2.5.2.5 Summary of Post-SRES Scenario Review

A new type of policy assessment has been conducted by the post-SRES activities, with nine modelling teams quantifying various simulation cases. Even though stabilization scenarios show a range among the models, several common trends and characteristics can be observed.

The different SRES baseline worlds require different technology and/or policy measures to stabilize at the same level. The A1F1, A1B, and A2 worlds require a wider range of stronger technology and/or policy measures than A1T, B1, and B2. For example, energy efficiency improvements in all sectors, the introduction of low-carbon energy, and afforestation would all be required in the A1F1, A1B, and A2 worlds in the first half of the 21^st century, with the additional introduction of advanced technologies in renewable energy and other energy sources in the second half of the 21^st century. The level of technology and/or policy measures in the beginning of this century would be significantly affected by the choice of development path over the next one hundred years. Higher emission worlds such as A1F1 and A2 require earlier reduction than low emission worlds such as A1T and B1.

The stabilization level chosen also significantly affects technology and/or policy measures and the timing of their introduction. More stringent stabilization targets require earlier emission reductions from baseline levels. The post-SRES scenario analysis suggests that stabilization at 450ppmv will require emissions reductions in Annex I countries that go significantly beyond the Kyoto Protocol commitments. It also suggests that maintaining emissions at the level of the Kyoto commitments may be adequate for achieving stabilization at 550ppmv or higher, although it should be recognized that several scenarios do indicate the need for significant emission reductions by 2020 in order to achieve these stabilization levels.

With respect to the important policy question of the role of developing countries in GHG emission mitigation, a preliminary finding of the post-SRES scenario analysis is that, assuming that the CO₂ emission reduction needed for stabilization occurs in Annex I countries only, per capita CO₂ emissions in Annex I countries would fall below per capita emissions in non-Annex I countries during the 21^st century except in some of A1T and B1 stabilization scenarios, and this occurs before 2050 in two-thirds of the scenarios. This suggests that, especially for more stringent stabilization targets and/or worlds with relatively high baseline emissions, there is a need for emissions to diverge from baseline levels in developing countries. The stabilization target and the baseline emission level were both important determinants of the timing when developing countries emissions might need to diverge from their baseline emissions.

No single measure will be sufficient for the timely development, adoption, and diffusion of mitigation options to stabilize atmospheric GHGs. Rather, a portfolio based on technological change, economic incentives, and institutional frameworks might be adopted. Large and continuous energy efficiency improvements and afforestation are common features of mitigation scenarios in all the different SRES worlds. Introduction of low-carbon energy is also a common feature of all scenarios, especially biomass energy introduction over the next one hundred years, as well as natural gas introduction in the first half of the 21^st century. Reductions in the carbon intensity of energy have a greater mitigation potential than reductions in the energy intensity of GDP in the latter half of the 21^st century, while energy intensity reduction is greater than carbon intensity reduction in the beginning of the century. This result appears to be robust across the storylines and stabilization levels, if drastic social changes are not assumed for energy efficiency improvement. In an A1B or A2 world, either nuclear power or carbon sequestration would become increasingly important for GHG concentration stabilization, the more so if stabilization targets are lower. Solar energy could play an important role in climate stabilization in the latter half of the 21^st century, especially for a higher emission baseline or lower stabilization levels.

Robust policy and/or technological options include technological efficiency improvements for energy supply and use, social efficiency improvements, renewable energy incentives, and the introduction of energy price incentives such as a carbon tax. Energy conservation and reforestation are reasonable first steps, but innovative supply-side technologies will eventually be required to achieve stabilization of atmospheric CO₂ concentration. Possibilities include using natural gas and combined-cycle technology to bridge the transition to more advanced fossil (fuel) and zero-carbon technologies such as hydrogen fuel cells. However, even with emissions control, some modellers found that energy systems would still be dependent on fossil fuels over the next century.

Integration between global climate policies and domestic air pollution abatement policies could effectively reduce GHG emissions in developing regions for the next two or three decades. However, control of sulphur emissions could amplify possible climate change, and partial trade-offs are likely to persist for environmental policies in the medium term.

Policies governing agriculture and land use and energy systems need to be linked for climate change mitigation. Failure to do this can lead to much larger than necessary costs. At tight levels of control, even some ability to acquire additional emissions capacity from land sequestration can have major cost-reducing impacts. Moreover, a high potential supply of biomass energy would ameliorate the burden of carbon emission reductions.

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