Emissions Scenarios

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5.1 Introduction

In this chapter emission estimates for radiatively important gases generated in 40 Special Report on Emission Scenarios (SRES) scenarios are present. These gases are carbon dioxide (CO2), methane (CH4 ), nitrous oxide (N2O), nitrogen oxides (NOx), carbon monoxide (CO), non-methane volatile organic compounds (NMVOCs), sulfur dioxide (SO2 ), chlorofluoro-carbons (CFCs) and hydrochlorofluorocarbons (HCFCs)1, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6 ) (see Table 5-1). Emission estimates presented here span the interval from 1990 to 2100 at the global level and at the level of four SRES macro-regions (OECD90, REF, ASIA, and ALM; see Appendix IV). In addition, sulfur emission estimates are presented in the regional gridded format to assist in quantifying the effects at the local level. Links between emissions and the underlying driving forces presented in Chapter 4 are illustrated also.

Table 5- 1: Overview of greenhouse gases (GHGs), ozone precursors, and sulfur emissions for the SRES scenario groups. Numbers are for the four markers and (in brackets) for the range across all scenarios from the same scenario group (standardized emissions, see also footnote 2).

(MtCH4 )
( MtN)
(MtC equiv.)
(Mt CO)

13.5 (13.5- 17.9)
289 (289- 640)
7.0 (5.8- 17.2)
824 combined
1663 (1080- 2532)
193 (133- 552)
40.2 (40.2- 77.0)
27.6 (27.6- 71.2)
(25.9- 36.7)
(392- 693)
(6.1- 16.2)
same as in A1
(2298- 3766)
(167- 373)
(63.3 -151.4)
(26.9- 83.3)
(28.2- 30.8)
(289- 735)
(5.9- 16.6)
same as in A1
(3260- 3666)
(192- 484)
(39.9 -132.7)
(27.4- 40.5)
(4.3- 9.1)
(274- 291)
(4.8- 5.4)
same as in A1
(1520- 2077)
(114 -128)
(28.1- 39.9)
(20.2- 27.4)
29.1 (19.6- 34.5)
889 (549- 1069)
16.5 (8.1- 19.3)
1096 combined
2325 (776- 2646)
342 (169- 342)
109.2 (70.9- 110.0)
60.3 (60.3- 92.9)
4.2 (2.7- 10.4)
236 (236- 579)
5.7 (5.3- 20.2)
386 combined
363 (363- 1871)
87 (58- 349)
18.7 (16.0- 35.0)
24.9 (11.4- 24.9)
13.3 (10.8- 21.8)
597 (465- 613)
6.9 (6.9- 18.1)
839 combined
2002 (661- 2002)
170 (130- 304)
61.2 (34.5- 76.5)
�47.9 (33.3- 47.9)

As a result of differences in modeling and estimation approaches, base-year (1990) emission values in SRES scenarios developed using different models show substantial variation. To facilitate both use of the scenarios and comparisons across scenarios and families, base-year emissions in all 40 SRES scenarios were standardized using one common set of values (see Box 5-1). With a few clearly indicated exceptions, only standardized emission values are discussed in this chapter. A complete set of standardized emissions, along with other quantitative scenario information, is provided in Appendix VII.

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 using different models. In addition, even with agreed reference values, it is time consuming and often impractical to fine-tune most integrated assessment models to reproduce a particular desired 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 selecting 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 SO2 , were set to be equal to the averages of initial values in the unadjusted four marker scenarios. This was carried out at the four-region level, and summed to obtain 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-specific 2000 value. The purpose 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 SO2 , 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.

The first section in this chapter presents a "roadmap" that serves as an orientation to the 40 SRES scenarios. The roadmap gives a simple taxonomy that compares input parameters, as represented by the four scenario families, and emission outputs, as represented by the 1990 to 2100 cumulative CO2 emissions. The subsequent sections discuss in detail emissions of each gas over the next 100 years to 2100.

5.2. Roadmap to the Scenarios

A classification scheme is presented here to assist the reader in understanding the links between driving forces and scenario outputs. This scheme can also be used to help select appropriate scenarios for further analysis (see Chapter 6).

The SRES scenarios were developed as quantitative interpretations of the four alternative storylines that represent possible futures with different combinations of driving forces. These broad scenario families are broken down further into seven scenario groups2, used here to classify the input driving forces (see also Table 4-20 in Chapter 4).

The scenario outputs of most interest are emissions of GHGs, SO2 , and other radiatively important gases. However, the categorization of scenarios based on emissions of multiple gases is quite difficult. All gases that contribute to radiative forcing should be considered, but methods of combining gases such as the use of global warming potentials (GWP) are appropriate only for near-term GHG inventories3. In addition, emission trajectories may display different dynamics, from monotonic increases to non-linear trajectories in which a subsequent decline from a maximum occurs. This particularly diminishes the significance of a focus on any given year, such as 2100. In light of these difficulties, the classification approach presented here uses cumulative CO2 emissions between 1990 and 2100. CO2 is the dominant GHG and cumulative CO2 emissions are expected to be roughly proportional to CO2 radiative forcing over the time scale considered (Houghton et al. 1996).

Total cumulative CO2 emissions from the 40 SRES scenarios fall into the range from 773 to 2538 gigatonnes of carbon (GtC) with a median of 1509 GtC. To represent this range, the scenario classification uses four intervals:


    Figure 5-1: Global cumulative CO2 emissions in the 40 SRES scenarios, classified into four scenario families (each denoted by a different color code: A1, red; A2, brown; B1, green; B2, blue). Marker scenarios are shown with thick lines without ticks, globally harmonized scenarios with thin lines, and non-harmonized scenarios with thin, dotted lines. Black lines show percentiles, means, and medians for the 40 SRES scenarios. For numbers on the two additional illustrative scenarios A1FI and A1T see Appendix VII.

    Less than 1100 GtC (low).
  • Between 1100 and 1450 GtC (medium-low).
  • Between 1450 and 1800 GtC (medium-high).
  • Greater than 1800 GtC (high).

Each CO2 interval contains multiple scenarios and scenarios from more than one family. Each category also includes one of the four marker scenarios. Figure 5-1 shows how cumulative CO2 emissions from the 40 SRES scenarios fit within the selected emission intervals.

Table 5-2 provides an overview of this scenario classification and links the scenario outcomes with factors that drive them, organized by family and scenario group (see also Table 4-20). The rows in Table 5-2 represent the emission categories, while the columns represent the scenario families. The analysis of Table 5-2 reveals two key results:

  • Similar future cumulative CO2 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 CO2 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 A1 family scenarios with high fossil-fuel use (A1C and A1G groups), for example in A1G-MiniCAM. Medium-high emissions are attained in most of the A1 group scenarios (including the A1B marker, A1B-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 A1 families. Finally, low emission levels result from almost all the B1 family scenarios (including the B1 marker, B1-IMAGE) as well as from scenarios that belong to the A1T high-technology scenario group (e.g., A1T-MARIA). In Table 5-2, italics are used for examples of scenarios within each emission category that illustrate alternative ways to achieve cumulative CO2 emissions similar to those of the marker scenarios.

Table 5-2: Scenario classification according to scenario family and cumulative total carbon dioxide emissions (fossil, industrial, and net deforestation) from 1990 to 2100. Markers are in bold print, the two additional illustrative scenarios of the A1 scenario group (see footnote 2 of text) in italics; all harmonized scenarios have shaded background; and examples of scenarios with similar CO emissions (see text) are underlined.
Cumulative Carbon Emission 1990-2100

Family A1    

Scenario Group A1C A1G A1B A1T A2 B1 B2

A1C- MiniCAM
A1G- MiniCAM



Medium- High
450- 1800GtC
A1B- MiniCAM
A1v1- MiniCAM
A1v2- MiniCAM


A2- MiniCAM


B2High- MiniCAM

Medium- Low
1100- 1450 GtC


A2- A1- MiniCAM


B1High- MiniCAM

B2- MiniCAM

<1100 GtC


B1- MiniCAM

Marker scenario 1499 1862 983 1164
Illustr. scenario 2189 1068
Harmonized scenarios 2127-2538 2178-2345 1301-2073 1068-1114 1732 773-1390 1359-1573
Other scenarios 2148 2189 1519-1731 1049 1352-1938 947-1201 1186-1686


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