Figure 6-1: Stabilizing CO₂ concentrations would require substantial reductions of emissions below current levels and would slow the rate of warming.

1. CO₂ emissions: The time paths of CO₂ emissions that would lead to stabilization of the concentration of CO₂ in the atmosphere at 450, 550, 650, 750, and 1,000 ppm are estimated for the WRE stabilization profiles using carbon cycle models. Lower CO₂ concentration levels would require an earlier reversal of emissions growth and earlier decreases to levels below current emissions. The shaded area illustrates the range of uncertainty in estimating CO₂ emissions corresponding to specified concentration time paths, as represented in carbon cycle models. Also shown for comparison are CO₂ emissions for three of the SRES scenarios (A1B, A2, and B1), which do not include greenhouse gas emission limits.
2. CO₂ concentrations: The CO₂ concentrations specified for the WRE profiles gradually approach stabilized levels that range from 450 to 1,000 ppm. Also shown for comparison are estimates of CO₂ concentrations that would result from three of the SRES projections of emissions (A1B, A2, and B1).
3. Global mean temperature changes: Global mean temperature changes are estimated for the WRE stabilization profiles using a simple climate model tuned in turn to each of several more complex models.Estimated warming slows as growth in the atmospheric concentration of CO₂ slows and warming continues after the time at which the CO₂ concentration is stabilized (indicated by black spots) but at a much diminished rate. It is assumed that emissions of gases other than CO₂ follow the SRES A1B projection until the year 2100 and are constant thereafter.This scenario was chosen as it is in the middle of the range of the SRES scenarios. The dashed lines show the temperature changes projected for the S profiles, an alternate set of CO₂ stabilization profiles (not shown in panels (a) or (b)). The shaded area illustrates the effect of a range of climate sensitivity across the five stabilization cases.The colored bars on the righthand side show, for each WRE profile, the range at the year 2300 due to the different climate model tunings and the diamonds on the righthand side show the equilibrium (very long-term) warming for each stabilization level using average climate model results. Also shown for comparison are temperature increases in the year 2100 estimated for the SRES emission scenarios (indicated by red crosses).