Elmar Kriegler
email: kriegler@pik-potsdam.de

Brian C O'neill
email: boneill@ucar.edu

Stéphane Hallegatte
email: hallegatte@centre-cired.fr

Tom Kram
email: tom.kram@pbl.nl

Richard H Moss

Robert Lempert
email: lempert@rand.org

Thomas J Wilbanks
email: wilbankstj@ornl.gov

Stephane Hallegatte

Stéphane Hallegatte

S Hallegatte

Socio-economic Scenario Development for Climate Change Analysis

Keywords: socio-economic scenario, climate change, impact, adaptation and vulnerability, integrated assessment modeling socio-economic scenario, climate change, impact, adaptation and vulnerability, integrated assessment modeling

Socio-economic scenarios constitute an important tool for exploring the long-term consequences of anthropogenic climate change and available response options. They have been applied for different purposes and to a different degree in various areas of climate change analysis, typically in combination with projections of future climate change. Integrated assessment modeling (IAM) has used them to develop greenhouse gas (GHG) emissions scenarios for the 21 st century and to investigate strategies for mitigating GHG emissions on a global scale. Analyses of climate change impacts, adaptation and vulnerabilities (IAV) depend heavily on assumptions about underlying socioeconomic developments, but have employed socio-economic scenarios to a lesser degree, due mainly to the multitude of contexts and scales of such analyses. A more consistent use of socio-economic scenarios that would allow an integrated perspective on mitigation, adaptation and residual climate impacts remains a major challenge.

We assert that the identification of a set of global narratives and socio-economic pathways offering scalability to different regional contexts, a reasonable coverage of key socio-economic dimensions and relevant futures, and a sophisticated approach to separating climate policy from counter-factual "no policy" scenarios would be an important step towards meeting this challenge. Such "Shared Socioeconomic Pathways" (SSPs) should be specified in an iterative manner and with close collaboration between IAM and IAV researchers to assure coverage of key dimensions, sufficient scalability and widespread adoption. They can be used not only as inputs to analyses, but also to collect the results of different climate change analyses in a matrix defined by two dimensions: climate exposure as characterized by a radiative forcing or temperature level and socio-economic development as classified by the SSPs. For some applications, SSPs may have to be augmented by "Shared Climate Policy Assumptions" (SPAs) capturing global components of climate policies that some studies may require as inputs. Finally, sufficient coverage of the relevant socio-economic dimensions for the analysis of mitigation, adaptation and residual climate impacts may be assessed by locating the SSPs along the dimensions of challenges to mitigation and to adaptation. We conclude that the development of SSPs, and integrated socio-economic scenarios more broadly, is a useful focal point for collaborative efforts between IAM and IAV researchers. This is likely to be a long-term and iterative enterprise comprising a collection of different activities: periodically taking stock of the evolving scenario work in both research communities, linking up individual efforts, and pursuing collaborative scenario work through appropriate platforms that still need to be established. In the short run, an important goal is to produce tangible outcomes that would allow the 5 th Assessment Report of the IPCC to take a more integrated perspective on mitigation, adaptation and residual climate impacts.

Le développement de scenarios socioéconomiques pour la recherche sur le changement climatique

Résumé

Les scénarios socio-économiques constituent un important outil d'exploration des conséquences de long terme du changement climatique d'origine anthropique, et des options disponibles pour y répondre. Ils ont été appliqués pour différents objectifs et à des degrés différents dans plusieurs domaines de l'analyse du changement climatique, typiquement en combinaison avec les projections de changements climatiques futurs. Les modèles d'évaluation intégrés (MEI) les ont utilisés pour développer des scénarios d'émissions de gaz à effet de serre (GES) pour le XXIe siècle et pour examiner les stratégies d'atténuation des émissions de GES à l'échelle planétaire. Les analyses sur les "impacts, l'adaptation, et la vulnérabilité" (IAV) du changement climatique dépendent fortement des hypothèses relatives aux développements socio-économiques sous-jacents, mais elles ont utilisé dans une moindre mesure les scénarios socio-économiques, principalement en raison de la multitude des contextes et des échelles de telles analyses. Il reste aujourd'hui difficile de mettre en place une utilisation plus cohérente des scénarios socio-économiques, qui autoriserait une vision plus intégrée de l'atténuation, de l'adaptation et des impacts climatiques résiduels. Nous soutenons qu'une étape importante pour relever ce défi est l'identification d'un ensemble de scénarios qualitatifs globaux et des trajectoires socio-économiques correspondantes, qui pourraient être déclinés à différentes échelles spatiales et couvriraient de manière convenable les différentes dimensions socioéconomiques des futurs possibles. De telles « trajectoires socio-économiques partagées » (TSP) doivent être spécifiées de façon interactive, par une étroite collaboration entre les chercheurs MEI et IAV pour garantir le traitement des dimensions les plus importantes, la possibilité de décliner les scénarios à différentes échelles, et pour favoriser une large utilisation des TSP par l'ensemble de la communauté. Ces TSP peuvent être utilisées non seulement comme une information d'entrée pour des analyses spécifiques, mais aussi pour rassembler les résultats de différentes études dans une matrice définie selon deux dimensions : l'exposition climatique, caractérisée par un forçage radiatif ou un niveau de température, et le développement socio-économique, décrit par les TSP. Les TSP pourront également être augmentées "d'hypothèses partagées de politique climatique" (HPP), qui décrivent les composantes globales des politiques climatiques, nécessaires à certaines études. Finalement, on pourra vérifier que les TSP réalisent une couverture suffisante des dimensions socioéconomiques pertinentes pour l'analyse de l'atténuation, de l'adaptation, et des impacts climatiques résiduels, en les situant selon deux dimensions, la première mesurant la capacité à l'atténuation et l'autre la capacité à s'adapter. Nous concluons que le développement des TSP, et plus largement celui des scénarios socioéconomiques intégrés, est un point central des efforts de collaboration entre les chercheurs des communautés MEI et IAV. C'est probablement une entreprise itérative et de long terme comportant un ensemble d'activités très différentes: faire périodiquement un bilan du travail des deux communautés de recherche sur les scénarios, rapprocher les efforts individuels sur ce thème, et mener un travail collaboratif sur les scénarios via des plates-formes appropriées qui restent encore à instituer. À court terme, un objectif important est la production de résultats tangibles permettant au cinquième rapport d'évaluation du GIEC d'adopter un point de vue plus intégré sur l'atténuation, l'adaptation et les impacts climatiques résiduels.

Mots-clés: Scénarios socio-économiques, Changement Climatique; Impact, adaptation et vulnérabilité, Modèles d'évaluation intégrés.

Goals and scope of the paper

The goals of this paper are

(1) to identify current challenges related to developing new socio-economic (SE) scenarios and narratives for the analysis of climate change impacts, adaptation and vulnerabilities (henceforth called IAV) and for the integrated assessment of climate response strategies (henceforth called IAM); and (2) to offer ideas on how to address these challenges in developing SE scenarios and narratives that can better facilitate IAM and IAV research, particularly research to be assessed and integrated in the IPCC 5 th Assessment Report (AR5) on climate change.

An effective strategy for developing new scenarios requires careful consideration of the needs of users (e.g. [START_REF] Parson | Useful Global Change Scenarios: Current Issues and Challenges[END_REF]. Two broad groups of users can be identified: end users, defined as "policy-and decision-makers who use scenario outputs and insights in various decision processes"; and intermediate users, defined as "researchers who use scenarios from a segment of the research community other than their own as inputs into their work" [START_REF] Moss | Towards New Scenarios for Analysis of Emissions, Climate Change, Impacts, and Response Strategies[END_REF]Section I.3, p. 6). In the context of climate change analysis, the number of different types of users is large and needs differ significantly across them. End users, for example, may aim to identify robust decisions that would lead to acceptable outcomes across a broad range of futures. A water manager facing a decision about system design in the face of uncertainty in future climate and water demand is a useful illustration. This type of use would benefit from a small set of scenarios derived from a large number of simulation model runs that vary in characteristics to which the particular type of decision under consideration could be sensitive [START_REF] Groves | A new analytic method for finding policy-relevant scenarios[END_REF]. However, the choice of such scenarios may prove very specific to the conditions facing each particular agency and the policies under consideration.

In contrast to end users, intermediate users often use perspectives about socio-economic futures individually or in small sets to carry out research on how development pathways might affect, or be affected by, future climate change and our ability to respond to it. Views about socio-economic futures are also used in research as a context for structuring assessments such as those that have been carried out by the IPCC or the Millennium Ecosystem Assessment on a global scale. By providing a small number of global-scale scenarios that many studies can draw upon, results across studies can be made more consistent and can therefore be brought together at the assessment stage in a more meaningful way. Researchers working on regional scales have often treated existing global scenarios as exogenous projections, adapted them, and used them as input to derive more detailed scenarios that suited the context and objective of their study. It should also be noted that end users and researchers are often interacting with each other during the scenario building process to jointly assure adequate content and context of scenarios (e.g. [START_REF] Garb | Scenarios in Society, Society in Scenarios: Toward a Social Scientific Analysis of Storyline-driven Environmental Modeling[END_REF].

In this paper we focus on challenges of socio-economic scenario design for intermediate users, that is, for research and assessment. This is not to say that scenarios for end users are less important, but only that their needs are sufficiently different to deserve a focused treatment of their own, which is beyond our scope here. We also focus on scenarios that contain a broadly defined, global framework, rather than those designed to directly address a specific local decision context. These broad global scenarios can and should be refined and linked at the regional and sectoral level, as we discuss below, so that they can be meaningfully related to issues of adaptation and vulnerability at the local level. However, the overarching global framework would provide a basis for global analyses and serve as a common background against which more focused studies can be carried out. This category of scenarios is envisioned to play an important role in the new scenario process and the next IPCC assessment.

In Section 2 we discuss existing challenges of using scenarios in climate change research analysis, and in particular for integrating research across the IAM, IAV, and climate modeling (CM) communities. We note that SE scenarios and narratives have been used in IAV and IAM research to different degrees, and have quite different relevance to the dominant modes of research in these areas. Therefore the starting points for new scenario development are very different between the two communities. In discussing these challenges, our goal is to provide a crystallization point for debate on how to advance the new scenario process. We hope that a shared understanding of the key remaining questions can provide a solid foundation for a constructive discussion that ultimately will yield the needed progress.

Section 3 discusses elements of the New Scenario Process. The need for new SE scenarios and narratives superseding the SRES scenarios has been identified by the IAM and IAV communities as well as the IPCC for its 5 th Assessment Report (AR5). At an IPCC expert meeting in Nordwijkerhout, the roadmap of a "New Scenario Process", to be conducted jointly by the IAM, IAV and Climate Modeling (CM) communities, was laid out [START_REF] Moss | Towards New Scenarios for Analysis of Emissions, Climate Change, Impacts, and Response Strategies[END_REF][START_REF] Moss | The next generation of scenarios for climate change research and assessment[END_REF]. This new scenario process started with a preparatory phase in which integrated assessment modeling teams produced detailed "Representative Concentration Pathways" (RCPs) to be used by the climate modeling teams for their projection of future climate change. However, the subsequent phases of the new scenario process, in particularly relating to the development of SE scenarios and narratives for framing the RCPs in a wider context remain to be operationalized.

In Section 4 we offer ideas on how a set of "Shared Socio-economic Pathways (SSPs)," containing both narratives and quantitative assumptions, could be constructed in a manner that would address several of the challenges we have identified and contribute to the new scenario process (for another discussion of ways forward for the new scenario process, see [START_REF] Van Vuuren | Developing new scenarios as a thread for future climate research[END_REF]. We do not expect nor intend to provide conclusive answers in this document. We expect that such answers will be explored in a focused discussion of the larger community in the coming months, and at the IPCC expert meeting on scenarios in November 2010.

Key challenges and questions for using socio-economic scenarios for climate change analysis and assessment

In this section we consider some of the major challenges in the use of socio-economic (SE) scenarios. Understanding these challenges is a precursor to identifying effective strategies for the development of new SE scenarios for use in future research. Here and in the following, we broadly define a SE scenario as a combination of quantitative projections and qualitative information (such as narratives) that jointly characterize a plausible future [START_REF] Carter | New assessment methods and the characterization of future conditions[END_REF]. We begin by discussing challenges related to the use of SE scenarios in IAM (section 2.1) and IAV research (section 2.2), and then turn to the question of how scenarios can be used for integrating work across the IAV, IAM, and CM communities (section 2.3). Due to the complexity of climate change analysis, a comprehensive integration is difficult to achieve by individual cross-cutting studies or by collecting results from independent studies differing in key assumptions and boundary conditions. We therefore raise the question of whether the integration of IAV and IAM research requires a shared set of assumptions about socio-economic pathways that can be adopted by both research streams (section 2.4). In this context, we define a "Shared Socio-Economic Pathway" (SSP) as consisting of a parsimonious narrative1 capturing the key dimensions of the underlying global scale SE development and a small collection of quantitative projections for global SE boundary conditions. Such pathways would reflect socioeconomic "baseline assumptions" describing a world without climate policy. As discussed in section 2.5, a variety of analyses will require input assumptions on climate policy as well as socio-economic baseline developments. It therefore may be necessary to augment SSPs that describe a world without climate policy with "Shared Climate Policy Assumptions" (SPAs).

Using scenarios in IAM research

In the analysis of climate change response strategies with integrated assessment models, the development of SE scenarios was driven by the need to specify emissions drivers and trends for the 21 st century for projecting future climate change [START_REF] Nakidenovid | Special Report on Emissions Scenarios[END_REF]. This led to a focus on quantitative global and centennial scale scenarios with an increasingly detailed description of trends in greenhouse gases, short-lived species and land use that were taken up by global atmospheric chemistry and climate models [START_REF] Moss | Towards New Scenarios for Analysis of Emissions, Climate Change, Impacts, and Response Strategies[END_REF]. IAM research provided scenario-based analysis of climate change response strategies, particularly concerning the mitigation of climate change for achieving climate stabilization targets (see Chapter 2 of the IPCC WGIII Third Assessment Report and Chapter 3 of the IPCC WGIII Fourth Assessment Report for overviews). Scenarios were used to structure the space of plausible socio-economic futures.

The Special Report on Emissions Scenarios (SRES; [START_REF] Nakidenovid | Special Report on Emissions Scenarios[END_REF], for example, combined a limited set of narratives derived from an intuitive logics approach 2 with a bundle of quantitative SE scenarios to cover the space of global SE futures. In addition, the value of scenarios for controlled comparison of model results and the analysis of model differences has long been widely recognized by the IAM community (e.g. [START_REF] Weyant | Multi-Greenhouse Gas Mitigation and Climate Policy[END_REF]Edenhofer et al., 2006Edenhofer et al., , 2010;;[START_REF] Clarke | International Climate Policy Architectures: Overview of the EMF 22 International Scenarios[END_REF]. However, the extended use of SE scenarios has also brought new challenges for IAM research. They include, e.g., the systematic exploration of the space of plausible SE futures [START_REF] Morgan | Improving the way we think about projecting future energy use and emissions of carbon dioxide[END_REF][START_REF] Schweizer | Using cross-impact balance analysis to improve future storyline scenarios[END_REF], the association of SE futures with likelihoods [START_REF] Schneider | What is 'dangerous' climate change?[END_REF][START_REF] Nakidenovid | Identifying dangers in an uncertain climate[END_REF], the treatment of the (climate) policy dimension in scenario-based approaches [START_REF] Parson | Useful Global Change Scenarios: Current Issues and Challenges[END_REF], and scaling issues as IAM studies are carried out at increasing resolution. Those challenges are relevant for the use of scenarios in climate change analysis in general, and thus are discussed in a broader context in Section 2.3. Some parts of the IAV community that are relatively model-oriented are also more scenariooriented, such as ecosystems, agriculture, energy demand, and food production; but they tend to limit their scenario attention to climate change or in some cases, sectoral development scenarios (although there have been efforts to develop conceptual frameworks for more integrated IAV analyses: e.g., [START_REF] Arnell | Climate and socio-economic scenarios for global-scale climate change impacts assessments: characterising the SRES storylines[END_REF][START_REF] Lorenzoni | A co-evolutionary approach to climate change impact assessment: part I: integrating socio-economic and climate change scenarios[END_REF]. In many cases, the climate change scenarios are regional rather than global. Global socio-economic scenarios such as the SRES scenarios have been used more rarely, often for large-scale boundary conditions such as population or GDP (e.g. [START_REF] Arnell | Climate change and global water resources: SRES emissions and socio-economic scenarios[END_REF]; [START_REF] Parry | Effects of climate change on global food production under SRES emissions and socio-economic scenarios[END_REF]Fischer et al. (2005); [START_REF] Nicholls | Coastal flooding and wetland loss in the 21 st century: changes under the SRES climate and socio-economic scenarios[END_REF]; [START_REF] Hanson | A Global Ranking of Port Cities with High Exposure to Climate Extremes[END_REF]).

Using scenarios in IAV research

(b) limited knowledge base Climate change impact and adaptation research deals with particularly complex issues (e.g., uncertainties about ecosystem response to warming, the importance of value judgment in cost assessment, great heterogeneity in impacts and adaptive capacity, a need for local and context-specific analyses, and long-range interactions through world markets). At the same time, in most parts of the world it has had very limited funding over the past two decades. As a result, it very often lacks either the data or the understanding of critical sensitivity parameters required for realistic use of large-scale scenarios and confidence in model results remain limited. In response to this lack to knowledge, most efforts have been devoted to sensitivity analyses as the critical pathways for improving knowledge. For instance, how can one project impacts of a change of temperature of, say, three degrees C, in 2080 without an understanding of how impacted systems change with changes in temperature?

(c) sensitivity to scenario assumptions The high sensitivity of IAV analysis results to contextual assumptions makes it difficult for a classical "causal-chain" approach (from scenario to impacts) to provide useful insights. The emerging perspective of the IAV community is that managing risks associated with a range of possible impact futures is a higher priority than focusing on a particular impact projection, and attention to relatively low-probability, high-consequence scenarios is often particularly important from a risk management point of view.

(d) importance of local context

Climate change vulnerabilities and responses depend not only on changes in climate parameters but on interactions between these parameters and changes over the same periods in local socioeconomic conditions, such as population size and distribution, economic activities, technologies, and institutions (IPCC AR4). Where available, socio-economic scenarios have generally been developed at a large scale (e.g., with nationally or regionally-aggregated information only) and do not include much of the relevant information for IAV information (e.g., detailed localization of activities, clustering of activities). The challenge of producing regional and sub-national scenarios at long timescales has been touched upon by a number of authors (see, among others, [START_REF] Gaffin | Downscaling and geo-spatial gridding of socioeconomic projections from the IPCC special report on emissions scenarios (SRES)[END_REF]Theobald, 2005;[START_REF] Bengtsson | A SRES-based gridded global population dataset for 1990-2100[END_REF][START_REF] Lempert | A general, analytic method for generating robust strategies and narrative scenarios[END_REF][START_REF] Grübler | Regional, National, and Spatially Explicit Scenarios of Demographic and Economic Change Based on SRES[END_REF][START_REF] Groves | A new analytic method for finding policy-relevant scenarios[END_REF][START_REF] Hallegatte | The Economics of Climate Change Impacts and Policy Benefits at City Scale: A Conceptual Framework[END_REF]Van Vuuren et al., 2010b). Local scenarios do exist, such as city scenarios designed to support urban planning. But these scenarios are not connected to global scenarios, in which global environmental change could be represented. Moreover, they usually consider time horizons of less than 30 years. Urban scenarios with a 2100 time horizon are not generally available so far, yet such scenarios would be of relevance to the understanding of urban-scale climate change impacts.

In a few cases, IAV research has been related to efforts to develop socioeconomic scenarios to accompany climate change scenarios, often based on regional knowledge. Examples include [START_REF] Berkhout | Non-Climate Futures Study: Socio-Economic Futures Scenarios for Climate Impact Assessment[END_REF][START_REF] Arnell | Climate and socio-economic scenarios for global-scale climate change impacts assessments: characterising the SRES storylines[END_REF][START_REF] Rounsevell | A coherent set of future land use change scenarios for Europe[END_REF]and 2008, and references therein). It is vitally important to assure that these two sets of perspectives do not diverge in the coming years, undermining the consistency of messages to policymakers and stakeholders by climate science. But this ambitious aim can also be an opportunity -to catalyze and strengthen linkages between the IAM and IAV communities -if it is approached in an inclusive and integrative way.

(a) Bridging scales Scale differences present a fundamental challenge to making SE scenarios useful for both IAM and IAV research. They must be scalable in the sense that even when covering the global scale with necessarily coarse resolution, they allow downscaling (to some extent) to the local context of IAV analysis. Narratives may offer such scalability, as local constituents may use a global narrative to explore implications for their specific location (e.g., the UKCIP national-scale scenarios; www.ukcip.org.uk). There is also a strong dependence between spatial detail and time horizon. Spatial detail may be most relevant over shorter time horizons, but can also matter in the long-run (e.g. hydropower dam safety, building construction in flood risk areas). However, the level of detail that can be provided by scenarios will clearly depend on how far into the future they reach. This is discussed in Section 4.1 in greater detail.

(b) Spanning the relevant range of futures Scenarios should ideally cover the range of plausible future developments in a comprehensible (i.e. a small number of scenarios) and at the same time comprehensive manner (i.e. achieving "acceptable" coverage of the space of plausible futures). These two requirements often conflict with each other, particularly when integrating scenarios for mitigation and adaptation analysis. Scenarios must be designed in such a way as to capture a broad range of conditions relevant to both mitigation and adaptation. One approach might be to consider scenarios describing worlds which combine challenges to mitigation that are larger (or smaller) with challenges to adaptation that are larger (or smaller) -an idea we discuss further in Section 4.4. In general, scenario design will need to grapple with the trade-off between scenario detail which may be needed for some analyses and scenario representativeness which will be need for making statements about uncertainty and robustness. Obviously, the greater the detail of a scenario, the less likely it can serve as a representative for a larger portion of the scenario space (Morgan and Keith, Climatic Change, 2007).

(c) Distinguishing climate policies from socio-economic baselines

The treatment of climate policies in climate change scenarios has been controversial. In economic analysis, policies are generally evaluated against a counterfactual "no policy" case.

The counterfactual no climate policy case usually provides the baseline scenario for IAM research, which then is compared with a climate policy scenario. In contrast, IAV researchers have often used a reference present-day case as baseline to compare with scenarios of future SE and climate change, which may include different sets of policy assumptions. Nevertheless, in order to study the effect of climate policies, it is necessary to separate them from other policy assumptions in the scenario configuration. This is clearly a difficult task given the range of policy contexts from local to global and from adaptation to mitigation. A key question is: How can a meaningful separation of the socio-economic baseline and climate policy dimensions be achieved?

Possible needs for shared socio-economic pathways (SSPs) for use by IAM and IAV research to construct integrated scenarios in climate change analysis

A comprehensive exploration of integrated scenarios of mitigation, adaptation and residual climate impacts will require significant contributions from all three IAV, IAM and CM research streams. For the time being, the problem is too complex to achieve such comprehensive integration within individual cross-cutting studies, although such studies will be of great value for fostering an integrated view on mitigation and adaptation. Therefore, the integration will have to rely on a multitude of IAM, IAV and CM studies which are more or less undertaken in parallel. The obvious question is whether integration of those studies would be feasible if their underlying assumptions are not harmonized to a minimal degree. And if such minimal harmonization is needed, how can it be achieved?

The most apparent possibility would be the specification of scenario elements like quantitative assumptions for socio-economic boundary conditions, representative concentration pathways (RCPs) and narratives to be taken up by IAM, IAV and CM research. The necessity of RCPs for structuring the interface to the climate modeling community has already been recognized, in good part owing to the very practical reason of the computational costs of running fully coupled global climate models. However, can a similar case be made for what we call "shared socioeconomic pathways (SSPs)" to be used by both IAV and IAM research? As described at the beginning of Section 2, SSPs may include both quantitative SE boundary conditions as well as narratives, adding socioeconomic context to the analyses of drivers, impacts, and responses to climate case.

A case for SSPs would rest on two requirements: (i) that it is possible to develop SSPs that are useful both in the IAM and IAV context, and (ii) that research based on SSPs could be assessed in a more integrative way than would otherwise be possible. SSPs would need to be specified and communicated in a timely and effective manner, and be recognized, correctly interpreted and taken up by both communities. This does not mean that the research needs of either community would be limited to the set of SSPs, but only that some important common needs to both would be met. Four different objectives for the use of SSPs can be identified:

 Consistency: SSPs will facilitate the integration of studies with similar assumptions (e.g. on GDP, population).

 Generating insight: Harmonization of some key assumptions and controlled variation of those assumptions across an ensemble of studies and/or models is very useful, if not a prerequisite, for generating a deeper understanding of the underlying reasons for results and their spread across studies / models. SSPs can greatly facilitate such an informed comparison of results. Harmonizing climate policy assumptions will provide additional value.

 Exploring the full space of possibilities: Uncoordinated research of IAV and IAM communities does not guarantee a comprehensive coverage of the space of key assumptions that may affect climate policies. [START_REF] Van Vuuren | Downscaling socioeconomic and emissions scenarios for global environmental change research: a review[END_REF] demonstrate, for example, that the particular SE scenarios underlying the construction of the four RCPs do not show a large spread in population pathways. SSPs can in some instances reduce the risk of settling on a too narrow area of the scenario space, although 'negotiated' common scenario assumptions have been often criticized for the opposite: narrowing the range of possible outcomes down to a few cases prematurely.

 Exploiting synergies to enable comprehensive research: It is already common practice in the research community to rely on a set of frequently used assumptions -provided e.g. by public datasets (IEA, World Bank, GTAP) or commonly used scenarios (SRES).

With the shrinking relevance of the SRES set of illustrative scenarios, the question emerges whether SSPs could produce new synergies for both IAV and IAM research by providing a set of off-the-shelf assumptions useful for and usable by researchers from both communities?

We will take up issues in applying and framing SSPs in greater detail in Section 4 of this paper. This includes a discussion of the nature of useful SSPs, how they might be specified, and how they can be related to the broader context of assessments informing climate policy.

Possible needs for shared climate policy assumptions (SPAs)

Ideally, in order to separate SE baseline developments from the effects of climate policy, SSPs should only include socio-economic "baseline assumptions". This will make climate policy analyses utilizing SSPs more flexible. policies. An example is international trade policies in a world with fragmented carbon markets.

Only policy variables of categories 1 and 3 can be included in the "no policy" baseline without affecting the baseline / climate policy separation. Obviously, climate policies (Category 2) should be excluded from the SE baseline to make it applicable to climate policy analysis. Finally, policy variables of category 4 would have to change between baseline and climate policy scenarios, making the separation of the two scenarios even more difficult. This separation is likely to be easier when mitigation efforts are limited; ambitious climate policies involving structural economic changes would obviously make it more difficult to distinguish between climate and non-climate policies.

SSPs are intended to be used in a broad range of analyses, many of which will involve assuming some type of mitigation policy, adaptation policy, or both. It may therefore be worthwhile to develop a small set of shared climate policy assumptions (SPAs) that could be used in common across a range of studies together with SSPs. The issue of how to specify and use SPAs will be taken up in Section 4.3.

Socio-economic scenarios and the new scenario process

This section describes the new scenario process as a key community process under way, and identifies key elements of it. The new scenario process has been described as a stool with three legs -climate/earth system modeling, integrated assessment modeling, and IAV research -in which each leg depends for its value and stability on strengths in the other two legs. We believe this is an immensely valuable bridging paradigm, and operationalizing it in any plans for moving forward is a high priority. However, the challenges discussed in Section 2 are also relevant for the new scenario process, and they will need to be tackled if progress is to be made.

A major stakeholder in the new scenario process is the IPCC with its upcoming 5 th Assessment Report (AR5). A central question for the assessment will be: for a given climate outcome, what mitigation actions might be required in order to produce this outcome, what will be the potential for adaptation, and what residual impacts might occur? Producing such information for a range of climate change outcomes will be critical information of relevance for climate change policy processes that must weigh the relative costs, benefits (including co-benefits), risks, and un-attended side effects of various levels and rates of climate change. IPCC author teams in the three working groups (WGs) will face the challenge to evaluate a very large number of studies from the IAM, IAV and CM communities, and to produce an assessment that is internally consistent and can be meaningfully combined across the three WGs.

The roadmap of the new scenario process was laid out in [START_REF] Moss | Towards New Scenarios for Analysis of Emissions, Climate Change, Impacts, and Response Strategies[END_REF], and summarized in [START_REF] Moss | The next generation of scenarios for climate change research and assessment[END_REF]. It consists of three phases -a preparatory, parallel and integration phase. Currently, IAM teams at IIASA, JGCRI-PNNL, PBL and NIES have produced the RCPs for use in the climate model runs, ensuring successful completion of the preparatory phase.

The traditional linear process of (i) identifying SE scenarios of emissions trends and drivers by the IAM community, (ii) forwarding them to atmospheric chemistry and climate models to augment them with climate change projections by the CM community, and then (iii) conducting IAV analyses on the basis of these scenarios and projections has proven difficult for various reasons, including an inability to "close the loop" from IAV studies back to socio-economic drivers, inhibition of collaboration across research communities, and length of time required to carry it out. For all these reasons, the new scenario process abandoned the linear approach in favor of a parallel and integration phase, where all three communities proceed with their analyses (more or less) in parallel.

The parallel phase contains four elements. Three of them are well under way. The IAM community has begun developing new scenarios, e.g., within the RCP replication process and other model (comparison) activities. The climate modeling community has started its climate model ensemble experiment (CMIP5) with exogenous climate forcing specified by the RCPs. In the meantime, IAV research based on existing scenarios and projections (SRES, CMIP4) continues. The remaining key challenge, labeled in [START_REF] Moss | Towards New Scenarios for Analysis of Emissions, Climate Change, Impacts, and Response Strategies[END_REF] as the task of developing storylines, is concerned with injecting a common structuring element in both the IAV and IAM contributions to the scenario process. Injecting such a common structuring element is particularly useful if the integration of IAV, IAM and climate modeling research, is to be pursued. Our discussion of challenges to the use of scenarios (as a structuring element) in climate change research in section 2 is broadly relevant to the storyline development aspect of the parallel phase.

Socio-economic scenario information will be employed in the integration phase to inform both IAM studies and IAV studies. Core questions are: how can the climate change patterns produced by the climate models driven by the RCP inputs, the socio-economic scenarios produced by IAMs, and the IAV studies be integrated most effectively? In particular, how can these various strands of work be brought together to provide a coherent view on mitigation and adaptation, and their costs, risks, and benefits, e.g. in IPCC AR5 or other assessment activities? Section 4 will propose some ideas to address the integration challenge.

Issues in framing and applying shared socio-economic pathways and climate policy assumptions

In this section, we discuss in greater detail what the role of shared socio-economic pathways (SSPs) and climate policy assumptions for integrating IAM and IAV research might be, and offer some ideas how they might be specified.

Because

Using SSPs for integrating IAM and IAV research

We first discuss a framework for conceptualizing how SSPs might be used to facilitate the integration of IAM and IAV research. Based on this framework, we then discuss two related issues: how climate policy assumptions could be related to SSPs, and the appropriate level of detail for SSPs.

Assume for a moment that the identification of SSPs useful to both the IAM and IAV communities by a process like the one described above would be feasible. One approach could then be to identify or develop a small set of SSPs that would structure the socio-economic space in a manner similar to the way RCPs structure the range of possible future forcing. Given a set of SSPs for use in both mitigation and IAV studies, one can then imagine the results of various mitigation and impact studies populating a matrix of combined socio-economic and climate change outcomes. Figure 1 presents a conceptual framework for thinking about how climate modeling, mitigation, and IAV studies could be integrated in a meaningful way. Each cell of the matrix represents a particular combination of climate change outcomes and socio-economic assumptions, and in principle it would contain the results of many studies of mitigation associated with achieving that climate outcome, adaptation measures that could be undertaken, and residual impacts. Each row of the matrix could be thought of as a summary of the implications of a range of possible future socio-economic conditions for a given level of climate change. Each column would describe the implications of increasing levels of climate change, or decreasing levels of mitigation effort, for a given set of socio-economic conditions (see van Vuuren et al., 2010a, for a related concept). Not all cells of the matrix have to contain a consistent scenario. For example, a SSP with rapid development of competitive renewable energies, low population growth and environmental orientation would be hard to reconcile with a 6 degree warming, even without climate policy.

The framework presented here would not exclude exploring the implications of socio-economic conditions other than those reflected in the SSPs, just as the development of the RCPs does not exclude exploring other forcing pathways. However, this framework, and the associated SSPs, would facilitate the development of a critical mass of studies sharing assumptions that are common enough to be able to meaningfully combine their results. Some key dimensions of those studies are likely important enough to anticipate highlighting. For example, the near-term (~2035) and long-term (~2100) time horizons identified would provide a natural dimension for characterizing the results from the literature (see Appendix 1 for a more detailed discussion). Another important dimension is the nature of underlying assumptions about the climate policy environment in scenarios.

Using shared climate policy assumptions together with SSPs

As discussed in Section 2, SSPs are envisaged to refer to socio-economic "baseline" developments. This allows greater flexibility to assess the climate policy dimension across and within SSPs. The question is whether there is also a need to specify shared climate policy assumptions (SPAs) that may be adopted by a wide range of studies in combination with SSPs.

There are two reasons that SPAs could prove useful. First, a given SSP will be paired with a wide range of climate change outcomes, some of which, such as the lowest RCP, will require a very large degree of mitigation effort. To illustrate this point, we have put an arrow indicating the direction of increasing mitigation effort with decreasing level of climate exposure next to the matrix in Figure 1. The approach to mitigation in cases with low climate exposure could be a major factor affecting how IAV analyses should be carried out. For example, if mitigation requires a large amount of land to be used for bioenergy production or afforestation, this will have major implications for impacts. Thus, it may be useful to ensure that there are enough studies that make common assumptions about mitigation policy in order to compare their results regarding impacts and adaptation.

Second, it may be that SSPs excluding climate policy will not be able to provide the full range of SE information that a large number of mitigation or adaptation studies at the local or regional level may need. Such analyses require assumptions about what is done in the rest of the world, including in terms of climate policies. For instance, mitigation policies implemented in a city will depend on the oil price, which in turn depends on global mitigation policies. Or consider insurance. Insurance is an adaptation option that is more or less adequate depending on whether the global reinsurance market face a large rise in demand or not. Local analysis may thus not only require baseline-SSPs, but also global climate-policy-SPAs.

In terms of the matrix representation discussed, the inclusion of SPAs would amount to adding a "climate policy" axis to the climate outcome axis and the SSP axis in Figure 1. Since the climate outcome axis already defines the level of ambition of global mitigation policy, there exists of course a close link to the climate policy dimension. This, however, is by no means a one-to-one mapping as many combinations of mitigation measures might yield the same level of warming.

Level of detail of SSPs and relation to marker scenarios

After having laid out a general framework for the application of SSPs, the question remains to what level of detail they should be specified. As discussed in Section 2.3 and 2.4, an assessment of the relevant uncertainty in future developments requires a broad coverage of the scenario space of plausible futures or at least the spanning of its boundaries. Broad coverage can only be achieved with broad-brush scenario characterizations which would allow categorizing entire groups of more detailed scenarios [START_REF] Schweizer | Using cross-impact balance analysis to improve future storyline scenarios[END_REF]. Parsimonious narratives can serve such a purpose. Adding quantitative projections as a further element will reduce the generality of a scenario significantly. But if limited to a small set of key variables like GDP, population, and perhaps a few others, it will still be possible to span the space of plausible futures with low, medium and high projections. This approach has been chosen for the development of the RCPs which were constructed to span the space of the future anthropogenic forcing of climate change. The combination of broad narratives and a very limited set of quantitative low, medium, high projections appears to be the appropriate level of specificity for SSPs. If SSPs provided more detail, they may not be able to span the scenario space sufficiently. If they provided less detail, they may be missing key information that would be needed by IAM and IAV research. Nevertheless, there will be a variety of studies in need of quantitative input that goes beyond what can be provided by SSPs. It may be necessary to construct more detailed quantitative marker scenarios for these studies to use. Those markers would be derived from model-based analysis and associated with a SSP. Since it is unlikely that a single marker can be representative for a given SSP, it may be useful to define a second somewhat orthogonal marker scenario in order to signify the ideosyncracy of individual markers associated with a SSP. Those markers would also include shared climate policy assumptions and populate the cells of the matrix in Figure 1, along with other studies employing the SSPs.

How might SSPs be specified?

Historically, socioeconomic futures developed for climate change research have consisted of qualitative narrative descriptions of future trends, quantitative assumptions about key socioeconomic variables, or both.

Approaches that combine narratives with quantitative assumptions (also called the storyline and simulation approach; [START_REF] Alcamo | The SAS approach: Combining qualitative and quantitative knowledge in environmental scenarios[END_REF] are most successful when these two elements are carried out iteratively in close collaboration, rather than independent from one another. Each element can inform and constrain the other, and therefore the internal consistency of the scenarios requires an integrative method.

In discussions of SSPs in support of the new scenarios process (section 3), an interest has been shown in taking the four RCP views of GHG concentration trends between now and 2100 as a starting point and developing socioeconomic scenarios to accompany the set of four climate futures. It is quite possible that several different socioeconomic scenarios might lead to a single RCP end point. Although an exploration of these possibilities (a process called "RCP replication" that is already underway in the IAM community) would appear to be of considerable value, the IAV community may not consider SSPs developed in this manner to be appropriate for their purposes. Rather, socio-economic and institutional conditions known to be important to IAV could be taken as a starting point for developing SSPs and their narratives, rather than beginning with concentration pathways.

Taking these views into account, here we discuss possible approaches to the development of both qualitative and quantitative elements of SSPs.

Approaching SSP development through structured views about alternative possible futures

One approach to developing SSPs is by identifying alternative possible futures as frames for discussion and working back toward rich descriptive narratives and, as possible, related quantitative scenarios. There have been several prominent examples of the use of such narrative-based approaches in scenario development. For example, the work by Paul Raskin and colleagues in the late 1990s on alternative sustainability transitions (NRC, 1999;[START_REF] Raskin | Great Transition: The Promise and Lure of the Times Ahead[END_REF][START_REF] Raskin | The Century Ahead: Searching for Sustainability[END_REF], based on earlier work by [START_REF] Gallopin | Branch Points: Global Scenarios and Human Choice[END_REF] identified three classes of socioeconomic scenarios for responses to sustainability challenges: conventional worlds, barbarization, and great transitions. Each class included two variants: conventional worlds (market forces, policy reform), barbarization (breakdown, fortress world), great transitions (eco-communalism, a new sustainability paradigm). Each of these six possible futures was associated with qualitative assumptions about trends in population, economy, environment, equity, technology, and governance/conflict. The six trends were, in a number of cases, estimated quantitatively in order to assess differences in such implications as resource requirements.

Another prominent example was the international Millennium Ecosystem Assessment (MEA), which developed four alternative scenarios: Global Orchestration (globalized with emphasis on economic growth and public goods), Order from Strength (regionalized with emphasis on national security and economic growth)), Adapting Mosaic (regionalized with emphasis on local adaptation and flexible governance), and TechnoGarden (globalized with emphasis on green technology). Quantitative models were used to assess differences between scenarios in gains, losses, and vulnerabilities for different regions and populations.

The SRES scenarios incorporated demographic change, socioeconomic development, and technological change in developing four families of climate change futures: A1, rapid change and global convergence, emphasizing technological development; A2, regional and local fragmentation; B1, global convergence, emphasizing sustainability; and B2, slower growth based on regional/local initiatives. These families, and subdivisions within them, were associated with quantitative assumptions about rates of population and economic growth, along with some assumptions about land use change [START_REF] Toth | Considering the Technical and Socio-economic Assumptions Embedded in the SRES Scenario Families[END_REF].

One feature of much of the literature, however, is that the logic of the narratives tends to be global, and challenges in associating them with regional variations have often not been met. An example where the end points were developed at a regional scale to fit regional views of possible futures was the MedAction project, supported by the EC in 2003, which used narratives as stimuli for participative local discussions of alternative futures and associated tradeoffs. It identified three qualitative scenarios: "Big is Beautiful", "Convulsive Change", and "Knowledge is King" [START_REF] Kok | Mediterranean scenarios. First Draft, MedAction Deliverable[END_REF]. Challenges included downscaling regional scenarios to a local scale, relationships to current experience, and constraining local creativity in considering other options.

Several open questions should be addressed in considering a structured approach to narratives. How useful have narratives proven for climate change analysis and assessment? One particular attraction of them may be that they offer scalability of global scale assumptions on regional to local scales in the context of individual IAV analysis. Have narratives lived up to this promise, and how would they need to be defined for offering such scalability? Finally, is there a way to systematize the intuitive logics approach to defining narratives so that a certain degree of comprehensiveness (in covering the space of plausible futures) can be achieved? Box 1 below provides an illustration how narratives could be specified.

It is likely that useful SSPs would include not just qualitative narratives, but quantitative specifications of some socio-economic conditions. One approach would be to provide quantifications of these factors at a broad global and regional level -so-called "boundary conditions" -so that more detailed quantifications could be developed by individual researchers at the local or sectoral level guided by an overarching quantitative framework.

BOX 1: ILLUSTRATIONS OF FRAMEWORKS FOR SHARED SOCIO-ECONOMIC PATHWAYS

In general, efforts to date to provide frameworks for thinking about alternative pathways for socioeconomic futures have been related to four major dimensions:

(1) economic change: e.g., rapid growth vs. modest growth, globalized markets vs. more localized systems (2) governmental/institutional change: e.g., effective in reaching goals vs. ineffective, democratic vs. authoritarian, participative vs. top-down (3) directions of change in social values: e.g., economic consumption vs. nature-society balance, social cooperation vs. conflict (4) technological change: significant, transformational, and green vs. slow and unequally distributed If a convenient framing would be based on two dimensions, which can be used to frame narratives about four different kinds of socioeconomic futures, one approach would be to use an economic dimension and a governance/institutional dimension, as in the case of the TERI scenarios of socioeconomic change in Northern India. The economic dimension would range from rapid economic growth through a globalized economy on one end of a continuum to modest economic growth based on localized systems on the other end, with technological change incorporated in the rate of growth. The institutional dimension would range from effective goal achievement and problem-solving in achieving development goals, with effective leadership and constituency support and cross-institutional cooperation, on one end of a continuum to institutional weakness, instability, fragmentation, and lack of cooperation and consensus on the other end, with social values incorporated in institutional performance.

Another approach might adopt as one dimension the achievement of social and economic development goals, for example as conceived by the UNDP Millennium Development Goals, which are aimed at cutting world poverty in half, saving tens of millions of lives, and spreading socioeconomic benefits to billions of people who are now disadvantaged (UNDP, 2010). One end of a continuum would be achievement of these goals, perhaps not by 2015 but by 2035, while the other end would be no significant progress toward achieving the goals. In a sense, this would move the governmental/institutional dimension toward a more operational conception, related to socioeconomic goals, against which could be set an economic change dimension, including technological change.

These two approaches would not have to be mutually exclusive. For instance, the approach related to socioeconomic goals could be taken as a framing for the relatively near future, while the broader approach could be taken as a framing for the longer term, on into the latter half of the century.

In either case, or in variations on these themes, narratives of four possible socioeconomic futures -HH, HL, LH, LL -would include descriptions of socioeconomic policies implied by such futures: e.g., international cooperation, resource management, and equity. But it would be important to recognize that different policy frameworks and power structures could lead toward the same kinds of socioeconomic outcomes.

As one example, in 2008-9, The Energy and Resources Institute (TERI) developed scenarios of socioeconomic change in Northern India for four cases related to permutations of two axes: policy approaches (market mechanisms vs. government policy-driven) and social values (emphasizing economic goals vs. emphasizing environmental goals). The four scenarios were then described by qualitative trends between now and 2050 in seven indicators, each based on a descriptive narrative: demographics, economy, social and cultural changes, conflict, technological change, environment, and governance (see below). Relating these indicators in turn with quantitative information about demographics and economic systems, TERI was able to produce quantitative estimations of differences in requirements by state for such resources as food, water, and energy.

Approaching SSP development through projections of socio-economic variables of interest

Alternatively, one can approach the development of SSPs by identifying a manageable number of socio-economic variables from a consideration of key factors that are often assumed to be exogenous to both IAV and IAM studies. It seems almost certain that SSP variables would include projections of demographic change, ideally distribution as well as total size, both regional and urban/rural, and projections of economic activity, ideally sectoral as well as aggregate. Other variables likely to be of interest include projected changes in land use and transportation patterns; changes in energy production and use, including technological innovation and change and considering energy resource potentials; changes in water supply and demand; changes in risk management practices; changes in institutions and governance, including the level of provision of public goods (e.g. education) and possible changes in how citizens view growth, environmental protection, and equity; and changes in information acquisition, access, and use with the evolution of IT technologies and society's response to them. Some of these variables, however, may be more tractable in qualitative narratives than in quantitative scenarios.

Analyses of climate policies need to take into account existing market failures in the economic system and cannot assume an ideal world in which markets would be complete and perfect. As a consequence, it would be useful for at least some SSP to include market failures and provide information about them. Examples include the unemployment level, the tax system structure and its imperfections, the share of informal economy, the barriers to capital flows and trade The determination of boundary conditions to be included in the SSPs would need to be the product of an intensive discussion between IAM and IAV researchers. This process would need to be not only collaborative but iterative as well. Initial brain-storming is likely to yield a large number of candidate factors for quantification, at a range of possible scales. Narrowing this list to a manageable number, and then combining assumptions into a small set of SSPs, would follow. The process would need to be iterated after considering whether the sets of boundary conditions covered a sufficient range of uncertainty for each factor (e.g., population growth, economic growth, etc.) and whether the logic of their combinations was sound.

The following key questions raised by this approach would need to be addressed:

 The fundamental question for exploration in the development of SSPs is whether there is a set of socioeconomic factors that would meet needs of both IAM and IAV analysis, perhaps nested among a larger range of factors that might differ in their relevance between IAM and IAV analysis or between regions.

 Do these sets of socio-economic factors bundled into SSPs lend themselves to a comprehensible (i.e. small number of SSPs) and fairly comprehensive exploration of the space of plausible socio-economic futures? What will be the logic for selecting a small set of SSPs from the many combinations likely to be plausible?

How can SPAs be specified?

As explained above, shared climate policy assumptions (SPAs) are a means to better separate the climate policy dimension from the socio-economic "baseline" dimension reflected in a SSP.

To allow, e.g., for a local analysis, especially on impacts and vulnerability, SSPs may have to be complemented with assumptions about global and national scale mitigation and adaptation policies, and SE variables that are directly affected by it. Local vulnerability will depend on whether ambitious mitigation policies are in place not only through its effect on the magnitude of climate change. For instance, a region might be more or less vulnerable to an increase in airconditioning energy demand, depending on its energy mix, which will be affected by global climate policies. Also, the global oil price is likely to be different if mitigation efforts are widespread. Local vulnerability will also depend on global-scale adaptation policies, e.g. through the availability of adaptation international financial support. If a selected set of climate policy assumptions can be shared between studies, it will allow for better integration of and comparison between sectoral and regional studies, and to investigate how local policies are influenced by global policies.

As discussed in Section 4.1., SPAs should first and foremost include policies that are implemented because of climate change, aiming either at adaptation or mitigation. Global policies that are implemented for reasons other than climate change, but are directly affected by climate policies constitute a complicated case. The change in these policies due to the implementation of climate policies may be best included in SPAs. An example is the amount and structure of development aid. This aid may not be directed primarily toward adaptation and mitigation but its amounts will clearly be influenced by climate change considerations. All other policy variables should be included in SSPs. This includes global policies implemented for reasons that are not related to climate change, but with obvious consequences for vulnerability, adaptation and mitigation. Of course, it will sometimes be difficult to draw a clear line between climate-related policies and other types of policies (e.g., adaptation and risk management; mitigation and energy policy).

As far as they relate to mitigation, SPAs can be defined in three main dimensions. First, the SPA should state the global "ambition" of policies, i.e. the policy targets in terms of emission reduction or in terms of stabilization concentration. For instance, a possible SPA ambition is the introduction of policies aiming at a stabilization of CO2 concentration at 450 ppm or of global temperature stabilization at 2°C above its pre-industrial level. Second, the SPA should state the "policy and measures" introduced to reach the target: carbon tax, energy tax, international trading scheme, R&D subsidy, norms and regulation, etc. Third, a SPA should include the "implementation limits and obstacles" that are considered. Examples of generic climate policy assumptions include domestic action on the basis of current ambitions, global coordination on 2 degree or 3 degree stabilization, etc. A SPA may consider an idealized case of all world countries implementing a carbon tax at the same date, or a fragmented international regime with different or zero carbon prices in different regions; it may also exclude or include suboptimalities in the implementation of policies (e.g., loophole in regulations). While specification of a limited number of such policy scenarios could be difficult to agree on, it could also provide substantial insight into the robustness of alternative policy designs.

The ambition of global mitigation policies is closely linked with the level of climate exposure represented by one axis of the matrix in Figure 1. A "mitigation SPA" consistent with a given SSP and a given climate outcome could, for example, be derived from a global-scale IAM simulation yielding specific manifestations of, e.g., global carbon and energy prices and land-use patterns. This SPA could then be assigned to a cell of the climate outcome-SSP matrix, but it would not be the only conceivable mitigation policy to reach the given mitigation target under a given SSP. In fact, it might represent as much the idiosyncrasies of a particular IAM, e.g. in its choice of land use patterns and low carbon energy mix, as the requirements of the target and the underlying SSP. To highlight the model dependence in the choice of a "mitigation SPA", it will be useful to compare with the results from other IAMs. This will produce a whole array of mitigation policies for a given SSP and climate outcome. The challenge then is to select a very small number of SPAs that best span the possible variation along a third "climate policy" dimension within selected cell of the climate outcome-SSP matrix. The so-called RCP replication process initiated by the IAM community is an obvious candidate for informing the "mitigation SPA" dimension in such a way. This would require, however, the incorporation of the SSPs in the process. Obviously, the selected "SSP-SPA-climate outcome" combinations need to be limited to a single digit number to be practical. This implies that only a small fraction of cells of the matrix in Figure 1 can be populated with one or two SPAs.

To highlight the dependence of mitigation policy on the assumptions about available policy levers, implementation limits and obstacles, it will also be useful to run the IAMs under different perfect and imperfect policy scenarios. From an economic perspective, the distinction between "1 st best" worlds (i.e. all market externalities can be removed by optimal policy choices) and "2 nd best" worlds (i.e. some market imperfections remain even under optimal policy) is another useful concept in this context. 2 nd best worlds may comprise such diverse issues like e.g. imperfect labor markets and underemployment (e.g., [START_REF] Guivarch | The costs of climate policies in a second best world with labour market imperfections[END_REF], limited technology availability, market power in the energy sector (e.g., [START_REF] Mathy | Climate policies in a second-best world -A case study on India[END_REF], imperfection in world food and natural resource markets, (monetary and nonmonetary) trade barriers, insufficient investments in transport, water and energy infrastructure, governance issues (e.g., absence of land tenure, inadequate risk management practices), regional or socio-category povery traps, and many others (see Appendix 2 for further explanation). Exploration of 1 st best and 2 nd best policy differences will be an important topic of many IAM studies to be carried out and eventually the IPCC 5 th assessment report.

In terms of adaptation, SPAs will be more difficult to define, as no adaptation efficiency metrics is widely accepted. Also, adaptation and development policies are even more difficult to distinguish than mitigation and development policies (e.g., better sanitation systems have positive consequences in terms of population health in absence of climate change, and are also a powerful adaptation action to cope with more intense precipitations). The intense overlap between adaptation and development policies and measures makes it particularly difficult to distinguish between policies that belong to the SSPs and those that belong to SPAs.

For local-scale IAV analysis, however, several global adaptation parameters will be important and need to be included in SPAs: (i) availability of international finance to fund adaptation actions; (ii) existence and robustness of global insurance and reinsurance markets; (iii) availability of adaptation-related technologies and information, including for instance water saving techniques, earth and climate observations and forecasts, and GIS software to support risk management; and (iv) existence and functioning of world-scale markets for food, energy, and other climate-related goods and services (including trade barriers and regulations). Like for mitigation, these policies can be assumed perfectly designed and implemented (e.g., international funding really dedicated to adaptation projects) or can include imperfections and sub-optimalities (e.g., non-monetary barriers to access to technologies, due to regulation or lack of information). The 1 st -best vs. 2 nd -best distinction is thus valid for adaptation as well.

Relating SSPs to broader contexts of assessment needs

Regardless of the approach taken to specify SSPs, it will be necessary to have some means of deciding whether a given SSP set is sufficient and appropriate to serve as a basis for a wide range of climate change analyses. A large number of socio-economic futures are possible to specify; which ones would be best to include in a small set of SSPs cannot be decided without some explicit criteria in mind. As a catalyst for discussion, we propose an approach to judging whether a set of SSPs is sufficient that begins with the fact that the scenarios will be used for both mitigation and IAV studies. How challenging it may be to reduce emissions in order to achieve a particular climate change pathway depends strongly on socio-economic conditions assumed to prevail. Similarly, how difficult it will be to adapt to the climate change implied by that pathway, and what the effect of impacts will be on society, will also depend crucially on assumptions about future socio-economic conditions. Therefore, a set of SSPs should be designed in such a way that it captures a wide range of conditions relevant to both mitigation and impacts, adaptation, and vulnerability. In this way, the SSPs will facilitate the development of a literature that explores a wide range of uncertainty in the implications of any given future climate change path (such as those implied by the RCPs).

To this end, we propose considering whether any given set of SSPs would span a space defined by two dimensions: challenges to mitigation and challenges to adaptation (see Figure 3).

Coverage of the full set of possible combinations would facilitate internally consistent analyses that covered the range of uncertainty relevant to both mitigation and IAV issues. Mitigation is more difficult if unabated emissions are high, and if the capacity to mitigate is low; therefore, by "challenges to mitigation", we mean SSPs whose socio-economic assumptions are ones that, in the absence of climate change policy, would lead to relatively high emissions and technological, institutional or other conditions that were poorly suited to implementing emissions reductions. Adaptation is more difficult if exposure and sensitivity to climate change is high, and the capacity to adapt is low. By "challenges to adaptation", we mean socio-economic conditions that, in the absence of climate-related policies, lead to higher vulnerability and less capacity to adapt to a given level of climate change. Importantly, we exclude the level of climate change from our definition of adaptation challenges, because SSPs are intended to be paired with a variety of different climate change outcomes and should not therefore include the level of climate change as part of their own definition.

Four SSPs are located within the space shown in the figure for illustrative purposes. For example, SSP 3 represents a world that faces large challenges to both adaptation and mitigation, while SSP 2 is a world in which these challenges are more manageable. SSPs 1 and 4 represent worlds in which challenges are large for either mitigation or adaptation, but not both. A set of SSPs with these characteristics (see Box 2 for an example) would cover a wide range of conditions relevant for consistently analyzing the implications of mitigation, adaptation, and impacts across a range of climate outcomes.

There are several important caveats to this conception of an overarching structure for SSPs. First, the two axes -challenges to mitigation and adaptation -are not socio-economic elements themselves that would be used to build up narratives, but rather are characterizations of the implications of those elements. That is, narratives would be constructed based on stories about possible trends in demographic, economic, technological, and institutional factors. The framework proposed here would serve to aid in the choice of a small set of combinations of such factors by ensuring that the resulting combinations covered a broad range of possible futures most relevant to both mitigation and adaptation.

Second, the dimensions "challenges to mitigation" or "challenges to adaptation" do not strongly constrain any individual socio-economic factors that might be considered for inclusion in a SSP. For example, high energy demand associated with a large challenge to mitigation could be consistent with a wide range of future population growth, economic growth, or urbanization

Challenges to Adaptation

Challenges to

Mitigation

SSP 2 SSP 4 SSP 3 SSP 1 futures considered individually. However, the two dimensions do constrain constellations of socio-economic factors. Combining high economic growth, rapid urbanization in developing countries, and energy intensive lifestyles would, as a group, be consistent with a world in which energy demand was high. Choosing constellations of factors would be further constrained by jointly considering their implications for vulnerability and adaptive capacity as well.

Third, the two axes are not necessarily independent. Some elements of the SSPs are likely to have implications for both mitigation and adaptation. Independence of the two axes is not strictly necessary for the approach to be useful, and the degree to which the two challenges overlap will to some extent remain a question to be answered by the new scenario process itself and related studies.

Finally, although this approach envisions narratives informed by challenges to mitigation and adaptation, rather than mitigative and adaptive activities themselves, there is likely some degree of overlap which may make it difficult to disentangle baseline and policy effects, complicating the interpretation of the SSPs.

Referring to the matrix in Figure 1, the general idea would be that this set of SSPs could be combined with various climate outcomes and their associated RCPs to assess mitigation and impacts/adaptation possibilities. The degree to which different SSP-RCP-climate change outcome combinations would be consistent would remain a research question. For example, one climate realization of RCP8.5 might lead to 3 or more degrees of warming by 2100, and this outcome would probably be consistent with SSP 1, in which the world would be reasonably well suited to adapt, but would find it difficult to mitigate emissions; both of these conditions could plausibly lead to a world willing to accept a relatively large magnitude of climate change. However, it is harder to envision this RCP and level of warming being combined with SSP 4; a world that would face difficultly adapting but would have a number of options to mitigate emissions is unlikely to be one that would allow emissions to grow at a rate that would produce the high radiative forcing of RCP8.5.

To locate the SSPs in the space of mitigation and adaptation prospects, the mitigation/adaptation challenge axes would need to be specified in terms of specific variables that would constitute these broad measures. In the tables below we indicate just a few that might be considered (IPCC WG 2 AR4 report, Chapter 18; [START_REF] Wilbanks | Integrating Mitigation and Adaptation as Responses to Climate Change: A Synthesis[END_REF][START_REF] Wilbanks | Integrating Mitigation and Adaptation as Responses to Climate Change: A Synthesis[END_REF][START_REF] Yohe | Mitigative capacity: The mirror image of adaptive capacity on the emissions side[END_REF]. Further extension of these lists would be an important next step in the development of this approach.

Challenges to adaptation

Limited research and technology development on climate change adaptation Limited knowledge of costs, benefits, prospects, and limits of adaptation options Limited evidence of effectiveness of options to adapt to climate change, because attribution is just emerging Limited adaptation capacities, especially in some developing regions Concentrations of population and economic activity in vulnerable areas, in some cases because of maladaptive policies

Challenges to mitigation

High demands for energy services and land use changes related to social and economic development aspirations Relatively low cost of fossil energy forms Large inertia in energy supply technology portfolios, together with limited capacities to reduce carbon emissions significantly Slow progress with realizing energy efficiency potentials Limited political will to enact significant mitigation policy initiatives

BOX II: EXAMPLE OF A SET OF SSPs COVERING THE SPACE OF CHALLENGES TO MITIGATION AND ADAPTATION DEPICTED ABOVE

To sketch an illustrative example of how four SSPs might fit the space depicted in the figure, SSP 1, as a world with large challenges to mitigation but reasonably well equipped to adapt, could be one in which, in the absence of climate policies, energy demand was high and most of this demand was met with carbon-based fuels (perhaps similar to the SRES A1FI scenario).

Investments in alternative energy technologies and the productivity of land were low. Thus, unmitigated emissions were high, and there were few readily available options for mitigation (including biomass). Nonetheless, development was relatively rapid (one driver of high energy demand) and itself was driven by high investments in human capital. Improved human capital also produced a more equitable distribution of resources, stronger institutions, and slower population growth, leading to a less vulnerable world better able to adapt to climate impacts.

SSP 2, in which the world is reasonably well suited to both mitigate and adapt, could be one in which development proceeds at a reasonably high pace, inequalities are lessened, technological change is rapid and directed toward environmentally friendly processes, including lower carbon energy sources and high productivity of land. An analog could be the SRES B1 scenario.

SSP 3, with large challenges to both mitigation and adaptation, could be a world in which unmitigated emissions were high due to moderate economic growth, a rapidly growing population, and slow technological change in the energy sector, making mitigation difficult (as, for example, in SRES A2). Investments in human capital were low, inequality was high, a regionalized world led to reduced trade flows, and institutional development was unfavorable, leaving large numbers of people vulnerable to climate change and many parts of the world with low adaptive capacity. Finally, SSP 4, in which mitigation might be relatively manageable while adaptation would be difficult and vulnerability high, could describe a mixed world, with relatively rapid technological development in low carbon energy sources in key emitting regions, leading to relatively large mitigative capacity in places where it mattered most to global emissions. However, in other regions development proceeded slowly, inequality remained high, and economies were relatively isolated, leaving these regions highly vulnerable to climate change with limited adaptive capacity.

Conclusion: Toward a synthesis of approaches or a shared agreement on framings?

We have presented an assessment of current challenges for the development and application of climate change scenarios, open questions to be addressed, and some ideas how to move forward. The following key points were made in Section 2 and 3 of this paper:

 Socio-economic scenarios in combination with climate change scenarios constitute an important tool for exploring the consequences of anthropogenic climate change and available response options.

 SE scenarios have been applied for different purposes and to a different degree in various areas of climate change analysis. Integrated assessment modeling has used them to develop GHG emissions scenarios for the 21 st century and to investigate strategies for mitigating GHG emissions on a global scale. Analyses of climate change impacts, adaptation and vulnerabilities depend heavily on assumptions about underlying socio-economic developments, but have employed SE scenarios to a lesser degree. This is due to the multitude of contexts and scales of such analyses.

 A more consistent use of socio-economic scenarios that would allow an integrated perspective on mitigation, adaptation and residual climate impacts remains a major challenge. Since the integration will have to rely on a multitude of studies from the IAM, IAV and CM communities, it is vitally important to assure that the different perspectives of these communities do not diverge in the coming years. The development of new SE scenarios can be an important contribution to catalyze and strengthen linkages between the IAM and IAV communities -if it is approached in an inclusive and integrative way.

 The obvious question is whether integration of studies from the IAM, IAV and CM communities would be feasible if their underlying assumptions are not harmonized to a minimal degree. And if such minimal harmonization is needed, how can it be achieved?

We assert that the identification of a set of global narratives and socio-economic pathways offering scalability to different regional contexts, a reasonable coverage of key socio-economic dimensions and relevant futures, and a sophisticated approach to separating climate policy from counter-factual "no policy" scenarios would be an important step towards a more integrated use of socio-economic scenarios.

In Section 4 of this paper, we have proposed some ideas for how a small set of socio-economic scenarios combining narratives and quantitative boundary conditions into Shared Socioeconomic Pathways (SSPs) could facilitate the process of integrative research and eventual assessment of research results. Such SSPs should be specified in an iterative manner and close collaboration between IAM and IAV researchers to assure coverage of key dimensions, sufficient scalability and widespread adoption. They can be used to collect different climate change analyses in a matrix defined by the two dimensions of climate outcome (as e.g. characterized by a radiative forcing or temperature level) and socio-economic development as classified by the SSPs. SSPs should be restricted to socio-economic baseline assumptions in order to allow for flexibility in analyzing the effect of different climate policies for a given SSP, or of a given climate policy across a range of different SSPs. Thus, for some applications, SSPs may have to be augmented by "Shared Climate Policy Assumptions" (SPAs) capturing global components of climate policies that some studies may require as inputs. Finally, sufficient coverage of the relevant socio-economic dimensions for the analysis of mitigation, adaptation and residual climate impacts may be assessed by locating the SSPs along the dimensions of challenges to mitigation and to adaptation.

We conclude that the development of SSPs and integrated socio-economic scenarios more broadly is a useful focal point for collaborative work between IAM and IAV researchers. However, it is an open question whether SSP development should seek a single approach or encourage, at least at the outset, an ensemble of approaches that share common features. This question can be viewed both under a long term (> 3 years) and short term perspective (until the IPCC 5 th Assessment report in 2013/14).

Long-term perspective on scenario development for integrated mitigation and adaptation analysis.

When taking a long term perspective, the focus should be on establishing platforms and processes for collaborative work of IAM and IAV researchers on integrated scenarios. As mentioned above, the quest for more integrated SE scenarios can be an important catalyst for strengthening the linkages between IAM and IAV research, but this is likely to be a gradual process. It will involve gradual adjustments of IAV and IAM research cultures towards a more broadly shared understanding of the utility of scenarios and their applications. We envisage that these gradual adjustments can be achieved by running through several iterative cycles of scenario work that allow gaining -with each cycle -greater insight into the nature of useful SSPs and SPAs as well as greater recognition of their value among both IAV and IAM researchers. Such a cycle may include (i) individual and initially separate scenario work from local to global scales, including attempts to adapt global scenarios to local contexts, (ii) a joint effort on taking stock of the evolving scenario database, its linkages, strengths and weaknesses, and finally -based on the insights gained -(iii) a deliberate and collaborative push of IAV and IAM researchers to develop next generation SSPs and SPAs on the global scale. This may later on be followed by an attempt to design standardized sets of scenario assumptions for a variety of national and local scales that are linked to the global scale SSPs.

These can then be scrutinized and improved in subsequent cycles. Phase (i) comprised of individual scenario work should provide an extensive test-bed for the usefulness of last generation SSPs and SPAs. Ideally this includes local, regional and context specific scenario work evaluating the scalability of SSPs. It should also test for the inclusion of key dimensions in SSPs, and their coverage of relevant futures.

The collaborative elements of the iterative process (Phases (ii) and (iii)) require platforms for IAM / IAV interaction that yet need to be established. Such platforms can take several forms, e.g.:  Dedicated working groups of organizations which are recognized by both IAM and IAV researchers  Regional working groups that assure participation of researchers from all regions in the scenario building process  Scenario data repositories that are used by both IAM and IAV researchers  A joint effort to build a global socio-economic database for the use of both IAM and IAV researchers.

In most cases, establishing such platforms needs resources, a common purpose and sense of urgency. In the short term, a sense of urgency and to some extent common purpose is provided by the 5 th Assessment Report of the IPCC. This creates a window opportunity to establish institutional structures for collaborative scenario work that may be able to sustain a scenario process even after the AR5 has been finalized.

Short term goals for integrated scenario development in the context of the new scenario process and aiming to inform the IPCC 5 th Assessment Report

In the short run, an immediate concern is to produce tangible outcomes that would allow the 5 th Assessment Report of the IPCC to take a more integrated perspective on mitigation, adaptation and residual climate impacts. The new scenarios process outlines a way forward, aiming for integration of research from the climate modeling, IAM, and IAV communities (NR07, [START_REF] Moss | The next generation of scenarios for climate change research and assessment[END_REF]. However, the new scenario process has yet to answer a series of important conceptual and methodological questions, and is still struggling to establish a platform for truly collaborative work between IAM and IAV researchers. One possibility, therefore, is to work toward joint IAM/IAV agreement on a shared set of questions to be answered by scenarios and/or narratives, along with a shared sense of socioeconomic and other contextual variables to be addressed. At a rather general level, this appears feasible in a matter of months rather than years.

The provision of more integrated scenarios on mitigation, adaptation and residual climate impacts for the IPCC 5 th Assessment Report should be widely recognized as an important goal that is shared by the IAV and IAM communities. This recognition will help to muster the will for quickly and jointly developing SSPs. The publication cut-off dates for AR5 set a very tight deadline. Papers to be included in the Working Group III report must be accepted by October 2013 and submitted in March 2013. This implies that IAV and IAM research based on SSPs and SPAs would need to commence around summer 2011 to allow at least 18 months for producing tangible results. At that time a large set of climate change projections from the coupled model intercomparison project for AR5 (CMIP5) will likely be available. By that time a set of SSPs, and to a smaller extent SPAs, would need to be developed, vetted and made available to the community, if they are to play a role in IAM and IAV research leading up to the AR5. Thus a decision on the role of SSPs in the new scenario process would have to be taken fairly soon. We expect that the topic will be taken up and discussed during the IPCC expert workshop on socioeconomic scenarios in November 2010.

Given this context, we suggest that next steps should include the following:

(1) Engage representatives of the IAM and IAV communities as soon as possible in a joint strategic planning effort to agree to key features of SSP development. These features would include the over-arching framework for a set of SSPs, basic features of their content, and a process for fleshing out further details. The features should reflect the agendas of each community and include strategies for collaboration in order to assure outcomes that integrate the priorities and perspectives of both communities. The IPCC scenarios workshop provides an excellent opportunity to pursue this step.

(2) Initiate joint SSP development involving both communities, including representation of regional IAV researchers in developing regions. An important aim would be to develop a set of broad-brush SSPs at the global and world region level by summer 2011, so that they are available for use at the same time that climate model runs based on the RCPs become available, and can inform research to be assessed in AR5.

(3) Identify or develop relevant information and data, including (as appropriate and as available) socio-economic projections and trends for regions of the world and possibly IAM model simulations in support of SSPs.

We conclude with a more general observation. In this paper, we discussed many challenges for the development of a new generation of climate change scenarios. We also laid out a set of ideas how to tackle these challenges. Tackling them will be important, because the stakes are high. New scenarios are needed for providing a more integrated perspective on mitigation, adaptation and residual climate impacts on all scales. This in turn will provide a better understanding of the many facets of climate change, and produce a more consistent picture of available response options. We are confident that the goal is large enough to engage researchers from the IAV and IAM communities in a serious collaborative effort. If this occurs scenarios will have shown their integrative power in yet another way. They are not only powerful tools for long-term strategic thinking from an integrated systems perspective. They are also powerful tools for initiating and framing a discourse integrating the views of many from different communities.

is the best one can do. On the other hand, if existing market imperfections (e.g., in the tax structure or in labor markets) can be addressed with climate policy, this may offer the opportunity for reduced mitigation costs. Thus, 2 nd best analysis of climate policy will give a more robust picture of feasibility and costs, will allow analyzing additional instruments to carbon pricing, like clean energies subsidies and efficiency regulation, in a consistent framework, and will be needed to identify double dividends and co-benefits of climate policy.

Important additional market failures and other limitations characterizing 2 nd best situations are:  Market power in the energy and other sectors (including fossil fuel markets),  Limited appropriability of private sector innovation (spill-over externality),  Distortionary taxation, subsidies and regulation, particularly in the labor, energy and agricultural sectors  (Under)provision of public goods (Transport and Energy Distribution infrastructure, Adaptation infrastructure like levees)  Limited technology availability due to failures in innovation process or political economy constraints  Limited flexibility in carbon pricing regimes, e.g. due to regional or sectoral fragmentation  Distorted insurance, capital and land markets  Imperfect labor market, with existing underemployment  Asymmetric information of firms, households and regulators  Trade barriers  Other pollution and resource externalities, including air pollution and overexploitation of natural resources More generally, we suggest that the concept of market failure, and a taxonomy of relevant market failures, will be an important dimension for clustering the results on mitigation, adaptation and residual climate impacts in a meaningful and policy relevant manner. This will require the identification of what market failures have been taken into account, and what not, by the individual studies to be assessed by AR5.

Figure 1 :

 1 Figure 1: Matrix of socio-economic "baseline" developments (characterized by SSPs) and climate change outcomes (determined by RCPs). White cells indicate that not all combinations of SSPs and climate change outcomes may provide a consistent scenario.

Figure 2 :

 2 Figure 2: Likely trends of development of indicators within each chosen scenario (TERI, 2009)

Figure 3 :

 3 Figure 3: Example of four SSPs spanning the space of challenges to mitigation and adaptation.

  Historically, the impact/adaptation/vulnerability (IAV) research community has seldom used scenarios of global futures as a basis for analyses of climate change impacts and adaptation potentials. Climate change impact studies have more typically analyzed the consequences of an assumed increment of a climate change parameter (e.g., +2°C). More recently, considering the heterogeneity and uncertainty of local climate changes, some studies have moved toward vulnerability analysis.

	analyses,
	most of the IAV research community in most parts of the world is accustomed to working
	through small-scale individual research rather than larger-group, quantitative model-based
	projections of initial conditions (in contrast to the climate science/ESM and IAM communities).

This does not mean, of course, that IAV research has ignored available global climate change or socioeconomic scenarios (where available) as useful ways to frame impact, adaptation, and vulnerability issues. For example, much of the research on implications of changes in temperature, precipitation, extreme events, and sea-level rise has been informed by discussions in the broader climate research and policy communities about risks of greatest concern, which in turn are informed by both global and regional climate scenarios.

The limited use of quantitative projections of climate parameters from global climate change scenarios in IAV research (or of contextual conditions from quantitative socioeconomic scenarios), however, reflects several realities (e.g.,

[START_REF] Rosenzweig | The State of Climate Change Vulnerability, Impacts, and Adaptation Research: Strengthening Knowledge Base and Community[END_REF]

):

(a) IAV research style Because climate change impact assessments need to start with local and sector scale

2.3. Using scenarios to integrate IAM, IAV and CM research

	changes in those parameters that are incorporated in climate change projections; and IAV
	research results that are not scenario-based can thereby be mapped onto scenarios. Much of
	this work has been carried out at a high level of quality, and it has built a general picture of
	climate change impacts of relatively modest climate change vs. relatively severe climate change
	that is viewed by the scientific community with a high degree of confidence, although specific
	impact projections are better considered illustrative than conclusive. A recent study by the US
	National Research Council (NRC, 2010) has addressed the painful fact that neither scientists nor
	policymakers have a clear picture of how the world would be different in the mid to long term
	under different climate change scenarios.
	Given these challenges, prospects for relating IAV research more often and more directly with
	climate change scenarios would seem to depend on three factors: (a) developing scenario
	information that is relevant to concerns of IAV research, such as information about extreme
	weather events or improved information about precipitation changes, (b) stimulating knowledge
	development and cultural changes in the IAV research community so that interactions with
	other parts of the climate change research effort are viewed as a higher priority and as adding
	value to IAV research, and (c) developing decision-making approaches that understand but are
	not impeded by uncertainties in using scenario-based IAV results.
	One consequence of these needs is the fact that two parallel and interlinked research lines are
	needed: one on how IAV analyses can incorporate global scenarios in their approaches and
	methods; the other on how global scenario can be developed to correspond to the needs of IAV
	analyses. Interactions between IAV community and scenario development is even more
	important because local IAV analyses using global socio-economic scenarios will necessarily lead
	to some inconsistencies between the assumptions (from scenarios) and results. In the
	development of global scenarios, therefore, it will be necessary to think of how to incorporate
	new information from local analysis when it becomes available. The idea would be to shift from
	a linear approach (global scenario  local scenario  local IAV analysis) to an interactive
	approach in which the local and global scales have two-way interactions.
	Society has three different response measures to climate change: mitigate, adapt and accept the
	residual climate damages. Obviously, the question is not what measure to choose over the
	other, but what mix of measures to adopt. This calls for an integrative assessment of results
	these are promising from IAV, IAM and CM research. However, the challenges of bridging the different perspectives,
	starting points for further advances in these directions. But the fact remains that there are no
	commonly shared sets of perspectives about socioeconomic futures across the world as
	counterparts of global climate change scenarios; and IAV researchers seldom have resources to
	develop them to support their work.
	Meanwhile, demand has been growing for assessments of impacts and adaptive responses
	implied by climate change scenarios, especially in IPCC assessments, even if most of the
	available IAV knowledge is not directly linked with climate change scenarios or informed by
	socioeconomic scenarios. The general response has been to extrapolate in order to produce
	preliminary IAV estimates (e.g., IPCC WG II AR 4, 2007; USGCRP, 2010). For instance, if IAV
	research postulates changes in climate parameters, these changes can be associated with

different scales and different needs of IAV and IAM research are considerable (e.g. O'Neill et al,

  It allows, e.g. studying the impact of different climate policies for a given SSP, or the impact of different SSPs on effects of climate policies Policy variables may be placed into four categories: 1. Policy variables that are largely unrelated to climate change, i.e. neither are they significantly affected by climate policy, nor would they replace or reinforce climate policies. An example may be retirement policies. 2. Policy variables directly related to climate such as carbon taxes, cap & trade, technology protocols, adaptation funds, etc. 3. Policy variables that are not mainly driven by considerations of climate change, but may have a substantial impact on climate policies, e.g. in terms of substituting or reinforcing them. An example is health policies lowering the vulnerability of a population to climate change. Urban plans and land-use plans are other good examples. 4. Policy variables outside climate policy that may be significantly affected by climate

  SSPs are linked to different needs, their development should begin with input from both the IAM and IAV communities about features that are essential to them. The development process should move toward agreement on a common set of features that would be shared by SSPs used by both communities and that would facilitate research across boundaries between IAM and IAV research. It should span a wide range of differences between the systems of greatest interest (e.g., sources of emissions vs. targets of impacts), geographic scales of interest (e.g., global vs. local), and different temporal scales of interest (e.g.., the next twenty years vs. the rest of this century). It should work toward a common set of data sources and working assumptions (e.g., about the variables of greatest interest in projecting demographic, economic, technological, and institutional futures). For example, can SSPs used by both communities be related to a limited number of general types of futures, defined in consistent and broadly understandable ways? If so, how many should be the goal? Defined according to what axes of major determinants?

Parsimonious narratives have been used as a means to convey qualitative information in scenarios for intermediate users (e.g. researchers; seeNakidenovid et al.,

2000). They need to be distinguished from the use of narratives in scenarios for decision makers, i.e. making scenarios sufficiently realistic and compelling to decision makers.2 Intuitive Logics is a holistic approach of deriving scenarios / storylines for strategic thinking by expert groups. It has been popularized by Royal Dutch Shell and futurologists such as Herman Kahn.

Acknowledgments:

We thank Timothy Carter, Kris Ebi, Mikiko Kainuma, Benjamin Preston, Keywan Riahi, Vanessa Schweizer, Detlef van Vuuren and Timm Zwickel for valuable comments that helped to improve this paper.

Appendix 1: Importance of short-vs long term time horizons for scenario development

The climate modeling community will produce high resolution climate change patterns out to 2035 for the 4.5 W/m2 RCP. This is also a time horizon over which it may be possible to produce more credible projections of socio-economic conditions, such as population and GDP density that may not have diverged substantially from current patterns. IAV studies may therefore be able to employ both more detailed climate and socio-economic information than is possible for long-term studies, and with less associated uncertainty. Over a longer time horizon, climate outcomes associated with RCPs will differ more significantly, and the potential for divergent socio-economic conditions is also greater. Given the difference in uncertainties and in the plausibility of higher resolution climate and socio-economic information, it may be appropriate for SSPs to be defined with different levels of detail for different time horizons. Since the spatial distribution of population and GDP will be tied to the present-day situation for the next decades, common gridded population and GDP scenarios out to 2035 might be a very useful tool for both IAV and IAM studies. These would need to be built on a harmonized and gridded database of present day (2010) distributions of population and GDP that could be used by IAV and IAM researchers as calibration point for the base year. In contrast, socio-economic projections out to 2100 are highly uncertain, and long term SSPs should thus be restricted to a few trajectories of fundamental socio-economic variables like GDP and population for large regions. Related questions are: How to harmonize assumptions for the short term (2035) and long term (2100)? How to assure coordination with RCP replications of the IAM community? A detailed SE data set for present day (e.g. base year 2010) may be of enormous use for many activities. Should there be a joint IAV / IAM effort to put such a dataset together?

Appendix 2: A summary of the 1 st vs. 2 nd best world distinction, and its importance for climate policy analysis

The feasibility and costs of mitigation and adaptation, as well as the damages from climate change will not only depend on socio-economic baseline trends, but also on policy environments that either nurture or impede the effective and efficient implementation of climate policies. Climate policy analysis often assumes that markets have worked efficiently in the past (i.e. market failures were negligible on a global scale), and are now faced with a climate externality.

Resulting optimal policies aim to internalize climate change (by means of greenhouse gas pricing) without any further needs for additional market interventions (like e.g. subsidies for clean energy, regulation etc.). In economic jargon, such situations of isolated market failures that can be removed with a single policy are called "1 st best", and the resulting optimal policies are called "first-best policies". A classic example of this approach is the work by [START_REF] Nordhaus | The Economics of Climate Change[END_REF][START_REF] Nordhaus | Warming the World[END_REF][START_REF] Nordhaus | A question of balance[END_REF]. Many mitigation cost calculations by integrated assessment models for AR4 were also conducted, more or less implicitly, with a first best philosophy. Such analyses are generally very optimistic about the prospects of climate policy, and there is a strong need to look into socalled "2 nd best" situations of multiple market failures [START_REF] Lipsey | The General Theory of Second Best[END_REF]) in greater detail. While a first-best policy removes all market failures, a second-best policy optimizes the outcome given the constraint that one (or some) of the market constraints or failures cannot be removed. The economic terms of "1 st best" and "2 nd best" should therefore not be confused with normative judgments of the goodness of a policy. In a 2 nd best situation, the 2 nd best policy