Emissions Scenarios - IPCC

170 An Overview of Scenarios
SRES Scenarios
Figure 4-1: The four SRES scenario famihes that share
common storyhnes are illustrated as branches of a twodimensional
tree. The two dimensions indicate the relative
orientation of the different scenario storylines toward
economic or environmental concerns and global and
regional scenario development pattems, respectively. There
is no implication that these two are mutually exclusive or
incompatible. In reality, the four scenarios share a space of
a much higher dimensionality given the numerous driving
forces and other assumptions needed to define any given
scenario in a particular modeling approach. The Al
storyline branches out into different groups of scenarios to
illustrate that altemarive development paths are possible
within one scenario family.
Group II Technical Support Unit (IPCC WGII TSU) sent letters
to Governments asking for nominations of modeling teams to
contribute to SRES and advertised in a number of scientific
journals for modelers to participate in SRES. Six different
modeling groups from Europe, North America, and Japan
volunteered to participate in the fonnulation and development
of the scenarios in response to the call. It is fortunate that they
are from three different continents and also include different
methodological approaches used in the literature to develop
quantitative emissions scenarios.
The six models have different regional aggregations. The
writing team decided to group the various global regions into
four "macro-regions" common to all the regional aggregations
across the six models (Box 4-1).
In response to a number of requests from potential user groups
within IPCC and in accordance with a decision of the IPCC
Bureau in 1998 to release draft scenarios to climate modellers
for their input in the Third Assessment Report, the writing team
chose one model run to characterize each scenaiio family.
Scenarios resulting from these runs are called "marker"
scenarios or simply "markers." There are four marker
scenarios, each considered characteristic for one of the four
scenario families. The rationale and process for designating
marker scenarios is discussed in more detail in Section 4.4.1.
The SRES scenario quantifications of the main indicators (such
as population and economic growth, characteristics of the
energy system, and the associated GHG emissions) all fall
within the range of studies published in the Hterature and
scenarios documented in the SRES database (see Chapter 2).
Quantitative indicators form an important part of each scenario
description. These indicators include gross world product,
population, supply and demand for principal energy forms,
energy resource characteristics, the breakdown of land use, and
emissions of various GHGs. The scenarios are designed so that
the evolution of their indicators over the 2P' centviry falls well
within the range represented by scenarios from the literature
and included in the SRES database (see Chapter 2 and Morita
and Lee, 1998; Nakicenovic, et al, 1998). More importantly,
they correspond to the qualitative characteristics of the
respective storylines. Also, they were revised iteratively within
the six modeling approaches to achieve internal consistency on
the basis of inputs from the entire SRES writing team and the
SRES open process.
Each storyline was characterized initially by two quantitative
"targets," namely global population (15, 10, and 7 billion by
2100 in scenarios A2, B2, and both Al and Bl, respectively)
and global gross domestic product (GDP) by 2100 (in 1990 US
dollars, US$550 trillion for Al, US$250 trillion for A2,
US$350 trifiion for Bl, and US$250 trillion for B2). These
quantitative targets guided the subsequent quantification of the
SRES scenarios with different model approaches. Generally,
the orders of magnitude of these original quantitative scenario
"guideposts" are reflected in the final SRES scenarios (see
Table 4-2) and have been adopted in a majority of SRES
scenarios. Evidently, the quantitative characteristics of the four
SRES scenario families comprise many more dimensions than
this, in particular regional pattems, differences in resource and
technology availability, land-use changes, non-carbon dioxide
(COj) GHGs, etc. These are discussed in the subsequent
Sections.
4.2. SRES Scenario Taxonomy
4.2.1. Storylines
The primary purpose of developing multiple scenario families
was to explore the uncertainties behind potential trends in
global developments and GHG emissions, as well as the key
drivers that influence these (see also Chapter 1, Section 1.7.2).
The writing team decided that narrative storylines, based on the
futures and scenario literature, would be the most coherent way
to describe their scenarios, for the following reasons.
• To help the team to think more coherently about the
complex interplay between scenario driving forces