2 

Highlights of Global Climate Change Impacts 

 in the United States

This booklet highlights key findings of Global Climate Change Impacts in the United States, 

a state of knowledge report about the observed and projected consequences of climate 

change for our nation and people. It is an authoritative scientific report written in plain 

language, with the goal of better informing public and private decision making at all levels.

The report draws from a large body of scientific information including the set of 21 synthesis 

and assessment products from the U.S. Global Change Research Program, the assessments 

of the Intergovernmental Panel on Climate Change, and much more. It also includes new 

information published since these assessments were released. 

While the primary focus of the report is on the impacts of climate change in the United 

States, it also discusses some of the actions society is already taking or can take to respond 

to the climate challenge. These include limiting climate change by, for example, reducing 

emissions of heat-trapping gases or increasing their removal from the atmosphere.

The importance of our current choices about heat-trapping emissions is underscored by 

comparing impacts resulting from higher versus lower emissions scenarios. Choices about 

emissions made now will have far-reaching consequences for climate change impacts, with 

lower emissions reducing the magnitude of climate change impacts and the rate at which 

they appear. 

The report also identifies examples of options currently being pursued to cope with or 

adapt to the impacts of climate change and/or other environmental issues. One example of 

adaptation is included in this booklet. There is generally insufficient information at present 

to evaluate the effectiveness, costs, and benefits of potential adaptation actions.

This booklet includes a brief overview of the 10 key findings of the 

report, using examples from the report to illustrate each finding. 

References for material in this booklet, including figures, 

can be found in the full report.

The full report can be found online at 

www.globalchange.gov/usimpacts

It is published by Cambridge University 

Press, with hard copies available at: 

www.cambridge.org

ISBN 978-0-521-14407-0
3

Agriculture ecosystems health society

water energy Transportation

Global Climate Change Impacts in the United States describes current and 

future impacts of climate change on various U.S. regions and sectors.

There are both important commonalities and important differences in the climate-related issues and 

consequences faced around the country. For example, water is a key issue in all regions, but the specific 

changes and impacts vary. Regional perspectives are thus critical to thinking through responses to the 

changes that will be faced around the nation.

While the impacts of climate change clearly vary among the regions, there are issues of 

national importance that transcend regional boundaries. Seven such “sectors” (shown 

below) are considered in this report, providing a more integrated national picture of 

impacts, although one with regional differences.

ISLANDS

©iStockphotos.com/Jeffrey Smith

©iStockphotos.com/luchschen ©iStockphotos.com/Clayton Hansen
4 

Forests

Forest growth is generally projected to 

increase in much of the East, but decrease 

in much of the West as water becomes even 

scarcer. Major shifts in species are expected, 

such as maple-beech-birch forests being 

replaced by oak-hickory in the Northeast. 

Insect infestations and wildfires are projected 

to increase as warming progresses. 

Coldwater Fish

Salmon, trout, and other 

coldwater fish will face 

additional stresses as water 

temperatures rise and 

summer streamflows decline. 

Ecosystems and the tourism 

and recreation they support 

will be adversely affected.

Interacting Stresses

Population shifts and development choices 

are making more Americans vulnerable to 

the impacts of climate change. An aging 

populace and continued population shifts to 

the Southeast, Southwest, and coastal cities 

amplify risks associated with extreme heat, 

sea-level rise, storm surge, and increasing 

water scarcity in some regions.

Sea Ice and Permafrost

Risks and costs in Alaska increase as 

thawing of permafrost damages roads, 

buildings, and forests, and declining sea ice 

increases coastal erosion and threatens 

the existence of some communities.

Coral Reefs

Rising water temperatures and 

ocean acidification threaten coral 

reefs and the rich ecosystems they 

support. These and other climaterelated impacts on coastal and 

marine ecosystems will have major 

implications for tourism and fisheries.

©iStockphotos.com/David Parsons

4

Impacts of Climate Change

Climate change is apparent now across our nation. Trends observed in recent decades include rising 

temperatures, increasing heavy downpours, rising sea level, longer growing seasons, reductions in 

snow and ice, and changes in the amounts and timing of river flows. These trends are projected to 

continue, with larger changes resulting from higher amounts of heat-trapping gas emissions, and 

smaller changes from lower amounts of these emissions. The observed changes in climate are already 

causing a wide range of impacts, and these impacts are expected to grow. Select examples follow.
5

Responding to Climate Change

Responses to climate change fall into two major categories. “Mitigation” focuses on reducing emissions of 

heat-trapping gases or increasing their uptake to reduce the amount and speed of climate change. “Adaptation” 

refers to changes made to better respond to present or future climate conditions in order to reduce harm or 

take advantage of opportunities. Both are necessary elements of a comprehensive response strategy.

Energy Supply

Warming will decrease 

demand for heating energy 

in winter and increase 

demand for cooling energy 

in summer. The latter will 

result in significant increases 

in electricity use and higher 

peak demand in most regions.

Agriculture

Increasing heat, pests, 

floods, weeds, and water 

stress will present increasing 

challenges for crop and 

livestock production. 

Heat Waves

Heat waves will become more 

frequent and intense, increasing 

threats to human health and quality 

of life, especially in cities.

Water Supply

Water supplies in the rapidly growing 

Southwest will become increasingly 

scarce, calling for difficult trade-offs 

among competing uses.

Heavy Downpours

More rain is already coming 

in very heavy events, and this 

trend is projected to increase 

across the nation. Such events 

are harmful to transportation 

infrastructure, agriculture, water 

quality, and human health. ©iStockphotos.com/JeremyWedel

Coastal Communities

Sea-level rise and storm surge 

will increase threats to homes 

and infrastructure including 

water, sewer, transportation, 

and communication systems. 

Many barrier islands and coastal 

marshes that protect the 

coastline and support healthy 

ecosystems will be lost. “copyright”©1993WarrenFaidley/weatherstock

Water and Energy 

Interactions

As warming increases 

competition for water, the 

energy sector will be strongly 

affected because power 

plants require large amounts 

of water for cooling. 

5
6 

The emissions responsible for humaninduced warming come primarily from 

the burning of fossil fuels (coal, oil, 

and gas) with additional contributions 

from the clearing of forests and 

agricultural activities. 

Global average temperature has risen 

by about 1.5oF since 1900. By 2100, it 

is projected to rise another 2 to 11.5oF. 

Increases at the lower end of this range 

are more likely if global heat-trapping 

gas emissions are cut substantially. If 

emissions continue to rise at or near 

current rates, temperature increases are 

more likely to be near the upper end of 

the range.

As a result of human activities, the 

present carbon dioxide concentration of

about 385 ppm is about 30 percent 

above its highest level over at least the 

last 800,000 years. In the absence of 

strong control measures, emissions 

projected for this century would result 

in the carbon dioxide concentration 

increasing to a level that is roughly 2 to 

3 times the highest level occurring over 

the last 800,000 or more years.

1 Global warming is unequivocal and primarily 

human-induced

Global Temperature and Carbon Dioxide

As the blue band indicates, without human influences, global average 

temperature would actually have cooled slightly over recent decades.

With human influences, it has risen strongly (black line), consistent 

with expectations from climate models (pink band).

Separating Human and

Natural Influences on Climate

Human activities have been 

clearly linked to many aspects 

of climate change including:

•	 Temperature

•	 Precipitation

•	 Ocean heat content

•	 Atmospheric moisture

•	 Arctic sea ice

TEN KEY FINDINGS

Global temperature has increased over the past 50 years. This observed increase is due 

primarily to human-induced emissions of heat-trapping gases. 
7

2 Climate changes are underway in the United States 

and are projected to grow

Changes observed in the U.S. include:

•	 Temperature rise

•	 Sea-level rise

•	 Increase in heavy downpours

•	 Rapidly retreating glaciers

•	 Thawing permafrost

•	 Longer growing season

•	 Longer ice-free season in the ocean 

and on lakes and rivers

•	 Earlier snowmelt

•	 Changes in river flows

Increases in Amounts of Very Heavy Precipitation 

1958 to 2007

Percentage change in very heavy precipitation

0 - 10% 10 - 20% 20 - 30% 30 - 40% 40 - 50% >60%

Heavy Downpours

One of the most striking changes in climate 

observed over the United States has been 

an increase in the frequency and intensity 

of heavy downpours. This increase was 

responsible for most of the observed increase 

in overall precipitation during the last 50 

years. The amount of precipitation falling in 

the heaviest 1 percent of rain events increased 

nearly 20 percent. During the past 50 years, 

the largest increases in heavy precipitation 

occurred in the Northeast and the Midwest.

Climate models project continued increases in the heaviest downpours during this century, while the 

lightest precipitation is projected to decrease. The chart below shows the projected changes in each 

category of precipitation, from lightest to heaviest, under increasing emissions of heat-trapping gases. 

Much larger increases in heavy downpours are projected under higher emissions scenarios than under lower 

emissions scenarios. 

A note on the emissions scenarios:

None of the emissions scenarios used in 

this report assumes any policies specifically 

designed to address climate change. All, 

including the lower emissions scenario, 

assume increases in heat-trapping gas 

emissions for at least the next few decades, 

though at different rates. The “lower 

emissions scenario” used in this report is the 

B1 scenario from the Intergovernmental Panel 

on Climate Change (IPCC). The “higher 

emissions scenario” is A2. Elsewhere, a third 

“even higher emissions scenario” is used, 

IPCC’s A1FI.

Projected Changes in Light, Moderate, and Heavy 

Precipitation by Late this Century
8 

Rising Temperature

U.S. average temperature has increased by about 2oF over the past 50 years, which is more than the global 

average temperature increase. In the next couple of decades, another degree or so of temperature rise is 

projected. The maps below show projected temperature increases for the middle and the end of this century, 

under both higher and lower emissions scenarios. As the maps illustrate, the higher the emissions, the higher 

the temperature increases. If actual emissions are lower than those represented in the lower emissions 

scenario, temperature increases would be smaller than those shown on the bottom set of maps. 

2 Climate changes are underway in the U.S. and are 

projected to grow

Lower Emissions Scenario

Mid-Century (2040-2059 average) End-of-Century (2080-2099 average)

 Projected Temperature Change (oF) from 1961-1979 Baseline

Higher Emissions Scenario

Mid-Century (2040-2059 average) End-of-Century (2080-2099 average)

The brackets on the thermometers represent 

the likely range of model projections, though 

lower or higher outcomes are possible.
9

Changing Precipitation Patterns

The maps below show projected future changes in precipitation relative to the recent past as simulated by 

15 climate models. The simulations are for late this century, under a higher emissions scenario, with blue 

indicating increasing precipitation, and brown indicating decreasing precipitation. For example, in the 

spring, climate models agree that northern areas are likely to get wetter, and southern areas drier. There is 

less confidence in exactly where the transition between wetter and drier areas will occur. Confidence in the 

projections is highest in the hatched areas.

Projected Change in North American Precipitation

 by 2080-2099
10 

Energy Supply and Use

• Warming will be accompanied by decreases in demand for 

heating energy and increases in demand for cooling energy. The 

latter will result in significant increases in electricity use and 

higher peak demand in most regions.

• Energy production is likely to be constrained by rising 

temperatures and limited water supplies in many regions.

• Energy production and delivery systems are exposed to sea-level 

rise and extreme weather events in vulnerable regions.

• Climate change is likely to affect some renewable energy sources 

across the nation, such as hydropower production in regions 

subject to changing patterns of precipitation or snowmelt.

Society

• Population shifts and development choices are 

making more Americans vulnerable to the 

expected impacts of climate change.

• Vulnerability is greater for people who have few 

resources and few choices.

• City residents and city infrastructure have unique 

vulnerabilities to climate change.

• Climate change affects communities through 

changes in climate-sensitive resources that occur 

both locally and at great distances.

• Insurance is one of the industries particularly 

vulnerable to increasing extreme weather events 

such as severe storms, but it can also help society 

manage the risks.

• The United States is connected to a world that is 

unevenly vulnerable to climate change and thus 

will be affected by impacts in other parts of the 

globe.

Philip Mote

AP Photo/Jeff Roberson

Tony A. Weyiouanna

 Nome Nugget News

Widespread climate-related impacts are occurring 3 now and are expected to increase
11

Ecosystems

• Ecosystem processes, such as those that 

control growth and decomposition, have 

been affected by climate change.

• Large-scale shifts have occurred in the 

ranges of species and the timing of the 

seasons and animal migration, and are very 

likely to continue.

• Fires, insect pests, disease pathogens, and 

invasive weed species have increased, and 

these trends are likely to continue.

• Deserts and drylands are likely to become 

hotter and drier, feeding a self-reinforcing 

cycle of invasive plants, fire, and erosion.

• Coastal and near-shore ecosystems are already under multiple stresses. Climate change and ocean 

acidification will exacerbate these stresses.

• Arctic sea ice ecosystems are already being adversely affected by the loss of summer sea ice and further 

changes are expected.

• The habitats of some mountain species and coldwater fish, such as salmon and trout, are very likely to 

contract in response to warming.

• Some of the benefits ecosystems provide to society will be threatened by climate change, while others 

will be enhanced.

Transportation

• Sea-level rise and storm surge will increase the risk of 

major coastal impacts, including both temporary and 

permanent flooding of airports, roads, rail lines, and 

tunnels.

• Flooding from increasingly intense downpours will 

increase the risk of disruptions and delays in air, rail, and 

road transportation, and damage from mudslides in some 

areas.

• The increase in extreme heat will limit some transportation 

operations and cause pavement and track damage. 

Decreased extreme cold will provide some benefits such as 

reduced snow and ice removal costs.

• Increased intensity of strong hurricanes would lead to 

more evacuations, infrastructure damage and failure, and 

transportation interruptions.

• Arctic warming will continue to reduce sea ice, lengthening 

the ocean transport season, but also resulting in greater 

coastal erosion due to waves. Permafrost thaw in Alaska 

will damage infrastructure. The ice road season will 

become shorter.

AP Photo/Jeff Roberson

Near Tucson, Arizona
12 

GREAT PLAINS

• Increasing temperature, 

evaporation, and drought 

frequency compound 

water scarcity problems

• Agriculture, ranching, and 

natural lands are stressed 

by limited water supplies 

and rising temperatures

• Alteration of key habitats 

such as prairie potholes 

affects native plants and 

animals

ALASKA

• Summers get hotter and drier, 

with increasing evaporation 

outpacing increased precipitation

• Wildfires and insect problems 

increase

• Lakes decline in area

• Permafrost thawing damages 

infrastructure

• Coastal storms increase risks to 

villages and fishing fleets

• Shifts in marine species affect 

fisheries

SOUTHWEST

• Scarce water supplies call for trade-offs 

among competing uses

• Increasing temperature, drought, 

wildfire, and invasive species accelerate 

landscape transformation

• Increased frequency and altered timing 

of flooding increases risks to people, 

ecosystems, and infrastructure

• Unique tourism and recreation 

opportunities are likely to suffer

ISLANDS

• Likely reductions in 

freshwater availability 

have significant 

impacts 

• Sea-level rise and 

storms threaten 

island communities

• Climate changes 

affecting coastal and 

marine ecosystems 

have major 

implications for 

tourism and fisheries

NORTHWEST

• Declining snowpack reduces 

summer streamflows, straining 

water resources including those 

needed for hydroelectric power

• Increasing wildfires, insects, and 

species shifts pose challenges 

for ecosystems and the forest 

products industry

• Rising water temperatures and 

declining summer streamflows 

threaten salmon and other 

coldwater fish species

• Sea-level rise increases erosion 

and land loss

3

GREAT PLAINS NORTHWEST

ALASKA

SOUTHWEST

ISLANDS

Widespread climate-related impacts are occurring 

now and are expected to increase
13

NORTHEAST

• Extreme heat and declining air quality are 

likely to pose increasing health risks

• Production of milk, fruits, and maple 

syrup is likely to be adversely affected 

• More frequent flooding due to sea-level 

rise, storm surge, and heavy downpours

• Reduced snow negatively affects winter 

recreation

• Lobster fishery continues northward 

shift; cod fishery further diminished

MIDWEST

• Heat waves, air quality problems, and 

insect and waterborne diseases increase

• Reduced water levels in the Great Lakes 

affect shipping, infrastructure, beaches, 

and ecosystems under a higher emissions 

scenario

• More periods of both floods and water 

deficits occur

• Floods, droughts, insects, and weeds 

challenge agriculture

• Diseases and invasive species threaten 

native fish and wildlife

SOUTHEAST

• Increases in air and water temperatures 

stress people, plants, and animals

• Decreased water availability is very likely to 

affect the economy and natural systems

• Sea-level rise and increases in hurricane 

intensity and storm surge cause serious 

impacts

• Thresholds are likely to be crossed, causing 

major disruptions to ecosystems and the 

benefits they provide to people 

• Severe weather events and reduced 

availability of insurance will affect coastal 

communities COASTS

• Significant sea-level rise increases risks 

to coastal cites

• More spring runoff and warmer water 

will increase the seasonal reduction of 

oxygen in coastal ecosystems

• Coral reefs will be affected by higher 

temperatures and ocean acidification

• Changing ocean currents will affect 

coastal ecosystems

SOUTHEAST

MIDWEST

NORTHEAST

SOUTHEAST
14 

4 Climate change will stress water resources

Climate change affects water resources:

•	 Water cycle is altered, affecting where, 

when, and how much water is available

•	 Floods and droughts become more 

common and intense as regional and 

seasonal precipitation patterns change 

and rainfall is more concentrated into 

heavy events with longer dry periods in 

between

•	 Less snow, more rain

•	 Wet areas get wetter, dry areas get drier

•	 Mountain snowpack declines, timing of 

runoff shifts to earlier in spring, and flows

are lower in summer

•	 Increased competition for water supplies

5 Crop and livestock production will be 

increasingly challenged

Water is an issue in every region, but the nature of the potential impacts varies. Drought – related to 

reduced precipitation, increased evaporation, and increased water loss from plants – is an important issue 

in many regions, especially in the West. Floods and water quality problems are likely to be amplified by 

climate change in most regions. Declines in 

mountain snowpack are important in the West 

and Alaska where snowpack provides vital 

natural water storage.

• Many crops show positive responses to 

elevated carbon dioxide and low levels of 

warming, but higher levels of warming often 

negatively affect growth and yields.

• Extreme events such as heavy downpours 

and droughts are likely to reduce crop yields 

because excesses or deficits of water have 

negative impacts on plant growth.

• Weeds, diseases, and insect pests benefit from 

warming, and weeds also benefit from a higher 

carbon dioxide concentration, increasing stress 

on crop plants and requiring more attention to 

pest and weed control. 

• Forage quality in pastures and rangelands 

generally declines with increasing carbon 

dioxide concentration because of the effects on 

plant nitrogen and protein content, reducing the land’s ability to supply adequate livestock feed. 

• Increased heat, disease, and weather extremes are likely to reduce livestock productivity.

Weed Growth is Promoted and 

Herbicide Loses Effectiveness at Higher CO2

Potential Future CO2

(680 ppm)

Current CO2

(380 ppm)

Runoff is the amount of precipitation that is not evaporated, stored 

as snowpack or soil moisture, or filtered down to groundwater. 

Projected changes in median runoff for 2041-2060, relative to a 

1901-1970 baseline, are mapped by water resource region. Colors 

indicate percentage changes in runoff. Hatched areas indicate greater 

confidence due to strong agreement among model projections. White 

areas indicate divergence among model projections. 

Projected Changes in Annual Runoff
15

Global sea level is rising due to the warminginduced expansion of ocean water, accelerated 

melting of most of the world’s glaciers, and 

loss of ice from the Greenland and Antarctic 

ice sheets. Additional warming will cause 

further sea-level rise over this century and 

beyond. 

Rising sea level is already eroding shorelines, 

drowning wetlands, and threatening homes, 

businesses, and infrastructure. The destructive 

potential of Atlantic hurricanes has increased 

in recent decades in association with 

increasing sea surface temperatures, and it is 

likely that hurricane rainfall and wind speeds 

will increase in response to global warming. 

Coastal water temperatures have risen and the 

geographic distributions of marine species 

have shifted.

Precipitation increases on land have increased 

river runoff, polluting coastal waters with 

more nitrogen and phosphorous, sediments, 

and other contaminants. Ocean acidification 

resulting from the uptake of carbon dioxide by 

ocean waters threatens corals, shellfish, and 

other living things that form their shells and 

skeletons from calcium carbonate. 

All of these forces converge and interact at 

the coasts, making these areas particularly 

sensitive to the impacts of climate change. 

6 Coastal areas are at increasing risk from sea-level rise, 

storm surge, and other climate-related stresses

Projected Sea-Level Rise

Based on the state of the science at the time, in 2007 the Intergovernmental Panel on Climate Change (IPCC) projected a rise 

of the world’s oceans from 8 inches to 2 feet by the end of this 

century (range shown as bars). However, they could not quantify the 

contributions to sea-level rise due to changes in ice sheet dynamics. 

More recent research has attempted to quantify this contribution 

by estimating future sea level based on its observed relationship to 

temperature. For example, the projections indicated by the light 

blue circles in the figure above estimate global average sea-level rise 

of almost 3.5 feet by the end of this century under a high emissions 

scenario. In areas where the land is sinking, such as the Atlantic and 

Gulf Coasts of the United States, sea-level rise will be higher than 

the global average.

Coastal impacts include:

•	 Sea-level rise leads to erosion and loss 

of land

•	 Storm surge causes flooding

•	 Increased pollution due to more runoff 

from the land

•	 	 Ocean acidification threatens coral

reefs and shellfish

Jack Pellette/NWS
16 

7 Threats to human health will increase

Number of Days Over 100oF

High air temperatures are uncomfortable, stressful, and 

increase the risk of heat-related illnesses. The number of 

days in which the temperature exceeds 100oF by late this 

century, compared to the 1960s and 1970s, is projected 

to increase strongly across the United States. 

Lower Emissions Scenario, 2080-2099

Recent Past, 1961-1979 

Higher Emissions Scenario, 2080-2099

Major health risks include:

•	 Heat stress

•	 Waterborne diseases (due to heavy 

downpours and higher temperatures)

•	 Poor air quality

•	 Extreme weather events

•	 Diseases caused by insects and rodents

•	 Pollen increase

•	 Children, the elderly, and the poor are 

most vulnerable to climate-related health 

effects

Climate change poses unique challenges to human 

health including heat waves and severe storms, ailments caused or exacerbated by air pollution and airborne allergens, and many climate-sensitive infectious 

diseases. Whether or not increased health risks due 

to climate change are realized will depend largely on 

societal responses and underlying vulnerability. The 

probability of exacerbated health risks due to climate 

change points to a need to maintain a strong public 

health infrastructure to help limit future impacts.

Increases in heat-related deaths are projected in cities 

around the nation, especially under higher emissions 

scenarios. This analysis included some adaptation 

measures. The graph shows the projected number of 

deaths per year, averaged over a three-decade period 

around 1975, 2055, and 2085 for the City of Chicago 

under lower and higher emissions.

Projected Increase in 

Heat-Related Deaths in Chicago
17

8 Climate change will interact with many 

social and environmental stresses

Social trends can increase our vulnerability to climate change. Recent trends include concentration 

of development along vulnerable coasts, aging of the U.S. population, increasing urbanization, and 

population growth in the South and West. Overlaying projections of future climate change and its impacts 

on expected changes in U.S. population and development patterns reveals a critical insight: More Americans 

will be living in the areas that are most vulnerable to the effects of climate change, from sea-level rise and 

hurricanes in the Southeast, to water scarcity in the Southwest.

Pollution and the resulting poor air quality is one of many human-caused stresses that interact with climate 

change to amplify impacts. The combination of air pollution with heat, drought, and stagnant air negatively 

affects human health and quality of life in cities.

Geographic Vulnerability of U.S. Residents to 

Selected Climate-Related Health Impacts

Location of Hurricane Landfalls,

1995 to 2000

Percentage of Population 

Aged 65 or Older, 2000

Locations of Extreme Heat Events,

1995 to 2000

West Nile Virus Cases,

2004

Maps indicating U.S. counties, or in some cases states, with existing vulnerability to climate-sensitive health 

outcomes. These examples demonstrate both the diversity of climate-sensitive health outcomes and the 

geographic variability of where they occur.
18 

Land subsidence (sinking) associated with the thawing of permafrost in Alaska, in addition to damaging forests and other natural systems, presents substantial challenges to engineers attempting to preserve 

infrastructure.

Cod in the North Atlantic are adapted to annual average temperatures near the seafloor ranging from 36 

to 54°F. Large populations of cod are generally not found above the 54°F threshold. Temperature also 

influences the location and timing of spawning, which in turn affect the growth and survival of young cod. 

Increases in bottom temperatures above 47°F are projected to lead to a decline in growth and survival. 

Projections of warming indicate that both the 47°F and the 54°F thresholds will be met or exceeded in this 

century under a higher emissions scenario.

Lobster catches in the southern part of the Northeast region have declined sharply in the past decade, 

associated with a temperature-sensitive bacterial shell disease. The southern extent of the commercial 

lobster harvest appears to be limited by this disease, the effects of which are expected to increase with 

rising near-shore water temperatures.

Crossing key temperature thresholds has contributed to major insect outbreaks that have destroyed millions 

of acres of trees in the United States. First, warmer winters allow larger populations of insects to survive 

the cold season that normally limits their numbers. Second, the longer warm season allows them to develop 

faster, sometimes completing 

two life cycles instead of one in 

a single growing season. Third, 

warmer conditions allow insects’ 

ranges to expand northward. 

Fourth, drought stress reduces 

trees’ ability to resist insect 

attack (for example, by pushing 

back against boring insects with 

the pressure of their sap). 

Spruce beetle, pine beetle, 

spruce budworm, and woolly 

adelgid (which attacks eastern 

hemlocks) are just some of the 

insects that are proliferating 

in the United States, causing 

devastation in many forests. 

These outbreaks are projected 

to increase with ongoing 

warming. Trees killed by insects 

also provide more dry fuel for 

wildfires.

9 Thresholds will be crossed, leading to large changes 

in climate and ecosystems

Alaska Spruce Beetle Infestation

Kenai Peninsula, 1972 to 1998

There are a variety of thresholds in the climate system and ecosystems. These thresholds 

determine, for example, the presence of sea ice and permafrost, and the survival of species, from 

fish to insect pests, with implications for society. With further climate change, the crossing of 

additional thresholds is expected.
19

Human-induced climate change is happening now, and impacts are already apparent. Greater impacts 

are projected, particularly if heat-trapping gas emissions continue unabated. Previous assessments have 

established these facts, and this report confirms, solidifies, and extends these conclusions for the United 

States. It reports the latest understanding of how climate change is already affecting important sectors 

and regions. In particular, it reports that some climate change impacts appear to be increasing faster than 

previous assessments had suggested.

Choices about emissions now and in the coming years 

will have far-reaching consequences for climate change 

impacts. A consistent finding of this assessment is that the 

rate and magnitude of future climate change and resulting 

impacts depend critically on the level of global atmospheric 

heat-trapping gas concentrations. Lower emissions of heattrapping gases will delay the appearance of climate change 

impacts and lessen their magnitude. Unless the rate of 

emissions is substantially reduced, impacts are expected to 

become increasingly severe for more people and places. 

Similarly, there are choices to be made about adaptation 

strategies that can help to reduce or avoid some of the 

undesirable impacts of climate change. One such example 

is shown below and other examples appear in the full report. 

There is much to learn about the effectiveness of the various 

types of adaptation responses and how they will interact with 

each other and with mitigation actions.

As our nation strives to develop effective strategies to 

respond to climate change, it is essential to have the latest and best scientific information about the impacts 

of climate change in the United States. The report highlighted in this booklet provides that information.

10 Future climate change and its impacts depend on 

choices made today

The amount and rate of future climate change depends primarily on current and future humancaused emissions of heat-trapping gases and airborne particles. Responses involve reducing 

emissions to limit future warming, and adapting to the changes that are unavoidable.

Global Average Temperature

1900-2100

Observed and projected changes in the global 

average temperature under three IPCC emissions 

scenarios that assume no policies to address 

climate change. The shaded areas show the likely 

ranges while the lines show the central projections 

from a set of climate models. A wider range of 

model types shows outcomes from 2 to 11.5oF. 

Changes are relative to the 1960-1979 average. 

Boston’s Deer Island sewage treatment plant was designed and built taking 

future sea-level rise into consideration. Because the level of the plant relative 

to the level of the ocean at the outfall is critical to the amount of rainwater 

and sewage that can be treated, the plant was built 1.9 feet higher than it would 

otherwise have been to accommodate the amount of sea-level rise projected 

to occur by 2050, the planned life of the facility. 

The planners recognized that the future would be different from the past and 

they decided to plan for the future based on the best available information. They assessed what could be easily 

and inexpensively changed at a later date versus those things that would be more difficult and expensive to 

change later. For example, increasing the plant’s height would be less costly to incorporate in the original design, 

while protective barriers could be added at a later date, as needed, at a relatively small cost.

Adaptation Example: Raising a Sewage Treatment Plant in Boston 

Massachusetts Water Resources Authority
U.S. Global Change Research Program

1717 Pennsylvania Avenue, NW • Suite 250 • Washington, DC 20006 USA

1-202-223-6262 (voice) • 1-202-223-3065 (fax)

http://www.globalchange.gov

Senior Science Writer and Lead Graphic Designer

Senior Science Writer

Susan J. Hassol, 

Climate Communication, LLC

Lead Graphic Designer

Sara W. Veasey, 

NOAA National Climatic Data Center

Federal Advisory Committee Authors

David M. Anderson,

NOAA World Data Center for 

Paleoclimatology

Donald F. Boesch,

University of Maryland Center for 

 Environmental Science

Virginia R. Burkett,

U.S. Geological Survey

Lynne M. Carter,

Adaptation Network

Stewart J. Cohen, 

Environment Canada and University of 

British Columbia

Nancy B. Grimm, 

Arizona State University

Jerry L. Hatfield,

U.S. Department of Agriculture

Katharine Hayhoe,

Texas Tech University

Anthony C. Janetos,

Pacific Northwest National Laboratory

Jack A. Kaye, 

National Aeronautics and Space

Administration

Jay H. Lawrimore, 

NOAA National Climatic Data Center

James J. McCarthy, 

Harvard University

A. David McGuire,

U.S. Geological Survey/University of 

Alaska Fairbanks

Edward L. Miles,

University of Washington

Evan Mills,

Lawrence Berkeley National Laboratory

Jonathan T. Overpeck,

University of Arizona

Jonathan A. Patz,

University of Wisconsin at Madison

Roger S. Pulwarty,

NOAA Climate Program Office and Earth 

System Research Laboratory

Benjamin D. Santer,

Lawrence Livermore National Laboratory

Thomas R. Karl, 

NOAA National Climatic Data Center

Jerry M. Melillo, 

Marine Biological Laboratory 

Thomas C. Peterson, 

NOAA National Climatic Data Center 

Author Team

Michael J. Savonis, 

U.S. Department of Transportation

H. Gerry Schwartz, Jr.,

Consultant/Transportation

Eileen L. Shea, 

NOAA National Climatic Data Center/

Integrated Data and Environmental 

Applications Center

John M.R. Stone,

Carleton University

Bradley H. Udall, 

University of Colorado/NOAA Earth 

System Research Laboratory

John E. Walsh,

University of Alaska Fairbanks

Michael F. Wehner, 

Lawrence Berkeley National Laboratory

Thomas J. Wilbanks,

Oak Ridge National Laboratory

Donald J. Wuebbles,

University of Illinois

Co-Chairs and Editors-in-Chief

June 2009