Climate policies can be characterized in terms of their attributes, such as the stringency of policy targets, the set of climate policy instruments employed to achieve the targets , and the time and place in which climate policy instruments are deployed. Following a proposal by Kriegler et al. (2012), we define Shared Climate Policy Assumptions as capturing key characteristics of mitigation and adaptation policies up to the global and century scale. The latter requirement emerges from the fact that SPAs should relate to the global scenario framework. At the same time they should be flexible enough to allow for regional differentiation of policies.
Concretely, SPAs should describe three attributes of climate policies (Kriegler et al. 2012). The first attribute is the global (collection of) “climate policy goals” such as emissions reductions targets, or different levels of ambition in limiting residual climate damages, e.g. in terms of development indicators that should not be jeopardized by climate change. However, there is a clear overlap of mitigation policy goals with the forcing (RCP) dimension of the scenario matrix, and of adaptation policy goals linked to achieving sustainable development and other societal goals described in the SSPs. RCP forcing levels can be used to directly describe long term mitigation targets (although not how to achieve those targets), and in any case have strong implications for the admissible global outcome of regional emissions reduction targets.
Thus, we have to distinguish two types of SPAs: a full SPA that includes all mitigation and adaptation policy targets, and thus embeds the forcing (RCP) dimension and possibly aspects of an SSP in it. And a reduced SPA that excludes the mitigation policy goals, at least as far as they relate to emissions reductions and global concentration and forcing outcomes, and the adaptation policy goals as far as they relate to development goals, and therefore is orthogonal to both the forcing (RCP) and the SSP axis of the matrix framework. Thus, the reduced SPA has to be used if variations of policy assumptions for a given RCP-SSP combination are to be explored. In this way, it adds a third axis to the scenario matrix. Both concepts—reduced and full SPA—can be entertained simultaneously. A full SPA may simply be the combination of a reduced SPA and an RCP forcing level. Or, if the mitigation targets are to be specified in terms of regional emissions reductions commitments, the RCP forcing level would determine the global cumulative amount of permitted emissions (and possibly some properties of the global emissions pathway), while the reduced SPA is free to include any distribution of these emissions permits across regions that adds up to the global total. Similarly, for adaptation policy goals, a full SPA would embed SSPs elements, such as development goals that are relevant for adaptation policies, while a reduced SPA would focus purely on the adaptation policy aspect. We note that this differentiation of full and reduced SPAs to ensure orthogonality of the SPA, SSP, and forcing (RCP) axes goes beyond what was proposed in Kriegler et al. (2012).
Second, the shared climate policy assumption should describe the characteristics of the global (collection of) “
policy regimes and measures
” introduced to reach the policy goals- On the mitigation side, such policy measures could be globally harmonized or regionally differentiated carbon taxes, an international emissions trading scheme with a particular burden sharing mechanism, a mix of different policy instruments ranging from emissions pricing to low carbon technology subsidies to regulatory policies, or a mix of different approaches in different sectors, e.g. including transport policies and schemes to protect tropical forest. SPAs may also relate to mitigation policy dimensions that are often not considered by model studies, but elsewhere in the literature. An example is how particular policies would be financed in practice (Clapp et al. 2012; Winkler et al. 2009). On the adaptation side, the SPA package may include, for example, the type of adaptation measures that are implemented (e.g. more efficient irrigation techniques or water recycling technologies) and the availability of various amounts of international support for adaptation in developing countries.
Third, a shared climate policy assumption should include the “
implementation limits and obstacles
” to the extent they are considered and are not part of an SSP. Those obstacles could be specified in terms of the exclusion of several policy options for some regions and sectors where they do not appear to be feasible. Such conditions could evolve over time such that initial limits might dissipate or intensify. For example, several land pools may be excluded from carbon pricing due to practical constraints of implementing such a pricing policy. Or a group of regions may be assumed to remain outside an international climate policy regime until some point in time. Likewise, adaptation effectiveness may differ in a model where behavioral biases in risk perceptions are accounted for, or where political economy or enforcement constraints makes it impossible to implement some policies (e.g., land-use regulations aiming at reducing disaster losses are difficult to implement in areas without official land tenure such as informal settlements).
Care needs to be taken to separate implementation limits and obstacles that are attributes of a SPA, and climate policy obstacles that are inherent in the socioeconomic reference environment described in an SSP. The latter can include market distortions, e.g. in energy and labor markets. Babiker and Eckaus (2007) and Guivarch et al. (2011) show that taking into account unemployment and friction in labor market adjustments can change in a significant way the assessment of mitigation costs. In developing countries in particular, existing economic distortions cannot be disregarded in the design of climate policies. The high level of unemployment (often over 25 %), the large share of the informal economy, and the difficulty to enforce regulation (e.g., in terms of land-use planning) and raise taxes create specific difficulties for climate policy. To the extent that such elements are considered, they will be part of an SSP rather than a reduced SPA. However, they could be added to a full SPA that aims to provide a complete picture on the three attributes of climate policy, i.e. goals, instruments, and obstacles.
The formulation of SPAs on a global and century scale can become complex. However, the goal of SPAs should not be to describe the climate policy landscape in every conceivable detail, but rather to summarize and make explicit the central policy assumptions that have to be made anyway by individual studies to produce climate policy scenarios. Seen through this lens, SPAs have been used for quite some time, for example, in integrated assessment model inter-comparison studies (e.g. Clarke et al. 2009; Riahi et al. 2014; Kriegler et al. 2014a) that needed to harmonize their climate policy assumptions in order to make results comparable. It is more and more recognized that such harmonization does not only involve the specification of a long term climate target as would be captured by the RCPs, but also, e.g., the basket of greenhouse gases and sectors to become subject to emissions pricing, the degree of global cooperation and the degree of overshoot of long term targets (Blanford et al. 2014).
A controlled variation of key climate policy assumptions, but not of all the details, will be required to make the scenario matrix approach fully operational. Knowledge about what type of SPAs have been used in modeling will be required to make appropriate comparisons across model analyses or other studies that explore a given combination of SSP and radiative forcing target (RCP). For instance, mitigation cost assessments by two models cannot be compared directly, if these two models make different assumptions on practical obstacles to policy implementation (e.g., if one model assumes a globally integrated carbon market and the other a regionally fragmented mitigation regime). Furthermore, the choice of SPAs will interact with the socioeconomic challenges to mitigation and adaptation that are engrained in the SSPs (O’Neill et al. 2014). For example, whether or not global coverage of emissions reductions can be achieved will affect the ability to reach a prescribed RCP forcing level. Likewise, whether or not a global adaptation fund is put in place will affect the ability of individual regions to adapt to climate change.