed by the ocean (9 out of 11 models) and the land (10 out of 11 models). In the case of the oceanic uptake, this is largely a consequence of the reduced buffering capacity as CO₂ increases, and therefore also occurs in the uncoupled C⁴MIP models.

Figure 7.13. Predicted increase in the fraction of total emissions that add to atmospheric CO₂. Changes in the mean partitioning of emissions as simulated by the C⁴MIP models up to 2000 (black symbols) and for the entire simulation period to 2100 (red symbols). The letters represent the models as given in Table 7.4. The box shown by the dotted line is a constraint on the historical carbon balance based on records of atmospheric CO₂ increase, and estimates of total emissions (fossil fuel plus land use emissions) and the oceanic uptake of anthropogenic CO₂ (Sabine et al., 2004a). The black and red diamonds show the model-mean carbon partitioning for the historical period and the entire simulation period, respectively. The red line shows the mean tendency towards an increasing airborne fraction through the 21st century, which is common to all models.