Emissions Scenarios - IPCC

Emission Scenanos 255
the base-year emissions of ПОП-СО2 GHGs are subject to
considerable uncertainty, in particular when it comes to
regional and sectoral breakdowns.
investigation and analysis is required to understand these issues
more fully.
Emissions of ПОП-СО2 radiatively important gases are subject
to considerable and unresolved uncertainties and are driven by
a more complex set of forces than CO, emissions. Therefore,
the types of models employed for the SRES analyses are not
expected to produce unambiguous and widely approved
estimates of emissions of these gases for a period of over a
century. Despite the limited knowledge, at some point in time
causal relationships between driving forces and non-COj
emissions need to be crafted mto the models for the sake of
completeness. Even if new insights are generated by specialist
researchers m certain fields of environmental science, and
these become accepted as the mainstream view, their adoption
in the models is often far from straightforward, as appropriate
links to drivers may not be readily available in the underlying
structure. Limited manpower and resources imply that
priorities must be assigned when deciding on further model
development, and as a consequence the models lag behind
"common wisdom" in certain areas. Of course, this does not
necessarily limit their abilities to capture major trends at a
more aggregate level, the mam purpose of these models.
In the following sections emission trajectories generated in the
SRES scenarios are presented and discussed. However, model
structures and properties, and exogenous assumptions made by
the modelers involved, may give rise to systematic deviations
within scenario families that may prove very significant
compared to average inter-family differences. Further
5.4.1. Methane
In the IPCC WGI SAR, anthropogenic CH^ emissions in the
year 1990 were estimated at 375 ± 75 MtCH^ (Houghton et al.,
1996) and are shown in Table 5-3. These emissions arise from
a variety of activities, dominated by biogenic processes that are
often subject to considerable uncertainties (see Chapter 3).
CH4 emissions across the six models used to generate the
SRES scenarios for 1990 range between 298 and 337 MtCH^.
After standardization (see Box 5-1), the base-year emissions in
the SRES scenarios were set to 310 MtCH4, within the range
mentioned above. About one-quarter of the total emissions are
related to fossil fuel extraction (CH^ emissions from coal
mines, CH4 venting from oil extracrion), transport and
distribution (e.g., leakage from pipelines), and consumption
(incomplete combustion). The biogenic sources include
agricuUure (enteric fermentation, rice paddies, and animal
waste), biomass buming, and waste management (landfills,
sewage). Based on this source list, future CH^ emission
trajectories depend upon such variables as volumes of fossil
fuels used in the scenarios, regional demographic and affluence
developments, and assumptions on preferred diets and
agricultural practices.
Results from the 40 SRES scenarios indicate that uncertainties
surrounding future CH^ emission levels are likely to increase
over time (Figure 5-5). By 2050 the range across all scenarios
1200
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Figure 5-5: Standardized global СН^ emissions for SRES scenarios, classified into four scenario families (each denoted by a
dilïerent color code - AI, red; A2, brown; Bl, green; B2, blue). Marker scenarios are shown with thick lines without ticks,
globally hannonized scenarios with thin lines, and non-harmonized scenarios with thin, dotted lines (see Table 4-3). Black
lines show percentiles, means, and medians for the SRES scenarios. For numbers on the two additional illustrative scenarios
AlFI and AIT see Appendix VII.