30

OUR CHANGING NATION

limate variability and change do not occur in isolation,but in an evolving, dynamic

social and economic context. This context is very likely to affect the character and

magnitude of climate impacts. Socioeconomic conditions are important drivers of climate

change,and also influence the way society responds to change. The prosperity and structure of the economy, the technologies available and in use,and the settlement patterns

and demographic structure of the population,are all very likely to contribute to how and

how much climate change will matter to Americans,and what they can and might wish to

do about it.

Thinking explicitly about socioeconomic futures is speculative,but doing a coherent

assessment of future climate impacts requires that potential future socioeconomic conditions be considered. Failing to explicitly consider these conditions risks making the

assumption that the future will be largely like the present – an assumption that is virtually

certain to be wrong. To see how wrong,one need only compare America’s society and

economy today to that of 100,50,or even 25 years ago.

To guide our thinking about socioeconomic futures,this Assessment developed three illustrative socioeconomic scenarios,which project high,medium,and low growth trends for

the US population and economy through the 21st century. These scenarios necessarily

involve uncertainties that grow large by the end of the century, as the figures show.

Nevertheless,they represent a plausible range of socioeconomic conditions that could

affect climate impacts and response capabilities. Using multiple scenarios avoids the

errors of attempting specific predictions,or assuming no change at all. Region and sector

C

Over the 21st century,

assuming no major

wars or other catastrophes, growth is likely to

continue. However, the

specifics of future US

growth depend upon

many uncertain factors

such as technological

change, world trade,

market conditions, and

immigration.

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

Future employment projections

track population,

and also include

varying rates of

employment by

older workers as

the US population

ages. 

Employment

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

Population

Through the 21st

century, US population projections range from

a 20% increase to

a doubling, principally due to

alternative

assumptions of

immigration

trends.

Low, Medium, and High Scenarios

21st Century Growth in America

700

600

500

400

300

200

100

0

350

300

250

200

150

100

50

0

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

The Assessment

considered high,

medium, and low

scenarios of

future US population and economic

growth. Future

trends in population, economic

growth, and technological change

will all shape our

contribution to climate change, our

vulnerability to it,

and our ability to

adapt. 
teams were asked to use these scenarios when their analyses required

demographic or economic inputs.

Growing Prosperity

he US economy and population

are growing. Barring major wars

or other catastrophes, growth is likely

to continue through the 21st century.

If economic growth is higher,society

is likely to be more able to take

advantage of the opportunities a

changing climate presents,and more

able to cope with its negative

impacts. Wealthier, industrialized

societies derive less of their incomes

from strongly climate-related activities than more traditional societies.

With more technology and infrastructure, wealthy societies also have more

resources to support adaptation,and

can more easily endure climate-related losses. Within societies,some will

very likely face greater burdens or

greater opportunities than others. It

is also possible that rapid economic

growth can increase vulnerability, by

increasing pollution (including greenhouse gas emissions),congestion,

demand for land and resources,and

stresses on natural ecosystems,and

possibly their vulnerability to climate

change.

Changing Technology

uch of the recent US economic growth has been fueled by

new technology. Although technological change can carry significant

social and environmental costs,in

aggregate it greatly increased

Americans’material well being over

the 20th century. For example,in the

past decade,new information and

communication technologies have

transformed many activities,bringing

increased productivity and new products and services.

Technology affects society’s relationship to climate in many ways. It is

very likely that technological change

will strongly influence the success of

any future efforts to control greenhouse gas emissions,and reduce vulnerability to climate change. For

example,it is possible that information technology, combined with new

cropping methods and advanced

crop varieties,will increase farmers’

ability to adapt to climate change or

variability. Similarly, advances in medicine,public health,and information

technology will likely strengthen our

abilities in the early detection,prevention,and treatment of disease.

Technology can also increase society’s vulnerability to unanticipated

extremes of climate. This can happen

because modern society is highly

interdependent,relying in critical

ways on electric power,transportation,and communications systems,all

of which can be disrupted by

extreme weather events if systems

have not been adequately designed to

deal with contingencies.

31

T

M

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

0.00

20.00

40.00

60.00

80.00

100.00

120.00

Projections of

21st century economic growth

span a wide

range, reflecting

the combined

effects of trends

in population and

productivity. 

Gross

Domestic

Product

The middle scenario holds annual growth in output per worker at

1.2%, the average

rate over the 20th

century. The high

and low scenarios double and

halve this annual

growth rate. 1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

400.0

Labor

Productivity

120

100

80

60

40

20

0

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

400

350

300

250

200

150

100

50

0

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

Low, Medium, and High Scenarios

21st Century Growth in America
32

A Growing, Aging, and Mobile Populace

he US population is projected to continue growing through the 21st century, but at

a declining rate. The scenarios used in this Assessment project a US population in

2100 that ranges from 353 to 640 million (representing average annual growth rates of

0.31% to 0.86% over the 21st century),with 494 million in the middle scenario. Most of

this uncertainty arises from alternative immigration assumptions.

The US population is aging. Over the 20th century, the fraction of Americans over age 65

increased from 1 in 25 to 1 in 8. Older people are physiologically more vulnerable to

heat stress. Without adaptive measures,a warmer climate would likely bring an increase

in heat-related illness and death,which society’s aging would compound. There is also

some chance that warming would reduce cold-related mortality, a trend that would also

interact with the aging of the population,although the data suggest a weaker effect than

for heat. Many older Americans prefer warmer climates,as the migration from northern

regions to the Sunbelt demonstrates. Widespread use of one technology, air conditioning,

powerfully advanced the growth of these southern regions. At the same time, rapid

growth in arid regions has sharply increased these regions’vulnerability to water shortages.

OUR CHANGING NATION

Each block on the map illustrates one county in the US. The height of

each block is proportional to that county's population density in the year

2000, so the volume of the block is proportional to the county’s total population. The color of each block shows the county's projected change in

population between 1970 and 2030, with shades of orange denoting

increases and blue denoting decreases. The patterns of recent population

change, with growth concentrated along the coasts, in cities, and in the

South and West, are projected to continue.

US Population and Growth Trends

Change in county population, 1970-2030

Some 53% of the

population now live in

the 17% of the land

area that comprises

the coastal zone, and

the largest population

growth for several

decades is projected

for coastal areas. 

T

Projected change in county population (percent), 1970 to 2030

>+250% (highest +3,877%)

+50% to +250%

+5% to +50%

-5% to +5%

-20% to -5%

-40% to -20%

<-40% (lowest -60%)
America is becoming more urban.

The fraction of Americans living in

cities increased from 40% in 1900 to

more than 75% today and this

increase is projected to continue.

Urbanization affects vulnerability to

climate and the capacity to adapt in

complex ways. City dwellers are less

dependent on climate-sensitive activities for their livelihoods,and have

more resources and social support

systems close at hand. But dense

concentrations of people and

property in coastal or riverside

metropolitan areas,dependent

on extensive fixed infrastructure

(including water,sewer, and

energy utilities, roads,tunnels,

and bridges) are likely to be

vulnerable to extreme events

such as floods,storms,storm

surges,and heat waves.

Combined with such other

urban stresses as congestion,

pollution,and the local heating that cities generate,it is

possible that climate

change could significantly

harm urban quality of life

and health.

Americans are also

moving to the coasts.

Some 53% of the

population now live in the 17% of

the land area that comprises the

coastal zone,and the largest population growth for several decades is

projected for coastal areas. Over the

next 25 years,population growth of

some 18 million is projected in the

coastal states of Florida,California,

Texas,and Washington. This trend is

exacerbating wetland loss and coastal

pollution. In addition,locating more

people and property in low-lying

coastal areas increases vulnerability

to storms,storm surges,erosion,and

sea-level rise – as several decades of

damage trends,and extreme recent

losses in Florida,Georgia,and the

Carolinas,all confirm.

Thinking about the Future:

Coping with Complexity

host of other factors are also

likely to affect the ease with

which society can adapt to,or take

advantage of, climate variability and

change. For particular regions or sectors,the factors likely to shape climate vulnerability include local zoning ordinances,housing styles,building codes,popular forms of recreation,the age and degree of specialization of capital in particular industries, world market conditions,and

the distribution of income. To further

complicate matters,many of these

factors are likely to be influenced by

climate variability and change,and to

influence each other. Trying to project all such relevant factors,or to

model their interactions, would be

impossible.

Rather, this first Assessment took a

highly simplified approach to projecting socioeconomic factors. When

teams needed more detailed socioeconomic projections than the scenarios of population and economic

growth provided,they were asked to

follow a standard procedure to generate and document the projections

they needed. They were asked to

select one or two additional factors –

such as development patterns,land

use,technology, or market conditions

– that they judged likely to have the

most direct effect on the issue they

were examining,and to vary these

factors through an uncertainty range

they judged plausible. This approach

has clear limitations.In fact,teams

found the complexity of even this

simplified approach challenging,and

made limited use of it beyond the

basic scenarios. It has,however,

allowed some preliminary investigations of the socioeconomic basis of

impacts and vulnerability, which can

be refined and extended as assessment methods and experience

advance.

33

A

0

10%

20%

30%

40%

50%

60%

70%

80%

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

0

20

40

60

80

100

120

140

160

180

200

% of US Population in Urban Areas

Total Rural Population % of US Population in Rural Areas

Total Urban Population

Essentially all 20th century US population growth has been in cities,

increasing the urban population fraction from 40% in 1900 to more than

75% in 1990. This move to the cities is projected to continue. 

US Rural and Urban Population 
34

OUR CHANGING NATION

Impacts, Adaptation, and Vulnerability

limate impacts,vulnerability, and adaptation are distinct but related concepts. Given

an assumed state of America’s society and economy, the impacts of a specified climate scenario are the differences it yields relative to a continuation of the present climate. These impacts may be beneficial or harmful,with most climate scenarios bringing

mixed effects:benefits to some people,places,and sectors,and harms to others. A system

is more or less sensitive to climate depending on whether a specified change in climate

brings large or small impacts.

People need not merely suffer the climate conditions they face,however, but can change

their practices,institutions,or technology to take maximum advantage of the opportunities the climate presents and limit the harms they suffer from its variations. Through such

adaptations,people and societies adjust to the average climate conditions and the variability of conditions they have experienced in the recent past. When habits,livelihoods,capital stock,and management practices are finely tuned to current climate conditions,the

direct effect of many types of change in these conditions,particularly if the change occurs

rapidly, is more likely to be harmful and disruptive than beneficial.

But just as societies adapt to the present climate,they can also adapt to changes in it.

Adaptation can be intentional or not,and can be undertaken either in anticipation of projected changes or in reaction to observed changes. Society’s capacity to adapt to future

climate change is a crucial uncertainty in determining what the actual consequences of

climate change will be. Societies and economies are vulnerable to climate change if they

face substantial unfavorable impacts,and have limited ability to adapt. Socioeconomic

conditions such as wealth,economic structure,settlement patterns,and technology play

strong roles in determining vulnerability to specified climate conditions,as the history of

US hurricane losses shows.

Human societies and economies have demonstrated great adaptability to wide-ranging

environmental and climatic conditions found throughout the world,and to historical variability. Wealthy industrial societies like the US function quite similarly in such divergent

climates as those of Fairbanks,Alaska and Orlando,Florida. While individual adaptability

also contributes,it is principally social and economic adaptations in infrastructure,capital,

technology, and institutions that make life in Orlando and Fairbanks so similar that individual Americans can move between them easily.

But adaptability has limits,for societies as for individuals,and individuals’ ability to move

through large climate differences tells us little about these limits. Moving between

Orlando and Fairbanks may be easy for an individual,but rapidly imposing the climate of

either place on the other would be very disruptive. The countless ways that particular

local societies have adapted to current conditions and their history of variability can be

changed,but not without cost,not all with equal ease,and not overnight. The speed of

climate change,and its relationship to the speed at which skills,habits,resource-management practices,policies,and capital stock can change,is consequently a crucial contributor to vulnerability. Moreover, however wisely we may try to adjust long-lived decisions to

anticipate coming climate changes, we will inevitability remain limited by our imperfect

projections of the coming changes.Effective adaptation may depend as much on our abiliWhile societies have

shown substantial

adaptability to climate

variability, the challenge of adapting to a

climate that is not stable, but evolving at an

uncertain rate, has

never been tested in an

industrialized society.

C
ty to devise responses that are robust to various possible changes,and adjustable as we

learn more,as on the quality of our projections at any particular moment. While societies

have shown substantial adaptability to climate variability, the challenge of adapting to a

climate that is not stable,but evolving at an uncertain rate,has never been tested in an

industrialized society.

Consequently, while adaptation measures can help Americans reduce harmful climate

impacts and take advantage of associated opportunities,one cannot simply assume that

adaptation will make the aggregate impacts of climate change negligible or beneficial.

Nor can one assume that all available adaptation measures will necessarily be taken; even

for such well-known hazards as fire, flood,and storms,people often fail to take inexpensive and easy risk-reduction measures in their choices of building sites,standards,and

materials,sometimes with grave consequences. In this first Assessment,potential climate

adaptation options were identified,but their feasibility, costs,effectiveness,or the likely

extent of their actual implementation were not assessed. Careful assessment of these will

be needed.

35

Total Property Losses

Millions of dollars, Constant $1992

0

1

2

3

4

5

6

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

1836

8000

0

100

200

300

400

500

600

700

800

900

Deaths

Number of hurricanes per year

28687

One cannot simply

assume that adaptation will make the

aggregate impacts of

climate change negligible or beneficial.

In this first

Assessment, potential

climate adaptation

options were identified, but their feasibility, costs, effectiveness, or the likely

extent of their actual

implementation were

not assessed.

The history of US losses due

to hurricanes over the 20th

century illustrates the importance of socioeconomic factors in determining vulnerability. Since 1900, economic

losses due to hurricanes have

increased markedly, while

deaths due to hurricanes have

decreased markedly – even

though there has been no significant trend in the number or

intensity of hurricanes. The

trends in economic losses and

deaths are explained primarily

by socioeconomic factors.

The decline in deaths reflects

the importance of improved

forecasts, warning systems,

and emergency preparedness

measures, an important set of

adaptation measures to

extreme weather events. The

increase in property losses

reflects the increasing concentration of valuable property

and infrastructure in low-lying

coastal regions in the path of

hurricanes. Many more people and much more property

are now located in harms way,

and while we have grown

much better at protecting the

people from hurricanes, we

cannot protect the property.

Hurricanes and their Impacts in the 20th Century

1900 - 1995
36

OUR CHANGING NATION

Multiple Stresses, Surprises, and Advancing Knowledge

limate change will occur together with many other economic,technological,and

environmental trends,which may stress the same ecological and social systems and

interact with climatic stresses. Human society has imposed various stresses on the environment,at diverse scales,for centuries. Over the 21st century some non-climatic stresses

will likely increase (such as loss of habitat) while others decrease (such as acidifying pollution); climate change is likely to compound some non-climatic stresses and mitigate others. Systems that are already bearing multiple other stresses are likely to be more vulnerable to climatic stress. This applies to communities and managed ecosystems,such as marginal agriculture or resource-based communities suffering job loss and out-migration. It

also applies to natural ecosystems,whose capacity for adaptation is,in general,likely to be

much more limited than that of human communities. Although the central importance of

considering interactions between multiple stresses is clear, present tools and methods for

doing this are limited;this limitation points to an important set of research needs.

Many climate changes and their impacts will likely be extensions of trends that are already

underway, and so are at least partly predictable,but some are not. We often expect natural

and social systems to change and respond continuously:push the system a little,and it

shifts a little. But complex climatic,ecological,and socioeconomic systems can sometimes respond in highly discontinuous ways:push the system a little more,and it might

shift to a completely new state. Such discontinuities or surprises can be seen clearly after

they happen,and attempting to explain them often generates important advances in our

understanding,but they are extremely difficult to predict. Several possible surprises and

discontinuities have been suggested for the Earth’s atmosphere,oceans,and ecosystems.

Still more potential for surprise arises from the intrinsic unpredictability of human

responses to the challenges posed by climate change. Even if the probability of any particular surprise occurring is low (which might not be the case),potential surprises are so

numerous and diverse that the likelihood of at least one occurring is much greater. We

The increase in harmful algal blooms along

nearly all US coastlines may be an ecological

effect of multiple environmental stresses.

The number and intensity of toxic algal

blooms, the areas and number of fisheries

affected, and the associated economic losses have all increased in recent decades.

The causes are not yet clear, but are suspected to reflect combined effects of pollution, excess nutrient input, transport of toxic

species, and climate conditions. Red tides,

which can render shellfish poisonous, are

one type of toxic algal bloom that appear to

increase with warmer ocean temperatures. 

Harmful Algal Blooms

California Florida New England

Past environmental

C

suprises have included the appearance of

the 1930s drought,

and the 1980s

appearance of the

Antarctic ozone hole.

Potential large-consequence surprises

present some of the

more worrisome concerns raised by climate change, and

pose the greatest

challenges for policy

and research. 
37

Continually advancing

knowledge and technology, and the social,

economic, and policy

conditions that support them, provide a

powerful foundation

for adapting to whatever climate changes

might come. 

have been surprised by environmental and socioeconomic changes many times.

Examples of past environmental surprises include the appearance of the 1930s

drought,and the 1980s appearance of the Antarctic ozone hole. Potential largeconsequence surprises present some of the more worrisome concerns raised

by climate change,and pose the greatest challenges for policy and research.

Surprises are inherently unpredictable. But two broad approaches can help us

prepare to live with a changing and uncertain climate, even considering the

possibility of surprise. First,some of our assessment effort can be devoted to

identifying and characterizing potential large-impact events, even if we presently judge their probability to be very small. Second,society can maintain a

diverse and advancing portfolio of scientific and technical knowledge,and policies that encourage the creation and use of new knowledge and technology.

This would provide a powerful foundation for adapting to whatever climate

changes might come.

Decreased

Snowpack

Increased

Precipitation

Increased

Temperature

Sea-level

Rise

Decreased

Soil Moisture

Increased

Run-off

Increased

Evaporation

Changing

Ecosystems Increased

Extreme Events Increased

Carbon Dioxide

Increased

UV Radiation

Climate Change and other Environmental Stresses

Although climate change has multiple linked impacts, climate change is itself just one of many forms of change

underway in the global environment, connected in complex and uncertain ways. The figure illustrates several of

the effects of climate change and several other forms of environmental change underway. Developing tools and

methods to assess the impacts of climate change in the context of other environmental changes will

be a high priority for future assessments.