Emissions Scenarios - IPCC

228 An Overview of Scenarios
• Largely stationary trend of global cropland areas (-39
million ha between 1990 and 2100, i.e., 3% of 1990
cropland areas).
• Decline in global forest cover by some 92 million ha.
• Increase of grasslands and biomass land-use of 188 and
552 million ha, respectively.
Land-use change patterns are more dynamic in the
intermediate time periods, and also display a wide variation
across different regions.
4.4.9.2. Harmonized and Other Al Scenarios
By and large, land-use changes in other Al scenarios show a
very wide range, being mostly model specific. As a rule, the Al
land-use change scenarios take an intermediate position
between the more extreme tendencies described by the
respective storylines of the A2 and В1 scenario families (see
Figure 4-12), which both result in large land-use changes
(albeit of an entirely different qualitative nature).
Corresponding scenario (model) differences are thus discussed
below for these two scenario families where they are largest.
4.4.9.3. A2 Scenarios
The ASF model used to produce the A2 marker scenario does
not generate estimates of the area covered by different
ecosystem types (e.g., forests, grasslands, etc.). However, the
ASF deforestation module estimates the area of different
tropical-forest types cleared annually for agricultural and other
puфoses (Lashof and Tiipak, 1990). This information is
subsequently used to estimate GHG emissions from
deforestation. The A2-ASF scenario also includes estimates of
natural carbon sinks and additional sinks attributable to reforestation
and afforestation activities. These estimates are
based on an extensive survey of available literature (Pepper et.
al., 1998).
By and large the land-use changes in the A2 marker scenario
reflect conventional wisdom - in a high-population and lowincome
world, natural land cover becomes progressively
depleted, which is only partially counterbalanced by re- and
afforestation activities. The size of the natural terrestrial carbon
For comparison, the corresponding productivity growth rates in the
other scenarios range from 1 % per year (B2-AIM) to 2% per year (B1 -
AIM),
consistent with their respective storylines. This emphasizes
productivity and efficiency (Bl) and more fragmented technology and
productivity growth (B2).
For A2-AIM, crop productivity growth rates
range between 1% and 1.5 % per year in the DEV and IND regions,
respectively; the difference is explained by only slowly closing
productivity gaps (approximated by GDP/capita), characteristic of this
scenario storyline. Similar differences also characterize other salient
scenario assumptions of importance to land-use changes (like biomass
yields, animal productivity, or the distribution of grain- versus rangefed
cattle). For instance, feed and protein yields from pasture land are
assumed to grow at 1.5 % per year and biomass yields at 0.5 % per
year in the Al scenario.
sink (including forests, grasslands, wetlands, and other
ecosystem types) in 1990 is estimated in ASF-A2 to amount to
1.8 GtC (1800 IVItC). By 2050 this sink reduces to 0.8 GtC and
by the end of the 2P' century terrestrial ecosystems become a
net source of carbon. On the other side of the natural carbon
balance is carbon sequestered by re- and afforestation. In A2-
ASF the respective (negative) carbon fluxes are estimated to
increase from 0.003 GtC in 1990, to 0.171 GtC by 2050, and
0.205 GtC by 2100.
4.4.9.4. Harmonized and Other A2 Scenarios
Out of the five A2 non-marker scenarios, A2-AIM and A2-
MiniCAM (and A2-A1-MINICAM) include explicit land-use
patterns. Table 4-18 illustrates that the major difference
between these quantifications is a higher energy biomass area
assumption in A2-AIM. Among other differences is a higher
percentage of cropland and grasslands in A2-MiniCAM and of
forestiand in A2-AIM.
4.4.9.5. Bl Scenarios
The most important indicators and assumptions made in the
IMAGE land-use model relate to agricultural yields and diets
that influence meat production, cattle population, and, in tum,
grasslands land cover. Cereal yields (to 2100) for REF, ASIA,
ALM, and the world average are assumed to increase by about
a factor of four, while cereal yields for OECD90 start from a
higher initial value and increase by only a factor of two. The
total cattle population in the IMAGE model includes dairy and
non-dairy cattle. Dairy cattle populations change as a result of
milk production per animal and the demand for milk. Nondairy
cattle populations change as a result of meat demand,
slaughter weight, and off-take rate. The number of slaughtered
animals (beef) increases in the period 1995 to 2060, the net
result of increasing animal productivity and increasing human
consumption of meat. The total number of cattie (dairy and
beef) shows a decreasing trend beyond the year 2000, with the
near-term increase in beef cattle more than offset by a decrease
in the number of dairy cattle. The forest area reflects the result
of increasing agricultural production and increasing
productivity. The major increase in demand for food in the
initial period 1995 to 2030 is nearly compensated by increasing
productivity, with a resultant slow decline in forest area, while
beyond 2030 forest areas start expand over abandoned
agricultural land. Overall, the net balance of land-use changes
in scenario Bl between 1990 and 2100 suggests a considerable
"greening" of the planet - a net increase in forest cover by
some 1260 million ha, a decrease in cropland of 390 million
ha, and an increase of about 200 million ha devoted to biomass
production. Grasslands decline by 1540 million ha, partly
because of the afforested areas mentioned above, but also
because they are converted to other land uses. Thus, while the
В1 scenario family is characterized by "high" rates of land-use
changes, the quality of these changes ("greening") is entirely
different compared to those of other scenario families (e.g.
A2).