The scientific community is in the midst of developing a new set of scenarios to underpin climate change research and permit the integrated analysis of future climate impacts, vulnerabilities, adaptation, and mitigation. These alternative pathways of plausible future development require the assessment of atmospheric composition; the magnitude and extent of future climate change; associated potential impacts on physical, natural, and human systems; the costs and possible effectiveness of mitigation and adaptation policies; the interactions among and trade-offs between climate change impacts and policies of adaptation and mitigation; and the relationship between climate change and socioeconomic development (Moss et al. 2008, 2010). These scenarios are being designed to represent a wide range of uncertainty in mitigation efforts required to achieve particular radiative forcing pathways, and in adaptation efforts that could be undertaken to prepare for and respond to the climatic changes and impacts associated with those pathways.
We summarize the rationale for and process of developing a conceptual framework for the new scenarios. We also discuss a number of important research directions required to further develop and apply the conceptual framework. The conceptual framework is described in the subsequent three papers in this Special Issue (van Vuuren et al. 2013; O’Neill et al. 2013; Kriegler et al. Submitted for publication in this special issue).
The new scenario process builds on previous processes of global scenario development, particularly those directed specifically at climate change, including the IS92 scenarios (Leggett et al. 1992) and the scenarios developed in the 1990s for the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenarios (SRES) (Nakicenovic et al. 2000). The SRES scenarios were developed to represent the range of driving forces and emissions in the scenario literature, including reflecting understanding about underlying uncertainties. They were developed by first creating internally consistent storylines of possible future worlds (Nakicenovic et al. 2000). The four main storylines (labeled using neutral identifiers A1, A2, B1 and B2) describe the relationships between driving forces of emissions of greenhouse gases and other radiatively active substances and their evolution over time. Each storyline represents different demographic, social, economic, technological, and environmental development pathways, intentionally designed to produce a wide range of future emissions pathways. For each storyline, several scenarios were developed using different integrated models of the global energy-economy-environment system to examine the range of possible outcomes associated with similar assumptions about driving forces. Altogether, 40 scenarios were quantified for the SRES report, six of which were selected as illustrative scenarios (one for each of the storylines, plus additional high and low emissions variants of the A1 storyline). By design, the SRES scenarios assumed no specifically targeted climate mitigation or adaptation policies and measures. Quantification of the storylines resulted in estimated emissions of greenhouse gases and sulfur that were used as input into climate models to project changes in patterns of climate variables such as temperature and precipitation. These projections have been extensively used to estimate possible impacts associated with each scenario.
Progress since the SRES in demographic and socioeconomic projections, and in impact, adaptation, and mitigation analyses, has fueled demand for this knowledge to be incorporated into new scenarios. The SRES scenarios are becoming dated in terms of scientific understanding and in their demographic and socioeconomic assumptions. New scenarios are needed to address more effectively questions concerning the policies and practice of preparing for and responding to climate change. Such scenarios also need to cover a wider range of greenhouse gas concentrations (including those that can be reached by implementing mitigation measures) and to facilitate improved integration of mitigation, adaptation, and impact analyses.
The new scenario process is not being organized or led by the IPCC, nor by any other international institution. Some reasons for this include the greater scientific credibility of scenarios developed by the research community, the potentially open-ended involvement of research groups across a wide range of disciplines and geographic regions, and greater control over a scenario building process free from institutional timetables. Moreover, it was felt that the community was mature enough to self-organize. None of these considerations exclude the IPCC or another institution from facilitating the process.
The process was designed based on extensive discussions in multiple fora, and was initiated in a 2006 workshop whose participants primarily included members of the climate and integrated assessment modeling communities (Meehl and Hibbard 2007). Discourse at subsequent workshops resulted in the preparation of a roadmap to develop new scenarios that was formalized at the Expert Meeting on Scenarios organized by the IPCC at Noordwijkerhout, Netherlands, in 2007 (Moss et al. 2008). This roadmap details a three-step, so-called parallel process (Moss et al. 2010) consisting of a preparatory phase (serving the needs of the climate modeling community), a parallel phase (climate modeling and socioeconomic scenario development), and an integration phase (development of scenarios addressing mitigation, adaptation and impacts). This process was designed to be responsive to the needs of national and international assessments; to develop a set of reference stabilization pathways as the first building block for the new community scenarios; and to address the climate modeling community interest in investigating a wider range of forcing outcomes and in having greenhouse gas emissions and concentrations prescribed to investigate climate-carbon cycle feedbacks.
The new scenario process follows a different logic than was used in the SRES. SRES scenarios were developed using a forward-looking logic that started by first describing driving forces, and then modeling the resulting emissions and atmospheric concentrations of greenhouse gases and aerosols. Climate models were then used to project the magnitude and pattern of climate change under different scenarios. In contrast, in the new scenario process, the community first agreed to establish a small number of pathways of atmospheric concentrations (and their associated radiative forcing) over the 21st century, and then to simultaneously develop climate change projections and socioeconomic pathways consistent with the concentration pathways (Meehl and Hibbard 2007; Hibbard et al. 2007). Based on this design, activities have been scheduled (and performed) as part of the three phases indicated above:
In phase 1, the integrated assessment modeling community developed four representative concentration pathways (RCPs) and the emissions that would produce them, taking into consideration the full basket of greenhouse gases, land use change, and other factors, and corresponding to the four chosen concentration levels. The development of the four RCPs is documented in a special issue of Climatic Change (Van Vuuren et al. 2011).
In the current phase (phase 2), the RCPs have been used as the basis for simulations with earth system models as part of the Climate Model Intercomparison Project (CMIP-5), producing projections of the magnitude and pattern of climate change over this century and, in some cases, to 2300 (Taylor et al. 2012). At the same time, the integrated assessment modeling (IAM) and impacts, adaptation, and vulnerability (IAV) Footnote 1 communities are developing new descriptions of future socioeconomic conditions, the Shared Socioeconomic Pathways (SSPs). O’Neill et al. (2013) describes the conceptual framework underlying the SSPs.
In phase 3, scenarios for use by the climate change science community will be created, integrating the descriptions of socioeconomic development with the climate change projections and with assumptions about climate mitigation and adaptation policies. The conceptual framework for this integration is discussed in van Vuuren et al. (2013).
An early question faced in this process was whether the approach of beginning with a set of pre-defined forcing pathways — the RCPs — and their climate change outcomes would constrain the range of future socioeconomic conditions that could be considered. An insight gained from the SRES (Nakicenovic et al. 2000) and confirmed by Van Vuuren et al. (2012) is that the magnitude and extent of greenhouse gas emissions does not have a one-to-one relationship with demographic and socioeconomic development. That is, multiple “reference” (i.e. no climate policy) socioeconomic pathways could lead to the same RCP, e.g. a development pathway with a large population and low emissions per capita can lead to the same emission or radiative forcing pathway as a development pathway with a smaller population but higher emissions per capita. Similarly, a particular socioeconomic pathway could be consistent with a wide range of emission or radiative forcing pathways, depending on the stringency and extent of mitigation efforts.
Therefore, a range of demographic and socioeconomic development pathways can be considered when creating scenarios in phase three of the parallel process by combining future socioeconomic conditions, as described in the SSPs, with possible mitigation and adaptation policies to reach radiative forcing pathways over the 21st century as defined by the range of the RCPs. The conceptual framework described in van Vuuren et al. (2013), O’Neill et al. (2013) and Kriegler et al. (Submitted for publication in this special issue) provides a flexible toolkit from which researchers can create scenarios to address specific research and policy-relevant questions.