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Emission Scenarios

What are Emissions Scenarios?

In order to determine the impact of climate change in the future, we need to have an idea of the concentrations of greenhouse gases and other pollutants in the atmosphere to which climate is sensitive, in the years to come.  These concentrations depend on their emissions from various sources, natural as well as man-made.

Emissions scenarios describe future releases into the atmosphere of greenhouse gases, aerosols, and other pollutants and, along with information on land use and land cover, provide inputs to climate models. They are based on assumptions about driving forces such as patterns of economic and population growth, technology development, and other factors. Levels of future emissions are highly uncertain, and so scenarios provide alternative images of how the future might unfold. They provide an appropriate tool with which to analyse how driving forces may influence future emission outcomes and to assess the associated uncertainties. They assist in climate change analysis, including climate modelling and the assessment of impacts, adaptation, and mitigation. The possibility that any single emissions path will occur as described in scenarios is highly uncertain.

The following emissions scenarios have been used by researchers and other analysts to project future climate change and develop mitigation strategies.

Background to Emissions Scenarios

A set of scenarios was developed to represent the range of driving forces and emissions in the scenario literature so as to reflect current understanding and knowledge about underlying uncertainties. They exclude only outlying “surprise” or “disaster” scenarios in the literature.

Any scenario necessarily includes subjective elements and is open to various interpretations. No judgment is offered by the IPCC as to the preference for any of the scenarios and they are not assigned probabilities of occurrence, neither should they be interpreted as policy recommendations.  The Special Report on Emissions Scenarios (SRES), published by the  IPCC in 2000, describes the emissions scenarios that have been used to make projections of possible future climate change, for the IPCC Third Assessment Report (TAR), published in 2001, and in the IPCC Fourth Assessment Report (AR4), published in 2007. The projected CO2 emissions for these scenarios are shown below.

Carbon emissions for four emission scenarios

Carbon emissions for four emission scenarios (Image: IPCC)

The SRES Scenarios

There are 40 different scenarios, each making different assumptions for future greenhouse gas pollution, land-use and other driving forces.  These emissions scenarios are organized into families, which contain scenarios that are similar to each other in some respects.  The following are the major families of SRES emissions scenarios:

  • 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).
  • 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.
  • 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 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.
  • 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.

[Further information] about how these Scenarios are used can be found in the climate projections webpage.

[Technical information] on SRES can be found in the IPCC’s special report on emission scenarios

Representative Concentration Pathways

The Representative Concentration Pathways (RCP) are based on selected scenarios from four modelling teams/models working on integrated assessment modelling, climate modelling, and modelling and analysis of impacts.  The RCPs are not new, fully integrated scenarios (i.e., they are not a complete package of socioeconomic, emissions, and climate projections). They are consistent sets of projections of only the components of radiative forcing (the change in the balance between incoming and outgoing radiation to the atmosphere caused primarily by changes in atmospheric composition) that are meant to serve as input for climate modelling. Conceptually, the process begins with pathways of radiative forcing, not detailed socioeconomic narratives or scenarios. Central to the process is the concept that any single radiative forcing pathway can result from a diverse range of socioeconomic and technological development scenarios. Four RCPs were selected, defined and named according to their total radiative forcing in 2100 (see table below). Climate modellers will conduct new climate model experiments using the time series of emissions and concentrations associated with the four RCPs, as part of the preparatory phase for the development of new scenarios for the IPCC's Fifth Assessment Report (expected to be completed in 2014) and beyond.

Table 1.1: Overview of Representative Concentration Pathways (RCPs)

RCP 8.5 Rising radiative forcing pathway leading to 8.5 W/m² in 2100.
RCP 6 Stabilization without overshoot pathway to 6 W/m² at stabilization after 2100
RCP 4.5 Stabilization without overshoot pathway to 4.5 W/m² at stabilization after 2100
RCP 3-PD2 Peak in radiative forcing at ~ 3 W/m² before 2100 and decline

[Technical information] The RCP data and information on their intended uses and limits can be found here.

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