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OCR for page 2
Evidence
Part I
for Human-Caused
Climate Change
T he overwhelming But how has this conclusion been reached? Climate science,
majority of climate like all science, is a process of collective learning that
scientists agree that
relies on the careful gathering and analyses of data,
the formulation of hypotheses, the development of
human activities, models to study key processes and make testable
predictions, and the combined use of observations
especially the burning
and models to test scientific understanding.
of fossil fuels (coal, oil, Scientific knowledge builds over time as new
and gas), are responsible observations and data become available.
Confidence in our understanding grows if multiple
for most of the climate lines of evidence lead to the same conclusions, or
change currently being if other explanations can be ruled out. In the case of
observed. climate change, scientists have understood for more
than a century that emissions from the burning of fossil
fuels could lead to increases in the Earth's average surface
temperature. Decades of research have confirmed and extended this
understanding.
2
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How do we know that Earth has warmed?
S cientists have been taking widespread measure-
ments of Earth's surface temperature since
around 1880. These data have steadily improved
changes in the instruments taking the measure-
ments or by other factors that affect local tempera-
ture, such as additional heat that has come from
and, today, temperatures are recorded by ther- the gradual growth of cities.
mometers at many thousands of locations, both on These analyses all show that Earth's average
the land and over the oceans. Different research surface temperature has increased by more than
groups, including the NASA Goddard Institute for 1.4°F (0.8°C) over the past 100 years, with much
Space Studies, Britain's Hadley Centre for Climate of this increase taking place over the past 35 years.
Change, the Japan Meteorological Agency, and A temperature change of 1.4°F may not seem like
NOAA's National Climate Data Center have used much if you're thinking about a daily or seasonal
these raw measurements to produce records of fluctuation, but it is a significant change when
long-term global surface temperature change you think about a permanent increase averaged
(Figure 1). These groups work carefully to make across the entire planet. Consider, for example,
sure the data aren't skewed by such things as that 1.4°F is greater than the average annual
E v i d e n c e f o r H u m a n - C a u s e d C l i m at e C h a n g e
FIGURE 1 FIGURE 2
NASA's Global Surface Temperature Record Esti- (bottom left) Climate monitoring stations on land and sea,
mates of global surface temperature change, relative such as the moored buoys of NOAA's Tropical Atmosphere
to the average global surface temperature for the Ocean (TAO) project, provide real-time data on tempera-
period from 1951 to 1980, which is about 14°C (57°F) ture, humidity, winds, and other atmospheric properties.
from NASA Goddard Institute for Space Studies show Image courtesy of TAO Project Office, NOAA Pacific Marine
a warming trend over the 20th century. The esti- Environmental Laboratory. (right) Weather balloons, which
mates are based on surface air temperature measure- carry instruments known as radiosondes, provide verti-
ments at meteorological stations and on sea surface cal profiles of some of these same properties throughout
temperature measurements from ships and satellites. the lower atmosphere. Image © University Corporation for
The black curve shows average annual temperatures, Atmospheric Research. (top left) The NOAA-N spacecraft,
and the red curve is a 5-year running average. The launched in 2005, is the fifteenth in a series of polar-
green bars indicate the margin of error, which has orbiting satellites dating back to 1978. The satellites carry
been reduced over time. Source: National Research instruments that measure global surface temperature and
Council 2010a other climate variables. Image courtesy NASA
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temperature difference between Washington, D.C., of the atmosphere and of the ocean and land
and Charleston, South Carolina, which is more surfaces. Satellite data are also used to study shifts in
than 450 miles farther south. Consider, too, that precipitation and changes in land cover.
a decrease of only 9°F (5°C) in global average Even though satellites measure temperature very
temperatures is the estimated difference between differently than instruments on Earth's surface, and
today's climate and an ice age. any errors would be of a completely different
In addition to surface temperature, other parts of nature, the two records agree. A number of other
the climate system are also being monitored carefully indicators of global warming have also been
(Figure 2). For example, a variety of instruments are observed (see pp.15-17). For example, heat waves
used to measure temperature, salinity, and currents are becoming more frequent, cold snaps are now
beneath the ocean surface. Weather balloons are shorter and milder, snow and ice cover are
used to probe the temperature, humidity, and winds decreasing in the Northern Hemisphere, glaciers
in the atmosphere. A key breakthrough in the ability and ice caps around the world are melting, and
to track global environmental changes began in the many plant and animal species are moving to cooler
1970s with the dawn of the era of satellite remote latitudes or higher altitudes because it is too warm
sensing. Many different types of sensors, carried to stay where they are. The picture that emerges
on dozens of satellites, have allowed us to build a from all of these data sets is clear and consistent:
truly global picture of changes in the temperature Earth is warming.
How do we know that greenhouse gases
lead to warming?
A s early as the 1820s, scientists began to ap-
preciate the importance of certain gases in
regulating the temperature of the Earth (see Box 1).
energy is then radiated upward from Earth's surface
in the form of heat. In the absence of greenhouse
gases, this heat would simply escape to space,
Greenhouse gases--which include carbon dioxide and the planet's average surface temperature
(CO2), methane, nitrous oxide, and water vapor-- would be well below freezing. But greenhouse
act like a blanket in the atmosphere, keep- gases absorb and redirect some of
ing heat in the lower atmosphere. this energy downward, keeping
Although greenhouse gases heat near Earth's surface. As
comprise only a tiny fraction concentrations of heat-
of Earth's atmosphere, they trapping greenhouse
are critical for keeping the gases increase in the
planet warm enough to atmosphere, Earth's
support life as we know it natural greenhouse
(Figure 3). effect is enhanced
Here's how the (like a thicker blanket),
"greenhouse effect" causing surface
works: as the Sun's energy temperatures to rise
hits Earth, some of it is (Figure 3). Reducing the
reflected back to space, but levels of greenhouse gases
most of it is absorbed by the in the atmosphere would
land and oceans. This absorbed cause a decrease in surface
temperatures.
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FIGURE 3
Amplification of the Greenhouse Effect The greenhouse effect is a natural phenomenon that is essential to
E v i d e n c e f o r H u m a n - C a u s e d C l i m at e C h a n g e
keeping the Earth's surface warm. Like a greenhouse window, greenhouse gases allow sunlight to enter and then
prevent heat from leaving the atmosphere. These gases include carbon dioxide (CO2), methane (CH4), nitrous
oxide (N2O), and water vapor. Human activities--especially burning fossil fuels--are increasing the concentrations
of many of these gases, amplifying the natural greenhouse effect. Image courtesy of the Marian Koshland Science
Museum of the National Academy of Sciences
Box 1
Early Understanding of Greenhouse Gases In 1824, French
physicist Joseph Fourier (top) was the first to suggest that the Earth's
atmosphere might act as an insulator of some kind--the first proposal
of what was later called the greenhouse effect. In the 1850s, Irish-
born physicist John Tyndall (middle) was the first to demonstrate
the greenhouse effect by showing that water vapor and other
atmospheric gases absorbed Earth's radiant heat. In 1896, Swedish
scientist Svante Arrhenius (bottom) was the first to calculate the
warming power of excess carbon dioxide (CO2). From his calculations,
Arrhenius predicted that if human activities increased CO2 levels in the
atmosphere, a warming trend would result.
5
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FIGURE 4
The Carbon Cycle Carbon is continually exchanged between the atmosphere, ocean, biosphere, and land on a
variety of timescales. In the short term, CO2 is exchanged continuously among plants, trees, animals, and the air
through respiration and photosynthesis, and between the ocean and the atmosphere through gas exchange. Other
parts of the carbon cycle, such as the weathering of rocks and the formation of fossil fuels, are much slower pro-
cesses occurring over many centuries. For example, most of the world's oil reserves were formed when the remains
of plants and animals were buried in sediment at the bottom of shallow seas hundreds of millions of years ago, and
then exposed to heat and pressure over many millions of years. A small amount of this carbon is released naturally
back into the atmosphere each year by volcanoes, completing the long-term carbon cycle. Human activities, espe-
cially the digging up and burning of coal, oil, and natural gas for energy, are disrupting the natural carbon cycle by
releasing large amounts of "fossil" carbon over a relatively short time period. Source: National Research Council
How do we know that humans are causing
greenhouse gas concentrations to increase?
D iscerning the human influence on greenhouse
gas concentrations is challenging because many
greenhouse gases occur naturally in Earth's atmo-
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
sphere. Carbon dioxide (CO2 ) is produced and con- and cutting down and burning of forests (deforesta-
sumed in many natural processes that are part of the tion), also add CO2 to the atmosphere.
carbon cycle (see Figure 4). However, once humans Until the 1950s, many scientists thought the
began digging up long-buried forms of carbon such oceans would absorb most of the excess CO2
as coal and oil and burning them for energy, addi- released by human activities. Then a series of
6
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FIGURE 5 FIGURE 6
Measurements of Atmospheric Carbon Dioxide Greenhouse Gas Concentrations for 2,000 Years
The "Keeling Curve" is a set of careful measurements Analysis of air bubbles trapped in Antarctic ice cores
of atmospheric CO2 that Charles David Keeling show that, along with carbon dioxide, atmospheric
began collecting in 1958. The data show a steady concentrations of methane (CH4) and nitrous oxide
annual increase in CO2 plus a small up-and-down (N2O) were relatively constant until they started to rise
sawtooth pattern each year that reflects seasonal in the Industrial era. Atmospheric concentration units
changes in plant activity (plants take up CO2 during indicate the number of molecules of the greenhouse
spring and summer in the Northern Hemisphere, gas per million molecules of air for carbon dioxide
E v i d e n c e f o r H u m a n - C a u s e d C l i m at e C h a n g e
where most of the planet's land mass and land and nitrous oxide, and per billion molecules of air for
ecosystems reside, and release it in fall and winter). methane. Image courtesy: U.S. Global Climate Research
Source: National Research Council, 2010a Program
scientific papers were published that examined the the Industrial Revolution, atmospheric CO2
dynamics of carbon dioxide exchange between concentrations were steady and then began to rise
the ocean and atmosphere, including a paper by sharply beginning in the late 1800s (Figure 6).
oceanographer Roger Revelle and Hans Seuss in Today, atmospheric CO2 concentrations exceed 390
1957 and another by Bert Bolin and Erik Eriksson parts per million--nearly 40% higher than
in 1959. This work led scientists to the hypothesis preindustrial levels, and, according to ice core data,
that the oceans could not absorb all of the CO2 higher than at any point in the past 800,000 years
being emitted. To test this hypothesis, Revelle's (see Figure 14, p.18).
colleague Charles David Keeling began collecting Human activities have increased the atmospheric
air samples at the Mauna Loa Observatory in Hawaii concentrations of other important greenhouse
to track changes in CO2 concentrations. Today, gases as well. Methane, which is produced by
such measurements are made at many sites around the burning of fossil fuels, the raising of livestock,
the world. The data reveal a steady increase in the decay of landfill wastes, the production and
atmospheric CO2 (Figure 5). transport of natural gas, and other activities,
To determine how CO2 concentrations varied increased sharply through the 1980s before
prior to such modern measurements, scientists have starting to level off at about two-and-a-half
studied the composition of air bubbles trapped in times its preindustrial level (Figure 6). Nitrous
ice cores extracted from Greenland and Antarctica. oxide has increased by roughly 15% since 1750
These data show that, for at least 2,000 years before (Figure 6), mainly as a result of agricultural
7
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+ =
Land Sink
Emissions Atmospheric
Concentration
Growth
Ocean Sink
FIGURE 7
Emissions Exceed Nature's CO2 Drain Emissions of CO2 due to fossil fuel burning and cement manufacture are
increasing, while the capacity of "sinks" that take up CO2--for example, plants on land and in the ocean--are
decreasing. Atmospheric CO2 is increasing as a result. Source: National Research Council, 2011a
fertilizer use, but also from fossil fuel burning and analyses show that about 45% of the CO2 emitted
certain industrial processes. Certain industrial by human activities remains in the atmosphere.
chemicals, such as chlorofluorocarbons (CFCs), Just as a sink will fill up if water is entering it faster
act as potent greenhouse gases and are long-lived than it can drain, human production of CO2 is
in the atmosphere. Because CFCs do not have outstripping Earth's natural ability to remove it
natural sources, their increases can be attributed from the air. As a result, atmospheric CO2 levels are
unambiguously to human activities. increasing (see Figure 7) and will remain elevated
In addition to direct measurements of CO2 for many centuries. Furthermore, a forensic-style
concentrations in the atmosphere, scientists have analysis of the CO2 in the atmosphere reveals the
amassed detailed records of how much coal, oil, chemical "fingerprint" of carbon from fossil fuels
and natural gas is burned each year. They also (see Box 2). Together, these lines of evidence prove
estimate how much CO2 is being absorbed, on conclusively that the elevated CO2 concentration in
average, by the oceans and the land surface. These the atmosphere is the result of human activities.
Box 2
Clues from the "fingerprint" of carbon dioxide. In a process that takes place over
millions of years, carbon from the decay of plants and animals is stored deep in the Earth's crust in
the form of coal, oil, and natural gas (see Figure 4). Because this "fossil" carbon is so old, it contains
very little of the radioisotope carbon-14--a form of the carbon that decays naturally over long time
periods. When scientists measure carbon-14 levels in the atmosphere, they find that it is much lower
than the levels in living ecosystems, indicating that there is an abundance of "old" carbon. While a
small fraction of this old carbon can be attributed to volcanic eruptions, the overwhelming majority
comes from the burning of fossil fuels. Average CO2 emissions from volcanoes are about 200 million
tons per year, while humans are emitting an estimated 36 billion tons of CO2 each year, 80-85% of
which are from fossil fuels.
8
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How much are human activities
heating Earth?
G reenhouse gases are referred to
as "forcing agents" because of
their ability to change the planet's
a cooling effect because they scatter a
portion of incoming sunlight back into
space (see Box 3). Human activities,
energy balance. A forcing agent especially the burning of fossil fuels,
can "push" Earth's temperature have increased the number of
up or down. Greenhouse gases aerosol particles in the atmosphere,
differ in their forcing power. especially over and around major
For example, a single methane urban and industrial areas.
molecule has about 25 times Changes in land use and land
the warming power of a single cover are another way that human
CO2 molecule. However, CO2 has a activities are influencing Earth's climate.
much larger overall warming effect than Deforestation is responsible for 10% to 20% of
methane because it is much more abundant and the excess CO2 emitted to the atmosphere each
stays in the atmosphere for much longer periods year, and, as has already been discussed, agriculture
of time. Scientists can calculate the forcing power contributes nitrous oxide and methane. Changes in
of greenhouse gases based on the changes in their land use and land cover also modify the reflectivity
concentrations over time and on physically based of Earth's surface; the more reflective a surface, the
calculations of how they transfer energy through the more sunlight is sent back into space. Cropland is
E v i d e n c e f o r H u m a n - C a u s e d C l i m at e C h a n g e
atmosphere. generally more reflective than an undisturbed forest,
Some forcing agents push Earth's energy balance while urban areas often reflect less energy than
toward cooling, offsetting some of the heating undisturbed land. Globally, human land use changes
associated with greenhouse gases. For example, are estimated to have a slight cooling effect.
some aerosols--which are tiny liquid or solid When all human and natural forcing agents are
particles suspended in the atmosphere, such as those considered together, scientists have calculated that
that make up most of the visible air pollution--have the net climate forcing between 1750 and 2005 is
Box 3
Warming and Cooling Effects of Aerosols Aerosols are tiny liquid or solid particles
suspended in the atmosphere that come from a number of human activities, such as fossil fuel
combustion, as well as natural processes, such as dust storms, volcanic eruptions, and sea spray
emissions from the ocean. Most of our visible air pollution is made up of aerosols. Most aerosols
have a cooling effect, because they scatter a portion of incoming sunlight back into space, although
some particles, such as dust and soot, actually absorb some solar energy and thus act as warming
agents. Many aerosols also enhance the reflection of sunlight back to space by making clouds
brighter, which results in additional cooling. Many nations, states, and communities have taken
action to reduce the concentrations of certain air pollutants such as the sulfate aerosols responsible
for acid rain. Unlike most of the greenhouse gases released by human activities, aerosols only
remain in the atmosphere for a short time--typically a few weeks.
9
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pushing Earth toward warming (Figure 8). The extra
energy is about 1.6 Watts per square meter of Earth's
surface. When multiplied by the surface area of
Earth, this energy represents more than 800 trillion
Watts (Terawatts)--on a per year basis, that's about
50 times the amount of power produced by all the
power plants of the world combined! This extra
energy is being added to Earth's climate system
every second of every day.
The total amount of warming that will occur in
response to a climate forcing is determined by a
variety of feedbacks, which either amplify or dampen
the initial warming. For example, as Earth warms,
polar snow and ice melt, allowing the darker colored
Energy (Watts/m2)
land and oceans to absorb more heat--causing Earth
to become even warmer, which leads to more snow
FIGURE 8
and ice melt, and so on (see Figure 9). Another impor-
Warming and Cooling Influences on Earth
Since 1750 The warming and cooling influences tant feedback involves water vapor. The amount of
(measured in Watts per square meter) of various cli- water vapor in the atmosphere increases as the ocean
mate forcing agents during the Industrial Age (from surface and the lower atmosphere warm up; warm-
about 1750) from human and natural sources has been
calculated. Human forcing agents include increases in ing of 1°C (1.8°F) increases water vapor by about
greenhouse gases and aerosols, and changes in land 7%. Because water vapor is also a greenhouse gas,
use. Major volcanic eruptions produce a temporary
this increase causes additional warming. Feedbacks
cooling effect, but the Sun is the only major natural
factor with a long-term effect on climate. The net ef- that reinforce the initial climate forcing are referred
fect of human activities is a strong warming influence to in the scientific community as positive, or ampli-
of more than 1.6 Watts per square meter. Source: Na-
fying, feedbacks.
tional Research Council, 2010a (Depiction courtesy U.S.
Global Climate Research Program) There is an inherent time lag in the warming that
is caused by a given climate forcing. This lag occurs
TEMPERATURES RISE
because it takes time for parts of Earth's climate
systems--especially the massive oceans--to warm
or cool. Even if we could hold all human-produced
forcing agents at present-day values, Earth would
continue to warm well beyond the 1.4°F already ob-
served because of human emissions to date.
Climate Feedback Loops The amount of warming
ARCTIC SEA
that occurs because of increased greenhouse gas
AS REFLECTIVE ICE MELTS emissions depends in part on feedback loops.
ICE DISAPPEARS, Positive (amplifying) feedback loops increase the
DARKER OCEAN net temperature change from a given forcing, while
WATER ABSORBS
MORE HEAT
negative (damping) feedbacks offset some of the
temperature change associated with a climate forcing.
The melting of Arctic sea ice is an example of a positive
feedback loop. As the ice melts, less sunlight is reflected
back to space and more is absorbed into the dark
ocean, causing further warming and further melting of
FIGURE 9 ice. Source: National Research Council, 2011d
10
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How do we know the current warming trend
isn't caused by the Sun?
A nother way to test a scientific theory is to in-
vestigate alternative explanations. Because
the Sun's output has a strong influence on Earth's
the Sun's output has not shown a net increase dur-
ing the past 30 years (Figure 10) and thus cannot be
responsible for the warming during that period.
temperature, scientists have examined records of Prior to the satellite era, solar energy output had
solar activity to determine if changes in solar output to be estimated by more indirect methods, such as
might be responsible for the observed global warm- records of the number of sunspots observed each
ing trend. The most direct measurements of solar year, which is an indicator of solar activity. These
output are satellite readings, which have been avail- indirect methods suggest there was a slight increase
able since 1979. These satellite records show that in solar energy reaching Earth during the first few
Measures of the Sun's Energy
Satellite measurements of the Sun's
energy incident on Earth, available since
1979, show no net increase in solar
forcing during the past 30 years. They
E v i d e n c e f o r H u m a n - C a u s e d C l i m at e C h a n g e
show only small periodic variations
associated with the 11-year solar cycle.
Source: National Research Council, 2010a
FIGURE 10
Warming Patterns in the Layers of
the Atmosphere Data from weather
balloons and satellites show a warming
trend in the troposphere, the lower
layer of the atmosphere, which extends
up about 10 miles (lower graph), and
a cooling trend in the stratosphere,
which is the layer immediately above
the troposphere (upper graph). This
is exactly the pattern expected from
increased greenhouse gases, which trap
energy closer to the Earth's surface.
Source: National Research Council, 2010a
FIGURE 11
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decades of the 20th century. This increase may have atmosphere since the late 1970s. Both of these data
contributed to global temperature increases during sets have been heavily scrutinized, and both show a
that period, but does not explain warming in the warming trend in the lower layer of the atmosphere
latter part of the century. (the troposphere) and a cooling trend in the
Further evidence that current warming is not upper layer (the stratosphere) (Figure 11). This is
a result of solar changes can be found in the exactly the vertical pattern of temperature changes
temperature trends in the different layers of the expected from increased greenhouse gases,
atmosphere. These data come from two sources: which trap energy closer to the Earth's surface. If
weather balloons, which have been launched an increase in solar output were responsible for
twice daily from hundreds of sites worldwide the recent warming trend, the vertical pattern of
since the late 1950s, and satellites, which have warming would be more uniform through the
monitored the temperature of different layers of the layers of the atmosphere.
How do we know that the current warming
trend is not caused by natural cycles?
D etecting climate trends is
complicated by the fact
that there are many natural
temperatures tend to be higher
during El Niño periods, such as
1998, and lower during La
variations in temperature, Niña years, such as 2008.
precipitation, and other However, these up-and-
climate variables. These down fluctuations are
natural variations are smaller than the 20th cen-
caused by many different tury warming trend; 2008
processes that can occur was still quite a warm year
across a wide range of in the long-term record.
timescales--from a particularly Natural climate variations can
warm summer or snowy winter also be forced by slow changes in
to changes over many millions of the Earth's orbit around the Sun that
years. affect the solar energy received by Earth, as
Among the most well-known short-term cli- is the case with the Ice Age cycle (see pp. 18-19)
matic fluctuations are El Niño and La Niña, which or by short-term changes in the amount of volca-
are periods of natural warming and cooling in the nic aerosols in the atmosphere. Major eruptions,
tropical Pacific Ocean. Strong El Niño and La Niña like that of Mount Pinatubo in 1991, spew huge
events are associated with significant year-to-year amounts of particles into the stratosphere that
changes in temperature and rainfall patterns across cool Earth. However, surface temperatures typically
many parts of the planet, including the United rebound in 2-5 years as the particles settle out of
States. These events have been linked to a number the atmosphere. The short-term cooling effects of
of extreme weather events, such as the 1992 flood- several large volcanic eruptions can be seen in the
ing in midwestern states and the severe droughts 20th century temperature record, as can the global
in southeastern states in 2006 and 2007. Globally, temperature variations associated with several
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strong El Niño and La Niña events, but an overall
warming trend is still evident (Figure 12).
In order to put El Niño and La Niña events and
other short-term natural fluctuations into perspec-
tive, climate scientists examine trends over several
decades or longer when assessing the human influ-
ence on the climate system. Based on a rigorous as-
sessment of available temperature records, climate
forcing estimates, and sources of natural climate
variability, scientists have concluded that there is a
more than 90% chance that most of the observed
global warming trend over the past 50 to 60 years
can be attributed to emissions from the burning of
fossil fuels and other human activities.
Such statements that attribute climate change
to human activities also rely on information from
Short-term Temperature Effects
of Natural Climate Variations
Natural factors, such as volcanic
eruptions and El Niño and La Niña
events, can cause average global
temperatures to vary from one year
E v i d e n c e f o r H u m a n - C a u s e d C l i m at e C h a n g e
to the next, but cannot explain the
long-term warming trend over the
past 60 years. Image courtesy of the
Marian Koshland Science Museum
FIGURE 12
Model Runs With and Without
Human Influences Model
simulations of 20th-century surface
temperatures more closely match
observed temperature when both
natural and human influences are
included in the simulations. The
black line shows an estimate of
observed surface temperatures
changes. The blue line shows results
from models that only include
natural forcings (solar activity and
volcanoes). The red-shaded regions
show results from models that
include both natural and human
forcings. Source: Meehl et al, 2011
FIGURE 13
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CLOUD S & WATE R VAPOR
RADIATIVE
EXCHANGE
EXCHANGE OF HEAT &
WIN D
GAS ES BETWEEN ATMOS P HERE,
S EA ICE & OCEAN
E VAPOTRAN SPIRATION EVAPORATION
CONDENSATION &
CONV ECTION
WAT E R S TORAGE AIR POLLUTION
I N I C E & SNOW
GLACIER MELT
TERRES TRIAL CARBON
CYCLE
S U R FAC E
R U N-OFF
OCEAN
OCEAN CARBON CYCLE
CIRCULATION
Box 4
What are climate models? For several decades, scientists have used the world's most ad-
vanced computers to simulate the Earth's climate. These models are based on a series of mathemati-
cal equations representing the basic laws of physics--laws that govern the behavior of the atmo-
sphere, the oceans, the land surface, and other parts of the climate system, as well as the interactions
among different parts of the system. Climate models are important tools for understanding past,
present, and future climate change. Climate models are tested against observations so that scientists
can see if the models correctly simulate what actually happened in the recent or distant past. Image
courtesy Marian Koshland Science Museum
climate models (see Box 4). Scientists have used turbed Earth" simulations predict that, in the ab-
these models to simulate what would have happened sence of human activities, there would have been
if humans had not modified Earth's climate during negligible warming, or even a slight cooling, over
the 20th century--that is, how global tempera- the 20th century. When greenhouse gas emissions
tures would have evolved if only natural factors and other activities are included in the models, how-
(volcanoes, the Sun, and internal climate variability) ever, the resulting surface temperature changes more
were influencing the climate system. These "undis- closely resemble the observed changes (Figure 13).
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What other climate changes and impacts
have already been observed?
R ising temperatures due to increasing greenhouse
gas concentrations have produced distinct pat-
terns of warming on Earth's surface, with stronger
lite monitoring began in 1978 (Figure 16). This
melting has been especially strong in late summer,
leaving large parts of the Arctic Ocean ice-free for
warming over most land areas and in the Arctic. weeks at a time and raising questions about effects
There have also been significant seasonal differences on ecosystems, commercial shipping routes, oil
in observed warming. For example, the second half and gas exploration, and national defense. Many
of the 20th century saw intense winter warming of the world's glaciers and ice sheets are melting in
across parts of Canada, Alaska, and northern Europe response to the warming trend, and long-term av-
and Asia, while summer warming was particularly erage winter snowfall and snowpack have declined
strong across the Mediterranean and Middle East in many regions as well, such as the Sierra Nevada
and some other places, including parts of the U.S. mountain range in the western United States.
west (Figure 15). Heat waves and record high tem- Much of the excess heat caused by human-emit-
peratures have increased across most regions of the ted greenhouse gases has warmed the world's
world, while cold snaps and record cold tempera- oceans during the past several decades. Water ex-
tures have decreased. pands when it warms, which leads to sea-level rise.
Global warming is also having a significant im- Water from melting glaciers, ice sheets, and ice caps
pact on snow and ice, especially in response to the also contributes to rising sea levels. Measurements
E v i d e n c e f o r H u m a n - C a u s e d C l i m at e C h a n g e
strong warming across the Arctic. For example, made with tide gauges and augmented by satellites
the average annual extent of Arctic sea ice has show that, since 1870, global average sea level has
dropped by roughly 10% per decade since satel- risen by about 8 inches (0.2 meters). It is estimated
FIGURE 15
Patterns of Warming in Winter and Summer Twenty-year average temperatures for 1986-2005 compared
to 1955-1974 show a distinct pattern of winter and summer warming. Winter warming has been intense across
parts of Canada, Alaska, northern Europe, and Asia, and summers have warmed across the Mediterranean and
Middle East and some other places, including parts of the U.S. west. Projections for the 21st century show a
similar pattern. Source: National Research Council, 2011a
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FIGURE 16
Loss of Arctic Sea Ice Satellite-based measurements
show a steady decline in the amount of September
(end of summer) Arctic sea ice extent from 1979 to
2009 (expressed as a percentage difference from 1979-
2000 average sea ice extent, which was 7.0 million
square miles). The data show substantial year-to-year
variability, but a long-term decline in sea ice of more
than 10% per decade is clearly evident, highlighted by
the dashed line. Source: National Research Council, 2010a
that roughly one-third of the total sea-level rise over
the past four decades can be attributed to ocean FIGURE 17
expansion, with most of the remainder due to ice Contributors to Sea-Level Rise
melt (Figure 17). Sea level has risen steadily over the past few decades
due to various contributors: thermal expansion in the
Because CO2 reacts in seawater to form carbonic upper 700 meters of ocean (red) and deeper ocean
acid, the acidification of the world's oceans is an- layers (orange), meltwater from Antarctic and Green-
land ice sheets (blue), meltwater from glaciers and
other certain outcome of elevated CO2 concentra-
ice caps (purple), and water storage on land (green).
tions in the atmosphere (Figure 18). It is estimated Source: National Research Council, 2011a
that the oceans have absorbed between one-quarter
and one-third of the excess CO2 from human activi-
ties, becoming nearly 30% more acidic than during the frequency and distribution of precipitation. Total
preindustrial times. Geologically speaking, this large precipitation in the United States has increased by
change has happened over a very short timeframe, about 5% over the past 50 years, but this has not
and mounting evidence indicates it has the poten- been geographically uniform--conditions are gener-
tial to radically alter marine ecosystems, as well as ally wetter in the Northeast, drier in the Southeast,
the health of coral reefs, shellfish, and fisheries. and much drier in the Southwest.
Another example of a climate change observed Warmer air holds more water vapor, which has led
during the past several decades has been changes in to a measurable increase in the intensity of precipita-
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Evidence of Ocean Acidification With
excess CO2 building up in the atmo-
sphere, scientists wanted to know if it
was also accumulating in the ocean.
Studies that began in the mid-1980s
show that the concentration of CO2 in
ocean water (in blue, calculated from
the partial pressure of CO2 in seawater)
has risen in parallel with the increase
in atmospheric CO2 (in red, part of the
Keeling curve). At the same time, the
ocean has become more acidic, because
the CO2 reacts with seawater to form
carbonic acid. The orange dots are direct
measurements of pH in surface seawater
(lower pH being more acidic), and the
green dots are calculated based on the
chemical properties of seawater. Source:
National Research Council, 2010d
FIGURE 18
tion events. In the United States, for example, the earlier, while other species are altering their seasonal
fraction of total precipitation falling in the heaviest breeding patterns. Global analyses show these
1% of rainstorm increased by about 20% over the behaviors occurred an average of 5 days earlier
past century, with the northeastern states experienc- per decade from 1970 to 2000. Such changes can
ing an increase of 54%. This change has increased disrupt feeding patterns, pollination, and other vital
the risk of flooding and puts additional stress on interactions between species, and they also affect
E v i d e n c e f o r H u m a n - C a u s e d C l i m at e C h a n g e
sewer and stormwater management systems. the timing and severity of insects, disease outbreaks,
As the climate has changed, many species have and other disturbances. In the western United
shifted their range toward the poles and to higher States, climate change has increased the population
altitudes as they try to stay in areas with the same of forest pests such as the pine beetle.
ambient temperatures. The timing of different The next section describes how observed climate
seasonal activities is also changing. Several plant trends and impacts are predicted to continue if
species are blooming earlier in Spring, and some emissions of human-produced greenhouse gases are
birds, mammals, fish, and insects are migrating maintained during the next century and beyond.
Climate change has increased the population of forest pests in the western United States. The red trees in this
photo of Dillon Reservoir in Colorado have died from an infestation of mountain pine beetle.
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The Ice Ages
Perhaps the most dramatic example of natural climate variability over long time
periods is the Ice Age cycle. Detailed analyses of ocean sediments, ice cores, and
other data show that for at least 800,000 years, and probably for the past 4 to 5
million years, the Earth has gone through extended periods when temperatures
were much lower than today and thick blankets of ice covered large areas of the
Northern Hemisphere. These long cold spells, which typically lasted for around
100,000 years, were interrupted by shorter warm "interglacial" periods, including the
past 10,000 years (Figure 14).
Through a convergence of theory, observations, and modeling, scientists have
deduced that the ice ages are caused by slight recurring variations in Earth's
orbit that alter the amount and seasonal distribution of solar energy reaching the
Northern Hemisphere. These relatively small changes in solar energy are reinforced
over thousands of years by gradual changes in Earth's ice cover (the cryosphere)
and ecosystems (the biosphere), eventually leading to large changes in global
800,000 Years of Temperature
and Carbon Dioxide Records
As ice core records from Vostok,
Antarctica, show, the tempera-
ture near the South Pole has
varied by as much as 20°F (11°C)
during the past 800,000 years.
The cyclical pattern of tempera-
ture variations constitutes the ice
age/interglacial cycles. During
these cycles, changes in carbon
dioxide concentrations (in red)
track closely with changes in
temperature (in blue), with CO2
lagging behind temperature
changes. Because it takes a while
for snow to compress into ice, ice
core data are not yet available
much beyond the 18th century at
most locations. However, atmo-
spheric carbon dioxide levels, as
measured in air, are higher today
than at any time during the past
800,000 years. Source: National
Research Council, 2010a
FIGURE 14
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temperature. The average global
temperature change during an ice age
cycle, which occur over about 100,000
years, is on the order of 9°F ± 2°F (5°C ±
1°C).
The data show that in past ice age
cycles, changes in temperature have
led--that is, started prior to--changes
in CO2. This is because the changes
in temperature induced by changes
in Earth's orbit around the Sun lead
to gradual changes in the biosphere
and the carbon cycle, and thus CO2,
reinforcing the initial temperature trend.
In contrast, the relatively rapid release of
CO2 and other greenhouse gases since
the start of the Industrial Revolution
from the burning of fossil fuel has,
E v i d e n c e f o r H u m a n - C a u s e d C l i m at e C h a n g e
in essence, reversed the pattern: the
additional CO2 is acting as a climate
forcing, with temperatures increasing
afterward.
The ice age cycles nicely illustrate
how climate forcing and feedback
The U.S. Geological Survey National Ice Core Lab
stores ice cores samples taken from polar ice caps effects can alter Earth's temperature, but
and mountain glaciers. Ice cores provide clues there is also direct evidence from past
about changes in Earth's climate and atmosphere
going back hundreds of thousands of years. climates that large releases of carbon
dioxide have caused global warming.
One of the largest known events of this type is called the Paleocene-Eocene
Thermal Maximum, or PETM, which occurred about 55 million years ago,
when Earth's climate was much warmer than today. Chemical indicators
point to a huge release of carbon dioxide that warmed Earth by another 9°F
and caused widespread ocean acidification. These climatic changes were
accompanied by massive ecosystem changes, such as the emergence of
many new types of mammals on land and the extinction of many bottom-
dwelling species in the oceans.
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