Emissions Scenarios - IPCC

252 Emission Scenarios
In addition to their cumulative emissions, the scenario families
are characterized by very different emissions trajectories. The
AI fossil fuel scenarios have continuously increasing
emissions, with rapid growth before 2050 and slower growth
thereafter. The AI scenarios with the "balanced" energy mix
(e.g. the marker AlB-AIM) typically have emissions that
decline after 2050, while the AIT technology scenarios have
slower growth prior to 2050, and a steeper decline after 2050.
As for the Al fossil fuel scenarios, A2 family scenarios are
characterized by high rates of growth in emissions prior to 2050
and subsequently continued growth but at lower rates. Unlike
the rest of the A2 scenarios, the A2-AI-MiniCAM has more
rapid growth in emissions after 2050 than before 2050, because
per capita incomes in a number of developing regions do not
reach a level at which per capita energy demands rise rapidly
until the middle of the 2Г' century. The Bl family is
characterized by lower growth in emissions prior to 2050,
mostly because of lower rates of growth in energy demand,
followed by declining emissions after 2050. Finally, the B2
family is characterized by relatively stable emissions post-2050,
after roughly doubling emissions between 1990 and 2050.
5.3.2. Land-Use Carbon Dioxide Emissions
Most changes in land use are induced by the demand for
cropland and grassland, which is driven by the demand for
food products, the extent of biomass energy use, and policies
and practices associated with forest management. The 1990
land-use COj emissions remain fairly uncertain, estimated at
1.6 ± 1.0 GtC (Watson et a!., 1996a); a similar level of
uncertainty is attached to current land-use emissions. This
uncertainty is reflected by the quantification of the SRES
storylines - the 1990 emission estimates from different models
range between 1.0 and 1.6 GtC, while the spread of estimates
at the four-region level is even larger. For the sake of
comparabifity, common, standardized (see Box 5-1 on
standardization) emissions are established at 1.1 GtC in 1990
and 1.0 GtC in 2000, to reflect the net carbon flux from
contemporary changes in forest cover.
Generally, the SRES models use different approaches to
estimate land-use change emissions - in some cases the only
source of emissions is tropical deforestation, while in other
cases more sources and sinks (including natural) are included.''
Moreover, méthodologie differences and uncertainties in
carbon content, carbon cycling, and land classification result in
seemingly inconsistent results between models that cover the
same land-use sources. These features complicate a
straightforward comparison between land-use emissions in
scenarios generated by different SRES models.
' The most comprehensive treatment is embedded in the IMAGE
model. For the sake of comparison the IMAGE team has made a
tentative estimate of those (net) emissions that are reasonably
consistent with what other models reported. These derived values are
used in this report.
Future trajectories of land-use COj emissions are shown in
Figure 5-4. Emissions in the 40 SRES scenarios range widely
in the same year and change significantly over time. In
scenarios with continued deforestation, emissions rise initially,
then reach a maximum, and finally decline with depletion of
forestland that can be cleared. At the other end of the scenario
spectrum, emissions turn negative and land-use changes
become an increasing CO2 sink through afforestation. By 2020,
the resultant uncertainty ranges between 0 and 3 GtC across all
40 SRES scenarios with a median of I.l GtC. By 2050, the
scenario range shifts to between -0.7 and 1.2 GtC (median, 0.5
GtC). By 2100 the scenario range lies between -2.8 and 2.2
GtC, with a median of 0.0 GtC. Interestingly, in specific years
(e.g., in 2050) scenarios from аИ four SRES families fall within
a relatively narrow emissions corridor (i.e. at least one scenario
from each of the four SRES scenario families falls within the
25"' and 75* percentiles of the emission range). This indicates
that similar levels of carbon fluxes related to land use could
arise from widely different socio-economic driving forces,
depending on future trends in food demand and dietary
pattems, agricultural productivity growth, forest practices, etc.
In general, the SRES COj land-use emissions follow the same
pattern as found in the literature (see Chapter 3) - initially
emissions increase because of continuing deforestation in
developing regions and subsequentiy they decrease following a
drop in population growth and increases in agricultural
productivity. The main difference between the SRES scenarios
and the literature reviewed in Chapter 3 lies in the maximum
emission values, which are significantly lower in the SRES
scenarios. Possibly this arises because the SRES models
explicitly simulate land-use change as a function of pressure on
the land (itself a function of agricultural productivity), while in
the literature land-use СО, emission scenarios are often based
on trend extrapolation or statistical relationships (e.g. with
population growth). The rapid economic development and
technological advances assumed in the SRES storylines thus
tend to mitigate CO, emissions from deforestation, and in
some cases lead to their reversal (e.g., tuming deforested lands
into carbon sinks).
As suggested by Figure 5-4, no simple relation exists between
the scenario families and land-use emissions. As in the case of
energy-related emissions, the AI family scenarios cover the
widest range of emissions and trajectories. In most cases,
emissions in these scenarios decline after 2030 to zero or
negative (carbon sinks) values. However, in two scenarios
(AIB-IMAGE and AlB-MARlA), emissions increase later in
the 2P' century, reflecting assumptions on additional land
pressures. As a rale, scenarios of the A2 family follow the same
convex trajectory as AI scenarios, also with two exceptions
(A2-IMAGE and A2-AIM). In accordance with the
sustainability emphasis of the ВI family storyline, scenarios
from this family have the lowest initial emissions, which by
2080 or earlier drop to zero or negative levels. The В1 family
marker scenario (BI-IMAGE) has negative land-use emissions
for most of the modeling period, a property directiy related to
Bl's "envhonmental conservation" emphasis.