12

OUR CHANGING CLIMATE

Climate and the Greenhouse Effect

arth’s climate is determined by complex interactions between the sun,oceans,atmosphere,land,and living things. The composition of the atmosphere is particularly

important because certain gases (including water vapor, carbon dioxide,methane,halocarbons,ozone,and nitrous oxide) absorb heat radiated from the Earth's surface. As the

atmosphere warms,it in turn radiates heat back to the surface,to create what is commonly called the "greenhouse effect." Changes in the composition of the atmosphere alter the

intensity of the greenhouse effect. Such changes,which have occurred many times in the

planet’s history, have helped determine past climates and will affect the future climate as

well.

Human Activities Alter the Balance

umans are exerting a major and growing influence on some of the key factors that

govern climate by changing the composition of the atmosphere and by modifying

the land surface. The human impact on these factors is clear. The concentration of carbon dioxide (CO2

) has risen about 30% since the late 1800s. The concentration of CO2is

now higher than it has been in at least the last 400,000 years. This increase has resulted

from the burning of coal,oil,and natural gas,and the destruction of forests around the

world to provide space for agriculture and other human activities. Rising concentrations

of CO2and other greenhouse gases are intensifying Earth’s natural greenhouse effect.

Global projections of population growth and assumptions about energy use indicate that

the CO2concentration will continue to rise,likely reaching between two and three times

its late-19th-century level by 2100. This dramatic doubling or tripling will occur in the

space of about 200 years,a brief moment in geological history.

E

H

Global projections

based on population

growth and assumptions about energy

use indicate that the

CO2concentration will

continue to rise, likely

reaching somewhere

between two and

three times its preindustrial level by

2100. 

About half the solar energy

absorbed at the surface

evaporates water, adding the

most important greenhouse

gas to the atmosphere.

When this water condenses

in the atmosphere, it releases the energy that powers

storms and produces rain

and snow.

The surface cools by radiating heat energy upward. The warmer the surface, the

greater the amount of heat energy that is

radiated upward.

Only a small amount of the heat

energy emitted from the surface

passes through the atmosphere

directly to space. Most is absorbed

by greenhouse gas molecules and

contributes to the energy radiated

back down to warm the surface and

lower atmosphere. Increasing the

concentrations of greenhouse gases

increases the warming of the surface

and slows loss of energy to space.

About 30% of incoming

solar energy is reflected

by the surface and the

atmosphere

The Earth’s Greenhouse Effect
The Climate Is Changing

s we add more CO2and other

heat-trapping gases to the

atmosphere,the world is becoming

warmer (which changes other

aspects of climate as well). Historical

records of temperature and precipitation have been extensively analyzed

in many scientific studies. These studies demonstrate that the global average surface temperature has

increased by over 1oF (0.6oC) during

the 20th century. About half this rise

has occurred since the late 1970s.

Seventeen of the eighteen warmest

years in the 20th century occurred

since 1980. In 1998,the global temperature set a new record by a wide

margin, exceeding that of the previous record year, 1997, by about 0.3oF

(0.2oC). Higher latitudes have

warmed more than equatorial

regions,and nighttime temperatures

have risen more than daytime temperatures.

As the Earth warms,more water evaporates from the oceans and lakes,

eventually to fall as rain or snow.

During the 20th centur y, annual precipitation has increased about 10% in

the mid- and high-latitudes. The

warming is also causing permafrost

to thaw, and is melting sea ice,snow

cover, and mountain glaciers. Global

sea level rose 4 to 8 inches (10-20

cm) during the 20th century because

ocean water expands as it warms and

because melting glaciers are adding

water to the oceans.

According to the Intergovernmental

Panel on Climate Change (IPCC),scientific evidence confirms that human

activities are a discernible cause of a

substantial part of the warming experienced over the 20th centur y. New

studies indicate that temperatures in

recent decades are higher than at any

time in at least the past 1,000 years.

It is very unlikely that these unusually

high temperatures can be explained 

solely by natural climate variations.

The intensity and pattern of temperature changes within the atmosphere

implicates human activities as a

cause.

The relevant question is not whether

the increase in greenhouse gases is

contributing to warming,but rather,

what will be the amount and rate of

future warming and associated climate changes,and what impacts will

those changes have on human and

natural systems.

A

0

1

2

3

4

5

6

7

8

9

Carbon Emissions

Fossil-fuels (Gt C)

Land-use Change (Gt C)

Records of Northern Hemisphere surface temperatures, CO2

concentrations, and carbon emissions show a close correlation. Temperature

Change: reconstruction of annual-average

Northern Hemisphere surface air temperatures

derived from historical records, tree rings, and

corals (blue), and air temperatures directly measured (purple). CO2 Concentrations: record

of global CO2concentration for the last 1000

years, derived from measurements of CO2concentration in air bubbles in the layered ice cores

drilled in Antarctica (blue line) and from atmospheric measurements since 1957. Carbon

Emissions: reconstruction of past emissions

of CO2as a result of land clearing and fossil fuel

combustion since about 1750 (in billions of metric tons of carbon per year). 

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

CO2 Concentrations

Temperature Change

1000 Years of Global CO2 and Temperature Change

1

0.8

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

380

360

340

320

300

280

260

9

8

7

6

5

4

3

2

1

0
or this study,three tools were

used to examine the potential

impacts of climate change on the US:

historical records,comprehensive

state-of-the-science climate simulation

models,and sensitivity analyses

designed to explore our vulnerability

to future climate change. These three

tools were used because prudent risk

management requires consideration

of a spectrum of possibilities.

Historical Records

ow do changes in climate affect

human and natural systems?

Records from the past provide an

informed perspective on this question. There have been a number of

climate variations and changes during

the 20th century. These include substantial warming,increases in precipitation,decade-long droughts,and

reduction in snow cover extent.

Analyzing these variations,and their

effects on human and natural systems,provides important insights into

how vulnerable we may be in the

future.

Climate Model Simulations

lthough Earth’s climate is

astoundingly complex,our ability to use supercomputers to simulate

the climate is growing. Today’s climate models are not infallible,but

they are powerful tools for understanding what the climate might be

like in the future.

A key advantage of climate models is

that they are quantitative and grounded in scientific measurements. They

are based on fundamental laws of

physics and chemistry, and incorporate human and biological interactions. They allow examination of a

range of possible futures that cannot

be examined experimentally.

Our confidence in the accuracy of climate models is growing. The best

models have been carefully evaluated

by the IPCC and have the ability to

replicate most aspects of past and

present climates. Two of these models have been used to develop climate change scenarios for this

Assessment.These scenarios should

be regarded as projections of what

might happen, rather than precise

predictions of what will happen.

Sensitivity Analyses

hat degree of climate change

would cause significant

impacts to natural and human systems? In other words,how vulnerable and adaptable are we? To help

answer such questions,scientists can

perform "sensitivity analyses" to

determine under what conditions and

to what degree a system is sensitive

to change. Such analyses are not predictions that such changes will,in

fact,occur;rather, they examine what

the implications would be if the specified changes did occur. For example,

an analyst might ask,"How large

would climate change have to be in

order to cause a specified impact?"

14

Climatologists use two types of data to monitor climate change. The first are historical measurements of temperature, precipitation,

humidity, pressure, and wind speed taken at thousands of locations across the globe. Because observing methods, instruments,

and station locations have changed over time, climatologists use various methods to crosscheck and corroborate these historical

data sets. For example, satellite and balloon records confirm that the planet has been warming for the past four decades, although

rates of atmospheric and surface warming differ somewhat from decade to decade. To peer further back into the past, climatologists

also analyze physical, biological, and chemical indicators. For example, past climate conditions can be inferred from the width of

tree rings, air trapped in ancient ice cores, and sediment deposited at the bottom of lakes and oceans. Taken together, this information demonstrates that the Earth’s climate over the past 10,000 years has been relatively stable compared to the 10,000 years that

preceeded this period and compared to the 20th century.

TOOLS FOR ASSESSING CLIMATE CHANGE IMPACTS 

F

H

A W

Climate Observations
15

Earth's climate is far too complex to

reproduce in a laboratory. An alternative is

to devise a mathematical representation, or model,

that can be used to simulate past, present, and future

climate conditions. These models incorporate the key physical parameters and processes that govern climate behavior. Once

constructed, they can be used to investigate how a change in greenhouse gases, or a volcanic eruption, might modify the climate. 

Computer models that simulate Earth's climate are called General Circulation Models or GCMs. The

models can be used to simulate changes in temperature, rainfall, snow cover, winds, soil moisture, sea

ice, and ocean circulation over the entire globe through the seasons and over periods of decades.

However, mathematical models are obviously simplified versions of the real Earth that cannot capture

its full complexity, especially at smaller geographic scales. Real uncertainties remain in the ability of

models to simulate many aspects of the future climate. The models provide a view of future climate

that is physically consistent and plausible, but incomplete. Nonetheless, through continual improvement over the last several decades, today’s GCMs provide a state-of-the-science glimpse into the next

century to help understand how climate change may affect the nation.

Climate Models

(Snow Cover,

 Vegetation, Reflectivity,

 Topography and Land Use)

Modeling the Climate System

Precipitation

Evaporation

Ocean Model

Layers

Stratus Clouds

Transition from

Solid to Vapor

Outgoing Heat

Energy

Snow Cover

Runoff

Evaporative

and Heat Energy

Exchanges

Incoming Solar

Energy

Atmosphere

(Temperature, Winds,

and Precipitation)

Cumulus

Clouds

Cirrus Clouds

Sea

Ice

Vertical

Overturning

Precipitation

& Evaporation

Heat & Salinity Ocean Exchange

(Currents, Temperature, and Salinity)

Atmospheric GCM

Winds

 Includes the Atmosphere,

 Land, Oceans, Ice, and Biosphere

 Soil

Moisture

Land Surface

Ocean

 Bottom

 Topography

Realistic Ocean GCM

Geography

Stratus

 Clouds
16

TOOLS FOR ASSESSING CLIMATE CHANGE IMPACTS 

Scenarios of the Future

nformation about the future is valuable, even if it is somewhat uncertain. For example,

many people plan their days around weather forecasts with uncertainty conveyed in

words or numbers. If there is "a 70% chance of rain" we might take an umbrella with us

to work. It may not rain,but if it does, we are prepared. Likewise,although the tools used

in this report to explore the possible range of climate change impacts – historical records,

computer simulations,and sensitivity analyses – contain uncertainties,their use still provides much valuable information for policymakers,planners,and citizens.

The fact that the climate is changing is apparent from detailed historical records of climate that provide a benchmark for assessing the future. Scientists’understanding of

America’s future climate – and of the impacts that this altered climate is likely to have on

agriculture,human health, water resources,natural ecosystems,and other key issues – has

been advanced by the use of computer simulations. Together,the historical record and

computer simulations indicate that America’s climate is very likely to continue changing

in the 21st centur y, and indeed,that these changes are likely to be substantially larger than

those in the 20th century, with significant impacts on our nation.

Climate Models used in the US Assessment

limate models continue to improve,and assumptions about future greenhouse gas

emissions continue to evolve. The two primary models used to project changes in

climate in this Assessment were developed at the Canadian Climate Centre and the Hadley

Centre in the United Kingdom. They have been peer-reviewed by other scientists and

both incorporate similar assumptions about future emissions (both approximate the midrange emissions scenario described on page 4).These models were the best fit to a list of

criteria developed for this Assessment.Climate models developed at the National Center

for Atmospheric Research (NCAR),NOAA's Geophysical Fluid Dynamics Laboratory

(GFDL), NASA's Goddard Institute for Space Studies (GISS),and Max Planck Institute (MPI)

in Germany, were also used in various aspects of the Assessment.

While the physical principles driving these models are similar,the models differ in how

they represent the effects of some important processes. Therefore,the two primary models paint different views of 21st century climate. On average over the US,the Hadley

model projects a much wetter climate than does the Canadian model,while the Canadian

model projects a greater increase in temperature than does the Hadley model. Both projections are plausible, given current understanding. In most climate models,increases in

temperature for the US are significantly higher than the global average temperature

increase. This is due to the fact that all models project the warming to be greatest at middle to high latitudes,partly because melting snow and ice make the surface less reflective

of sunlight,allowing it to absorb more heat. Warming will also be greater over land than

over the oceans because it takes longer for the oceans to warm.

I

C

On average over the

US, the Hadley model

projects a wetter climate than does the

Canadian model,

while the Canadian

model projects a

greater increase in

temperature than

does the Hadley

model.
Uncertainties about future climate

stem from a wide variety of factors,

from questions about how to represent clouds and precipitation in climate models to uncertainties about

how emissions of greenhouse gases

will change. These uncertainties

result in differences in climate model

projections. Examining these differences aids in understanding the range

of risk or opportunity associated with

a plausible range of future climate

changes. These differences in model

projections also raise questions about

how to interpret model results,especially at the regional level where projections can differ significantly.

17

Changes in Temperature over the US

Simulated by Climate Models

Simulations from leading climate models of changes in decadal average surface temperature for

the conterminous US (excluding Alaska and Hawaii) based on historic and projected changes in

atmospheric concentrations of greenhouse gases and sulfate aerosols. The heavy red and black

lines indicate the primary models used by the National Assessment. For the 20th century, the

models simulate a US temperature rise of about 0.7 to 1.9 ̊F, whereas estimates from observations

range from 0.5 to 1.4 ̊F; estimates for the global rise are 0.9 to 1.4 ̊F for models and 0.7 to 1.4 ̊F for

observations, suggesting reasonable agreement. For the 21st century, the models project warming ranging from 3 to 6 ̊F for the globe and 3 to 9 ̊F for the US. The two models at the low end of

this range assume lower emissions of greenhouse gases than do the other models.

Observed and Modeled 

Average Annual Temperature

Observed 1961-1990 Average

Canadian Model 1961-1990 Average

Hadley Model 1961-1990 Average

The observed temperature averages for 1961-1990 are similar to the temperatures simulated by the Canadian and

Hadley models for the same time period. These are the two

primary models used to develop climate change scenarios

for this Assessment.

100oF

90oF

80oF

70oF

60oF

50oF

40oF

30oF

20oF

10oF

0oF

+100oF

90oF

80oF

70oF

60oF

50oF

40oF

30oF

20oF

10oF

0oF

+100oF

90oF

80oF

70oF

60oF

50oF

40oF

30oF

20oF

10oF

0oF

Range of Projected Warming

in the 21st Centur y

*Hadley Centre - version 2

*Canadian Centre

Max Planck Institute

Geophysical Fluid Dynamics Laboratory

Hadley Centre - version 3

NCAR - parallel version

NCAR - climate system model

*Hadley Model +5oF +5oF

*Canadian Model +8oF +9oF

MPI, GFDL and +3 to 6oF +3 to 9oF

NCAR Models 

Global US

The two primary climate models used

in this Assessment

have been peerreviewed and both

incorporate similar

assumptions about

future emissions

(both approximate

the IPCC "IS92a"

scenario with a 1%

per year increase in

greenhouse gases

and growing sulfur

emissions).

1850 1900 1950 2000 2050 2100

9

8

7

6

5

4

3

2

1

0

-1

-2

-3

-4

-5

*The two primary models used in the Assessment.
18

TOOLS FOR ASSESSING CLIMATE CHANGE IMPACTS 

Interpreting Climate Scenarios

ur level of confidence in climate scenarios depends on what aspect is being considered,and over what spatial scale and time period.Increases in greenhouse gases will

cause global temperatures to increase. There is less certainty about the magnitude of the

increase,because we lack complete knowledge of the climate system and because we do

not know how human society and its energy systems will evolve. Similarly, we are confident that higher surface temperatures will cause an increase in evaporation,and hence in

precipitation,but less certain about the distribution and magnitude of these changes.

The most certain climate projections are those that pertain to large-scale regions,are

given as part of a range of possible outcomes,and are applied to trends over the next century. Model projections of continental-scale and century-long trends are more reliable

than projections of shorter-term trends over smaller scales. Projections on a decade-bydecade basis,and projections of transient weather phenomena such as hurricanes,are

considerably less certain. Two examples serve to illustrate this point. Most climate models project warming in the eastern Pacific, resulting in conditions that look much like current El Niño conditions. When today’s existing El Niño pattern is superimposed on this El

Niño-like state,El Niño events would likely be more intense,as would their impacts on US

weather. Some recent studies suggest that El Niño and La Niña conditions are likely to

become more frequent and intense.Other studies suggest little overall change. While

these projections must be interpreted with caution,prudent risk management suggests

considering the possibility of increases in El Niño and La Niña intensity and frequency.

The projections are less certain regarding changes in the incidence of tropical storms and

hurricanes. Some recent studies suggest that hurricanes will become more intense,while

others project little change. It is possible that a 5-10% increase in hurricane wind speed

will occur by 2100;confirming this remains an important research issue. Perhaps a more

important concern is rainfall during hurricanes.

One set of model simulations projects that peak

precipitation rates during hurricanes will increase

25-30% by the end of the 21st century. Today, El

Niño conditions are associated with increased

Pacific and decreased Atlantic hurricane frequencies. La Niña is associated with increased Atlantic

hurricane frequencies. However, hurricane formation is dependent on a large number of atmospheric and surface conditions. Given these complex dynamics,projections for changes in the frequency and paths of tropical storms must be

viewed with caution.

O

1

2

3

4

5

6

7

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

El Niño and La Niña Effects on the Chance of

Landfalling Hurricanes over the 20th Centur y

During El Niño and La Niña years, the chance of landfalling hurricanes on the Gulf and Atlantic coasts changes

dramatically, as seen in this chart based on data since

1900. During El Niño years the chance of hurricanes is

greatly reduced; no more than two hurricanes have ever

made landfall during an El Niño year. On the other hand,

during La Niña years, the chance of hurricanes greatly

increases; there has been nearly a 40% chance of three or

more hurricanes making landfall during a La Niña year.

Model projections

of continental-scale

and century-long

trends are more reliable than projections of shorter-term

trends over smaller

scales.
19

A Continually Changing Climate and the Potential for Surprises

t is essential to note that the 21st century’s climate,unlike that of the preceeding thousand years,is not expected to be stable but is very likely to be in a constant state of

change. For example,the duration and amount of ice in the Great Lakes is expected to

decrease. It is possible that in the short term an increase in "lake effect" snows would be

a consequence during mid-winter, though they would likely decrease in the long term.

Across the nation,as climate continues to warm,precipitation is very likely to increasingly

fall as rain rather than snow. Such continuously changing climate presents a special challenge for human adaptation.

In addition,there is the potential for "surprises." Because climate is highly complex,it is

important to remember that it might surprise us with sudden or discontinuous change,or

by otherwise evolving quite differently from what is expected. Surprises challenge

humans' ability to adapt,because of how quickly and unexpectedly they occur. For example,what if the Pacific Ocean warms in such a way that El Niño events become much

more extreme? This could reduce the frequency, but perhaps not the strength,of hurricanes along the East Coast,while on the West Coast,more severe winter storms, extreme

precipitation events,and damaging winds could become common. What if large quantities of methane,a potent greenhouse gas currently frozen in icy Arctic tundra and sediments,began to be released to the atmosphere by warming,potentially creating an amplifying "feedback loop" that would cause even more warming? We simply do not know

how far the climate system or other systems it affects can be pushed before they respond

in unexpected ways.

There are many examples of potential surprises,each of which would have large consequences. Most of these potential outcomes are rarely reported,in this study or elsewhere.

Even if the chance of any particular surprise happening is small,the chance that at least

one such surprise will occur is much greater. In other words,while we can't know which

of these events will occur, it is likely that one or more will eventually occur.

Another caveat is appropriate: climate scenarios are based on emissions scenarios for various gases. The development of new energy technologies,the speed of population growth,

and changes in consumption rates each have the potential to alter these emissions in the

future,and hence the rate of climate change.

A continuously changing climate presents a

special challenge for

human adaptation. 

Because climate is

highly complex, it is

important to remember that it might surprise us with sudden

or discontinuous

change.

We simply do not

know how far the climate system or other

systems it affects can

be pushed before they

respond in unexpected ways.

I

Water temperature profile in the Pacific Ocean, January 1997. Water temperature profile in the Pacific Ocean, November 1997.

During El Niño conditions, the equatorial pool of warm water (shown in red) expands and moves eastward to span the entire equatorial Pacific east of

the dateline. This dramatic warming affects global atmospheric circulation including effects on the jet stream, winter storms, and tropical storms.
How to read these maps: The color scale indicates changes in

temperature in oF over a 100 year period. For example, at 0oF

there is no change; at +10oF there is a 10oF increase from the

begining to the end of the century.

Past and Future US

Temperature Change

bservations from 1200 weather

stations across the US show

that temperatures have increased

over the past centur y, on average by

almost 1oF (0.6 ̊C).The coastal

Northeast,the upper Midwest,the

Southwest,and parts of Alaska have

experienced increases in the annual

average temperature approaching 4oF

(2oC) over the past 100 years. The

rest of the nation has experienced

less warming. The Southeast and

southern Great Plains have actually

experienced a slight cooling over the

20th century, but since the 1970s

have had increasing temperatures as

well. The largest observed warming

across the nation has occurred in

winter.

Average warming in the US is projected to be somewhat greater than for

the world as a whole over the 21st

century. In the Canadian model scenario,increases in annual average

temperature of 10oF (5.5oC) by the

year 2100 occur across the central US

with changes about half this large

along the east and west coasts.

Seasonal patterns indicate that projected changes will be particularly

large in winter, especially at night.

Large increases in temperature are

projected over much of the South in

summer, dramatically raising the heat

index (a measure of discomfort based

on temperature and humidity).

In the Hadley model scenario,the

eastern US has temperature increases

of 3-5oF (2-3oC) by 2100 while the

rest of the nation warms more,up to

7oF (4oC),depending on the region.

In both models,Alaska is projected to

experience more intense warming

than the lower 48,and in fact,this

warming is already well underway. In

contrast,Hawaii and the Caribbean

islands are likely to experience less

warming than the continental US,

because they are at lower latitudes

and are surrounded by ocean,which

warms more slowly than land.

20

Both the Canadian and Hadley model

scenarios project substantial warming

during the 21st century. The warming

is considerably greater in the Canadian

model, with most of the continental US

experiencing increases from 5 to 15 ̊F.

In this model, the least warming occurs

in the West and along the Atlantic and

Gulf Coasts. In the Hadley model,

annual temperatures are projected to

increase from 3 to 7 ̊F, with the largest

warming occurring in the western half

of the country. 

LOOKING AT AMERICA’S CLIMATE

Temperature Change

Observed 20th Century

Canadian Model 21st Centur y

Hadley Model 21st Century

The change in the annual average temperature over the

20th century has a distinctive pattern. Most of the US has

warmed, in some areas by as much as 4 ̊F. Only portions

of the southeastern US have experienced cooling, and this

was primarily due to the cool decades of the 1960s and

1970s. Temperatures since then have reached some of the

highest levels of the century.

+15oF

+10oF

+5oF

0o

-5oF

+15oF

+10oF

+5oF

0o

-5oF

+15oF

+10oF

+5oF

0o

-5oF

O
Summer Maximum and Winter Minimum Temperature Change

Canadian Model 21st Century Summer Maximum

+15oF

+10oF

+5oF

0o

-5oF

Hadley Model 21st Century Summer Maximum

+15oF

+10oF

+5oF

0o

-5oF

Hadley Model 21st Centur y Winter Minimum

+15oF

+10oF

+5oF

0o

-5oF

Canadian Model 21st Centur y Winter Minimum

+15oF

+10oF

+5oF

0o

-5oF

Maximum Temperature

in the US (annual average)

Minimum Temperature

in the US (annual average)

The annual average of minimum and maximum temperatures are compiled from the

daily lows and highs. These graphs show

the lows and highs, averaged over the year

and over the lower 48 states. The green line

shows observed temperatues while the red

and blue lines are model projections for the

future.

The minimum and maximum temperatures

are important because, far more than the

average, they influence such things as

human comfort, heat and cold stress in

plants and animals, maintenance of snowpack, and pest populations (many pests are

killed by low temperatures; a rise in the minimum often allows more pests to survive).

Hadley Model

Canadian Model

Observations

Hadley Model

Canadian Model

Observations

21

Average US warming is 

projected to be somewhat

greater than global average

warming over the 21st century.

Large increases in temperature

are projected over much of the

South in summer, dramatically

raising the heat index (a measure of discomfort based on temperature and humidity).

Canadian Model 21st Century Hadley Model 21st Centur y

The projected changes in the heat index for the

Southeast are the most dramatic in the nation with the

Hadley model suggesting increases of 8 to 15oF for the

southernmost states, while the Canadian model projects increases above 25oF for much of the region.

July Heat

Index Change

+25oF

+20oF

+15oF

+10oF

+5oF

0oF

1900 1950 2000 2050 2100 1900 1950 2000 2050 2100

55

50

45

40

35

80

75

70

65

60
22

Changes in Precipitation

verage US precipitation has

increased by 5-10% over the last

century with much of that due to an

increase in the frequency and intensity of heavy rainfall. Precipitation

increases have been especially noteworthy in the Midwest,southern

Great Plains,and parts of the West

and Pacific Northwest.Decreases

have been observed in the northern

Great Plains.

For the 21st century,the Canadian

model projects that percentage

increases in precipitation will be

largest in the Southwest and

California,while east of the Rocky

Mountains,the southern half of the

nation is projected to experience a

decrease in precipitation. The percentage decreases are projected to be

particularly large in eastern Colorado

and western Kansas,and across an

arc running from Louisiana to

Virginia. Projected decreases in precipitation are most evident in the

Great Plains during summer and in

the East during both winter and summer. The increases in precipitation

projected to occur in the West,and

the smaller increases in the

Northwest,are projected to occur

mainly in winter.

In the Hadley model,the largest percentage increases in precipitation are

projected to be in the Southwest and

Southern California,but the increases

are smaller than those projected by

the Canadian model. In the Hadley

model,the entire US is projected to

have increases in precipitation,with

the exception of small areas along

the Gulf Coast and in the Pacific

Northwest.Precipitation is projected

to increase in the eastern half of the

nation and in southern California and

parts of Nevada and Arizona in summer, and in every region during the

winter, except the Gulf States and

northern Washington and Idaho.

In both the Hadley and Canadian

models,most regions are projected to

experience an increase in the frequency of heavy precipitation events.

This is especially notable in the

Hadley model,but the Canadian

model shows the same characteristic.

While the actual amounts are modest,

the large percentage increases in rainfall projected for the Southwest are

related to increases in atmospheric

moisture and storm paths. A warmer

Pacific would pump moisture into

the region and there would also be a

southward shift in Pacific Coast storm

activity. In the Sierra Nevada and

Rocky Mountains, much of the

increased precipitation is likely to fall

as rain rather than snow, causing a

reduction in mountain snow packs.

LOOKING AT AMERICA’S CLIMATE

Precipitation Change

Observed 20th Century

Canadian Model 21st Centur y

Significant increases in precipitation have occurred across much of

the US in the 20th century. Some localized areas have experienced

decreased precipitation. The Hadley and Canadian model scenarios

for the 21st century project substantial increases in precipitation in

California and Nevada, accelerating the observed 20th century trend

(some other models do not simulate these increases). For the eastern two-thirds of the nation, the Hadley model projects continued

increases in precipitation in most areas. In contrast, the Canadian

model projects decreases in precipitation in these areas, except for

the Great Lakes and Northern Plains, with decreases exceeding 20%

in a region centered on the Oklahoma panhandle. Trends are calculated relative to the 1961-90 average.

100%

75%

50%

25%

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A

Hadley Model 21st Century
23

This would tend to increase wintertime river flows and decrease summertime flows in the West. Across

the Northwest,and the central and

eastern US,the two model projections of precipitation change are in

less agreement. These differences

will be resolved only by improvements in climate modeling.

Changes in Soil Moisture

oil moisture is critical for both

agriculture and natural ecosystems. Soil moisture levels are determined by an intricate interplay

among precipitation, evaporation,

run-off, and soil drainage. By itself, an

increase in precipitation would

increase soil moisture. However,

higher air temperatures will increase

the rate of evaporation and,in some

areas,remove moisture from the soil

faster than it can be added by precipitation. Under these conditions, some

regions are likely to become drier

even though their rainfall increases.

In fact,soil moisture has already

decreased in portions of the Great

Plains and Eastern Seaboard,where

precipitation has increased but air

temperature has risen.

Since soil moisture projections reflect

both changes in precipitation and in

evaporation associated with warming,the differences between the two

models are accentuated in the soil

moisture projections. For example,in

the Canadian model,soil moisture

decreases of more than 50% are common in the Central Plains due to the

combination of precipitation reductions exceeding 20% and temperature

increases exceeding 10oF. In the

Hadley model,this same region experiences more modest warming of

about 5oF and precipitation increases

of around 20%, generally resulting in

soil moisture increases.

Increased drought becomes a national problem in the Canadian model.

Intense drought tendencies occur in

the region east of the Rocky

Mountains and throughout the MidAtlantic-Southeastern states corridor.

Increased tendencies toward drought

are also projected in the Hadley

model for regions immediately east of

the Rockies. California and Arizona,

plus a region from eastern Nebraska

to Virginia's coastal plain, experience

decreases in drought tendency. The

differences in soil moisture and

drought tendencies will be significant

for water supply, agriculture, forests,

and lake levels.

Summer Soil Moisture Change

(Relative to the 1961-90 Average)

Observed 20th Century

Canadian Model 21st Centur y

Hadley Model 21st Centur y

Soil moisture has tended to increase in the central US with decreases in some localized areas. In the Northeast and in the western third of

the country, there has been less change in soil moisture, despite the

increase in precipitation, due to compensating temperature increases.

The Hadley and Canadian models project strong increases in soil moisture in the Southwest. For the rest of the nation, the Hadley model projects mostly increases while the Canadian model projects mostly decreases, with large decreases in the Central Plains. The contrasts between the

two models result from the combination of greater precipitation in the

Hadley model and higher air temperatures in the Canadian model.

+100%

+75%

+50%

+25%

0

-25%

-50%

-75%

-100%

+100%

+75%

+50%

+25%

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S

In both the Hadley and Canadian

models, most regions are projected

to see an increase in the frequency

of heavy precipitation events. 

Higher air temperatures will

increase the rate of evaporation

and, in some areas, remove moisture from the soil faster than it can

be added by precipitation.
24

ECOSYSTEMS IN THE FUTURE

he natural vegetation covering about 70% of the US land surface is strongly influenced both by the climate and by the atmospheric carbon dioxide (CO2

) concentration. To provide a common base of information about potential changes in vegetation

across the nation for use in the regional and sector studies,specialized ecosystem models were run using the two major climate model scenarios selected for this Assessment.

A summary of the national level results follows. Agricultural and production forestry

systems are the focus of separate sections of this Overview report.

What are Ecosystems?

cosystems are communities of plants,animals,microbes,and the physical environment in which they exist. They can be characterized by their biological richness,

by the magnitude of flows of energy and materials between their constituent species

and their physical environment,and by the interactions among the biological species

themselves,that is, by which species are predators and prey, which are competitors,and

which are symbiotic.

Ecologists often categorize ecosystems by their dominant vegetation – the deciduous broad-leafed forest ecosystems of New England,the short-grass prairie

ecosystems of the Great Plains,the desert ecosystems of the Southwest. The

term "ecosystem" is used not only to describe natural systems (such as coral

reefs,alpine meadows,old growth forests,or riparian habitats),but also for plantation forests and agricultural systems,although these ecosystems obviously differ in many important ways from the natural ecosystems they have replaced.

Ecosystems Supply Vital Goods and Services

hile we value natural ecosystems in their own right,ecosystems of all

types,from the most natural to the most extensively managed,produce a

variety of goods and services that benefit humans. Some of these enter the market and contribute directly to the economy. Thus, forests as sources of timber

and pulpwood,and agro-ecosystems as sources of food are important to us. But

ecosystems also provide a set of un-priced services that are valuable,but that

typically are not traded in the marketplace. There is no current market, for

example,for the services that forests and wetlands provide for improving water

quality, regulating stream flow, and providing some measure of protection from

floods. However, these services are very valuable to society.

Ecosystems are also valued for recreational,aesthetic,and ethical reasons. These

are also difficult to value monetarily, but are nevertheless important. The bird

life of the coastal marshes of the Southeast and the brilliant autumn colors of

the New England forests are treasured components of our regional heritages,

and important elements of our quality of life.

T

E

W

What to Expect 

with Climate Change

• Changes in the productivity and

carbon storage capacity of

ecosystems, decreases in some

places and increases in others,

are very likely. 

• Shifts in the distribution of

major plant and animal species

are likely.

• Some ecosystems such as

alpine meadows are likely to disappear in some places because

the new local climate will not support them or there are barriers to

their movement.

• In many places, it is very likely

that ecosystem services, such as

air and water purification, landscape stabilization against erosion, and carbon storage capacity

will be reduced. These losses will

likely occur in the wake of episodic, large-scale disturbances that

trigger species migrations or local

extinctions.

• In some places, it is very likely

that ecosystems services will be

enhanced where climate-related

stresses are reduced.
Tropical Rainforest

Temperate Rainforest

Temperate Forest

Taiga

Tundra

Grassland

Desert

Tropical Seasonal Forest

450

400

350

300

250

200

150

100

50

30 25 20 15 10 5 0 -5 -10 -15

Mean Annual Temperature,  ̊C

Tropical Subtropical Warm Temperate Cold Temperate Arctic-Alpine

25

The species in some

ecosystems are so

strongly influenced by

the climate to which

they are adapted that

they are vulnerable

even to modest

climate changes.

Climate and Ecosystems

limatic conditions determine where individual species of plants and animals can live,

grow, and reproduce. Thus,the collections of species that we are familiar with – the

southeastern mixed deciduous forest,the desert ecosystems of the arid Southwest,or the

productive grasslands of the Great Plains – are influenced by climate as well as other factors such as land-use. The species in some ecosystems are so strongly influenced by the

climate to which they are adapted that they are vulnerable even to modest climate

changes. For example,alpine meadows at high elevations in the West exist where they do

entirely because the plants that comprise them are adapted to the cold conditions thatwould be too harsh for other species in the region. The desert vegetation of the

Southwest is adapted to the high summer temperatures and aridity of the region. Forests

in the east are adapted to relatively high rainfall and soil moisture;if drought conditions

were to persist, grasses and shrubs could begin to out-compete tree seedlings,leading to

completely different ecosystems.

There are also many freshwater and marine examples of sensitivities to climate variability

and change. In aquatic ecosystems, for example,many fish can breed only in water that

falls within a narrow range of temperatures. Thus,species of fish that are adapted to cool

waters can quickly become unable to breed successfully if water temperatures rise.

Wetland plant species can adjust to rising sea levels by dispersing to new locations,within

limits. Too rapid sea-level rise can surpass the ability of the plants to disperse,making it

impossible for coastal wetland ecosystems to re-establish themselves.

C

Both temperature and precipitation limit the distribution of plant communities. The climate

(temperature and precipitation) zones of some

of the major plant communities (such as temperate forests, grasslands, and deserts) in the

US are shown in this figure. Note that grasslands' zone encloses a wide range of environments. This zone can include a mixture of

woody plants with the grasses. The shrublands

and woodlands of the West are examples of

grass/woody vegetation mixes that occur in the

zone designated as grasslands.

With climate change, the areas occupied by

these zones will shift relative to their current

distribution. Plant species are expected to shift

with their climate zones. The new plant communities that result from these shifts are likely

to be different from current plant communities

because individual species will very likely

migrate at different rates and have different

degrees of success in establishing themselves

in new places.

Distribution of Plant Communities

Tropical Subtropical Warm Temperate Cold Temperate Arctic-Alpine

450

400

350

300

250

200

150

100

50

Mean Annual Temperature oC
26

ECOSYSTEMS IN THE FUTURE

Species in current

ecosystems can differ

substantially in their

tolerances of changes

in temperature and

precipitation, and in

their responses to

changes in CO2; thus,

new climate conditions are very likely to

result in current

ecosystems breaking

apart, and new

assemblages of

species being created.

Effects of Increased CO2 Concentration on Plants

he ecosystem models used in this Assessment consider not only changes in climate,

but also increases in atmospheric CO2. The atmospheric concentration of CO2

affects plant species in ecosystems since it has a direct physiological effect on photosynthesis,the process by which plants use CO2

to create new biological material. Higher

concentrations of CO2generally enhance plant growth if the plants also have sufficient

water and nutrients,such as nitrogen,to sustain this enhanced growth. For this reason,

the CO2levels in commercial greenhouses are sometimes boosted in order to stimulate

plant growth. In addition,higher CO2levels can raise the efficiency with which plants

use water. Different types of plants respond at different rates to increases in atmospheric

CO2, resulting in a divergence of growth rates due to CO2increase. Some species grow

faster, but provide reduced nutritional value. The effects of increased CO2level off at

some point;thus,continuing to increase CO2levels will not result in increased plant

growth indefinitely. There is still much we do not understand about the CO2“fertilization”effect,its limits,and its direct and indirect implications.

Species Responses to Changes in Climate and CO2

he responses of ecosystems to changes in climate and CO2are made up of the individual responses of their constituent species and how they interact with each other.

Species in current ecosystems can differ substantially in their tolerances of changes in

temperature and precipitation,and in their responses to changes in CO2; thus,new climate conditions are very likely to result in current ecosystems breaking apart,and new

assemblages of species being created. Current ecosystem models have great difficulty in

predicting these kinds of biological and ecological responses,thus leading to large uncertainties in projections.

What the Models Project

odeling results to date indicate that natural ecosystems on land are very likely to be

highly sensitive to changes in surface temperature,precipitation patterns,other climate parameters,and atmospheric CO2

concentrations. Two types of models utilized in

this Assessment to examine the ecological effects of climate change are biogeochemistry

models and biogeography models. Biogeochemistry models simulate changes in basic

ecosystem processes such as the cycling of carbon, nutrients,and water (ecosystem function). Biogeography models simulate shifts in the geographic distribution of major plant

species and communities (ecosystem structure).

T

T

M
The biogeochemistry models used in

this analysis generally simulate increases in the amount of carbon in vegetation and soils over the next 30 years

for the continental US as a whole.

These probable increases are small –

in the range of 10% or less,and are

not uniform across the country. In

fact,for some regions the models simulate carbon losses over the next 30

years. One of the biogeochemistry

models,when operating with the

Canadian climate scenario,simulates

that by about 2030,parts of the

Southeast will likely lose up to 20% of

the carbon from their forests. A carbon loss by a forest is treated as an

indication that it is in decline. The

same biogeochemistry model,when

operating with the Hadley climate scenario,simulates that forests in the

same part of the Southeast will likely

gain between 5 and 10% in carbon in

trees over the next 30 years.

Why do the two climate scenarios

result in opposite ecosystem responses in the Southeast? The Canadian climate scenario shows the Southeast as

a hotter and drier place in the early

decades of the 21st century than does

the Hadley scenario. With the

Canadian scenario, forests will be

under stress due to insufficient moisture,which causes them to lose more

carbon in respiration than they gain in

photosynthesis. In contrast,the

Hadley scenario simulates relatively

plentiful soil moisture, robust tree

growth,and forests that accumulate

carbon.

27

Changes in Vegetation Carbon 

Hadley Model 2030s

Canadian Model 2030s

The maps above show projections of relative changes in vegetation carbon between 1990 and

the 2030s for two climate scenarios. Under the Canadian model scenario, vegetation carbon

losses of up to 20% are projected in some forested areas of the Southeast in response to

warming and drying of the region by the 2030s. A carbon loss by forests is treated as an indication that they are in decline. Under the same scenario, vegetation carbon increases of up to

20% are projected in the forested areas in the West that receive substantial increases in precipitation. Output from TEM (Terrestrial Ecosystem Model) as part of the VEMAP II (Vegetation

Ecosystem Modeling and Analysis Project) study. 

>10% decrease

up to 10% decrease

no change

up to 10% increase

>10% increase
28

ECOSYSTEMS IN THE FUTURE

Ecosystem Models

Current Ecosystems

Maps of current and projected

potential vegetation distribution

for the conterminous US.

Potential vegetation means the

vegetation that would be there in

the absence of human activity.

Changes in vegetation distribution by the end of the 21st century are in response to two climate

scenarios, the Canadian and the

Hadley. Output is from MAPSS

(Mapped Atmosphere-Plant-Soil

System).

Tundra

Taiga / Tundra

Conifer Forest

Northeast Mixed Forest

Temperate Deciduous Forest

Southeast Mixed Forest

Tropical Broadleaf Forest

Savanna / Woodland

Shrub / Woodland

Grassland

Arid Lands

rolonged stress due to insufficient soil moisture can make trees more susceptible to

insect attack,lead to plant death,and increase the probability of fire as dead plant

material adds to an ecosystem's "fuel load." The biogeography models used in this analysis

simulate at least part of this sequence of climate-triggered events in ecosystems as a prelude to shifts in the geographic distribution of major plant species. One of the biogeography models,when operating with the Canadian climate scenario,simulates that towards

the end of the 21st century, a hot dry climate in the Southeast will result in the replacement of the current mixed evergreen and deciduous forests by savanna/woodlands and

grasslands,with much of the change involving fire. This change in habitat type in the

Southeast would imply that the animal populations of the region would also change,

although the biogeography models are not designed to simulate these changes. The same

biogeography model,when operating with the Hadley scenario,simulates a slight northward expansion of the mixed evergreen and deciduous forests of the Southeast with no

significant contraction along the southern boundary. Other biogeography models show

similar results.

Major Uncertainties

ajor uncertainties exist in the biogeochemistry and biogeography models. For

example,ecologists are uncertain about how increases in atmospheric CO2affect

the carbon and water cycles in ecosystems. What they assume about these CO2effects

can significantly influence model simulation results. One of these models was used to

show the importance of testing these assumptions.Consideration of climate change alone

M

Will disturbances caused P

by climate change be

regular and small or will

they be episodic and

large? The latter category of disturbances is

likely to have a negative

impact on ecosystem

services; the ability of

ecosystems to cleanse

the air and water, stabilize landscapes against

erosion, and store carbon, for example, are

very likely to be diminished.
29

Canadian Model 

Hadley Model 

Tundra

Taiga / Tundra

Conifer Forest

Northeast Mixed Forest

Temperate Deciduous Forest

Southeast Mixed Forest

Tropical Broadleaf Forest

Savanna / Woodland

Shrub / Woodland

Grassland

Arid Lands

A substantial portion of the

Southeast’s mixed forest is

replaced by a combination of

savanna and grassland in

response to fire caused by warming and drying of the region as

projected by the Canadian

model. The Hadley climate projection leads to a simulated

northward expansion of the

mixed forest. 

These particular model runs

show the response of vegetation

to atmospheric concentrations of

CO2that have stabilized at about

700 parts per million, approximately twice the present level.

In the Southwest, large areas of

arid lands are replaced with

grassland or shrub/woodland in

response to increases in precipitation projected by both models.

results in a 10% decrease in plant productivity. Consideration of both climate

and CO2effects results in an increase in plant productivity of 10%.This illustrates the importance of resolving uncertainties about the effects of CO2

on

ecosystems.

With respect to biogeography models,scientists are uncertain about the frequency and size of disturbances produced by factors such as fire and pests that

initiate changes in the distribution of major plant and animal species. Will disturbances caused by climate change be regular and small or will they be

episodic and large? The latter category of disturbances is likely to have a negative impact on ecosystems services; the ability of ecosystems to cleanse the air

and water,stabilize landscapes against erosion,and store carbon, for example,

are very likely to be diminished.

Ecologists are uncertain

about how increases in

atmospheric CO2affect

the carbon and water

cycles in ecosystems.

What they assume about

these CO2effects can

significantly influence

model simulation results.