Emissions Scenarios - IPCC

An Overview of the Scenario Literature 79
2.1. Introduction
Presented in tliis cliapter is tlie assessment of more than 400
global and regional greenhouse gas (GHG) emissions scenarios
based on an extensive literature review. Emissions scenarios
provide an important input for the assessment of future climate
change. Future anthropogenic GHG emissions depend on
numerous driving forces, including population growth,
economic development, energy supply and use, land-use
pattems, and a host of other human activities. These main
driving forces that determine the emissions trajectories in the
scenarios often also provide input to assess possible emissions
mitigation strategies and possible impacts of unabated
emissions. In view of the many different uses, it is not
surprising that numerous emissions scenarios are presented in
the literature and that the number of regional and global
emissions scenarios is growing.
An important characteristic of the scenarios in this Special Report
on Emissions Scenarios (SRES) is that they reflect the underlying
uncertainty, part of which derives from the range of emissions in
the literature. The objective was to encompass the variation within
the most important scenario driving forces and emissions, the
complexity of possible relationships between driving forces and
emissions, and the associated uncertainties that characterize
altemative future developments. The SRES scenarios cover most
of the range of the GHG emissions scenarios found in the
Hterature, including the International Panel on CHmate Change
(IPCC) 1992 Scenarios (IS92) series (Leggett et al., 1992). The
writmg team considered the literature in creating a new set of
scenarios. Importantly, however, the literature on existing
scenarios provides only a general framework to aid analysis; it is
mformative, but not determinative.
The literature review consists of four parts;
• documentarion of as many as possible of the
quantitative global and regional emissions scenarios
available both in the open literature and from
international activities that involve documentation of
submitted scenarios;
• development of a scenario database to document the
more than 400 emissions scenarios collected during the
literature review;
• evaluation of the ranges and relationships of the main
scenario driving forces and the resultant emissions for
the scenarios documented in the database; and
• assessment of the scenario submissions received
tlirough the SRES "open process."
These four components of the literature review are well suited
to document and assess the (quantitative) scenarios that assign
numeric values to describe the future evolution of driving
forces and emissions.
Central to this assessment of emissions scenarios and their
main driving forces is a unique scenario database developed by
the National Institute for Environmental Studies (NIES) in
Japan for SRES (Morita and Lee, 1998). The database
version of 3 April 1998, which is assessed in this chapter,
includes 416 different scenarios. The current database
version can be accessed through an ftp-site
(www-cger.nies.go.jp/cger-e/db/ipcc.html). It is the most
comprehensive collection of emissions scenarios in the
publicly available literature. It includes most of the recent
global and regional scenarios and all of the scenarios used in
the latest IPCC evaluation of emissions scenarios (Alcamo et
al., 1995). Therefore, the emissions scenarios documented in
the database are representative of the literature in general.
However, there are a number of ways in which the coverage
of the scenarios in the database could be extended in the
future. For example, inclusion of long-term emissions
scenarios for individual countries, when available, would
improve the regional coverage (e.g., Parikh, 1996; Murthy et
al., 1997). Also, a large majority of the scenarios report only
energy-related carbon dioxide (CO2) emissions (230
scenarios), while only some report non-energy CO, and other
GHG emissions. This shortcoming of the emissions scenarios
in the literature was identified in the last IPCC evaluation
(Alcamo et al., 1995).
The scenarios in the database were collected from 171 different
literature sources and other scenario-evaluafion activities, such
as the Energy Modeling Forum (EMF; see Weyant, 1993) and
the International Energy Workshop (lEW; see Manne and
Schrattenholzer, 1996, 1997). The scenarios span a wide range
of assumptions about demographic trends, levels of economic
development, energy consumption and efficiency pattems, and
other factors. The aim of this chapter is to show how the
database can be utilized for the analysis of GHG emissions
ranges and their main driving forces. Part of this assessment of
the emissions scenarios is based on an earlier publication on
the analysis of scenarios documented in the SRES database
(Nakicenovic etal, 1998a).
The scenarios in the database display a large range of future
GHG emissions. Part of the range can be attributed to the
different methods and models used to formulate the scenarios,
which include simple spreadsheet models, economic models,
and systems-engineering models. However, most of the range
results from differences in the input assumptions of the
scenarios, in particular those of the main scenario driving
forces. In addition, simply to compare altemative emissions
levels across different scenaiios is not sufficient to shed light
on internal consistency, plausibility, and comparability of the
assumptions behind the scenarios. Analysis of the underlying
driving forces is thus also an important part of the evaluation.
This chapter provides an analysis of the main driving forces,
such as population growth, economic growth, energy
consumption, and energy and carbon intensities. Some of these
driving forces are specified as model inputs, and some are
derived from model outputs, so it is necessaiy to determine the
assumed relationships among the main driving forces.
Although the scenario database is well suited for the
documentation of quantitative scenarios, there is also a