Working Group I: The Scientific Basis

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The Special Report on Emissions Scenarios (SRES) (IPCC, 2000b) produced a series of scenarios, of which six are used here, representing outcomes of distinct narratives of economic development and demographic and technological change. In ISAM model runs with these scenarios, past fossil emissions (see Section 3.4.1), CO2 concentrations (Enting et al., 1994; Keeling and Whorf, 2000) and mean global temperatures (Jones et al., 2000) were specified up to and including 1999; scenario-based analyses started in 2000. In the Bern-CC model runs, observed CO2 (Etheridge, et al., 1996, Keeling and Whorf, 2000) and past fossil emissions (Marland et al., 1999) were prescribed, and historical temperature changes were modelled, based on radiative forcing from greenhouse gases and aerosols; again, scenario-based analyses started in 2000. Past emissions from changing land use were calculated in order to balance the carbon budget.

The six scenarios lead to substantial differences in projected CO2 concentration trajectories (Figure 3.12). Significant uncertainties are introduced by the range of model parametrizations considered, so that the trajectories calculated for “adjacent” scenarios overlap, especially during the first half-century. The reference cases of the six scenarios account for a range of 2100 CO2 concentrations from 541 to 963 ppm in the Bern-CC model and 549 to 970 ppm in the ISAM model. The uncertainties around the 2100 values due to model parametrizations are -12 to +10 % (ISAM) and -14 to +31 % (Bern-CC).

These uncertainties reflect incomplete understanding of climate sensitivity and the carbon cycle. They substantially limit our current ability to make quantitative predictions about the future consequences of a given emissions trajectory. Nevertheless, the results show that higher emissions are always expected to lead to higher projected atmospheric concentrations. They also show that the range of emissions scenarios currently accepted as plausible leads to a range of CO2 concentrations that exceeds the likely upper bound of uncertainties due to differences among model parameterizations and assumptions.

Figure 3.12: Projected CO2 concentrations resulting from six SRES scenarios. The SRES scenarios represent the outcome of different assumptions about the future course of economic development, demography and technological change (see Appendix II). Panel (a) shows CO2 emissions for the selected scenarios and panels (b) and (c) show resulting CO2 concentrations as projected by 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, and each scenario the reference case is shown by a black line, the upper bound (high-CO2 parametrization) is indicated by the top of the coloured area, and the lower bound (low-CO2 parametrization) by the bottom of the coloured area or (where hidden) by a dashed coloured line.

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