3.6 Other Gas Emissions
Driving forces of emissions other than CO2 or those of agriculture or land-use
changes are discussed here. The direct GHGs N2O and CH4 are discussed first,
followed by the indirect GHGs, which include sulfur and the ozone precursors
NOx, CO, and volatile organic compounds (VOCs). Finally, the many various powerful
GHGs, including ozone-depleting substances (ODS), are discussed.
The sources and sinks for these gases continue to be highly uncertain. Little
research has been carried out to evaluate the influences of socio-economic and
technological driving forces on long-term emission trends of these gases. As
a rule, future emissions of these gases are included in long-term emission models
on the basis of simple relationships to aggregate economic or sector-specific
activity drivers, not least because individual source strengths continue to
be highly uncertain. Notable exceptions are emissions of sulfur and ODS, which
have been more intensively studied in connection with non-climate policy analysis
in the domains of regional acidification and stratospheric ozone depletion.
3.6.2. Nitrous Oxide
Natural and agricultural soils are the dominant sources of N2O emissions, so
future emission levels are governed by the land-use changes and changes in agricultural
output and practices discussed in Section 3.5.2. Nevertheless,
other sources are also important and are discussed here.
The dominant industrial sources are the production of HNO3 and adipic acid.
The key driver for the production of HNO3 is the demand for fertilizer. Hence
this emission source is closely related to the agricultural production driving
forces discussed in Section 3.5, as well as to improvements
in production technologies. Adipic acid, (CH2) 4 (COOH)2 , is a feedstock for
nylon production and one of the largest-volume synthetic chemicals produced
in the world each year - current annual global production is 1.8 million metric
tons (Stevens III, 1993). Production has an associated by-product of 0.3 kg
N2O/kg adipic acid for unabated emission, which at present results in a global
emission of about 0.4 MtN as N2O annually. Emissions mostly arise in the OECD
countries, which accounted for some 95% of global adipic acid production in
1990 (Davis and Kemp, 1991). Fenhann (2000) reviews the (sparse) scenario literature
and concludes that future emissions will be determined mostly by two variables
- demand growth as a result of growth in economic activity and progressively
phased-in emission controls.
By the early 1990s, it was estimated that about one-third of OECD emissions
had been abated (Stevens III, 1993). This abatement is an accidental result
of the treatment of flue-gases in a reductive furnace (thermal destruction)
to reduce NOx emissions, which coincidentally also converts about 99% of the
N2O into nitrogen gas (N2). In other regions only about 20% of emissions had
been abated by the early 1990s.
Major adipic acid producers worldwide have agreed to substantially reduce N2O
emissions by 1996 to 1998. In July 1991 they formed an inter-industry group
to share information on old and new technologies developed for N2O abatement,
such as improved thermal destruction, conversion into nitric oxide for recycling,
and the promising low-temperature N2O catalytic decomposition into N2 currently
being developed by DuPont. The introduction of all three technologies could
result in a 99% reduction of N2O emissions from adipic acid production (Storey,
1996). They are expected to be introduced at plants owned by Asahi (Japan),
BASF and Bayer (Germany), DuPont (US), and Rhône-Poulenc (France) (Chemical
Week, 1994). After the planned changes, US producers will have abated over
90% of the N2O emissions from adipic acid production. In recent years nylon-6.6
production dropped in the US, Western Europe, and Japan, largely in response
to capacity and production in other Asian countries. By 2000 production is expected
to recover in these countries (Storey, 1996).
Another major source of N2O is the transport sector. Gasoline vehicles without
catalytic converters have very low, sometimes immeasurably small, emissions
of N2O. However, vehicles equipped with three-way catalytic converters have
N2O emissions that range from 0.01 to 0.1 g/km in new catalysts, and from 0.16
to 0.22 g/km in aging catalysts (IPCC, 1996). Emission levels also depend on
precise engine running conditions. At the upper end of the emission range from
aging catalysts, N2O emissions contribute around 25% of the in-use global warming
impact of driving (Michaelis et al., 1996).
The introduction of catalytic converters as a pollution control measure in
the majority of industrialized countries is resulting in a substantial increase
in N2O emissions from gasoline vehicles. Several Annex I countries include projections
of N2O from this source in their national communications to the UNFCCC, using
a variety of projection methods (for example, Environment Canada, 1997; UNFCCC,
1997; VROM, 1997). The projections from these counties differ substantially
in the contribution that transport is expected to make to their national N2O
emissions in 2020, ranging from about 10% in France to over 25% in Canada. They
anticipate that mitigation measures will be much more effective in reducing
industrial and agricultural emissions of N2O than mobile source emissions. Indeed,
little research has been carried out to identify catalytic converter technologies
that result in lower N2O emissions. However, emissions are likely to be lower
in countries that require regular emission inspections and replacement of faulty
pollution control equipment.
Agricultural and land-use change emission drivers are discussed in Section
3.5.2. The other major sources are from the use of fossil fuels and the
disposal of waste, for which the driving forces are briefly reviewed here. The
earlier literature is reviewed in Barnes and Edmonds (1990). A more detailed
recent literature review is given in Gregory (1998).
Emissions from the extraction, processing, and use of fossil fuels will be
driven by future fossil fuel use. CH4 emissions from venting during oil and
gas production may decrease because of efforts to reduce them (IGU, 1997b).
Flaring and venting volumes from oil and gas operations peaked in 1976 to 1978,
but a gradual reduction in volumes of gas flared and vented has occurred over
the past 20 years (Boden et al., 1994, Marland et al., 1998; Stern
and Kaufmann, 1998). Shell International Ltd. (1998) estimated a reduction in
its own emissions from venting by 1 MtCH4 per year to 0.367 MtCH4 in the five
years to 1997. The IEA Greenhouse Gases R&D Programme (1997) notes that emission
reductions from the oil and gas sector would yield a high economic return. Additionally,
new natural gas developments generally use the latest technology and are almost
leak free compared to older systems. Taking all these factors into account,
it seems plausible that CH4 emissions from the oil and gas sector should fall
as the 21st century progresses. Nonetheless, the primary driver (oil and gas
production) is likely to expand significantly in the future, depending on resource
availability and technological change. A representative range from the literature,
for example the scenarios described in Nakicenovic et al. (1998a),
indicates substantial uncertainty in which future levels of oil and gas production
could range between 130 and some 900 EJ. Assuming a constant emission factor,
future CH4 emissions from oil and gas could range from a decline compared to
current levels to a fourfold increase. With the more likely assumption of declining
emission factors, future emission levels would be somewhat lower than suggested
by this range.
The concentrations of CH4 in coal seams are low close to the surface, and hence
emissions from surface mining are also low (IEA CIAB, 1992). Concentrations
at a few hundred meters or deeper can be more significant; releases from these
depths are normally associated with underground mining. Emissions per ton of
coal mined can vary widely both from country to country and at adjacent mines
within a country (IEA Greenhouse Gases R&D Programme, 1996a). CH4 mixed with
air in the right proportions is an explosive mixture and a danger to miners.
Measures to capture and drain the CH4 are common in many countries - the captured
CH4, if of adequate concentration, can be a valuable energy source. The techniques
currently used reduce total emissions by about 10%. Many older, deeper coal
mines in Europe are being closed, which will reduce emissions. Replacement coal
mines tend to be in exporting countries with low cost reserves near the surface,
so the emissions will be low. For the future, emissions will depend principally
on the proportion of coal production from deep mines and on total coal production.
A representative range of future coal production scenarios given in Nakicenovic
et al. (1998a) indicates a very wide range of uncertainty. Future coal
production levels could range anywhere from 14 to well over 700 EJ, between
a sevenfold decrease to an eightfold increase compared to 1990 levels. Conversely,
CH4 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
CH4 capture for non-climate reasons are likely to characterize the 21st 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 CO2 and CH4 on decomposition (IEA Greenhouse Gases R&D Programme, 1996b).
If oxygen is present, most of the waste degrades by aerobic micro-organisms
and the main product is CO2. If no oxygen is present, different micro-organisms
become active and a mixture of CO2 and CH4 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 CO2. 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 CH4 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 CH4 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 CH4
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 CH4. Again, capture and use of
some of the CH4 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 CH4 production, and the extent to
which the CH4 produced is captured and used.
Several authors, including Rudd et al. (1993) and Fearnside (1995), note that
some hydroelectric schemes result in emissions of CH4 from decaying vegetation
trapped by water as the dams fill; these emissions climatically exceed those
of a thermopower plant delivering the same electricity. Rosa et al. (1996),
Rosa and Schäffer (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 CH4 and
N2O 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 CH4 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 CH4 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 CH4 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 CH4 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.