Some of the main findings regarding different categories of land-use emissions
are outline below.
Global scenarios of CO2 emissions from deforestation have their widest
range around the middle of the 21st 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 21st century.
Most global scenarios of CH4 emissions from rice cultivation show an
upward trend until the middle of the 21st 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 CH4 emissions from enteric fermentation show
an upward trend until the end of the 21st 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 that N2O emissions from fertilized soils continue
to increase up to the end of the 21st century, and the range of estimates of
normalized emissions in 2100 exceeds a factor of two.
Three 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.