 Section 0 of the Fourth National Climate Assessment, Vol. 2, Impact, Risks, and Adaptation in the United States. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recording by Warren Coddy, Gurney, Illinois. Fourth National Climate Assessment, Vol. 2, Impact, Risks, and Adaptation in the United States by the U.S. Global Change Research Program. Section 0, Front Matter. About this report. The National Climate Assessment. The Global Change Research Act of 1990 mandates that the U.S. Global Change Research Program, USGCRP, deliver a report to Congress and the President no less than every four years that, quote, 1. integrates, evaluates, and interprets the findings of the program. 2. Analyzes the effects of global change on the natural environment, agriculture, energy production and use, land and water resources, transportation, human health and welfare, human social systems, and biological diversity. And 3. Analyzes current trends in global change, both human induced and natural, and projects major trends for the subsequent 25 to 100 years. End quote. The Fourth National Climate Assessment, NCA4, fulfills that mandate in two volumes. This report, Vol. 2, draws on the foundational science described in Vol. 1, the Climate Science Special Report, CSSR. Vol. 2 focuses on the human welfare, societal, and environmental elements of climate change and variability for 10 regions and 18 national topics. With particular attention paid to observed and projected risks, impacts, consideration of risk reduction, and implications under different mitigation pathways. Where possible, NCA4 Vol. 2 provides examples of actions underway in communities across the United States to reduce the risks associated with climate change, increase resilience, and improve livelihoods. This assessment was written to help inform decision makers, utility and natural resource managers, public health officials, emergency planners, and other stakeholders by providing a thorough examination of the effects of climate change on the United States. Climate Science Special Report, NCA4 Vol. 1. The Climate Science Special Report, CSSR, published in 2017, serves as the first volume of NCA4. It provides a detailed analysis of how climate change is affecting the physical Earth system across the United States and provides the foundational physical science upon which much of the assessment of impacts in this report is based. The CSSR integrates and evaluates current findings on climate science and discusses the uncertainties associated with these findings. It analyzes trends in climate change, both human-induced and natural, and projects major trends to the end of this century. Projected changes in temperature, precipitation patterns, sea-level rise, and other climate outcomes are based on a range of scenarios widely used in the climate research community, referred to as representative concentration pathways, RCPs. As an assessment and analysis of the physical science, the CSSR provides important input to the development of other parts of NCA4 and their primary focus on the human welfare, societal, economic, and environmental elements of climate change. A summary of the CSSR is provided in Chapter 2, Our Changing Climate, of this report. The full report can be accessed at science2017.globalchange.gov. Report Development, Review, and Approval Process The National Oceanic and Atmospheric Administration, NOAA, served as the administrative lead agency for the preparation of this report. A federal steering committee, composed of representatives from USGCRP agencies, oversaw the report's development. A team of more than 300 federal and non-federal experts, including individuals from federal, state, and local governments, tribes and indigenous communities, national laboratories, universities, and the private sector, volunteered their time to produce the assessment, with input from external stakeholders at each stage of the process. A series of regional engagement workshops reached more than 1,000 individuals in over 40 cities, while listening sessions, webinars, and public comment periods provided valuable input to the authors. Participants included decision makers from the public and private sectors, resource and environmental managers, scientists, educators, representatives from businesses and non-governmental organizations, and the interested public. NCA4 Volume 2 was thoroughly reviewed by external experts and the general public, as well as the federal government, that is, the NCA4 Federal Steering Committee, and several rounds of technical and policy review by the 13 federal agencies of the USGCRP. An expert external peer review of the whole report was performed by an ad hoc committee of the National Academies of Sciences, Engineering, and Medicine, N-A-S-E-M. Additional information on the development of this assessment can be found in Appendix 1, Report Development Process. Sources used in this report The findings in this report are based on an assessment of the peer-reviewed scientific literature complemented by other sources, such as gray literature, where appropriate. In addition, authors used well-established and carefully evaluated observational and modeling data sets, technical input reports, USGCRP's sustained assessment products, and a suite of scenario products. Each source was determined to meet the standards of the Information Quality Act, see Appendix 2, Information in the Fourth National Climate Assessment. Sustained Assessment Products The USGCRP's sustained assessment process facilitates and draws upon the ongoing participation of scientists and stakeholders enabling the assessment of new information and insights as they emerge. The USGCRP led the development of two major sustained assessment products, as inputs to NCA4. The impacts of climate change on human health in the United States, a scientific assessment, and the second state of the Carbon Cycle Report. In addition, USGCRP agencies contributed products that improved the thoroughness of this assessment, including the US Department of Agriculture's Scientific Assessment, Climate Change, Global Food Security, and the US Food System. NOAA's Climate Resilience Toolkit, Climate Explorer, and State Climate Summaries. The US Environmental Protection Agency's updated economic impacts of climate change reports and a variety of USGCRP indicators and scenario products that support the evaluation of climate-related risks. See Appendix 3, Data Tools and Scenario Products. USGCRP Scenario Products As part of the sustained assessment process, federal inter-agency groups developed a suite of high-resolution scenario products that span a range of plausible future changes through at least 2100 in key environmental parameters. This new generation of USGCRP Scenario Products, hosted at https://scenarios.globalchange.gov, includes changes in average and extreme statistics of key climate variables, for example temperature and precipitation, changes in local sea level rise along the entire US coastline, changes in population as a function of demographic shifts and migration, and changes in land use driven by population changes. USGCRP Scenario Products help ensure consistency in underlying assumptions across the report and, therefore, improve the ability to compare and synthesize results across chapters, where possible, authors have used the range of these scenario products to frame uncertainty in future climate and associated effects as it relates to the risks that are the focus of their chapters. As discussed briefly elsewhere in this front matter, and in more detail in Appendix 3, Data Tools and Scenario Products, future scenarios referred to as RCPs provide the global framing for NCA4 Vol 1 and 2. RCPs focus on outputs, such as emissions and concentrations of greenhouse gases and particulate matter, that are, in turn, fed into climate models. As such, a wide range of future socioeconomic assumptions at the global and national scale, such as population growth, technological innovation and carbon intensity of energy mix, could be consistent with the RCPs used throughout NCA4. For this reason, further guidance on US population and land use assumptions was provided to authors. See Appendix 3, Data Tools and Scenario Products, including Table A3.1, for additional detail on these scenario products. Guide to the Report Summary Findings The 12 summary findings represent a very high level synthesis of the material in the underlying report. They consolidate key messages and supporting evidence from 16 underlying national-level topic chapters, 10 regional chapters, and two response chapters. Overview The overview presents the major findings alongside selected highlights from NCA4 Vol 2, providing a synthesis of material from the underlying report chapters. Chapter Text Key Messages and Traceable Accounts Chapters are centered around key messages, which are based on the author's expert judgment of the synthesis of the assessed literature. With a view to presenting technical information in a manner more accessible to a broad audience, this report aims to present findings in the context of risks to natural and or human systems. Assessing the risks to the nation posed by climate change and the measures that can be taken to minimize those risks helps users weigh the consequences of complex decisions. Since risk can most meaningfully be defined in relation to objectives or societal values, key messages in each chapter of this report aim to provide answers to specific questions about what is at risk in a particular region or sector and in what way. The text supporting each key message provides evidence, discusses implications, identifies intersections between systems or cascading hazards, and points out paths to greater resilience. Where a key message focuses on managing risk, authors considered the following questions. What do we value? What is at risk? What outcomes do we wish to avoid with respect to these valued things? What do we expect to happen in the absence of adaptive action and or mitigation? How bad could things plausibly get? Are there important thresholds or tipping points in the unique context of a given region, sector, and so on? These considerations are encapsulated in a single question. What keeps you up at night? Importantly, climate is only one of many drivers of change and risk. Where possible, chapters provide information about the dominant sources of uncertainty, such as scientific uncertainty or socioeconomic factors, as well as information regarding other relevant non-climate stressors. Each key message is accompanied by a traceable account that restates the key message found in the chapter text with calibrated confidence and likelihood language. See table one. These traceable accounts also document the supporting evidence and rationale the authors used in reaching their conclusions. While also providing information on sources of uncertainty, more information on traceable accounts is provided below. Our changing climate. USGCRP oversaw the production of the Climate Science Special Report, CSSR, NCA4 Volume 1, which assesses the current state of science related to climate change and its physical impacts. The CSSR is a detailed analysis of how climate change affects the physical earth system across the United States. It presents foundational information and projections for climate change that improve consistency across analyses in NCA4 Volume 2. The CSSR is the basis for the Physical Climate Science Summary presented in Chapter 2, Our Changing Climate, of this report. National Topic Chapters The National Topic Chapters summarize current and future climate change related risks and what can be done to reduce those risks. These National Chapters also synthesize relevant content from the Regional Chapters. New National Topic Chapters for NCA4 include Chapter 13, Air Quality, Chapter 16, Climate Effects on US International Interests, and Chapter 17, Sector Interactions, Multiple Stressors, and Complex Systems. Regional Chapters Responding to public demand for more localized information and because impacts and adaptation tend to be realized at a more local level, NCA4 provides greater detail in the Regional Chapters compared to the National Topic Chapters. The Regional Chapters assess current and future risks posed by climate change to each of NCA4's 10 regions. See Figure 1 and what can be done to minimize risk. Challenges, opportunities, and success stories for managing risk are illustrated through case studies. The regions defined in NCA4 are similar to those used in the Third National Climate Assessment, NCA3, with these exceptions. The Great Plains region, formerly stretching from the border of Canada to the border of Mexico, is now divided into the Northern Great Plains and Southern Great Plains along the Nebraska-Kansas border. And content related to the US Caribbean Islands is now found in its own chapter, distinct from the Southeast region. Response Chapters The response chapters assess the science of adaptation and mitigation, including benefits, tradeoffs, and best practices of ongoing adaptation measures and quantification of economic damages that can be avoided by reducing greenhouse gas emissions. The National Climate Assessment does not evaluate or recommend specific policies. Economic Estimates To the extent possible, economic estimates in this report have been converted to 2015 dollars using the US Bureau of Economic Affairs implicit price deflaters for gross domestic product, Table 1.1.9. For more information, please visit https colon double backslash b-e-a dot gov backslash national backslash index dot htm. Where documented in the underlying literature, discount rates in specific estimates in this assessment are noted next to those projections. Use of Scenarios Climate Modeling Experts develop climate projections for a range of plausible futures. These projections capture variables such as the relationship between human choices, greenhouse gas, GHG, and particulate matter emissions, GHG concentrations in our atmosphere, and the resulting impacts, including temperature change and sea level rise. Some projections are consistent with continued dependence on fossil fuels while others are achieved by reducing GHG emissions. The resulting range of projections reflects in part the uncertainty that comes with quantifying future human activities and their influence on climate. The most recent set of climate projections developed by the international scientific community is classified under four representative concentration pathways or RCPs. A wide range of future socioeconomic assumptions could be consistent with the RCPs used throughout NCA4. NCA4 focuses on RCP 8.5 as a higher scenario associated with more warming and RCP 4.5 as a lower scenario with less warming. Other RCP scenarios, e.g. RCP 2.6, a very low scenario, are used where instructive, such as in analyses of mitigation science issues. To promote understanding while capturing the context of the RCPs, authors use the phrases a higher scenario RCP 8.5 and a lower scenario RCP 4.5. RCP 8.5 is generally associated with higher population growth, less technological innovation, and higher carbon intensity of the global energy mix. RCP 4.5 is generally associated with lower population growth, more technological innovation, and lower carbon intensity of the global energy mix. NCA4 does not evaluate the feasibility of the socioeconomic assumptions within the RCPs. Future socioeconomic conditions, and especially the relationship between economic growth, population growth, and innovation, will have a significant impact on which climate change scenario is realized. The use of RCP 8.5 and RCP 4.5 as core scenarios is broadly consistent with the range used in NCA3. For additional detail on these scenarios and what they represent, please see Appendix 3, Data Tools and Scenario Products, as well as Chapter 4 of the Climate Science Special Report. Treatment of Uncertainties, Risk Framing, Confidence, and Likelihood Risk Framing. In March 2016, NASEM convened a workshop characterizing risk in climate change assessments to assist NCA4 authors in their analyses of climate-related risks across the United States. To help ensure consistency and readability across chapters, USG-CRP developed guidance on communicating the risks and opportunities that climate change presents, including the treatment of scientific uncertainties. Where supported by the underlying literature, authors were encouraged to describe the full scope of potential climate change impacts, both negative and positive, including more extreme impacts that are less likely but would have severe consequences, and communicate the range of potential impacts and their probabilities of occurrence. Describe the likelihood of the consequences associated with the range of potential impacts, the character and quality of the consequences, both negative and positive, and the strength of available evidence. Communicate cascading effects among and within complex systems and quantify risks that could be avoided by taking action. Additional detail on how risk is defined for this report, as well as how risk-based framing was used, is available in Chapter 1, Overview, C-Box 1.2 Evaluating Risks to Inform Decisions. Traceable Accounts, Confidence, and Likelihood Throughout NCA4's assessment of climate-related risks and impacts, authors evaluated the range of information in the scientific literature to the fullest extent possible, arriving at a series of key messages for each chapter. Drawing on guidance developed by the Intergovernmental Panel on Climate Change, IPCC, chapter authors further described the overall reliability in their conclusions using these metrics in their chapter's traceable accounts. Confidence in the validity of a finding based on the type, amount, quality, strength, and consistency of evidence, such as mechanistic understanding, theory, data, models, and expert judgment, the skill, range, and consistency of model projections, and the degree of agreement within the body of literature. Likelihood, which is based on measures of uncertainty expressed probabilistically, in other words, based on statistical analysis of observations or model results, or on the author's expert judgment. The author team's expert assessment of confidence for each key message is presented in the chapter's traceable accounts. Where the authors consider it is scientifically justified to report the likelihood of a particular impact within the range of possible outcomes, key messages in the traceable accounts also include a likelihood designation. Traceable accounts describe the process and rationale the authors used in reaching their conclusions, as well as their confidence in these conclusions. They provide additional information about the quality of information used and allow traceability to data and resources. Table 1. This table describes the meaning of the various categories of confidence level and likelihood assessment used in NCA4. The levels of confidence are the same as they appear in the CSSR, NCA4 Volume 1. And while the likelihood scale is consistent with the CSSR, there are fewer categories, as that report relies more heavily on quantitative methods and statistics. This binning of likelihood is consistent with other USG-CRP sustained assessment products, such as the climate and health assessment, and NCA3. Confidence level. Confidence level very high. Strong evidence, established theory, multiple sources, confident results, well documented and accepted methods, etc. High consensus. Confidence level high. Moderate evidence, several sources, some consistency, methods vary and or documentation limited, etc. Medium consensus. Confidence level medium. Suggestive evidence, a few sources, limited consistency, models incomplete, methods emerging, etc. Competing schools of thought. Confidence level low. Inconclusive evidence, limited sources, extrapolations, inconsistent findings, poor documentation and or methods not tested, etc. Disagreement or lack of opinions among experts. Likelihood. Likelihood very likely. Greater than or equal to 9 in 10. Likelihood likely. Greater than or equal to 2 in 3. Likelihood as likely as not, equals 1 in 2. Likelihood unlikely. Less than or equal to 1 in 3. Likelihood very unlikely. Less than or equal to 1 in 10. End of Section 0. Section 1 of the Fourth National Climate Assessment, Volume 2. Impacts, risks and adaptation in the United States. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recording by Warren Coddy, Gurney, Illinois. Fourth National Climate Assessment, Volume 2. Impacts, risks and adaptation in the United States by the U.S. Global Change Research Program, Chapter 1. Overview, Part 1. Introduction. Earth's climate is now changing faster than at any point in the history of modern civilization, primarily as a result of human activities. The impacts of global climate change are already being felt in the United States and are projected to intensify in the future. But the severity of future impacts will depend largely on actions taken to reduce greenhouse gas emissions and to adapt to the changes that will occur. Americans increasingly recognize the risks climate change poses to their everyday lives and livelihoods and are beginning to respond. Figure 1.1. Water managers in the Colorado River Basin have mobilized users to conserve water in response to ongoing drought intensified by higher temperatures. And an extension program in Nebraska is helping ranchers reduce drought and heat risks to their operations. The state of Hawaii is developing management options to promote coral reef recovery from widespread bleaching events caused by warmer waters that threaten tourism, fisheries, and coastal protection from wind and waves. To address higher risks of flooding from heavy rainfall, local governments in southern Louisiana are pooling hazard reduction funds, and cities and states in the northeast are investing in more resilient water, energy, and transportation infrastructure. In Alaska, a tribal health organization is developing adaptation strategies to address physical and mental health challenges driven by climate change and other environmental changes. As Midwestern farmers adopt new management strategies to reduce erosion and nutrient losses caused by heavier rains, forest managers in the northwest are developing adaptation strategies in response to wildfire increases that affect human health, water resources, timber production, fish and wildlife, and recreation. After extensive hurricane damage, fueled in part by a warmer atmosphere and warmer higher seas, communities in Texas are considering ways to rebuild more resilient infrastructure. In the U.S. Caribbean, governments are developing new frameworks for storm recovery based on lessons learned from the 2017 hurricane season. Climate-related risks will continue to grow without additional action. Decisions made today determine risk exposure for current and future generations, and will either broaden or limit options to reduce the negative consequences of climate change. While Americans are responding in ways that can bolster resilience and improve livelihoods, neither global efforts to mitigate the causes of climate change nor regional efforts to adapt to the impacts currently approach the scales needed to avoid substantial damages to the U.S. economy, environment, and human health and well-being over the coming decades. Climate shapes where and how we live and the environment around us. Natural ecosystems, agricultural systems, water resources, and the benefits they provide to society are adapted to past climate conditions and their natural range of variability. A water manager may use past or current stream flow records to design a dam. A city could issue permits for coastal development based on current flood maps, and an electric utility or a farmer may invest in equipment suited to the current climate, all with the expectation that their investments and management practices will meet future needs. However, the assumption that current and future climate conditions will resemble the recent past is no longer valid, Chapter 28, Adaptation, Key Message 2. Observations collected around the world provide significant clear and compelling evidence that global average temperature is much higher and is rising more rapidly than anything modern civilization has experienced with widespread and growing impacts. Figure 1.2. Climate Science Special Report. Chapter 1.9 The warming trend observed over the past century can only be explained by the effects that human activities, especially emissions of greenhouse gases, have had on the climate. Chapter 2. Climate. Key Message 1. And Figure 2.1 Climate change is transforming where and how we live and presents growing challenges to human health and quality of life, the economy, and the natural systems that support us. Risks posed by climate variability and change vary by region and sector and by the vulnerability of people experiencing impacts. Social, economic, and geographic factors shape the exposure of people and communities to climate-related impacts and their capacity to respond. Risks are often highest for those that are already vulnerable, including low income communities, some communities of color, children, and the elderly. Chapter 14. Human Health Key Message 2. Chapter 15. Tribes. Key Messages 1-3 Chapter 28. Adaptation. Introduction Climate change threatens to exacerbate existing social and economic inequalities that result in higher exposure and sensitivity to extreme weather and climate-related events and other changes. Chapter 11. Urban. Key Message 1 Marginalized populations may also be affected disproportionately by actions to address the underlying causes and impacts of climate change if they are not implemented under policies that consider existing inequalities. Chapter 11. Urban. Key Message 4. Chapter 28. Adaptation. Key Message 4 This report draws a direct connection between the warming atmosphere and the resulting changes that affect Americans' lives, communities, and livelihoods now and in the future. It documents vulnerabilities, risks, and impacts associated with natural climate variability and human-caused climate change across the United States and provides examples of response actions underway in many communities. It concludes that the evidence of human-caused climate change is overwhelming and continues to strengthen, that the impacts of climate change are intensifying across the country, and that climate-related threats to Americans' physical, social, and economic well-being are rising. These impacts are projected to intensify, but how much they intensify will depend on actions taken to reduce global greenhouse gas emissions and to adapt to the risks from climate change now and in the coming decades. Chapter 28. Adaptation. Introduction. Chapter 29. Mitigation. Key Messages 3 and 4 Our changing climate. Observations, causes, and future change. Observed change. Observations from around the world show the widespread effects of increasing greenhouse gas concentrations on Earth's climate. High temperature extremes and heavy precipitation events are increasing. Glaciers and snow cover are shrinking, and sea ice is retreating. Seas are warming, rising, and becoming more acidic, and marine species are moving to new locations toward cooler waters. Flooding is becoming more frequent along the U.S. coastline. Growing seasons are lengthening, and wildfires are increasing. These and many other changes are clear signs of a warming world. Figure 1.2. Chapter 2. Climate Box 2.2. Appendix 3. Data and Scenarios. See also the USG-CRP Indicators and EPA Indicators' websites. Causes of Change. Scientists have understood the fundamental physics of climate change for almost 200 years. In the 1850s, researchers demonstrated that carbon dioxide and other naturally occurring greenhouse gases in the atmosphere prevent some of the heat radiating from Earth's surface from escaping to space. This is known as the greenhouse effect. This natural greenhouse effect warms the planet's surface about 60 degrees Fahrenheit above what it would be otherwise, creating a habitat suitable for life. Since the late 19th century, however, humans have released an increasing amount of greenhouse gases into the atmosphere through burning fossil fuels and, to a lesser extent, deforestation and land use change. As a result, the atmospheric concentration of carbon dioxide, the largest contributor to human-caused warming, has increased by about 40% over the industrial era. This change has intensified the natural greenhouse effect, driving an increase in global surface temperatures and other widespread changes in Earth's climate that are unprecedented in the history of modern civilization. Global climate is also influenced by natural factors that determine how much of the sun's energy enters and leaves Earth's atmosphere, and by natural climate cycles that affect temperatures and weather patterns in the short term, especially regionally. However, the unambiguous long-term warming trend in global average temperature over the last century cannot be explained by natural factors alone. Greenhouse gas emissions from human activities are the only factors that can account for the observed warming over the last century. There are no credible alternative human or natural explanations supported by the observational evidence. Without human activities, the influence of natural factors alone would actually have had a slight cooling effect on global climate over the last 50 years. Chapter 2, Climate, Key Message 1, Figure 2.1 Future Change Greenhouse gas emissions from human activities will continue to affect Earth's climate for decades and even centuries. Humans are adding carbon dioxide to the atmosphere at a rate far greater than it is removed by natural processes, creating a long-lived reservoir of the gas in the atmosphere and oceans that is driving the climate to a warmer and warmer state. Some of the other greenhouse gases released by human activities, such as methane, are removed from the atmosphere by natural processes more quickly than carbon dioxide. As a result, efforts to cut emissions of these gases could help reduce the rate of global temperature increases over the next few decades. However, longer-term changes in climate will largely be determined by emissions and atmospheric concentrations of carbon dioxide and other longer-lived greenhouse gases. Chapter 2, Climate, Key Message 2 Climate models representing our understanding of historical and current climate conditions are often used to project how our world will change under future conditions. See Chapter 2, Climate, Box 2.7 Climate is defined as whether conditions over multiple decades and climate model projections are generally not designed to capture annual or even decadal variation in climate conditions. Instead, projections are typically used to capture long-term changes such as how the climate system will respond to changes in greenhouse gas levels over the century. Scientists test climate models by comparing them to current observations and historical changes. Confidence in these models is based in part on how well they reproduce these observed changes. Climate models have proven remarkably accurate in simulating the climate change we have experienced to date, particularly in the past 60 years or so when we have greater confidence in observations. See Climate Science Special Report, Chapter 4.3.1 The observed signals of a changing climate continue to become stronger and clearer over time, giving scientists increased confidence in their findings even since the third national climate assessment was released in 2014. Today, the largest uncertainty in projecting future climate conditions is the level of greenhouse gas emissions going forward. Future global greenhouse gas emissions levels and resulting impacts depend on economic, political and demographic factors that can be difficult to predict with confidence far into the future. Like previous climate assessments, NCA4 relies on a suite of possible scenarios to evaluate the implications of different climate outcomes and associated impacts throughout the 21st century. These representative concentration pathways, RCPs, capture a range of potential greenhouse gas emissions pathways and associated atmospheric concentration levels through 2100. RCPs drive climate model projections for temperature, precipitation, sea level and other variables under futures that have either lower or higher greenhouse gas emissions. RCPs are numbered according to changes in radiative forcing by 2100 relative to pre-industrial conditions, plus 2.6, plus 4.5, plus 6.0 or plus 8.5 watts per square meter. Each RCP leads to a different level of projected global temperature change. Higher numbers indicate greater projected temperature change and associated impacts. The higher scenario, RCP 8.5, represents a future where annual greenhouse gas emissions increase significantly throughout the 21st century before leveling off by 2100. Whereas the other RCPs represent more rapid and substantial mitigation by mid-century, with greater reductions thereafter, current trends in annual greenhouse gas emissions globally are consistent with RCP 8.5. Of the two RCPs predominantly referenced throughout this report, the lower scenario, RCP 4.5, envisions about 85% lower greenhouse gas emissions than the higher scenario, RCP 8.5, by the end of the 21st century. In some cases throughout this report, a very low scenario, RCP 2.6, that represents more immediate, substantial and sustained emissions reductions is considered. Each RCP could be consistent with a range of underlying socioeconomic conditions or policy choices. See the Scenario Products section of Appendix 3 in this report, as well as Climate Science Special Report Chapters 4.2.1 and 10.2.1 for more detail. The effects of different future greenhouse gas emissions levels on global climate become most evident around 2050, when Temperature, Figure 1.3, Chapter 2, Climate, Figure 2.2, Precipitation and Sea level rise, Figure 1.4, Chapter 2, Climate, Figure 2.3, projections based on each scenario begin to diverge significantly. With substantial and sustained reductions in greenhouse gas emissions, e.g. consistent with the very low scenario, RCP 2.6, the increase in global annual average temperature relative to pre-industrial times could be limited to less than 3.6 degrees Fahrenheit, 2 degrees Celsius, Chapter 2, Climate, Box 2.4, Climate Science Special Report, Chapter 4.2.1. Without significant greenhouse gas mitigation, the increase in global annual average temperature could reach 9 degrees Fahrenheit or more by the end of this century, Chapter 2, Climate, Key Message 2. For some aspects of Earth's climate system that take longer to respond to changes in atmospheric greenhouse gas concentrations, such as global sea level, some degree of long-term change will be locked in for centuries to come, regardless of the future scenario. Sea Climate Science Special Report, Chapter 12.5.3 Early greenhouse gas emissions mitigation can reduce climate impacts in the nearer term, such as reducing the loss of Arctic sea ice and the effects on species that use it, and in the longer term, by avoiding critical thresholds, such as marine ice sheet instability and the resulting consequences for global sea level and coastal development. Chapter 29. Mitigation, Timing and Magnitude of Action Annual average temperatures in the United States are projected to continue to increase in the coming decades. Regardless of future scenario, additional increases in temperatures across the contiguous United States of at least 2.3 degrees Fahrenheit relative to 1986 to 2015 are expected by the middle of this century. As a result, recent record-setting hot years are expected to become common in the near future. By late this century, increases of 2.3 to 6.7 degrees Fahrenheit are expected under a lower scenario, RCP 4.5, and 5.4 degrees to 11 degrees Fahrenheit under a higher scenario, RCP 8.5, relative to 1986 to 2015. Figure 1.3 Chapter 2 Climate Key Message 5 Figure 2.4 Alaska has warmed twice as fast as the global average since the mid-20th century. This trend is expected to continue. Chapter 26 Alaska Background High temperature extremes, heavy precipitation events, high tide flooding events along the US coastline, ocean acidification and warming, and forest fires in the western United States and Alaska are all projected to continue to increase, while land and sea ice cover, snowpack and surface soil moisture are expected to continue to decline in the coming decades. These and other changes are expected to increasingly impact water resources, air quality, human health, agriculture, natural ecosystems, energy and transportation infrastructure, and many other natural and human systems that support communities across the country. The severity of these projected impacts and the risks they present to society is greater under futures with higher greenhouse gas emissions, especially if limited or no adaptation occurs. Chapter 29 Mitigation Key Message 2 Box 1.1 Confidence and Uncertainty in Climate Science Many of the decisions we make every day are based on less than perfect knowledge. For example, while GPS based applications on smartphones can provide a travel time estimate for our daily drive to work, an unexpected factor like a sudden downpour or fender bender might mean a ride originally estimated to be 20 minutes could actually take longer. Fortunately, even with this uncertainty, we are confident that our trip is unlikely to take less than 20 minutes, or more than half an hour, and we know where we are headed. We have enough information to plan our commute. Uncertainty is also a part of science. A key goal of scientific research is to increase our confidence and reduce the uncertainty in our understanding of the world around us. Even so, there is no expectation that uncertainty can be fully eliminated, just as we do not expect a perfectly accurate estimate for our drive time each day. Studying Earth's climate system is particularly challenging, because it integrates many aspects of a complex natural system, as well as many human-made systems. Climate scientists find varying ranges of uncertainty in many areas, including observations of climate variables, the analysis and interpretation of those measurements, the development of new observational instruments, and the use of computer-based models of the processes governing Earth's climate system. While there is inherent uncertainty in climate science, there is high confidence in our understanding of the greenhouse effect and the knowledge that human activities are changing the climate in unprecedented ways. There is enough information to make decisions based on that understanding. Where important uncertainties do exist, efforts to quantify and report those uncertainties can help decision-makers plan for a range of possible future outcomes. These efforts also help scientists advance understanding and ultimately increase confidence in and the usefulness of model projections. Assessments like this one explicitly address scientific uncertainty associated with findings and use specific language to express it to improve relevance to risk analysis and decision-making. See Front Matter and Box 1.2 End of Box 1.1 Box 1.2 Evaluating Risks to Inform Decisions In this report, risks are often defined in a qualitative sense as threats to life, health, and safety, the environment, economic well-being, and other things of value to society. Chapter 28 Adaptation, Introduction In some cases, risks are described in quantitative terms, estimates of how likely a given threat is to occur, probability, and the damages that would result if it did happen. Consequences Climate change is a risk management challenge for society. It presents uncertain and potentially severe consequences for natural and human systems across generations. It is characterized by multiple intersecting and uncertain future hazards and, therefore, acts as a risk multiplier that interacts with other stressors to create new risks or to alter existing ones. See Chapter 17 Complex Systems, Key Message 1 Current and future greenhouse gas emissions, and thus mitigation actions to reduce emissions, will largely determine future climate change impacts and risks to society. Mitigation and adaptation activities can be considered complementary strategies. Mitigation efforts can reduce future risks, while adaptation can minimize the consequences of changes that are already happening as a result of past and present greenhouse gas emissions. Adaptation entails proactive decision making and investments by individuals, businesses, and governments to counter specific risks from climate change that vary from place to place. Climate risk management includes some familiar attributes and tactics for most businesses and local governments, which often manage or design for a variety of weather-related risks, including coastal and inland storms, heat waves, threats to water availability, droughts, and floods. Measuring risk encompasses both likelihoods and consequences of specific outcomes and involves judgments about what is of value, ranking of priorities, and cost-benefit analyses that incorporate the trade-offs among climate and non-climate-related options. This report characterizes specific risks across regions and sectors in an effort to help people assess the risks they face, create and implement a response plan, and monitor and evaluate the efficacy of a given action. See Chapter 28, Adaptation, Key Message 1, Figure 28.1 End of BOX 1.2 End of Chapter 1, Part 1 Section 2 of the Fourth National Climates Assessment, Volume 2 by USG-CRP This LibriVox recording is in the public domain. Recording by Warren Coddy, Gurney, Illinois. Chapter 1 Overview, Part 2 Climate change in the United States, current and future risks. Some climate-related impacts, such as increasing health risks from extreme heat, are common to many regions of the United States. Chapter 14, Human Health, Key Message 1. Others represent more localized risks, such as infrastructure damage caused by thawing of permafrost, long-frozen ground, in Alaska, or threats to coral reef ecosystems from warmer and more acidic seas in the US Caribbean, as well as Hawaii and the US-affiliated Pacific Islands. Chapter 26, Alaska, Key Message 2 Chapter 20, US Caribbean, Key Message 2 Chapter 27, Hawaii and Pacific Islands, Key Message 4 Risks vary by both a community's exposure to physical climate impacts and by factors that influence its ability to respond to changing conditions and to recover from adverse weather and climate-related events, such as extreme storms or wildfires. Chapter 14, Human Health, Key Message 2 Chapter 15, Tribes, State of the Sector, Key Message 1 and 2 Chapter 28, Adaptation, Key Message 4 Many places are subject to more than one climate-related impact, such as extreme rainfall combined with coastal flooding, or drought coupled with extreme heat, wildfire and flooding. The compounding effects of these impacts result in increased risks to people, infrastructure, and interconnected economic sectors. Chapter 11, Urban, Key Message 1 Impacts affecting interconnected systems can cascade across sectors and regions, creating complex risks and management challenges. For example, changes in the frequency, intensity, extent, and duration of wildfires can result in a higher instance of landslides that disrupt transportation systems and the flow of goods and services within or across regions. Box 1.3 Many observed impacts reveal vulnerabilities in these interconnected systems that are expected to be exacerbated as climate-related risks intensify. Under a higher scenario, RCP 8.5, it is very likely that some impacts, such as the effects of ice sheet disintegration on sea-level rise and coastal development, will be irreversible for many thousands of years, and others, such as species extinction, will be permanent. Chapter 7. Ecosystems, Key Message 1 Chapter 9. Oceans, Key Message 1 Chapter 29. Mitigation, Key Message 2 Economy and Infrastructure Without more significant global greenhouse gas mitigation and regional adaptation efforts, climate change is expected to cause substantial losses to infrastructure and property, and impede the rate of economic growth over the century. Chapter 4. Energy, Key Message 1 Chapter 8. Coastal, Key Message 1 Chapter 11. Urban, Key Message 2 Chapter 12. Transportation, Key Message 1 Regional Chapters 18-27 Regional economies and industries that depend on natural resources and favorable climate conditions, such as agriculture, tourism and fisheries, are increasingly vulnerable to impacts driven by climate change. Chapter 7. Ecosystems, Key Message 3 Chapter 10. Agriculture, Key Message 1 Reliable and affordable energy supplies, which underpin virtually every sector of the economy, are increasingly at risk from climate change and weather extremes. Chapter 4. Energy, Key Message 1 The impacts of climate change beyond our borders are expected to increasingly affect our trade and economy, including import and export prices and U.S. businesses with overseas operation and supply chains. Box 1.4. Chapter 16. International, Key Message 1 Chapter 17. Complex Systems, Key Message 1 Some aspects of our economy may see slight improvements in a modestly warmer world. However, the continued warming that is projected to occur without significant reductions in global greenhouse gas emissions is expected to cause substantial net damage to the U.S. economy, especially in the absence of increased adaptation efforts. The potential for losses in some sectors could reach hundreds of billions of dollars per year by the end of this century. Chapter 29. Mitigation, Key Message 2 Existing water, transportation, and energy infrastructure already face challenges from heavy rainfall, inland and coastal flooding, landslides, drought, wildfire, heat waves, and other weather and climate events. Figures 1.5 through 1.9 Chapter 11. Urban, Key Message 2 Chapter 12. Transportation, Key Message 1 Many extreme weather and climate-related events are expected to become more frequent and more intense in a warmer world, creating greater risks of infrastructure disruption and failure that can cascade across economic sectors. Chapter 3. Water, Key Message 2 Chapter 4. Energy, Key Message 1 Chapter 11. Urban, Key Message 3 Chapter 12. Transportation, Key Message 2 For example, more frequent and severe heat waves and other extreme events in many parts of the United States are expected to increase stresses on the energy system, amplifying the risk of more frequent and longer-lasting power outages and fuel shortages that could affect other critical sectors and systems such as access to medical care. Chapter 17. Complex Systems, Box 17.5 Chapter 4. Energy, Key Message 1 Chapter 8. Coastal, Key Message 1 Chapter 11. Urban, Key Message 3 Chapter 12. Transportation, Key Message 3 Current infrastructure is typically designed for historical climate conditions, Chapter 12. Transportation, Key Message 1 and development patterns. For instance, coastal land use generally do not account for changing climate. Chapter 5. Land Changes, State of the Sector Resulting in increasing vulnerability to future risks from weather extremes and climate change, Chapter 11. Urban, Key Message 2 Infrastructure, age and deterioration make failure or interrupted service from extreme weather even more likely, Chapter 11. Urban, Key Message 2 Climate change is expected to increase the costs of maintaining, repairing and replacing infrastructure with differences across regions. Chapter 12. Transportation, Regional Summary Recent extreme events demonstrate the vulnerabilities of interconnected economic sectors to increasing risks from climate change. C. Box 1.3 In 2017, Hurricane Harvey dumped an unprecedented amount of rainfall over the Greater Houston area, some of which has been attributed to human-induced climate change. Chapter 2. Climate, Box 2.5 Resulting power outages had cascading effects on critical infrastructure facilities, such as hospitals and water and wastewater treatment plants. Reduced oil production and refining capacity in the Gulf of Mexico caused price spikes regionally and nationally from actual and anticipated gasoline shortages. Figure 1.6 Chapter 17. Complex Systems, Key Message 1 In the U.S. Caribbean, hurricanes Irma and Maria caused catastrophic damage to infrastructure, including the complete failure of Puerto Rico's power grid and the loss of power throughout the U.S. Virgin Islands, as well as extensive damage to the region's agricultural industry. The death toll in Puerto Rico grew in the three months following Maria's landfall on the island due in part to the lack of electricity and potable water as well as access to medical facilities and medical care. Chapter 20. U.S. Caribbean, Box 20.1, Key Message 5 Climate-related risks to infrastructure, property, and the economy vary across regions. Along the U.S. coastline, public infrastructure and $1 trillion in national wealth held in coastal real estate are threatened by rising sea levels, higher storm surges, and the ongoing increase in high tide flooding. Figures 1.4 and 1.8 Chapter 8. Coastal, Key Message 1 Coastal infrastructure provides critical lifelines to the rest of the country, including energy supplies and access to goods and services from overseas trade. Increased damage to coastal facilities is expected to result in cascading costs and national impacts. Chapter 8. Coastal, Key Message 1 Chapter 4. Energy, State of the Sector, Key Message 1 High tide flooding is projected to become more disruptive and costlier as its frequency, depth, and inland extent grow in the coming decades. Without significant adaptation measures, many coastal cities in the southeast are expected to experience daily high tide flooding by the end of the century. Chapter 8. Coastal, Key Message 1 Chapter 19. Southeast, Key Message 2 Higher sea levels will also cause storm surge from tropical storms to travel farther inland than in the past, impacting more coastal properties and infrastructure. Chapter 8. Coastal, Key Message 1 Chapter 19. Southeast, Key Message 2 Oil, natural gas, and electrical infrastructure located along the coasts of the Atlantic Ocean and Gulf of Mexico are at increased risk of damage from rising sea levels and stronger hurricanes. Regional disruptions are expected to have national implications. Chapter 4. Energy, State of the Sector, Key Message 1 Chapter 18. Northeast, Key Message 3 Chapter 19. Southeast, Key Message 2 Hawaii and the US-affiliated Pacific Islands and the US Caribbean also face high risks to critical infrastructure from coastal flooding, erosion, and storm surge. Chapter 4. Energy, State of the Sector, Chapter 20 US Caribbean, Key Message 3 Chapter 27. Hawaii and Pacific Islands, Key Message 3 In the western United States, increasing wildfire is damaging ranches and rangelands as well as property and cities near the wildland-urban interface. Drier conditions are projected to increase the risk of wildfires and damage to property and infrastructure, including energy production and generation assets, and the power grid. Chapter 4. Energy, Key Message 1 Chapter 11. Urban, Regional Summary Chapter 24. Northwest, Key Message 3 In Alaska, thawing of permafrost is responsible for severe damage to roads, buildings, and pipelines that will be costly to replace, especially in remote parts of Alaska. Alaska oil and gas operations are vulnerable to thawing permafrost, sea level rise, and increased coastal exposure due to declining sea ice. However, a longer ice-free season may enhance offshore energy operations and transport. Chapter 4. Energy, State of the Sector Chapter 26. Alaska, Key Message 2 These impacts are expected to grow with continued warming. U.S. agriculture and the communities it supports are threatened by increases in temperatures, drought, heavy precipitation events, and wildfire on rangelands. Figure 1.10. Chapter 10. Ag and Rural, Key Messages 1 and 2 Case Study. Groundwater Depletion in the Agalalo Aquifer Region Chapter 23. South Great Plains, Key Message 1 Case Study. The Edwards Aquifer Yields of major U.S. crops such as corn, soybeans, wheat, rice, sorghum, and cotton are expected to decline over this century as a consequence of increases in temperatures and possibly changes in water availability and disease and pest outbreaks. Chapter 10. Ag and Rural, Key Message 1 Increases in growing season temperatures in the Midwest are projected to be the largest contributing factor to declines in U.S. agricultural productivity. Chapter 21. Midwest, Key Message 1 Climate change is also expected to lead to large-scale shifts in the availability and prices of many agricultural products across the world with corresponding impacts on U.S. agricultural producers and the U.S. economy. Chapter 16. International, Key Message 1 Extreme heat poses a significant risk to human health and labor productivity in the agricultural, construction, and other outdoor sectors. Chapter 10. Ag and Rural, Key Message 3 Under a higher scenario, RCP 8.5, almost two billion labor hours are projected to be lost annually by 2090 from the impacts of temperature extremes costing an estimated $160 billion in lost wages. Chapter 14. Human Health, Key Message 4 States within the Southeast, Chapter 19, Southeast, Key Message 4 And Southern Great Plains, Chapter 23, Southern Great Plains, Key Message 4 Regions are projected to experience some of the greatest impacts. See Figure 1.21 Natural Environment and Ecosystem Services Climate change threatens many benefits that the natural environment provides to society, safe and reliable water supplies, clean air, protection from flooding and erosion, and the use of natural resources for economic, recreational, and subsistence activities. Valued aspects of regional heritage and quality of life tied to the natural environment, wildlife, and outdoor recreation will change with the climate. And as a result, future generations can expect to experience and interact with natural systems in ways that are much different than today. Without significant reductions in greenhouse gas emissions, extinctions and transformative impacts on some ecosystems cannot be avoided, with varying impacts on the economic, recreational, and subsistence activities they support. Changes affecting the quality, quantity, and availability of water resources, driven in part by climate change, impact people and the environment, Chapter 3, Water, Key Message 1 Dependable and safe water supplies for U.S. Caribbean, Hawaii, and U.S. affiliated Pacific Island communities and ecosystems are threatened by rising temperatures, sea level rise, saltwater intrusion, and increased risks of drought and flooding. Chapter 3, Water, Regional Summary, Chapter 20, U.S.-Caribbean, Key Message 1 Chapter 27, Hawaii and Pacific Islands, Key Message 1 In the Midwest, the occurrence of conditions that contribute to harmful algal blooms, which can result in restrictions to water usage for drinking and recreation, is expected to increase. Chapter 3, Water, Regional Summary, Chapter 21, Midwest, Key Message 3 In the Southwest, water supplies for people in nature are decreasing during droughts due in part to climate change. Intensifying droughts, heavier downpours, and reduced snowpack are combining with other stressors such as groundwater depletion to reduce the future reliability of water supplies in the region with cascading impacts on energy production and other water-dependent sectors. Chapter 3, Water, Regional Summary, Chapter 4, Energy, State of the Sector, Chapter 25, Southwest, Key Message 5 In the Southern Great Plains, current drought and projected increases in drought length and severity threaten the availability of water for agriculture, figures 1.11 and 1.12, Chapter 23, Southern Great Plains, Key Message 1 Reductions in mountain snowpack and shifts in snowmelt timing are expected to reduce hydropower production in the Southwest and the Northwest, Chapter 24, Northwest, Key Message 3, Chapter 25, Southwest, Key Message 5. Drought is expected to threaten oil and gas drilling and refining, as well as thermoelectric power plants that rely on a study supply of water for cooling. Chapter 4, Energy, State of the Sector, Key Message 1, Chapter 22, Northern Great Plains, Key Message 4, Chapter 23, Southern Great Plains, Key Message 2, Chapter 25, Southwest, Key Message 5 Tourism, outdoor recreation, and subsistence activities are threatened by reduced snowpack, increases in wildfire activity, and other stressors affecting ecosystems and natural resources, figure 1.2D, 1.2K, and 1.13, Chapter 7, Ecosystems, Key Message 3 Increasing wildfire frequency, Chapter 19, Southeast, Case Study, Prescribed Fire Pest and disease outbreaks, Chapter 21, Midwest, Case Study, Adaptation and Forestry, and other stressors are projected to reduce the ability of US forests to support recreation as well as economic and subsistence activities. Chapter 6, Forests, Key Messages 1 and 2, Chapter 19, Southeast, Key Message 3, Chapter 21, Midwest, Key Message 2 Increases in wildfire smoke events, driven by climate change, are expected to reduce the amount and quality of time spent in outdoor activities. Chapter 13, Air Quality, Key Message 2, Chapter 24, Northwest, Key Message 4 Projected declines in snowpack in the western United States and shifts to more precipitation falling as rain than snow, and the cold season in many parts of the Central and Eastern United States are expected to adversely impact the winter recreation industry. Chapter 18, Northeast, Key Message 1, Chapter 22, Northern Great Plains, Key Message 3, Chapter 24, Northwest, Key Message 1, Box 24.7 In the Northeast, activities that rely on natural snow and ice cover may not be economically viable by the end of the century without significant reductions in global greenhouse gas emissions. Chapter 18, Northeast, Key Message 1, Diminished snowpack, increased wildfire, pervasive drought, flooding, ocean acidification, and sea level rise directly threatened the viability of agriculture, fisheries, and forestry enterprises on tribal lands across the United States and impact tribal tourism and recreation sectors. Chapter 15, Tribes, Key Message 1 Climate change has already had observable impacts on biodiversity and ecosystems throughout the United States that are expected to continue. Many species are shifting their ranges, figure 1.2H, and changes in the timing of important biological events, such as migration and reproduction, are occurring in response to climate change, Chapter 7, Ecosystems, Key Message 1. Climate change is also aiding the spread of invasive species, Chapter 21, Midwest, Case Study, Adaptation, and Forestry, Chapter 22, Northern Great Plains, Case Study, Crow Nation, and the spread of invasive species. Recognized as a major driver of biodiversity loss and substantial ecological and economic costs globally, Chapter 7, Ecosystems, Invasive Species. As environmental conditions change further, mismatches between species and the availability of the resources they need to survive are expected to occur, Chapter 7, Ecosystems, Key Message 2. Without significant reductions in global greenhouse gas emissions, extinctions and transformative impacts on some ecosystems cannot be avoided in the long term. Chapter 9, Oceans, Key Message 1 While some new opportunities may emerge from ecosystem changes, economic and recreational opportunities and cultural heritage based around historical use of species or natural resources in many areas are at risk. Chapter 7, Ecosystems, Key Message 3. Chapter 18, Northeast, Key Messages 1 and 2, Box 18.6 Ocean warming and acidification pose high and growing risks for many marine organisms and the impacts of climate change on ocean ecosystems are expected to lead to reductions in important ecosystem services such as aquaculture, fishery productivity, and recreational opportunities. Chapter 9, Oceans, Key Message 2 While climate change impacts on ocean ecosystems are widespread, the scope of ecosystem impacts occurring in tropical and polar areas is greater than anywhere else in the world. Ocean warming is already leading to reductions in vulnerable coral reef and sea ice habitats that support the livelihoods of many communities. Chapter 9, Oceans, Key Message 1 Decreasing sea ice extent in the Arctic represents a direct loss of important habitat for marine mammals causing declines in their populations. Figure 1.2f Chapter 26, Alaska, Box 26.1 Changes in spring ice melt have affected the ability of coastal communities in Alaska to meet their walrus harvest needs in recent years. Chapter 26, Alaska, Key Message 1 These changes are expected to continue as sea ice declines further. Chapter 2, Climate, Key Message 7 In the tropics, ocean warming has already led to widespread coral reef bleaching and or outbreaks of coral diseases off the coastlines of Puerto Rico, the U.S. Virgin Islands, Florida, and Hawaii and the U.S. affiliated Pacific Islands. Chapter 20, U.S. Caribbean, Key Message 2 Chapter 27, Hawaii and Pacific Islands, Key Message 4 By mid-century, widespread coral bleaching is projected to occur annually in Hawaii and the U.S. affiliated Pacific Islands. Figure 1.14 Bleaching and ocean acidification are expected to result in loss of reef structure leading to lower fishery shields and loss of coastal protection and habitat with impacts on tourism and livelihoods in both regions. Chapter 20, U.S. Caribbean, Key Message 2 Chapter 27, Hawaii and Pacific Islands, Key Message 4 While some targeted response actions are underway, Figure 1.15, many impacts, including losses of unique coral reef and sea ice ecosystems, can only be avoided by significantly reducing global greenhouse gas emissions, particularly carbon dioxide. Chapter 9, Oceans, Key Message 1 Human Health and Well-Being Higher temperatures, increasing air quality risks, more frequent and intense extreme weather and climate-related events, increases in coastal flooding, disruption of ecosystem services, and other changes increasingly threaten the health and well-being of the American people, particularly populations that are already vulnerable. Future climate change is expected to further disrupt many areas of life, exacerbating existing challenges and revealing new risks to health and prosperity. Rising temperatures pose a number of threats to human health and quality of life, Figure 1.16. Higher temperatures in the summer are linked directly to an increased risk of illness and death, particularly among older adults, pregnant women and children. Chapter 18, Northeast, Box 18.3 With continued warming, cold-related deaths are projected to decrease and heat-related deaths are projected to increase. In most regions, the increases in heat-related deaths are expected to outpace the reductions in cold-related deaths. Chapter 14, Human Health, Key Message 1 Rising temperatures are expected to reduce electricity generation capacity while increasing energy demands and costs, which can in turn lead to power outages and blackouts. Chapter 4, Energy, Key Message 1.11, Urban, Regional Summary, Figure 11.2 These changes strain household budgets, increase people's exposure to heat, and limit delivery of medical and social services. Risks from heat stress are higher for people without access to housing with sufficient insulation or air conditioning. Chapter 11, Urban, Key Message 1 Changes in temperature and precipitation can increase air quality risks from wildfire and ground-level ozone, smog. Projected increases in wildfire activity due to climate change would further degrade air quality, resulting in increased health risks and impacts on quality of life. Chapter 13, Air Quality, Key Message 2, Chapter 14, Human Health, Key Message 1 Unless counteracting efforts to improve air quality are implemented, climate change is expected to worsen ozone pollution across much of the country with adverse impacts on human health. Figure 1.21, Chapter 13, Air Quality, Key Message 1 Earlier spring arrival, warmer temperatures, changes in precipitation, and higher carbon dioxide concentrations can also increase exposure to airborne pollen allergens. The frequency and severity of allergic illnesses, including asthma and hay fever, are expected to increase as a result of a changing climate. Chapter 13, Air Quality, Key Message 3 Rising air and water temperatures and changes in extreme weather and climate-related events are expected to increase exposure to waterborne and foodborne diseases, affecting food and water safety. The geographic range and distribution of disease-carrying insects and pests are projected to shift as climate changes, which could expose more people in North America to ticks that carry Lyme disease and mosquitoes that transmit viruses such as West Nile, Chikungangya, Dengue, and Zika. Chapter 14, Human Health, Key Message 1 Chapter 16, International, Key Message 4 Mental health consequences can result from exposure to climate or extreme weather-related events, some of which are projected to intensify as warming continues. Chapter 14, Human Health, Key Message 1 Coastal city flooding, as a result of sea-level rise in hurricanes, for example, can result in forced evacuation with adverse effects on family and community stability, as well as mental and physical health. Chapter 11, Urban, Key Message 1 In urban areas, disruptions in food supply or safety related to extreme weather or climate-related events are expected to disproportionately impact those who already experience food insecurity. Chapter 11, Urban, Key Message 3 Indigenous peoples have historical and cultural relationships with ancestral lands, ecosystems, and culturally important species that are threatened by climate change. Chapter 15, Tribes, Key Message 1 Chapter 19, Southeast, Key Message 4 Case Study, Mountain Ramps Chapter 24, Northwest, Key Message 5 Climate change is expected to compound existing physical health issues in Indigenous communities, in part due to the loss of traditional foods and practices, and in some cases, the mental stress from permanent community displacement. Chapter 14, Human Health, Key Message 2 Chapter 15, Tribes, Key Message 2 Throughout the United States, Indigenous peoples are considering or actively pursuing relocation as an adaptation strategy in response to climate-related disasters, more frequent flooding, loss of land due to erosion, or as livelihoods are compromised by ecosystem shifts, linked to climate change. Chapter 15, Tribes, Key Message 3 In Louisiana, a federal grant is being used to relocate the tribal community of Isle-de-Jean Charles in response to severe land loss, sea-level rise, and coastal flooding. Figure 1.17 Chapter 19, Southeast, Key Message 2 Case Study, A Lesson Learned for Community Resettlement In Alaska, coastal native communities are already experiencing heightened erosion, driven by declining sea ice, rising sea levels, and warmer waters. Figure 1.18 Coastal and river erosion and flooding in some cases will require parts of communities, or even entire communities, to relocate to safer terrain. Chapter 26, Alaska, Key Message 2 Combined with other stressors, sea-level rise, coastal storms, and the deterioration of coral reef and mangrove ecosystems put the long-term habitability of coral atolls in the Hawaii and US-affiliated Pacific Islands region at risk, introducing issues of sovereignty, human and national security, and equity. Chapter 27, Hawaii and Pacific Islands, Key Message 6 Box 1.3 Interconnected Impacts of Climate Change The impacts of climate change and extreme weather on natural and built systems are often considered from the perspective of individual sectors. How does a changing climate impact water resources, the electric grid, or the food system? None of these sectors, however, exists in isolation. The natural, built, and social systems we rely on are all interconnected, and impacts and management choices within one sector may have cascading effects on the others. Chapter 17, Complex Systems, Key Message 1 For example, wildfire trends in the western United States are influenced by rising temperatures and changing precipitation patterns, pest populations, and land management practices. As humans have moved closer to forest lands, increased fire suppression practices have reduced natural fires and led to denser vegetation, resulting in fires that are larger and more damaging when they do occur. Figures 1.5 and 1.2k. Chapter 6, Forests, Key Message 1 Warmer winters have led to increased pest outbreaks and significant tree kills, with varying feedbacks on wildfire. Increased wildfire driven by climate change is projected to increase costs associated with health effects, loss of homes, and other property, wildfire response, and fuel management. Failure to anticipate these interconnected impacts can lead to missed opportunities for effectively managing risks within a single sector and may actually increase risks to other sectors. Planning around wildfire risk and other risks affected by climate change entails the challenge of accounting for all of these influences and how they interact with one another. See Chapter 17, Complex Systems, Box 17.4 New to this edition of the NCA, Chapter 17, Complex Systems, highlights several examples of interconnected impacts and documents how a multi-sector perspective and joint management of systems can enhance resilience to a changing climate. It is often difficult or impossible to quantify and predict how all relevant processes and interactions in interconnected systems will respond to climate change. Non-climate influences, such as population changes, add to the challenges of projecting future outcomes. Chapter 17, Complex Systems, Key Message 2 Despite these challenges, there are opportunities to learn from experience to guide future risk management decisions. Valuable lessons can be learned retrospectively. After Superstorm Sandy in 2012, for example, the mayor of New York City initiated a climate change adaptation task force that brought together stakeholders from several sectors such as water, transportation, energy, and communications to address the interdependencies among them. Chapter 17, Complex Systems, Box 17.1, Key Message 3 End of Box 1.3 Box 1.4, How Climate Change Around the World Affects the United States The impacts of changing weather and climate patterns beyond U.S. international borders affect those living in the United States, often in complex ways that can generate both challenges and opportunities. The International Chapter, Chapter 16, new to this edition of the NCA, assesses our current understanding of how global climate change, natural variability, and associated extremes are expected to impact, and in some cases are already impacting U.S. interests both within and outside of our borders. Current and projected climate-related impacts on our economy include increased risks to overseas operations of U.S. businesses, disruption of international supply chains, and shifts in the availability and prices of commodities. For example, severe flooding in Thailand in 2011 disrupted the supply chains for U.S. electronics manufacturers, Chapter 16, International, Figure 16.1. U.S. firms are increasingly responding to climate-related risks, including through their financial disclosures and partnerships with environmental groups, Chapter 16, International, Key Message 1. Impacts from climate-related events can also undermine U.S. investments in international development by slowing or reversing social and economic progress in developing countries, weakening foreign markets for U.S. exports, and increasing the need for humanitarian assistance and disaster relief efforts. Predictive tools can help vulnerable countries anticipate natural disasters, such as drought, and manage their impacts. For example, the United States and international partners created the Famine Early Warning Systems Network, FEWSNet, which helped avoid severe food shortages in Ethiopia during a historic drought in 2015. Chapter 16, International, Key Message 2. Natural variability and changes in climate increase risks to our national security by affecting factors that can exacerbate conflict and displacement outside of U.S. borders, such as food and water insecurity and commodity price shocks. More directly, our national security is impacted by damage to U.S. military assets such as roads, runways, and waterfront infrastructure from extreme weather and climate-related events, figures 1.8 and 1.9. The U.S. military is working to both fully understand these threats and incorporate projected climate changes into long-term planning. For example, the Department of Defense has performed a comprehensive, scenario-driven examination of climate risks from sea level rise to all of its coastal military sites, including atolls in the Pacific Ocean. Chapter 16, International, Key Message 3. Finally, the impacts of climate change are already affecting the ecosystems that span our nation's borders and the communities that rely on them. International frameworks for the management of our shared resources continue to be restructured to incorporate risks from these impacts. For example, a joint commission that implements water treaties between the United States and Mexico is exploring adaptive water management strategies that account for the effects of climate change and natural variability on Colorado River water. Chapter 16, International, Key Message 4. End of BOX 1.4 End of Section 2.