Emissions Scenarios - IPCC

Emission Scénarios 241
5.1 Introduction
In this chapter emission estimates for radiatively important
gases generated in 40 Special Report on Emission Scenarios
(SRES) scenarios are present. These gases are carbon dioxide
(COj), methane (CH^), nitrous oxide (N2O), nitrogen oxides
(NOj,), carbon monoxide (CO), non-methane volatile organic
compounds (NMVOCs), sulfur dioxide (SO,), chlorofluorocarbons
(CFCs) and hydrochlorofluorocarbons (HCFCs),^
hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and
sulfur hexafluoride (SFg) (see Table 5-1). Emission estimates
presented here span the interval from 1990 to 2100 at the
global level and at the level of four SRES macro-regions
(OECD90, REF, ASIA, and ALM; see Appendix IV). In
addition, sulfur emission estimates are presented in the
regional gridded format to assist in quantifying the effects at
the local level. Links between emissions and the underlying
driving forces presented in Chapter 4 are illustrated also.
As a result of differences in modeling and estimation
approaches, base-year (1990) emission values in SRES
scenarios developed using different models show substantial
variation. To facilitate both use of the scenarios and
comparisons across scenarios and families, base-year
emissions in all 40 SRES scenarios were standardized using
one common set of values (see Box 5-1). With a few clearly
indicated exceptions, only standardized emission values are
discussed in this chapter. A complete set of standardized
emissions, along with other quantitative scenario information,
is provided in Appendix VII.
The first section in this chapter presents a "roadmap" that
serves as an orientation to the 40 SRES scenarios. The
roadmap gives a simple taxonomy that compares input
parameters, as represented by the four scenario families, and
emission outputs, as represented by the 1990 to 2100
cumulative CO^ emissions. The subsequent sections discuss in
detail emissions of each gas over the next 100 years to 2100.
5.2. Roadmap to the Scenarios
A classification scheme is presented here to assist the reader in
understanding the links between driving forces and scenario
outputs. This scheme can also be used to help select
appropriate scenarios for further analysis (see Chapter 6).
The SRES scenarios were developed as quantitative
interpretations of the four alternative storylines that represent
possible futures with different combinations of driving forces.
These broad scenario families are broken down further into
seven scenario groups,- used here to classify the input driving
forces (see also Table 4-20 in Chapter 4).
' Emission trajectories of these ozone-depleting substances (ODSs)
are not developed by the SRES team, but adopted from WMO/UNEP
(1998).
The scenario outputs of most interest are emissions of GHGs,
SOj, and other radiatively important gases. However, the
categorization of scenarios based on emissions of multiple
gases is quite difficult. All gases that contribute to radiative
forcing should be considered, but methods of combining gases
such as the use of global warming potentials (GWP) are
appropriate only for near-term GHG inventories.-' In addition,
emission trajectories may display different dynamics, from
monotonie increases to non-Unear trajectories in which a
subsequent decline from a maximum occurs. This particularly
diminishes the significance of a focus on any given year, such
as 2100. In light of these difficulties, the classification
approach presented here uses cumulative COj emissions
between 1990 and 2100. COj is the dominant GHG and
cumulative CO2 emissions are expected to be roughly
proportional to CO2 radiative forcing over the time scale
considered (Houghton et al. 1996).
Total cumulative COj emissions from the 40 SRES scenarios
fall into the range from 773 to 2538 gigatonnes of carbon (GtC)
with a median of 1509 GtC. To represent this range, the
scenario classification uses four intervals:
• Less than 1100 GtC (low).
• Between 1100 and 1450 GtC (medium-low).
• Between 1450 and 1800 GtC (medium-high).
• Greater than 1800 GtC (high).
Each CO2 interval contains multiple scenarios and scenarios
from more than one family. Each category also includes one of
the four marker scenarios. Figure 5-1 shows how cumulative
CO2 emissions from the 40 SRES scenarios fit within the
selected emission intervals.
Table 5-2 provides an overview of this scenario classification
and links the scenario outcomes with factors that drive them,
organized by family and scenario group (see also Table 4-20).
The rows in Table 5-2 represent the emission categories, while
the columns represent the scenario families. The analysis of
Table 5-2 reveals two key resuhs:
^ During the approval process of the Summary for Policymakers at the
5th Session of Working Group III (WGIII) of the IPCC from 8 to 11
March 2000 in Katmandu, Nepal, h was decided to combine the AlC
and AIG groups into one "fossil intensive" group AlFI in contrast to
the non-fossil group AIT, and to select two illustrative scenarios from
these two Al groups to facilitate use by modelers and policy makers.
This leads to six scenario gi-oups that constitute the four scenario
families, three of which are in the Al family. All scenarios are equally
sound.
In particular, the IPCC Working Group 1 (WGI) Second Assessment
Report (SAR) GWPs are calculated for constant concentrations. In
long-tei-m scenarios, concentrations may change significantly, as do
GWP values. It is unclear how to apply GWPs to long-tetm scenarios
in a meaningful manner. In addition, the GWP approach is not
applicable to gases such as SO^ and ozone precursors.