Emissions Scenarios - IPCC

Scénario Driving Forces 147
Global scenaiios of CO2 emissions from deforestation have
their widest range around the middle of the 2P' century and
converge on zero toward the end of the century. The eventual
decrease in emissions computed by the scenarios results in part
from the assumed slowing of agricultural land expansion in
tropical regions. Another reason is some scenarios assume that
forests will nearly disappear in Asia and Africa before or
around the middle of the 2P' century.
Most global scenarios of CH^ emissions from rice cultivation
show an upward trend until the middle of the 2P' century and
then stabilize. The global trend is chiefly influenced by
estimates for Asia, where more than 80% of these emissions
currently originate. Normalized global emissions range by a
factor of three in the year 2100. The wide range has mostly to
do with different estimates of the future rice cropland area,
which is influenced largely by different assumptions about
future rice productivity.
All global scenarios of CH^ emissions from enteric
fermentation show an upward trend until the end of the 2P'
century. The maximum range of normalized emissions is by a
factor of 2.0 (which occurs in the year 2100), the smallest
range of the four categories of emissions examined. Also, these
emissions have the smallest range of current estimates in the
literature, and the smallest range of base-year estimates in the
scenarios. Most scenarios of emissions in industrial regions
show a stabilizing or decreasing trend, because of the
assumption that the number of livestock will continue to
decline with decreasing demand for beef and increasing animal
productivity. Meanwhile, the assumed economic development
in the developing regions will stimulate demand for beef,
which leads to an increase in livestock (despite improvements
in animal productivity) and higher emissions.
Most global scenarios show tliat N2O emissions from fertilized
soils continue to increase up to the end of the 2P' century, and
the range of estimates of normalized emissions in 2100
exceeds a factor of two.
Tliree of the four categories of emissions show increasing
global trends up to the end of the 21st century. The exception
is CO2 from deforestation (see above). Hence it is likely that
land-use emissions will continue to contribute significantly to
the build-up of GHGs in the atmosphere, especially to levels of
CH4 and N2O. Studies of mitigation of climate change should
take this into account and scenarios of land-use emissions
should be included in these studies.
Regarding regional scenarios, land-use emissions stabilize or
decrease in industrial regions, and increase substantially in
Africa, but less so in Asia. Emission trends in Latin America
are between those of industrial and developing regions. These
regional trends reflect the stabilizing demand for agricultural
products and agricultural land in industrial countries, and the
assumed continuation of agricultural development elsewhere.
3.6 Other Gas Emissions
3.6.1. Introduction
Driving forces of emissions other than CO2 or those of
agriculture or land-use changes are discussed here. The direct
GHGs N2O and CH^ are discussed first, followed by the
indirect GHGs, which include sulfur and the ozone precursors
NO J,, 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. Littie 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 nonclimate
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 landuse
changes and changes in agricultural output and practices
discussed in Section 3.5.2. Nevertheless, other sources ai'c 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 N,OAg
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 (spai'se) scenario
literature and concludes that future emissions will be
determined mostiy 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 N0^