and Climate
Climate Change Scenarios: IPCC story lines, models, downscaling

    Sources used:
IPCC Scenarios IPCC climate models Regional scenarios Local downscaling

IPCC SRES (Special Report on Emissions Scenarios - SRES) scenarios were constructed to explore future developments in the global environment with special reference to the production of greenhouse gases and aerosol precursor emissions.

The IPCC SRES scenarios contain various driving forces of climate change, including population growth and socio-economic development. These drivers encompass various future scenarios that might influence greenhouse gas (GHG) sources and sinks, such as the energy system and land use change. The evolution of driving forces underlying climate change is highly uncertain. This results in a very wide range of possible emissions paths of greenhouse gases.

The SRES team defined four narrative storylines (see Figure 1), labeled A1, A2, B1 and B2, describing the relationships between the forces driving greenhouse gas and aerosol emissions and their evolution during the 21st century for large world regions and globally. Each storyline represents different demographic, social, economic, technological, and environmental developments that diverge in increasingly irreversible ways.

  • A1: globalization, emphasis on human wealth Globalized, intensive (market forces)
    The A1 storyline and scenario family describes a future world of very rapid economic growth, global population that peaks in mid-century and declines thereafter, and the rapid introduction of new and more efficient technologies. Major underlying themes are convergence among regions, capacity building, and increased cultural and social interactions, with a substantial reduction in regional differences in per capita income. The A1 scenario family develops into three groups that describe alternative directions of technological change in the energy system. The three A1 groups are distinguished by their technological emphasis: fossil intensive (A1FI), non-fossil energy sources (A1T), or a balance across all sources.

  • A2: regionalization, emphasis on human wealth Regional, intensive (clash of civilizations)
    The A2 storyline and scenario family describes a very heterogeneous world. The underlying theme is self-reliance and preservation of local identities. Fertility patterns across regions converge very slowly, which results in continuously increasing global population. Economic development is primarily regionally oriented and per capita economic growth and technological change are more fragmented and slower than in other storylines.

  • B1: globalization, emphasis on sustainability and equity Globalized, extensive (sustainable development)
    The B1 storyline and scenario family describes a convergent world with the same global population that peaks in midcentury and declines thereafter, as in the A1 storyline, but with rapid changes in economic structures toward a service and information economy, with reductions in material intensity, and the introduction of clean and resource-efficient technologies. The emphasis is on global solutions to economic, social, and environmental sustainability, including improved equity, but without additional climate initiatives.

  • B2: regionalization, emphasis on sustainability and equity Regional, extensive (mixed green bag)
    The B2 storyline and scenario family describes a world in which the emphasis is on local solutions to economic, social, and environmental sustainability. It is a world with continuously increasing global population at a rate lower than A2, intermediate levels of economic development, and less rapid and more diverse technological change than in the B1 and A1 storylines. While the scenario is also oriented toward environmental protection and social equity, it focuses on local and regional levels.

Causes of climate change: driving forces and emissions

The IMAGE 2.2 model (Integrated Model for Assessment of the Greenhouse Effect) considers two major closely interlinked driving forces:

  • Population development
  • World Economy
which, in turn drive the major "emitters"
  • The energy system
  • Land use (change)

World Economy: growth, distribution, technology

Economic development is expressed in GNP (Gross National Product). The SRES scenarios span a wide range of future levels of economic activity. The highest overall prediction is for the A1 scenario; an estimated GNP of US$529 trillion (1990 US dollars) in 2100. The lowest overall prediction is for the B2 scenario; an estimated GNP of US$235 trillion in 2100. This means that globalization combined with an emphasis on wealth would generate the highest economic growth. This is mainly because population growth is lower in a global scenario, causing a narrower division of the GNP. The emphasis on wealth rather than on sustainability also increases the GNP. It is estimated that the future income gap between developed and developing countries will be smaller than was initially estimated in the IS92 scenarios.

A key element, however, is technological development and the implied energy efficiency of future economic development.

Population development

Population growth is determined by fertility and mortality rates. Global population projections range from 7,1 to 15 billion people (or more than double the minimum estimate, i.e., an "uncertainty" of a factor of two in this key driving variable) by 2100 across the scenarios, depending on the rate and extent of the demographic transition. Figure 3 shows that population growth is strongest in the regional and material scenario (A2) for 1992 results. Regionalization causes more population growth than does globalization.

Figure 3 shows Population projections - historical data from 1900 to 1990 (based on Durand, 1967; Demeny; 1990; UN, 1998), and SRES scenarios (based on Lutz, 1996, for high and low, and UN, 1998, for medium) and IPCC IS92 scenarios (Leggett et al., 1992; Pepper et al., 1992) from 1990 to 2100. (

Emission, clearly are nor only driven by the basi number, but at least as much by lifestyles, and the associated economic actitives and technologies used. Pattern of production and consumption are bound to change with changin population densitiies, and the ratio between rural, agricultural population and activities and the urban. industrial or service sector employed (or unemployed) parts of the total population also determice the resulting emission patterns.


Energy system

The impact of future energy use will largely depend on the fuel type. Both global scenarios depict a transition towards more non-fossil fuel sources. In the regional sustainable scenario the transition towards non-fossil fuel sources is much more gradual. The regional wealth scenario marks a stark transition back to fossil fuels. In all scenarios the share of oil and gas declines and more coal will be used for energy generation in the future.

Figure 4 summarizes the 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 on the top, coal to 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 A1B, A2, B1, and B2 with respective markers shown as lines. In addition, the four scenario groups within the A1 family A1B, A1C, A1G, and A1T, which explore different technological developments in the energy systems, are shaded individually. In the SPM, A1C and A1G are combined into one fossil-intensive group A1FI. 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.

Land use change

There are many different land use types. The main land use types that are considered by the IPCC are forests, arable land and grassland. Land use change is largely related to demands for food by a growing population and changing diets.

Currently there is a lot of deforestation. In most SRES scenarios, the current trend of deforestation is eventually reversed because of slower population growth and increased agricultural productivity. Reversals of deforestation trends are strongest in the globalized scenarios. In the globalized sustainable scenario pasture lands decrease significantly because of increased productivity in livestock management and dietary shifts away from meat.

Figure 5 shows global land-use patterns, shares (%) of croplands and energy biomass, forests, and other categories including grasslands - historical development from 1970 to 1990 (based on B1-IMAGE) and in SRES scenarios. As for the energy triangle in Figure 4, each corner corresponds to a hypothetical situation in which land use is dedicated to a much greater extent than today to one category - 60% to cropland and energy biomass at the top, 80% to forests to the left, and 80% to other categories (including grasslands) to the right. Constant shares in total land area of cropland and energy biomass, forests, and other categories are denoted by their respective isoshare lines. For 1990 to 2100, alternative trajectories are shown for the SRES scenarios. The three marker scenarios A1B, B1, and B2 are shown as thick colored lines, and other SRES scenarios as thin colored lines. The ASF model used to develop the A2 marker scenario projects only land-use change related GHG emissions. Comparable data on land cover changes are therefore not available.The trajectories appear to be largely model specific and illustrate the different views and interpretations of future land-use patterns across the scenarios (e.g. the scenario trajectories on the right that illustrate larger increases in grasslands and decreases in cropland are MiniCAM results).

Table 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).

scenario CO2
(MtCH4 )
( MtN)
(MtC equiv.)
(Mt CO)
A1B 13.5
(13.5- 17.9)
(289- 640)
(5.8- 17.2)
(1080- 2532)
(133- 552)
(40.2- 77.0)
(27.6- 71.2)
A1C (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)
A1G (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)
A1T (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)
A2 29.1
(19.6- 34.5)
(549- 1069)
(8.1- 19.3)
(776- 2646)
(169- 342)
(70.9- 110.0)
(60.3- 92.9)
B1 4.2
(2.7- 10.4)
(236- 579)
(5.3- 20.2)
(363- 1871)
(58- 349)
(16.0- 35.0)
(11.4- 24.9)
B2 13.3
(10.8- 21.8)
(465- 613)
(6.9- 18.1)
(661- 2002)
(130- 304)
(34.5- 76.5)
(33.3- 47.9)

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