Emissions Scenarios - IPCC

Scénario Driving Forces 149
anywhere from 14 to well over 700 EJ, between a sevenfold
decrease to an eightfold increase compared to 1990 levels.
Conversely, CH^ capture, either during mining or prior to
mining, not only reduces risk to miners but also provides a
valuable energy source. Thus, rising levels of CH^ capture for
non-climate reasons are likely to characterize the 2P' century.
This would in particular apply to high coal production
scenarios, in which most of the coal will need to come from
deep mining once the easily accessible surface mine deposits
have become exhausted. Growth in future emissions from coal
mining is therefore likely to be substantially lower than growth
in coal production.
Domestic and some industrial wastes contain organic matter
that emits a combination of CO, and CH_j on decomposition
(TEA Greenhouse Gases R&D Programme, 1996b). If oxygen
is present, most of the waste degrades by aerobic microorganisms
and the main product is CO^. If no oxygen is
present, different micro-organisms become active and a
mixture of CO^ and CH^ is produced. Decay by this
mechanism can take months or even years (US EPA, 1994).
Traditionally, waste has been dumped in open pits and this is
still the main practice in most developing countries. Thus,
oxygen is present and the main decay product is CO^. In recent
decades, health and local environmental concerns in developed
countries have resulted in better waste management, with lined
pits and a cap of clay, for example, added regularly over newer
dumps. This prevents fresh supplies of oxygen becoming
available so the subsequent decay process is anaerobic and CH^
is produced. Williams (1993) notes that landfill sites are
complex and highly variable biologic systems and many
factors can lead to a wide variability in CH^ production. For the
future, increasing wealth and urbanization in developing
countries may lead to more managed landfill sites and to more
CH4 production. However, the CH^ produced can be captured
and utilized as a valuable energy source, or at least flared for
pollution and safety reasons; indeed, this is a legal requirement
in the USA for large landfills. Future emissions are therefore
unlikely to evolve linearly with population growth and waste
generation, but the scenario literature is extremely sparse on
this subject - the major source remains the previous IS92
scenario series (Pepper et al., 1992).
Different methods are used to treat domestic sewage, some of
which involve anaerobic decomposition and the production of
СЩ. Again, capture and use of some of the CH^ produced
limits emissions. For the future, emissions will depend on the
extension of sewage treatment in developing countries, the
extent to which the techniques used enhance or limit CH^
production, and the extent to which the CH^ produced is
captured and used.
Several authors, including Rudd et al. (1993) and Fearnside
(1995) , note that some hydroelectric schemes resuh in
emissions of CH^ from decaying vegetation trapped by water
as the dams fill; these emissions climatically exceed those of a
thennopower plant delivering the same electricity. Rosa et al.
(1996) , Rosa and Schaffer (1994), and Gagnon and van de Vate
(1997) point out that the two schemes discussed by Rudd et al.
(1993) and Fearnside (1995) may be exceptional, with very
large reservoir surface areas, a high density of organic matter,
and low power output. Gagnon and van de Vate (1997) estimate
the combined CH^ and NjO emissions from hydroelectric
schemes at 5.5 gC equivalent per kWh compared to a range of
80 to 200 gC equivalent per kWh for a modern fossil power
station (Rogner and Khan, 1998); that is, hydroelectric power
emits less than 3% and 7%, respectively. While some GHG
emissions from new hydroelectric schemes are expected in the
future, especially in tropical settings (Galy-Lacaux et al.,
1999), in the absence of more comprehensive field data, such
schemes are regarded as a lower source of CH^ emissions
compared to those of other energy sector or agricultural
activities. Hydroelectric power is therefore not treated as a
separate emission category in SRES.
In summary, numerous factors could lead to increases in
emissions of CH^ in the future, primarily related to the
expansion of agricultural production and greater fossil fuel use.
Recent studies also identify a number of processes and trends
that could reduce CH^ emission factors and hence may lead to
reduced emissions in the future. These trends are not yet
sufficiently accounted for in the literature, in which CH^
emission factors typically are held constant. The overall
consequence is to introduce additional uncertainty into
projections, as the future evolution of such emission factors is
unclear. However, from the above discussion, the least likely
future is one of constant emission factors and the range of
future emissions is likely to be lower than those projected in
previous scenarios with comparable growth in primary activity
drivers.
3.6.4. Sulfur Dioxide
Two major sets of driving forces influence future SOj
emissions:
• Level and structure of energy supply and end-use, and
(to a lesser extent) levels of industrial output and
process mix.
• The degree of SOj-control policy intervention assumed
(i.e., level of environmental policies implemented to
limit SO2 emissions).
Grübler (1998c) reviewed the literature and empirical
evidence, and showed that both clusters of driving forces are
linked to the level of economic development. With increasing
affluence, energy use per capita rises and its structure changes
away from traditional solid fuels (coal, lignite, peat, fuelwood)
toward cleaner fuels (gas or electricity) at the point of end-use.
This structural shift combined with the greater emphasis on
urban air quality that accompanies rising incomes results in a
roughly inverted U (lU) pattern of SOj emissions and/or
concentrations. Emissions rise initially (with growing per
capita energy use), pass through a maximum, and decline at
higher income levels due to structural change in the end-use