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In order to determine how climate may change in the future we need to know how the concentrations of those atmospheric components which affect the Earth's energy balance may change. Gases such as water vapour, carbon dioxide, methane and nitrous oxide (the greenhouse gases) absorb long-wave (heat) radiation emitted from the Earth's surface and re-emit this energy, ultimately resulting in raised surface temperatures. Whilst these greenhouse gases occur naturally, human activities since the beginning of the industrial revolution have resulted in large increases in the atmospheric concentrations of these gases and it is now widely accepted that this has affected global climate.
Trying to determine how atmospheric composition may change in the future is fraught with uncertainty since it is necessary to make assumptions about how both the natural and anthropogenic emissions of these greenhouse gases will change which, in turn, is dependent on assumptions regarding population growth, economic activity, energy use, land use change, etc.
For the IPCC Third Assessment Report (IPCC, 2001), a new set of emissions scenarios was commissioned to replace the six IS92 emissions scenarios (Leggett et al., 1992) detailed in the 1992 Supplement (IPCC, 1992) to the IPCC First Assessment Report (IPCC, 1990). Until recently, the IS92a scenario, a 'business-as-usual' type scenario, had been in wide use by the climate modelling and vulnerability, impacts and adaptation communities.
The IPCC Special Report on Emissions Scenarios (SRES; Nakicenovic et al., 2000) details 4 storylines, narratives of qualitative (e.g., political, social, cultural and educational conditions) emissions drivers. The SRES emissions scenarios are the quantitative interpretations of these qualitative storylines. Six international modelling teams (see Table 1) were involved in quantifying the SRES storylines, which resulted in the formulation of 40 alternative SRES scenarios, of which no single scenario is treated as more or less probable than others belonging to the same scenario family. In order to reduce the number of scenarios to be used in climate change studies, six marker, or illustrative, scenarios have been selected based on the consensus opinion of the modelling teams. These are A1FI, A1T and A1B from the A1 family, and A2, B1 and B2.
Table 1: Modelling teams involved in quantifying the SRES storylines
The Six SRES Marker Scenarios
Only a brief introduction to the SRES emissions scenarios is given here. Full details can be found in the Special Report on Emissions Scenarios (Nakicenovic et al., 2000). (Full text for other IPCC Special Reports and also the Third Assessment Report can be found on the IPCC web site.)
A very simplistic representation of the six SRES Marker Scenarios is given in Figure 1.
Figure 1: A schematic representation of the SRES scenario family. The A1 and A2 families have a more economic focus than B1 and B2, which are more environmental, whilst the focus of A1 and B1 is more global compared to the more regional A2 and B2.
The quantitative inputs for each scenario are, for example, regionalised measures of population, economic development and energy efficiency, the availability of various forms of energy, agricultural production and local pollution controls. Explicit policies to limit greenhouse gas emissions or to adapt to the expected global climate change are NOT included. Details of these inputs (population, energy use etc.) for each scenario can be found in Appendix VII: Data Tables of the SRES.
A1FI, A1T and A1B
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 (A1B; where balanced is defined as not relying too heavily on one particular energy source, on the assumption that similar improvement rates apply to all energy supply and end use technologies).
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 population. Economic development is primarily regionally oriented and per capita economic growth and technological change more fragmented and slower than other storylines.
The B1 storyline and scenario family describes a convergent world with the same global population that peaks in mid-century and declines thereafter, as in the A1 storyline, but with rapid change 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.
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 towards environmental protection and social equity, it focuses on local and regional levels.
Changes in global-mean temperature associated with each of the six marker scenarios are illustrated in Figure 2. The response of global-mean temperature to the different emissions scenarios can be determined by using a relatively simple upwelling diffusion energy balance (UD/EB) climate model, such as the one developed by Wigley and Raper (1992). This model distinguishes between land and ocean and between the hemispheres, but simulates only the underlying signal in response to external forcing and not the variability.
Figure 2: Global-mean temperature change (°C) associated with the six SRES marker scenarios, A1FI, A1T, A1B, A2, B1 and B2. These figures have been derived using a simple climate model. The 'several models all SRES envelope' shows the temperature projections for the simple model when tuned to a number of complex models with a range of climate sensitivities. [Source: IPCC WGI Summary for Policymakers]
IS92 emissions scenariosIn the 1992 Supplement (IPCC, 1992) to the IPCC First Assessment Report (IPCC, 1990), Leggett et al. (1992) proposed six emissions scenarios, the IS92 scenarios, which reflected the large uncertainty associated with, for example, the evolution of population and economic growth, technological advances, technology transfer and responses to environmental, economic or institutional constraints.
IS92a: a middle of the range scenario in which population rises to 11.3 billion by 2100, economic growth averages 2.3% year -1 between 1990 and 2100 and a mix of conventional and renewable energy sources are used. Only those emissions controls internationally agreed upon and national policies enacted into law, e.g., London Amendments to the Montreal Protocol, are included.
IS92b: population rises to 11.3 billion by 2100 and the current emissions control policies are enlarged to include stated policies beyond those legally adopted, e.g., all CO2 commitments of OECD member countries are included along with an assumption of world-wide ratification and compliance with the amended Montreal Protocol.
IS92c: economic growth averages 1.2% year-1 between 1990 and 2100 and population is forecast to be 6.4 billion by 2100, with population decreasing in the 21st century. As well as assuming lower growth in GNP per capita than IS92a and IS92b, low oil and gas resource availability results in higher prices which promote the expansion of nuclear and renewable energy. Lower population growth results in slower deforestation rates.
IS92d: another low scenario, but more optimistic than IS92c. The trend is towards increasing environmental protection but only actions that could be taken due to concerns about local or regional air pollution and waste disposal are included. Population is forecast to be 6.4 billion by 2100 and would be associated with lower natality, falling below the replacement rate late in the 21st century, due, for example, to improvements in per capita income or increased family planning. Low fossil fuel resource availability means that there is greater market penetration of renewable energy and safe nuclear power. A 30% environmental surcharge on fossil energy use is levied to meet the costs of more stringent local pollution controls. Greater well-being is assumed to lead to voluntary actions to halt deforestation, to adopt CFC substitutes with no radiative or other adverse effects and to recover and efficiently use the methane from coal mines and land fills.
IS92e: results in the highest CO2 emissions. Economic growth averages 3% year-1, between 1990 and 2100 and the population is forecast to reach 11.3 billion by 2100. Fossil resources are plentiful but, due to assumed improvements in living standards, environmental surcharges are imposed on their use. Nuclear energy is phased out by 2075 and, although CFC substitute assumptions are the same as those of IS92d, the plentiful fossil fuel resources discourage the additional used of coal mine methane for energy supply. Deforestation proceeds at the same pace as IS92a.
IS92f: falls below IS92e, has high population growth (17.6 billion by 2100), but lower assumptions of improvements in GNP per capita than IS92a. Other assumptions are high fossil fuel resource availability, increasing costs of nuclear power and less improvement in renewable energy technologies and costs.
These six emissions scenarios were considered to be equally likely.
Figure 3 illustrates the global-mean temperature change associated with the six IS92 scenarios.
Figure 3: Global-mean temperature change (°C) associated with the IS92 emissions scenarios. IS92a is shown in bold.
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Updated Wed Sep 17 16:23:34 2003
IMPORTANT NOTE: CICS operated from 1993-2006. These pages serve only to document the projects, scenarios, publications, and products undertaken during the period when CICS was active. In 2006, CICS became the Pacific Climate Impacts Consortium (PCIC).
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