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CHAPTER TWO
Vulnerabilities and Impacts
A
daptation is intended to reduce climate change vulnerabilities and impacts.
That means any consideration of adaptation planning must begin with consid-
eration of risks associated with climate change vulnerabilities and impacts, to
the extent that these can be anticipated.
More specifically, adaptation includes (1) the strategies, policies, and measures imple-
mented to avoid, prepare for, and effectively respond to the adverse impacts of climate
change on natural and human systems (to the extent that they can be anticipated),
and (2) the social, cultural, economic, geographic, ecological, and other factors that de-
termine the vulnerability of places, systems, and populations. Climate-related changes
can create new or interact with existing vulnerabilities to cause impacts, including
changes in:
• Temperature, both averages and extremes;
• Precipitation, both averages and extremes;
• The intensity, frequency, duration, and/or location of extreme weather events;
• Sea level; and
• Atmospheric carbon dioxide (CO2) concentrations.
Vulnerability is often defined as the capacity to be harmed. It is a function of the
character, magnitude, and rate of climate variation to which a system is exposed, its
sensitivity, and its adaptive capacity (Clark et al., 2000; IPCC, 2007a; Turner et al., 2003).
Vulnerabilities can be reduced by limiting the magnitude of climate change through
actions to limit greenhouse gas (GHG) emissions (ACC: Limiting the Magnitude of Future
Climate Change; NRC, 2010c), reducing sensitivity (the underlying social, cultural,
economic, geographic, ecological, and other factors that interact with exposures to
determine the magnitude and extent of impacts), or improving coping capacity (the
ability to avoid, prepare for, and respond to an impact so that it is not seriously disrup-
tive). Actions to reduce sensitivity and increase coping capacity are keys to effective
adaptation to climate change.
A risk perspective (Chapter 4) considers the probability of an exposure and its con-
sequences, including uncertainties in projecting the magnitude, rate, and extent of
climate change. It also considers factors that shape sensitivities and coping capacities,
which are as important as exposures in determining impacts. Later chapters of this
report consider options for reducing risks by reducing sensitivities and improving cop-
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A D A P T I N G T O T H E I M PA C T S O F C L I M AT E C H A N G E
ing capacities. This chapter provides the context by summarizing what is known about
current and projected climate change impacts and vulnerabilities in the United States.
PROJECTED u.S. CLIMATE CHANgES THAT COuLD REQuIRE ADAPTIvE RESPONSES
Climate-related impacts that require adaptation are already being observed in the
United States and its coastal waters (USGCRP, 2009), and empirical evidence suggests
that many of these and other impacts will grow in severity in the future (USGCRP,
2009). Over the past 50 years:
• Average temperature in the United States increased more than 2°F (1°C).
• Precipitation in the United States increased an average of 5 percent, and the
intensity of precipitation events also increased.
• Many types of extreme weather events increased in frequency and intensity;
hurricanes, although not more frequent, increased in destructive energy.
• Sea level increased along most of the U.S. coast over the past 50 years, with
some areas along the Atlantic and Gulf coasts experiencing increases of
greater than 8 inches.
• Arctic sea ice extent decreased 3 to 4 percent per decade, with end-of-summer
ice declining at 11 percent per decade.
These changes are causing impacts that should promote adaptation regardless of
whether the trends are permanent. In many circumstances, projected increases in the
frequency and intensity of many extreme weather events over the next several de-
cades will initially drive adaptation more than changes in mean weather variables. Im-
ages from Alaska provide a vivid example of observed climate change impacts in the
United States (Figure 2.1). These impacts already require adaptation in many locations
and economic sectors.
Effective adaptation depends on understanding projected climatic changes at geo-
graphic and temporal scales appropriate for the needed response. The report Global
Climate Change Impacts in the United States (USGCRP, 2009) was based on two pro-
jected climate change scenarios: one of relatively moderate changes in the event that
GHG emissions peak before the middle of the century and decline thereafter (lower
emissions scenario), and another of relatively severe changes in the event that GHG
emissions continue to grow at current rates without aggressive actions to limit them
(higher emissions scenario). Prospects for adaptation to keep disruption from climate
change impacts at socially acceptable levels depend very substantially on what hap-
pens with efforts to limit emissions. At moderate rates and levels of climate change,
adaptation can be very effective. At severe rates and levels of climate change, limits of
0
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Vulnerabilities and Impacts
FIguRE 2.1 The Arctic village of Shismaref: Rising sea levels and fierce storms have eroded the shoreline
near this coastal Inupiat village, breaking down sea walls and washing away homes. Residents decided
to relocate farther inland for safety, giving up their traditional fishing, sealing, and home-building sites.
SOURCE: Photo by Edward W. Lempinen/AAAS. © 2006 AAAS.
many adaptation options are likely to be reached, and resulting adaptations are likely
to be much more disruptive.
A key fact about climate change impacts is that stabilization of atmospheric GHG con-
centrations will not immediately stabilize the climate, which will continue to change
for some time because of the delayed response of the climate to the buildup of GHGs
emitted in the recent past. A companion to this report (ACC: Limiting the Magnitude
of Future Climate Change; NRC, 2010c) details the challenges and choices the nation
and the world face in sufficiently limiting GHG emissions to keep climatic changes
at a relatively moderate level.1 It also concludes that stabilizing emissions at moder-
ate levels is becoming increasingly difficult in the face of U.S. and global inaction. The
U.S. Global Change Research Program’s (USGCRP’s) characterizations of two possible
futures (lower or higher emissions) show that an effective response to climate change
must include both adaptation and mitigation (e.g., Wilbanks and Sathaye, 2007).
Much of the current knowledge about projected climate changes in the United States
comes from an assessment process mandated by the U.S. Congress in the Global
1
For a discussion linking emission rates and atmospheric concentrations of GHGs to changes in global
mean temperature, see Chapter 2 of ACC: Limiting the Magnitude of Future Climate Change (NRC, 2010c).
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A D A P T I N G T O T H E I M PA C T S O F C L I M AT E C H A N G E
Change Research Act of 1990 (P.L. 101-606), including the U.S. National Assessment
(USGCRP, 2001) and conclusions from 21 widely peer-reviewed Synthesis and Assess-
ment Products (SAPs) produced by the U.S. Climate Change Science Program between
2006 and 2009 on specific topics ranging from knowledge of the physical climate
system to the interface between climate change and society. The SAPs were sum-
marized and updated in Global Climate Change Impacts in the United States (USGCRP,
2009). According to this summary report, future climate change impacts in the United
States will include warmer average temperatures, changes in precipitation patterns,
more frequent heat waves and severe storms, rising sea level, and decreases in sea ice
and permafrost, which will be particularly rapid in the Arctic.
The average temperature in the United States will continue to rise with climate
change, but the magnitude of the increase depends primarily on the amount of heat-
trapping GHGs emitted globally and how sensitive the climate is in responding to
those emissions. Figure 2.2 shows projected temperature change under the higher
and lower emissions scenarios in midcentury and at the end of the century. The brack-
ets on the thermometers represent the likely range of model projections, although
lower or higher outcomes are possible. By the end of the century, the average U.S.
temperature is projected to increase approximately 7°F to 11°F (4°C to 6°C) under the
higher emissions scenario and approximately 4°F to 6.5°F (2°C to 4°C) under the lower
emissions scenario (USGCRP, 2009).
Projections of future precipitation generally indicate that northern areas (higher
latitudes) will receive more precipitation, and southern areas, particularly in the West,
will become drier (USGCRP, 2009). However, the mechanisms by which human-induced
climate change affects precipitation are subtler than those of temperature and paint a
more complex picture (e.g., Zhang et al., 2007). Figure 2.3 shows projected changes by
2080-2099 under the higher emissions scenario; these are sample results from climate
models, not projections of certainty for the future.
The amount of rain falling in the heaviest downpours has already increased approxi-
mately 20 percent on average in the past century, and this trend is very likely to con-
tinue, with the largest increases in the wettest places (USGCRP, 2009). Figure 2.4 shows
projected changes from the 1990s average to the 2090s average in the amount of
precipitation falling in light, moderate, and heavy events. The lightest precipitation is
projected to decrease, while the heaviest will increase, continuing the observed trend.
Many types of extreme weather events, such as heat waves, have become more fre-
quent and intense during the past 40 to 50 years, while cold extremes have become
less frequent (USGCRP, 2009). In the future, currently rare extreme events (for example
a 1-in-20-year event) are projected to become more commonplace (Figure 2.5), al-
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Vulnerabilities and Impacts
FIguRE 2.2 Projected temperature change (°F) from 1961-1979 baseline. NOTE: These results are derived
from global models whose spatial resolution is insufficient to resolve important details like mountain
ranges. SOURCE: USGCRP (2009) (http://www.globalchange.gov).
though these projected increases will not be uniformly distributed over temporal and
spatial scales. For example, a day so hot that it is currently experienced once every 20
years would likely occur every other year or more frequently by the end of the century
under the higher emissions scenario. Although uncertainties remain about whether
the number of hurricanes could increase with climate change, the destructive energy
of Atlantic hurricanes is likely to increase in this century as sea surface temperature
rises (USGCRP, 2009) (Figure 2.6). In addition, cold-season storm tracks are shifting
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A D A P T I N G T O T H E I M PA C T S O F C L I M AT E C H A N G E
FIguRE 2.3 Projected change in North American precipitation by 2080-2099. NOTE: Cross-hatching indi-
cates areas in which climate models do not agree. SOURCE: USGCRP (2009) (http://www.globalchange.gov).
northward, and the strongest storms are likely to become stronger and more frequent
(USGCRP, 2009).
The ocean is warming and glaciers and polar ice sheets are melting, causing sea level
to continue to rise, most likely at a faster rate than in recent history. Globally, under
the higher emissions scenarios, average sea level is estimated to rise by 3 to 4 feet
(USGCRP, 2009). How much land will become submerged will vary regionally, depend-
ing on the regional tectonics and geomorphology (land masses can be in the process
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Vulnerabilities and Impacts
FIguRE 2.4 Projected changes in light, moderate, and heavy precipitation from the 1990s average to the
2090s average in North America. As shown here, the lightest precipitation is projected to decrease, while
the heaviest will increase, continuing the observed trend. The higher emissions scenario yields larger
changes. Projections are based on the models used in the IPCC (2007) Synthesis Report. NOTE: “Lower
emissions scenario” refers to IPCC SRES B1, “higher emissions scenario” refers to A2, and “even higher emis-
sions scenario” refers to A1FI. SOURCE: USGCRP (2009) (http://www.globalchange.gov).
of rising or sinking relative to sea level) and ocean currents (which can cause the
ocean surface to rise or sink relative to the average global sea level).
DETERMININg vuLNERAbILITIES TO PROJECTED CLIMATE CHANgES
As defined earlier, vulnerability is a function of the character, magnitude, and rate of
climate change to which a system is exposed, as well as the system’s sensitivity and its
adaptive capacity. Therefore, vulnerability can be assessed through the examination of
these three factors. Assessing exposure to climate change reveals regional differences
in the climate-related impacts that the United States will experience. Table 2.1 summa-
rizes climate-related exposures and the regions that will most likely be affected.
Vulnerability can also be examined through the sensitivity and adaptive capacities of
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A D A P T I N G T O T H E I M PA C T S O F C L I M AT E C H A N G E
FIguRE 2.5 Projected frequency of extreme heat (2080-2099 average). SOURCE: USGCRP (2009) (http://
www.globalchange.gov).
a particular community, system (i.e., economic, ecosystem, etc.), or sector. Vulnerability
encompasses the risk and protective factors that ultimately determine whether a sub-
population experiences adverse outcomes due to climate change (Balbus and Malina,
2009). For example, Table 2.2 summarizes various subpopulations that are particularly
vulnerable to multiple climate-related exposures to health risks.
Vulnerabilities of human systems are shaped by a wide variety of nonclimatic condi-
tions. Although important, limited access to financial resources is not the only source
of vulnerability. Examples of other sources include population shifts and development
choices, such as dense urban development in drought-prone areas; and places and
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Vulnerabilities and Impacts
FIguRE 2.6 Projected sea surface temperature change. SOURCE: USGCRP (2009) (http://www.global
change.gov).
communities that are especially dependent on climate-sensitive industries such as
agriculture, forestry, and tourism. In the built environment, each city’s residents and
infrastructures will be affected in unique ways (USGCRP, 2009). The vulnerability of
natural systems, on the other hand, depends primarily on an ecosystem’s resilience
to change. Changes in ecosystem function, in turn, affect human communities that
depend on natural ecosystems to maintain clean water supplies, soil fertility, and other
vital services (USGCRP, 2009).
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TAbLE 2.1 Summary of regional climate-related impacts
Climate-Related Impacts
Extreme
Urban Rainfall Sea
Early Degraded Air Heat Heat Tropical with Level
United States Census Regions Snowmelt Quality Island Wildfires Waves Drought Storms Flooding Rise
New England • • • • • • •
ME VT NH MA RI CT
Middle Atlantic • • • • • • • •
NY PA NJ DE MD
East North Central • • • • • •
WI MI IL IN OH
West North Central • • • • •
ND MN SD IA NE KS MO
South Atlantic • • • • • • • •
WV VA NC SC GA FL DC
East South Central • • • • • •
KY TN MS AL
West South Central • • • • • • • •
TX OK AR LA
Mountain • • • • • • •
MT ID WY NV UT CO AZ NM
Pacific • • • • • • • • •
AK CA WA OR HI
SOURCE: Adapted from CCSP (2008f ).
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Vulnerabilities and Impacts
TAbLE 2.2 Summary of vulnerability to climate-sensitive health outcomes by
subpopulation
Groups with Increased Vulnerability Climate-Related Exposures
Infants and children Heat stress, ozone air pollution, water- and food-borne illnesses,
Lyme disease, dengue
Pregnant women Heat stress, extreme weather events, water- and food-borne
illnesses
Elderly/Chronic medical conditions Heat stress, air pollution, extreme weather events, water- and
food-borne illnesses, dengue
Impoverished/Low socioeconomic Heat stress, extreme weather events, air pollution, vector-borne
status infectious diseases
Outdoor workers Heat stress, ozone air pollution, Lyme disease, other vector-
borne infectious diseases
HOW CHANgINg CLIMATE CONDITIONS AND
vuLNERAbILITIES IMPACT DIFFERENT u.S. SECTORS
Climate Change Will Interact with Many Social and Environmental Stresses
Society, its infrastructure, and its policies were developed in a relatively stable climate.
Although climate change will create advantages for some locations and populations,
on average, climate change is expected to adversely affect water resources, ecosys-
tems, human health, energy, transportation, and other sectors. The expectation of
adverse impacts stems in part from the fact that these systems were designed during
a period of relatively stable climate conditions and in part from the accelerating rate
of change, which presents a novel challenge for adaptation. Recent events suggest
that changes in extreme weather events, including heat waves, floods, droughts, wind-
storms, and wildfires, will likely be particularly challenging for communities and sec-
tors to adapt to. The combination of climate change and trends in population growth
also poses serious adaptation challenges. For example, population growth in the
past century has been greatest in the South, along the coast, and in larger cities; this
trend aligns somewhat with places where the threats of future heat waves and severe
storms are greatest (USGCRP, 2009). With most of the U.S. population residing in urban
areas, vulnerabilities associated with aging urban infrastructure, traffic congestion, air
quality, social inequities, and other variables exacerbate the challenges of adapting to
climate change. Key conclusions about how climate change will interact with social
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A D A P T I N G T O T H E I M PA C T S O F C L I M AT E C H A N G E
adaptation at the national level has been difficult; but at a local level, most decision
makers and stakeholders contemplating climate change actions find it unwarranted
to consider one strategy apart from the other (see also Chapter 5). A key issue is how
mitigation and adaptation actions relate to each other. Some options offer comple-
mentarities and synergies, while some work at cross-purposes with each other. For
example, increasing the efficiency and affordability of space cooling helps to extend
the benefits of cooling to a wider range of the residents of warming settlements; at
the same time, it also reduces requirements for electricity generation to enable those
services. On the other hand, choices between growing biomass for energy produc-
tion and growing biomass as a GHG sink, both of which can be mitigation strategies,
relate in different ways to adaptation strategies. For instance, bioenergy production
can add to challenges in adapting to water scarcity in some regions. In addition, grow-
ing biomass intended for long-term carbon storage can be complicated by climate
change impacts on regional ecological systems, along with associated adaptive land
use strategies.
Other possible impacts of climate change policies—not all of them negative—include
effects on choices of energy production and use technologies, on environmental emis-
sions, and on international energy technology and service markets (for additional de-
tails see ACC: Limiting the Magnitude of Future Climate Change; NRC, 2010c). Yet another
issue is possible side effects of “geoengineering” options, should they be implemented.
In general, geoengineering options intended to reduce the amount of solar radiation
reaching the Earth—such as by creating a sulfate cloud in the atmosphere—would be
virtually certain to affect vegetation growth and rainfall regimes, although the mag-
nitude and geographic distribution of the potential effects are not well understood.
Options intended to reduce current levels of CO2 in the atmosphere would require
extensive carbon storage in places such as underground geologic formations, a possi-
bility that presents a different range of impact and adaptation concerns. In either case,
both known and unintended impacts could require adaptations in response.
COMPARATIvE METRICS OF IMPACTS AND vuLNERAbILITIES
In order to determine tradeoffs between various climate change policies and actions,
scientists have attempted to find objective measures for dangerous climate interfer-
ence (as prescribed in the United Nations Framework Convention for Climate Change
1992) that might push the system beyond its adaptive capacity. To date, such an ob-
jective characterization of “dangerous” climate interference has not been developed.
Nevertheless, a framework to consider global key vulnerabilities was developed for the
Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (Smith
0
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Vulnerabilities and Impacts
Reasons For Concern for the United States
5
Large Risks to Net Negative High Large
Increase in Global Mean Temperature above circa 1990 (˚C)
Increase Many Negative for Most Increase
in All Regions
Metrics
4
3
Future
2
Negative 1
for Some
Positive or Regions;
Negative Positive
Market
Risks to for
Impacts Low Increase
Increase Some Others
0
Past
-0.6
Risks to Aggregate Distribution Risks of Large National
Risk of
Unique Impacts of Impacts Scale Security
Extreme
and Discontinuities Concerns
Weather
Threatened
Events
Systems
FIguRE 2.9 Risks from climate change for the United States. Climate change consequences for the
United States are plotted against increases in global mean temperature (°C) after 1990. Each column
represents country-specific outcomes associated with increasing global mean temperature for each of
the six reasons for concern. The color scheme represents progressively increasing levels of risk: white in-
Figure 2-9
dicates neutral or small negative or positive impacts or risks, yellow indicates negative impacts for some
systems or low risks, and red means negative impacts or n that are more widespread and/or greater in
vector versio risks
magnitude. Orange indicates a range of transition from risks calibrated in the modest risks of yellow and
replaced from original source
those calibrated in more severe and/or widespread risks of red. SOURCE: Yohe (2010); for details related
to the assumptions in this figure see Appendix D.
et al., 2001; IPCC, 2001b; and updated in Smith et al., 2009a). Following Yohe (2010), the
panel responded to Woolsey (2009), Peters (2009), and Burke et al. (2009) by adding a
sixth “reason for concern” related to the national security interests of the United States.
To be precise, the aggregate metrics, as applied to the United States in Figure 2.9,
include the following:
1. Risk of extreme weather events. The likelihood of extreme events with sub-
stantial consequences for societies and natural systems such as increases in
frequency or intensity of heat waves, floods, droughts, wildfires, or tropical
cyclones, etc.
2. Risk to unique and threatened systems. The likelihood of imposing increased
damage or irreparable loss to unique and threatened systems such as coral
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A D A P T I N G T O T H E I M PA C T S O F C L I M AT E C H A N G E
reefs, tropical glaciers, endangered species, unique ecosystems, biodiversity
hotspots, indigenous communities, etc.
3. Aggregate impacts. The likelihood of recognizing damages in aggregate. There
are impacts distributed across the economy that can be aggregated into a
single metric. Again, this reason for concern traditionally reported aggregate
economic damages reports as, for example, the social cost of carbon.
4. Distribution of impacts. The likelihood of disparities of impacts (positive or
negative) across regions or sectors. Some regions, sectors, or communities
could face harm from climate change while others could even benefit. While
this reason for concern historically focused primarily on economic metrics,
recent work has aggregated subnational alternative metrics.
5. Risks of largescale discontinuities. The likelihood of certain “threshold” phe-
nomena that may have very large impacts. Examples include partial or com-
plete deglaciation of the West Antarctic or Greenland ice sheets (that could
lead to rapid increases in sea level), substantial reduction in the strength of
the North Atlantic Meridional Overturning Circulation (that could result in a
relatively rapid change in the climate system due to redistribution of heat in
the oceans), and a “runaway greenhouse effect” (featuring more rapid warm-
ing) driven by methane emissions from melting permafrost.
6. National security concerns. The likelihood that growing attention to climate
change risks and vulnerabilities that will occur beyond national borders will
nonetheless require response by defense and other mission agencies within
the U.S. government (for additional details, see Chapter 6).
It should be emphasized that this figure calibrates risks to increases in global mean
temperature. It follows that the depicted transitions from low to high levels of concern
do not necessarily reflect how risks might change at different rates of warming, nor
do they necessarily indicate when impacts might be realized and how vulnerabilities
might be influenced by alternative development pathways and the exercise of adap-
tive capacity. When applied to the globe and here to the United States, the underlying
“reasons for concern” framework nonetheless continues to be a viable mechanism
with which to describe key climate risks and thus help to identify priorities with regard
to ongoing and future research initiatives and attractive foci for policy discussion and
implementation (IPCC, 2001a, 2007a).
In summary, two qualitative conclusions emerge from the expert judgments that are
embodied in Figure 2.9. Both reflect the evolving changes in climate variability that
will be driven by long-term climate change.
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Vulnerabilities and Impacts
1. If policy makers were provided with only aggregate economic metrics when
they asked to be informed about the significance and timing of impacts, then
they would miss many if not all of the other risks that are captured in five
other equally appropriate reasons for concerns. Indeed, Figure 2.9 suggests
that decision makers could, as a result, come to the erroneous conclusion that
it may take quite some time for the country as a whole to experience the rami-
fications of “dangerous anthropogenic interference” with the climate system
that has attracted the attention of many other countries. Of course, distinct
localities and regions within the United States will have to cope with a diverse
set of climate-driven vulnerabilities, and decision makers who work in these
arenas will have an incentive to consider more focused economic aggregates.
Even in these cases, though, interpreting aggregate economic indicators can
be difficult, especially if those indicators ignore economic damages that will
occur beyond specified borders—damages that are certainly part of a full and
complete characterization of the potential economic risk to the nation.
2. Conversely, dangerous anthropogenic interference in the climate system will
likely be discovered at all levels as climate change alters the intensities, fre-
quencies, and regional distributions of extreme weather events. It is in these
areas where investing in adaptive capacity and exercising adaptation options
at the local level play their most critical roles; and it is through these manifes-
tations that diversity in the climate risks facing various geographic regions
scattered across the country and various climate-sensitive sectors scattered
throughout the economy supports bottom-up approaches to evaluating ad-
aptation needs.
The first conclusion is almost a corollary of the observation that aggregate economic
estimates of damages too often ignore low-probability risks. Indeed, this deficiency
is just one of a growing list of concerns about relying too heavily on monetary
estimates—estimates that, for the most part, miss many nonmarket damages and
nearly all consequences from social contingent consequences (see, e.g., Yohe, 2009a;
Yohe and Tirpak, 2008). The second follows from the expectation that reasons for
concern, by offering alternative but nonetheless aggregate metrics, communicate the
diversity of those risks more effectively.
Care needs to be taken in interpreting these conclusions. Authors of the various ver-
sions of the reasons for concern have emphasized that they cannot be the sole basis of
policy; too many assumptions and limits of knowledge are either buried or missing. It
is important to understand, for example, that these six aggregate “reasons for concern”
reflect adaptation only to the extent the capacity to respond is included in the under-
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A D A P T I N G T O T H E I M PA C T S O F C L I M AT E C H A N G E
lying literature. It has long been understood that the capacity to adapt depends on
development pathways that cannot be reflected in simple calibrations of changes in
global mean temperature; it is now understood that the ability to exercise the capacity
to adapt is very site specific. It follows that it was impossible, in this and other “embers”
exercises, to include depictions of where existing or potential “coping ranges” might
be exceeded by changes in global mean temperature.
Reasons for concern are best viewed as suggestions of where one might discover vul-
nerabilities and impacts that some or even most might consider “dangerous.” Superim-
posed against ranges of temperature trajectories (as in Figure SPM-2 or SPM-3 in IPCC,
2001a), they might even suggest when such danger might begin to occur. It follows
that reasons for concern, when properly applied, can help scientists and decision mak-
ers identify areas where more detailed analyses of vulnerabilities, and the associated
opportunities for effective adaptation, might be most productive in directing research
and informing policy design and implementation.
MAJOR SCIENTIFIC CHALLENgES IN ASSESSINg CLIMATE CHANgE IMPACTS
AND vuLNERAbILITIES AND THEIR IMPLICATIONS FOR ADAPTATION
At this early stage in analyzing adaptation needs and potentials, many scientific chal-
lenges remain in assessing vulnerabilities and impacts associated with climate change
(ACC: Advancing the Science of Climate Change; NRC, 2010b). Six of the most significant
of these challenges include:
1. The level of scientific confidence in understanding and projecting climate change
increases with spatial scale while the relevance and value of the projections for
society declines.
A branch of climate science called detection and attribution (D&A) seeks to under-
stand the causes of observed changes in climate by comparing observed changes
with those simulated by climate models under rising atmospheric GHG concentrations
and against background climate variability in model simulations with no rising GHGs.
For statistical reasons, D&A is most successful at large spatial scales. It was first used to
identify human influence on globally averaged temperature over the 20th century.
Difficulties remain in attributing temperature changes on smaller than continental
scales and over time scales of less than 50 years (IPCC, 2007b). Although the level of
scientific confidence in climate change projections decreases at smaller spatial and
temporal scales, the societal value increases. For example, while there is limited value
in using the global averaged temperature for planning adaptation, information such
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Vulnerabilities and Impacts
as the projected ranges in possible changes in 100-year flood risk on a particular river
at a given location can be very useful even if highly uncertain.
2. A finerscale understanding of climate change risks and vulnerabilities is needed.
Impacts and adaptation are often local issues because the actual climate change
impacts experienced will result from interactions of a specific climatic exposure with
a specific population, sector, or system sensitive to that exposure, as well as the ability
of that population, sector, or system to avoid, prepare for, and effectively respond to
the risk. Thus, the same climatic exposure can have different consequences in differ-
ent locations, and even in the same location at different time periods. Improvements
are needed in the ability to project climate change at local and regional scales and
to increase understanding of risks and the ability to design efficient and effective
responses. Significant scientific challenges also remain in our limited understand-
ing of the social, environmental, economic, institutional, and other factors that could
interact with climatic changes to create impacts in any given location. Although Hur-
ricane Katrina cannot be attributed to climate change, it demonstrated how hazard
predictions with high certainties might fail to elicit proactive, necessary adaptations,
partially due to a lack of understanding of local vulnerabilities and partially due to
the difficulties of incorporating the latest natural and social sciences knowledge into
practice and pre-disaster planning (NRC, 2006). Social groups particularly vulnerable
to extreme weather events include the elderly, pregnant women, children, people with
chronic medical conditions, people with mobility and cognitive constraints, and the
urban and rural poor—all groups that are disproportionately represented within low-
income communities (Balbus and Malina, 2009). Understanding such vulnerabilities in
advance, and implementing appropriate strategies to increase resilience, affects the
magnitude and extent of climate impacts (Kates et al., 2006; NRC, 2006).
Projected increases in the frequency and intensity of extreme weather events high-
light the need to increase understanding of those most at risk, both today and in
future societies with possibly different risk profiles. However, research on vulnerability
and impacts (and associated sustainability indices) has frequently been at scales too
large to incorporate social justice issues. One counterexample is heat waves, where re-
search at finer scales has shown that poorer areas of cities often have higher tempera-
tures because of fewer green spaces, and that the residents of these areas may have
less access to air conditioning or may not open windows during heat waves for fear of
crime, thus increasing vulnerability.
3. Multiple stresses will interact with the impacts of climate change, leading to differ
ent vulnerabilities to the same climate condition in different locations and a need
for different adaptive responses.
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Actual impacts will depend not only on climate but also on changes in other stresses
over the same period. For example, impacts of climate change on vulnerable coast-
lines in 2070 will be shaped not only by changes in sea level, storm tracks, and storm
intensities but also by land subsidence, changes in population size and distribution,
economic activities and wealth, technology, and institutional structures. Understand-
ing how interactions with these other factors accentuate or ameliorate climate change
impacts is important for adaptation planning. For example, detailed projections of
socioeconomic scenarios are often not available beyond several decades into the
future (and if they are available, they are highly speculative), which limits our ability for
integrated modeling to understand how interactions could play out over time.
4. Adapting to changes in averages versus changes in extremes results in a funda
mental scientific and policy challenge.
Projections of the impacts of climate change tend to focus on changes in average
weather variables, particularly changes in average temperature. As important as these
changes are likely to be—for example, how increasing average temperature affects
the suitability of particular cereal crops for a given region—the actions required
for adapting to averages can be different from the actions required for adapting to
extreme events. Strategies to manage the risks of climate change need to address
projected increases in the frequency and intensity of extreme weather events, as well
as unexpected threshold events. Depending on the cost of adaptation options versus
the cost of impacts they are designed to avert, it may be helpful to prepare for low-
probability/high-consequence events. Science and engineering needs include reeval-
uation of boundaries of flood plains, better flood maps, and redesign or retrofitting
of hospitals and other critical infrastructure so that services would not be disrupted
during an extreme weather event. Effective adaptation will thus require consideration
of climate change risks along multiple dimensions: increasing resilience to warmer
temperatures and average changes in the water cycle while at the same time increas-
ing resilience to extreme weather events—and doing both while considering current
and future changes in other driving forces.
5. Interactions and integration across regions and sectors cause considerable
complexity and will lead to unanticipated consequences of both impacts and
adaptations.
Climate change impacts in one sector or region usually spread secondary and ter-
tiary impacts elsewhere. For instance, reducing impacts of summer warming on the
quality of life of urban populations is likely to call for more air conditioning in homes
and places of work. This will impact the energy sector by adding peak demands for
electricity production, which can in turn impact the water sector by requiring more
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cooling water for thermal power plant operation. Likewise, agricultural adaptations in
a region may call for increased use of irrigation, while water resources adaptation in
that region may call for decreased water availability for irrigation.
Impacts can cross regional boundaries as well. For instance, more intense storms in
vulnerable areas can mean flows of evacuees to other regions, along with at least
temporary shortages (or increases in the price) of products and services disrupted by
the storms, as was the case with energy products after Hurricane Katrina (Bamberger
and Kumins, 2005). Further research is needed to improve our understanding of how
to effectively develop cross-regional and cross-sectoral adaptation plans.
6. The types of impacts, vulnerabilities, and adaptation options are different for
natural and human systems.
Both national and international assessments have described the broad patterns of
recent and projected responses of natural systems and biodiversity to climate change
(IPCC, 2007a; MEA, 2005; USGCRP, 2009). It is highly likely that most natural systems are
sensitive to climate change. Much of our current understanding of ecosystem dynam-
ics, however, is based on observations and models that assume less dramatic direc-
tional changes in environment and ecosystem dynamics. These models and theories
provide an important starting point for understanding rapid change, but they will
undoubtedly require reassessment as new patterns of environmental and ecological
controls emerge.
These changes are likely to result in the loss of some ecosystems and the formation of
novel ecosystems due to the loss of some species and arrival of others. Loss of biodi-
versity is quite likely, including both loss of rare species and loss or reduced impor-
tance of keystone species. During these times of rapid biological adjustment, meta-
population dynamics (i.e., interactions among partially isolated subpopulations) and
migration of species across increasingly fragmented and human-modified landscapes
are likely to exert greater influence on ecosystem structure and functioning than in
the past. All of these changes could reduce the resilience of natural ecosystems and
make them more vulnerable to threshold changes. These and other broad changes in
the ground rules by which ecosystems operate create significant scientific challenges
in understanding and predicting the patterns and consequences of changes in natural
ecosystems.
The prospect of widespread ecological change also raises two pragmatic questions
that represent additional research challenges: How can the rates of undesirable
ecological change (e.g., loss of biodiversity) be minimized? And how can the flow of
essential ecosystem services on which society depends be sustained in the face of
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rapid ecological change? Both questions will require improved understanding of the
dynamics of social-ecological systems and collaborations.
ADAPTATION AND uNCERTAINTy
Adapting to climate change impacts will require doing our best to understand the
factors that drive both the impacts and our ability to respond. This reality has led to
urgent calls for more information about the range of possible impacts and the level of
certainty in our projections of the future. It is clear that society cannot avoid the risks
of climate change entirely. One challenge for decision makers will be the limits to our
ability to identify and reduce uncertainties related to climate change.
Major uncertainties in determining future climate include the natural internal vari-
ability of the climate system, the trajectories of future emissions of GHGs and aerosols,
and the response of the global climate system to any given set of future emissions
(see also Chapter 4 and Meehl et al., 2007; NRC, 2010c). The magnitude and sources
of these uncertainties can be explored using global climate models. These models
have become more sophisticated and accurate over time in replicating the historical
record. However, it is unlikely that climate models will be able to predict the future on
fine spatial scales with a high degree of accuracy on long time scales. At best, climate
models can provide insights about the range of possible futures.
Lack of certainty about future conditions is commonly, but often inappropriately, used
as a rationale for inaction. In fact, improving our understanding of the kinds of uncer-
tainties that we face will be helpful in risk-management decisions, even if the uncer-
tainties cannot be readily quantified. For example, some uncertainties result from pro-
cesses that are still missing from the climate models but are potentially resolvable in
the future (e.g., changes in climate that result from changing land use and land cover).
There are other uncertainties that are inherent in the complexity of the climate system
itself, and it is unlikely that those kinds of uncertainty will be reduced significantly. For
example, the uncertainty of the long-term trajectory of GHGs is very likely not to be
reducible (CCSP, 2009c).
Another source of uncertainty comes from the fact that current global climate mod-
els operate at relatively coarse spatial scales (hundreds of kilometers or miles), and
thus do not accurately represent conditions in specific places; rather, they represent
average conditions across broad regions such as the entire Southwest.2 This prob-
2 However, it should be noted that the resolution of global models is increasing, and some of the simula-
tions for the next IPCC report may be run at 50 km for the near-term future (next 20 years) .
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lem of spatial scale is overcome through the application of various “downscaling”
techniques—ways to generate information at higher spatial resolution from coarse-
scale global model output. There are three primary methods of downscaling: (1)
simple downscaling, where the coarse-resolution information is simply interpolated
to higher resolution, or the coarse-scale changes in climate are used in the context of
higher-resolution observed data; (2) statistical downscaling, which relies on statisti-
cal relationships between historically observed large-scale climate variables and local
climate (e.g., daily temperature in a specific city) that are then applied to the climate
change context; and (3) dynamical downscaling techniques, such as regional mod-
eling, where a higher-resolution climate model is applied to just part of the Earth’s
surface (e.g., the western United States) and is “nested” in the global models.
Regional climate models can better represent smaller-scale processes such as those
related to complex terrain (e.g., mountains) and provide data at scales closer to those
at which decisions are made (within a watershed, for example). Regional climate mod-
els are useful in trying to understand the physical processes that control regional cli-
mate and the likely impacts of climate change within regions and sectors for risk-man-
agement planning. However, “downscaled” climate data can introduce other sources of
uncertainty and are not yet the panacea that many resource managers hope they will
be (Wang et al., 2004).3 Making adaptation decisions in the context of uncertainty will
remain a challenge, but one that can be overcome with careful attention to improv-
ing the understanding and characterization of—and the ability to communicate—the
nature and sources of uncertainty.
CONCLuSIONS
The United States is already experiencing impacts of climate change that require
adaptation. Some of these impacts are already testing, or soon will seriously test, the
nation’s coping mechanisms. In summary, the panel finds that climate change impacts
are certain to increase throughout this century, requiring significant effort to adapt
in order to avoid socially, economically, and environmentally disruptive changes in
systems with high value to society. Adaptation options need to address current and
projected changes in mean weather variables as well as increases in the frequency
and intensity of many extreme events.
3 Different regional climate models produce different responses to the boundary conditions from the
global models, presenting another source of uncertainty. Also, high-resolution modeling must be considered
in the context of the other uncertainties mentioned above. Nesting one regional model inside one global
model, regardless of how high the resolution, will not provide important information about the larger-scale
uncertainties, and can even be misleading and create a “false certainty.”
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Impacts later this century will be notably greater if GHG emissions are not stabilized at
a moderate level. If the magnitude of climate change is relatively severe, as depicted in
the USGCRP higher projection, then regions, sectors, and systems will be hard pressed
to cope with impacts and their costs. In addition, impacts of climate change are highly
diverse and disaggregated, in many cases playing out at localized geographic, sectoral,
and societal scales. As a result, effective approaches to adaptation will likely vary from
case to case.
In most cases, impacts are imbedded in interactions between climatic changes per se
and other driving forces, such as changes in demographics, economics, land use, and
technology, which also vary from case to case. Therefore, impacts and vulnerability are
place-based and fundamentally driven by the scale at which the impact occurs. Many
scientific challenges remain in assessing impacts and vulnerabilities and providing the
specific and localized information needed to guide adaptation decisions.
Conclusion: Many current and future climate change impacts require immediate
actions to improve the ability of the nation to adapt. Because some impacts
may not require immediate attention, possible adaptation options need to be
prioritized based on where and when urgent action is needed. This highlights the
need to identify vulnerabilities, impacts, and adaptation options across the nation,
at all levels of decision making.
Conclusion: Gaps in the knowledge required to link anticipated impacts with
appropriate adaptation strategies and actions need to be addressed as a high
national research priority.
Conclusion: It is inadequate to provide policy makers with only aggregate
economic metrics to convey the significance and timing of climate change
impacts. Aggregated data miss most nonmarket damages and nearly all social
contingent consequences that society might deem unacceptable, including those
from outside our borders.
Conclusion: Uncertainty about the nature of future climate change impacts in
specific locations is not a rationale for inaction but a call for better understanding
and communication of the sources and nature of uncertainty in the context of
decision making.
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