FOURTH NATIONAL CLIMATE ASSESSMENT
EXECUTIVE SUMMARY: REPORT FINDINGS

Communities

Climate change exacerbates existing vulnerabilities in communities across the United States, jeopardizing people, economic growth, and quality of life and creates new risks that are projected to intensify without adaptation and mitigation.

Weather and climate extremes, expected to intensify under a changing climate, are causing cascading disruptions and damages in interdependent networks of infrastructure, ecosystems, and social systems that provide essential goods and services to society. Future climate change will further disrupt many areas of life, exacerbating existing challenges to prosperity posed by aging infrastructure, stressed ecosystems, and social inequality. Risks posed by sea level rise, more intense weather and climate extremes, heightened exposure to climate-related health hazards, and disruptions in livelihoods and ecosystem services will continue to grow without adaptation and mitigation.

Economy

Losses to infrastructure, property, and productivity driven by the impacts of climate change are expected to increasingly disrupt the U.S. economy, even with mitigation and adaptation efforts.

Critical systems that support economic activity are increasingly disrupted by the impacts of climate change and extreme weather events, many of which are expected to continue growing in frequency and intensity over the coming century. U.S. business operations overseas and U.S. competitiveness will increasingly be affected by weather and climate extremes. Industries that depend on natural resources and favorable climate conditions, such as agriculture, tourism, and fisheries, are increasingly vulnerable to the impacts of climate change. While a few aspects of our economy may see slight improvements in a warmer world, without efforts to reduce carbon emissions and adapt to climate impacts, climate change is projected to cause substantial damage to the U.S. economy, with the potential for annual losses in some sectors reaching hundreds of billions of dollars by the end of the century.

Water

The quality and quantity of water available for use by humans and ecosystems across the country is being affected by the impacts of climate change, presenting an increasing risk to people and the environment.

The interaction of climate change, climate variability, and trends in water and land usage is leading to imbalances in the timing of water supply and demand, increased risk of drought, and declines in surface water quality, posing challenges to water managers across the country. Extreme precipitation events are anticipated to increase in a warming climate, leading to more severe floods and a higher risk of infrastructure failure. Aging water infrastructure, typically designed for past weather and climate conditions, compounds the climate risk faced by society; water sector adaptation to a changing climate requires strategies that are robust to a wide range of climate futures.

Health

Climate change-driven impacts on extreme weather, air quality, and the transmission of disease through insects, food, and water increasingly threaten the health and well-being of the American people, particularly populations that are already vulnerable.

Changes in temperature and precipitation driven by climate change increase air quality risks from wildfire and surface ozone pollution and heighten exposure to allergens. Rising air and water temperatures and more intense extreme events are expected to increase exposure to waterborne and foodborne diseases, affecting food and water safety. Climate change is expected to alter the geographic range and distribution of disease-carrying insects and pests, exposing more people to ticks that carry Lyme disease and mosquitoes that carry diseases such as Zika, West Nile virus, and dengue. Older adults, children, low-income communities, and communities of color are disproportionately affected by, and less resilient to, the health impacts of climate change. Adaptation policies and programs that help individuals, communities, and states prepare for and manage a changing climate reduce the number of injuries, illnesses, and deaths from climate-related health outcomes.

Infrastructure

Our Nation’s aging infrastructure is further stressed by increases in heavy precipitation, coastal flooding, heat, and other extreme events as well as changes to average precipitation and temperature. Without adaptation, climate change will continue to degrade infrastructure performance over the rest of the century, with the potential for cascading impacts that threaten our economy, built environment, and health and well-being.

Climate change and extreme weather events increasingly disrupt our Nation’s energy and transportation systems, threatening more frequent and longer-lasting power outages, fuel shortages, and service disruptions with cascading impacts on other critical sectors. The continued increase in the frequency and extent of tidal flooding due to sea level rise threatens America’s trillion-dollar coastal property market and public infrastructure, with cascading impacts to the larger economy. In Alaska, rising temperatures and erosion are causing damage to infrastructure that will be costly to repair or replace, particularly in rural areas. Infrastructure design, planning, and operational measures and standards can reduce exposure and vulnerability to the impacts of climate change while providing additional near-term benefits.

Ecosystems and Ecosystem Services

Ecosystems and the benefits they provide to society are being altered by climate change; these impacts are projected to continue. Without significant reductions in carbon emissions, transformative impacts on some ecosystems will occur.

Climate change has already had observable impacts on biodiversity and ecosystems throughout the United States, including changes in the characteristics of marine and terrestrial species that affect human interactions with them and the benefits they provide to society. Where changes occur too quickly for species to adapt, local extinctions can occur; without significant reductions in carbon emissions, extinctions and transformative impacts on some ecosystems cannot be avoided. 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 we do today. Adaptation strategies that are flexible and coordinated at landscape-scales are being implemented to address emerging impacts of climate change on valued natural resources.

Agriculture

Rising temperatures, extreme heat, drought, and heavy rainfall increasingly disrupt American agriculture, reducing crop yields and quality, increasing crop and livestock exposure to pests, and threatening the economy.

Climate change presents numerous challenges to crop productivity, livestock health, and the health of rural communities. Flooding caused by heavy rainfall, projected to intensify in a changing climate, can erode soils, pollute water supplies, and damage crops, pastures, and rural infrastructure. Increases in temperatures, drought, and heavy rainfall are expected to disrupt crop growth, reduce the quality of crops and grazing land, and increase disease and pest infestations in crops and livestock. While some regions may see conditions conducive to expanded (or alternative crop) productivity, overall, yields from major U.S. crops are expected to decline as a consequence of increases in temperatures, and possibly changes in water availability, diseases, and pest infestations. These changes threaten commodity grain production and put the economies of agricultural regions at risk. However, there are numerous adaptation strategies available to cope with adverse impacts of climate variability and change, requiring varying levels of investment.

Tourism and Recreation

Many essential services provided by ecosystems and the environment that support tourist economies, outdoor recreation, and quality of life will be degraded by the impacts of climate change.

Climate change poses risks to seasonal or outdoor economies in many communities across the United States. In turn, this affects the health and well-being of the people who make their living supporting these economies, including rural, coastal, and indigenous communities. Changes affecting water, land, and other natural resources (such as salmon and timber), as well as infrastructure and related services are projected to continue. These impacts are expected to result in decreased tourism revenue in some places and, for some communities, loss of identity. While some new opportunities may emerge from these ecosystem changes, cultural heritage and economic and recreational opportunities based around historical use of species or natural resources in many areas are at risk.

Oceans and Coasts

Coastal communities and the ecosystems that support them are increasingly threatened by climate-driven impacts; without significant reductions in carbon emissions and adaptation measures, many communities will be transformed by the latter part of this century, with cascading impacts to the larger economy.

Climate change-related impacts such as rising water temperatures, ocean acidification, sea level rise, tidal flooding, coastal erosion, and storm surge threaten our oceans and coasts. These effects are projected to continue even under a low emissions scenario; putting ocean and marine species at risk; decreasing the productivity of fisheries; and threatening the communities that rely on fisheries for livelihoods, tourism, and recreation. Lasting damage to coastal property and infrastructure driven by tidal flooding and sea level rise is expected to lead to personal financial loss and higher insurance premiums. Actions to plan for and adapt to more frequent and severe coastal flooding may decrease direct losses and cascading economic impacts.

Indigenous Peoples

Climate change increasingly threatens tribal and Indigenous communities’ livelihoods, economies, health, and cultural identities through disruption of interconnected social, physical, and ecological systems.

Climate change disrupts agriculture, fishing, forestry, recreation, and tourism; the tribal and Indigenous communities that rely on these resources for their economic, social, and physical well-being will face a number of challenges as temperatures rise and ecosystems are affected. While economic, political, and infrastructure limitations may affect these communities’ ability to adapt, tightly-knit social and cultural networks present opportunities to build community capacity and increase resilience.

Interconnected Impacts

As climate risks intensify, the interdependent systems on which we rely are vulnerable to cascading impacts across sectors, threatening essential services and sectors within and beyond the Nation’s borders.

Climate change is an added stressor in the complex web of natural, built, and social systems that supports our economy, health, and well-being. This presents a number of additional risks, some not immediately apparent due to the interconnected nature of these systems. Events that lead to disruption and damage can result in more frequent and longer-lasting disruptions, which lead to cascading impacts on a number of other critical sectors, including international trade and national security. Efforts to address these interconnected vulnerabilities can help communities and sectors of the American economy increase their resilience.

Adaptation and Mitigation

Communities and businesses are working to reduce climate change-related risks and their associated costs through the adoption of proactive management and adaptation strategies that are robust to a wide range of climate futures.

Effective responses to climate-change impacts and associated risks involve planning for an uncertain future. Many communities are undertaking activities to address climate-related challenges, and this can result in increased benefits for other sectors. In the absence of intervention, climate change is projected to result in increasing vulnerability for communities and businesses across the United States.

Introduction

Earth’s climate is now changing faster than at any point in human history. The impacts of this global climate change are underway in the United States, and Americans are responding to rapid changes affecting their everyday lives and livelihoods. While reservoir managers in the Colorado River Basin adjust to lower water levels, cities along the Gulf and Atlantic Coasts are adapting to more frequent flooding and more powerful storm surge driven in part by rising seas and heavier rainfall. As farmers in the Midwest adopt new crop management strategies, communities in the western U.S. are responding to rising forest fire damage driven by the interaction among land management practices, higher temperatures, severe drought, and insect outbreaks. Alaskans are coping with infrastructure damaged by thawing ground and heightened coastal erosion, and fishermen in the Northeast are adjusting to more frequent ocean heat waves that harm valuable fisheries. Climate change poses significant risks to our everyday lives, but Americans are responding to change in ways that can reduce climate-related risks, bolster resilience to change, and improve livelihoods.

Climate shapes where and how we live and the environment around us. Natural ecosystems, agriculture, water resources, and the services they provide to society are adapted to past climate conditions and their range of variability. For thousands of years, humanity has benefited from a relatively stable climate, and society has developed under the assumption that past climate is a reliable predictor of the future. A water manager may use past or current streamflow records to design a dam, a city could issue permits for coastal development based on current flood maps, and a farmer may invest in equipment to grow a particular crop, all with the expectation that current and future climate conditions will resemble that of the recent past and that their investments will meet future needs. Even within this natural range, climate variability imposes significant costs: agricultural losses from drought, infrastructure damage from flooding, and loss of life during extreme heat events, among others.

The assumption that current and future climate conditions will be similar to the historical record is no longer reliably true. Observations collected around the world—of elevated atmospheric concentrations of greenhouse gases, rising ocean and air temperatures, melting glaciers, shrinking snow and sea ice cover, rising sea levels, increases in high temperature extremes and heavy precipitation, changes in the ranges of plant and animal species, and longer growing seasons—provide clear evidence of a rapid warming trend that is pushing the climate system beyond the range of natural variability that modern civilization has experienced. These observations, along with an understanding of patterns of natural variability and other external forces that influence climate, allow scientists to identify the human fingerprint on climate. The long-term warming trend observed over the past century can only be explained by the effect that human activities, especially emissions of greenhouse gases from burning fossil fuels and clearing forests, have had on the climate.

This Fourth National Climate Assessment draws a direct connection between the warming atmosphere and 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. 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 these trends are projected to continue in the absence of actions to reduce greenhouse gas emissions.

Climate change puts many things Americans care about at risk, both now and in the future, and risks will intensify without action. Many options are available to reduce risks, and choices made today will determine the magnitude of future risks. Some changes will still occur even if all greenhouse gas emissions stopped today, and some of those changes will be irreversible. Some changes may be beneficial in the short term, but over the long term, the benefits of a changing climate are expected to be greatly outweighed by adverse impacts. While the American economy has continued to grow and some measures of human well-being have improved over the past several decades, many communities, ecosystems, and economic sectors have already experienced negative impacts and they remain at great risk as warming trends continue. Many of these economic gains are expected to be surpassed by cumulative losses by the end of the century without adequate response measures.

Climate change exacerbates challenges that Americans already face and introduces new risks. Risks range from the inconvenient, such as increasing high tide flooding along the East Coast related to sea level rise, to the existential, such as the forced relocation of coastal communities in Alaska and along the Gulf Coast. Risks posed by climate variability and change interact with existing stressors, such as economic inequality, unequal access to resources, and unequal distribution of environmental risks. Social and economic factors often shape exposure to climate-related risks and a community’s capacity to adapt to them, and very often, risks are highest for populations that are already vulnerable—the poor, the sick, the very young and the very old, some communities of color, and others—and have lower capacity to adapt to change. Climate change threatens to exacerbate these existing inequalities that result in higher exposure and sensitivity to extreme events and other changes. Some actions to address the underlying causes of climate change, if not implemented under equitable policy considerations, may also weigh heavily on marginalized populations. Measures underway to reduce the risks from climate change vary across the country and face different challenges in urban, tribal, coastal, rural, and many other communities (Ch. 28: Mitigation, Ch. 29: Adaptation).

Global climate change is projected to continue throughout this century and beyond under all greenhouse gas emissions scenarios, but actions taken now can reduce future risks while offering other benefits in the near-term. There is a strong and growing economic case for reducing the risks of climate change by cutting greenhouse gas emissions and taking steps to adapt to changes that are unavoidable as a result of past and present emissions.

This report documents the changes already observed and those projected for the future. It evaluates the risks that can be avoided or mitigated by actions to reduce atmospheric concentrations of greenhouse gases and actions to adapt to the impacts of climate change. Since the release of the Third National Climate Assessment (NCA3) in 2014, the scale and scope of adaptation implementation in the United States and worldwide has grown significantly. In some parts of the United States, extreme events influenced by climate change (such as hurricanes, floods, and wildfires), or more gradual changes (such as increases in high tide flooding related to sea level rise), have driven action to adapt and improve resilience to present and future changes. However, consideration of climate risk is not yet standard practice in decision-making, and many opportunities to reduce risks remains. This report provides examples of actions underway in communities across the United States to reduce the risks associated with climate change, increase resilience, and improve livelihoods.

1.2 Global Climate Change: Observations and Attribution

Global annual average temperatures have increased by about 1.5°F since the beginning of the 20th century (see “Weather and Climate” section below and Ch. 2: Our Changing Climate). The global long-term warming trend has been accentuated in recent years with record-breaking annual average global surface temperatures and continued sharp declines in sea ice cover in the Arctic Ocean. Since the publication of NCA3 in 2014, that year became the warmest on record globally; 2015 surpassed 2014 by a wide margin; and 2016 became the third consecutive year to set a new record. Sixteen of the last 17 years are the warmest years on record for the globe. Many lines of evidence demonstrate that human activities are the primary reason for this increase.

Scientists’ understanding of current and future climate change has been shaped by many decades of research into the causes of past changes in climate, the buildup of greenhouse gases in the atmosphere from human activities, and the linked changes observed in the oceans, atmosphere, snow and ice, land, and ecosystems.

Scientists have long understood the role of naturally occurring greenhouse gases in Earth’s climate. By trapping heat, these gases warm the planet’s surface enough to create a suitable climate for human habitation: this is known as the greenhouse effect. Since the late 19th century, however, humans have released an increasing amount of greenhouse gases into the atmosphere, primarily through the burning of fossil fuels, the clearing of forests, and other widespread changes in land use. The concentration of carbon dioxide in the atmosphere has increased by 40% over the industrial era (Figure 1.1a). This increase has intensified the natural greenhouse effect, driving a rapid increase in global surface temperatures and other widespread changes in the Earth’s climate system. Observations collected around the world provide consistent evidence of a warming planet, including reductions in both sea and land ice, longer growing seasons, warming and rising seas, and increases in heavy precipitation across most of the United States (Figure 1.1).

Natural factors also influence climate, but natural factors alone cannot explain the observed rapid change in global temperatures and other climate variables. Natural climate cycles such as the El Niño Southern–Oscillation can affect temperatures and precipitation patterns in the short term, especially regionally, and year-to-year variability in climate is expected even as global temperatures rise steadily over longer time periods. Other factors influence climate, as well. For example, over timescales of many thousands of years, periodic shifts in Earth's orbit have a significant effect on global climate. Over timescales of a few years, the release of small particles from volcanic eruptions can have a temporary cooling effect.

Scientists use many sources of evidence to understand the drivers of climate change, including long-term observations collected around the world and computer models of the climate system. Climate model simulations estimating the influence of both natural and human factors on the climate can be compared with observed climate over time, allowing scientists to explore the sources of change (Figure 1.2). No combination of natural factors is found in the observational record that would account for the current warming trend, and without human activities, the influence of natural factors alone would actually have had a slight cooling effect on global climate over the last fifty years (Ch. 2: Our Changing Climate).

Sophisticated computer models of the Earth’s climate system allow scientists to explore the effects of both natural and human factors. In all three panels of this figure, the black line shows the observed annual average global surface temperature for 1880–2016 as a difference from the average value for 1880–1910.

The top panel (a) shows the temperature changes simulated by a climate model when only natural factors (dark red line) are considered. The other lines show the individual contributions to the overall effect from observed changes in Earth’s orbit (brown line), the amount of incoming energy from the sun (purple line), and changes in emissions from volcanic eruptions (green line). Note that no long-term trend in globally-averaged surface temperature would be expected from natural factors alone.

The middle panel (b) shows the simulated changes in global temperature when considering only human influences (dark red line), including the contributions from emissions of greenhouse gases (purple line) and small particles (referred to as aerosols, brown line) as well as changes in ozone levels (orange line) and changes in land cover, including deforestation (green line). Changes in aerosols and land cover have had a net cooling effect in recent decades, while changes in near-surface ozone levels have had a small warming effect. These smaller effects are dominated by the large warming influence of greenhouse gases such as carbon dioxide and methane. Note that the net effect of human factors (dark red line) explains most of the long-term warming trend.

The bottom panel (c) shows the temperature change (dark red line) simulated by a climate model when both human and natural influences are included. The result matches the observed temperature record closely, particularly since 1950, making the dominant role of human drivers plainly visible.

Researchers do not expect climate models to exactly reproduce the specific timing of actual weather events or short-term climate variations, but they do expect the models to capture how the whole climate system behaves over long periods of time. The simulated temperature lines represent the average values from a large number of simulation runs. The red hatching represents uncertainty bands based on those simulations. For any given year, 95% of the simulations will lie inside the red bands.

Source: NASA GISS. Data from NASA Goddard Institute for Space Studies (GISS) ModelE2; data and experimental design from Miller et al. 2014 and Marvel et al. 2016.

1.3 Climate Change Impacts in the United States: Current Risks

Global climate change affects all life on Earth. Warmer surface temperatures trigger changes throughout the climate system that impact water resources, air quality, human health, agriculture, natural ecosystems, infrastructure, and many other systems that support society and the economy. Some of these impacts, such as increasing health risks from extreme heat, are common to many regions of the United States (Ch. 7: Human Health). Others, such as damage to infrastructure caused by thawing of long-frozen ground in Alaska, represent more localized risks (Ch. 26: Alaska). Many impacts are interconnected and can cascade across other sectors and regions, creating complex risks and management challenges, such as changes in heavy rainfall events and resulting flooding that can disrupt the flow of goods and services within or across regions. Many observed impacts reveal vulnerabilities in these interconnected systems that are expected to be exacerbated as climate-related risks intensify (Ch. 17: Complex Systems).

Weather and Climate

Annual average temperature in the continental United States has increased by 1.2°F over the last few decades and by 1.7°F since the beginning of the 20th century (Figure 1.1b), and the past decade was the warmest on record for the Nation. Alaska, the Northwest, the Southwest, and the Northern Great Plains saw the largest increases in average temperature, while the Southeast experienced the smallest increase. The Dust Bowl era of the 1930s remains the peak period for extreme heat since 1900, but heat waves have become more frequent since the 1960s, while extreme cold waves are less frequent and less intense. The length of the growing season (the number of days between the last spring frost and the first autumn frost each year) has increased in all regions of the United States (Figure 1.1g) (Ch. 2: Our Changing Climate).

Higher temperatures and more frequent and more intense extreme heat events are already affecting water resources, air quality, human health, infrastructure, and natural ecosystems and the services they provide to society. High temperatures in the summer are conclusively linked to an increased risk of illness and death, particularly among older adults, pregnant women, and children, with growing evidence of elevated risks for a range of illnesses. Climate change is increasing the risk of adverse respiratory and cardiovascular effects, including premature death, due in part to higher concentrations of air pollutants in many parts of the United States (Ch. 14: Human Health). Warmer springs, longer summer dry seasons, and drier soils and vegetation have already lengthened the wildfire season. In the western United States, more frequent wildfires, driven in part by drought and higher temperatures, are increasing air quality risks (Ch. 13: Air Quality). In Alaska, climate-driven increases in air pollution are linked to the increases in wildfire frequency and intensity (Ch. 26: Alaska). Longer growing seasons are increasing exposure to allergens. Warmer temperatures are altering the geographic range and distribution of disease-carrying insects and pests, exposing more people to ticks that carry Lyme disease and mosquitoes that carry diseases such as Zika, West Nile virus, and dengue (Ch. 14: Human Health).

Annual average surface temperatures in Alaska and the Arctic have increased more than twice as fast as the global average over the last 50 years, accompanied by rapid rates of warming and thawing of frozen ground known as permafrost that underlies about half of Alaska. While increasing winter temperatures are reducing heating costs throughout the state, changes in permafrost present growing risks. Construction in the Arctic depends on permafrost integrity, and subsidence (sinking of the ground) due to permafrost thaw is already causing damage to roads, buildings, and other infrastructure that will be costly to replace, especially in remote parts of Alaska. Permafrost also supports food storage in traditional underground ice cellars used by native communities. With warming climate conditions, many of these ice cellars are beginning to thaw, increasing the risks for food-borne illness, food spoilage, and even injury from structural failure (Ch. 26: Alaska).

Climate change has already had observable impacts on biodiversity and ecosystems throughout the United States, including changes in the characteristics of species that affect how humans interact with them and the benefits they provide to society. Climate change is producing large-scale shifts in the distribution and abundance of species and is altering ecosystems on land and in the oceans. Many species are shifting their ranges in response to climate change, and changes in the timing of important biological events are occurring. Climate change is also aiding the spread of invasive species, which is recognized as a major driver of biodiversity loss and produces substantial ecological and economic costs globally (Ch. 7: Ecosystems).

Snow and Ice

Alaska and the Arctic are undergoing particularly dramatic change. Rising temperatures are not only reducing ice volume and surface extent on land, lakes, and in the sea but also the annual duration over which ice is present in those areas. Since the early 1980s, the annual minimum sea ice extent (observed in September each year) in the Arctic Ocean has declined between 10.7% and 15.9% per decade (Figure 1.1e) (Ch. 2: Our Changing Climate). These changes affect both regional and global climate, key marine ecosystems and species, and the communities that depend on them.

Decreasing sea ice extent in the Arctic represents a direct loss of important habitat for animals like polar bears and ringed seals that use ice for hunting, shelter, migration, and reproduction, causing declines in their populations. Changes in spring ice melt have affected the ability of Alaskan coastal communities to meet their walrus harvest needs, resulting in low harvest levels in several recent years.

Reduced sea ice cover and thickness also increases coastal exposure to waves and storms. Coastal communities in Alaska are already experiencing heightened erosion driven by greater exposure to storm activity, rising sea levels, and warmer waters. The number of coastal erosion events has increased significantly as protective sea ice is no longer present during the fall months. Rates of erosion vary throughout the state, with the highest rates measured on the arctic coastline at more than 59 feet per year (Ch. 26: Alaska).

Warmer temperatures also impact snow and ice cover on land. Increasing air temperatures have substantially affected how much winter precipitation falls as snow versus rain, particularly over the western United States. Warming has resulted in a shift in the timing of snowmelt runoff to earlier in the year, presenting a risk to water supplies for use by humans and ecosystems. Declining snowpack can also threaten hydropower generation. For example, severe extended drought caused California’s hydropower output to decline 59% in 2015 compared to the average annual production over the two prior decades (Ch. 4: Energy).

Land and Water

A warmer atmosphere can hold more moisture, resulting in changes in the global water cycle that affect the quality and quantity of water available for use by people and ecosystems. Consequences include longer and more frequent droughts and less frequent but more severe precipitation events and storm activity. Global climate change is already increasing the frequency and intensity of some types of extreme weather events. In the United States, the frequency and intensity of heavy precipitation has increased since 1901, with the largest increases in the Northeast and Midwest (Figure 1.1c). Recent droughts and associated heat waves have reached record intensity in some regions of the United States, particularly the West. As yet, there is no detectable change in long-term U.S. drought trends that can be attributed to climate change (Figure 1.1i), but there is strong evidence for human influence on surface soil moisture deficits linked to higher temperatures. These decreases in soil moisture contribute to conditions of agricultural drought that are of concern to food producers (Ch. 2: Our Changing Climate).

The Nation’s land and water resources, and the infrastructure built up around them, are at increasing risk from weather and climate extremes (Ch. 11: Built Environment). Such events are causing cascading disruptions and damages in interdependent networks of infrastructure, ecosystems, and social systems that provide essential goods and services to society. Recent events across the country demonstrate the sensitivity of the built environment and related services to weather and climate-related events. Hurricane Irene (2011) and Superstorm Sandy (2012) highlighted the inadequacy of aging urban infrastructure for managing current and future storm events. In South Carolina in 2015, extreme rainfall (more than 20 inches in three days) caused widespread damage to homes, businesses, public buildings, and infrastructure, and the failure of 49 state-regulated dams. In 2016, extreme rainfall (nearly 20 inches in three days) triggered widespread flooding in Louisiana that damaged at least 60,000 homes and led to 13 deaths. In February 2017, heavy rainfall across northern and central California led to substantial property and infrastructure damage from record flooding, landslides, and erosion (Ch. 3: Water).

The interaction among climate change, climate variability, and trends in water and land usage is also leading to imbalances in the timing of water supply and demand, increased drought risk, and declines in surface water quality, posing challenges to water managers across the country. Higher temperatures result in increased human use of water, particularly through increased water demand for agriculture; in some regions of the United States, water supplies are already stressed by increasing consumption. Drought or heat events, coupled with changes to population, urbanization, and economic development can have significant impacts on the demand for limited energy, water, or food resources, all of which are demanded by the other sectors as well. These effects were evidenced in recent droughts in California (2012–2016), the Southern Great Plains (2011), Texas (2012–2015), and the “snow drought” in Oregon (2015). Damage from the 2012 drought is estimated at $31.8 billion nationwide. Surface water quality is also declining as water temperature increases and more frequent high-intensity rainfall events mobilize pollutants such as excess sediments and nutrients (Ch. 3: Water).

Ecosystem primary production, a measure of how much energy ecosystems produce through photosynthesis, has increased over the 20th century, primarily due to the fertilizing (or “greening”) effect of increasing atmospheric carbon dioxide. Increasing primary production has helped land-based ecosystems absorb one-third of the total carbon emitted by human activities (Ch. 7: Ecosystems).

Increased temperatures and earlier snowmelt have led to an increase in the length of the wildfire season by 80 days in some parts of the western United States, and land management practices over the past century have contributed to high fire risk. Over the past two decades, a warmer and drier climate has increased the area burned across the Nation. Large, intense wildfires in some locations have been difficult to suppress, increasing risks to property and lives. The cost of fire suppression has also increased over time, partially driven by the high cost of protecting property at the wildland–urban interface. During the past 30 years, tree deaths caused by bark beetles in the western United States have exceeded those caused by wildfire. Many bark beetle outbreaks have been associated with drought and elevated temperatures (allowing pests to persist longer) and these outbreaks can negatively affect timber prices and the economic well-being of forest landowners and wood processors (Ch. 6: Forests).

Oceans and Coasts

Coastal communities and the ecosystems that support them are increasingly threatened by climate change related impacts, including rising water temperatures, ocean acidification and deoxygenation, sea level rise, and increased high tide and storm surge flooding, erosion, and saltwater intrusion (Ch. 8: Coastal Effects).

The world’s oceans have absorbed about 93% of the excess heat caused by greenhouse gas warming since the mid-20th century and are currently absorbing more than a quarter of the carbon dioxide emitted to the atmosphere annually by human activities, making them warmer and more acidic, which in turn can harm vulnerable marine ecosystems. (Figure 1.1k and 1.1l). Major oxygen losses have occurred over the last 50 years in inland seas, estuaries, and in the coastal and open oceans, driven by climate change alongside other stressors, including nutrient run-off, with potential detrimental effects on marine ecosystems. Ocean ecosystems are being transformed by the combined stressors of warming waters, ocean acidification, and deoxygenation linked to climate change(Ch. 2: Our Changing Climate; Ch. 9: Oceans & Marine Resources).

The impact of warming on fish stocks is becoming more severe. Recent “marine heatwave” events along the Northeast Coast in 2012 and along the entire West Coast in 2014–2016 produced high ocean temperatures similar to the average conditions expected later this century under future emissions scenarios (Ch. 18: Northeast; Ch. 24: Northwest). Resulting ecosystem changes included the appearance of warm-water species not normally seen in these locations, increased mortality of marine mammals, and an unprecedented harmful algal bloom along the West Coast. These factors combined to produce economic stress in some of the Nation’s most valuable fisheries (Ch. 9: Oceans & Marines Resources). Shellfish populations, an important subsistence and commercial resource along the Alaskan coast, have been declining for more than 20 years throughout coastal Alaska, with ocean warming and acidification being contributing factors to the decline (Ch. 26: Alaska).

Tropical marine ecosystems are among the most vulnerable to the impacts of climate change. Warming has led to mass bleaching and/or outbreaks of coral diseases off the coastlines of Puerto Rico, the U.S. Virgin Islands, Florida, Hawai”˜i and the U.S.-Affiliated Pacific Islands (Ch. 20: U.S. Caribbean; Ch. 27: Hawai”˜i and the U.S.-Affiliated Pacific Islands). Loss of reef-building corals alters the entire reef ecosystem, leading to changes in the communities of fish and invertebrates that inhabit reefs. These changes directly impact coastal communities that depend on reefs for food, income, storm protection, tourism-related revenue and other services (Ch. 9: Oceans and Marines Resources; Ch. 8: Coastal Effects).

Global average sea level has risen by about 7–8 inches since 1900 as oceans have warmed and land ice has melted, with almost half (about 3 inches) of this rise occurring since 1993 (Figure 1.1j) (Ch. 2: Our Changing Climate). Although storms, floods, and erosion have always been hazards, in combination with rising sea levels, they now threaten U.S. coastal real estate worth about $1 trillion as well as the continued viability of coastal communities that depend upon coastal water, land, and other natural resources for economic health and cultural integrity. Sea level rise is contributing to increases in high tide flooding that threaten public infrastructure and property. Annual occurrences of daily high tide flooding exceeding local elevation thresholds for minor impacts to infrastructure have already increased 5- to 10-fold since the 1960s in several U.S. coastal cities, and rates are accelerating in cities along the Atlantic and Gulf Coasts due to rising sea levels. Climate change exacerbates the risks to many coastal zones that are already vulnerable to flooding due to other factors such as subsidence (local land sinking relative to surrounding areas, such as from subsurface extractive practices), the nature of development in the area, and the occurrence of weather-driven inundation events (Ch. 8: Coastal Effects).

1.4 Climate Change Impacts in the United States: Future Risks and Vulnerabilities

The long lifetime of carbon dioxide in the atmosphere and the delay in the response of the climate system to changes in atmospheric greenhouse gas concentrations means that near-term changes in climate are largely already locked into place by past and present emissions. Even if greenhouse gas emissions from human activities were to suddenly drop to zero tomorrow, it is estimated that historical emissions would commit the world to an additional 1.1°F of warming over this century relative to the last few decades. The scale of warming beyond that will mostly be determined by choices that society makes today and over the next few decades and by the extent to which processes in Earth’s climate system amplify human-caused warming. With significant reductions in emissions, global temperature increase relative to preindustrial times could be limited to 3.6°F or less. Without major reductions, the increase in annual average global temperatures could reach 9°F or more by the end of this century.

To evaluate the potential effects of future climate change across the United States, NCA4 relies on “Representative Concentration Pathways” (RCPs) that capture uncertainties in emissions levels throughout the 21st century (see Figure 1.3). These emissions scenarios drive climate model simulations that are used for impacts, adaptation, and vulnerability analyses, allowing an assessment of impacts across regions and sectors, and of costs that can be avoided by response actions, for a range of scenarios and temperature thresholds. This report focuses primarily on two of the RCP scenarios: a “lower” scenario (RCP4.5) representing some emissions reductions and a “higher” scenario (RCP8.5) representing continued high emissions growth. A “very low” scenario (RCP2.6) is used in some select instances (see Figure 1.3 and the “Scenario Products” section of the Front Matter for more detail).

All RCP scenarios project similar temperature and global sea level rise outcomes over the next few decades. For example, annual average temperatures in the continental United States are expected to rise at least 2.5°F over the next couple decades, relative to 1976–2005, no matter which RCP is considered. The effects of potential emissions reductions on global climate become evident around 2050. Around mid-century, temperature, precipitation, and sea level rise outcomes from different scenarios begin to diverge significantly. Much larger rises in global average temperature are expected by late century under both scenarios: by 4.6°–9.3°F under a higher scenario (RCP8.5) or 1.9°–5.0°F under a lower scenario (RCP4.5) for the period 2081–2100 relative to 1986–2005. Under a very low scenario (RCP2.6), global average temperature increases could be limited to 0.6°–3.4°F (Ch. 2: Our Changing Climate).

Weather and Climate

Annual average temperatures are projected to increase across the United States, with increases by late in the 21st century of 2.8°–7.3°F under a lower scenario (RCP4.5) and 5.8°–11.9°F under a higher scenario (RCP8.5) relative to 1976–2005. Extreme high temperatures are projected to increase even more than average temperatures. Cold waves are projected to become less intense, while heat waves are projected to become more frequent and more intense. Recent record-setting hot years are expected to become common in the near future (Ch. 2: Our Changing Climate).

Extreme temperatures pose a significant threat to human health and productivity in the United States (Ch. 29: Mitigation). 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. In a study of 49 large cities in the United States, changes in extreme temperatures (hot and cold) are anticipated to result in more than 9,000 additional premature deaths per year under a higher scenario (RCP8.5) by the end of the century, though this number would be lower if considering acclimatization or other adaptations (for example, use of air conditioning). Under a lower scenario (RCP4.5), more than half these deaths could be avoided each year. The annual economic damages associated with the additional extreme temperature-related deaths were estimated at $140 billion by 2090 under a higher scenario (RCP8.5) and $60 billion under a lower scenario (RCP4.5), indicating a reduction of approximately $80 billion in economic damages under RCP4.5 compared to RCP8.5 from avoided loss of life (Ch. 14: Human Health).

Under a higher scenario (RCP8.5), 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. States within the Southeast (Ch. 19: Southeast) and Southern Great Plains (Ch. 23: Southern Great Plains) regions are projected to experience higher impacts, with labor productivity in high-risk sectors such as agricultural, construction, utilities, and manufacturing projected to decline by 3%. Some counties in Texas and Florida are projected to experience more than 6% losses in annual labor hours by the end of the century (Ch. 14: Human Health).

More frequent and severe heat waves in many parts of the United States will also increase stresses on electric power, increasing the risk of cascading failures within the electric power network that could propagate into other critical infrastructure such as telecommunications, information technology infrastructure, transportation systems, and water and wastewater treatment (Ch. 12: Transportation; Ch. 17: Complex Systems).

Climate change is expected to alter the geographic range, seasonal distribution, and abundance of vector-borne pathogens, exposing more people in North America to ticks that carry Lyme disease and mosquitoes that carry diseases like West Nile virus, Chikungunya, dengue, and Zika. Exposure to the mosquito Aedes aegypti, which can transmit dengue, Zika, chikungunya, and yellow fever viruses, is projected to increase across the globe by the end of the century due to climate, demographic, and socioeconomic changes, with some of the largest increases in human exposure projected to occur in North America. Similarly, changes in extreme precipitation and temperature may influence the range of tick species that transmit common pathogens. Annual national cases of West Nile neuroinvasive disease are projected to more than double by 2050 due to increasing temperatures, among other factors, resulting in approximately $1 billion per year in hospitalization costs and premature deaths. In 2090, an additional 3,300 annual cases of West Nile are projected under a higher scenario (RCP8.5), with $3.3 billion per year in costs. Under a lower scenario (RCP4.5), approximately half of these cases and costs would be avoided (Ch. 14: Human Health).

The frequency and severity of allergic illnesses, including asthma and hay fever, is expected to increase as a result of a changing climate. Earlier spring arrival, warmer temperatures, changes in precipitation, and higher carbon dioxide concentrations can increase exposure to airborne pollen allergens.

Higher average and extreme temperatures also impact agriculture. While some regions may see conditions conducive to expanded (or alternative crop) productivity, overall, yields from major U.S. crops are expected to decline as a consequence of increases in temperatures and possibly changes in water availability, air quality, diseases, and pest infestations. These changes threaten commodity grain production and, consequently, threaten the economic growth of America’s agricultural regions.

Projections suggest that cropland suitability will increase at higher latitudes, and croplands in southern portions of the country may shift to livestock grazing. For high-latitude regions, climate change could result in large-scale transformation from naturally vegetated ecosystems to systems dominated by agriculture. Such changes could have profound effects on the regional and cultural identities many communities have shared for generations (Ch. 10: Agriculture and Rural Communities).

Snow and Ice

The volume and extent of ice cover on land, lakes, and sea is expected to continue to decline throughout the 21st century. Glacier losses observed over recent decades are expected to continue. Nearly ice-free late summers are anticipated in the Arctic Ocean by mid-century under both higher and lower scenarios (Ch. 2: Our Changing Climate).

Reduced arctic sea ice extent creates opportunities for increased vessel traffic of various types to pass through the Bering Strait to or from the Northern Sea Route, the Northwest Passage, and by mid-century, directly across the Arctic Ocean. Significant effects on shipping are likely several decades away, and new trans-arctic shipping will likely have little economic effect within Alaska, but will bring environmental risks to fisheries and subsistence resources (Ch. 26: Alaska). Declining sea ice may restrict access to resources in some coastal Alaska Native communities, while in others, it may increase access and enhance economic opportunities through cultural and recreational tourism (Ch. 15: Tribal & Indigenous Communities). Increased vessel traffic may further impact subsistence livelihoods by introducing environmental stressors such as invasive aquatic and plant species.

Coastal and riverbank erosion due to a combination of rising sea levels, thawing permafrost, reduced sea ice, and fall storms pose threats to infrastructure in Alaska. Coastal erosion and flooding in some cases will require that entire communities, or portions of communities, relocate to safer terrain (Ch. 26: Alaska).

In western snow-dominated watersheds, declining mountain snowpack and shifts in snowmelt timing affect water availability for hydropower production. Energy systems in the Northwest and Southwest are likely to experience the most severe impacts of changing water availability, as reductions in mountain snowpack and shifts in snowmelt timing affect hydropower production (Ch. 24: Northwest and Ch. 25: Southwest).

Land and Water

The impacts of climate change and extreme weather events, many of which are expected to continue growing in frequency and intensity over the coming century, are expected to lead to more severe floods and greater risk of infrastructure failure in some regions. Infrastructure design, operation, financing principles, and regulatory standards, which typically do not account for a changing climate, pose a risk to existing infrastructure systems (Ch. 11: Built Environment, Urban Systems, and Cities).

The frequency and intensity of heavy precipitation events in the United States are projected to continue to increase over the 21st century, although there are important regional differences (Ch. 2: Our Changing Climate). These increases in extreme precipitation and flooding, combined with inadequate water and sewer infrastructure, may contribute to viral and bacterial contamination from combined sewage overflows and a lack of access to potable drinking water, increasing exposure to pathogens that lead to gastrointestinal illness. Runoff from more frequent and intense rainfall will increasingly compromise recreational waters and sources of drinking water through increased introductions of pathogens and toxic algal blooms (Ch. 14: Human Health).

Surface soil moisture over most of the United States is likely to decrease, driven largely by increased evaporation rates due to warmer temperatures. This means that, all else being equal, future droughts in most regions are expected to be stronger and potentially last longer. These trends are projected to be strongest in the Southwest and Southern Great Plains where precipitation is projected to decrease in most seasons and droughts may become more frequent.. Projections indicate large declines in snowpack in the western United States and shifts to more precipitation falling as rain than snow in the cold season in many parts of the central and eastern United States. Under a higher scenario (RCP8.5), and assuming no change to current water resources management, chronic, long-duration hydrological drought is increasingly possible by the end of this century (Ch. 2: Our Changing Climate).

Ecosystem primary productivity is expected to increase, on balance across the United States, through the end of the century under a higher scenario (RCP8.5), though there are important regional differences. In many arid regions, including the Southwest, worsening droughts may decrease ecosystem productivity. A warming climate may increase tree growth in some forests, but decrease tree growth in others that are water-limited. Elevated atmospheric carbon dioxide can increase tree growth in some instances, but this effect may be negated by other impacts of climate change such as increased ground-level air pollutants. Drought and extreme temperatures can cause heat-related stress in vegetation, in turn reducing forest productivity and increasing tree death (Ch. 6: Forests).

Overall, climate change is expected to decrease the ability of American forests to provide ecosystem services to society. More frequent extreme weather events are expected to increase the frequency and intensity of severe ecological disturbances such as wildfire and insect outbreaks, driving rapid and often long-term changes in forest ecosystems alongside more gradual changes. Tree growth and carbon storage are expected to decrease in most locations as a result of higher temperatures, more frequent drought, and increased forest disturbances (Ch. 6: Forests).

The incidence of large forest fires in the western United States and Alaska is projected to increase further as the climate changes, with profound changes to regional ecosystems and the potential to have a self-reinforcing effect on the climate system that would result in additional warming. By 2100, the annual area burned in the United States could increase 2–6 times from the present, depending on the geographic area, ecosystem, and local climate, increasing risks in forests and affecting how local communities can use the land (Ch. 5: Land Cover and Land Use Change). A projected lengthening in the fire season and additional large fires increase the amount of smoke in the air, with negative consequences for human health. Smoke from wildfires would impair visibility in populated regions and of scenic vistas in national parks. More prevalent wildfires are expected to increase the rate at which sporting events and other outdoor recreational activities are canceled because of the health hazard of wildfire smoke (Ch. 13: Air Quality; Ch. 14: Human Health). Wildfires also present the risk of increasing property damage, ecosystem damage, and loss of life.

Species range shifts are expected to continue, with species generally moving northward or to higher elevations, and aquatic organisms to greater depths. When changes occur too quickly for species to adapt, local extinctions can occur. Without significant reductions in greenhouse gas emissions, extinctions and transformative impacts on some ecosystems cannot be avoided. 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 we do today (Ch. 7: Ecosystems).

Oceans and Coasts

Climate change related impacts such as rising water temperatures, ocean acidification, deoxygenation, sea level rise, high tide flooding, coastal erosion, and storm surge are projected to continue even under a lower scenario (RCP4.5), putting ocean and marine species at risk, decreasing fisheries productivity, and threatening the communities that rely on them for livelihoods, tourism, and recreation.

Global average sea level is expected to continue to rise by at least several inches in the next 15 years and by 1–4 feet relative to present-day levels, with above-average changes on the East and Gulf Coasts of the United States driven in part by changes in ocean circulation and local land sinking relative to other areas (Figure 1.4). Regardless of future emissions scenario, it is expected that global mean (average) sea level will continue to rise beyond 2100. Recent science suggests that global mean sea level rise of up to 6 feet by 2100 is physically possible under the lower scenario (RCP4.5), and up to 10 feet under the higher scenario (RCP8.5) cannot be ruled out if collapse of major ice sheets occurs.

Sea level rise and intensified coastal flooding have the potential to transform life in many coastal communities. High tide flooding along the Atlantic and Gulf Coasts will continue increasing in depth, frequency, and extent this century. Lasting damage to coastal property and infrastructure driven by chronic high tide flooding and sea level rise is expected to lead to personal financial loss and higher insurance premiums even under lower scenarios (RCP4.5 or RCP2.6). Coastal risks may be further exacerbated as sea level rise increases the frequency and extent of extreme flooding associated with U.S. coastal storms such as hurricanes and nor’easters. Damage to supply chains, energy production facilities, and other infrastructure has the potential for cascading impacts across sectors and on other parts of the country. Sea level rise may result in or exacerbate saltwater intrusion into coastal rivers and aquifers, with impacts on drinking water supplies and coastal and estuarine ecosystems (Ch. 8: Coastal Effects).

By 2100, under a higher scenario (RCP8.5), global average sea surface temperature is projected to increase by 3.6°–6.2°F as compared to late 20th century values, ocean oxygen levels are projected to decrease by 3.5%, and global average surface ocean acidity is projected to increase by 100% to 150% (Ch. 2: Our Changing Climate). Ocean warming, deoxygenation, and acidification are projected to increase changes in fishery-related species, reduce catches in some areas, and challenge effective management of marine fisheries and protected species. Ocean acidification is expected to reduce harvest of U.S. shellfish in the Northeast, Northwest, and Southeast/Gulf of Mexico by the end of the century and is expected to increase prices for consumers (Ch. 9: Oceans and Marine Resources). Modest to moderate declines in ocean ecosystem primary production are projected for most low- to mid-latitude oceans over the next century, but regional patterns of change are less certain. Increasing primary productivity in the Arctic is expected due to decreasing ice cover, which increases exposure to sunlight, thereby allowing more photosynthesis (Ch. 7: Ecosystems).

Climate-induced disruption to ocean ecosystems will lead to reductions in important ecosystem services such as aquaculture, fishery productivity, recreational opportunities, and protection from coastal erosion. Because tropical regions are already some of the warmest, there are few species available to replace species that move to cooler water; as a result, fishing communities in Hawai”˜i and the Pacific Islands (Ch. 27: Hawai”˜i & Pacific Islands), the Caribbean (Ch. 20: U.S. Caribbean), and the Gulf of Mexico are particularly vulnerable to climate-driven changes in fish populations. Indigenous peoples in the Pacific Islands also work disproportionately in the tourism sector, which may be affected by impacts on commercial and subsistence fishing and agriculture and by other climate impacts that influence tourism, such as sea level rise, coral bleaching, and marine species mortality as water warms. In the absence of significant reductions in carbon emissions, transformative impacts on ocean ecosystems cannot be avoided (Ch. 15: Tribal & Indigenous).

1.5 Compound Stressors and Vulnerable Populations

Climate change and extreme weather impacts are often considered from a single-sector perspective (for example, agriculture or water resources). However, the sectors affected by climate risks also interact with one another and with other sectors (Ch. 17: Complex Systems). Weather and climate-related risks such as heat waves, floods, and droughts can result in disruptions to critical systems including electricity generation, transportation, and the flow of goods and services. These disruptions can also lead to cascading impacts on other critical sectors such as public health and safety, food production, and water supply. These systems and sectors are also subject to a range of other stressors, such as population growth, community development, technological change, and infrastructure degradation, that can compound or otherwise interact with climate-related risks.

Assessing individual sectors and stressors might not capture interactions among sectors or the risk of cascading failures, which occur when failure in one system leads to increased risks or failures in other systems. Choices in any one sector to confront the many climate-related hazards facing that sector have the potential to cause cascading impacts across the full, connected system. For example, the use of groundwater in California as an agricultural backstop in the recent drought may alter California’s resilience to future droughts.

Despite the challenge of managing system interdependencies, there are opportunities to learn from experience to guide future risk management decisions. For instance, the Department of Defense integrates consideration for the implications of climate change and variability for food, water, energy, human migration, supply chains, conflict, and disasters into decision-making and operations around the world (Ch. 16, Climate Effects on U.S. International Interests). However, it is often difficult or impossible to quantify all relevant processes and interactions, and a strong analytical basis for predicting these dynamics is still in development (Ch. 17: Complex Systems).

While many sectors face large economic risks from climate change, other impacts can have significant societal or cultural values that are harder to quantify. Climate change also interacts with social and economic factors that influence how risks are distributed; the risks of climate change are not felt equally by all. Communities that are already disproportionately affected by the impacts of extreme weather events may experience increased impacts due to local changes in the occurrence of extreme events. Conclusive research shows that low-income communities and communities of color that are often already overburdened with poor environmental conditions are disproportionately affected by, and less resilient to, the health impacts of climate change. Without action, risks will continue to increase disproportionately for communities that are already most vulnerable (Ch. 14: Human Health).

Climate change adversely affects cultural identities, food security, and the determinants of physical and mental health for Indigenous peoples and communities through disruption of interconnected social, physical, and ecological systems. Indigenous peoples’ livelihoods and economies, including agriculture, fishing, forestry, recreation, and tourism, are threatened by the impacts of climate change. Climate impacts on traditional subsistence economies are well documented. Increased wildfire, diminished snowpack, pervasive drought, flooding, ocean acidification, and sea level rise directly threaten the viability of agriculture, fisheries, and forestry enterprises on Indigenous lands across the United States and impact tribal tourism and recreational areas. Indigenous agriculture is being adversely affected by rising temperatures and changing regional patterns of flooding, drought, and dust storms, all of which can decrease crop yields and increase the need for irrigation.

Many Indigenous peoples are now facing relocation due to climate-related disasters, more frequent flooding, loss of land from erosion, or due to livelihoods that are being compromised by shifts in ecologies linked to climate change. In nearly every region of the United States, there are Indigenous peoples considering relocation or actively pursuing relocation as an adaptation strategy, including communities in Alaska, the Southeast, the Pacific Islands, and the Pacific Northwest (Ch. 15: Tribal and Indigenous Communities).

Avoiding, Reducing, and Managing the Risks of Climate Change

Future risks from climate change are tied directly to decisions made in the present. Some level of climate change is projected to continue throughout this century and beyond, but actions taken now (both in terms of mitigation to reduce atmospheric greenhouse gas concentrations and adaptation to prepare for the present and future impacts of climate change) can reduce the magnitude of risks beyond the next few decades while offering other benefits in the near-term. Mitigation and adaptation measures can be considered complements due to unavoidable climate changes from past and present emissions and the inability of mitigation efforts to avoid all climate risks.

The longer emissions reductions are delayed, the more abrupt and costly reductions will need to be in order to avoid the most dangerous impacts from climate change. Acting sooner rather than later generally results in lower costs overall for both adaptation and mitigation efforts. Under a higher scenario (RCP8.5), some impacts, such as the effects of ice sheet disintegration on sea level rise and coastal development, will be irreversible for thousands of years, while others will be permanent (Ch. 28: Adaptation; Ch. 29: Mitigation).

1.6.1 Mitigation

Global-scale mitigation of greenhouse gas emissions can significantly reduce climate change impacts and economic damages across the United States, though the magnitude of avoided risk varies by sector and region. The observed acceleration in annual greenhouse gas emissions over the past 15–20 years is consistent with a higher scenario (RCP8.5), but since 2014, annual growth in global emissions has slowed—a sign that economic growth has been largely decoupled from greenhouse gas emissions. However, even if emissions continue at the recently stabilized levels, the resulting global temperature increase would still be more than 3.6°F above preindustrial levels.

Despite the greenhouse gas reductions announced as part of recent international agreements, there is still uncertainty about future emissions levels and resulting impacts due to changing economic, political, and demographic factors. This Fourth National Climate Assessment (NCA4) quantifies possible climate changes under a range of future greenhouse gas mitigation scenarios through 2100 (see section 1.4 and Scenarios in Front Matter) and assesses how potential mitigation pathways may avoid or reduce the long-term risks of climate change within the United States. NCA4 does not evaluate technology options, costs, or the adequacy of existing or planned mitigation efforts relative to meeting specific policy targets, nor does it evaluate or recommend policy options.

The best available science concludes that substantial global-scale greenhouse gas emissions reductions would significantly reduce climate change impacts and economic damages across the United States, though the magnitude and timing of avoided risks varies by sector and region. By the end of the century, reduced climate change under a lower scenario (RCP4.5) compared to a higher one (RCP8.5, and in the absence of significant adaptation measures) avoids thousands to tens of thousands of deaths per year from extreme temperatures, hundreds to thousands of deaths per year from poor air quality, and the annual loss of hundreds of millions of labor hours from extreme temperatures. Each of these avoided impacts represent domestic economic benefits of mitigation on the order of tens to hundreds of billions of dollars per year.

Many activities are underway in the public and private sectors to reduce greenhouse gas emissions. At the Federal level, a number of measures have been implemented to promote advanced, low-carbon energy technologies and fuels, including energy efficiency, as well as measures to address greenhouse gas emissions sources other than fossil fuel combustion, including agriculture and forestry programs to increase soil and forest carbon storage. State, local, and tribal government approaches include both technology-specific policies and technology-neutral emissions reduction strategies (see Figure 1.5). Several states have adopted mandates to increase low-carbon electricity use and/or to increase energy efficiency. States in the Northeast participate in a market-based effort to reduce power sector emissions, and California has a legal mandate to reduce emissions across the state’s economy by 40% below 1990 levels by 2030. At the city and county levels, 132 local governments have set greenhouse gas emissions reductions targets, and several U.S. territories have adopted renewable energy portfolio standards. Many tribes are prioritizing energy efficiency and using renewable energy sources to meet their energy needs. In the private sector, many companies are implementing voluntary efforts to reduce emissions and/or manage climate risks to their operations.

Market forces and technological change, particularly within the electric power sector, as well as policy decisions, have contributed to a decline in U.S. greenhouse gas emissions over the past decade. In 2015, U.S. emissions were at their lowest levels since 1994. The power sector accounts for the largest emissions reductions for a sector, a 20% reduction from 2005 to 2015. Lower carbon-emitting natural gas generation has displaced coal generation due to the rising production of low-cost, unconventional natural gas, in part supported by Federal investment in research and development. Generation from renewable energy, particularly wind and solar, has grown over the same time period.

Recent studies suggest that considering the indirect effects of mitigation could significantly reduce–or possibly even completely offset–the potential costs associated with cutting greenhouse gas emissions in the United States. Beyond reduction of climate pollutants, there are many benefits associated with greenhouse gas emissions reductions, often immediate, such as improving air quality and public health, increased reliance on local sources of energy thereby increasing energy independence and security, and the creation of new, high-paying jobs (Ch. 29: Mitigation).

1.6.2 Adaptation

The reduction of climate change risk due to mitigation also depends on assumptions about how adaptation changes the exposure and vulnerability of people, livelihoods, ecosystems, and infrastructure in the United States. Adaptation refers to actions taken at the local, regional, and national scale to reduce risks from today’s altered climate conditions and to prepare for further changes in the future. Many types of adaptation actions exist, including changes to business operations, adjustments to natural resource management strategies, targeted capital investments across diverse sectors, policy changes, and other steps.

Adaptation can be thought of as having five general stages: 1) awareness, 2) assessment, 3) planning, 4) implementation and monitoring, and 5) evaluation and response. The Third National Climate Assessment, published in 2014, found substantial activity across the United States in the early adaptation stages of awareness, assessment, and planning. Since then, implementation of adaptation actions has significantly increased. This Fourth National Climate Assessment finds that many adaptation planning and implementation activities are taking place across the United States by organizations, communities, businesses, and others; however, implementation is not yet commonplace—and evaluation is even more limited (Ch. 28: Adaptation).

Adaptation actions have increased in part due to growing awareness of climate-related threats and impacts and the risks these pose to business operations and supply chains, critical public infrastructure and communities, natural areas and public lands, and ecosystem services and in part due to increasing recognition that investment in adaptation provides economic and social benefits that can significantly exceed the costs. Most adaptation actions taken to date address current climate variability; fewer address future change. Such proactive actions can prove compelling if they generate both near- and long-term benefits; if they manage long-lived assets such as infrastructure and public lands for the future; if climate adaptation becomes part of public expectations and professional standards; or when effective planning can reduce the near-term costs of addressing future change through means such as flexible designs and iterative adaptive management.

The benefits of adaptation can often be achieved by integrating climate considerations into organizations’ current best-practice activities. Communities face many stressors, and the same approaches for managing existing stressors well can also apply to risks related to climate change.

Despite challenges, this mainstreaming of climate adaptation into existing decision processes has begun in many areas, for instance in financial risk reporting, engineering standards, military planning, and disaster risk management. A growing number of jurisdictions address climate risk in their land use, hazard mitigation, capital improvement, and transportation plans. For instance, a handful of cities explicitly link their coastal plans and their hazard mitigation plans using a common, climate-informed vulnerability analysis to support both types of plans. All branches of the U.S. military now routinely integrate climate risks into their analyses, plans, and programs. Particular attention is paid to climate change effects on force readiness, which is related to the resilience of military infrastructure such as bases and training ranges.

However, over the long-term and especially under a higher scenario (RCP8.5), reducing climate-related risks and taking advantage of the opportunities derived from risk reduction requires more significant changes, both in the types of actions taken, and in the institutional, cultural, and organizational practices needed to implement and derive benefits from those actions. Sustained financing and communication among networks, including the sharing of best practices and having champions espouse such work, have been shown to be valuable components of successful adaptation.

Many adaptation initiatives—including changes to policies, business operations, capital investments, and other steps—yield benefits in excess of their costs in the near-term, as well as over the long-term. The current adaptation response literature offers considerable guidance on actions whose benefits exceed their costs in some sectors (such as adaptation responses to storms and rising seas in coastal zones, to riverine and extreme precipitation flooding, and for agriculture at the farm level), but less so in others (such as such as those aimed at addressing many health risks, biodiversity, other infrastructure impacts, and business services and industry) that are less well understood. Direct and indirect benefits may include many aspects of well-being such as economic, ecological, health, social, and security improvements. In some cases, proactive adaptive actions taken specifically to prepare for future change will yield benefits relative to the costs over the life of the investment. Such proactive action is often important for large-scale infrastructure, which is expected to endure for decades, or for crafting laws and regulations associated with property rights in areas that may be engulfed by sea level rise.

Benefit–cost analysis provides one important, but not the only, means to evaluate alternative adaptation actions. Effective adaptation can provide a broad range of benefits, including improving economic opportunity, health, equity, security, education, social connectivity, sense of place, safeguarding cultural resources and practices, and environmental quality, that may be difficult to aggregate into a single monetary value. More fundamentally, different people may value benefits differently. For instance, climate change can have significant impacts on equity and ecosystems, and individuals can have strongly divergent views on distributional justice and the intrinsic value of nature.

Poor or marginalized populations often face a higher risk from climate change because they live in areas with higher exposure, are more sensitive to climate impacts, or lack adaptive capacity. Prioritizing adaptation actions for such populations may provide ancillary societal benefits, leading, for instance, to improved infrastructure in their communities and increased focus on efforts to promote social cohesion and broader community improvement. Equity considerations can also lead to expanded participation of poor or marginalized populations in adaptation planning efforts. This can not only enhance the fairness of the process, but also affect choices regarding the appropriate balance between the resources invested in reducing climate risk and those towards other social goals such as job creation, access to quality healthcare access, and achievement of high education standards (Ch. 28: Adaptation).