Introduction: Summary of the Second Assessment Report and progress since this
This chapter reviews three scenario literatures: general mitigation scenarios
produced since the Second Assessment Report (SAR), narrative-based scenarios
found in the general futures literature, and mitigation scenarios based on the
new reference scenarios developed in the Intergovernmental Panel on Climate
Change (IPCC) Special Report on Emissions Scenarios (SRES).
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 into
Each mitigation scenario describes a future world with particular economic,
social, and environmental characteristics, and therefore implicitly or explicitly
contains information about development, equity, and sustainability (DES). Since
the difference between reference case scenarios and their corresponding mitigation
scenarios is simply the addition of deliberate climate policy, it can be the
case that the differences in emissions among reference case scenarios are greater
than between any one such scenario and its mitigation version.
General Greenhouse Gas Emissions Mitigation Scenarios
This chapter considers the results of 519 quantitative emission 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
These mitigation scenarios include concentration stabilization scenarios, emission
stabilization scenarios, tolerable windows/safe emission corridor scenarios,
and other mitigation scenarios. They all include energy-related carbon dioxide
(CO2) emissions; several also include CO2 emissions from
land-use changes and industrial processes and other important greenhouse gases
Mitigation 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 to reflect measures that are taken to implement
such options. Regional targets are introduced in the models with regional disaggregation.
Emission trading is introduced in more recent work. Some models employ 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
at 550ppmv (and their respective baseline scenarios) yielded several insights1.
There was 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; 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
show that emission trading lowers overall mitigation cost by shifting mitigation
to non-OECD countries, where abatement costs are assumed to be lower. 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 is due to the structure of the models, which do not
assume major technological breakthroughs. Conversely, baseline scenarios with
high carbon intensity reductions show larger carbon intensity reductions in
their corresponding mitigation scenarios. Global macroeconomic costs of mitigation
in the reviewed scenarios range from 0% to 3.5% of gross domestic product (GDP),
while a few simple models estimate more increase in the second half of the 21st
century. No clear relationship was discovered between the GDP loss and the GDP
growth assumptions in the baselines.
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 reduce emissions sufficiently to meet assumed mitigation targets;
and that methane (CH4) mitigation can be carried out more rapidly,
with a more immediate impact on the atmosphere, than CO2 mitigation.
In most cases it is clear that mitigation scenarios and mitigation policies
are strongly related to their baseline scenarios, but no systematic analysis
in this class of literature has been published on the relationship between mitigation
and baseline scenarios.
Global Futures Scenarios
Global futures scenarios do not specifically or uniquely consider GHG emissions.
Instead, they are more general stories of possible future worlds.
They can complement the more quantitative emission scenario assessment because
they consider dimensions that elude quantification, such as governance and social
structures and institutions, but which are nonetheless important to the success
of mitigation policies. Addressing these issues reflects the different perspectives
presented in Chapter 1 on cost-effectiveness, equity,
A survey of this literature has yielded a number of insights. First, a wide
range of future conditions has been identified by futurists, ranging from variants
of sustainable development to collapse of social, economic, and environmental
systems. Since the underlying socio-economic drivers of emissions may vary widely
in the future, it is important that climate policies should be designed so that
they are resilient against widely different future conditions.
Second, the global futures scenarios that show falling GHG emissions tend to
show improved governance, increased equity and political participation, reduced
conflict, and improved environmental quality. They also tend to show increased
energy efficiency, shifts to non-fossil energy sources, and/or shifts to a post-industrial
economy. Furthermore, population tends to stabilize at relatively low levels,
in many cases as a result of increased prosperity, expanded provision of family
planning, and improved rights and opportunities for women. A key implication
is that sustainable development policies can make a significant contribution
to emission reduction.
Third, different combinations of driving forces are consistent with low emission
scenarios. The implication of this would seem to be that it is important to
consider the linkage between climate policy and other policies and conditions
associated with the choice of future paths in a general sense.
Special Report on Emission Scenarios
Six new GHG emission reference scenario groups (not including specific climate
policy initiatives), organised into 4 scenario families, were developed
by the IPCC and published as the Special Report on Emission Scenarios (SRES).
Scenario families A1 and A2 emphasize economic development but differ with respect
to the degree of economic and social convergence; B1 and B2 emphasize sustainable
development but also differ in terms of degree of convergence. In all, six models
were used to generate 40 scenarios that comprise the six scenario groups. In
each group of scenarios, which should be considered equally sound, one illustrative
case was chosen to illustrate the whole set of scenarios. These six scenarios
include marker scenarios for each of the scenario families as well as two scenarios,
A1FI and A1T, which illustrate alternative energy technology developments in
the A1 world.
The SRES scenarios lead to the following findings:
- Alternative combinations of driving-force variables can lead to similar
levels and structure of energy use, land-use patterns and emissions.
- Important possibilities for further bifurcations in future development trends
exist within each scenario family.
- Emissions profiles are dynamic across the range of SRES scenarios. They
portray trend reversals and indicate possible emissions cross-over among different
- Describing potential future developments involves inherent ambiguities and
uncertainties. One and only one possible development path (as alluded to for
instance in concepts such as business-as-usual scenario) simply
does not exist. The multi-model approach increases the value of any scenario
set, since uncertainties in the choice of model input assumptions can be more
explicitly separated from the specific model behaviour and related modelling
Review of Post-SRES Mitigation Scenarios
Recognizing the importance of multiple baselines in evaluating mitigation strategies,
recent studies analyze and compare mitigation scenarios using as their baselines
the new SRES scenarios. This allows for the assessment in this report of 76
Post-SRES Mitigation Scenarios produced by nine modelling teams.
These mitigation scenarios were quantified on the basis of storylines for each
of the six SRES scenarios which describe the relationship between the kind of
future world and its capacity for mitigation.
Quantifications differ with respect to the baseline scenario including assumed
storyline, the stabilization target, and the model that was used. The post-SRES
scenarios cover a very wide range of emission trajectories but the range is
clearly below the SRES range. All scenarios show an increase in CO2
reduction over time. Energy reduction shows a much wider range than CO2
reduction, because in many scenarios a decoupling between energy use and carbon
emissions takes place as a result of a shift in primary energy sources.
In general, the lower the stabilization target and the higher the level of
baseline emissions, the larger the CO2 divergence from the baseline
that is needed, and the earlier that it must occur. The A1FI, A1B, and A2 worlds
require a wider range and more strongly implemented technology and/or policy
measures than A1T, B1, and B2. The 450 ppmv stabilization case requires very
rapid emission reduction over the next 20 to 30 years.
A key policy question is what kind of emission reductions in the medium term
(after the Kyoto protocol commitment period) would be needed. Analysis of the
post-SRES scenarios (most of which assume developing country emissions to be
below baselines by 2020) suggests that stabilization at 450ppmv will require
emissions reductions in Annex I countries after 2012
that go significantly beyond their Kyoto Protocol commitments. It also suggests
that it would not be necessary to go much beyond the Kyoto commitments for Annex
I countries by 2020 to achieve stabilization at 550ppmv or higher. However,
it should be recognized that several scenarios indicate the need for significant
Annex I emission reductions by 2020 and that none of
the scenarios introduces other constraints such as a limit to the rate of temperature
An important policy question already mentioned concerns the participation of
developing countries in emission mitigation. A preliminary finding of the post-SRES
scenario analysis is that, if it is assumed that the CO2 emission
reduction needed for stabilization would occur in Annex
I countries only, Annex I per capita CO2
emissions would fall below non-Annex I per capita emissions
during the 21st century in nearly all of the stabilization scenarios,
and before 2050 in two-thirds of the scenarios. This suggests that the stabilization
target and the baseline emission level are both important determinants of the
timing when developing countries emissions might need to diverge from
Climate policy would reduce per capita final energy consumption in the economy-emphasized
worlds (A1FI, A1B, and A2), but not in the environment-emphasized worlds (B1
and B2). The reduction in energy use caused by climate policies would be larger
in Annex I than in non-Annex I.
However, the impact of climate policies on equity in per capita final energy
use would be much smaller than that of the future development path.
No single measure will be sufficient for the timely development, adoption,
and diffusion of mitigation options to stabilize atmospheric GHGs. Instead,
a portfolio based on technological change, economic incentives, and institutional
frameworks could be adopted. Combined use of a broad array of known technological
options has a long-term potential which, in combination with associated socio-economic
and institutional changes, is sufficient to achieve stabilization of atmospheric
CO2 concentrations in the range of 450550ppmv or below.
Assumed mitigation options differ among scenarios and are strongly dependent
on the model structure. However, common features of mitigation scenarios include
large and continuous energy efficiency improvements and afforestation as well
as low-carbon energy, especially biomass, over the next one hundred years and
natural gas in the first half of the 21st century. Energy conservation
and reforestation are reasonable first steps, but innovative supply-side technologies
will eventually be required. Possible robust options 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. Solar energy
along with either nuclear energy or carbon removal and storage would become
increasingly important for a higher emission world or lower stabilization target.
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. Supply of biomass energy as well as biological
CO2 sequestration would broaden the available options for carbon
emission reductions, although the post-SRES scenarios show that they cannot
provide the bulk of the emission reductions required. That has to come from