# U.S. National Climate Assessment
U.S. Global Change Research Program 

# Climate Change Impacts in the United States 

# OVERVIEW

### 2 3 The National Climate Assessment assesses the science of climate change and its impacts across the United States, now and throughout this century. It documents climate change related impacts and responses for various sectors and regions, with the goal of better informing public and private decision-making at all levels.

A team of more than 300 experts (see full report), guided by a 60-member National Climate Assessment and Development Advisory Committee (listed on the inside back cover) produced the full report – the largest and most diverse team to produce a U.S. climate assessment. Stakeholders involved in the development of the assessment included decision-makers from the public and private sectors, resource and environmental managers, researchers, representatives from businesses and non-governmental organizations, and the general public. More than 70 workshops and listening sessions were held, and thousands of public and expert comments on the draft report provided additional input to the process.

The assessment draws from a large body of scientific peer-reviewed research, technical input reports, and other publicly available sources; all sources meet the standards of the Information Quality Act. The report was extensively reviewed by the public and experts, including a panel of the National Academy of Sciences, the 13 Federal agencies of the U.S. Global Change Research Program, and the Federal Committee on Environment, Natural Resources, and Sustainability.



## About the 

# HIGHLIGHTS

The 148-page Highlights book presents the major findings and selected highlights from Climate Change Impacts in the United States , the third National Climate Assessment.

The Highlights report is organized around the National Climate Assessment’s 12 Report Findings, which take an overarching view of the entire report and its 30 chapters. All material in the Highlights report is drawn from the full report. The Key Messages from each of the 30 report chapters appear throughout the Highlights

. 

### Online at: nca2014.globalchange.gov



### Online at: nca2014.globalchange.gov /highlights 

## About the 

# NATIONAL CLIMATE ASSESSMENT

About the 

# OVERVIEW

This booklet provides a high level compendium of Climate Change Impacts in the United States , the Third National Climate Assessment. The Overview covers the most important impacts at the national level but does not attempt to provide a comprehensive summary of the entire assessment. Numbered references can be found in the Highlights

. To supplement this Overview , regional fact sheets are available that offer highlights from each of the eight regions. These and other resources can found at the website nca2014.globalchange.gov

. 

### Our Changing Climate

Water Resources

Energy Supply and Use

Transportation

Agriculture

Forests

Ecosystems and Biodiversity

Human Health

Energy, Water, and Land Use

Urban Systems and Infrastructure Indigenous Peoples, Lands, and Resources

Land Use and Land Cover Change

Rural Communities

Biogeochemical Cycles

Oceans and Marine Resources

Coastal Zones

Decision Support

Mitigation

Adaptation Climate Change Impacts in the United States describes current and future impacts on various U.S. regions and sectors. It also describes some of the responses underway and planned. The eight regions are shown on the map, and icons and titles below identify the various subjects, sectors, cross-sector topics, and responses to climate change covered in the assessment.

Photo Credits: Cover–Sandbagging: DoD photo by Staff Sgt. Michael Crane, U.S. Air Force; Oil rig and wind turbine: ©Jim West/imagebroker/Corbis; Fireman: ©AP Photo/The Press-Enterprise/Terry Pierson; Solar panel: ©Dennis Schroeder, NREL; Blue-green textured background on front and back covers and on page vii: ©iStockPhoto.com/javaman3. Pg. 14–Chicago sunset: ©Bill Ross/Corbis; Farm during drought: ©Scott Olson/Getty Images; North Atlantic hurricane: Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC caption; Supercell thunder

storm over a plain: ©Roger Hill/Science Photo Library/Corbis; Blue marble globe: courtesy NASA. Pg. 15– Blue marble globe: courtesy NASA; Clouds with precipitation: ©Eric Raptosh Photography/Blend Images/Corbis; Car in flooded road: ©James Borchuck/ZUMA Press/Corbis; field: ©AgStock Images/Corbis; Ice melt: ©Steve Morgan/epa/Corbis; Beach waves near city: ©Joe Raedle/Getty Images; Dissolved shell in acidified ocean water: David Liittschwager , National Geographic Images. Pg. 16–Person pumping gas: Charles Minshew/KOMU; People cooling off during heatwave: ©Julie Jacobson/AP/Corbis; Smog over city: ©iStockPhoto.com/Daniel Stein; Child blowing nose: ©Stockbyte/Getty Images. Pg. 17–Mosquito biting finger: ©James Gathany, CDC; Road washed out due to flooding: ©John Wark/AP/Corbis; Mountain stream: ©Dan Sherwood/Design Pics/Corbis; Farmer with corn: ©iStockPhoto.com/Valentin

Russanov. Pg. 18–Person building house: ©Aaron Huey/National Geographic Society/Corbis; Bear: ©Chase Swift/Corbis; Manatee: US Fish and Wildlife Service; Person with solar panels: ©Dennis Schroeder, NREL. Back cover: Blue-green textured background on front and back covers and on page vii: ©iStockPhoto.com/javaman3, Field: ©Timothy Hearsum/AgStock Images/Corbis; Woman and solar panel: ©Bill Miles/Mint Images/Corbis; Sea ice melt: ©Steve Morgan/epa/Corbis; Flood rescue workers and victim: ©Adam Hunger/Reuters/Corbis 

### Climate Science Supplement Frequently Asked Questions

### 4 5 

# CLIMATE CHANGE AND

C limate change, once considered an issue for a distant future, has moved firmly into the present. Corn producers in Iowa, oyster growers in Washington State, and maple syrup producers in Vermont are all observing climate-related changes that are outside of recent experience. So, too, are coastal planners in Florida, water managers in the arid Southwest, city dwellers from Phoenix to New York, and Native Peoples on tribal lands from Louisiana to Alaska. This National Climate Assessment concludes that the evidence of human-induced climate change continues to strengthen and that impacts are increasing across the country.

Americans are noticing changes all around them. Summers are longer and hotter, and extended periods of unusual heat last longer than any living American has ever experienced. Winters are generally shorter and warmer. Rain comes in heavier downpours. People are seeing changes in the length and severity of seasonal allergies, the plant varieties that thrive in their gardens, and the kinds of birds they see in any particular month in their neighborhoods.

Other changes are even more dramatic. Residents of some coastal cities see their streets flood more regularly during storms and high tides. Inland cities near large rivers also experience more flooding, especially in the Midwest and Northeast. Insurance rates are rising in some vulnerable locations, and insurance is no longer available in others. Hotter and drier weather and earlier snow melt mean that wildfires in the West start earlier in the spring, last later into the fall, and burn more acreage. In Arctic Alaska, the summer sea ice that once protected the coasts has receded, and autumn storms now cause more erosion, threatening many communities with relocation.

Scientists who study climate change confirm that these observations are consistent with significant changes in Earth’s climatic trends. Long-term, independent records from weather stations, satellites, ocean buoys, tide gauges, and many other data sources all confirm that our nation, like the rest of the world, is warming. Precipitation patterns are changing, sea level is rising, the oceans are becoming more acidic, and the frequency and intensity of some extreme weather events are increasing. Many lines of independent evidence demonstrate that the rapid warming of the past half-century is due primarily to human activities.

The observed warming and other climatic changes are triggering wide-ranging impacts in every region of our country and throughout our economy. Some of these changes can be beneficial over the short run, such as a longer growing season in some regions and a longer shipping season on the Great Lakes. But many more are detrimental, largely because our society and its infrastructure were designed for the climate that we have had, not the rapidly changing climate we now have and can expect in the future. In addition, climate change does not occur in isolation. Rather, it is superimposed on other stresses, which combine to create new challenges.

This National Climate Assessment collects, integrates, and assesses observations and research from around the country, helping us to see what is actually happening and understand what it means for our lives, our livelihoods, and our future. The report includes analyses of impacts on seven sectors – human health, water, energy, transportation, agriculture, forests, and ecosystems – and the interactions among sectors at the national level. The report also assesses key impacts on all U.S. regions: Northeast, Southeast and Caribbean, Midwest, Great Plains, Southwest, Northwest, Alaska, Hawai`i and Pacific Islands, as well as the country’s coastal areas, oceans, and marine resources.

Over recent decades, climate science has advanced significantly. Increased scrutiny has led to increased certainty that we are now seeing impacts associated with human-induced climate change. With each passing year, the accumulating evidence further expands our understanding and extends the record of observed trends in temperature, precipitation, sea level, ice mass, and many other variables recorded by a variety of measuring systems and analyzed by independent research groups from around the world. It is notable that as these data records have grown longer and climate models have become more comprehensive, earlier predictions have largely been confirmed. The only real surprises have been that some changes, such as sea level rise and Arctic sea ice decline, have outpaced earlier projections.

What is new over the last decade is that we know with increasing certainty that climate change is happening now. While scientists continue to refine projections of the future, observations unequivocally show that climate is changing and that the warming of the past 50 years is primarily due to human-induced emissions of heat-trapping gases. These emissions come mainly from burning coal, oil, and gas, with additional contributions from forest clearing and some agricultural practices.

Global climate is projected to continue to change over this century and beyond, but there is still time to act to limit the amount of change and the extent of damaging impacts.

This report documents the changes already observed and those projected for the future.

It is important that these findings and response options be shared broadly to inform citizens and communities across our nation. Climate change presents a major challenge for society. This report advances our understanding of that challenge and the need for the American people to prepare for and respond to its far-reaching implications. 

# THE AMERICAN PEOPLE

### 6 7 

# C limate change is already affecting the American people in far-reaching ways. Certain types of extreme weather events with links to climate change have become more frequent and/or intense, including prolonged periods of heat, heavy downpours, and, in some regions, floods and droughts. In addition, warming is causing sea level to rise and glaciers and Arctic sea ice to melt, and oceans are becoming more acidic as they absorb carbon dioxide. These and other aspects of climate change are disrupting people’s lives and damaging some sectors of our economy.

Climate Change: Present and Future Evidence for climate change abounds, from the top of the atmosphere to the depths of the oceans. Scientists and engineers from around the world have meticulously collected this evidence, using satellites and networks of weather balloons, thermometers, buoys, and other observing systems. Evidence of climate change is also visible in the observed and measured changes in location and behavior of species and functioning of ecosystems. Taken together, this evidence tells an unambiguous story: the planet is warming, and over the last half century, this warming has been driven primarily by human activity.

Multiple lines of independent evidence confirm that hu

man activities are the primary cause of the global warm

ing of the past 50 years. The burning of coal, oil, and gas, and clearing of forests have increased the concentration

These are just some of the indicators measured globally over many decades that show that the Earth’s climate is warming. White arrows indicate increasing trends; black arrows indicate decreasing trends. All the indicators expected to increase in a warming world are increasing, and all those expected to decrease in a warming world are decreasing. (Figure source: NOAA NCDC, based on data updated from Kennedy et al. 2010 a ). 

## Ten Indicators of a Warming World of carbon dioxide in the atmosphere by more than 40% since the Industrial Revolution, and it has been known for almost two centuries that this carbon dioxide traps heat. Methane and nitrous oxide emissions from agriculture and other human activities add to the atmospheric burden of heat-trapping gases. Data show that natural factors like the sun and volcanoes cannot have caused the warming observed over the past 50 years. Sensors on satellites have measured the sun’s output with great accuracy and found no overall increase during the past half century. Large vol

canic eruptions during this period, such as Mount Pinatubo in 1991, have exerted a short-term cooling influence. In fact, if not for human activities, global climate would actu

ally have cooled slightly over the past 50 years. The pat

tern of temperature change through the layers of the at

mosphere, with warming near the surface and cooling higher up in the stratosphere, further confirms that it is the buildup of heat-trapping gases (also known as “greenhouse gases”)
that has caused most of the Earth’s warming over the past half century.

Because human-induced warming is superimposed on a background of natural variations in climate, warm

ing is not uniform over time.
Short-term fluctuations in the long-term upward trend are thus natural and expected. For example, a recent slowing in the rate of surface air temperature rise appears to be related to cyclic changes in the oceans and in the sun’s energy out

put, as well as a series of small volcanic eruptions and other factors. Nonetheless, global tempera

tures are still on the rise and are expected to rise
further.

U.S. average temperature has increased by 1.3°F to 1.9°F since 1895, and most of this increase has
occurred since 1970. The most recent decade was the nation’s and the world’s hottest on record, and 2012 was the hottest year on record in the conti

nental United States. All U.S. regions have experi

enced warming in recent decades, but the extent of warming has not been uniform. In general, temperatures are rising more quickly in the north. Alaskans have experienced some of the largest increases in temperature between 1970 and the present. People living in the Southeast have experi

enced some of the smallest temperature increases over this period.

Temperatures are projected to rise another 2°F to
4°F in most areas of the United States over the next few decades. Reductions in some short-lived human-induced emis

sions that contribute to warming, such as black carbon (soot) and methane, could reduce some of the projected warming over the next couple of decades, because, unlike carbon dioxide, these gases and particles have relatively short atmo

spheric lifetimes.

The amount of warming projected beyond the next few decades is directly linked to the cumulative global emissions of heat-trapping gases and particles. By the end of this century, a roughly 3°F to 5°F rise is projected under a lower emis

sions scenario, which would require substantial reductions in emissions (referred to as the “B1 scenario”), and a 5°F to 10°F rise for a higher emissions scenario assuming continued increases in emissions, predominantly from fossil fuel com

bustion (referred to as the “A2 sce

nario”). These projections are based
on results from 16 climate models
that used the two emissions scenarios in a formal inter-model comparison study. The range of model projections
for each emissions scenario is the re

sult of the differences in the ways the models represent key factors such as water vapor, ice and snow reflectivity, and clouds, which can either dampen or amplify the initial effect of human influences on temperature. The net effect of these feedbacks is expected to amplify warming. More informa

tion about the models and scenarios used in this report can be found in Appendix 5 of the full report. 1 Different amounts of heat-trapping gases released into the atmosphere by human activ

ities produce different projected increases in Earth’s temperature. The lines on the graph represent a central estimate of global average temperature rise (relative to the 1901-1960 average) for the two main scenarios used in this report. A2 assumes continued increases in emissions throughout this century, and B1 assumes significant emissions reductions, though not due explicitly to climate change policies. Shading indicates the range (5 th to 95 th percentile) of results from a suite of cli

mate models. In both cases, temperatures are expected to rise, although the difference be

tween lower and higher emissions pathways is substantial. (Figure source: NOAA NCDC / CICS-NC). 

## Projected Global Temperature Change The green band shows how global average temperature would have changed over the last century due to natural forces alone, as simulated by climate models. The blue band shows model simulations of the effects of human and natural forces (including solar and volcanic activity) combined. The black line shows the actual observed global average temperatures. Only with the inclu

sion of human influences can models reproduce the observed temperature changes. (Figure source: adapted from Huber and Knutti 2012 b ). 

## Separating Human and Natural Influences on Climate

Coal-fired power plants emit heat-trapping carbon dioxide to the atmosphere. 

## OVERVIEW

### 8 9 Prolonged periods of high temperatures and the per

sistence of high nighttime temperatures have increased in many locations (especially in urban areas) over the past half century. High nighttime temperatures have wide

spread impacts because people, livestock, and wildlife get no respite from the heat. In some regions, prolonged periods of high temperatures associated with droughts contribute to conditions that lead to larger wildfires and longer fire seasons. As expected in a warming climate, recent trends show that extreme heat is becoming more common, while extreme cold is becoming less common.
Evidence indicates that the human influence on climate has already roughly doubled the probability of extreme heat events such as the record-breaking summer heat experienced in 2011 in Texas and Oklahoma. The incidence of record-breaking high temperatures is projected to rise. 2

Human-induced climate change means much more than just hotter weather. Increases in ocean and freshwater temperatures, frost-free days, and heavy downpours have all been documented. Global sea level has risen, and there have been large reductions in snow-cover extent, glaciers, and sea ice. These changes and other climatic changes have affected and will continue to affect human health, water supply, agriculture, transportation, energy, coastal areas, and many other sectors of society, with increasingly adverse impacts on the American economy and quality of life. 3

Some of the changes discussed in this report are common to many regions. For example, large increases in heavy precipitation have occurred in the Northeast, Midwest, and Great Plains, where heavy downpours have fre

quently led to runoff that exceeded the capacity of storm drains and levees, and caused flooding events and accel

erated erosion. Other impacts, such as those associated with the rapid thawing of permafrost in Alaska, are unique to a particular U.S. region. Permafrost thawing is causing extensive damage to infrastructure in our nation’s largest

state. 4

Some impacts that occur in one region ripple beyond that region. For example, Percent changes in the amount of precipitation falling in very heavy events (the heaviest 1%) from 1958 to 2012 for each region. There is a clear national trend toward a greater amount of precipitation being concentrated in very heavy events, particularly in the Northeast and Midwest. (Figure source: updated from Karl et al. 2009 c ). 

## Observed Change in Very Heavy Precipitation coastal communities, especially along the Gulf Coast, the Atlantic seaboard, and in Alaska. Coastal infrastructure, including roads, rail lines, energy infrastructure, airports, port facilities, and military bases, are increasingly at risk from sea level rise and damaging storm surges. Sea level is projected to rise by another 1 to 4 feet in this century, although the rise in sea level in specific regions is expected to vary from this global average for a number of reasons. A wider range of scenarios, from 8 inches to more than 6 feet by 2100, has been used in risk-based analyses in this report. In general, higher emissions scenarios that lead to more warming would be expected to lead to higher amounts of sea level rise. The stakes are high, as nearly five million Americans and hundreds of billions of dollars of property are locatedin areas that are less than four feet above the local hightide level. 6

In addition to causing changes in climate, increasing levels of carbon dioxide from the burning of fossil fuels and
other human activities have a direct effect on the world’s
oceans. Carbon dioxide interacts with ocean water to form carbonic acid, increasing the ocean’s acidity. Ocean surface waters have become 30% more acidic over the last 250 years as they have absorbed large amounts of carbon dioxide from the atmosphere. This ocean acidification makes water more corrosive, reducing the capacity of marine organisms with shells or skeletons made of calcium carbonate (such as corals, krill, oysters, clams, and crabs) to survive, grow, and reproduce, which in turn will affect
the marine food chain. 7

Widespread Impacts Impacts related to climate change are already evident in many regions and sectors and are expected to become increasingly disruptive across the nation throughout this century and beyond. Climate changes
interact with other environmental and societal factors in ways that can either moderate or intensify these impacts.

Some climate changes currently have beneficial effects for specific sectors or regions. For example, current benefits of warming include longer growing seasons for agriculture and longer ice-free periods for shipping on the Great Lakes. At the same time, however, longer growing seasons, along with higher temperatures and carbon dioxide levels, can increase pollen production, intensifying and lengthening the allergy season. Longer ice-free periods
on the Great Lakes can result in more lakeeffect snowfalls. the dramatic decline of summer sea ice in the Arctic – a loss of ice cover roughly equal to half the area of the continental United States – exacerbates global warming by reducing the reflectivity of Earth’s surface and increasing the amount of heat absorbed. Similarly, smoke from wildfires in one location can contribute to poor air quality in faraway regions, and evidence suggests that particulate matter can affect atmospheric properties and therefore weather patterns. Major storms and the higher storm surges exacerbated by sea level rise that hit the Gulf Coast affect the entire country through their cascading effects on oil and gas production and distribution. 5

Water expands as it warms, causing global sea levels to rise; melting of land-based ice also raises sea level by adding water to the oceans. Over the past century, global average sea level has risen by about 8 inches. Since 1992, the rate of global sea level rise measured by satellites has been roughly twice the rate observed over the last century, providing evidence of acceleration. Sea level rise, combined with coastal storms, has increased the risk of erosion, storm surge damage, and flooding for

The correlation between rising levels of carbon dioxide in the atmosphere (red) with rising carbon dioxide levels (blue) and falling pH in the ocean (green). As carbon dioxide accumulates in the ocean, the water becomes more acidic (the pH declines). (Figure source: modified from Feely et al. 2009 d ). 

## As Oceans Absorb CO 2 

## They Become More Acidic Pteropods, or “sea butterflies,” are eaten by a variety of marine species ranging from tiny krill to salmon to whales. The photos show what happens to a pteropod’s shell in seawater that is too acidic. On the left is a shell from a live pteropod from a region in the Southern Ocean where acidity is not too high. The shell on the right is from a pteropod in a region where the water is more acidic. (Figure source: (left) Bednaršek et al. 2012 e (right) Nina Bednaršek). 

## Shells Dissolve in Acidified Ocean Water

### 10 11 

## Northeast Communities are affected by heat waves, more extreme precipitation events, and coastal flooding due to sea level rise and storm surge.



## Southeast and Caribbean Decreased water availability, exacerbated by population growth and land-use change, causes increased competition for water. There are increased risks associated with extreme events such as hurricanes.



## Midwest Longer growing seasons and rising carbon dioxide levels increase yields of some crops, although these benefits have already been offset in some instances by occurrence of extreme events such as heat waves, droughts, and floods.



## Great Plains Rising temperatures lead to increased demand for water and energy and impacts on agricultural practices.



## Southwest Drought and increased warming foster wildfires and increased competition for scarce water resources for people and ecosystems.



## Northwest Changes in the timing of streamflow related to earlier snowmelt reduce the supply of water in summer, causing far-reaching ecological and socioeconomic consequences.



## Alaska Rapidly receding summer sea ice, shrinking glaciers, and thawing permafrost cause damage to infrastructure and major changes to ecosystems. Impacts to Alaska Native
communities increase.



## Hawai‘i and Pacific Islands Increasingly constrained freshwater supplies, coupled with increased temperatures, stress both people and ecosystems and decrease food and water security.



## Coasts Coastal lifelines, such as water supply infrastructure and evacuation routes, are increasingly vulnerable to higher sea levels and storm surges, inland flooding, and other climate-related changes.



## Oceans The oceans are currently absorbing about a quarter of human-caused carbon dioxide emissions to the atmosphere and over 90% of the heat associated with global warming, leading to ocean acidification and the alteration of marine ecosystems. Observed and projected climate change impacts vary across the regions of the United States. Selected impacts emphasized in the regional chapters are shown below, and many more are explored in detail in this report. Sectors affected by climate changes include agriculture, water, human health, energy, transportation, forests, and ecosystems. Climate change poses a major challenge to U.S. agriculture because of the critical dependence of agricultural systems on climate. Climate change has the potential to both positively and negatively affect the location, timing, and productivity of crop, livestock, and fishery systems at local, national, and global scales. The United States produces nearly $330 billion per year in agricultural commodities. This productivity is vulnerable to direct impacts on crops and livestock from changing
climate conditions and extreme weather events and indi

rect impacts through increasing pressures from pests and pathogens. Climate change will also alter the stability of food supplies and create new food security challenges for
the United States as the world seeks to feed nine billion people by 2050. While the agriculture sector has proven to be adaptable to a range of stresses, as evidenced by continued growth in production and efficiency across the United States, climate change poses a new set of challenges. 8

Water quality and quantity are being affected by climate change. Changes in precipitation and runoff, combined with changes in consumption and withdrawal, have reduced surface and groundwater supplies in many areas. These trends are expected to continue, increasing the likelihood of water shortages for many uses. Water quality is also diminishing in many areas, particularly due to sediment and contaminant concentrations after heavy downpours. Sea level rise, storms and storm surges, and changes in surface and groundwater use patterns are expected to compromise the sustainability of coastal freshwater aquifers and wetlands. In most U.S. regions, water resources managers and planners will encounter new risks, vulnerabilities, and opportunities that may not be properly managed with existing practices. 9

Climate change affects human health in many ways. For example, increasingly frequent and intense
heat events lead to more heatrelated illnesses and deaths and, over time, worsen drought and wildfire risks, and intensify air pollution. Increasingly frequent extreme precipitation and associated flooding can lead to injuries and increases in waterborne disease. Rising sea surface temperatures have been linked with increasing levels and ranges of diseases. Rising sea levels intensify coastal flooding and storm surge, and thus exacerbate threats to public safety during storms. Certain groups of people are more vulnerable to the range of climate change related health impacts, including the elderly, children, the poor, and the sick. Others are vulnerable because of where they live, including those in floodplains, coastal zones, and some urban areas. Improving and properly supporting the public health infrastructure will be critical to managing the potential health impacts of climate change. 10

Climate change also affects the living world, including people, through changes in ecosystems and biodiversity. Ecosystems provide a rich array of benefits and services to humanity, including habitat for fish and wildlife, drinking water storage and filtration, fertile soils for growing crops, Certain groups of people are more vulnerable to the range of climate change related health impacts, including the

elderly, children, the poor, and the sick. Climate change can exacerbate respiratory and asthma-relat

ed conditions through increases in pollen, ground-level ozone, and wildfire smoke.

Increasing air and water temperatures, more intense precipita

tion and runoff, and intensifying droughts can decrease water quality in many ways. Here, middle school students in Colorado test water quality.

### 12 13 The amount of future climate change will still largely be determined by choices society makes about emissions. buffering against a range of stressors including climate change impacts, and aesthetic and cultural values. These benefits are not always easy to quantify, but they support jobs, economic growth, health, and human well-being. Climate change driven disruptions to ecosystems have direct and indirect human impacts, including reduced water supply and quality, the loss of iconic species and landscapes, effects on food chains and the timing and success of species migrations, and the potential for extreme weather and cli

mate events to destroy or degrade the ability of ecosystems to provide societal benefits. 11

Human modifications of ecosystems and landscapes often increase their vulnerability to damage from extreme weather events, while simultaneously reducing their nat

ural capacity to moderate the impacts of such events. For example, salt marshes, reefs, mangrove forests, and barrier islands defend coastal ecosystems and infrastructure, such as roads and buildings, against storm surges. The
loss of these natural buffers due to coastal development, erosion, and
sea level rise increases the risk of catastrophic damage during or after extreme weather events. Although floodplain wetlands are greatly reduced from their his

torical extent, those that remain still absorb floodwaters and reduce the effects of high flows on river-margin lands. Extreme weather events that produce sudden increases in water flow, often carrying debris and pollutants, can decrease the natural capacity of ecosystems to cleanse
contaminants. 12

The climate change impacts being felt in the regions and sectors of the United States are affected by global trends and economic decisions. In an increasingly interconnect

ed world, U.S. vulnerability is linked to impacts in other nations. It is thus difficult to fully evaluate the impacts of climate change on the United States without considering consequences of climate change elsewhere.

Response Options As the impacts of climate change are becoming more prevalent, Americans face choices. Especially because of past emissions of long-lived heat-trapping gases, some additional climate change and related impacts are now unavoidable. This is due to the long-lived nature of many of these gases, as well as the amount of heat absorbed and retained by the oceans and other responses within the climate system. The amount of future climate change, however, will still largely be determined by choices society makes about emissions. Lower emissions of heat-trappinggases and particles mean less future warming and lesssevere impacts; higher emissions mean more warming and more severe impacts. Efforts to limit emissions or increase carbon uptake fall into a category of response options known as “mitigation,” which refers to reducing the amount and speed of future climate change by reducing emissions of heat-trapping gases or removing carbon dioxide from the atmosphere. 13

The other major category of response options is known as “adaptation,” and refers to actions to prepare for and adjust to new conditions, thereby reducing harm or taking advantage of new opportunities. Mitigation and adaptation actions are linked in multiple ways, including that effective mitigation reduces the need for adaptation in the future. Both are essential parts of a com

prehensive climate change response strategy. The threat of irreversible im

pacts makes the timing of mitigation efforts particularly critical. This report includes chapters on Mitigation, Adaptation, and Decision Support that offer an overview of the options and activities being planned or implemented around the coun

try as local, state, federal, and tribal governments, as well as businesses, organizations, and individuals begin to respond to climate change. These chapters conclude that while re

sponse actions are under development, current implemen

tation efforts are insufficient to avoid increasingly negative social, environmental, and economic consequences. 14

Large reductions in global emissions of heat-trapping gases, similar to the lower emissions scenario (B1) analyzed in this assessment, would reduce the risks of some of the worst impacts of climate change. Some targets called for in international climate negotiations to date would require even larger reductions than those outlined in the B1 scenar

io. Meanwhile, global emissions are still rising and are on a path to be even higher than the high emissions scenario (A2) analyzed in this report. The recent U.S. contribution to annual global emissions is about 18%, but the U.S. contri

bution to cumulative global emissions over the last century is much higher. Carbon dioxide lasts for a long time in the atmosphere, and it is the cumulative carbon emissions that determine the amount of global climate change. After decades of increases, U.S. CO 2 emissions from energy use (which account for 97% of total U.S. emissions) declined by around 9% between 2008 and 2012, largely due to a shift from coal to less CO 2 -intensive natural gas for electricity production. Governmental actions in city, state, regional, and federal programs to promote energy efficiency have also contributed to reducing U.S. carbon emissions. Many, if not most of these programs are motivated by other policy objectives, but some are directed specifically at greenhouse gas emissions. These U.S. actions and others that might be undertaken in the future are described in the Mitigation chapter of this report. Over the remainder of this century, aggressive and sustained greenhouse gas emission reduc

tions by the United States and by other nations would be needed to reduce global emissions to a level consistent with the lower scenario (B1) analyzed in this assessment. 15

With regard to adaptation, the pace and magnitude of observed and projected changes emphasize the need to be prepared for a wide variety and intensity of impacts. Because of the growing influence of human activities, the climate of the past is not a good basis for future planning. For example, building codes and landscaping ordinances could be updated to improve energy efficiency, conserve water supplies, protect against insects that spread disease (such as dengue fever), reduce susceptibility to heat stress, and improve protection against extreme events. The fact that climate change impacts are increasing points to the urgent need to develop and refine approaches that enable decision-making and increase flexibility and resilience in the face of ongoing and future impacts. Reducing non-cli

mate-related stresses that contribute to existing vulnera

bilities can also be an effective approach to climate change adaptation. 16

Adaptation can involve considering local, state, regional, national, and international jurisdictional objectives. For example, in managing water supplies to adapt to a changing climate, the implications of international treaties should be considered in the context of managing the Great Lakes, the Columbia River, and the Colorado River to deal with increased drought risk. Both “bottom up” community plan

ning and “top down” national strategies may help regions deal with impacts such as increases in electrical brownouts, heat stress, floods, and wildfires. 17

Proactively preparing for climate change can reduce impacts while also facilitating a more rapid and efficient response to changes as they happen. Such efforts are beginning at the federal, regional, state, tribal, and local levels, and in the corporate and non-governmental sectors, to build adaptive capacity and resilience to climate change impacts. Using scientific information to prepare for climate changes in advance can provide economic opportunities, and proactively managing the risks can reduce impacts and
costs over time. 18

There are a number of areas where improved scientific information or understanding would enhance the capacity to estimate future climate change impacts. For example, knowledge of the mechanisms controlling the rate of ice loss in Greenland and Antarctica is limited, making it difficult for scientists to narrow the range of expected future sea level rise. Improved understanding of ecological and social responses to climate change is needed, as is understanding of how ecological and social responses will
interact. 19

A sustained climate assessment process could more effi

ciently collect and synthesize the rapidly evolving science and help supply timely and relevant information to deci

sion-makers. Results from all of these efforts could continue to deepen our understanding of the interactions of human and natural systems in the context of a changing climate, enabling society to effectively respond and prepare for our
future. 20

The cumulative weight of the scientific evidence contained in this report confirms that climate change is affecting the American people now, and that choices we make will affect our future and that of future generations.

Cities providing transportation options including bike lanes, buildings designed with energy saving features such as green roofs, and houses elevated to allow storm surges to pass underneath are among the many response options being pursued around the country.

### 14 15 

# CLIMATE TRENDS

Extreme Weather There have been changes in some types of extreme weather events over the last several decades. Heat waves have become more frequent and intense, especially in the West. Cold waves have become less frequent and intense across the nation. There have been regional trends in floods and droughts. Droughts in the Southwest and heat waves everywhere are projected to become more intense, and cold waves less intense everywhere.



## Hurricanes The intensity, frequency, and duration of North Atlantic hurricanes, as well as the frequency of the strongest (Category 4 and 5) hurricanes, have all increased since the early 1980s. The relative contributions of human and natural causes to these increases are still uncertain. Hurricane-associated storm intensity and rainfall rates are projected to increase as the climate continues to warm.



## Severe Storms Winter storms have increased in frequency and intensity since the 1950s, and their tracks have shifted northward over the United States. Other trends in severe storms, including the intensity and frequency of tornadoes, hail, and damaging thunderstorm winds, are uncertain and are being studied intensively. 

## Temperature U.S. average temperature has increased by 1.3°F to 1.9°F since record keeping began in 1895; most of this increase has occurred since about 1970. The most recent decade was the nation’s warmeston record. Temperatures in the United States are expected to continue to rise. Because humaninduced warming is superimposed on a naturally varying climate, the temperature rise has not been, and will not be, uniform or smooth across the country or over time.



## Ocean Acidification The oceans are currently absorbing about a quarter of the carbon dioxide emitted to the atmosphere annually and are becoming more acidic as a result, leading to concerns about intensifying impacts on marine ecosystems. 

## Global climate is changing and this change is apparent across a wide range of observations. The global warming of the past 50 years is primarily due to human activities. Global climate is projected to continue to change over this century and beyond. The magnitude of climate change beyond the next few decades depends primarily on the amount of heat-trapping gases emitted globally, and how sensitive the Earth’s climate is to those emissions. Heavy Downpours Heavy downpours are increasing nationally, especially over the last three to five decades. Largest increases are in the Midwest and Northeast. Increases in the frequency and intensity of extreme precipitation events are projected for all U.S. regions.



## Frost-free Season The length of the frost-free season (and the corresponding growing season) has been increasing nationally since the 1980s, with the largest increases occurring in the western United States, affecting ecosystems and agriculture. Across the United States, the growing season is projected to continue to lengthen.



## Sea Level Global sea level has risen by about 8 inches since reliable record keeping began in 1880. It is projected to rise another 1 to 4 feet by
2100. 

## Ice Melt Rising temperatures are reducing ice volume and surface extent on land, lakes, and sea. This loss of ice is expected to continue. The Arctic Ocean is expected to become

essentially ice free in summer before mid-century. 

## Precipitation Average U.S. precipitation has increased since 1900, but some areas have had increases greater than the national average, and some areas have had decreases. More winter and spring precipitation is projected for the northern United States, and less for the Southwest, over this century. These two pages present the Key Messages from the “Our Changing Climate” chapter of the full report. They pertain to Report Findings 1, 2, and 3.

### 16 17 These findings distill important results that arise from this National Climate Assessment. They do not represent a full summary of all of the chapters’ findings, but rather a synthesis of particularly noteworthy conclusions.



### 1. Global climate is changing and this is apparent across the United States in a wide range of observations. The global warming of the past 50 years is primarily due to human activities, predominantly the burning of fossil fuels.

Many independent lines of evidence confirm that human activities are affecting climate in unprecedented ways. U.S. average temperature has increased by 1.3°F to 1.9°F since record keeping began in 1895; most of this increase has occurred since about 1970. The most recent decade was the warmest on record. Because human-induced warming is superimposed on a naturally varying climate, rising temperatures are not evenly distributed across the country or
over time. 21



### 4. Impacts related to climate change are already evident in many sectors and are expected to become increasingly disruptive across the nation throughout this century and beyond.

Climate change is already affecting societies and the natural world. Climate change interacts with other environmental and societal factors in ways that can either moderate or intensify these impacts. The types and magnitudes of impacts vary across the nation and through time. Children, the elderly, the sick, and the poor are especially vulnerable. There is mounting evidence that harm to the nation will increase substantially in the future unless global emissions of heat-trapping gases are greatly reduced. 24 

### 3. Human-induced climate change is projected to continue, and it will accelerate significantly if global emissions of heat-trapping gases continue to increase.

Heat-trapping gases already in the atmosphere have committed us to a hotter future with more climate-related impacts over the next few decades. The magnitude of climate change beyond the next few decades depends primarily on the amount of heat-trapping gases that human activities emit globally, now and in the future. 23 

### 2. Some extreme weather and climate events have increased in recent decades, and new and stronger evidence confirms that some of these increases are related to human activities.

Changes in extreme weather events are the primary way that most people experience climate change. Human-induced climate change has already increased the number and strength of some of these extreme events. Over the last 50 years, much of the United States has seen an increase in prolonged periods of excessively high temperatures, more heavy downpours, and in some regions, more severe droughts. 22 

# REPORT FINDINGS

5. Climate change threatens human health and well-being in many ways, including through more extreme weather events and wildfire, decreased air quality, and diseases transmitted by insects, food, and water.

Climate change is increasing the risks of heat stress, respiratory stress from poor air quality, and the spread of waterborne diseases. Extreme weather events often lead to fatalities and a variety of health impacts on vulnerable populations, including impacts on mental health, such as anxiety and post-traumatic stress disorder. Large-scale changes in the environment due to climate change and extreme weather events are increasing the risk of the emergence or reemergence of health threats that are currently uncommon in the United States, such as dengue fever. 25



### 8. Climate disruptions to agriculture have been increasing and are projected to become more severe over this century.

Some areas are already experiencing climate-related disruptions, particularly due to extreme weather events. While some U.S. regions and some types of agricultural production will be relatively resilient to climate change over the next 25 years or so, others will increasingly suffer from stresses due to extreme heat, drought, disease, and heavy downpours. From mid-century on, climate change is projected to have more negative impacts on crops and livestock across the country – a trend that could diminish the security of our food supply. 28 

### 7. Water quality and water supply reliability are jeopardized by climate change in a variety of ways that affect ecosystems and livelihoods.

Surface and groundwater supplies in some regions are already stressed by increasing demand for water as well as declining runoff and groundwater recharge. In some regions, particularly the southern part of the country and the Caribbean and Pacific Islands, climate change is increasing the likelihood of water shortages and competition for water among its many uses. Water quality is diminishing in many areas, particularly due to increasing sediment and contaminant concentrations after heavy downpours. 27 

### 6. Infrastructure is being damaged by sea level rise, heavy downpours, and extreme heat; damages are projected to increase with continued climate change.

Sea level rise, storm surge, and heavy downpours, in combination with the pattern of continued development in coastal areas, are increasing damage to U.S. infrastructure including roads, buildings, and industrial facilities, and are also increasing risks to ports and coastal military installations. Flooding along rivers, lakes, and in cities following heavy downpours, prolonged rains, and rapid melting of snowpack is exceeding the limits of flood protection infrastructure designed for historical conditions. Extreme heat is damaging transportation infrastructure such as roads, rail lines, and airport runways. 26

### 18 199. Climate change poses particular threats to Indigenous Peoples’ health, wellbeing, and ways of life.

Chronic stresses such as extreme poverty are being exacerbated by climate change impacts such as reduced access to traditional foods, decreased water quality, and increasing exposure to health and safety hazards. In parts of Alaska, Louisiana, the Pacific Islands, and other coastal locations, climate change impacts (through erosion and inundation) are so severe that some communities are already relocating from historical homelands to which their traditions and cultural identities are tied. Particularly in Alaska, the rapid pace of temperature rise, ice and snow melt, and permafrost thaw are significantly affecting critical infrastructure and
traditional livelihoods. 29



### 12. Planning for adaptation (to address and prepare for impacts) and mitigation (to reduce future climate change, for example by cutting emissions) is becoming more widespread, but current implementation efforts are insufficient to avoid increasingly negative social, environmental, and economic consequences.

Actions to reduce emissions, increase carbon uptake, adapt to a changing climate, and increase resilience to impacts that are unavoidable can improve public health, economic development, ecosystem protection, and quality of life. 32 

### 11. Ocean waters are becoming warmer and more acidic, broadly affecting ocean circulation, chemistry, ecosystems, and marine life.

More acidic waters inhibit the formation of shells, skeletons, and coral reefs. Warmer waters harm coral reefs and alter the distribution, abundance, and productivity of many marine species. The rising temperature and changing chemistry of ocean water combine with other stresses, such as overfishing and coastal and marine pollution, to alter marine-based food production and harm fishing communities. 31 

### 10. Ecosystems and the benefits they provide to society are being affected by climate change. The capacity of ecosystems to buffer the impacts of extreme events like fires, floods, and severe storms is being overwhelmed.

Climate change impacts on biodiversity are already being observed in alteration of the timing of critical biological events such as spring bud burst and substantial range shifts of many species. In the longer term, there is an increased risk of species extinction. These changes have social, cultural, and economic effects. Events such as droughts, floods, wildfires, and pest outbreaks associated with climate change (for example, bark beetles in the West) are already disrupting ecosystems. These changes limit the capacity of ecosystems, such as forests, barrier beaches, and wetlands, to continue to play important roles in reducing the impacts of these extreme events on infrastructure, human communities, and other valued

resources. 30 

## REPORT FINDINGS

Chair Jerry Melillo, Marine Biological Laboratory

Vice-Chairs Terese (T.C.) Richmond, Van Ness Feldman, LLP Gary Yohe, Wesleyan University

Committee Members Daniel Abbasi, GameChange Capital, LLC E. Virginia Armbrust, University of Washington Timothy (Bull) Bennett, Kiksapa Consulting, LLC Rosina Bierbaum, University of Michigan and PCAST Maria Blair, Independent James Buizer, University of Arizona Lynne M. Carter, Louisiana State University F. Stuart Chapin III, University of Alaska Camille Coley, Florida Atlantic University Jan Dell, ConocoPhillips Placido dos Santos, WestLand Resources, Inc. Paul Fleming, Seattle Public Utilities Guido Franco, California Energy Commission Mary Gade, Gade Environmental Group Aris Georgakakos, Georgia Institute of Technology David Gustafson, Monsanto Company David Hales, Second Nature Sharon Hays, Computer Sciences Corporation Mark Howden, CSIRO Anthony Janetos, Boston University Peter Kareiva, The Nature Conservancy Rattan Lal, Ohio State University Arthur Lee, Chevron Corporation Jo-Ann Leong, Hawai‘i Institute of Marine Biology Diana Liverman, University of Arizona and Oxford University Rezaul Mahmood, Western Kentucky University Edward Maibach, George Mason University Michael McGeehin, RTI International Susanne C. Moser, Susanne Moser Research & Consulting and Stanford University Richard Moss, University of Maryland and PNNL Philip Mote, Oregon State University Jayantha Obeysekera, South Florida Water Management District Marie O’Neill, University of Michigan Lindene Patton, Zurich Financial Services John Posey, East-West Gateway Council of Governments Sara Pryor, Indiana University Andrew Rosenberg, University of New Hampshire and Union of Concerned Scientists Richard Schmalensee, Massachusetts Institute of Technology Henry Schwartz, HGS Consultants, LLC Joel Smith, Stratus Consulting Donald Wuebbles, University of Illinois

Ex Officio Committee Members Ko Barrett, U.S. Department of Commerce Katharine Batten, U.S. Agency for International Development Virginia Burkett, U.S. Department of the Interior Patricia Cogswell, U.S. Department of Homeland Security Gerald Geernaert, U.S. Department of Energy John Hall, U.S. Department of Defense Leonard Hirsch, Smithsonian Institution William Hohenstein, U.S. Department of Agriculture Patricia Jacobberger-Jellison, National Aeronautics and Space Administration Thomas R. Karl, Subcommittee on Global Change Research, U.S. Department of Commerce George Luber, U.S. Department of Health and Human Services C. Andrew Miller, U.S. Environmental Protection Agency Robert O’Connor, National Science Foundation Susan Ruffo, White House Council on Environmental Quality Arthur Rypinski, U.S. Department of Transportation Trigg Talley, U.S. Department of State Federal National Climate Assessment and Development Advisory Committee (NCADAC)

John Holdren, Assistant to the President for Science and Technology and Director, White House Office of Science and Technology Policy Katharine Jacobs, Director, National Climate Assessment, White House Office of Science and Technology Policy (through December 2013) Thomas Armstrong, Director, U.S. Global Change Research Program National Coordination Office, White House Office of Science and Technology Policy Thomas R. Karl, Chair, Subcommittee on Global Change Research, U.S. Department of Commerce Tamara Dickinson, Principal Assistant Director for Environment and Energy, White House Office of Science and Technology Policy Fabien Laurier, Director, Third National Climate Assessment, White House Office of Science and Technology Policy Glynis C. Lough, NCA Chief of Staff, U.S. Global Change Research Program David Easterling, NCA Technical Support Unit Director, NOAA NCDC Federal Executive Team

Highlights and Report Production Team Susan Joy Hassol, Senior Science Writer Brooke Stewart, Science Editor/Production Coordinator Tom Maycock, Technical Editor Daniel Glick, Editor Sara W. Veasey, Creative Director Jessicca Griffin, Lead Graphic Designer

Report Authors and Additional Staff, see nca2014.globalchange.gov Assessment Support Staff
Fred ric Lipschultz, Senior Scientist, Regional Coordinator Susan Aragon-Long, Senior Scientist, Sector Coordinator Emily Therese Cloyd, Public Participation/Engagement Coordinator Ilya Fischhoff, Program Coordinator Bryce Golden-Chen, Program Coordinator Julie Maldonado, Engagement Assistant, Tribal Coordinator Alison Delgado, Scientist, Sector Coordinator

### 20 

## U.S. Global Change Research Program 1717 Pennsylvania Avenue, NW • Suite 250 • Washington, DC 20006 USA http://www.globalchange.gov This report summarizes the impacts of climate change on the United States, now and in the future. 

## U.S. National Climate Assessment