2 Greenhouse Gas Emissions Scenarios
A long-term view of a multiplicity of future possibilities is required to consider
the ultimate risks of climate change, assess critical interactions with other
aspects of human and environmental systems, and guide policy responses. Scenarios
offer a structured means of organizing information and gleaning insight on the
Each mitigation scenario describes a particular future world, with particular
economic, social, and environmental characteristics, and they therefore implicitly
or explicitly contain information about DES. Since the difference between reference
case scenarios and stabilization and mitigation scenarios is simply the addition
of deliberate climate policy, it can be the case that the differences in emissions
among different reference case scenarios are greater than those between any
one such scenario and its stabilization or mitigation version.
This section presents an overview of three scenario literatures: general mitigation
scenarios produced since the SAR, narrative-based scenarios found in the general
futures literature, and mitigation scenarios based on the new reference scenarios
developed in the IPCC SRES.
2.2 Greenhouse Gas Emissions Mitigation Scenarios
This report considers the results of 519 quantitative emissions scenarios from
188 sources, mainly produced after 1990. The review focuses on 126 mitigation
scenarios that cover global emissions and have a time horizon encompassing the
coming century. Technological improvement is a critical element in all the general
Based on the type of mitigation, the scenarios fall into four categories: concentration
stabilization scenarios, emission stabilization scenarios, safe emission corridor
scenarios, and other mitigation scenarios. All the reviewed scenarios include
energy-related carbon dioxide (CO2) emissions; several also include
CO2 emissions from land-use changes and industrial processes, and
other important GHGs.
Policy options used in the reviewed mitigation scenarios take into account
energy systems, industrial processes, and land use, and depend on the underlying
model structure. Most of the scenarios introduce simple carbon taxes or constraints
on emissions or concentration levels. Regional targets are introduced in the
models with regional disaggregation. Emission permit trading is introduced in
more recent work. Some models employ policies of supply-side technology introduction,
while others emphasize efficient demand-side technology.
Allocation of emission reduction among regions is a contentious issue. Only
some studies, particularly recent ones, make explicit assumptions about such
allocations in their scenarios. Some studies offer global emission trading as
a mechanism to reduce mitigation costs.
Technological improvement is a critical element in all the general mitigation
Detailed analysis of the characteristics of 31 scenarios for stabilization
of CO2 concentrations at 550 ppmv4
(and their baseline scenarios) yielded several insights:
- There is a wide range in baselines, reflecting a diversity of assumptions,
mainly with respect to economic growth and low-carbon energy supply. High
economic growth scenarios tend to assume high levels of progress in the efficiency
of end-use technologies; however, carbon intensity reductions were found to
be largely independent of economic growth assumptions. The range of future
trends shows greater divergence in scenarios that focus on developing countries
than in scenarios that look at developed nations. There is little consensus
with respect to future directions in developing regions.
- The reviewed 550ppmv stabilization scenarios vary with respect to reduction
time paths and the distribution of emission reductions among regions. Some
scenarios suggested that emission trading may lower the overall mitigation
cost, and could lead to more mitigation in the non-OECD countries. The range
of assumed mitigation policies is very wide. In general, scenarios in which
there is an assumed adoption of high-efficiency measures in the baseline show
less scope for further introduction of efficiency measures in the mitigation
scenarios. In part this results from model input assumptions, which do not
assume major technological breakthroughs. Conversely, baseline scenarios with
high carbon intensity reductions show larger carbon intensity reductions in
their mitigation scenarios.
Only a small set of studies has reported on scenarios for mitigating non-CO2
gases. This literature suggests that small reductions of GHG emissions can be
accomplished at lower cost by including non-CO2 gases; that both
CO2 and non-CO2 emissions would have to be controlled
in order to slow the increase of atmospheric temperature sufficiently to achieve
climate targets assumed in the studies; and that methane (CH4) mitigation
can be carried out more rapidly, with a more immediate impact on the atmosphere,
than CO2 mitigation.
Generally, it is clear that mitigation scenarios and mitigation policies are
strongly related to their baseline scenarios, but no systematic analysis has
been published on the relationship between mitigation and baseline scenarios.