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Review of the Draft Fifth National Climate Assessment (2023)

Chapter: 2 Overarching Comments and Recommendations

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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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2
Overarching Comments and Recommendations

The Committee applauds the Fifth National Climate Assessment (NCA5) authors for an impeccably researched, assembled, and interpreted vast body of literature on an extremely complex and rapidly changing topic—climate change impacts, adaptation, and mitigation in the United States. This is no easy task and is increasingly challenging as the knowledge base on climate change (e.g., the literature, action on climate change mitigation and adaptation) has dramatically expanded in recent decades, particularly since the last National Climate Assessment (NCA) report was released in 2018. The Committee also commends the NCA process for the inclusion of traceable accounts sections at the end of each chapter to describe the process and rationale authors used to develop the chapter and reach consensus on key messages. These sections support the ability of the draft NCA5 report to accurately document the state of knowledge—including recent additions and remaining gaps in knowledge—regarding the impacts of climate change.

The Committee has many recommendations on how the draft NCA5 report can be revised to more clearly and credibly communicate its contents and add emphasis to better balance the state of knowledge on climate change. These recommendations are intended to support the overall goal of NCA5, which is to report on: the scientific understanding of climate change, how climate change already has and is expected to further affect the United States, and what can be done to mitigate and adapt to climate change. Before discussing the recommendations for revising the draft NCA5 report, the Committee outlines some of the many aspects of the report that deserve note and praise.

NOTES OF PRAISE

The draft NCA5 report gives significant attention to matters of equity and justice. This is a substantial and long-overdue improvement over past NCAs, and the Committee appreciates the attention to evaluating climate change impacts and responses through the lens of equity and justice. This attention to equity and justice is notable in many ways, including in the identification of overburdened communities, disproportionate consequences, and systemic drivers of vulnerability, particularly for tribal and other frontline communities in urban and rural areas. The Committee commends the NCA5 authors for using traditional knowledge in its assessment such as in Chapters 16 (Tribes and Indigenous Peoples), 29 (Alaska), and 30 (Hawai‘i and US-Affiliated Pacific Islands).

The Committee commends the NCA5 authors for identifying progress the United States has made in adapting to climate change, particularly at the state and local levels. The regional chapters review state-level adaptations and highlight promising examples of local-level adaptations, including actions of tribal communities. Chapter 31 (Adaptation) strikes the appropriate tone describing how adaptation measures to date appear to be insufficient to address the growing risks from climate change and that transformative adaptation as well as increased funding and technical support will be needed. Given the strong attention the report devotes to adaptation, there is an opportunity to give a similar treatment to mitigation actions, noted below.

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Lastly, the draft NCA5 report has done a better job than previous NCAs of characterizing the Representative Concentration Pathway (RCP) scenarios used in recent Intergovernmental Panel on Climate Change (IPCC) assessments and NCAs. The use of terms such as “intermediate” and “very high” to characterize the RCPs better reflects the likelihood of these radiative forcing scenarios happening and makes these scenarios more accessible to general audiences. Consistent use of these more accessible terms beyond the introductory chapters would more clearly and effectively communicate the implications of each scenario throughout the report.

ADDRESSING THE MANDATE IN THE GLOBAL CHANGE RESEARCH ACT OF 1990

Overall, the Committee finds the draft NCA5 report addresses the mandate of Section 106 of the Global Change Research Act (GCRA) (Box 2-1). The draft NCA5 report integrates, evaluates, and interprets the science on climate change, and the Committee finds that the breadth of coverage of key climate change topics is impressive. The draft NCA5 report has done a masterful job pulling diverse information on the complex topic of global change together. In subsequent sections and in Chapter 3 of this report, the Committee offers specific comments on how the information base is interpreted and opportunities for improvement.

Regarding subsection 1 of the GCRA, the draft NCA5 report addresses the state of knowledge about the science of climate change, including how climate change is already happening as a result of greenhouse gas (GHG) emissions and how the climate is projected to continue changing; impacts of climate change on sectors and regions of the United States; and how those impacts are projected to change in the future as a result of varying intensities of climate change mitigation and adaptation efforts. Although the Committee has recommendations on how to improve the draft NCA5 report, it is important to note that it is comprehensive, thorough, and well documented. In addition, although the Committee has recommendations on how to better integrate uncertainties into the text supporting key messages, overall, the NCA5

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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authors have made a substantial effort to assess the state of knowledge and report scientific uncertainties associated with their report findings, providing credibility and transparency to the report.

Consistent with previous NCAs, the draft NCA5 report focuses on climate change and does not focus on other global change issues. In many cases, the draft NCA5 report analyzes the interactions of climate change with other global change issues—particularly how other global changes intensify the effects of climate change. For example, Chapter 6 (Land Cover and Land-Use Change) considers how land cover and land-use change can lower the resilience of ecosystems and agriculture to climate change impacts. Chapter 8 (Ecosystems, Ecosystem Services, and Biodiversity) effectively discusses the interactions of climate change with non-climate stressors including land-use change, urbanization, pollution, and overharvesting, and Chapter 7 (Forests) discusses the impact of fire and land-use change on the terrestrial carbon sink. Additionally, some regional chapters discuss other changes that have important implications for local vulnerability. Chapter 29 (Alaska) touches on non-climate stressors, including food insecurity, limited employment, and cost of living (Key Message 29.7), and Chapter 22 (Southeast) discusses urbanization in the region and how it changes exposure to climate change. However, important global change trends are not discussed in other regional and national chapters. The Focus on Compound and Complex Events defines “complex events” as those where climatic and non-climatic stressors interact in ways that exacerbate climate hazards, but these events are not addressed throughout the Focus. Other changes, such as baseline conditions, may also be important to discuss to better understand how the vulnerability of regions and sectors are changing. For example, the state of the public health system is a key factor affecting the vulnerability of the health sector to climate change but is not discussed in Chapter 15 (Human Health). Overall, the draft NCA5 report could more intentionally integrate how climate change interacts with the full suite of global changes and non-climate stressors.

The draft NCA5 report covers all of the topics enumerated in subsection 2 of the GCRA, including individual chapters on climate change impacts on natural ecosystems and biological diversity (Chapter 8 [Ecosystems, Ecosystem Services, and Biodiversity]), agriculture (Chapter 11 [Agriculture, Food Systems, and Rural Communities]), energy production and use (Chapter 5 [Energy Supply, Delivery, and Demand]), land-use (Chapter 6 [Land Cover and Land-Use Change]), water resources (Chapter 4 [Water]), transportation (Chapter 13 [Transportation]), human health (Chapter 15 [Human Health]), and social systems (Chapter 20 [Social Systems and Justice]), and many other chapters including the regional chapters touch on these issues as well. Several chapters are highly relevant to the scope of the GCRA, though not enumerated in subsection 2, including Chapter 7 (Forests), Chapter 12 (Built Environment, Urban Systems, and Cities), and Chapter 17 (Climate Effects on US International Interests).

Even though NCA5 has a chapter on climate trends (Chapter 2), projections of impacts 25 years to 100 years into the future—as noted in subsection 3 of the GCRA—are not consistently provided throughout the draft NCA5 report. For example, Chapter 15 (Human Health) says little about the projected impacts of climate change on human health. There is some discussion of projected impacts by mid-century and end of the century in most national and regional chapters. However, the Committee did not note any projections or discussions of impacts 100 years from the present (i.e., 2025), though this is likely due to the focus on estimating impacts out to the end of the 21st century in the scientific literature. This has been the case for decades (e.g., National Assessment Synthesis Team, 2001), but as time has passed, the end of the century is less than 100 years after the publication of NCAs, and therefore does not

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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satisfy subsection 3 of the GCRA. As described in the Committee’s review of Appendix 3 and in Recommendation 8 below, there are difficulties in extending projections beyond 2100, but where this information is available, it should be included. Where there are gaps in the literature in which projections 100 years in the future have not been considered, these gaps should be noted in the “Major Uncertainties and Research Gaps” section of individual chapter traceable accounts.

Some chapters (e.g., Chapter 15 [Human Health]) present future impacts and risks as a function of global warming levels rather than tying them to specific emission scenarios and time frames; both can be effective ways to communicate projections and vulnerabilities into the future. For example, Figure 4.7 in Chapter 4 (Water) displays projected mid-21st century changes in runoff under the “intermediate” emissions scenario, while Figure 8.3 in Chapter 8 (Ecosystems, Ecosystem Services, and Biodiversity), identifies increases in global mean temperature that could lead to a suite of adverse global ecological impacts. Chapters could more consistently discuss first the diagnostic state of the science on impacts (i.e., past and current, including attribution of impacts to climate change) and then the forward-looking prognostic state of science on future impacts, rather than frequently switching between discussions of historical and projected changes.

RECOMMENDATIONS FOR CONSISTENT STRUCTURES

Clear and consistent structures support NCA5 in reaching its target audiences effectively, thereby enhancing the credibility and usability of the report. In the section that follows, the Committee outlines recommendations that address the structure of chapters across the entire draft NCA5 report. Overall, the Committee recommends that consistent structures be followed for key messages, traceable accounts, chapter introductions, and figures. These recommendations considered together would improve the readability of the draft NCA5 report for the range of audiences described in Chapter 1 of this report. Additionally, this section notes a number of content inconsistencies across chapters and provides recommendations for remedies.

Key Messages

Key messages form the backbone of NCA5 and should capture the state of knowledge around climate change and identify important research gaps and possible next steps for advancing the knowledge base. Therefore, it is vital that key messages throughout the report are crafted consistently and carefully. Key messages provide an opportunity for NCA5 to provide credible, salient, and tailored information to audiences in ways that are not policy prescriptive (Farrell and Jäger, 2006). While challenging to write, key messages should incorporate and express the knowledge base through the use of appropriate confidence and likelihood levels and careful writing of findings that can be understood by broad audiences and not taken out of context. As they are currently written, there is significant variation in the structure of the key messages throughout the draft NCA5 report. NCA5 authors may consider the key messages as having three parts: the label (i.e., Key Message 2.1); title (i.e., Climate Is Changing and Scientists Understand Why); and message (see Box 2-2).

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Recommendation 1: Key message “labels” and “titles” should follow a common structure throughout the report.

Key message titles should communicate enough information to engage readers and use a consistent hierarchy for the information presented, following best practices in design (Seddon and Waterhouse, 2009). Therefore, the titles would be more effective if written as short, informative statements—instead of just one or two words—that convey a simplified but accurate message. Key message titles in Chapter 5 (Energy Supply, Delivery, and Demand) all convey information—for example, Key Message 5.2, “Compounding Factors Affect Energy System and Community Vulnerabilities.” In contrast, in Chapter 17 (Climate Effects on US International Interests), the title of Key Message 17.2 could be rephrased from “National Security” to “Destabilization of Other Countries by Climate Change Affects US National Security.” Additionally, the Committee appreciates that authors include the words “Key Message” in each label to cue to audiences of what they are reading and its importance. If the authors write titles as brief statements, they should be carefully written to be consistent with the knowledge base in the key message and traceable accounts. The Committee provides more specific suggestions for many of the key message titles in Chapter 3 of this report.

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Recommendation 2: The “message” part of each key message should have a consistent reading level, length, and voice throughout the report.

The message part of each key message (see Box 2-2) varies significantly across the draft NCA5 report. The messages are the text that will be read and quoted the most by a wide range of audiences, including those that lack familiarity with the jargon and technical language of climate change; thus, the messages should be the most accessible parts of the report. Each message should be written simply and clearly, using short, uncomplicated sentences, omitting as much specialized jargon or needlessly complex language as possible (e.g., choose a word like “use” over “utilize”) (Somerville and Hassol, 2011; van der Linden et al., 2015). This practice will ensure comprehension by broad audiences. It is similarly recommended by major medical associations that health communication materials target their intended audiences and use appropriate language for broad comprehension (Badarudeen and Subharwal, 2010). The messages also vary in length, ranging from one sentence to more than five sentences. The NCA5 authors may consider selecting a range for a number of words or sentences to standardize key messages so that they are a consistent length. Because of the web-first format, the authors could consider consulting with designers on the best length.

The messages would be more effectively communicated if they were more balanced; some are very broad, and some are very specific. On the one hand, broad messages can seem unclear or not specific enough (e.g., Key Message 4.1, 15.3, 17.1, 18.1, 21.1, 25.4, 28.1). For example, Key Message 28.2 includes “disproportionate” with no explanation of its meaning in the context of the key message. On the other hand, specific messages often have too much jargon and technical language. For example, Key Message 6.3 uses technical phrases such as “crop yield improvements,” “animal-sourced foods,” “agriculture system resilience,” and “biomass crop cultivation” that could be simplified to language that could be more easily understood by general audiences (Somerville and Hassol, 2011). Key Message 28.41 is an example of a key message that says a lot but successfully uses plain language. The authors should also carefully consider the use of terms that can be imprecise, such as “historically,” “already,” and “currently,” ensuring that they are used in consistent, accurate ways across key messages. Many language issues occur across multiple chapters, and NCA5 authors may find it useful to look at the full list of key messages together to ensure that terms are being used harmoniously.

When possible, messages and titles should use the active voice (i.e., subject followed by a verb followed by the object of the verb) over the passive voice (i.e., object followed by a verb with or without a subject). Passive voice can be hard for a reader to understand because there is a less clear relationship between the subject and verb, making the relationship between the confidence/likelihood rating and climate impact unclear.2 For example, in Chapter 1 (Overview), the section 1.5 header, “Deep cuts in emissions would be required to meet national commitments” could be re-phrased to demonstrate who or what is making deep cuts in emissions

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1 Key Message 28.4. Demographics and Human Health. Increases in extreme heat, drought, and wildfire activity are negatively impacting the physical health Southwest residents (high confidence). Climate change is also shaping the demographics of the region by spurring the migration of people, primarily from Central America to the Southwest (medium confidence). Individuals particularly vulnerable to increasing climate change impacts include the elderly, outdoor workers, and people with low income (high confidence). Local, state, and federal adaptation initiatives are working to respond to these climatic and demographic changes and help people and communities become more resilient (medium confidence).

2 See https://owl.purdue.edu/owl/general_writing/academic_writing/active_and_passive_voice/active_versus_passive_voice.html.

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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using active voice. For example, the section header could be revised to: “Local, state, federal, tribal, and Indigenous communities are pursuing mitigation actions that reduce emissions across the country.” However, in using an active voice, NCA5 authors should be careful not to make statements policy prescriptive.

Additionally, some messages use a first-person voice (e.g., “Our Future,” Key Message 29.7; “We Know How to Drastically Reduce Emissions,” Key Message 32.2). In writing, the first-person point of view is used to tell a story from the author’s perspective. Because this is an assessment and review of the state of knowledge rather than original research where the authors could use the first-person to explain their results (i.e., “our findings suggest”), the Committee suggests NCA5 authors avoid first-person language. The Committee also encourages authors to avoid first-person language because it is unclear who the “our” or “we” refers to. It is also important to carefully replace first person language when rewriting these statements rather than simply using encompassing phrases such as “people in the US,” because that can also be inaccurate. Instead of using first-person language, where social science is available to support the key message, references to specific groups of people may be an acceptable replacement. Otherwise, the authors may consider selecting a more generic term such as “people” or “Americans,” but this should be defined in the beginning of the report (i.e., Americans could reference anyone living in the United States or may only refer to citizens, problematically excluding some people who also live in the United States but who are not citizens, such as undocumented immigrants).

Additionally, authors should pay special attention to statements that may be perceived as policy prescriptive or advocating for a specific position, such as declarative statements that may not be adequately supported. In general, evidence that is policy relevant should be emphasized and statements that are normative should be deemphasized. For example, in Chapter 2 (Climate Trends), a subsection in Key Message 2.3 is titled, “The Nation Has No Choice But to Adapt to a Changing Climate” and Key Message 31.3 states “To minimize the potential for adaptation actions to benefit some at the expense of others, adaptation processes must emphasize collaboration, centralize equity and justice, and incorporate a wide range of values and knowledge sources.” These statements should be rephrased so as not to be perceived as recommending certain policies or conveying a level of certainty not reflective of US policy. For example, the subsection in Chapter 2 (Climate Trends) could be re-phrased: “Adaptation can offset the adverse effects of climate change.”

Regarding the text supporting the key messages, the key message should drive the supporting discussion, not the other way around. Many key messages read as though they were distilled after the supporting text was drafted. Instead, the key message should include the key points that reflect the most important or new findings for each chapter. The supporting text should support the entire key message, including providing context and noting uncertainties, as appropriate, that can be elaborated on in the traceable accounts. Attention is needed to ensure text supporting the key messages adequately achieves these goals consistently across NCA5.

Recommendation 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.

Confidence and likelihood statements convey important context to readers about the claim being made. Confidence is intended to be a qualitative statement based on the amount and

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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consistency of information available (i.e., the weight of evidence). Likelihood is intended to be a quantitative statement based on observations and model projections or an assessment of such quantitative information across sources (i.e., the probability that an impact has happened, is happening, or will happen). It is often unclear whether the NCA5 authors distinguish between the qualitative versus quantitative natures of the two types of assessments.

In general, confidence and likelihood definitions should be provided for general audiences in the Front Matter, in both written and numerical format (Budescu et al., 2014) and consistently assigned in each key message across NCA5. Each claim in the key message should include confidence at minimum and likelihood if—and only if—a quantitative assessment of probability can be made from the source material. Presently, some messages make multiple claims, but do not include confidence and likelihood at all or only include one statement of confidence/likelihood at the end of the message. This leaves the reader to wonder if the rating is applied to one claim, all claims, or just the most recent claim in the message. The draft NCA5 report in general seldom includes likelihood statements—these should be consistently considered across the document so that the lack of likelihood statements reflects the threshold defined in the Front Matter rather than an oversight. Box 2-3 provides an example of a successful use of confidence/likelihood statements and an example of a key message where the use of confidence/likelihood statements could be improved.

Additionally, likelihood statements are often provided without a corresponding explanation of quantitative support in traceable accounts sections. The likelihood statements correspond to specific probability ranges per the definitions provided in the Front Matter, which require care to justify the difference between levels of likelihood (e.g., very likely at >90% probability versus likely at >66% probability). Expert judgment can be part of the likelihood assessment, but the assessment should be based on more than belief, and the basis for such quantitative judgments should be adequately described in the traceable accounts. When quantitative likelihood assessment is not possible, it is preferable to provide only a confidence statement consistent with the knowledge base. In some cases, the knowledge base is limited or emerging, and this should be reflected in the confidence language and further expounded on in the traceable accounts. Because general audiences may only read the main text and not the more technical traceable accounts, it may also be appropriate to highlight key uncertainties briefly in the text supporting the key messages. Specifically, providing uncertainty or knowledge gaps with scientific claims makes the information more useful to decision makers and increases transparency, which can contribute to trustworthiness.

In some cases, the draft NCA5 report ascribes likelihood language to findings that are not in dispute (i.e., where the likelihood probability, based on the knowledge base, is greater than 99%). In such cases, the Committee suggests NCA5 authors follow the IPCC Sixth Assessment Report (AR6) language to classify these findings as “unequivocal” or “established fact” rather than “very high confidence.” For example, Key Message 2.1 states, “It is virtually certain that human activities have increased atmospheric levels of carbon dioxide and other greenhouse gases (very high confidence)” whereas IPCC AR6 states: “Observed increases in well-mixed greenhouse gas (GHG) concentrations since around 1750 are unequivocally caused by human activities” (IPCC, 2021a). The use of term “unequivocal” is appropriate when discussing facts where there is no doubt regarding the findings.

The language that scientists use to describe uncertainty, including the words “confidence” and “likelihood,” is jargon. It is the specialized language of science and the words confidence and likelihood have different meanings and interpretations in everyday use (Somerville and

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Hassol, 2011). In the Front Matter, the concepts of confidence and likelihood could be expanded somewhat to make them more accessible to readers unfamiliar with this language based on results from Budescu et al. (2014). The authors may also consider including a visual representation of confidence and likelihood, to supplement the text and numerical definitions in the Front Matter (Kause et al., 2022; Mastrandrea et al., 2010). The IPCC report is a similarly technical report, so simply presenting these terms and definitions with reference to the IPCC report is insufficient. Authors may consider a brief explanation about the nature of science, and how a single study or finding may advance understandings, that repeated replications of that work, scrutiny by the community of experts, and the convergence of expert agreement of particular findings and claims increase confidence in those understandings (Oreskes, 2021). This need not be extensive because that work has been done elsewhere, but this brief context is important for a less technical reader. In the web design, NCA5 authors may consider including definitions of confidence or likelihood ratings when the mouse arrow hovers over the phrase to remind readers of the meaning of these terms or navigate back to the Front Matter. Additionally, authors may consider including the standard definitions of confidence and likelihood at the beginning of each chapter as a standalone section. Many readers may never read the Front Matter where these terms are defined.

Traceable Accounts

Recommendation 4: A consistent framework for traceable accounts sections would better support the key messages and build consistency and credibility across chapters.

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
×

Traceable accounts written for each key message are an essential component of NCA5, providing both credibility and transparency to each key message and associated text and, therefore, the report. Overall, the traceable accounts include information about the state of knowledge and uncertainties that strengthen the quality of NCA5. The Committee’s review of the traceable accounts emphasized, to the extent possible, whether the evidence presented for each key message consistently and adequately supported the assigned confidence and likelihood designations and the associated text rather than rigorously assessing the credibility of the determinations made by the authors. The Committee commends the use of confidence and likelihood statements consistent with the IPCC Fifth Assessment Report (AR5) definitions; however, there are significant inconsistencies in the traceable accounts sections across chapters, both in structure and how the evidence is presented. Chapter-specific comments are presented in Chapter 3 of this report, but here the Committee notes some of the major inconsistencies and provides recommendations to build a framework for NCA5 authors as they revise the traceable accounts sections.

Overall, the traceable accounts are appropriately written at a more technical level than the rest of the report and summarily discuss the literature base for their key messages. However, traceable accounts sections inconsistently satisfy their intended purpose, as defined by USGCRP (2018a): to detail how the authors arrived at their confidence and likelihood statements, and what evidence was used. The process for writing the traceable accounts is an analytical one about the state of the science in which authors should apply robust evidence (i.e., literature cited) to justify both the confidence and likelihood assigned to each statement within each key message. Thus, within the traceable accounts section, broad statements about the literature do not suffice and should be removed, citations should be included, and the substantive scope of the traceable accounts should be limited to the scope of the main text of the chapter. Additionally, where in the supporting text, authors are only able to make brief statements with a citation due to chapter word limits, they may provide greater depth in the traceable accounts, expanding on the main text to include additional context and discussion, specifically describing how that detail, nuance, or gap in understanding factored into the confidence and likelihood rating. For example, a discussion of which projections were and were not available for the chapter authors to consider and an explanation of model uncertainties are appropriate to include in the traceable accounts. There are many dense and technical passages in the text supporting the key messages that may be better suited for traceable accounts sections to enhance the readability of the main text (e.g., the text describing advances in the understanding of equilibrium climate sensitivity in Chapter 3 [Earth System Processes], pages 3-9 to 3-10).

There are many instances across the draft NCA5 report where traceable accounts sections do not provide any citations to support their statements (e.g., Chapters 4 [Water], 21 [Northeast], and 23 [US Caribbean]) and/or use broad statements such as “authors used their collective expertise and weighed the overall literature” to reach conclusions (e.g., Chapters 17 [Climate Effects on US International Interests], 23 [US Caribbean], and 25 [Northern Great Plains]). This kind of broad statement is descriptive of the process used to synthesize the literature and is not a substitute for identifying the literature (and should, likewise, be removed), nor does it provide sufficient detail required or support for the key messages. Section 106 of the GCRA does not require a summary report of the authors’ collective expertise but rather an integration, evaluation, and assessment of the credible scientific base. It is appropriate for process details to be included; however, this should be done in a consistent way (e.g., some chapters provide a list of authors’ expertise while others do not). Traceable accounts sections should be revised to

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
×

demonstrate which references support each confidence and likelihood statement under each key message. Examples of thoroughly written traceable accounts can be found in Chapter 5 of the Climate and Health Assessment, Vector-Borne Diseases (Beard et al., 2016) or the Energy Chapter of the Fourth National Climate Assessment (NCA4) (USGCRP, 2018a). In the draft NCA5 report, the traceable accounts for Chapter 19 (Economics) are strong overall, though there is room for improvement in the use of citations, described in Chapter 3 of this report.

The Committee recommends the following structure for traceable accounts that could be used throughout NCA5:

  • Provide the full key message at the start of each traceable account.
  • Summarize which references support each confidence and likelihood claim.
    • Include citations in “Description of Evidence Base.”
    • Include citations to “Major Uncertainties and Research Gaps.”
  • Include a “therefore” statement describing why authors designated confidence and likelihood findings. Authors should describe why the confidence is not lessened due to research gaps and cite literature that supports this decision only if that literature is cited in the main text.
    • Include citations to “Description of Confidence and Likelihood,” if relevant.

Chapter Introductions

Recommendation 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.

NCA5 authors should assume that readers will read chapters that they are interested in without reading the Front Matter, Chapter 1 (Overview), or other introductory material. In general, the introduction section of the chapter should provide the reader with a brief background and context on the chapter theme and its relevance to climate change, a brief statement about what was covered in NCA4 and what new knowledge is being added in NCA5—including the key messages—and what the chapter will not cover. 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. The consistent inclusion of sub-headers throughout chapters could also help to support this goal.

Many of the chapter introductions would benefit from a high-level introductory sentence to explain why climate change is relevant to the chapter’s topic or themes. In most introductory paragraphs, it is not explicit how the introduction section is related to the key messages. In introducing a chapter’s contents, the introduction should not read as an abstract of the chapter (e.g., Chapters 2 [Climate Trends], 4 [Water], and 28 [Southwest]). Chapter authors could consider using a model like Chapter 17 (Climate Effects on US International Interests) where the introduction is directly related to the key messages and key message titles are in parenthesis. Chapter 29 (Alaska) also provides a successful example that includes a table of the key messages with examples of climate drivers/impacts and responses; this type of summary or roadmap could be replicated in other chapters. In addition, chapter introductions should explicitly note what

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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topics are covered in the chapter and what topics are not. For example, Chapter 19 (Economics) does not discuss the economics of mitigation, and Chapter 17 (Climate Effects on US International Interests) does not review the international impacts of climate change—these choices should be explained in the introductions.

Introductory sections vary widely with respect to the use of technical language and could better engage the target audiences if they were less technical. The Committee highlights chapter-specific instances where jargon is used in Chapter 3 of this report. Similarly, when introducing new or complicated terms, authors should assume readers are encountering them for the first time, including seemingly basic terms (e.g., adaptation, mitigation). For commonly used terms across the whole report, a consistent definition should be selected and used throughout and referenced in the glossary currently under development for the final report but not available in the draft NCA5 report. For example, Chapter 31 (Adaptation) details the choice of “adaptation” over the commonly used “resilience” whereas “resilience” is used in many other chapters. Chapter 20 (Social Systems and Justice) uses “overburdened communities” over other terms frequently used in the report, such as “marginalized” or “underrepresented.” The NCA5 authors should make decisions about commonly used terms and use them consistently throughout the report. In the web design, rollover definitions could be provided for common phrases or words that are defined in the glossary.

The introductions to each chapter also use in-text citations and figures inconsistently. While figures may be more appropriate for some chapter introductions over others, there are opportunities to build consistency—for example, all regional chapters could include a map of the region with place names and locations referenced throughout the chapter. Introductions that use figures could consistently place them at the end of the section rather than in the middle because they are typically short sections. The NCA5 authors may also consider a word range for the introduction sections so that they are all roughly the same length throughout the report. The NCA5 authors should expand upon the glossary definitions in the chapter when the additional context or an expanded definition is needed to present a more precise definition tailored to the chapter readers.

Graphics and Boxes

Recommendation 6: All figures, figure captions, and figure citations should be consistent and self-contained.

Overall, the Committee is impressed with the graphics throughout the draft NCA5 report and understands that many graphics are still being designed or were unavailable to review due to the timing of copyright clearances. As the NCA5 authors make revisions, the Committee suggests they consider that many readers will only look at the figures and read the captions and may download and use the figures for a range of uses (e.g., reports, power points, lectures). Therefore, the Committee recommends making graphics self-contained such that the reader can easily understand the key ideas related to the figure without looking in the text. On the other hand, many figures are not adequately discussed in the body of the chapters and are at most pointed to in a parenthetical or not at all. All figures should be integrated meaningfully into the text to justify their inclusion in the chapter.

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Based on recommendations by Gerst et al. (2021), Harold et al. (2016), and Rougier et al. (2014), the Committee encourages authors to ensure that each figure adheres to some basic principles of design and include at a minimum:

  • Consistent use of titles;
  • Legible text;
  • Use of color and contrast, accessible to people who have colorblindness;
  • Legend, if applicable;
  • Sufficiently detailed caption; and
  • Reference to the figure source(s), if applicable

To expand on the checklist above, there are many opportunities to introduce more consistency across the figures throughout the report. Furthermore, Gerst et al. (2021) explains how to improve many NCA figures based on experimental studies. Regarding embedded figure titles, NCA5 authors could consider choosing a consistent format for titles and making sure titles above and embedded in figures match. Regarding the color palette, the NCA5 authors could consider a consistent palette across the report figures, and if possible, use colors consistently to communicate particular ideas (i.e., ensure icons related to water are the same shade of blue). Regarding symbols and icons, NCA5 authors could consider adopting a similar style of icons across the report, and use them consistently (e.g., a fish icon representing fisheries in one figure will be the same fish icon representing fisheries in another graphic) and define these icons using legends in individual figures, where appropriate.

Figure caption length and amount of detail varies greatly across the report. Captions should sufficiently communicate the key ideas in the figure, explaining to the reader the important trends, takeaway points, data sources, or other information to inform readers on how the figure was created and how it should be read (Rougier et al., 2014). Captions should begin with a nontechnical explanation of what the figure shows and then, to a limited extent, the technical underpinnings of the figure. Especially in the case of a schematic illustration—a type of figure that presents a scene or story with text boxes or callouts of important facts—the captions should have sufficient references to the source material used to create the figure (Perra and Brinkman, 2021). Many (e.g., Figures 10.5, 11.5) only list the agencies or organizations that created the data or figure, making it difficult for audiences to identify the source for a claim communicated by, or data used, to generate a figure. In addition, captions should always include dates of the data or information being presented. The web version of the report could use hover-over popups to include more technical underpinnings of the figure.

The draft NCA5 report uses maps throughout to communicate a lot of information. Consistent with the recommendation about icons, the Committee suggests the report use consistent map features (e.g., colors and symbols) so that readers can easily understand any map they see in the report. Special attention is needed to establish a consistent arrangement of the contiguous states, Alaska, Hawai‘i and the US-Affiliated Pacific Islands, and the US Caribbean (Puerto Rico and the US Virgin Islands) on maps to ensure the full geographic domain of NCA5 is depicted. In the draft NCA5 report, Alaska and Hawai‘i are placed on maps in various locations (e.g., sometimes in their geographically accurate locations and sometimes below Florida), and in some cases, are not included at all. The US Caribbean is rarely included. If a geographic region is omitted, there should be an explanation in the caption as to why (e.g., no data). Additionally, there are issues related to scale with Alaska, Hawai‘i, the US-Affiliated

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Pacific Islands, and the US Caribbean, and when that is the case, appendixes could be used for maps that are too small or out of scale. Gerst et al. (2021) should also be referenced for guidance on developing maps.

Recommendation 7: Boxes within chapters should be used to highlight specific examples or to summarize or improve message accessibility.

The use and content of boxes in chapters varies considerably across the draft NCA5 report. Boxes could be more effective if reserved for specific types of content (e.g., firsthand stories, examples, complex concepts, an important figure, or to summarize or otherwise improve message accessibility) in order to highlight specific items and/or deliver content more rapidly. Assessments of this nature inevitably provide a high-level summary of the state of the science, which can be rather abstract. Boxes are an opportunity to provide specific examples that can help readers connect the material to their everyday lives or policies and decisions they may hear about on the news, thus reducing the psychological distance (van der Linden et al., 2015). At the same time, boxes should at least mention their connection to climate change and related discussions in the chapters—for example, Boxes 28.2 and 4.3 do not mention the role of climate change.

When possible, boxes could be better utilized to communicate a message that is not clearly expressed in the main text or needs amplification. Boxes should include the relevant who, what, when, where, why, and how to provide a more tangible connection for readers. For example, Boxes 14.1 and 18.1 both tell compelling stories with data. Chapter 29 (Alaska) has many boxes with firsthand accounts from residents, which are excellent for making abstract phenomena more concrete and integrating Indigenous knowledge, but could be enhanced with some additional information (i.e., a description or graph showing the climate trend) and context (i.e., a few introductory sentences to describe the setting).

ADDITIONAL INCONSISTENCIES BETWEEN CHAPTERS

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. As a web-first report, it is unlikely NCA5 will be read by audiences cover-to-cover; thus, it is appropriate for chapter authors to design their chapters to meet their specific topical needs. However, there are certain content areas, described below, that would benefit from more consistent treatment throughout the draft NCA5 report to strengthen the credibility of key messages and enhance clarity and readability. Where inconsistencies in structure and content between chapters cannot otherwise be resolved, the Front Matter could explain these differences (e.g., the structure of each chapter was decided by the chapter authors). In the section below, the Committee outlines specific inconsistencies that stood out and, if resolved, would improve the draft NCA5 report.

Recommendation 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.

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Treatment of Adaptation and Mitigation

Adaptation and mitigation are not treated consistently across chapters. Adaptation is a major topic in all chapters, except in Chapter 32 (Mitigation), but it is not addressed in a consistent manner (e.g., sometimes together with mitigation). For example, in Chapter 7 (Forests), Key Message 7.3 focuses on adaptation, while Chapter 12 (Built Environment, Urban Systems, and Cities) has Key Message 12.3 on mitigation and adaptation measures in the built environment. Both sectors have GHG emissions or removals and are affected by climate change and thus address both mitigation and adaptation. Regional chapters in particular inconsistently include mitigation in key messages. There are also inconsistencies in how the regional chapters describe adaptation efforts or obscure what adaptation means by using examples such as forest management. Some chapters, such as Chapters 21 (Northeast) and 25 (Northern Great Plains), have key messages on adaptation, while most other regional chapters discuss adaptation within sectors and themes. Mitigation is sometimes mentioned together with adaptation, but examples used are typically only related to adaptation (e.g., Chapters 15 [Human Health], 21 [Northeast]). Often, chapter key messages include adaptation but omit discussions of mitigation entirely (e.g., Chapters 25 [Northern Great Plains], 16 [Tribes and Indigenous Peoples]).

In addition to emphasizing mitigation more prominently, GHG emissions and removals should be described in a consistent way across the report. Specifically, total emissions and net-zero emissions are often blurred together, as are removals and carbon sequestration (e.g., Figure 32.1 in Chapter 32 [Mitigation]). Furthermore, Figure 32.1. is confusing in this regard. The historical record is shown as gross emissions, with net-zero in 2050 shown as these emissions trending to zero emissions. Should the forest sink be included and continued, 2050 would be net negative, while “net-zero” would be reached around 2040. The statement that forests are a sink of 0.8 is not consistent with Chapter 7 (Forests), which reports the sink as 0.4. This confusion extends from what appears to be the absence of a standard inventory of emissions and removals across the entirety of the report. For example, NCA5 authors could select a consistent US emissions dataset and time frame based on the latest EPA national GHG inventory for 1990-2020 (EPA, 2022) with a standard benchmark year. Likewise, “key categories” should consistently refer to the chosen dataset. Such a dataset should be introduced to the reader (e.g., in the Front Matter or Chapter 2 [Climate Trends]), including appropriate caveats and limitations, and consistently cross-referenced throughout.

A key area of advancement since NCA4 is in negative emission technologies (e.g., carbon dioxide removal). Relevant to the discussions of adaptation and mitigation, these technologies should also be discussed as part of the broader portfolio of options to achieve net-zero GHG emissions, including relevant references (e.g., NASEM, 2019, 2021, 2022a).

Distinction Between Natural Variability and Climate Change

The draft NCA5 report frequently describes “change” but does not consistently distinguish between natural variability and climate change. This lack of clarity can be misleading for the reader by creating the misconception that some impacts that are the result of the combination of natural variability and climate change are instead solely due to climate change (e.g., Chapters 4 [Water], 6 [Land Cover and Land-Use Change], 9 [Coastal Effects], 28 [Southwest], 30 [Hawai‘i and US-Affiliated Pacific Islands]). Consistent with the mandate of the

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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GCRA, it is important for NCA5 authors to accurately identify the multiple drivers of change (including global change), in addition to climate change, and accurately attribute impacts. Clearly describing impacts due to natural variability, climate change, or a combination of the two is important not only for informing policy decisions, but also for building public understand and trust in the report’s messages. Climate change and natural variability should be clearly defined in the glossary and natural variability could be introduced in chapter introductions when the concept is important for understanding key messages.

Use of Scenarios

Where projections from multiple climate change scenarios are available, (e.g., RCPs 4.5 and 8.5), NCA5 should strive to report results from multiple scenarios. This is particularly important because the divergence between projected climate impacts based on different scenarios becomes more pronounced decades from now, especially after mid-century. Projections of sea-level rise from different climate models should also be included when such results are available. Specifically, regional chapters should utilize results from multiple emission scenarios (e.g., RCPs), sea-level rise scenarios, and climate models where such results are available. The Committee recognizes that many studies of climate change impacts only use the “Very High Scenario” (RCP 8.5). In such cases, only the projections from that scenario can be reported in NCA5, but the limited use of emissions or sea-level rise scenarios and climate models should be clearly reflected in confidence and likelihood levels and explained in the traceable accounts. Findings based on a single scenario and one or a few models should, all else being equal, receive lower confidence and likelihood than findings based on the application of multiple scenarios and models.

Data, models, and projections used to formulate likelihood statements should be standardized across the chapters. Appendix 3 describes the standard datasets provided to the NCA5 authors: downscaled model-based projections of temperature and precipitation and sea-level rise scenarios. However, the individual chapters deviate from these standard datasets in many cases, drawing from a variety of sources, including local assessments and past NCAs. Each chapter should, at minimum, provide statements based on the standard sources meant to be common across NCA5. If other sources provide different or more nuanced information then these sources can be utilized as well—but always as a supplement to the standard sources in Appendix 3, not in place of the standard sources.

Treatment of Focus On… Features

The Focus On… Features are topics chosen because of their cross-cutting importance but could be better integrated into the national and regional chapters. These features address interesting topics, and the Committee applauds their inclusion in the draft NCA5 report. However, they are only mentioned in passing in some relevant chapters (e.g., Focus on Western Wildfire in Chapter 7 [Forests], Focus on Risks to Supply Chains in Chapter 19 [Economics]). More importantly, the relevant chapters do not substantively integrate the Focus On… Features (e.g., Chapter 16 [Tribes and Indigenous Peoples] discusses COVID-19 and does not cite to the Focus on COVID-19 and Climate Change; Chapter 8 [Ecosystems, Ecosystem Services, and

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Biodiversity] discusses wildfires but does not cite the Focus On Western Wildfires; Chapter 18 [Sector Interactions, Multiple Stressors, and Complex Systems] does not mention the Focus on Compound and Complex Events), and there are opportunities to use these features to reduce redundancies and point the reader to sections, and vice versa, with more detail. For example, Chapter 30 (Hawai‘i and US-Affiliated Pacific Islands) contains an entire box on Blue Carbon Ecosystems (Box 30.4). The chapter may more effectively reference the Focus on Blue Carbon and devote that box space to another relevant example.

Additionally, the Focus On… Features should reflect a structure similar to that of the national and regional chapters to effectively communicate the chosen topics. The bolded statements in each Focus On… Feature should be treated as key messages, be clearly and carefully written in concise, accurate sentences, and they should include confidence and likelihood statements. The traceable accounts should include a brief discussion of authorship, discuss the knowledge base, include references, and detail the rationale for confidence and likelihood statements, in line with the recommendations on traceable accounts above.

Use of Terminology

In general, careful attention to consistent terminology and precision of language is needed. The draft NCA5 report does not consistently use the metric or Imperial system between and within chapters. Chapter 1 (Overview) introduces the convention of Fahrenheit (Celsius) but does not use this convention consistently, which causes reader confusion. There are even inconsistencies within sentences, for example, in Chapter 2 (Climate Trends): “For every additional 1°C of global warming, the average US temperature is projected to increase by around 2.5°F.” Terminology for similar concepts is also used differently from chapter to chapter—for example, adaptation versus resilience, ocean economy versus marine economy, citizen versus resident, nature-based solutions versus natural climate solutions, carbon sequestration versus carbon removal, and land-use change versus land system change. The Committee acknowledges the challenges in coming to a consensus on acceptable definitions for terms used commonly and interchangeably across scientific disciplines. The Committee suggests defining terms where they are meant to have a distinct meaning in the report text and the glossary currently under development (e.g., explain the difference between carbon dioxide removal, carbon sequestration, and carbon capture use and sequestration/storage) and consistently use one of the terms (e.g., use adaptation instead of resilience) across the report where multiple terms convey the same meaning. Throughout the report, imprecise language is also frequently used—for example, “significantly,” “will,” and “driving.” This language makes assumptions about decision making, and in some cases, may suggest levels of scientific confidence that are not supported by the text, citations, and traceable accounts.

Recommendation 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.

Projected sea-level rise (SLR) is not treated consistently across national and regional chapters and is often inconsistent with confidence and likelihood levels defined in the Front Matter. Some regional chapters (e.g., Chapter 22 [Southeast]) utilize only discrete SLR scenarios

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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from the Sea-Level Rise Technical Report (Sweet et al., 2022), while others use projections from emissions scenarios in IPCC AR6 (e.g., Chapter 23 [US Caribbean]), and still others attempt to relate the two but do so incorrectly (e.g., Chapter 30 [Hawai‘i and US-Affiliated Pacific Islands], 28 [Southwest], 27 [Northwest]). An underlying cause of this inconsistency is the overlapping naming conventions between the SLR and emissions scenarios. For example, the Intermediate Scenario for SLR from Sweet et al. (2022) most closely corresponds to IPCC SSP5-8.5, which is the Very High Emissions Scenario, while the Intermediate Emissions Scenario corresponds to IPCC SSP2-4.5. Additional confusion arises from the discrete nature of the SLR scenarios, which contrasts with the ensemble-based IPCC projections used for all other climate indicators; misunderstanding these differences leads to inaccurate and poorly formed likelihood statements that cannot be appropriately derived from a discrete trajectory.

In general, NCA5 should utilize and distinguish between the different use cases of SLR projections versus SLR scenarios. Projections offer information about the range of possible SLR outcomes for a specific emissions scenario, while the scenarios provide a range of discrete decision-relevant timelines for the purpose of developing adaptation approaches. It is not good practice—nor is it necessary—to utilize only one type of SLR information to satisfy the dual needs of making likelihood statements and providing decision-relevant information. The Committee suggests two possible ways the draft NCA5 report can be improved in this regard. The first (and preferred) option is to standardize the use of the ensemble-based IPCC SLR projections to make likelihood statements—similar to how temperature and precipitation are handled—and to standardize the use of the Sweet et al. (2022) scenarios for use in discussing decision-relevant timelines. In this case, statements such as the one made on Page 9-20 Lines 7-8 would be formulated similar to those in IPCC AR6 Chapter 9—for example, “Considering only processes for which projections can be made with at least medium confidence, global mean sea-level will increase from 2020 to 2050 by Y [X to Z, likely range] meters for SSP1-1.9 and by B [A to C, likely range] meters for SSP5-8.5.” The second option is for NCA5 authors to narrowly focus on the Sweet et al. (2022) scenarios. In this case, clear guidance should be provided to chapter authors for how to formulate reasonable likelihood statements around these discrete timelines. One way to do this is to emphasize the use of Table 2.4 in Sweet et al. (2022), whereby a possible likelihood statement made using the scenarios in combination with NCA5 confidence/likelihood language could be, “The Intermediate SLR Scenario is unlikely to be exceeded for 3ºC of global average surface warming by 2100 and is about as likely as not to be exceeded under Very High Emissions (SSP5-8.5) when including Low-Confidence Processes.” Regardless of which of the above options is chosen, Appendix 3 detailing the relationship between IPCC projections and the Sweet et al. (2022) SLR scenarios should be expanded as it does not adequately explain the relationship, nor does it adequately detail the difference between the framing and appropriate use cases of each source of information. Additional comments regarding the handling of SLR projections and scenarios are provided in Chapter 3 of this report.

Recommendation 10: Individual chapters across the draft NCA5 report—particularly national and regional chapters—should be better integrated and cross-referenced.

The structure of the draft NCA5 report begins with the physical science chapters (Chapters 2-3) that provide a foundation for the national chapters (Chapters 4-20), which support the regional (Chapters 21-30) and responses chapters (Chapters 31 and 32). The national chapters lay out foundational concepts and the regional chapters build on that information to explore

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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specific regional impacts. Better integration of sector and regional chapters would lead to a more coherent NCA5 that more clearly highlights the integrated complexities associated with climate change.

The national chapters appropriately cover issues at the scale of the United States. Some regional chapters raise topics that are not necessarily specific to that region but could be covered in appropriate national chapters. For example, Key Message 21.5 in Chapter 21 (Northeast) discusses whether finance for climate action plans is adequate. The text in this key message addresses both national and regional concerns, and the matter of adequacy of finance for mitigation and adaptation is certainly not limited to the Northeast region. Rather, this issue is of national importance, and it may be more appropriate to address the adequacy of finance in Chapters 31 (Adaptation) and 32 (Mitigation) and, if appropriate, have the regional chapters cover finance issues specific to the respective regions.

Cross-referencing chapters across the report demonstrates the inherent interconnections and complexities of climate change and its impacts. Overall, stronger cross-referencing between chapters would reduce redundancies and inconsistencies, create more space for authors to expand on certain topics, and strengthen connections between topics. For example, some regional chapters (e.g., Chapter 30 [Hawai‘i and US-Affiliated Pacific Islands]) appear siloed from the rest of the report and would benefit from comprehensive two-way cross-referencing with relevant national chapters. While all chapters may not have been available to authors at the time the draft NCA5 report was developed, there is an opportunity for authors to leverage information included in different chapters as the draft NCA5 report is revised.

Relatedly, regional authors should be mindful when citing studies that cover the nation rather than the region (e.g., Chapter 25 [Northern Great Plains] discussion of mental health) not to create an ecological fallacy when national analyses are downscaled to regions. Similarly, national chapters should be mindful when citing global studies (e.g., Figure 8.3). The NCA5 authors should identify global or national-level studies as such and use appropriate confidence levels when applying results to smaller geographic scales. Each chapter should discuss these limitations in the literature (as mentioned above) in their traceable accounts sections.

RECOMMENDATIONS ON EQUITY AND JUSTICE

Recommendation 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.

The Committee commends the NCA5 authors for incorporating equity and justice into the report; however, there are opportunities to highlight related issues more intentionally and consistently across the chapters. Notions of “equity” and “justice,” along with their related dimensions, are critical when considering climate change impacts, mitigation, and adaptation. Acknowledgment emphasizes the rights and interests of populations that have traditionally been excluded due to gender, age, race/ethnicity, poverty, or other social factors, and helps to identify key questions and indicators that can be used to assess progress, issues of access, the distribution of costs and benefits, competing interests, as well as just transitions. Though these and related

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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terms will take on different meanings depending on the context, they can be applied in an appropriate manner across all chapters for consistency and cohesiveness.

Dimensions to consider include distributional, procedural, and recognitional justice, as described in Chapter 1 (Overview) (page 1-40). As the dimensional overview on page 1-40 is critical for consistent integration across the report, the Committee suggests a citation for the definitions provided. In addition, the concept of intergenerational justice should be added. For consistency in term usage, NCA5 authors may reference the definitions in the Chapter 1 (Overview) (page 1-40) or develop and reference a more robust glossary with cited definitions (see Recommendation 12 below). To address different dimensions of justice, authors may highlight how resources, impacts, vulnerabilities, benefits, and burdens vary across populations or geographies; recognize who is typically involved in the decision-making processes; highlight historical norms regarding inclusion and representation; and identify who is typically left out of such processes. “Intergenerational” equity considers fairness or justice between present and future generations, including the burdens and costs that will be incurred by future generations as a result of action taken (or not taken) at present. Issues of access, including appropriate data and governance are also important to consider to support inclusiveness and equity. As climate change impacts, mitigation, and adaptation sometimes expose competing priorities, recognition of tradeoffs, including equity and justice concerns, should be addressed when applicable.

Overall, there is substantial variation in how equity and justice issues are integrated across chapters. Some chapters, for example Chapter 9 (Coastal Effects), use the introduction to strongly frame the chapter in the context of equity and justice, while other chapters such as Chapters 6 (Land Cover and Land-Use Change), 7 (Forests), and 8 (Ecosystems, Ecosystem Services, and Biodiversity) make no mention of equity or justice in the introductions. Chapter 15 (Human Health) frames health in the context of equity and justice in the introduction and goes on to consistently integrate related issues throughout the chapter, including systemic connections, depth of reporting on disproportionate impacts (rather than a blanket statement), and related dimensions of equity. Chapter 18 (Sector Interactions, Multiple Stressors, and Complex Systems) strongly illustrates the systemic interconnections relating climate change to equity and justice, and uniquely—but importantly—integrates governance and data justice issues. Other chapters would benefit from following these examples. The Committee recommends including equity and justice framing in chapter introductions and prioritizing the integration of related issues throughout the chapters where possible, rather than stand-alone sections addressing the topic. Doing so would highlight systemic interconnections and help avoid the appearance of addressing the topic as an afterthought. Additional considerations for addressing equity and justice throughout NCA5 are outlined in Box 2-4.

Recommendation 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.

To ensure consistency throughout the report, the Committee recommends the development or adoption of a glossary of terms related to equity and justice. Care should be taken to avoid terminology that is harmful to impacted communities, particularly the use of deficit language. For example, many chapters use the phrase “marginalized communities,” but referring to people as “marginalized” can perpetuate perceptions of inadequacy and lack of autonomy within the communities being discussed. Generally, it is best to be as specific as

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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possible when describing equity-and justice-related issues—including populations—rather than relying on jargon and terms. The Committee recommends following a similar process as was used in the New York State Climate Impact Assessment3 process, in which a glossary of terms related to equity and justice was developed from respected information sources. In the draft NCA5 report, there is inconsistent use of terminology around climate and gentrification (e.g., eco-gentrification, green gentrification, environmental gentrification) and terminology on race and socioeconomic status (e.g., minority, racialized minority, BIPOC, low income, low wealth). For example, Chapter 15 (Human Health) uses both “low-income” and “low-wealth,” while most of the other chapters use “low-income.” These two terms are distinct and should be used appropriately based on the intended meaning. Agreement on one respected reference for each term, such as “low-income” and “low-wealth,” would help to ensure consistent use of terminology across the report.

Recommendation 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.

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3 See https://nysclimateimpacts.org.

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Access to climate observations, projections, and assessments is not equitable across US territories, particularly for US-affiliated islands in the Pacific and Caribbean, which is noted in the respective regional chapters. NCA5 provides an opportunity to increase equity in this area, and every effort should be made to do so. Many figures in national chapters do not include the US Caribbean, and while many figures do include Hawai‘i, virtually no figures include the other US-Affiliated Pacific Islands despite the availability of information to do so. Whenever satellite data or global model output is used to generate national figures (e.g., Figures 2.4, 2.5, 3.7, 4.3), NCA5 should include a complete map of all island territories. If no data are available for these regions, the figure caption should state this to be the case (e.g., Figure 2.10). The Committee recognizes that including all island territories is not always optimal when attempting to produce legible figures for the contiguous United States due to the geographic extent of the US-Affiliated Pacific Islands. In such cases, the Committee recommends that separate figures be created for regions that are not included, that these figures be aggregated in an appendix, and that figure captions in national chapters point to this appendix for maps of regions not shown. Finally, the national chapters should strive to be more inclusive of island territories in textual statements and assessments of climate-related impacts and risks.

AREAS TO ADD EMPHASIS

Recommendation 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.

In reviewing the draft NCA5 report as a whole, the Committee identified a number of topical areas where relatively more emphasis could be placed. In this section, the Committee outlines topics that should receive more complete attention across the report and identifies specific examples or inconsistencies across the report.

Mitigation

The treatment of GHG mitigation is uneven across the draft NCA5 report and should receive more attention in many of the chapters, consistent with its treatment in Chapter 1 (Overview). On the one hand, Chapter 32 of the draft NCA5 report is devoted to mitigation. In contrast, national chapters do not consistently emphasize mitigation or draw connections to Chapter 32 (Mitigation). Most notably, chapters on sectors that have significant GHG emissions or removals treat mitigation inconsistently. For example, Chapters 12 (Built Environment, Urban Systems, and Cities) and 13 (Transportation) have key messages on mitigation (i.e., Key Message 12.3, Key Message 13.1). On the other hand, Chapters 7 (Forests) and 11 (Agriculture, Food Systems, and Rural Communities) address GHG emissions with a focus on removals in text boxes, and Chapter 5 (Energy Supply, Delivery, and Demand) addresses GHG mitigation together with adaptation but does not include emissions or mitigation in any of the key messages. Some chapters discuss adaptation and mitigation as a grouped topic (e.g., Chapter 16 [Tribes and Indigenous Peoples], Chapter 15 [Human Health]), but then only examples of adaptation are provided with no specific discussion on mitigation. The inconsistent attention to mitigation

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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extends to the regional chapters. While Chapter 21 (Northeast) summarizes state GHG mitigation plans in a box and Chapters 26 (Southern Great Plains) and 28 (Southwest) summarize mitigation actions in the regions in figures, the other regional chapters say very little or nothing, or may discuss emissions reductions but do not refer to the actions as “mitigation” (e.g., Chapter 25 [Northern Great Plains]). Given the importance of mitigation in the consideration of climate change impacts in the future and the substantial knowledge base on mitigation, a greater emphasis on mitigation in the United States is appropriate.

Adaptation Successes and Shortcomings

Significant adaptation efforts are happening across the United States at all different scales. Summarizing the breadth of adaptation efforts is difficult because the United States lacks transparent and credible mechanisms to aggregate and assess the combined effectiveness of these measures regionally and nationally. The draft NCA5 report provides a competing dual analysis of adaptation. Many chapters describe specific adaptations, such as Chapter 21 (Northeast), that lists state and tribal adaptation plans. While recent work on adaptation is impressive, the recitation of adaptations does not assess whether and to what extent these measures will be successful in reducing risks from climate change. In contrast, Chapter 31 (Adaptation) finds that without a more dedicated effort toward transformation, the current trajectory of adaptations will be inadequate to fully mitigate the increasing risks from climate change and examines the need for transformative adaptation. Both views provide important insights—adaptations are being made but appear inadequate to fully adapt to how the climate may change. Chapter 31 (Adaptation) should better integrate these two points, and the synthesis of these two approaches to assessing adaptation should be carefully reflected in Chapter 1 (Overview). It would be appropriate for national and regional chapters to coordinate their discussions of adaptation to be consistent with the framework outlined in Chapter 31 (Adaptation). It may also be appropriate for NCA5 authors to acknowledge that adaptation actions related to infrastructure, human health, local economies, ecosystems, and emergency management are often about changing an element of a larger project that is already scheduled to occur, making projects challenging to assess as standalone adaptation actions.

Projected Impacts of Climate Change

The impacts of climate change are increasing, resulting in greater impacts on health, the built environment, and natural systems. However, the focus of many key messages across the draft NCA5 report is on recent observed changes and impacts. As noted above, a key requirement of the GCRA is to report on projected climate change impacts, specifically 25 to 100 years into the future, and relatively more emphasis on projected impacts is needed. For example, some sector chapters, such as Chapter 12 (Built Environment) and Chapter 13 (Transportation) devote little attention to projected climate change impacts. Many regional chapters focus only on recent impacts and would benefit from discussing projections and key uncertainties (e.g., Chapters 27 [Northwest], 29 [Alaska]). Furthermore, there is a tendency in some key messages (e.g., Key Message 4.1) to conflate or switch between current and projected changes in a confusing way. Care should be given not to intermix statements about observed and projected

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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changes in key messages. Recommendation 8 above details the suggested use of scenarios when projections are included.

Attribution of Extreme Events

The Committee commends the discussion in the draft NCA5 report on recent observed extreme events that identify how vulnerable society and nature are to climate change. Attribution of such events is an emerging field of research (e.g., NASEM, 2016). In many cases in the draft NCA5 report, extreme climate or weather are discussed (e.g., Chapters 4 [Water], 11 [Agriculture, Food Systems, and Rural Communities], 12 [Built Environment, Urban Systems, and Cities], 28 [Southwest]) but there is little or no discussion disentangling natural variability from climate change as causes of extreme events, as discussed above. For example, Chapter 28 (Southwest) discusses the current mega-drought affecting the entire region and cites Williams et al. (2022) but does not mention the causes of the drought or that the reference concludes that while climate change is making the drought more severe and long-lasting, natural variability has a relatively greater contribution to the severity of the drought than does climate change. As another example, the text in Chapter 1 (Overview) and Figure 1.4 are misleading because they imply the increase in the number of billion-dollar disasters is solely due to more intense weather and climate events. However, societal exposure to such events caused by population and property increases in hazardous areas also plays a significant role (see Figure 1.4 and explanatory text; Figure 2.6 and accompanying text; and Appendix 4, Figure A4.5). Chapters should be carefully written to emphasize that many extreme events have a higher likelihood of occurring due to climate change, rather than directly attributing extreme events to climate change, unless there is evidence to support the statement. Without precise attribution of extreme events, readers could be left with the impression that these events are entirely the result of climate change. There is an opportunity to expand the discussion of attribution of extreme events in the main text of Chapter 3 (Earth System Processes) to describe the concept in a more accessible way, the use of attribution studies and their limitations, and progress in the knowledge base since NCA4.

Rural Analysis

Rural communities are covered in Chapter 11 (Agriculture, Food Systems, and Rural Communities) and urban communities are covered in Chapter 12 (Built Environment, Urban Systems, and Cities). Impacts on urban communities are also addressed in other chapters of the draft NCA5 report, but the unique challenges and opportunities for rural communities are not adequately addressed. A more balanced discussion of rural communities would support the framework recommended for equity and justice, provided above. For example, Chapter 13 (Transportation) focuses on transportation networks in cities and omits discussions of rural mobility. Chapter 14 (Air Quality) covers environmental justice and air pollution risks in urban areas, with little attention to such risks in rural areas. Chapter 15 (Human Health) makes passing mention of rural farmers but does not discuss issues specific to rural communities (e.g., access to health care). Even though Chapter 12 (Built Environment, Urban Systems, and Cities) is focused on the built environment in urban areas, the built environment in rural areas is not covered in the

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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draft NCA5 report. The regional chapters generally do a better job of commenting on rural areas, though there are opportunities, for example, in Chapter 28 (Southwest), to place more emphasis on rural areas.

Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Suggested Citation:"2 Overarching Comments and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2023. Review of the Draft Fifth National Climate Assessment. Washington, DC: The National Academies Press. doi: 10.17226/26757.
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Roughly every four years, the U.S. Global Change Research Program produces a congressionally mandated assessment of global change science and the impacts, adaptation, and mitigation of climate change in the United States. The draft Fifth National Climate Assessment (NCA5), released publicly in November 2022, covers a wide range of U.S. impacts, from human health and community well-being to the built environment, businesses and economies, and ecosystems and water resources. NCA5 had the largest scale of collaboration to date in the series, with input from hundreds of experts from all levels of governments, academia, non-government organizations, the private sector, and the public. The National Academies report provides an independent, comprehensive review and makes recommendations to strengthen the accuracy, credibility, and accessibility of the draft NCA5 report.

The National Academies’ review of the draft NCA5 report finds that it successfully meets the requirements of the federal mandate, provides accurate information, and effectively communicates climate science to the public, decision makers, and other stakeholders. The review makes recommendations for ways the draft NCA5 report could be strengthened, including: adopting more clear and consistent structure for key messages and figures across the report; resolving inconsistencies between chapters in how terms and topics are discussed, for example the use of scenarios and projections; intentionally applying an equity and justice lens across chapters; and increasing emphasis on certain topical areas.

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