Summary Findings

Summary Findings

NCA4 Summary Findings
These Summary Findings represent a high-level synthesis of the material in the underlying
report. The findings consolidate Key Messages and supporting evidence from 16 national-level
topic chapters, 10 regional chapters, and 2 chapters that focus on societal response strategies(mitigation and adaptation). Unless otherwise noted, qualitative statements regarding future
conditions in these Summary Findings are broadly applicable across the range of different
levels of future climate change and associated impacts considered in this report.

1. Communities
Climate change creates new risks and exacerbates existing vulnerabilities in communities acrossthe United States, presenting growing challenges to human health and safety, quality of life, andthe rate of economic growth.
The impacts of climate change are alreadybeing felt in communities across the country.
More frequent and intense extreme weather
and climate-related events, as well as changes
in average climate conditions, are expected tocontinue to damage infrastructure, ecosystems,
and social systems that provide essential benefits to communities. Future climate change
is expected to further disrupt many areas oflife, exacerbating existing challenges to prosperity posed by aging and deteriorating infrastructure, stressed ecosystems, and economic
inequality. Impacts within and across regions

will not be distributed equally. People who are
already vulnerable, including lower-income andother marginalized communities, have lower
capacity to prepare for and cope with extreme
weather and climate-related events and are expected to experience greater impacts. Prioritizing adaptation actions for the most vulnerable
populations would contribute to a more equitable future within and across communities.
Global action to significantly cut greenhousegas emissions can substantially reduce climate-related risks and increase opportunities
for these populations in the longer term.

2. Economy
Without substantial and sustained global mitigation and regional adaptation efforts, climatechange is expected to cause growing losses to American infrastructure and property and impede
the rate of economic growth over this century.
In the absence of significant global mitigation
action and regional adaptation efforts, risingtemperatures, sea level rise, and changes inextreme events are expected to increasingly
disrupt and damage critical infrastructure andproperty, labor productivity, and the vitalityof our communities. Regional economies andindustries that depend on natural resources and favorable climate conditions, such as

U.S. Global Change Research Program

agriculture, tourism, and fisheries, are vulnerable to the growing impacts of climate change.
Rising temperatures are projected to reduce
the efficiency of power generation while increasing energy demands, resulting in higher
electricity costs. 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

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Summary Findings
with overseas operations and supply chains.
Some aspects of our economy may see slightnear-term improvements in a modestly warmer
world. However, the continued warming thatis projected to occur without substantial andsustained reductions in global greenhouse gas
emissions is expected to cause substantial net
damage to the U.S. economy throughout thiscentury, especially in the absence of increased
adaptation efforts. With continued growth inemissions at historic rates, annual losses insome economic sectors are projected to reachhundreds of billions of dollars by the end of the
century—more than the current gross domestic
product (GDP) of many U.S. states.

3. Interconnected Impacts
Climate change affects the natural, built, and social systems we rely on individually and throughtheir connections to one another. These interconnected systems are increasingly vulnerable tocascading impacts that are often difficult to predict, threatening essential services within andbeyond the Nation’s borders.
Climate change presents added risks to interconnected systems that are already exposed
to a range of stressors such as aging and deteriorating infrastructure, land-use changes,
and population growth. Extreme weather andclimate-related impacts on one system can result in increased risks or failures in other critical systems, including water resources, foodproduction and distribution, energy and transportation, public health, international trade,and national security. The full extent of climatechange risks to interconnected systems, manyof which span regional and national boundaries,
is often greater than the sum of risks to individual sectors. Failure to anticipate interconnected
impacts can lead to missed opportunities for
effectively managing the risks of climate change
and can also lead to management responses
that increase risks to other sectors and regions.
Joint planning with stakeholders across sectors, regions, and jurisdictions can help identify
critical risks arising from interaction amongsystems ahead of time.

4. Actions to Reduce Risks
Communities, governments, and businesses are working to reduce risks from and costs associated with climate change by taking action to lower greenhouse gas emissions and implement
adaptation strategies. While mitigation and adaptation efforts have expanded substantially inthe last four years, they do not yet approach the scale considered necessary to avoid substantial
damages to the economy, environment, and human health over the coming decades.
Future risks from climate change depend
primarily on decisions made today. The integration of climate risk into decision-making
and the implementation of adaptation activities
have significantly increased since the Third
National Climate Assessment in 2014, including

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in areas of financial risk reporting, capital investment planning, development of engineering
standards, military planning, and disaster risk
management. Transformations in the energy sector—including the displacement of coalby natural gas and increased deployment of26

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Summary Findings
renewable energy—along with policy actions
at the national, regional, state, and local levels are reducing greenhouse gas emissions in
the United States. While these adaptation andmitigation measures can help reduce damages
in a number of sectors, this assessment showsthat more immediate and substantial global
greenhouse gas emissions reductions, as well as
regional adaptation efforts, would be needed to

avoid the most severe consequences in the longterm. Mitigation and adaptation actions alsopresent opportunities for additional benefits
that are often more immediate and localized,
such as improving local air quality and economies through investments in infrastructure.
Some benefits, such as restoring ecosystems
and increasing community vitality, may beharder to quantify.

5. Water
The quality and quantity of water available for use by people and ecosystems across the country
are being affected by climate change, increasing risks and costs to agriculture, energy production,industry, recreation, and the environment.
Rising air and water temperatures and changes in precipitation are intensifying droughts,
increasing heavy downpours, reducing snowpack, and causing declines in surface water
quality, with varying impacts across regions.
Future warming will add to the stress on watersupplies and adversely impact the availabilityof water in parts of the United States. Changes
in the relative amounts and timing of snow andrainfall are leading to mismatches between water availability and needs in some regions, posing threats to, for example, the future reliabilityof hydropower production in the Southwest
and the Northwest. Groundwater depletion isexacerbating drought risk in many parts of theUnited States, particularly in the Southwest andSouthern Great Plains. Dependable and safe
water supplies for U.S. Caribbean, Hawai‘i, andU.S.-Affiliated Pacific Island communities are
threatened by drought, flooding, and saltwatercontamination due to sea level rise. Most U.S.
power plants rely on a steady supply of waterfor cooling, and operations are expected to beaffected by changes in water availability andtemperature increases. Aging and deterioratingwater infrastructure, typically designed for pastenvironmental conditions, compounds the climate risk faced by society. Water management
strategies that account for changing climate
conditions can help reduce present and futurerisks to water security, but implementation ofsuch practices remains limited.

6. Health
Impacts from climate change on extreme weather and climate-related events, air quality, and thetransmission of disease through insects and pests, food, and water increasingly threaten thehealth and well-being of the American people, particularly populations that are already vulnerable.
Changes in temperature and precipitation are
increasing air quality and health risks fromwildfire and ground-level ozone pollution.
Rising air and water temperatures and more

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intense extreme events are expected to increase exposure to waterborne and foodborne
diseases, affecting food and water safety. With
continued warming, cold-related deaths are

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Summary Findings
projected to decrease and heat-related deaths
are projected to increase; in most regions,increases in heat-related deaths are expected
to outpace reductions in cold-related deaths.
The frequency and severity of allergic illnesses, including asthma and hay fever, areexpected to increase as a result of a changing
climate. Climate change is also projected toalter the geographic range and distribution of
disease-carrying insects and pests, exposing
more people to ticks that carry Lyme disease
and mosquitoes that transmit viruses such
as Zika, West Nile, and dengue, with varying
impacts across regions. Communities in theSoutheast, for example, are particularly vulnerable to the combined health impacts from

vector-borne disease, heat, and flooding. Extreme weather and climate-related events canhave lasting mental health consequences in affected communities, particularly if they result
in degradation of livelihoods or community
relocation. Populations including older adults,
children, low-income communities, and somecommunities of color are often disproportionately affected by, and less resilient to, thehealth impacts of climate change. Adaptationand mitigation policies and programs that help
individuals, communities, and states prepare
for the risks of a changing climate reduce thenumber of injuries, illnesses, and deaths fromclimate-related health outcomes.

7. Indigenous Peoples
Climate change increasingly threatens Indigenous communities’ livelihoods, economies, health,and cultural identities by disrupting interconnected social, physical, and ecological systems.
Many Indigenous peoples are reliant on natural resources for their economic, cultural,and physical well-being and are often uniquely affected by climate change. The impacts ofclimate change on water, land, coastal areas,
and other natural resources, as well as infrastructure and related services, are expected toincreasingly disrupt Indigenous peoples’ livelihoods and economies, including agriculture andagroforestry, fishing, recreation, and tourism.
Adverse impacts on subsistence activities havealready been observed. As climate changes continue, adverse impacts on culturally significant
species and resources are expected to resultin negative physical and mental health effects.
Throughout the United States, climate-related

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impacts are causing some Indigenous peoples
to consider or actively pursue community relocation as an adaptation strategy, presenting
challenges associated with maintaining culturaland community continuity. 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 andincrease resilience. Many Indigenous peoples
are taking steps to adapt to climate change
impacts structured around self-determinationand traditional knowledge, and some tribes are
pursuing mitigation actions through development of renewable energy on tribal lands.

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Summary Findings
8. Ecosystems and Ecosystem Services
Ecosystems and the benefits they provide to society are being altered by climate change, andthese impacts are projected to continue. Without substantial and sustained reductions in global
greenhouse gas emissions, transformative impacts on some ecosystems will occur; some coral
reef and sea ice ecosystems are already experiencing such transformational changes.
Many benefits provided by ecosystems and theenvironment, such as clean air and water, protection from coastal flooding, wood and fiber,crop pollination, hunting and fishing, tourism,
cultural identities, and more will continue tobe degraded by the impacts of climate change.
Increasing wildfire frequency, changes in insect
and disease outbreaks, and other stressors are
expected to decrease the ability of U.S. forests to support economic activity, recreation,and subsistence activities. Climate change has
already had observable impacts on biodiversity,
ecosystems, and the benefits they provide tosociety. These impacts include the migrationof native species to new areas and the spreadof invasive species. Such changes are projected to continue, and without substantial andsustained reductions in global greenhousegas emissions, extinctions and transformative

impacts on some ecosystems cannot be avoided in the long term. Valued aspects of regional
heritage and quality of life tied to ecosystems,
wildlife, and outdoor recreation will changewith the climate, and as a result, future generations can expect to experience and interact
with the natural environment in ways that are
different from today. Adaptation strategies,
including prescribed burning to reduce fuel for
wildfire, creation of safe havens for important
species, and control of invasive species, arebeing implemented to address emerging impacts of climate change. While some targeted
response actions are underway, many impacts,
including losses of unique coral reef and sea ice
ecosystems, can only be avoided by significantly reducing global emissions of carbon dioxide
and other greenhouse gases.

9. Agriculture and Food
Rising temperatures, extreme heat, drought, wildfire on rangelands, and heavy downpours are
expected to increasingly disrupt agricultural productivity in the United States. Expected increases in challenges to livestock health, declines in crop yields and quality, and changes in extreme
events in the United States and abroad threaten rural livelihoods, sustainable food security, andprice stability.
Climate change presents numerous challenges
to sustaining and enhancing crop productivity,
livestock health, and the economic vitality ofrural communities. While some regions (suchas the Northern Great Plains) may see conditions conducive to expanded or alternative
crop productivity over the next few decades,
overall, yields from major U.S. crops are expected to decline as a consequence of increases in

U.S. Global Change Research Program

temperatures and possibly changes in water
availability, soil erosion, and disease and pest
outbreaks. Increases in temperatures during
the growing season in the Midwest are projected to be the largest contributing factor to
declines in the productivity of U.S. agriculture.
Projected increases in extreme heat conditions
are expected to lead to further heat stress forlivestock, which can result in large economic

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Summary Findings
losses for producers. 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. These changes threaten future gains
in commodity crop production and put rural
livelihoods at risk. Numerous adaptation strategies are available to cope with adverse impactsof climate variability and change on agricultural
production. These include altering what is produced, modifying the inputs used for production, adopting new technologies, and adjusting
management strategies. However, these strategies have limits under severe climate change
impacts and would require sufficient long- andshort-term investment in changing practices.

10. Infrastructure
Our Nation’s aging and deteriorating infrastructure is further stressed by increases in heavy precipitation events, coastal flooding, heat, wildfires, 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, national security, essential services, and health and well-being.
Climate change and extreme weather eventsare expected to 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. Infrastructure currently
designed for historical climate conditions ismore vulnerable to future weather extremes
and climate change. The continued increase in
the frequency and extent of high-tide flooding
due to sea level rise threatens America’s trillion-dollar coastal property market and public
infrastructure, with cascading impacts to thelarger economy. In Alaska, rising temperaturesand erosion are causing damage to buildings
and coastal infrastructure that will be costly
to repair or replace, particularly in rural areas;
these impacts are expected to grow without

U.S. Global Change Research Program

adaptation. Expected increases in the severity
and frequency of heavy precipitation events
will affect inland infrastructure in every region,
including access to roads, the viability of bridges, and the safety of pipelines. Flooding fromheavy rainfall, storm surge, and rising high tides
is expected to compound existing issues withaging infrastructure in the Northeast. Increased
drought risk will threaten oil and gas drilling
and refining, as well as electricity generationfrom power plants that rely on surface waterfor cooling. Forward-looking infrastructure
design, planning, and operational measures andstandards can reduce exposure and vulnerability to the impacts of climate change and reduceenergy use while providing additional nearterm benefits, including reductions in greenhouse gas emissions.

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Summary Findings
11. Oceans and Coasts
Coastal communities and the ecosystems that support them are increasingly threatened by the
impacts of climate change. Without significant reductions in global greenhouse gas emissions
and regional adaptation measures, many coastal regions will be transformed by the latter part ofthis century, with impacts affecting other regions and sectors. Even in a future with lower greenhouse gas emissions, many communities are expected to suffer financial impacts as chronic
high-tide flooding leads to higher costs and lower property values.
Rising water temperatures, ocean acidification,
retreating arctic sea ice, sea level rise, high-tide
flooding, coastal erosion, higher storm surge,
and heavier precipitation events threaten our
oceans and coasts. These effects are projected
to continue, putting ocean and marine species
at risk, decreasing the productivity of certain
fisheries, and threatening communities that
rely on marine ecosystems for livelihoods andrecreation, with particular impacts on fishingcommunities in Hawai‘i and the U.S.-AffiliatedPacific Islands, the U.S. Caribbean, and the Gulf
of Mexico. Lasting damage to coastal property
and infrastructure driven by sea level rise andstorm surge is expected to lead to financial
losses for individuals, businesses, and communities, with the Atlantic and Gulf Coasts facing
above-average risks. Impacts on coastal energy
and transportation infrastructure driven by sea
level rise and storm surge have the potential

for cascading costs and disruptions across thecountry. Even if significant emissions reductions occur, many of the effects from sea level
rise over this century—and particularly throughmid-century—are already locked in due to historical emissions, and many communities are
already dealing with the consequences. Actions
to plan for and adapt to more frequent, widespread, and severe coastal flooding, such as
shoreline protection and conservation of coastal ecosystems, would decrease direct losses andcascading impacts on other sectors and partsof the country. More than half of the damages
to coastal property are estimated to be avoidable through well-timed adaptation measures.
Substantial and sustained reductions in global
greenhouse gas emissions would also significantly reduce projected risks to fisheries andcommunities that rely on them.

12. Tourism and Recreation
Outdoor recreation, tourist economies, and quality of life are reliant on benefits provided by our
natural environment that will be degraded by the impacts of climate change in many ways.
Climate change poses risks to seasonal andoutdoor economies in communities across theUnited States, including impacts on economies
centered around coral reef-based recreation,
winter recreation, and inland water-based
recreation. In turn, this affects the well-beingof the people who make their living supportingthese economies, including rural, coastal, andIndigenous communities. Projected increases

U.S. Global Change Research Programin wildfire smoke events are expected to impair
outdoor recreational activities and visibility
in wilderness areas. Declines in snow and ice
cover caused by warmer winter temperatures
are expected to negatively impact the winterrecreation industry in the Northwest, Northern Great Plains, and the Northeast. Somefish, birds, and mammals are expected to shift
where they live as a result of climate change,

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Fourth National Climate Assessment

Summary Findings
with implications for hunting, fishing, and otherwildlife-related activities. These and other climate-related impacts are expected to result in
decreased tourism revenue in some places and,for some communities, loss of identity. Whilesome new opportunities may emerge fromthese ecosystem changes, cultural identities
and economic and recreational opportunities

U.S. Global Change Research Program

based around historical use of and interaction
with species or natural resources in many areas
are at risk. Proactive management strategies,
such as the use of projected stream temperatures to set priorities for fish conservation, canhelp reduce disruptions to tourist economies
and recreation.

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Fourth National Climate Assessment