Emissions Scenarios - IPCC

An Overview of the Scenario Literature 85
Box 2-1: Range of Land Use COj Emissions in the Database
About 23% of the current total anthropogenic COj emissions arise from land-use change (Pepper et al., 1992), which makes
it an hnportant drivmg force. Direct comparison of absolute levels of land-use CO^ emissions between scenarios in the
database is difficult because of variations in how models depict deforestation and in how modelers classify anthropogenic and
natural land-use fluxes. In addition, models are based on different base-year data, which further complicates comparison. For
example, the 1990 base-year emissions estimates from land-use change (e.g., deforestation) range from 0.6 to 1.4 gigatons of
elemental carbon (GtC). However, by indexing emissions to 1990, it is possible to make more meaningful comparisons of
trends among the 26 scenarios in the SRES database that report land-use CO^ emissions. It is important to note tiiat only three
modeling groups produced the 26 scenarios described here. Clearly, emissions from land-use change have not been as well
explored by the modeling community as energy-related emissions.
All 26 scenarios show a decrease in COj emissions from land-use change over time and are below current levels by 2100; some
models even report emissions reductions below zero, which suggests CO^ sequestration (e.g., through afforestation). The
emissions range is very wide during the next few decades, but narrows considerably around mid-century. For example, there is
more than a factor ten difference (after normalizing for base-year differences) between the highest and lowest scenarios in 2020.
(For reference, the IS92a scenario falls slightly below the median of the range.) By 2050, however, the gap between the extremes
narrows, and by 2100 the range is very small indeed: the highest scenario shows COj emissions from land use only 2.4 times as
great as those found in the lowest scenario. All 26 scenarios in the database report COj emissions of less than 1 GtC originating
from land-use change m 2100, and 23% of the scenarios indicate COj sequestration by the end of the 2P' century.
1900 1950 2000 2050 2100
Figure 2-la: Global energy-related and industrial CO^ emissions-'* - historical development and future scenarios, shown as an index
(1990 = 1). Median (50*), 5*, 25^^ 75*, and 95* percentiles of the frequency distribution are shown. The statistics associated with
scenarios from the literature do not imply probability of оссштепсе (e.g., the frequency distribution of the scenarios may be
influenced by the use of IS92a as a reference for many subsequent studies). The vertical bars on the right side of the figure indicate
the ranges for intervention, non-intervention, and non-classified scenario samples, respectively. The emissions paths indicate a wide
range of future emissions. The range is also large in the base year 1990 and is indicated by an "error" bar (see also Figure 2-lb). To
separate the variation due to base-year specification from different future paths, emissions are indexed for the year 1990, when
actual global energy-related and industrial CO, emissions were about 6 GtC. The actual coverage of COj emissions sources may
vary across the 256 different scenarios from the database included in the figure. The scenario samples used vary across the time
steps (for 1990 256 scenarios, for 2020 and 2030 247, for 2050 211, and for 2100 190 scenarios). As a result of software limitations,
only 250 scenarios can be plotted on the graph. However, the scenarios not shown are included in the assessment and have almost
identical trajectories to the sample of 250 scenarios shown. Data sources: Morita and Lee, 1998; Nakicenovic et al, 1998a.