The Initiative for Climate Action Transparency (ICAT) has prepared an assessment guide called the ICAT Non-State and Subnational Action (NSA) Guide (ICAT NSA guide) for aggregating the ex-ante GHG emissions impact of non-state and subnational climate actions, but variations in assumptions and methodology still occur in practice. This makes it difficult to communicate and interpret the results of individual assessments and determine the extent to which the results may be comparable. The natural variation in approach can stem from different assessment objectives that strongly influence the method or because of using different data, depending on access and availability.

The assumptions made in core areas will affect the conservativeness, limitations, and probability of assessment results. For example, including a broad scope of NSA actions in an assessment may demonstrate a larger potential emissions reduction, but may carry limitations if the included actions are not likely to be achieved, or if missing data must be filled by gap-filling methods. On the other hand, applying a high degree of conservativeness may yield more realistic results in the absence of further intervention but may also risk excluding actors that are ambitious but require support to achieve their aims. Clearly defining the objective at the outset of the assessment can guide the process.

The purpose of this supplemental methodological paper is to support users of the ICAT NSA guide with key decision points during the assessment process. This paper does not indicate which specific approach is best but, rather, highlights the use cases and implications of key decisions on the assessment outcome. This paper is directly aimed at users of the ICAT NSA guide in designing their assessments. It may also be useful to the media and other reviewers of assessment studies seeking to understand how assessments were performed and whether and how the results of different studies may be compared. The figure below provides an overview of key assumption areas discussed in this paper.

Key Findings:

  • A growing body of research exists that explores the potential impact of climate-change mitigation efforts taken by non-state and subnational actors, including cities, states, and businesses.
  • Methodologies for assessing the collective impact of multiple actors are evolving and may require numerous assumptions and considerations to calculate potential emission reductions and avoid overlaps and double-counting between actors.
  • This paper builds on the Initiative for Climate Action Transparency (ICAT) Non-State and Subnational Action Guide to elaborate the key decision points and various assumptions that may be applied when calculating the ex-ante collective impact of non-state and subnational actors’ mitigation policies, actions, and targets.
  • The specific assumptions and decisions depend on several conditions related to the assessment boundary and objectives and should be recorded and transparently communicated to contextualize and enhance the credibility of results.

Executive Summary:

Users of the ICAT NSA guide face several decisions that will affect the overall assessment process, the degree of conservativeness, limitations, and the results. On the surface, different study results may appear comparable if they include the same actor groups or target types and even if they follow the same steps within the ICAT NSA guide; but, in fact, they may apply very different methodological assumptions and thus produce different results. It is critically important that studies clearly and transparently communicate their assumptions and equally important that the target audience review the results along with underlying assumptions. Assessments of non-state and subnational climate actions are critical to understand who is taking action, how it supports national governments in meeting or exceeding NDCs, and where greater ambition is required.

To provide structure to the process of aggregating ex-ante GHG emissions impacts, the Initiative for Climate Action Transparency (ICAT) developed a guide, the Non-State and Subnational Action (NSA) Guide. It provides a step-by-step approach for national policymakers and analysts to collect data, assess the impact of non-state and subnational actions and transparently communicate assessment results. ICAT also provides an excel-based Climate Action Aggregation Tool (CAAT) to support users in implementing the guide (ICAT 2021).

While many challenges are identified in the ICAT NSA guide, the different approaches to address them are not fully explained. Building on the lessons and experience of real-world assessment efforts, this complementary document follows the steps of the ICAT NSA guide and provides further guidance to assist users in designing their methodology and selecting assumptions in each step to minimize limitations and enable transparent reporting of results. The paper should be used in combination with the ICAT NSA guide to weigh various options under each step.