This chapter reviews three scenario literatures, which span a range from more quantitative scenario analysis to analysis that is based more on narrative descriptions (see Figure 2.1). At the quantitative end of the spectrum are the “general mitigation scenarios” reviewed in this section, which consist mainly of quantitative descriptions of driving forces and emission profiles.

More than 500 emission scenarios have already been quantified, including non-mitigation (non-intervention) scenarios and mitigation (intervention) scenarios that assume policies to mitigate climate change. These scenarios have been published in the literature or reported in conference proceedings, and many of them were collected in the IPCC SRES database (Morita & Lee, 1998a) and made available through the Internet (Morita & Lee, 1998b). Using this database, a systematic review of non-mitigation scenarios has already been reported in the SRES (Nakicenovic et al., 2000). However, several mitigation and other scenarios were missing from this database and new emission scenarios have been quantified since the SRES review. Accordingly, the missing scenarios and new scenarios were collected and the database revised for this new review of mitigation scenarios (Rana and Morita, 2000).

The current database collection, covered in this report, contains the results of a total of 519 scenarios from 188 sources. These scenarios were mainly produced after 1990. Two questionnaires were sent to representative modellers in the world, and sets of scenarios from the International Energy Workshop (IEW) and Energy Modelling Forum (EMF) comparison programmes were collected. The database is intended to include only scenarios that are based on quantitative models. Therefore, it does not include scenarios produced using other methods; for example, heuristic estimations such as Delphi.

Of the 519 scenarios, a total of 380 were global GHG emission scenarios, most of which were disaggregated into several regional emission profiles. Of these 380 global emission scenarios, a total of 150 were mitigation (climate policy) scenarios. This review focuses on mitigation scenarios that cover global emissions and also have a time horizon encompassing the coming century. Of the 150 mitigation scenarios, a total of 126 long-term scenarios that cover the next 50 to 100 years were selected for this review. 24 scenarios were excluded on the basis of their short time coverage.

Table 2.1 presents an outline of several representative scenarios in this review; these scenarios exemplify the modelling literature. Columns 1 and 2 of the table show the main identifiers of the scenarios, namely, the model name and source and the policy scenario name, as given by the modellers. The third and fourth columns show the policy scenario type and specific scenario assumptions. The remaining columns contain additional important features of the policy scenarios, including reduction time-paths and burden sharing, GHGs analyzed, policy options and approaches, and feedback. Only five studies among the selected sources of Table 2.1 have detailed policies. Most of the other scenarios assume very simple policy options such as carbon taxes and simple constraints.

Table 2.1: Overview of mitigation scenarios: the main futures of representive scenarios from 26 sources
Model name and source	Policy scenario name	Policy scenario type	Specific scenario assumptions2	Reduction time paths and burden sharing	GHGs analyzed	Sectors in which policies are introduced	Feedbacks3
1 ASF	RCWP	Emission stab.	475ppm	Based on policy scenario	CO2, CO, CH4, N2O	Detailed policy scenario	EP to M
EPA (1990)	RCWR	Emission stab.	350ppm		NOx, CFCs	Energy supply; Land use; End use	EP to M
2 ASF/ IMAGE	Control policy (2x CO2 by 2090)	Conc. stab.	540ppm	Based on policy scenario	CO2, CO, CH4, N2O	Detailed policy scenario	EP to M
IPCC (1990)	Accelerated control ( < 2x CO2)	Conc. stab.	465ppm		NOx, CFCs	Energy supply; Land use; End use
3 ASF	IS92b	Other mitigation1	18.6BtC		CO2, CO, CH4, N2O,	Energy supply; Industrial processes	EP to M
IPCC (1992)			(CO2 emissions)		VOC. SOx, CFCs, NOx
4 MESSAGE	ECS’92 +	Other mitigation1			CO2	Energy supply; Industrial processes;
Nakicenovic et al. (1993)						End use
5 DICE    Nordhaus (1994)	Optimal policy;	Other mitigation1		Utility maximization	CO2, CFCs	Energy	C to M
	10- yr delay of optimal policy	Other mitigation1		Utility maximization	Other GHGs are
	emission stabilization	Emission stab.	8BtC/ yr (CO2 +CFCs)	Based on policy scenario			I to M
	20% emission cut	Other mitigation1	6BtC/ yr (CO2 +CFCs)	Based on policy scenario			C to M
	Geoengineering	Other mitigation		Based on policy scenario			I to M
	Climate stabilization	Slow global temp. increase	0.2° C/ decade	Based on policy scenario
6 CETA	“Selfish” case	Emissions cont. by OECD		Cost minimization (regional)	CO2, CO, CH4, N2O,	Energy	C to M
Peck and Tiesberg (1995)	“Altruistic” case	Emissions cont. by OECD		Cost minimization (global)	CFCs
	“Optimal” case	Emissions cont. by both		Cost minimization (global)
7 LESS    (IPCC, 1996)	LESS Constructions	Other mitigation1			CO2	Energy supply
8 Manne et al. (1995)	Delayed tax; Early tax	Other mitigation1	750 ppm; 540 ppm	Utility maximization	CO2, CH4, N2O	Energy	C to M
MERGE	Emission stab.	Emission stab.	540 ppm	Utility maximization
	Conc. stab.	Conc. stab.	415 ppm	Utility maximization
9 MESSAGE	Case C	Other mitigation1	430 ppm	Based on policy scenario	CO2, CO, CH4, N2O,	Energy supply
WEC (1995)	Ecologically driven				SOx, CFCs NOx, VOC.	End use
10 WBGU (1995)      (German Adv. Council)	Tolerable temp. window	Safe corridor temp. rise constraint	deltaT = 1° C (upper limit)	Temp. rise constraint	CO2
11 AIM/ Top- down	Negotiable safe emiss. corridor	Safe corridor	deltaT = 1- 2° C	Temp. rise constraint	CO2	Energy	EP to M
Matsuoka et al. (1996)		Temperature rise const.
12 DICE/ RICE	Cooperative RICE	Other mitigation1		Global welfare optimization	CO2	Energy; Land use	C to M
Nordhaus and Yang (1996)	Non- cooperative RICE	Other mitigation1	Regional welf. optimization				I to M
13 IMAGE 2	Stab 350– 650 ppm	Conc. stab.	367– 564 ppm		CO2, CH4, N2O	Energy supply; Industrial processes;
Alcamo and Kreileman (1996)	Stab yr 1990	Conc. stab.	354 ppm			Land use
	St2000-a - St2000-e	Other mitigation1	633– 433 ppm
	Safe emissions corridor	Safe corridor	deltaT = 1– 2° C deg	Temp. rise constraint
14 MiniCAM	Adv. tech	Other mitigation1		Based on policy scenario	CO2, CH4, N2O, SOx, aerosols,	Energy supply	EP to M
Edmonds et al.	(5 Cases using different technologies)				Halocarbons
15 YOHE      Yohe and Wallace (1996)	Stabilization	Conc. stab.		Based on policy scenario	CO2	Energy	C to M
16 DIAM      Ha- Duong, et al. (1997)	450A- D; 550A- D; 650A	Conc. stab.		Cost minimization	CO2	Energy
17 FUND 1.6	Non- cooperative optimum	Other mitigation1		Regional welf. optimization	CO2, CH4, N2O
Tol (1997)	Cooperative optimum	Other mitigation1		Generational welf. optim.
18 MERGE 3.0      Manne and Richels (1997)	Range of scenarios 350 to 750 ppm	Conc. stab.	350 to 750ppm depending on scenario	Utility maximization (non- Annex I begin limit in 2030)	CO2	Energy	C to M
19 SGM      Edmonds et al. (1997)	M1990 ; M1990+ 10%; M1990– 10%; M1995	Other mitigation1			CO2 , CO, CH2 , N2O, NOx, VOC, SOx.	Energy	EP to M
20 ABARE/ GTEM      Tulpule et al. (1998)	Independent abatement; Annex B trading; Double bubble	Other mitigation1	Kyoto targets		CO2	Energy	C to M
21 AIM/ Top- down      Kainuma et al. (1998)	No trading; Annex I Trading; Global trading; Double bubble; Annex I + Chn& Ind; No trading 5% offset	Other mitigation1	Kyoto targets	Based on policy scenario			C to M
22 G- CUBED	Annex I trading; Double bubble	Other mitigation1			CO2	Energy	C to M
McKibbin (1998)	Global permit trade
23 MARIA	Case B	Emission stab.	1990 level			Energy supply; End use; Land use	C to M
Mori and Takahashi (1998)
24 NE21	Conc. regulation	Conc. stab.	Below 550ppm	Cost minimization	CO2	Energy	C to M
Fujii and Yamaji (1998)
25 WorldScan	No Trade; Full trade; Clubs	Other mitigation1	Kyoto targets		CO2		EP to M
Bollen et al. (1996)	Restricted trade; CDM
26 FUND 1.6      Tol (1999)	EMF- 14 scenarios (WRE/ WGI+ 450/ 550/ 650+ NC/ C)	Conc. stab.	Various	Various
Notes: 1 Other mitigation means emission reduction not necessarily leading to stabilization. 2 Specific scenario assumption indicates year 2100 level of emissions/conc. unless specified otherwise. 3 EP: Energy price; M: Macro economy; C: Cost; I: Impact. Conc.stab. : concentration of stabilization Emissions cont.: emissions continuing

Based on the type of mitigation, the scenarios can be classified into four categories: concentration stabilization scenarios, emission stabilization scenarios, safe emission corridor (tolerable windows/safe landing) scenarios, and other mitigation scenarios.

Scenarios for concentration stabilization account for a large proportion of the mitigation scenarios, with 47 of the 126 mitigation scenarios being classified into this type. Many scenarios of this type were quantified in the process of the EMF comparison (Weyant and Hill, 1999) where a systematic guideline was prepared for stabilization quantification. Of the 47 scenarios, two-thirds are intended to stabilize atmospheric concentrations of CO₂ at 550ppmv. The concentration of 550ppmv was used as a benchmark for stabilization in the previous studies on mitigation scenarios. This number may be related to the frequent references made to it in political discussions. The adoption by the European Union of a maximum increase in global average temperature of 2°C above pre-industrial levels is roughly equivalent to a stabilization level of 550ppmv CO₂ equivalent or 450ppmv CO₂. It does not imply an agreed-upon desirability of stabilization at this level. In fact, environmental groups have argued for desirable levels well below 550ppmv, while other interest groups and some countries have questioned the necessity and/or feasibility of achieving 550ppmv. Scenarios with levels of concentration stabilization other than 550ppmv are contained in IPCC (1990), Manne et al. (1995), Alcamo and Kreileman (1996), Ha-Duong et al. (1997), Manne and Richels (1997), and Fujii and Yamaji (1998).

The emission stabilization scenarios account for 20 of the 126 mitigation scenarios. Most scenarios of this type are intended to stabilize at 1990 emission levels in Annex I or the Organization for Economic Co-operation and Development (OECD) countries. Some scenarios have emissions stabilizing at other levels, for example, the emissions stabilization scenario of DICE (Nordhaus, 1994) aims at a level of 8GtC/yr of CO₂ and chlorinated fluorocarbons (CFCs) by 2100. Other stabilization scenarios, namely the “Safe Emissions Corridor” or “Tolerable Windows” (WBGU, 1995; Alcamo and Kreileman, 1996; Matsuoka et al., 1996) and “Climate Stabilization” (Nordhaus, 1994) scenarios, determine the upper limit of emissions based on a constraint of some natural threshold, such as global mean temperature increase rate. Only a few studies are based on such scenarios.

Other scenarios based on DICE (Nordhaus, 1994), MERGE (Manne and Richels, 1997) and MARIA (Mori and Takahashi, 1998) determine the level of emission reduction based on net benefit maximization, which is estimated as the benefit produced by climatic policy minus the policy implementation cost. In addition to the above, the low CO₂-emitting energy supply system (LESS) constructions should be noted. These scenarios were developed on the basis of detailed assessments of technological potentials, and can therefore be distinguished from many other mitigation scenarios (see Box 2.2).

Of the remaining mitigation scenarios, a total of 50 adopt other criteria to reduce GHGs. Some of these scenarios assume the introduction of specific policies such as a constant carbon tax, while others assume the Kyoto Protocol targets for Annex I countries up to 2010 and a stabilization of their emissions thereafter at 2010 levels.

While all the scenarios deal necessarily with energy-related CO₂ emissions that have the most significant influence on climate change, several models include CO₂ emissions from land use changes and industrial processes (e.g., IPCC, 1992; Nakicenovic et al., 1993; Matsuoka et al., 1995; Alcamo and Kreileman, 1996). Some of them include other important GHGs in their calculations, such as methane (CH₄) and nitrous oxide (N₂O) (e.g., EPA, 1990; IPCC, 1990; Manne et al., 1995; Tol, 1997), and a few go even further to include sulphates, volatile organic compounds (VOCs), and halocarbons (e.g. IPCC, 1992; WEC, 1995; Edmonds et al., 1996, 1997). With respect to the policy options used in the scenario quantifications, three fields are taken into account in the reviewed studies: energy systems (including both supply and demand), industrial processes (including cement and metal production), and land use (including agriculture and forest management).

Since most of the modelling exercises have been carried out to study the CO₂ emissions from human activities linked to the use of energy, energy supply and end-use are naturally the areas where policy is applied. Energy supply options include natural gas, renewable energy, and commercial biomass; introduction of new technologies; and so on. End-use options chiefly pertain to increased energy efficiency in industry, transport, and residential and/or commercial applications.

The policy instruments analyzed depend on the underlying model structure. Most of the scenarios introduce policies such as simple carbon taxes or a constraint on emissions or concentration levels for achieving the desired reduction or stabilization. How the constraint is imposed varies from scenario to scenario. Among the models with regional disaggregation, a few regional targets have been introduced (e.g., Nordhaus, 1994; Tol, 1999). Regional disaggregation also allows modellers to let the regions trade in emission permits. Permit trading is introduced in more recent work, especially just before and after the Third Conference of the Parties to the United Nations Framework Convention on Climate Change in Kyoto (December 1997). Some studies offer permit trading as a mechanism to reduce the overall costs of abatement. Much of the work done in the early 1990s led to the development of detailed scenarios for introducing such policies (EPA, 1990; IPCC, 1990, 1992). Some models employ policies of supply-side technology introduction (Nakicenovic et al., 1993; Edmonds et al., 1996; Fujii and Yamaji, 1998), while other models emphasize the introduction of efficient demand-side technology (EPA, 1990; Kainuma et al., 1999a).

The issue of burden sharing among regions is a contentious one and it was sparsely treated in the first half of the 1990s. Most discussions about burden sharing are of a qualitative and partial nature and are not related to model-based mitigation scenarios. A few studies (most notably Rose and Stevens, 1993; Enquete Commission, 1995; and Manne and Richels, 1997) present a set of burden-sharing rules in their scenarios. Of late, the EMF exercises looking at the Kyoto scenarios have treated this issue better than in the past (Weyant, 1999).

The time-paths of emission reduction are determined in three ways in the reviewed studies. First, the emission trajectories are determined by policy scenarios that have been designed in detail for regions over the time frame (EPA, 1990; IPCC, 1990; WEC, 1995; Edmonds et al., 1996; Yohe and Wallace, 1996; Kainuma et al., 1998). Second, dynamic optimization models automatically determine these reduction time-paths by global cost minimization over time (e.g., Peck and Tiesberg, 1995; Fujii and Yamaji, 1998) or economic welfare maximization (Nordhaus, 1994; Manne et al., 1995). Third, mitigation scenarios of tolerable windows/safe landing, or safe emission corridors, can fix the time series of emission reduction by introducing a specific constraint of the rate of change in natural systems including the global temperature change rate (e.g., Alcamo and Kreileman, 1996).

Finally, there are differences in the treatment of feedback to the macro-economy in the models. While most bottom-up models have no feedback from cost to the macro-economy, top-down models allow for the feedback of energy prices to the macro-economy. The MERGE (Manne et al., 1995) and CETA (Peck and Tiesberg, 1995) models also have feedback from impacts to the macro-economy.

Technological improvement is a critical element in all the general mitigation scenarios. This is apparent when the detailed policy options are studied, where such literature is available. For instance, Nakicenovic et al. (1993) (using MESSAGE) incorporated policies of dematerialization and recycling, efficiency improvements and industrial process changes, and fuel-mix changes in the industrial sector; fuel efficiency improvements, modal split changes, behavioural change, and technological change in the transport sector; and efficiency improvements of end-use conversion technologies, fuel-mix changes, and demand-side measures in the household and services sector. It should be noted that efficiency improvement through technological advancement is emphasized in all sectors. Similar policies leading to efficiency improvement were also underlined in earlier modelling studies such as EPA (1990), IPCC (1990), and IPCC (1992).

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