Report Guide

The following describes the format of the report and the structure of each chapter.

Executive Summary

The Executive Summary describes the impacts of climate change on the health of the American public. It summarizes the overall findings and represents each chapter with a brief overview, the Key Findings, and a figure from the chapter.


Key Findings and Traceable Accounts

Topical chapters include Key Findings, which are based on the authors’ consensus expert judgment of the synthesis of the assessed literature. The Key Findings include confidence and likelihood language as appropriate (see “Documenting Uncertainty” below and Appendix 4: Documenting Uncertainty).

Each Key Finding is accompanied by a Traceable Account which documents the process and rationale the authors used in reaching these conclusions and provides additional information on sources of uncertainty. The Traceable Accounts can be found in the Supporting Evidence section of each chapter.

Chapter Text

Each chapter assesses the state of the science in terms of observed and projected impacts of climate change on human health in the United States, describes the link between climate change and health outcomes, and summarizes the authors’ assessment of risks to public health. Both positive and negative impacts on health are reported as supported by the scientific literature. Where appropriate and supported by the literature, authors include descriptions of critical non-climate stressors and other environmental and institutional context; social, behavioral, and adaptive factors that could increase or moderate impacts; and underlying trends in health that affect vulnerability (see “Populations of Concern” below). While the report is designed to inform decisions about climate change, it does not include an assessment of literature on climate change mitigation, adaptation, or economic valuation, nor does it include policy recommendations.

Exposure Pathway Diagram

Each topical chapter includes an exposure pathway diagram (see Figure 1). These conceptual diagrams illustrate a key example by which climate change affects health within the area of interest of that chapter. These diagrams are not meant to be comprehensive representations of all the factors that affect human health. Rather, they summarize the key connections between climate drivers and health outcomes while recognizing that these pathways exist within the context of other factors that positively or negatively influence health outcomes.


Figure 1: Understanding the Exposure Pathway Diagrams

Figure 1: Understanding the Exposure Pathway Diagrams
The center boxes include selected examples of climate drivers, the primary pathways by which humans are exposed to health threats from those drivers, and the key health outcomes that may result from exposure. The left gray box indicates examples of the larger environmental and institutional context that can affect a person’s or community’s vulnerability to health impacts of climate change. The right gray box indicates the social and behavioral context that also affects a person’s vulnerability to health impacts of climate change. This path includes factors such as race, gender, and age, as well as socioeconomic factors like income and education or behavioral factors like individual decision making. The examples listed in these two gray boxes can increase or reduce vulnerability by influencing the exposure pathway (changes in exposure) or health outcomes (changes in sensitivity or adaptive capacity ). The diagram shows that climate change can affect health outcomes directly and by influencing the environmental, institutional, social, and behavioral contexts of health.

The exposure pathway diagram in Chapter 1: Introduction: Climate Change and Human Health is a high-level overview of the main routes by which climate change affects health, summarizing the linkages described in the following chapters. Because the exposure pathway diagrams rely on examples from a specific health topic area, a diagram is not included in Chapter 9: Populations of Concern, as that chapter describes crosscutting issues relevant to all health topics.

Research Highlights

Four chapters include research highlights: Temperature-Related Death and Illness, Air Quality Impacts, Vector-Borne Disease, and Water-Related Illness. Six research highlight sections across these four chapters describe the findings of recently published quantitative analyses of projected impacts conducted for inclusion in this report. Each analysis is summarized with a brief description of the study’s 1) Importance, 2) Objectives, 3) Methods, 4) Results, and 5) Conclusions. The analyses are all published in external peer-reviewed sources, and the full description of modeling methods and findings can be found in those citations. While authors of these analyses were provided with modeling guidance and conferred on opportunities for consistency in approach, no comprehensive set of assumptions, timeframes, or scenarios were applied across modeling analyses. Therefore, these six studies do not represent an integrated modeling assessment. The findings of these analyses are considered as part of the overall assessment of the full body of literature when developing the chapter Key Findings. For more information on modeling methods see Appendix 1: Technical Support Document.

Populations of Concern

One of the main goals of this assessment was to identify populations that are particularly vulnerable to specific health impacts associated with climate change. Each chapter includes discussion of this topic in addition to the full chapter devoted to populations of concern. In these discussions, the authors identify segments of the general population that the peer-reviewed literature has identified as being at increased risk for health-related climate impacts, now or in the future.

Emerging Issues

The Emerging Issues sections briefly describe emerging areas of research including areas of potential future concern; health impacts not currently prevalent or severe in the United States but with potential to become a health concern; or areas where the links between climate change and a human health outcome are in early stages of study and for which a more comprehensive synthesis is outside the scope of this report.

Research Needs

While the goal of this assessment is to highlight the current state of the science on climate impacts on health, research needs identified through the development of this assessment are briefly summarized in each chapter. These research needs could inform research beyond the current state of the science or outside the scope of this report.

Supporting Evidence

The Traceable Accounts supporting each Key Finding are provided at the end of each chapter in the Supporting Evidence section.

Documenting Uncertainty: Confidence and Likelihood

Two kinds of language are used when describing the uncertainty associated with specific statements in this report: confidence language and likelihood language (see table below and Appendix 4: Documenting Uncertainty). Confidence in the validity of a finding is based on the type, amount, quality, strength, and consistency of evidence and the degree of expert agreement on the finding. Confidence is expressed qualitatively and ranges from low confidence (inconclusive evidence or disagreement among experts) to very high confidence (strong evidence and high consensus).

Likelihood language describes the likelihood of occurrence based on measures of uncertainty expressed probabilistically (in other words, based on statistical analysis of observations or model results or based on expert judgment). Likelihood, or the probability of an impact, is a term that allows a quantitative estimate of uncertainty to be associated with projections. Thus, likelihood statements have a specific probability associated with them, ranging from very unlikely (less than or equal to a 1 in 10 chance of the outcome occurring) to very likely (greater than or equal to a 9 in 10 chance).

Likelihood and Confidence Level


Very Likely
≥9 in 10
≥2 in 3
As Likely as Not
≈ 1 in 2
≤ 1 in 3
Very Unlikely
≤1 in 10

Confidence Level

Very High Strong evidence (established theory, multiple sources, consistent results, well documented and accepted methods, etc.), high consensus
High Moderate evidence (several sources, some consistency, methods vary and/or documentation limited, etc.), medium consensus
Medium Suggestive evidence (a few sources, limited consistency, models incomplete, methods emerging, etc.), competing schools of thought
Low Inconclusive evidence (limited sources, extrapolations, inconsistent findings, poor documentation and/or methods not tested, etc.), disagreement or lack of opinions among experts

All Key Findings include a description of confidence. Where it is considered scientifically justified to report the likelihood of particular impacts within the range of possible outcomes, Key Findings also include a likelihood designation. Confidence and likelihood levels are based on the expert assessment and consensus of the chapter author teams. The author teams determined the appropriate level of confidence or likelihood by assessing the available literature, determining the quality and quantity of available evidence, and evaluating the level of agreement across different studies. For specific descriptions of the process by which each chapter author team came to consensus on the Key Findings and assessment of confidence and likelihood, see the Traceable Account section for each chapter. More information is also available in Appendix 1: Technical Support Document and Appendix 4: Documenting Uncertainty.