7.3.5.4 Summary of Coupling Between the Carbon Cycle and Climate

7.3.5.4.1 Robust findings

Results from the coupled climate-carbon cycle models participating in the C⁴MIP project support the following statements:

• All C⁴MIP models project an increase in the airborne fraction of total anthropogenic CO₂ emissions through the 21st century.

• The CO₂ increase alone will lead to continued uptake by the land and the ocean, although the efficiency of this uptake will decrease through the carbonate buffering mechanism in the ocean, and through saturation of the land carbon sink.

• Climate change alone will tend to suppress both land and ocean carbon uptake, increasing the fraction of anthropogenic CO₂ emissions that remain airborne and producing a positive feedback to climate change. The magnitude of this feedback varies among the C⁴MIP models, ranging from a 4 to 44% increase in the rate of increase of CO₂, with a mean (± standard deviation) of 18 ± 11%.

7.3.5.4.2 Key uncertainties

The C⁴MIP models also exhibit uncertainties in the evolution of atmospheric CO₂ for a given anthropogenic emissions scenario. Figure 7.14 shows how uncertainties in the sensitivities of ocean and land carbon processes contribute to uncertainties in the fraction of emissions that remain in the atmosphere. The confidence limits were produced by spanning the range of sensitivities diagnosed from the 11 C⁴MIP models (Tables 7.4 and 7.5). In the absence of climate change effects (lowest three bars), models simulate increased uptake by ocean and land (primarily as a result of CO₂ enhancement of NPP), with a slight offset of the land uptake by enhancement of the specific heterotrophic respiration rate (see Section 7.3.5.3.2). However, there is a wide range of response to CO₂, even in the absence of climate change effects on the carbon cycle. Climate change increases the fraction of emissions that remain airborne by suppressing ocean uptake, enhancing soil respiration and reducing plant NPP. The sensitivity of NPP to climate change is especially uncertain because it depends on changing soil water availability, which varies significantly between General Circulation Models (GCMs), with some models suggesting major drying and reduced productivity in tropical ecosystems (Cox et al., 2004). The transient climate sensitivity to CO₂ is also a major contributor to the overall uncertainty in the climate-carbon cycle feedback (top bar).

Other potentially important climate-carbon cycle interactions were not included in these first generation C⁴MIP experiments. The ocean ecosystem models used in C⁴MIP are at an early stage of development. These models have simple representations of the biological fluxes, which include the fundamental response to changes in internal nutrients, temperature and light availability, but for most models do not include the more complex responses to changes in ecosystem structure. Changes in ecosystem structure can occur when specific organisms respond to surface warming, acidification, changes in nutrient ratios resulting from changes in external sources of nutrients (atmosphere or rivers) and changes in upper trophic levels (fisheries). Shifts in the structure of ocean ecosystems can influence the rate of CO₂ uptake by the ocean (Bopp et al., 2005).

Figure 7.14. Uncertainties in carbon cycle feedbacks estimated from analysis of the results from the C⁴MIP models. Each effect is given in terms of its impact on the mean airborne fraction over the simulation period (typically 1860 to 2100), with bars showing the uncertainty range based on the ranges of effective sensitivity parameters given in Tables 7.4 and 7.5. The lower three bars are the direct response to increasing atmospheric CO₂ (see Section 7.3.5 for details), the middle four bars show the impacts of climate change on the carbon cycle, and the top black bar shows the range of climate-carbon cycle feedbacks given by the C⁴MIP models.

The first-generation C⁴MIP models also currently exclude, by design, the effects of forest fires and prior land use change. Forest regrowth may account for a large part of the land carbon sink in some regions (e.g., Pacala et al., 2001; Schimel et al., 2001; Hurtt et al., 2002; Sitch et al., 2005), while combustion of vegetation and soil organic matter may be responsible for a significant fraction of the interannual variability in CO₂ (Cochrane, 2003; Nepstad et al., 2004; Kasischke et al., 2005; Randerson et al., 2005). Other important processes were excluded in part because modelling these processes is even less straightforward. Among these are N cycling on the land (which could enhance or suppress CO₂ uptake by plants) and the impacts of increasing ozone concentrations on plants (which could suppress CO₂ uptake).