Uncertainties in climate stabilization

The atmospheric composition, temperature and sea level implications out to 2300 of new reference and cost-optimized stabilization emissions scenarios produced using three different Integrated Assessment (IA) models are described and assessed. Stabilization is defined in terms of radiative forcing targets for the sum of gases potentially controlled under the Kyoto Protocol. For the most stringent stabilization case (“Level 1” with CO₂ concentration stabilizing at about 450 ppm), peak CO₂ emissions occur close to today, implying (in the absence of a substantial CO₂ concentration overshoot) a need for immediate CO₂ emissions abatement if we wish to stabilize at this level. In the extended reference case, CO₂ stabilizes at about 1,000 ppm in 2200—but even to achieve this target requires large and rapid CO₂ emissions reductions over the twenty-second century. Future temperature changes for the Level 1 stabilization case differ noticeably between the IA models even when a common set of climate model parameters is used (largely a result of different assumptions for non-Kyoto gases). For the Level 1 stabilization case, there is a probability of approximately 50% that warming from pre-industrial times will be less than (or more than) 2°C. For one of the IA models, warming in the Level 1 case is actually greater out to 2040 than in the reference case due to the effect of decreasing SO₂ emissions that occur as a side effect of the policy-driven reduction in CO₂ emissions. This effect is less noticeable for the other stabilization cases, but still leads to policies having virtually no effect on global-mean temperatures out to around 2060. Sea level rise uncertainties are very large. For example, for the Level 1 stabilization case, increases range from 8 to 120 cm for changes over 2000 to 2300.