Emissions Scenarios - IPCC

142 Scenario Driving Forces
Table 3-7: Overview of scenarios presented in Section 3.5.
Scenario Scenario Identification Type(*) Reference
Number
M IS92a IPCC 1992 GR Leggett ff al. (1992)
1-2 IS92b IPCC 1992 GR Leggett ef al. (1992)
1-3 IS92c IPCC 1992 GR Leggett et al. (1992)
1-4 IS92d IPCC 1992 GR Leggett et al. (1992)
1-5 IS92e IPCC 1992 GR Leggett а/. (1992)
1-6 IS92f IPCC 1992 GR Leggett et al. (1992)
1-7 IS92 SI : IPCC 1992 Sensitivity 1
(High Deforestation, High Biomass) GR Leggett eí а/. (1992)
1-8 IS92 S4 : IPCC 1992 Sensitivity 4
(Halt Deforestation, High Plantation) GR Leggett et al. (1992)
2-1 Baseline A IMAGE 2.1 GR Alcamo, etal. (1996)
2-2 Baseline В IMAGE 2.1 GR Alcamo, et al. (1996)
2-3 Baseline С IMAGE 2.1 GR Alcamo, et al. (1996)
2-4 Less В1 Changed Trade GR Leemans, etal. (1996)
2-5 Less В1 No Biofuels GR Alcamo and Kreileman (1996)
2-6 Stab 350 All GR Alcamo and Kreileman (1996)
3-1 AIM, Asian Pacific Integrated Model
Land use emission scenario GR Matsuoka and Morita (1994)
7-1 EPA-SCW EPA (Slowly Changing World) GR Lashof and Tirpak (1990)
7-2 EPA-RCW EPA (Rapidly Changing World) GR Lashof and Tirpak (1990)
7-3 EPA-High Reforestation EPA (Halt GR Lashof and Tirpak (1990)
Deforestation, High Reforestation)
8-1 HI Houghton-Population G Houghton (1991)
8-2 H2 Houghton-Exponential Extrapolation G Houghton (1991)
* G = global, R = regional.
3.5.2. Carbon Dioxide Emissions from Anthropogenic
Land-Use Change
A variety of changes in land use can result in anthropogenic
COj emission or absorption. These changes most obviously
include pemanent deforestation or afforestation. However,
many changes in land-management practices also contribute to
COj fluxes because of changes in standing biomass densities or
in soil carbon. Empirical studies of such CO^ fluxes are rare, so
that information on current emissions is very poor. Whereas
comprehensive information exists for forests globally, only a
few countries have detailed information on forest and
agricultural land-management practices. Hence, global
estimates of COj emissions from anthropogenic land-use
change, including those in the SRES, tend to be based entirely
on net deforestation-afforestation and on average figures for
carbon storage per hectare in forests.
Emissions of COj from deforestation arise mostly from the
burning of trees and other vegetafion in tropical forests cleared
for agricultural use. These emissions also stem from the
decomposifion of trees harvested for lumber, the buming of
wood for fuel, and soil respiration. If harvested wood is
replaced by new seedlings, it is normally assumed that the
amount of CO-, released by decomposifion or buming is
compensated by the COj taken up during growth of the
seedlings and, therefore, that the net emissions of harvested
trees is zero. Where net afforestation occurs, net emissions are
taken to be negative (i.e. afforestation acts as a sink).
As a consequence of inconsistencies in base-year estimates for
net changes in forest biomass, emission estimates are
normalized relative to their 1990 value before comparison with
each other (Figure 3-14). Figure 3-14 also clearly depicts the
relative change of emissions with time (Alcamo, et ai, 1995;
Nakicenovic et al., 1998b).
The scenarios of CO, emissions from land-use change have quite
different temporal paths, and show their widest range before the
middle of the 2P' century (Figure 3-14). Nearly all the scenarios
then converge to very low emissions by the end of the century. At
their widest point, the scenarios span about a factor of 14.
The different sets of scenarios can be grouped into roughly two
typical paths: One set declines smoothly after 1990, while the
other sharply increases for a few decades after 1990. After the
middle of the 2V- century most scenarios stabilize or continue
to decline because either the driving forces of deforestation
equilibrate or because forests are depleted. These processes are
discussed further below. By 2100, COj scenarios of