The Gap and CO2 Stabilization

We have used the term "the gap" to refer to the difference between CO2 emissions associated with reference scenarios, with their attendant technological progress, and emissions along a path that stabilizes the concentration of atmospheric CO2. Historic emissions from 1850 through 1999 and the variety of SRES reference emissions paths are shown in Figure 4.4.

The Wigley, Richels, and Edmonds (1996, WRE) emissions paths (Figure 4.5) trace emissions trajectories that are consistent with stable atmospheric concentrations of CO2 at five alternative levels. These paths exhibit the peak and decline pattern associated with the limit on cumulative emissions for atmospheric CO2 concentrations. The higher the stabilization concentration, the later and higher is the peak in emissions. For most of the scenarios in the open literature, the dramatic advances in anticipated technology developments are insufficient to stabilize the concentration of atmospheric CO2.

The magnitudes of the "gaps" between WRE trajectories for stabilization of atmospheric CO2 concentrations at levels ranging from 350 ppm to 750 ppm are shown in Figure 4.6 and in Tables 4.1 and 4.2. The range of emissions reductions, relative to the

Figure 4.5. WRE CO2 emissions trajectories for five alternative CO2 concentrations.

Global Human CO2 Emissions: 1850-1999 WRE 350 WRE 450 WRE 550 'WRE 650 WRE 750

1875 1900 1925 1950 1975 2000 2025 2050 2075 Year reference case, that are required to attain a WRE concentration stabilization path is very wide (Figure 4.6). For some of the illustrative scenarios, events unfold in such a way as to stabilize the concentration of CO2 at approximately 550 ppm without the need for explicit policies to limit greenhouse gas emissions. By construction, these scenarios assume that sustainable development is a priority and that policy measures ensure that non-carbon-emitting technologies are developed and deployed in preference to fossil energy technology. For most of the SRES reference scenarios, such good outcomes are not anticipated. Most of the scenarios require additional emissions reductions to stabilize the atmospheric concentration of CO2.

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