Climate Change: Lines of Evidence
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Transcript of Climate Change: Lines of Evidence
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answers to common questions about the science
of climate change
Climate
Change
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Contents
Part I. Evidence for Human-Caused Climate Change 2
How do we know that Earth has warmed? 3
How do we know that greenhouse gases lead to warming? 4
How do we know that humans are causing greenhouse gases to increase? 6
How much are human activities heating Earth?9
How do we know the current warming trend isnt caused by the Sun? 11
How do we know the current warming trend isnt caused by natural cycles? 12
What other climate changes and impacts have been observed? 15
The Ice Ages 18
Part II. Warming, Climate Changes, and Impactsin the 21st Century and Beyond 20
How do scientists project future climate change? 21
How will temperatures be affected? 22
How is precipitation expected to change? 23
How will sea ice and snow be affected? 26
How will coastlines be affected? 26
How will ecosystems be affected? 28
How will agriculture and food production be affected? 29
Part III. Making Climate Choices 30
How does science inform emissions choices? 31
What are the choices for reducing greenhouse gas emissions? 32
What are the choices for preparing for the impacts of climate change? 34
Why take action if there are still uncertainties about the risks of climate change? 35
Conclusion 36
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Just what is climate? Climate is commonly thought o as the expected weather conditions
at a given location over time. People know when they go to New York City in winter, they
should take a coat. When they visit the Pacic Northwest, they take an umbrella. Climate
can be measured at many geographic scalesor example, cities, countries, or the
entire globeby such statistics as average temperatures, average number o rainy days,
and the requency o droughts. Climate changereers to changes in these statistics
over years, decades, or even centuries.
Enormous progress has been made in increasing our understanding o climate change
and its causes, and a clearer picture o current and uture impacts is emerging. Research
is also shedding light on actions that might be taken to limit the magnitude o climate
change and adapt to its impacts.
This booklet is intended to help people understand what is known about climate change.First, it lays out the evidence that human activities, especially the burning o ossil uels,
are responsible or much o the warming and related changes being observed around
the world. Second, it summarizes projections o uture climate changes and impacts
expected in this century and beyond. Finally, the booklet examines how science can help
inorm choices about managing and reducing the risks posed by climate change. The
inormation is based on a number o National Research Council reports (see inside back
cover), each o which represents the consensus o experts who have reviewed hundreds o
studies describing many years o accumulating evidence.
Climate Change
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But how has this conclusion been reached? Climate science,
like all science, is a process o collective learning that
relies on the careul gathering and analyses o data,
the ormulation o hypotheses, the development o
models to study key processes and make testable
predictions, and the combined use o observations
and models to test scientifc understanding.
Scientifc knowledge builds over time as new
observations and data become available.
Confdence in our understanding grows i multiple
lines o evidence lead to the same conclusions, or
i other explanations can be ruled out. In the case o
climate change, scientists have understood or more
than a century that emissions rom the burning o ossil
uels could lead to increases in the Earths average surace
temperature. Decades o research have confrmed and extended this
understanding.
The overwhelming
majority o climate
scientists agree that
human activities,
especially the burning
o ossil uels (coal, oil,
and gas), are responsible
or most o the climatechange currently being
observed.
EvidEncEfor Human-Caused
Climate ChangePart I
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Scientists have been taking widespread measure-
ments o Earths surace temperature since
around 1880. These data have steadily improved
and, today, temperatures are recorded by ther-
mometers at many thousands o locations, both on
the land and over the oceans. Dierent research
groups, including the NASA Goddard Institute or
Space Studies, Britains Hadley Centre or Climate
Change, the Japan Meteorological Agency, and
NOAAs National Climatic Data Center have used
these raw measurements to produce records o
long-term global surace temperature change
(Figure 1). These groups work careully to makesure the data arent skewed by such things as
changes in the instruments taking the measure-
ments or by other actors that aect local tempera-
ture, such as additional heat that has come rom
the gradual growth o cities.
These analyses all show that Earths average
surace temperature has increased by more than
1.4F (0.8C) over the past 100 years, with much
o this increase taking place over the past 35 years.
A temperature change o 1.4F may not seem like
much i youre thinking about a daily or seasonal
fuctuation, but it is a signicant change when
you think about a permanent increase averaged
across the entire planet. Consider, or example,
that 1.4F is greater than the average annual
FIGURE 2FIGURE 1
NASAs Global Surace Temperature Record Esti-mates o global surace temperature change, relative
to the average global surace temperature or theperiod rom 1951 to 1980, which is about 14C (57F)rom NASA Goddard Institute or Space Studies showa warming trend over the 20th century. The esti-mates are based on surace air temperature measure-ments at meteorological stations and on sea suracetemperature measurements rom ships and satellites.The black curve shows average annual temperatures,and the red curve is a 5-year running average. Thegreen bars indicate the margin o error, which hasbeen reduced over time. Source: National ResearchCouncil 2010a
(bottom let) Climate monitoring stations on land and sea,such as the moored buoys o NOAAs Tropical Atmosphere
Ocean (TAO) project, provide real-time data on tempera-ture, humidity, winds, and other atmospheric properties.Image courtesy o TAO Project Oce, NOAA Pacic MarineEnvironmental Laboratory. (right) Weather balloons, whichcarry instruments known as radiosondes, provide verti-cal proles o some o these same properties throughoutthe lower atmosphere. Image University Corporation orAtmospheric Research. (top let) The NOAA-N spacecrat,launched in 2005, is the teenth in a series o polar-orbiting satellites dating back to 1978. The satellites carryinstruments that measure global surace temperature andother climate variables. Image courtesy NASA
How do we know that Earth has warmed?
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temperature dierence between Washington, D.C.,
and Charleston, South Carolina, which is more
than 450 miles arther south. Consider, too, that
a decrease o only 9F (5C) in global average
temperatures is the estimated dierence between
todays climate and an ice age.In addition to surace temperature, other parts o
the climate system are also being monitored careully
(Figure 2). For example, a variety o instruments are
used to measure temperature, salinity, and currents
beneath the ocean surace. Weather balloons are
used to probe the temperature, humidity, and winds
in the atmosphere. A key breakthrough in the ability
to track global environmental changes began in the
1970s with the dawn o the era o satellite remote
sensing. Many dierent types o sensors, carried
on dozens o satellites, have allowed us to build atruly global picture o changes in the temperature
o the atmosphere and o the ocean and land
suraces. Satellite data are also used to study shits in
precipitation and changes in land cover.
Even though satellites measure temperature very
dierently than instruments on Earths surace, and
any errors would be o a completely dierentnature, the two records agree. A number o other
indicators o global warming have also been
observed (see pp.15-17). For example, heat waves
are becoming more requent, cold snaps are now
shorter and milder, snow and ice cover are
decreasing in the Northern Hemisphere, glaciers
and ice caps around the world are melting, and
many plant and animal species are moving to cooler
latitudes or higher altitudes because it is too warm
to stay where they are. The picture that emerges
rom all o these data sets is clear and consistent:Earth is warming.
How do we know that greenhouse gaseslead to warming?
As early as the 1820s, scientists began to ap-preciate the importance o certain gases inregulating the temperature o the Earth (see Box 1).Greenhouse gaseswhich include carbon dioxide
(CO2), methane, nitrous oxide, and water vapor
act like a blanket in the atmosphere, keep-
ing heat in the lower atmosphere.
Although greenhouse gases
comprise only a tiny raction
o Earths atmosphere, they
are critical or keeping the
planet warm enough to
support lie as we know it
(Figure 3).Heres how the
greenhouse eect
works: as the Suns energy
hits Earth, some o it is
refected back to space, but
most o it is absorbed by the
land and oceans. This absorbed
energy is then radiated upward rom Earths surace
in the orm o heat. In the absence o greenhouse
gases, this heat would simply escape to space,and the planets average surace temperature
would be well below reezing. But greenhouse
gases absorb and redirect some o
this energy downward, keeping
heat near Earths surace. As
concentrations o heat-
trapping greenhouse
gases increase in the
atmosphere, Earths
natural greenhouse
eect is enhanced(like a thicker blanket),
causing surace
temperatures to rise
(Figure 3). Reducing the
levels o greenhouse gases
in the atmosphere would
cause a decrease in surace
temperatures.
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Amplication o the Greenhouse Eect The greenhouse eect is a natural phenomenon that is essential tokeeping the Earths surace warm. Like a greenhouse window, greenhouse gases allow sunlight to enter and thenprevent heat rom leaving the atmosphere. These gases include carbon dioxide (CO2), methane (CH4), nitrousoxide (N2O), and water vapor. Human activitiesespecially burning ossil uelsare increasing the concentrationso many o these gases, ampliying the natural greenhouse eect. Image courtesy o the Marian Koshland Science
Museum o the National Academy o Sciences
FIGURE 3
Box 1
Early Understanding o Greenhouse Gases In 1824, Frenchphysicist Joseph Fourier (top) was the rst to suggest that the Earthsatmosphere might act as an insulator o some kindthe rst proposalo what was later called the greenhouse eect. In the 1850s, Irish-born physicist John Tyndall (middle) was the rst to demonstrate
the greenhouse eect by showing that water vapor and otheratmospheric gases absorbed Earths radiant heat. In 1896, Swedishscientist Svante Arrhenius (bottom) was the rst to calculate thewarming power o excess carbon dioxide (CO2). From his calculations,Arrhenius predicted that i human activities increased CO2 levels in theatmosphere, a warming trend would result.
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Discerning the human infuence on greenhousegas concentrations is challenging because manygreenhouse gases occur naturally in Earths atmo-
sphere. Carbon dioxide (CO2)is produced and con-
sumed in many natural processes that are part o the
carbon cycle (see Figure 4). However, once humans
began digging up long-buried orms o carbon such
as coal and oil and burning them or energy, addi-
tional CO2 began to be released into the atmosphere
much more rapidly than in the natural carbon cycle.
Other human activities, such as cement production
and cutting down and burning o orests (deoresta-
tion), also add CO2 to the atmosphere.
Until the 1950s, many scientists thought the
oceans would absorb most o the excess CO2
released by human activities. Then a series o
FIGURE 4
The Carbon Cycle Carbon is continually exchanged between the atmosphere, ocean, biosphere, and land on avariety o timescales. In the short term, CO2 is exchanged continuously among plants, trees, animals, and the airthrough respiration and photosynthesis, and between the ocean and the atmosphere through gas exchange. Otherparts o the carbon cycle, such as the weathering o rocks and the ormation o ossil uels, are much slower pro-cesses occurring over many centuries. For example, most o the worlds oil reserves were ormed when the remainso plants and animals were buried in sediment at the bottom o shallow seas hundreds o millions o years ago, and
then exposed to heat and pressure over many millions o years. A small amount o this carbon is released naturallyback into the atmosphere each year by volcanoes, completing the long-term carbon cycle. Human activities, espe-cially the digging up and burning o coal, oil, and natural gas or energy, are disrupting the natural carbon cycle byreleasing large amounts o ossil carbon over a relatively short time period. Source: National Research Council
How do we know that humans are causinggreenhouse gas concentrations to increase?
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scientic papers were published that examined the
dynamics o carbon dioxide exchange between
the ocean and atmosphere, including a paper by
oceanographer Roger Revelle and Hans Seuss in
1957 and another by Bert Bolin and Erik Eriksson
in 1959. This work led scientists to the hypothesis
that the oceans could not absorb all o the CO2
being emitted. To test this hypothesis, Revelles
colleague Charles David Keeling began collecting
air samples at the Mauna Loa Observatory in Hawaii
to track changes in CO2 concentrations. Today,such measurements are made at many sites around
the world. The data reveal a steady increase in
atmospheric CO2 (Figure 5).
To determine how CO2 concentrations varied
prior to such modern measurements, scientists have
studied the composition o air bubbles trapped in
ice cores extracted rom Greenland and Antarctica.
These data show that, or at least 2,000 years beore
the Industrial Revolution, atmospheric CO2
concentrations were steady and then began to rise
sharply beginning in the late 1800s (Figure 6).
Today, atmospheric CO2 concentrations exceed 390
parts per millionnearly 40% higher than
preindustrial levels, and, according to ice core data,
higher than at any point in the past 800,000 years
(see Figure 14, p.18).
Human activities have increased the atmospheric
concentrations o other important greenhouse
gases as well. Methane, which is produced bythe burning o ossil uels, the raising o livestock,
the decay o landll wastes, the production and
transport o natural gas, and other activities,
increased sharply through the 1980s beore
starting to level o at about two-and-a-hal
times its preindustrial level (Figure 6). Nitrous
oxide has increased by roughly 15% since 1750
(Figure 6), mainly as a result o agricultural
FIGURE 5 FIGURE 6
Measurements o Atmospheric Carbon DioxideThe Keeling Curve is a set o careul measurementso atmospheric CO2 that Charles David Keelingbegan collecting in 1958. The data show a steadyannual increase in CO2 plus a small up-and-downsawtooth pattern each year that refects seasonalchanges in plant activity (plants take up CO2 duringspring and summer in the Northern Hemisphere,where most o the planets land mass and landecosystems reside, and release it in all and winter).Source: National Research Council, 2010a
Greenhouse Gas Concentrations or 2,000 YearsAnalysis o air bubbles trapped in Antarctic ice coresshow that, along with carbon dioxide, atmosphericconcentrations o methane (CH4) and nitrous oxide(N2O) were relatively constant until they started to risein the Industrial era. Atmospheric concentration unitsindicate the number o molecules o the greenhousegas per million molecules o air or carbon dioxideand nitrous oxide, and per billion molecules o air ormethane. Image courtesy: U.S. Global Climate ResearchProgram
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8
ertilizer use, but also rom ossil uel burning and
certain industrial processes. Certain industrial
chemicals, such as chlorofuorocarbons (CFCs),
act as potent greenhouse gases and are long-lived
in the atmosphere. Because CFCs do not have
natural sources, their increases can be attributed
unambiguously to human activities.
In addition to direct measurements o CO2
concentrations in the atmosphere, scientists haveamassed detailed records o how much coal, oil,
and natural gas is burned each year. They also
estimate how much CO2 is being absorbed, on
average, by the oceans and the land surace. These
analyses show that about 45% o the CO2 emitted
by human activities remains in the atmosphere.
Just as a sink will ll up i water is entering it aster
than it can drain, human production o CO2 is
outstripping Earths natural ability to remove it
rom the air. As a result, atmospheric CO2 levels are
increasing (see Figure 7) and will remain elevated
or many centuries. Furthermore, a orensic-style
analysis o the CO2
in the atmosphere reveals the
chemical ngerprint o carbon rom ossil uels
(see Box 2). Together, these lines o evidence prove
conclusively that the elevated CO2 concentration in
the atmosphere is the result o human activities.
+ =
Emissions AtmosphericConcentration
Growth
Land Sink
Ocean Sink
FIGURE 7
Emissions Exceed Natures CO2 Drain Emissions o CO2 due to ossil uel burning and cement manuacture areincreasing, while the capacity o sinks that take up CO2or example, plants on land and in the oceanare
decreasing. Atmospheric CO2 is increasing as a result. Source: National Research Council, 2011a
Clues rom the fngerprint o carbon dioxide.In a process that takes place overmillions o years, carbon rom the decay o plants and animals is stored deep in the Earths crust inthe orm o coal, oil, and natural gas (see Figure 4). Because this ossil carbon is so old, it containsvery little o the radioisotope carbon-14a orm o the carbon that decays naturally over long timeperiods. When scientists measure carbon-14 levels in the atmosphere, they nd that it is much lowerthan the levels in living ecosystems, indicating that there is an abundance o old carbon. While asmall raction o this old carbon can be attributed to volcanic eruptions, the overwhelming majoritycomes rom the burning o ossil uels. Average CO2 emissions rom volcanoes are about 200 milliontons per year, while humans are emitting an estimated 36 billion tons o CO2 each year, 80-85% owhich are rom ossil uels.
Box 2
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Greenhouse gases are reerred toas orcing agents because otheir ability to change the planets
energy balance. A orcing agent
can push Earths temperature
up or down. Greenhouse gases
dier in their orcing power.
For example, a single methane
molecule has about 25 times
the warming power o a single
CO2 molecule. However, CO2 has amuch larger overall warming eect than
methane because it is much more abundant and
stays in the atmosphere or much longer periods
o time. Scientists can calculate the orcing power
o greenhouse gases based on the changes in their
concentrations over time and on physically based
calculations o how they transer energy through the
atmosphere.
Some orcing agents push Earths energy balance
toward cooling, osetting some o the heating
associated with greenhouse gases. For example,
some aerosolswhich are tiny liquid or solid
particles suspended in the atmosphere, such as those
that make up most o the visible air pollutionhave
a cooling eect because they scatter a
portion o incoming sunlight back into
space (see Box 3). Human activities,
especially the burning o ossil uels,
have increased the number o
aerosol particles in the atmosphere,
especially over and around major
urban and industrial areas.
Changes in land use and land
cover are another way that human
activities are infuencing Earths climate.
Deorestation is responsible or 10% to 20% o
the excess CO2 emitted to the atmosphere each
year, and, as has already been discussed, agriculture
contributes nitrous oxide and methane. Changes in
land use and land cover also modiy the refectivity
o Earths surace; the more refective a surace, the
more sunlight is sent back into space. Cropland is
generally more refective than an undisturbed orest,
while urban areas oten refect less energy than
undisturbed land. Globally, human land use changes
are estimated to have a slight cooling eect.
When all human and natural orcing agents are
considered together, scientists have calculated that
the net climate orcing between 1750 and 2005 is
How much are human activitiesheating Earth?
Warming and Cooling Eects o Aerosols Aerosols are tiny liquid or solid particlessuspended in the atmosphere that come rom a number o human activities, such as ossil uelcombustion, as well as natural processes, such as dust storms, volcanic eruptions, and sea sprayemissions rom the ocean. Most o our visible air pollution is made up o aerosols. Most aerosolshave a cooling eect, because they scatter a portion o incoming sunlight back into space, althoughsome particles, such as dust and soot, actually absorb some solar energy and thus act as warmingagents. Many aerosols also enhance the refection o sunlight back to space by making cloudsbrighter, which results in additional cooling. Many nations, states, and communities have takenaction to reduce the concentrations o certain air pollutants such as the sulate aerosols responsibleor acid rain. Unlike most o the greenhouse gases released by human activities, aerosols onlyremain in the atmosphere or a short timetypically a ew weeks.
Box 3
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pushing Earth toward warming (Figure 8). The extra
energy is about 1.6 Watts per square meter o Earths
surace. When multiplied by the surace area o
Earth, this energy represents more than 800 trillion
Watts (Terawatts)on a per year basis, thats about
50 times the amount o energy people consume
rom all energy sources combined! This extra energy
is being added to Earths climate system every
second o every day.
The total amount o warming that will occur in
response to a climate orcing is determined by a
variety o eedbacks, which either ampliy or dampen
the initial warming. For example, as Earth warms,
polar snow and ice melt, allowing the darker colored
land and oceans to absorb more heatcausing Earth
to become even warmer, which leads to more snow
and ice melt, and so on (see Figure 9). Another impor-
tant eedback involves water vapor. The amount o
water vapor in the atmosphere increases as the ocean
surace and the lower atmosphere warm up; warm-
ing o 1C (1.8F) increases water vapor by about
7%. Because water vapor is also a greenhouse gas,
this increase causes additional warming. Feedbacks
that reinorce the initial climate orcing are reerred
to in the scientic community as positive, or ampli-
ying, eedbacks.
There is an inherent time lag in the warming that
is caused by a given climate orcing. This lag occurs
because it takes time or parts o Earths climate
systemsespecially the massive oceansto warm
or cool. Even i we could hold all human-produced
orcing agents at present-day values, Earth would
continue to warm well beyond the 1.4F already ob-
served because o human emissions to date.
Warming and Cooling Infuences on EarthSince 1750 The warming and cooling infuences(measured in Watts per square meter) o various cli-mate orcing agents during the Industrial Age (romabout 1750) rom human and natural sources has beencalculated. Human orcing agents include increases ingreenhouse gases and aerosols, and changes in landuse. Major volcanic eruptions produce a temporarycooling eect, but the Sun is the only major naturalactor with a long-term eect on climate. The net e-ect o human activities is a strong warming infuence
o more than 1.6 Watts per square meter. Source: Na-tional Research Council, 2010a (Depiction courtesy U.S.Global Climate Research Program)
Energy (Watts/m2)
FIGURE 8
TEMPERATURES RISE
AS REFLECTIVE
ICE DISAPPEARS,
DARKER OCEAN
WATER ABSORBS
MORE HEAT
ARCTIC SEA
ICE MELTS
Climate Feedback Loops The amount o warmingthat occurs because o increased greenhouse gasemissions depends in part on eedback loops.Positive (ampliying) eedback loops increase thenet temperature change rom a given orcing, whilenegative (damping) eedbacks oset some o thetemperature change associated with a climate orcing.The melting o Arctic sea ice is an example o a positiveeedback loop. As the ice melts, less sunlight is refectedback to space and more is absorbed into the darkocean, causing urther warming and urther melting oice. Source: National Research Council, 2011dFIGURE 9
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11
Another way to test a scientic theory is to in-vestigate alternative explanations. Becausethe Suns output has a strong infuence on Earths
temperature, scientists have examined records o
solar activity to determine i changes in solar output
might be responsible or the observed global warm-
ing trend. The most direct measurements o solar
output are satellite readings, which have been avail-
able since 1979. These satellite records show that
the Suns output has not shown a net increase dur-
ing the past 30 years (Figure 10) and thus cannot be
responsible or the warming during that period.
Prior to the satellite era, solar energy output had
to be estimated by more indirect methods, such as
records o the number o sunspots observed each
year, which is an indicator o solar activity. These
indirect methods suggest there was a slight increase
in solar energy reaching Earth during the rst ew
Measures o the Suns EnergySatellite measurements o the Sunsenergy incident on Earth, available since1979, show no net increase in solarorcing during the past 30 years. Theyshow only small periodic variationsassociated with the 11-year solar cycle.Source: National Research Council, 2010a
Warming Patterns in the Layers othe Atmosphere Data rom weatherballoons and satellites show a warmingtrend in the troposphere, the lowerlayer o the atmosphere, which extendsup about 10 miles (lower graph), anda cooling trend in the stratosphere,which is the layer immediately abovethe troposphere (upper graph). Thisis exactly the pattern expected rom
increased greenhouse gases, which trapenergy closer to the Earths surace.Source: National Research Council, 2010a
FIGURE 11
FIGURE 10
How do we know the current warming trend
isnt caused by the Sun?
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12
decades o the 20th century. This increase may have
contributed to global temperature increases during
that period, but does not explain warming in the
latter part o the century.
Further evidence that current warming is nota result o solar changes can be ound in the
temperature trends in the dierent layers o the
atmosphere. These data come rom two sources:
weather balloons, which have been launched
twice daily rom hundreds o sites worldwide
since the late 1950s, and satellites, which have
monitored the temperature o dierent layers o the
atmosphere since the late 1970s. Both o these data
sets have been heavily scrutinized, and both show a
warming trend in the lower layer o the atmosphere
(the troposphere) and a cooling trend in the
upper layer (the stratosphere) (Figure 11). This isexactly the vertical pattern o temperature changes
expected rom increased greenhouse gases,
which trap energy closer to the Earths surace. I
an increase in solar output were responsible or
the recent warming trend, the vertical pattern o
warming would be more uniorm through the
layers o the atmosphere.
How do we know that the current warmingtrend is not caused by natural cycles?
Detecting climate trends iscomplicated by the actthat there are many natural
variations in temperature,
precipitation, and otherclimate variables. These
natural variations are
caused by many dierent
processes that can occur
across a wide range o
timescalesrom a particularly
warm summer or snowy winter
to changes over many millions o
years.
Among the most well-known short-term cli-
matic fuctuations are El Nio and La Nia, which
are periods o natural warming and cooling in the
tropical Pacic Ocean. Strong El Nio and La Nia
events are associated with signicant year-to-year
changes in temperature and rainall patterns across
many parts o the planet, including the United
States. These events have been linked to a number
o extreme weather events, such as the 1992 food-
ing in midwestern states and the severe droughts
in southeastern states in 2006 and 2007. Globally,
temperatures tend to be higher
during El Nio periods, such as
1998, and lower during La
Nia years, such as 2008.
However, these up-and-down fuctuations are
smaller than the 20th cen-
tury warming trend; 2008
was still quite a warm year
in the long-term record.
Natural climate variations can
also be orced by slow changes in
the Earths orbit around the Sun that
aect the solar energy received by Earth, as
is the case with the Ice Age cycle (see pp. 18-19)
or by short-term changes in the amount o volca-
nic aerosols in the atmosphere. Major eruptions,
like that o Mount Pinatubo in 1991, spew huge
amounts o particles into the stratosphere that
cool Earth. However, surace temperatures typically
rebound in 2-5 years as the particles settle out o
the atmosphere. The short-term cooling eects o
several large volcanic eruptions can be seen in the
20th century temperature record, as can the global
temperature variations associated with several
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strong El Nio and La Nia events, but an overall
warming trend is still evident (Figure 12).
In order to put El Nio and La Nia events and
other short-term natural fuctuations into perspec-
tive, climate scientists examine trends over severaldecades or longer when assessing the human infu-
ence on the climate system. Based on a rigorous as-
sessment o available temperature records, climate
orcing estimates, and sources o natural climate
variability, scientists have concluded that there is a
more than 90% chance that most o the observed
global warming trend over the past 50 to 60 years
can be attributed to emissions rom the burning o
ossil uels and other human activities.
Such statements that attribute climate change
to human activities also rely on inormation rom
FIGURE 12
Short-term Temperature Eectso Natural Climate VariationsNatural actors, such as volcaniceruptions and El Nio and La Niaevents, can cause average globaltemperatures to vary rom one yearto the next, but cannot explain thelong-term warming trend over thepast 60 years. Image courtesy o theMarian Koshland Science Museum
FIGURE 13
Model Runs With and WithoutHuman Infuences Modelsimulations o 20th-century suracetemperatures more closely matchobserved temperature when bothnatural and human infuences are
included in the simulations. Theblack line shows an estimate oobserved surace temperatureschanges. The blue line shows resultsrom models that only includenatural orcings (solar activity andvolcanoes). The red-shaded regionsshow results rom models thatinclude both natural and humanorcings. Source: Meehl et al, 2011
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climate models (see Box 4). Scientists have usedthese models to simulate what would have happened
i humans had not modied Earths climate during
the 20th centurythat is, how global tempera-
tures would have evolved i only natural actors
(volcanoes, the Sun, and internal climate variability)
were infuencing the climate system. These undis-
turbed Earth simulations predict that, in the ab-sence o human activities, there would have been
negligible warming, or even a slight cooling, over
the 20th century. When greenhouse gas emissions
and other activities are included in the models, how-
ever, the resulting surace temperature changes more
closely resemble the observed changes (Figure 13).
14
What are climate models? For several decades, scientists have used the worlds most ad-vanced computers to simulate the Earths climate. These models are based on a series o mathemati-cal equations representing the basic laws o physicslaws that govern the behavior o the atmo-sphere, the oceans, the land surace, and other parts o the climate system, as well as the interactionsamong dierent parts o the system. Climate models are important tools or understanding past,present, and uture climate change. Climate models are tested against observations so that scientistscan see i the models correctly simulate what actually happened in the recent or distant past. Imagecourtesy Marian Koshland Science Museum
Box 4
EVAPORATION
CONDENSATION &
CONVECTION
EXCHANGE OF HEAT &
GASES BETWEEN ATMOSPHERE,
SEA ICE & OCEAN
GLACIER MELT
RADIATIVE
EXCHANGE
TERRESTRIAL CARBON
CYCLE
OCEAN CARBON CYCLEOCEAN
CIRCULATION
WATER STORAGE
IN ICE & SNOW
SURFACE
RUN-OFF
AIR POLLUTION
WIND
CLOUDS & WATER VAPOR
EVAPOTRANSPIRATION
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R ising temperatures due to increasing greenhousegas concentrations have produced distinct pat-terns o warming on Earths surace, with stronger
warming over most land areas and in the Arctic.
There have also been signicant seasonal dierences
in observed warming. For example, the second hal
o the 20th century saw intense winter warming
across parts o Canada, Alaska, and northern Europe
and Asia, while summer warming was particularly
strong across the Mediterranean and Middle East
and some other places, including parts o the U.S.
west (Figure 15). Heat waves and record high tem-
peratures have increased across most regions o the
world, while cold snaps and record cold tempera-
tures have decreased.
Global warming is also having a signicant im-
pact on snow and ice, especially in response to the
strong warming across the Arctic. For example,
the average annual extent o Arctic sea ice has
dropped by roughly 10% per decade since satel-
lite monitoring began in 1978 (Figure 16). This
melting has been especially strong in late summer,
leaving large parts o the Arctic Ocean ice-ree or
weeks at a time and raising questions about eects
on ecosystems, commercial shipping routes, oil
and gas exploration, and national deense. Many
o the worlds glaciers and ice sheets are melting in
response to the warming trend, and long-term av-
erage winter snowall and snowpack have declined
in many regions as well, such as the Sierra Nevada
mountain range in the western United States.
Much o the excess heat caused by human-emit-
ted greenhouse gases has warmed the worlds
oceans during the past several decades. Water ex-
pands when it warms, which leads to sea-level rise.
Water rom melting glaciers, ice sheets, and ice caps
also contributes to rising sea levels. Measurements
made with tide gauges and augmented by satellites
show that, since 1870, global average sea level has
risen by about 8 inches (0.2 meters). It is estimated
15
FIGURE 15
Patterns o Warming in Winter and SummerTwenty-year average temperatures or 1986-2005 comparedto 1955-1974 show a distinct pattern o winter and summer warming. Winter warming has been intense acrossparts o Canada, Alaska, northern Europe, and Asia, and summers have warmed across the Mediterranean andMiddle East and some other places, including parts o the U.S. west. Projections or the 21st century show asimilar pattern. Source: National Research Council, 2011a
What other climate changes and impactshave already been observed?
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that roughly one-third o the total sea-level rise over
the past our decades can be attributed to ocean
expansion, with most o the remainder due to ice
melt (Figure 17).
Because CO2 reacts in seawater to orm carbonic
acid, the acidication o the worlds oceans is an-
other certain outcome o elevated CO2 concentra-
tions in the atmosphere (Figure 18). It is estimated
that the oceans have absorbed between one-quarterand one-third o the excess CO2 rom human activi-
ties, becoming nearly 30% more acidic than during
preindustrial times. Geologically speaking, this large
change has happened over a very short timerame,
and mounting evidence indicates it has the poten-
tial to radically alter marine ecosystems, as well as
the health o coral rees, shellsh, and sheries.
Another example o a climate change observed
during the past several decades has been changes in
the requency and distribution o precipitation. Total
precipitation in the United States has increased by
about 5% over the past 50 years, but this has not
been geographically uniormconditions are gener-
ally wetter in the Northeast, drier in the Southeast,
and much drier in the Southwest.
Warmer air holds more water vapor, which has led
to a measurable increase in the intensity o precipita-
FIGURE 16
Loss o Arctic Sea Ice Satellite-based measurementsshow a steady decline in the amount o September(end o summer) Arctic sea ice extent rom 1979 to2009 (expressed as a percentage dierence rom 1979-2000 average sea ice extent, which was 7.0 millionsquare miles). The data show substantial year-to-year
variability, but a long-term decline in sea ice o morethan 10% per decade is clearly evident, highlighted bythe dashed line. Source: National Research Council, 2010a
FIGURE 17
Contributors to Sea-Level RiseSea level has risen steadily over the past ew decadesdue to various contributors: thermal expansion in theupper 700 meters o ocean (red) and deeper oceanlayers (orange), meltwater rom Antarctic and Green-land ice sheets (blue), meltwater rom glaciers andice caps (purple), and water storage on land (green).Source: National Research Council, 2011a
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tion events. In the United States, or example, the
raction o total precipitation alling in the heaviest
1% o rainstorm increased by about 20% over the
past century, with the northeastern states experienc-
ing an increase o 54%. This change has increased
the risk o fooding and puts additional stress on
sewer and stormwater management systems.
As the climate has changed, many species have
shited their range toward the poles and to higher
altitudes as they try to stay in areas with the sameambient temperatures. The timing o dierent
seasonal activities is also changing. Several plant
species are blooming earlier in Spring, and some
birds, mammals, sh, and insects are migrating
earlier, while other species are altering their seasonal
breeding patterns. Global analyses show these
behaviors occurred an average o 5 days earlier
per decade rom 1970 to 2000. Such changes can
disrupt eeding patterns, pollination, and other vital
interactions between species, and they also aect
the timing and severity o insects, disease outbreaks,
and other disturbances. In the western United
States, climate change has increased the population
o orest pests such as the pine beetle.The next section describes how observed climate
trends and impacts are predicted to continue i
emissions o human-produced greenhouse gases are
maintained during the next century and beyond.
FIGURE 18
Evidence o Ocean Acidication Withexcess CO2 building up in the atmo-sphere, scientists wanted to know i itwas also accumulating in the ocean.
Studies that began in the mid-1980sshow that the concentration o CO2 inocean water (in blue, calculated romthe partial pressure o CO2 in seawater)has risen in parallel with the increasein atmospheric CO2 (in red, part o theKeeling curve). At the same time, theocean has become more acidic, becausethe CO2 reacts with seawater to ormcarbonic acid. The orange dots are directmeasurements o pH in surace seawater(lower pH being more acidic), and thegreen dots are calculated based on thechemical properties o seawater. Source:National Research Council, 2010d
Climate change has increased the population o orest pests in the western United States. The red trees in thisphoto o Dillon Reservoir in Colorado have died rom an inestation o mountain pine beetle.
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Perhaps the most dramatic example o natural climate variability over long timeperiods is the Ice Age cycle. Detailed analyses o ocean sediments, ice cores, and
other data show that or at least 800,000 years, and probably or the past 4 to 5
million years, the Earth has gone through extended periods when temperatures
were much lower than today and thick blankets o ice covered large areas o the
Northern Hemisphere. These long cold spells, which typically lasted or around
100,000 years, were interrupted by shorter warm interglacial periods, including the
past 10,000 years (Figure 14).
Through a convergence o theory, observations, and modeling, scientists have
deduced that the ice ages are caused by slight recurring variations in Earths
orbit that alter the amount and seasonal distribution o solar energy reaching theNorthern Hemisphere. These relatively small changes in solar energy are reinorced
over thousands o years by gradual changes in Earths ice cover (the cryosphere)
and ecosystems (the biosphere), eventually leading to large changes in global
the iCe ages
800,000 Years o Temperatureand Carbon Dioxide RecordsAs ice core records rom Vostok,Antarctica, show, the tempera-ture near the South Pole hasvaried by as much as 20F (11C)
during the past 800,000 years.The cyclical pattern o tempera-ture variations constitutes the iceage/interglacial cycles. Duringthese cycles, changes in carbondioxide concentrations (in red)track closely with changes intemperature (in blue), with CO2lagging behind temperaturechanges. Because it takes a whileor snow to compress into ice, icecore data are not yet availablemuch beyond the 18th century atmost locations. However, atmo-spheric carbon dioxide levels, as
measured in air, are higher todaythan at any time during the past800,000 years. Source: NationalResearch Council, 2010a
FIGURE 14
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temperature. The average globaltemperature change during an ice age
cycle, which occur over about 100,000
years, is on the order o 9F 2F (5C
1C).
The data show that in past ice age
cycles, changes in temperature have
ledthat is, started prior tochanges
in CO2. This is because the changes
in temperature induced by changes
in Earths orbit around the Sun leadto gradual changes in the biosphere
and the carbon cycle, and thus CO2,
reinorcing the initial temperature trend.
In contrast, the relatively rapid release o
CO2 and other greenhouse gases since
the start o the Industrial Revolution
rom the burning o ossil uel has,
in essence, reversed the pattern: the
additional CO2 is acting as a climate
orcing, with temperatures increasing
aterward.
The ice age cycles nicely illustrate
how climate orcing and eedback
eects can alter Earths temperature, but
there is also direct evidence rom past
climates that large releases o carbon
dioxide have caused global warming.
One o the largest known events o this type is called the Paleocene-Eocene
Thermal Maximum, or PETM, which occurred about 55 million years ago,
when Earths climate was much warmer than today. Chemical indicators
point to a huge release o carbon dioxide that warmed Earth by another 9F
and caused widespread ocean acidifcation. These climatic changes were
accompanied by massive ecosystem changes, such as the emergence o
many new types o mammals on land and the extinction o many bottom-
dwelling species in the oceans.
The U.S. Geological Survey National Ice Core Labstores ice cores samples taken rom polar ice capsand mountain glaciers. Ice cores provide cluesabout changes in Earths climate and atmospheregoing back hundreds o thousands o years.
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Fortunately, scientists have made great strides in
predicting the amount o temperature change that
can be expected or dierent amounts o uture
greenhouse gas emissions and in understanding
how increments o globally averaged
temperaturesincreases o 1C, 2C, 3C and
so orthrelate to a wide range o impacts.
Many o these projected impacts pose serious
risks to human societies and things people care
about, including water resources, coastlines,
inrastructure, human health, ood security, and
land and ocean ecosystems.
Warming, Climate Changesand impacts
in the 21st Century and BeyondPart II
In order to respondeectively to the risksposed by uture climate
change, decision makers
need inormation on the
types and severity o
impacts that might
be expected.
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The biggest actor in determining uture globalwarming is projecting uture emissions o CO2and other greenhouse gaseswhich in turn depend
on how people will produce and use energy, what
national and international policies might be imple-
mented to control emissions, and what new tech-
nologies might become available. Scientists try to
account or these uncertainties by developing dier-
ent scenarios o how uture emissionsand hence
climate orcingwill evolve. Each o these scenarios
is based on estimates o how dierent socioeco-
nomic, technological, and policy actors will change
over time, including population growth, economic
activity, energy-conservation practices, energy tech-
nologies, and land use.
Scientists use climate models (see Box 4, p.14)
to project how the climate system will respond to
dierent scenarios o uture greenhouse gas concen-trations. Typically, many dierent models are used,
each developed by a dierent modeling team.
Each model uses a slightly dierent set o mathe-
matical equations to represent how the atmosphere,
oceans, and other parts o the climate system inter-
act with each other and evolve over time. Models
are routinely compared with one another and tested
against observations to evaluate the accuracy and
robustness o model predictions.
The most comprehensive suite o modeling ex-
periments to project global climate changes was
completed in 2005.1 It included 23 dierent models
rom groups around the world, each o which used
the same set o greenhouse gas emissions scenarios.
Figure 19 shows projected global temperature
changes associated with high, medium-high, and
low uture emissions (and also the committed
FIGURE 19
Projected temperature change or threeemissions scenarios Models project globalmean temperature change during the 21stcentury or dierent scenarios o utureemissionshigh (red), medium-high (green)and low (blue) each o which is based ondierent assumptions o uture populationgrowth, economic development, lie-stylechoices, technological change, and avail-
ability o energy alternatives. Also shown arethe results rom constant concentrationscommitment runs, which assume that atmo-spheric concentrations o greenhouse gasesremain constant ater the year 2000. Eachsolid line represents the average o modelruns rom dierent modeling using the samescenario, and the shaded areas provide ameasure o the spread (one standard devia-tion) between the temperature changesprojected by the dierent models. Source:National Research Council, 2010a
1The modeling experiments were part o the World Cli-mate Research Programmes Coupled Model Intercom-
parison Project phase 3 (CMIP3) in support o the Inter-governmental Panel on Climate Change (IPPC) FourthAssessment Report.
How do scientists projectfuture climate change?
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22
warmingwarming that will occur as a result o
greenhouse gases that have already been emitted).
Continued warming is projected or all three uture
emission scenarios, but sharp dierences in global
average temperature are clearly evident by the endo the century, with a total temperature increase in
2100, relative to the late 20th century, ranging rom
less than 2F (1.1C) or the low emissions scenario
to more than 11F (6.1C) or the high emissions
scenario. These results show that human decisions
can have a very large infuence on the magnitude outure climate change.
How will temperatures be affected?
L ocal temperatures vary widely rom day to day,week to week, and season to season, but howwill they be aected on average? Climate modelers
have begun to assess how much o a rise in averagetemperature might be expected in dierent regions
(Figure 20). The local warmings at each point
on the map are rst divided by the correspond-
ing amount o global average warming and then
scaled to show what pattern o warming would be
expected. Warming is greatest in the high latitudes
o the Northern Hemisphere and is signicantlylarger over land than over ocean.
As average temperatures continue to rise, the
number o days with a heat index above 100F
FIGURE 20
Projected warming or three emissions scenarios Models project the geographical pattern o annual averagesurace air temperature changes at three uture time periods (relative to the average temperatures or the period19611990) or three dierent scenarios o emissions. The projected warming by the end o the 21st century is lessextreme in the B1 scenario, which assumes smaller greenhouse gas emissions, than in either the A1B scenario orthe A2 business as usual scenario. Source: National Research Council 2010a
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23
(the heat index combines temperature and humid-
ity to determine how hot it eels) is projected to
increase throughout this century (Figure 21). By
the end o the century, the center o the United
States is expected to experience 60 to 90 ad-
ditional days per year in which the heat index is
more than 100F. Heat waves also are expected
to last longer as the average global temperature
increases. It ollows that as global temperatures
rise, the risk o heat-related illness and deaths also
should rise. Similarly, there is considerable con-
dence that cold extremes will decrease, as will
cold-related deaths. The ratio o record high tem-
peratures to record low temperatures, currently
2 to 1, is projected to increase to 20 to 1 by mid-
century and 50 to 1 by the end o the century or
a mid-range emissions scenario. FIGURE 21
Projections o Hotter Days Model projections sug-gest that, relative to the 1960s and 1970s, the numbero days with a heat index above 100F will increasemarkedly across the United States. Image courtesy U.S.Global Climate Research Program
How is precipitation expected to change?Global warming is ex-pected to intensiyregional contrasts in precipi-
tation that already exist: dry
areas are expected to get
even drier, and wet areas
even wetter. This is because
warmer temperatures tend
to increase evaporation romoceans, lakes, plants, and
soil, which, according to both theory and observa-
tions, will boost the amount o water vapor in the
atmosphere by about 7% per 1C (1.8F) o warm-
ing. Although enhanced evaporation provides more
atmospheric moisture or rain and snow in some
downwind areas, it also dries out the land surace,
which exacerbates the impacts o drought in some
regions.
Using the same general
approach as or tempera-
tures, scientists can project
regional and seasonal per-
centage change in precipita-
tion expected or each 1C
(1.8F) o global warming
(Figure 22). The results show
that the subtropics, where
most o the worlds deserts
are concentrated, are likely to see 5-10% reductions
in precipitation or each degree o global warming.
In contrast, subpolar and polar regions are expected
to see increased precipitation, especially during win-
ter. The overall pattern o change in the continental
United States is somewhat complicated, as it lies
between the drying subtropics o Mexico and the
Caribbean and the moistening subpolar regions o
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25
expected to increase annually by two to our times
per degree o warming (Figure 24). At the same time,
areas dominated by shrubs and grasses, such as parts
o the Southwest, may experience a reduction in re
over time as warmer temperatures cause shrubs and
grasses to die out. In this case, the potential societal
benets o ewer res would be countered by the losso existing ecosystems.
FIGURE 24
Increased Risk o Fire Rising temperatures and in-creased evaporation are expected to increase the risk ore in many regions o the West. This gure shows thepercent increase in burned areas in the West or a 1Cincrease in global average temperatures relative to themedian area burned during 1950-2003. For example,re damage in the northern Rocky Mountain orests,marked by region B, is expected to more than doubleannually or each 1C (1.8F) increase in global averagetemperatures. Source: National Research Council, 2011a
% change in runoff per degree warming(relative to 1971-2001)
FIGURE 23
Changes in Runo per Degree Warming Enhanced evaporation caused by warming is projected to decrease the
amount o runothe water fowing into rivers and creeks in many parts o the United States. Runo is a keyindex o the availability o resh water. The gure shows the percent median change in runo per degree o globalwarming relative to the period rom 1971 to 2000. Red areas show where runo is expected to decrease, greenwhere it will increase. Source: National Research Council, 2011a
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26
A
s global warming continues, the
planets many orms o ice aredecreasing in extent, thickness,
and duration. Models indicate
that seasonally ice-ree condi-
tions in the Arctic Ocean are
likely to occur beore the end
o this century and suggest
about a 25% loss in Septem-
ber sea-ice extent or each
1C (1.8F) in global warming.
In contrast to the Arctic, sea
ice surrounding Antarctica has, onaverage, expanded during the past
several decades. This increase may be linked
to the stratospheric ozone hole over the Antarc-
tic, which developed because o the use o ozone-
depleting chemicals in rerigerants and spray cans.
The ozone hole allows more damaging UV light to
get to the lower atmosphere and, in the Antarctic,
may have also resulted in lower temperatures as
more heat escapes to space. However, this eect is
expected to wane as ozone returns to normal levels
by later this century, due in part to the success othe Montreal Protocol, an international treaty that
banned the use o ozone-depleting
chemicals. Still, Antarctic sea icemay decrease less rapidly than
Arctic ice, in part because the
Southern Ocean stores heat
at greater depths than the
Arctic Ocean, where the
heat cant melt ice as easily.
In many areas o the
globe, snow cover is expect-
ed to diminish, with snowpack
building later in the cold season
and melting earlier in the spring.According to one sensitivity analysis,
each 1C (1.8F) o local warming may lead
to an average 20% reduction in local snowpack in
the western United States. Snowpack has impor-
tant implications or drinking water supply and
hydropower production. In places such as Siberia,
parts o Greenland, and Antarctica, where tem-
peratures are low enough to support snow over
long periods, the amount o snowall may increase
even as the season shortens, because the increased
amount o water vapor associated with warmertemperatures may enhance snowall.
How will sea ice and snow be affected?
How will coastlines be affected?
Some o Earths most densely populated regionslie at low elevation, making rising sea levela cause or concern. Sea-level rise is projected
to continue or centuries in response to human-
caused increases in greenhouse gases, with an
estimated 0.5-1.0 meter (20-39 inches) o mean
sea-level rise by 2100. However, there is evidence
that sea-level rise could be greater than expected
due to melting o sea ice. Recent studies have
shown more rapid than expected melting rom
glaciers and ice sheets. Observed sea-level rise has
been near the top o the range o projections that
were made in 1990 (Figure 25).
Quantiying the uture threat posed to particular
coastlines by rising seas and foods is challeng-
ing. Many nonclimatic actors are involved, such
as where people choose to build homes, and the
risks will vary greatly rom one location to the next.
Moreover, inrastructure damage is oten triggered
by extreme events, or example hurricanes and
earthquakes, rather than gradual change. However,
there are some clear hot spots, particularly in
large urban areas on coastal deltas, including those
o the Mississippi, Nile, Ganges, and Mekong rivers.
I average sea level rises by 0.5 meters (20 inches)
relative to a 1990 baseline, coastal fooding could
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27
aect 5 million to 200 million people worldwide. Up
to 4 million people could be permanently displaced,
and erosion could claim more than 250,000 square
kilometers o wetland and dryland (98,000 square
miles, an area the size o Oregon). Relocations are
already occurring in towns along the coast o Alaska,
where reductions in sea ice and melting permarost
allow waves to batter and erode the shoreline.
Coastal erosion eects at 1.0 meter o sea-level
rise would be much greater, threatening many
parts o the U.S. coastline (Figure 26).
FIGURE 25
Comparison o Projected and Observed Sea-LevelRise Observed sea-level change since 1990 has beennear the top o the range projected by the Intergov-ernmental Panel on Climate Change Third Assessment
Report, published in 1990 (gray-shaded area). The redline shows data derived rom tide gauges rom 1970 to2003. The blue line shows satellite observations o sea-level change. Source: National Research Council, 2011a
FIGURE 26
Projected Eects o Sea-LevelRise on the U.S. East and GulCoasts I sea level were to rise asmuch as 1 meter (3.3-eet), theareas in pink would be susceptibleto coastal fooding. With a 6-me-ter (19.8-oot) rise in sea level,areas shown in red would also besusceptible. The pie charts showthe percentage area o some citiesthat are potentially susceptible at1-meter and 6-meter sea-level rise.Source: National Research Council,2010a
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Whether marine or terrestrial, all organisms
attempt to acclimate to a changing environ-ment or else move to a more avorable location
but climate change threatens to push some species
beyond their ability to adapt or move. Special
stress is being placed on cold-adapted species on
mountain tops and at high latitudes. Shits in the
timing o the seasons and lie-cycle events such as
blooming, breeding, and hatching are causing mis-
matches between species that disrupt patterns o
eeding, pollination, and other key aspects o ood
webs. The ability o species to move and adapt also
are hampered by human inrastructural barriers(e.g., roads), land use, and competition or interac-
tion with other species.
In the ocean, circulation changes will be a key
driver o ecosystem impacts. Satellite data show that
warm surace waters are mixing less with cooler,
deeper waters, separating near-surace marine lie
rom the nutrients below and ultimately reducing
the amount o phytoplankton, which orms the
base o the ocean ood web (Figure 27). Climate
change will exacerbate this problem in the tropics
and subtropics. However, in temperate and polar
waters, vertical mixing o waters could increase,
especially with expected losses in sea ice. At the
same time, ocean warming will continue to push the
ranges o many marine species toward the poles.
Changing ocean chemistry can result in other
impactswarmer waters could lead to a decline
in subsurace oxygen, boosting the risk o dead
zones, where species high on the ood chain are
largely absent because o a lack o oxygen. Ocean
acidication, brought on as the oceans take in more
o the excess CO2 will threaten many species over
time, especially mollusks and coral rees. But not all
lie orms will suer: some types o phytoplankton
and other photosynthetic organisms may benet
rom increases in CO2. Ocean acidication will
continue to worsen i CO2 emissions continue un-
abated in the decades ahead.
How will ecosystems be affected?
FIGURE 27
Eects on the Ocean Food Web The growth rateo marine phytoplankton, which orm the base othe ocean ood web, is likely to be reduced over timebecause o higher ocean surace temperatures. This
creates a greater distance between warmer suracewaters and cooler deep waters, separating upper ma-rine lie rom nutrients ound in deep water. The gureshows changes in phytoplankton growth (verticallyintegrated annual mean primary production, or PP),expressed as the percentage dierence between 2090-2099 and 1860-1869) per 1C (1.8F) o global warm-ing. Source: National Research Council, 2011a
The American pika
is a cold-adaptedspecies that isbeing isolatedon mountaintopislands by risingtemperatures.Image courtesyo J. R. Douglass,YellowstoneNational Park.
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The stress o climate changeon arming may threatenglobal ood security. Although
an increase in the amount o
CO2 in the atmosphere avors
the growth o many plants, it
does not necessarily translate into
more ood. Crops tend to grow
more quickly in higher temperatures,
leading to shorter growing periods
and less time to produce grains. In addition,a changing climate will bring other hazards,
including greater water stress and the risk o higher
temperature peaks that can quickly damage crops.
Agricultural impacts will vary across regions and by
crop. Moderate warming and associated increases
in CO2 and changes in precipitation are expected
to benet crop and pasture lands in middle to high
latitudes but decrease yield in seasonally dry and
low-latitude areas. In Caliornia, where hal the
nations ruit and vegetable crops are grown, climate
change is projected to decrease yields o almonds,
walnuts, avocados, and table grapes by up to 40
percent by 2050. Regional assessments or other
parts o the world consistently conclude that climate
change presents serious risk to critical staple crops
in sub-Saharan Arican and in places that rely on
water resources rom glacial melt and
snowpack.
Modeling indicates that the
CO2-related benets or some
crops will largely be outweighed
by negative actors i global tem-
perature rises more than 1.0C
(1.8F) rom late 20th-century values
(Figure 28), with the ollowing project-
ed impacts:
For each degree of warming, yields of corn in
the United States and Africa, and wheat in
India, drop by 5-15%
Crop pests, weeds, and disease shift in
geographic range and frequency
If 5C (9F ) of global warming were to be
reached, most regions of the world would
experience yield losses, and global grain prices
would potentially double
Growers in prosperous areas may be able to adapt
to these threats, or example by varying the crops
which they grow and the times at which they are
grown. However, adaptation may be less eective
where local warming exceeds 2C (3.6F) and will be
limited in the tropics, where the growing season is
restricted by moisture rather than temperature.
Loss o Crop Yields per DegreeWarming Yields o corn in the UnitedStates and Arica, and wheat in India,are projected to drop by 5-15% per de-gree o global warming. This gure alsoshows projected changes in yield perdegree o warming or U.S. soybeansand Asian rice. The expected impacts oncrop yield are rom both warming andCO2 increases, assuming no crop adap-tation. Shaded regions show the likelyranges (67%) o projections. Values oglobal temperature change are relativeto the preindustrial value; current globaltemperatures are roughly 0.7C (1.3F)above that value. Source: National Re-search Council, 2011aFIGURE 28
How will agriculture and
food production be affected?
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30
As a result, decision makers o all typesincluding
individuals, businesses, and governments at all levels
are taking or planning actions to respond to climate
change. Depending on how much emissions are
curtailed, the uture could bring a relatively mild
change in climate or it could deliver extreme
changes that could last thousands o years. Thenations scientifc enterprise can contribute both by
continuing to improve understanding o the causes
and consequences o climate change and by improving
and expanding the options available to limit the magnitude o
climate change and to adapt to its impacts.
MakingclecHOicEsPart III
Astrong body o
evidence shows that
climate change is occurring,
is caused largely by human
activities, and poses signif-
cant risks or a broad range
o human and naturalsystems.
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31
A s discussed in Part II o this booklet, improve-ments in the ability to predict climate changeimpacts per degree o warming has made it easier
to evaluate the risks o climate change. Policymak-
ers are let to address two undamental questions:
(1) at what level o warming are risks acceptable
given the cost o limiting them; and (2) what level
o emissions will keep Earth within that level o
warming? Science cannot answer the rst question,
because it involves many value judgments outside
the realm o science. However, much progress has
been made in answering the second.
Even with expected improvements in energy e-
ciency, i the world continues with business as
usual in the way it uses and produce energy, CO2
emissions will continue to accumulate in the atmo-
sphere and warm Earth.2 As illustrated in Figure 29,
to keep atmospheric concentrations o CO2 roughly
steady or a ew decades at any given level to avoid
increasing climate change impacts, global emissions
would have to be reduced by 80%.
Another helpul concept is that the amount o
warming expected to occur rom CO2 emissionsdepends on the cumulative amount o carbon emis-
sions, not on how quickly or slowly the carbon is
added to the atmosphere (Figure 30). Humans have
emitted about 500 billion tons (gigatonnes) o car-
bon to date. Best estimates indicate that adding
about 1,150 billion tons o carbon to the air would
lead to a global mean warming o 2C (3.6F).
How does science informemissions choices?
FIGURE 29
FIGURE 30
Illustrative Example: How Emissions Relate to CO2
Concentrations. Sharp reductions in emissions areneeded to stop the rise in atmospheric concentrationso CO2 and meet any chosen stabilization target. Thegraphs show how changes in carbon emissions (toppanel) are related to changes in atmospheric concen-trations (bottom panel). It would take an 80% reduc-tion in emissions (green line, top panel) to stabilizeatmospheric concentrations (green line, bottom panel)or any chosen stabilization target. Stabilizing emis-sions (blue line, top panel) would result in a continuedrise in atmospheric concentrations (blue line, bottompanel), but not as steep as a rise i emissions continueto increase (red lines). Source: National Research Council,2011a
2Other greenhouse gases are a actor, but CO2 is by arthe most important greenhouse gas in terms o long-termclimate change eects.
Cumulative Emissions and Increases in Global MeanTemperature Recent studies show that or a particularchoice o climate stabilization temperature, there wouldbe only a certain range o allowable cumulative carbonemissions. Humans have emitted a total o about 500 bil-lion tons (gigatonnes) o carbon emissions to date. Theerror bars account or estimated uncertainties in boththe carbon cycle (how ast CO2 will be taken up by theoceans) and in the climate responses to CO2 emissions.Source: National Research Council, 2011a
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32
Adding CO2 more quickly would bring tempera-
tures to that value more quickly, but the value itsel
would change very little.
Because cumulative emissions are what matters,
policies oriented toward the very long term (several
decades into the uture) might be able to ocus less
on speciying exactly when reductions must take
place and more on how much total emissions are al-
lowed over a long periodin eect, a carbon budget.
Such a budget would speciy the amount o total
greenhouse gas that can be emitted during a speci-
ed period o time (say, between now and 2050).
Meeting any specic emissions budget is more
likely the earlier and more aggressively work is done
to reduce emissions (Figure 31). Its like going on a
diet. I a person wants to lose 40 pounds by a cer-
tain event in the uture, it would be much easier to
reach that goal i he or she begins eating less and
exercising more as soon as possible, rather than
waiting to start until the month beore the event.
Meeting an Emissions Budget Meeting any emissionsbudget will be easier the sooner and more aggressivelyactions are taken to reduce emissions. Source: NationalResearch Council
FIGURE 31
As discussed earlier, to limit climatechange in the long term, the most
important greenhouse gas to control
is carbon dioxide, which in the
United States is emitted primarily
as a result o burning ossil uels.
Figure 32 shows the relative
amount o emissions rom resi-
dential, commercial, industrial,
and transportation sources. Its
not really a matter o doing with-
out, but being smarter about how weproduce and use energy.
The United States is responsible or about
hal o the human-produced CO2 emissions already
in the atmosphere and currently accounts or
roughly 20% o global CO2 emissions, despite hav-
ing only 5% o the worlds population. The U.S.
percentage o total global emissions is projected to
decline over the coming decades as emissions
rom rapidly developing nations such as China and
India will continue to grow. Thus,reductions in U.S. emissions
alone will not be adequate to
avert climate change risks.
However, strong U.S. lead-
ershipdemonstrated
through strong domestic
actions, may help infu-
ence other countries to
pursue serious emission
reduction eorts as well.
Several key opportunities toreduce how much carbon dioxide
accumulates in the atmosphere are
available (Figure 33), including:
Reduce underlying demand or goods and ser-
vices that require energy, or example, expand
education and incentive programs to infuence
consumer behavior and preerences; curtail sprawl-
ing development patterns that urther our depen-
dence on petroleum.
What are the choices for reducinggreenhouse gas emissions?
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33
Improve the eciency with which energy is
used, or example, use more ecient methods or
insulating, heating, cooling, and lighting buildings;
upgrade industrial equipment and processes to be
more energy ecient; and encourage the purchase
o ecient home appliances and vehicles.
Expand the use o low- and zero-carbon energy
sources, or example, switch rom coal and oil to
natural gas, expand the use o nuclear power and
renewable energy sources such as solar, wind, geo-
thermal, hydropower, and biomass; capture and
sequester CO2 rom power plants and actories.Capture and sequester CO2 directly rom the
atmosphere, or example, manage orests and soils
to enhance carbon uptake; develop mechanical
methods to scrub CO2 directly rom ambient air.
Advancing these opportunities to reduce emis-
sions will depend to a large degree on private sector
investments and on the behavioral and consumer
choices o individual households. Governments at
ederal, state, and local levels have a large role to
play in infuencing these key stakeholders through
eective policies and incentives. In general, there are
our major tool chests rom which to select policiesor driving emission reductions:
Pricing o emissions such as by means o a car-
bon tax or cap-and-trade system;
mandates or regulations that could include
direct controls on emitters (or example,
through the Clean Air Act) or mandates such as
automobile uel economy standards, appliance
eciency standards, labeling requirements,
building codes, and renewable or low-carbon
portolio standards or electricity generation;
public subsidies or emission-reducing choices
through the tax code, appropriations, or loan
guarantees; and
providing inormation and education and pro-
moting voluntary measures to reduce emissions.
A comprehensive national program would likely
use tools rom all o these areas. Most economists
and policy analysts have concluded, however, that
putting a price on CO2 emissions that is suciently
high and rises over time is the least costly path to
signicantly reduce emissions; and it is the most e-cient incentive or innovation and the long-term
investments necessary to develop and deploy energy
ecient and low-carbon technologies and inrastruc-
ture. Complementary policies may also be needed,
however, to ensure rapid progress in key areas.
U.S. greenhouse gas emissions in 2009 show the rela-tive contribution rom our end-uses: residential, com-mercial (e.g., retail stores, oce buildings), industrial,and transportation. Electricity consumption accountsor the majority o energy use in the residential andcommercial sectors. Image courtesy: U.S. EnvironmentalProtection Agency
FIGURE 32
FIGURE 33
Key Opportunities or ReducingEmissions A chain o actors deter-
mine how much CO2 accumulates inthe atmosphere. Better outcomes(gold ellipses) could result i thenation ocuses on several opportunitieswithin each o the blue boxes. Source:National Research Council, 2010b
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34
Opportunities to Reduce Other Human-
Produced Warming Agents
There are opportunities to reduce emissions o non-
CO2 gases, such as methane, nitrous oxide, and
some industrial gases (e.g., hydrofuorocarbons),
which comprise at least 15% o U.S. greenhouse gas
emissions. Molecule or molecule, these gases are
generally much stronger climate orcing agents than
CO2, although carbon dioxide is the most important
contributor to climate change over the long-term
because o its abundance and long lietime.
Some non-CO2 greenhouse gases can be re-
duced at negative or modest incremental costs. For
example, reducing methane leaks rom oil and gas
systems, coal mining, and landlls is cost-eective
because there is a market or the recovered gas.
Reducing methane also improves air quality.
The largest overall source o non-CO2 green-
house emissions is rom agriculture, in particular,
methane produced when livestock digest their
ood, and also nitrous oxide and methane rom
manure and nitrogen ertilizer. These emissions can
be reduced in many ways, including by employing
precision agriculture techniques that help armers
minimize the over-ertilization practices that lead to
emissions, and by improving livestock waste man-agement systems.
Some shortlived pollutants that are not green-
house gases also cause warming. One example is
black carbon, or soot, emitted rom the burning
o ossil uels, biouels, and biomass (or example,
the dung used in cookstoves in many developing
countries). Black carbon can cause strong local or
regional-scale atmospheric warming where it is
emitted. It can also ampliy warming in some re-
gions by leaving a heat-absorbing black coating on
otherwise refective suraces such as arctic ice andsnow. Reducing emissions o these short-lived warm-
ing agents could help ease climate change in the
near term.
What are the choices for preparing for theimpacts of climate change?
A lthough adaptation planning andresponse eorts are under wayin a number o states, counties, and
communities, much o the nations
experience is in protecting its peo-
ple, resources, and inrastructure
are based on the historic record o
climate variability during a time o
relatively stable climate. Adaptation to
climate change calls or a dierent para-digmone that considers a range o possible
uture climate conditions and associated impacts,
some well outside the realm o past experience.
Adaptation eorts are hampered by a lack o solid
inormation about benets, costs, and the potential
and limits o dierent responses. This is due in part
to the diversity o impacts and vulnerabilities across
the United States and the relatively small body o re-
search that ocuses on climate change
adaptation actions. In the short term,
adaptation actions most easily de-
ployed include low-cost strategies
that oer near-term co-benets, or
actions that reverse maladaptive
policies and practices. In the longer
term, more dramatic, higher cost
responses may be required. Table 1
provides a ew examples o short-term ac-tions that might be considered to address some
o the expected impacts o sea-level rise.
Even though there are still uncertainties regarding
the exact nature and magnitude o climate change
impacts, mobilizing now to increase the nations
adaptive capacity can be viewed as an insurance pol-
icy against climate change risks. The ederal govern-
ment could play a signicant role as a catalyst and
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35
coordinator o local and regional eorts by providing
technical and scientic resources, incentives to begin
adaptation planning, guidance across jurisdictions,
a platorm to share lessons learned, and support o
scientic research to expand knowledge o impacts
and adaptation. In addition to the direct impacts o
climate change, the United States can be indirectly
aected by the impacts o climate change occurring
elsewhere in the world. Thus, it is in the countrys
interest to help enhance the adaptive capacity o
other nations, particularly developing countries that
lack resources and expertise.
TABLE 1. Examples o some adaptation options or one expected outcome o sea-level rise.
Why take action if there are still uncertaintiesabout the risks of climate change?
Further research will never completely eliminateuncertainties about climate change and its risks,given the inherent complexities o the climate sys-
tem and the many behavioral, economic, and tech-
nological actors that are dicult to predict into the
uture. However, uncertainty is not a reason or inac-
tion, and there are many things we already know
about climate change that we can act on. Reasons
or taking action include the ollowing:
The sooner that serious eorts to reduce green-
house gas emissions proceed, the lower the risks
posed by climate change and the less pressure
there will be to make larger, more rapid, and
potentially more expensive reductions later.
Some climate change impacts, once maniest-
ed, will persist or hundreds or even thousands
o years and will be dicult or impossible to
undo. In contrast, many actions taken to re-
spond to climate change could be reversed or
scaled back i they somehow prove to be more
stringent than actually needed.
Each day around the world, major investments
are being made in equipment and inrastruc-
ture that can lock in commitments to more
greenhouse gas emissions or decades to
come. Getting the relevant incentives and poli-
cies in place now will provide crucial guidance
or these investment decisions.
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36
Many actions that could be taken to reduce
vulnerability to climate change impacts are
common sense investments that also will oer
protection against natural climate variations
and extreme events.
The challenge or society is to weigh the risks andbenets and make wise choices even knowing there
are uncertainties, as is done in so many other realms,
or example, when people buy home insurance. A
valuable ramework or supporting climate choices is
an iterative risk management approach. This
reers to a process o systematically identiying risks
and possible response options; advancing a portolio
o actions that are likely to reduce risks across a
range o possible utures; and adjusting responses
over time to take advantage o new knowledge, in-
ormation, and technological capabilities.
ConclusionResponding to climate change is about makingchoices in the ace o risk. Any course o actioncarries potential risks and costs; but doing nothing
may pose the greatest risk rom climate change and
its impacts. Americas climate choices will be made
by elected ocials, business leaders, individuals,
and other decision makers across the nation; and
those choices will involve numerous value judg-
ments beyond the reach o science. However,
robust scientic knowledge and analyses are a
crucial oundation or inorming choices.
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RefeRences
National Research Council, 2010a, Advancing the Science o Climate Change
National Research Council, 2010b, Limiting the Magnitude o Climate Change
National Research Council, 2010c, Adapting to the Impacts o Climate Change
National Research Council, 2011d, Inorming an Eective Response to Climate Change
National Research Council, 2010e, Ocean Acidifcation: A National Strategy to Meet theChallenges o a Changing Ocean
National Research Council 2011a, Climate Stabilization Targets: Emissions, Concentrations,and Impacts or Decades to Millennia
National Research Council, 2011c, Americas Climate Choices
For more inormation, contact the Board on Atmospheric Sciences and Climate at202-334-3512 or visit http://dels.nas.edu/basc. A video based on Part I o this booklet isavailable at http://americasclimatechoices.org.
This booklet was prepared by the National Research Council with support rom the National Oceanic andAtmospheric Administration (NOAA).