Emissions Scenarios - IPCC

Emission Scenarios 243
Box 5-1: Scenario Standardization
One of the primary reasons for developing emissions scenarios is to enable coordinated studies of climate change, climate
impacts, and mitigation options and strategies. With the multi-model approach used in the SRES process, 1990 and 2000
emissions do not agree in scenarios developed usmg different models. In addition, even with agreed reference values, it is time
consuming and often impractical to fine-tune most mtegrated assessment models to reproduce a particular desned result.
Nevertheless, differences in the base year and 2000 emissions may lead to confusion among the scenario users. Therefore, the
1990 and 2000 emission estimates were standardized in all the SRES scenarios, with emissions diverging after the year 2000.
The procedure for selectmg 1990 and 2000 emission values and the subsequent adjustments to scenario emissions are described
in this box.
The standardized scenarios share the same values for emissions in both 1990 and 2000. Emissions for the year 2000 are, of
course, not yet known and 1990 emissions are also uncertain. The 1990 and 2000 emission estimates for all gases, except SOj,
were set to be equal to the averages of initial values in the unadjusted four marker scenarios. This was carried out at the fourregion
level, and summed to obtahi the standardized global totals. The resultant estimates are within relevant uncertainty ranges
for each substance, but should not be interpreted as "endorsed" by the Intergovernmental Panel on Climate Change (IPCC) to
represent values for either global or regional emissions. Rather, they are the standardized base-year estimates used for the
emissions scenarios.
From 2000 to 2100, emissions in all the scenarios (except CO2 emissions from land-use and SO2 emissions) were adjusted by
applying a constant offset equal to the difference between the standardized 2000 value and the scenario-speciñc 2000 value.
The puфose was to smooth scenario trajectories between 2000 and 2010.
This procedure results in small distortions for those emissions that rise with time, or at least that do not ultimately decrease by
a large amount as compared to the base year. However, for emissions that fall significantly over time, such as those from
deforestation or of SOj, this procedure can cause more significant distortions and can even change the sign of the emission
estimates at later times (i.e., change positive emission estimates into negative ones and vice versa). To avoid these distortions,
for the aforementioned emissions, the year 2000 offset was reduced by 10% per decade, cumulatively, to make the offset zero
by the year 2100. This allows preservation of the shape of emission trajectories and still ensures the 2000 standardization.
The non-standardized scenario values are available from the modeling teams upon request, although the standardized values
should be used for most purposes.
• Similar future cumulative COj emissions may emerge
from different sets of driving forces;
• Conversely, similar future states of the world with
respect to socio-economic development may yield
different outcomes in terms of cumulative CO^
emissions.
Similar cumulative CO2 emissions can be attained in very
different social, economic, and technological circumstances.
High emission levels of the A2 marker scenario (A2-ASF) are
also attained in all the Al family scenarios with high fossil-fuel
use (AIC and AIG groups), for example in AIG-MiniCAM.
Medium-high emissions are attained in most of the Al group
scenarios (including the AIB marker, AlB-AIM), but also in
scenarios from the B2 scenario family with high fossil-fuel use
(e.g., B2-ASF). Medium-low emissions, which are
characteristic of the B2 family, including the B2 marker (B2-
MESSAGE), are also attained in the A2-A1-MiniCAM
scenario, which illustrates the transition between the A2 and
Al families. Finally, low emission levels resuh from almost all
the Bl family scenarios (including the ВI marker, Bl-IMAGE)
as well as from scenarios that belong to the AIT hightechnology
scenario group (e.g., AIT-MARIA). In Table 5-2,
italics are used for examples of scenarios within each emission
category that illustrate altemative ways to achieve cumulative
COj emissions similar to those of the marker scenarios.
5.3. Carbon Dioxide
COT is the largest contributor to anthropogenic radiative
forcing of the atmosphere. As described in more detail in
Chapter 3, the main sources of anthropogenic COj emissions
are fossil fuel combustion and the net release of carbon from
changes in teirestrial ecosystems, commonly refen-ed to as
land-use changes. To a lesser extent, CO^ is emitted by
industrial activities, in particular by cement production (Table
5-3).
5.3.1. Carbon Dioxide Emissions from Fossil Fuels and
Industry
As shown in Table 5-3, fossil fuels were the main source of
CO, emissions in 1990. Therefore, it is expected that future
CO2 emission levels will depend primarily on the total energy
consumption and the structure of energy supply. The total