3.7.3.4 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 3.7.3.2, 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.

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