Climate Change 2001:
Working Group I: The Scientific Basis
Other reports in this collection Stabilisation scenarios and their implications for futureCO2 emissions

Stabilisation scenarios illustrate implied rates of CO2 emission that would arrive at various stable CO2 concentration levels. These have been projected using a similar methodology to that applied in the analysis of emissions scenarios. The WRE trajectories follow CO2 concentrations consistent with the IS92a scenario beginning in 1990 and branch off to reach constant CO2 concentrations of 450, 550, 650, 750 and 1,000 ppm (Wigley et al., 1996). The rationale for various alternative time trajectories and stabilisation levels is discussed in Chapter 2 of the IPCC WGIII Third Assessment Report (Morita et al., 2001). Differences in emissions pathways for different time trajectories leading to a certain stabilisation target (e.g., S versus WRE profiles) are discussed in Schimel et al., (1997). Here, we have calculated emissions for one set of emission profiles to illustrate differences in implied emissions that arise from updating models since the SAR.

As in Section, the models were initialised up to present. Then anthropogenic emissions for the prescribed CO2 stabilization profiles were calculated; deduced emissions equal the change in modeled ocean and terrestrial carbon inventories plus the prescribed change in atmospheric CO2 content. To estimate the strength of carbon cycle-climate feedbacks, global temperature (ISAM) and changes in the fields of temperature, precipitation and cloud cover (Bern-CC) were projected from CO2 radiative forcing only, neglecting effects of other greenhouse gases and aerosols which are not specified in the WRE profiles. The results for the reference cases are not substantially different from those presented in the SAR (Figure 3.13). However, the range based on alternative model parametrizations is larger than presented in the SAR, mainly due to the range of simulated terrestrial CO2 uptake. CO2 stabilisation at 450, 650 or 1,000ppm would require global anthropogenic CO2 emissions to drop below 1990 levels, within a few decades, about a century, or about two centuries, respectively.

In all cases, once CO2 concentration becomes constant, the implied anthropogenic emission declines steadily. This result was expected. It highlights the fact that to maintain a constant future CO2 concentration, anthropogenic CO2 emissions would ultimately have to be reduced to the level of persistent natural sinks. Persistent terrestrial sinks are not well quantified; peatlands may be a candidate, but the gradual rise in atmospheric CO2 concentration during the present interglacial (Figure 3.2) argues against any such sink. Estimates of current uptake by peatlands are <0.1 PgC/yr (Clymo et al., 1998). Mixing of ocean DIC between surface and deep waters should continue to produce ocean uptake for several centuries after an input of anthropogenic atmospheric CO2 (Siegenthaler and Oeschger, 1978; Maier-Reimer and Hasselmann, 1987; Sarmiento et al., 1992). This mixing is the main reason for continued uptake (and therefore positive calculated emissions) after stabilisation. However, the main, known natural sink expected to persist longer than a few centuries is that due to dissolution of CaCO3 in ocean sediments, which increases ocean alkalinity and thereby allows additional CO2 to dissolve in the ocean. For CO2 concentrations about 1,000 ppm, this sink is estimated to be smaller than about -0.1 PgC/yr (Archer et al., 1998). Thus, for any significant CO2 emissions to persist over centuries without continuing to increase atmospheric CO2 would require some method of producing an artificial carbon sink.

Figure 3.13: Projected CO2 emissions leading to stabilisation of atmospheric CO2 concentrations at different final values. Panel (a) shows the assumed trajectories of CO2 concentration (WRE scenarios; Wigley et al., 1996) and panels (b) and (c) show the implied CO2 emissions, as projected with two fast carbon cycle models, Bern-CC and ISAM (see Box 3.7 and Figure 3.11). The ranges represent effects of different model parametrizations and assumptions as indicated in the text and in the caption to Figure 3.11. For each model, the upper and lower bounds (corresponding to low- and high-CO2 parametrizations, respectively) are indicated by the top and bottom of the shaded area. Alternatively, the lower bound (where hidden) is indicated by a dashed line.

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