Emissions Scenarios - IPCC

Technical Summary 41
3000
e
о
2500
a
2000
High > 1800 GtC
Al FI
A2
о
s
о
-£
1500
Medium-Hii >h 1450- ISOOGtG
••••••••••••••
Medium-Lo w 1100- 450 GtC
AlB
IS92 range
>
1000 -
Low< llOOGlC
Bl
"3
s
s
500 -
u
0
1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
H
Figure TS-9: Global cumulative CO2 emissions (GtC, standardized). The ranges of cumulative emissions for the SRES
scenarios are shown. Scenarios are grouped into four categories: low, medium-low, medium-high, and high emissions. Each
category contains one marker scenario plus alternatives that lead to comparable cumulative emissions, although often through
different driving forces. The ranges of cumulative emissions of the six SRES scenario groups are shown as colored vertical
bars and the range of the IS92 scenarios as a black vertical bar.
toward higher emissions (SRES maximum of 2570 GtC
compared to 2140 GtC for IS92), but not toward lower
emissions. The lower bound for both scenario sets is just below
800 GtC.
This categorization can guide comparisons using either
scenarios with different driving forces yet similar emissions,
or scenarios with similar driving forces but different
emissions. This characteristic of SRES scenarios also has very
important implications for the assessment of climate-change
impacts, mitigation, and adaptation strategies. Two future
worlds with fundamentally different characteristic features,
such as the AlB and B2 marker scenarios, also have different
cumulative CO2 emissions and radiative forcing, but very
similar CO2 emissions in 2100. In contrast, scenarios that are
in the same category of cumulative emissions can have
fundamentally different driving forces and different CO2
emissions in 2100, but very similar cumulative emissions and
radiative forcing. Presumably, adverse impacts and effective
adaptation measures would vary among the scenarios from
different families that share similar cumulative emissions but
have different demographic, socio-economic, and
technological driving forces. This is another reason for
considering the entire range of emissions in future
assessments of climate change.
9.2. Other Greenhouse Gases
Of the GHGs, CO2 is the main contributor to anthropogenic
radiative forcing because of changes in concentrations from
pre-industrial times. According to Houghton et al. (1996) wellmixed
GHGs (CO2, CH4, N2O, and the halocarbons) induced
additional radiative forcing of around 2.5 W/m^ on a global and
annually averaged basis. CO2 accounted for 60% of the total,
which indicates that the other GHGs are significant as well.
Whereas CO^ emissions are by-and-large attributable to two
major sources, energy consumption and land-use change, other
emissions arise from many different sources and a large number
of sectors and applications (e.g. see Table 5-3 in Chapter 5).
The SRES emissions scenarios also have different emissions
for other GHGs and chemically active species such as CO,
NO J,, nitrogen oxides, and NMVOCs. The uncertainties that
surround the emissions sources of the other GHGs, and the
more complex set of driving forces behind them are
considerable and unresolved. Therefore, the models and
approaches employed for the SRES analyses cannot produce
unambiguous and generally approved estimates for different
sources and world regions over a century. Keeping these
caveats above in mind, Table TS-4 (see later) shows the
emissions of all relevant direct and indirect GHGs for the four
marker scenarios and, in brackets, the range of the other
scenarios in the same family (or scenario groups for the AI