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Emissions Scenarios


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4.4.8. Prospects for Future Energy Systems

In the energy systems models used to generate the scenarios reported here, the entire energy systems structure is represented from primary energy extraction, through conversion, transport, and distribution, all the way to the provision of energy services. Primary energy harnessed from nature (e.g., coal from a mine, hydropower, biomass, solar radiation, produced crude oil, or natural gas) is converted in refineries, power plants, and other conversion facilities to give secondary energy in the form of fuels and electricity. This secondary energy is transported and distributed (including trade between regions) to the point of final energy use. Final energy is transformed into useful energy (i.e., work or heat) in appliances, machines, and vehicles. Finally, application of useful energy results in delivered energy services (e.g., the light from a light bulb, mobility).

Box 4-9: Dynamics of Technological Change in the MESSAGE-Based Quantifications for the Four SRES Marker Scenarios.

Technological change in energy supply and end-use technologies has historically been a main driver of structural changes in energy systems, efficiency improvements, and improved environmental compatibility. Yet, despite its crucial role, the mechanisms that underlie technological innovation and diffusion of new technologies remain poorly understood, so modeling technological change as an endogenous process to the economy and society is still in its infancy. Historically, the track record of technology forecasts has at best been mixed, with a number of notable failures particularly in the energy sector. In the 1960s, for instance, R&D in the US attempted to develop nuclear-propelled aircraft, and nuclear electricity was anticipated to become "too cheap to meter." Conversely, the dynamic technological changes in microprocessors, information technologies, and aeroderivative turbines (and their combination with the steam cycle in the form of combined cycle gas turbines) were largely underestimated. This is similar to the pessimistic market outlook for gasoline-powered cars at the end of the 19th and start of the 20th centuries.

In recognition of the considerable uncertainty in describing future technological trends, a scenario approach was adopted to vary technology-specific assumptions in the MESSAGE model runs of the SRES scenarios. Depending on the specific interpretation of the four SRES scenario storylines, alternative technologies and alternative ranges of their future characteristics were assumed as model inputs.

Two guiding principles determined the choice of particular technology assumptions in MESSAGE.

First, technologies not yet demonstrated to function on a prototype scale were excluded. Therefore, for instance, nuclear fusion is excluded from the technology portfolio of all SRES scenarios calculated with the MESSAGE model. However, production of hydrogen- or biomass-based synfuels (e.g. ethanol) or advanced nuclear and solar electricity generation technologies are included, as they have demonstrated their physical feasibility at least on a laboratory or prototype scale, or in some specific niche markets (even if they are uneconomic at currently prevailing energy prices). Second, the range of technology-specific assumptions is empirically derived. Statistical distributions of technology characteristics based on a large technology inventory (consisting of 1600 technologies) and developed at IIASA (Messner and Strubegger, 1991; Strubegger and Reitgruber, 1995) were used. Means, maxima, and minima from these distributions (e.g. of estimated future technology costs) guided which particular values to adopt across scenarios on the basis of the scenario taxonomy suggested by the scenario storylines (ranging from conservative to optimistic).

Tables 4-13a to 4-13e summarize the technology characteristics and resultant diffusion rates across the four SRES scenario families and their scenario groups. Table 4-13a presents a brief overview of a selection of major energy technologies represented in the MESSAGE model. (Being a detailed "bottom-up" model, MESSAGE literally contains hundreds of individual technologies, too many to summarize here; instead, only the most important technology groups, aggregated across many individual technologies, are presented.) Table 4-13b summarizes salient technology characteristics in terms of levelized costs (investment and operating costs levelized per unit energy output, excluding fuel costs) and Table 4-13c summarizes the resultant marker deployment (diffusion) of these technologies by 2050 and 2100 for the B2-MESSAGE marker scenario. This scenario is characterized by intermediate levels of growth in energy demand and conservative assumptions as to future technological change. The latter were adopted based on a literature survey (Strubegger and Reitgruber, 1995) as well as an expert opinion poll.

In particular, the B2-MESSAGE scenario adopted technology characteristics of the equally conservative IIASA-WEC Scenario B (Nakicenovic et al., 1998), which was based on the Strubegger and Reitgruber (1995) analysis, complemented by a review of some 100 energy experts assembled by WEC. Table 4-13d indicates how technology costs in the other MESSAGE scenarios differ from those of the B2 scenario. (The prevalence of negative values in Table 4-13d indicates that most scenarios are more optimistic concerning cost improvements of future technology than the MESSAGE B2 scenario.) Finally, Table 4-13e indicates the difference in market deployment (diffusion) of the other MESSAGE SRES scenarios compared to that of the B2 scenario. Positive values indicate higher market deployment, and negative ones show lower diffusion. However, differences across scenarios in terms of technology diffusion are not governed by technology costs alone. Other technology characteristics (such as efficiency and infrastructure availability) and market (demand) growth are also important in determining market deployment rates and diffusion potentials of energy technologies.




Table 4-13a: Overview of selected energy technologies represented in MESSAGE.

Technology Aggregates Including:

Centralized Electricity Generation:  
Coal conventional Conventional coal power plants with DESOX (flue-gas desulfurization, FGD) and DENOX (flue-gas denitrification)
IGCC Integrated coal Gasification Combined Cycle
Coal fuel cell Coal-based high-temperature fuel cell (internal reforming)
Oil New standard oil power plant (Rankine cycle, low NOx and with FGD); existing crude oil and light oil engine-plants; light oil combined cycle power plants
Gas standard Standard gas power plant (Rankine cycle, potential for cogeneration)
NGCC Natural Gas-fired Combined Cycle power plant with DENOX
NGFC Natural Gas-powered high-temperature Fuel Cell, cogeneration possibilities
Bio New biomass-fired power plant (Rankine cycle, cogeneration possibilities); advanced biomass power plants (gasified biomass is burned in combined cycle gas turbines)
Nuclear Conventional, existing nuclear power plants
Advanced nuclear/other Nuclear high-temperature reactors for electricity and hydrogen coproduction, future inherently safe nuclear reactor designs, and other future zero-carbon electricity-generating technologies for base load
Hydro Hydropower plants (low and high cost)
Wind Wind power plant
Other renewables Geothermal power plant (cogeneration potential); grid-connected solar photo-voltaic power plant (no storage); solar thermal power plants with storage, and solar thermal power plant for hydrogen production
Decentralized Electricity Generation:  
Hydrogen fuel cell Decentralized stationary and mobile hydrogen fuel cells (cogeneration systems or off-hours electricity generation)
Photo-voltaics On-site solar photo-voltaic power plant in the residential and/or commercial sectors, and in the industrial sector
Synfuels:  
Coal synliquids Light oil and methanol production from coal
Biomass synliquids Ethanol production from biomass
Gas synliquids Methanol production from natural gas
Syngases Syngases from various sources, including biomass and coal gasification
Hydrogen, H2(1) Hydrogen production from fossil fuels (coal or gas)
Hydrogen, H2(2),(3), Non-fossil hydrogen production: H2(2): from biomass and electricity, H2(3): from nuclear and solar




Table 4-13b: Levelized costs (1990US$/GJ) of selected energy technologies (excluding fuel costs) in B2-MESSAGE (minima and maxima for eleven world regions).

  1990 2050 2100

 
min
max
min
max
min
max
Coal conversion
3.6
7.5
4.4
7.8
4.4
7.8
IGCC
9.4
9.4
8.3
8.6
6.9
8.6
Coal fuel cell
11.9
11.9
11.9
11.9
11.9
11.9
Oil
3.9
28.9
3.3
5.3
3.3
5.3
Gas standard
3.6
8.3
3.9
4.7
3.9
4.7
NGCC
4.9
5.0
3.3
3.3
2.8
2.8
NGFC
8.4
8.4
6.7
6.7
6.7
6.7
Biofuel
5.8
9.2
5.8
8.3
5.8
8.3
Nuclear
6.7
9.7
7.2
9.7
7.2
9.7
Advanced nuclear/other
10.8
10.8
10.6
10.6
10.6
10.6
Hydro
2.5
15.8
2.5
22.2
2.5
22.2
Wind
15.8
15.8
9.4
9.4
9.4
9.4
Other renewables
6.4
29.8
7.2
10.8
7.2
10.8
Hydrogen fuel cell
8.4
8.4
6.7
6.7
6.3
6.3
Photo-voltaic
20.4
29.8
8.1
11.7
8.1
11.7
Coal synliquids
6.9
6.9
6.4
7.0
6.4
7.0
Biomass synliquids
7.1
7.1
4.8
4.8
4.8
4.8
Gas synliquids
3.7
3.7
2.6
2.6
2.6
2.6
Syngases
4.6
4.6
3.4
4.1
3.4
4.1
Hydrogen H2(1)
5.6
5.6
1.7
3.9
1.7
3.9
Hydrogen H2(2)
4.9
4.9
1.5
3.2
1.5
3.2
Hydrogen H2(3)
11.9
11.9
8.4
12.6
8.4
12.6




Table 4-13c: Energy output (EJ) of selected energy technologies in B2-MESSAGE.

 
1990
2050
2100

Coal conversion
16.2
9.7
0.0
IGCC
0.0
15.9
65.1
Coal fuel cell
0.0
0.0
0.0
Oil
4.8
0.1
0.0
Gas standard
5.7
0.4
0.0
NGCC
0.6
45.0
72.7
NGFC
0.0
5.1
0.0
Biofuel
0.5
2.8
22.3
Nuclear
5.7
18.9
50.6
Advanced nuclear/other
1.3
27.9
88.7
Hydro
7.9
19.7
28.4
Wind
0.011
11.5
17.2
Other renewables
0.11
18.4
50.4
Hydrogen fuel cell
0.000
10.5
11.4
Photo-voltaic
0.001
25.2
57.3
Coal synliquids
0.0
4.2
71.8
Biomass synliquids
1.5
31.8
34.9
Gas synliquids
0.0
13.0
39.5
Syngases
0.0
0.1
0.0
Hydrogen H2(1)
0.0
36.4
0.0
Hydrogen H2(2)
0.0
10.4
0.0
Hydrogen H2(3)
0.0
0.0
0.0




Table 4-13d : Levelized costs (1990US$/ GJ) of selected energy technologies (excluding fuel costs) in MESSAGE scenarios relative to the costs in the B2- MESSAGE marker scenario (minima and maxima for eleven world regions). The A1C and A1G scenario groups have been combined into the fossil- intensive A1FI group in the SPM (see also footnote 1).

  2050 2100

  B1 A1B A1C A1G A1T A2a B1 A1B A1C A1G A1T A2a
 
  min 
  max
  min
  max
  min
  max
  min
  max
  min
  max
  min
  max
  min
  max
  min
  max
  min
  max
  min
  max
  min
  max
  min
  max

Coal conversion
0
0
0
0
0
0
0
0
0
0
-0.6
0
0
0
0
0
0
0
0
0
0
0
-0.6
0
IGCC
-0.8
-1.1
-0.6
-0.8
-0.6
-0.8
1.1
0.8
-0.6
-0.8
-0.8
-0.3
0.0
-1.7
0.3
-1.1
0.8
-0.8
2.5
0.8
0.3
-1.1
0
-0.6
Coal fuel cell
-2.2
-2.2
-2.2
-2.2
-2.2
-2.2
-0.3
-0.3
-2.2
-2.2
0
0
-2.5
-2.5
-2.5
-2.5
-2.5
-2.5
-0.3
-0.3
-2.5
-2.5
0
0
Oil
-1.1
0
-1.1
0
0
0
-1.1
0
-1.1
0
0.3
-0.6
-1.1
0
-1.1
0
0
0
-1.1
0
-1.1
0
0.3
-0.6
Gas standard
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
NGCC
-1.4
-0.6
-1.4
-0.6
-0.3
0.6
-1.1
-0.6
-1.1
-0.6
-0.3
0.6
-0.8
0
-0.8
0
0.3
1.1
-0.6
0.0
-0.6
0
0
1.1
NGFC
-1.4
-1.4
-1.4
-1.4
0
0
-1.1
-1.1
-1.4
-1.4
0
0
-2.2
-2.2
-2.2
-2.2
0
0
-1.1
-1.1
-2.2
-2.2
0
0
Biofuel
-0.6
-1.7
-0.6
-1.7
0.0
0.0
-0.3
-1.1
-0.6
-1.7
0
0
-1.4
-2.8
-1.4
-2.8
0
0
-0.3
-1.1
-1.4
-2.8
0
0
Nuclear
0
0
0
0
1.4
0
0
0
0
0
0
0
0
0
0
0
1.4
0
0
0
0
0.0
0
0
Advanced nuclear/other
-2.2
0.6
-3.9
0.6
-1.1
3.3
-1.1
1.9
-3.6
0.6
0
0
-5.3
2.2
-6.4
-2.5
-2.2
3.3
-2.2
-0.3
-5.8
-1.4
0
0
Hydro
0
0
0
0
0
0
0
0
0
0
0.3
-5.6
0
0
0
0
0
0
0
0
0
0
0.3
-5.6
Wind
-2.8
-2.8
-4.7
-4.7
3.3
3.3
-2.8
-2.8
-4.7
-4.7
0.0
0.6
-4.2
-4.2
-6.4
-6.4
3.3
3.3
-2.8
-2.8
-6.4
-6.4
0.0
0.6
Other renewables
-3.3
-2.8
-4.4
-2.8
0.8
12.8
-3.9
-2.8
-4.4
-2.8
0.8
13.3
-5.3
-2.8
-6.1
-2.8
0.8
12.8
-4.2
-2.8
-6.1
-2.8
0.8
10.8
Hydrogen fuel cell
-1.3
-1.3
-1.2
-1.2
0
0
-0.9
-0.9
-1.5
-1.5
0
0
-2.1
-2.1
-1.8
-1.8
0
0
-0.8
-0.8
-2.3
-2.3
0
0
Photo-voltaic
-4.2
-5.8
-5.3
-7.5
8.1
11.9
0.0
0.0
-5.3
-7.5
0.6
11.1
-6.1
-5.8
-6.7
-9.4
8.1
11.9
0.0
0.0
-6.7
-9.4
0.6
11.1
Coal synliquids
-1.3
-1.8
-1.7
-0.9
-2.1
-0.9
-1.7
-0.9
-1.7
-0.9
-1.3
-0.9
-1.3
-1.8
-1.7
-0.9
-2.4
-0.9
-1.7
-0.9
-1.7
-0.9
-1.3
-0.9
Bio synliquids
-1.7
-1.7
-1.7
-1.7
0.0
0.0
-0.8
-0.8
-1.7
-1.7
0.0
0.0
-2.4
-1.7
-2.3
-1.7
0.0
0.0
-0.8
-0.8
-2.4
-1.7
0.0
0.0
Gas synliquids
-0.5
-0.5
1.1
1.1
0.0
0.0
1.0
1.0
1.1
1.1
0.0
0.0
-0.8
-0.5
1.1
1.1
0.0
0.0
1.0
1.0
1.1
1.1
0.0
0.0
Syngases
-0.5
-1.0
-0.5
-1.0
0.0
-0.5
-0.5
0.6
-0.5
-1.0
0.0
0.0
-0.6
-1.0
-0.5
-1.0
0.0
-0.5
-0.5
0.6
-0.6
-1.0
0.0
0.0
Hydrogen H2(1)
-0.3
-0.7
-0.3
-0.7
0.8
1.6
-0.3
-0.6
-0.3
-0.7
0.0
0.0
-0.7
-0.7
-0.7
-0.7
0.8
1.6
-0.3
-0.6
-0.7
-0.7
0.0
0.0
Hydrogen H2(2)
0.0
-0.4
-0.2
-0.4
0.3
1.4
0.0
-0.3
-0.2
-0.4
0.0
0.0
0.0
-0.4
-0.5
-0.4
0.1
1.4
0.0
-0.3
-0.5
-0.4
0.0
0.0
Hydrogen H2(3)
-3.0
-1.6
-5.0
-7.4
5.5
1.3
-8.4
-12.6
-5.0
-2.3
7.1
8.6
-3.0
-1.6
-5.5
-7.4
5.5
1.3
-8.4
-12.6
-5.5
-2.3
7.1
8.6

A. Cost variations refer to a four- region model only. The spread across regions is therefore somewhat smaller than in the other scenarios.



Table 4- 13e: Energy output (EJ) of selected energy technologies in MESSAGE scenarios relative to the B2- MESSAGE marker scenario. The A1C and A1G scenario groups have been combined into the fossil- intensive A1FI group in the SPM (see also footnote 1).

  2050 2100

 
B1
A1B
A1C
A1G
A1T
A2a
B1
A1B
A1C
A1G
A1T
A2a

Coal conversion
-8.6
6.5
36.0
26.9
4.7
16.9
0.0
1.8
6.5
7.5
0.1
7.2
IGCC
-15.6
8.8
-4.7
-2.9
-12.3
25.2
-65.1
-26.3
-17.6
-65.1
-65.1
62.7
Coal fuel cell
0.0
2.2
12.9
2.4
0.0
0.0
0.0
0.1
204.9
0.0
0.0
0.0
Oil
0.0
0.0
0.0
7.8
0.0
0.0
0.0
0.0
0.0
1.0
0.0
0.0
Gas standard
-0.3
-0.3
-0.3
3.8
0.7
-0.3
0.0
0.0
0.0
0.0
0.0
0.0
NGCC
-15.1
34.4
-7.7
13.0
5.5
-18.1
-64.7
102.8
-67.8
143.4
-43.5
-2.0
NGFC
-5.1
10.0
10.3
26.2
-1.6
-3.6
0.0
0.0
0.0
60.5
0.0
0.0
Biofuel
-2.2
5.0
6.7
12.1
0.4
-0.5
-19.1
55.3
-19.3
-13.4
-15.9
3.9
Nuclear
-13.1
-9.2
-11.1
-5.3
-11.3
13.4
-50.0
-41.2
-50.6
-22.5
-50.6
31.2
Advanced nuclear/other
-4.0
44.8
83.6
47.2
44.8
-14.7
-52.0
255.0
253.5
211.1
16.2
-38.4
Hydro
0.4
8.7
7.2
7.3
3.5
1.4
-3.6
11.3
12.5
8.2
-2.6
3.8
Wind
-0.2
5.5
-0.6
2.5
4.4
1.7
-7.9
14.6
1.7
1.9
0.3
1.0
Other renewables
1.9
9.2
-12.8
3.0
8.5
-10.0
3.8
21.2
-10.4
2.0
18.8
7.2
Hydrogen fuel cell
57.0
5.2
-10.5
9.8
41.3
-8.7
84.2
92.7
-11.4
28.5
309.7
-9.4
Photo-voltaic
-1.8
12.9
8.5
14.2
9.7
-2.9
-29.3
58.3
42.2
45.9
34.3
-0.4
Coal synliquids
1.2
34.7
177.0
29.3
24.8
48.4
-69.4
-57.0
421.3
-48.9
-57.7
258.0
Biomass synliquids
-5.6
-30.4
1.3
-30.7
-22.2
-17.1
-9.0
-34.6
-6.5
7.9
-20.4
34.9
Gas synliquids
-3.3
-3.2
24.7
-6.5
-5.3
-5.1
-27.9
-33.1
137.3
-5.1
-29.1
64.0
Syngases
0.5
6.4
2.0
0.5
-0.1
0.5
60.6
854.7
84.3
0.0
990.2
35.6
Hydrogen H2(1)
75.2
-8.4
-24.3
45.9
-6.5
-15.2
0.0
0.0
0.0
0.0
0.0
0.0
Hydrogen H2(2)
17.3
-5.6
-9.6
-10.1
-9.5
-5.8
0.0
0.0
0.0
0.0
0.0
0.0
Hydrogen H2(3)
62.7
97.9
5.1
0.0
125.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0

A. Calculated with a four- region model. The spread across regions is therefore somewhat smaller than in the other scenarios.



Important differences exist in accounting conventions on how to calculate the primary energy equivalent of particularly renewable and nuclear energy (see Watson et al., 1996). To assure comparability of model results, the SRES writing team agreed to adopt as a common accounting methodology the direct equivalent method for all non-thermal uses of renewables and nuclear. The primary energy equivalence of these energy forms is accounted for at the level of secondary energy, that is, the first usable energy form or "currency" available to the energy system. For instance, the primary energy equivalence of electricity generated from solar photo-voltaics or nuclear power plants is set equal to their respective gross electricity output, not to the heat equivalent of radiation energy from fissile reaction, the solar radiance that falls onto a photo-voltaic panel and is converted into electricity with efficiencies that range from 10% to 15%, or the heat that would have to be generated by burning fossil fuels to produce the same amount of electricity as generated in a photo-voltaic cell or a nuclear reactor (as used in the so-called "substitution" accounting method). This common 33 SRES accounting convention must be borne in mind when comparing the primary energy-use figures of this report with those of other studies, which invariably use different accounting methods depending on the organization that produces the scenario. An illustration of the sensitivity of different accounting methods on estimates of primary energy use in long-term energy scenarios is given in Nakicenovic et al. (1998). (See also the discussion in Chapter 2, in which scenario comparisons are based on index numbers rather than absolute figures to account for these definitional differences.)


Table 4-14:Primary energy use (EJ) for the four SRES marker scenarios and all SRES scenarios in 1990, 2020, 2050, and 2100. The range for 1990 illustrates the differences in base-year calibration across the models and uncertainties that stem from the inclusion or exclusion of non-commercial energy use, which is particularly important for developing countries.

2050 2100

Region
1990
A1
A2
B1
B2
A1
A2
B1
B2

OECD90
151-182
267
(184-315)
266
(207-300)
166
(134-233)
236
(189-236)
397
(181-607)
418
(267-496)
126
(126-274)
274
(197-274)
REF
69-95
103
(83-267)
93
(57-116)
64
(50-79)
97
(53-117)
139
(70-290)
155
(61-457)
39
(25-80)
125
(40-328)
IND
227-252
370
(303-532)
359
(264-406)
230
(203-303)
334
(255-339)
536
(275-896)
573
(385-847)
164
(164-345)
399
(237-593)
ASIA
49-79
440
(293-789)
335
(249-449)
272
(204-537)
319
(284-411)
838
(308-965)
581
(477-753)
154
(154-434)
521
(309-562)
ALM
35-49
538
(235-634)
278
(166-354)
312
(176-312)
217
(137-254)
852
(391-1109)
563
(437-662)
196
(196-446)
437
(300-538)
DEV
84-123
977
(606-1278)
612
(415-740)
583
(406-837)
536
(421-660)
1639
(700-2074)
1144
(914-1375)
350
(350-880)
959
(609-1096)
WORLD
326-368
1347
(913-1611)
971
(679-1059)
813
(642-1090)
869
(679-966)
2226
(1002-2737)
1717
(1304-2040)
514
(515-1157)
1357
(846-1625)


Table 4-14 gives an overview of primary energy use in the four SRES marker scenarios and the range of all SRES scenarios.

Figure 4-11: Global primary energy structure, shares (%) of oil and gas, coal, and non-fossil (zero-carbon) energy sources - historical development from 1850 to 1990 and in SRES scenarios. Each corner of the triangle corresponds to a hypothetical situation in which all primary energy is supplied by a single source - oil and gas, coal at the left, and non-fossil sources (renewables and nuclear) to the right. Constant market shares of these energies are denoted by their respective isoshare lines. Historical data from 1850 to 1990 are based on Nakicenovic et al. (1998). For 1990 to 2100, alternative trajectories show the changes in the energy systems structures across SRES scenarios. They are grouped by shaded areas for the scenario families A1, A2, B1, and B2 with respective markers shown as lines. In addition, the four scenario groups within the A1 family (A1, A1C, A1G, and A1T) that explore different technological developments in the energy systems are shaded individually. The A1C and A1G scenario groups have been merged into one fossil-intensive A1FI scenario group in the SPM (see footnote 1). For comparison the IS92 scenario series are also shown, clustering along two trajectories (IS92c,d and IS92a,b,e,f). For model results that do not include non-commercial energies, the corresponding estimates from the emulations of the various marker scenarios by the MESSAGE model were added to the original model outputs.

Figure 4-11 illustrates both the historical change of world primary energy structure over time and future changes as given in the SRES scenarios. Each corner of the triangle corresponds to a hypothetical situation in which all primary energy is supplied by a single source - oil and gas at the top, coal at the left, and non-fossil sources, renewables (including wood), and nuclear at the right. The historical change reflects major technology shifts from the traditional use of renewable energy flows to the coal and steam age of the 19 th century, and subsequently to the dominance of oil and internal combustion engines in the 20 th century. In around 1850 (lower right of Figure 4-11), only about 20% of world primary energy was provided by coal; the other 80% was provided by traditional renewable energies (biomass, hydropower, and animal energy). With the rise of industrialization, coal substituted for traditional renewable energy forms, and by 1910 (lower left of Figure 4-11) around three-quarters of world primary energy use relied on coal. The second major transition was the replacement of coal by oil and later by gas. By the early 1970s (see 1970 point labeled on Figure 4-11), 56% of global primary energy use was based on oil and gas. From the early 1970s to 1990, the global primary energy structure has changed little, although efforts to substitute for oil imports have led to an increase in the absolute amount of coal used and to the introduction of non-fossil alternatives in the OECD countries (e.g., nuclear energy in France). Rapid growths in energy demand and coal use, particularly in Asia, have outweighed structural changes in the OECD countries.

Figure 4-11 also gives an overview of the evolution of the global energy system between 1990 and 2100 as reflected in the SRES scenarios. The four marker scenarios are shown as thick lines. In addition, for each scenario family the area spanned by all the SRES scenarios in that family is marked in the same color as the trajectory for the respective marker. The SRES scenarios cover a wider range of energy structures than the previous IS92 scenario series, reflecting advances in knowledge on the uncertainty ranges of future fossil resource availability and technological change. Scenarios B1, B2, A1T, and to some extent A1B follow a trend toward increasing shares of zero-carbon options in the long term. A1G more or less follows an oil-gas isoshare line that perpetuates the current dominance of oil and gas in the global energy balance far into the 21st century. Scenarios in group A1C indicate a near doubling of coal's share in primary energy use. Also of interest is the trajectory of the A2 marker scenario, which returns in its energy structure by 2100 (over 50% coal share) to the situation that prevailed almost 200 years before (i.e., around 1900). However, even with similar fuel shares, the technologies, end-use fuels, and applications projected in the A2 scenario are radically different from those of the past.



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