Summary

The US Global Change Research Program (USGCRP) is mandated to develop an assessment of global change every four years or less. The Fifth National Climate Assessment (NCA5) is the latest in a series of sustained assessments that evaluate the state of global change science and analyze the wide range of impacts of climate change in the United States. The assessments represent a consensus-based view of the state of knowledge, relevant for policy and decision making.

The draft NCA5 report reviewed here addresses a wide range of topics of high importance to the United States and society more broadly, extending from human health and community well-being to the built environment, to businesses and economies, and to ecosystems and water resources. NCA5 is a unique opportunity to examine the impacts of current and projected climate change on each region of the United States. Hundreds of experts representing federal, state, and local governments; academia; non-governmental organizations; and the private sector developed the draft NCA5 report, with further input from community engagement events and public comment. The scale of collaboration is the largest for any National Climate Assessment (NCA) to date. The impressive and rich array of perspectives introduced through the NCA5 writing process provides an opportunity to develop a foundational climate change report that informs and highlights adaptation and mitigation efforts and serves as a valuable resource for broad audiences.

The National Academies of Sciences, Engineering, and Medicine convened an ad hoc Committee to Review the Draft Fifth National Climate Assessment (the “Committee”). The Committee is charged with providing a comprehensive, independent review of the draft NCA5 report, concurrent with the public comment period. In its review, the Committee evaluated the draft NCA5 report to determine whether it meets the requirements of the federal mandate; provides accurate information grounded in the scientific literature; and effectively communicates climate science, impacts, and responses for general audiences including the public, decision makers, and other stakeholders (see Chapter 1 of this report for the Committee’s complete Statement of Task). Chapter 2 of this report synthesizes the Committee’s overarching comments on the draft NCA5 report and makes recommendations for improvement. Chapter 3 provides detailed comments on each individual chapter of the draft NCA5 report, and Appendix A includes detailed line-by-line comments. The Committee’s approach to this review was to provide constructive comments and specific suggestions to strengthen the accuracy, consistency, credibility, and accessibility of the key messages and supporting text in the draft NCA5 report.

The Committee applauds the NCA5 authors for their outstanding job assembling, researching, and interpreting the vast knowledge of an extremely complex and rapidly changing topic—climate change impacts, adaptation, and mitigation in the United States. The Committee also commends the NCA process for its use of traceable accounts to accurately document the state of knowledge—including emerging and remaining gaps in knowledge—regarding the impacts of climate change. The draft NCA5 report gives significant attention to matters of equity and justice—a substantial and long-overdue improvement over past NCAs. The Committee appreciates the attention to evaluating climate change impacts and responses through the lens of equity and justice. The Committee also commends the NCA5 authors for identifying progress the United States has made in adapting to climate change, particularly providing examples of successful adaptations by local and tribal communities.

Clear and consistent structures support the ability of NCA5 to reach its target audiences effectively, thereby enhancing the credibility and usability of the report. In order for the draft NCA5 report to better provide a clear, thorough, and credible encapsulation of the knowledge base of the impacts of climate change on the United States, the Committee recommends that consistent structures be adopted to make the report accessible to the target audiences:

1. Key message “labels” and “titles” should follow a common structure throughout the report.
2. The “message” part of each key message should have a consistent reading level, length, and voice throughout the report.
3. Confidence and likelihood statements should be used consistently for each claim across all key messages, and readers should be able to readily understand what is meant by confidence and likelihood.
4. A consistent framework for traceable accounts sections would better support the key messages and build consistency and credibility across chapters.
5. The introduction to each chapter should be consistent in length, depth of content, relationship to key messages, and voice; new terminology should also be introduced in a consistent manner.

It is vital that key messages throughout the report are crafted consistently and carefully. Key messages should incorporate and express the knowledge base through the use of appropriate confidence and likelihood levels and be carefully written so that findings can be understood by broad audiences and not taken out of context. Special attention is needed to ensure confidence and likelihood statements convey important context to readers about claims made in the key messages. Similarly, traceable accounts provide credibility and transparency to each key message and associated text; thus, a framework is recommended to build consistency and credibility for this important section. While it is appropriate for the text supporting the key messages to look different from one chapter to another, consistent introductory sections would provide readers with the appropriate context and a roadmap to understand the rest of the chapter.

Overall, the Committee is impressed with the graphics throughout the draft NCA5 report. Considering that figures and boxes are opportunities to meaningfully convey information to the broadest possible audiences, the Committee recommends the following:

6. All figures, figure captions, and figure citations should be consistent and self-contained.
7. Boxes within chapters should be used to highlight specific examples or to summarize or improve message accessibility.

The Committee recommends making figures and their captions self-contained such that the reader can easily understand the key ideas related to the figure without reading the supporting chapter text. The Committee also suggests that figures adhere to basic principles of design, including through the use of consistent titles, colors, and symbols. The use and content of boxes could also be more effective if reserved for specific types of content (i.e., firsthand stories, examples, complex concepts, an important figure). When possible, boxes could be better utilized to communicate a message that is not clearly expressed in the main text or needs amplification.

In addition to the structural issues raised above, there are a number of other inconsistencies across the draft NCA5 report that pertain to the ways topics and terms are structured and discussed. While it is appropriate for chapter authors to design their chapters to meet specific topical needs, there are certain content areas that would benefit from more consistent treatment throughout the draft NCA5 report to strengthen the credibility of key messages and enhance clarity and readability:

8. The draft NCA5 report should be revised to address inconsistencies across chapters, including treatment of adaptation and mitigation, distinction between natural variability and climate change, use of scenarios, treatment of Focus On… Features, and the use of terminology.
9. The draft NCA5 report should be revised to utilize and distinguish between the different use cases of sea-level rise projections and sea-level rise scenarios clearly and consistently.
10. Individual chapters across the draft NCA5 report—particularly national and regional chapters—should be better integrated and cross-referenced.

Where inconsistencies in structure and content between chapters cannot otherwise be resolved, the Front Matter could explain these differences. Overall, stronger cross-referencing and integration between chapters would reduce redundancies and inconsistencies, create more space for authors to expand on certain topics, and strengthen connections between national and regional topics.

The Committee commends the NCA5 authors for incorporating equity and justice into the report; however, there are opportunities to more intentionally and consistently integrate issues related to equity and justice throughout the report to highlight systemic interconnections. To that end, the Committee makes the following recommendations:

11. For consistency, chapter introductions should include context on equity and justice as related to the chapter topic. To move beyond general statements, different dimensions of justice, including distributional, procedural, recognitional, and intergenerational justice, should be addressed as appropriate throughout the chapter text. Related gaps in the literature should be identified in the text and traceable accounts.
12. A glossary of terms related to equity and justice should be adopted and/or developed to ensure consistent use of terminology across NCA5. Associated references should be provided.
13. To provide equitable access to climate-related information across US communities, national chapters should include all US islands and territories in their figures and assessments whenever possible.

Finally, in reviewing the draft NCA5 report as a whole, the Committee recommends more thorough attention should be given to certain topical areas:

14. Relatively more emphasis should be placed on the topics of: mitigation, adaptation successes and shortcomings, projected impacts of climate change, attribution of extreme events, and rural analysis.

This page intentionally left blank.