TO WHAT EXTENT CAN HUMANS AFFECT CLIMATE?
H
uman beings have impacted Earth’s environment since early man caused mass extinctions of large animals, especially in North America. Ancient Romans smelted
so much lead that the effects can be traced all over the northern hemisphere. Since
the beginning of the Industrial Age, human activity has had ever increasing environmental
consequences. Until recently, these consequences have been manifested on local and regional scales only. Today, however, there is growing evidence that industrial and other
human activities may affect the global climate. Climate is extremely complex, and it is a
challenge to separate natural effects from human effects.
This theme (Climate - Human Interactions) discusses various evidence for human impact on
the environment and the ability of science to understand and make predictions for a system as
complex as a planet’s climate.
Related Themes:
• Climate changes are examined in Climate - Process and Change.
• How satellites are used to study climate is presented in Climate - Measurements.
• Trends in mean sea level change and their relationship to global climate are outlined
in Climate - Process and Change.
• How ocean circulation, upwelling, and downwelling affect climate is explained in
Climate - Systems and Interactions.
• How phytoplankton communities convert carbon dioxide into oxygen is addressed
in the Life - Scale and Structure and Life - Systems and Interactions.
• Why measuring ocean currents provides benefits to humankind is addressed in the
Oceans - Human Interactions.
Related Activities:
• Making a Greenhouse
INTRODUCTION
The burgeoning population of Earth and rapid industrialization have led to growing concerns over the impact of human activity on Earth’s environment. Air and water pollution at the
local level--i.e., within particular cities or regions--has been the major focus of environmental
interest through most of the last half of this century. But in recent years, environmental research
has revealed evidence that global industrial activity has caused measured increases in atmospheric carbon dioxide (CO2) and partial depletion of Earth’s ozone layer at polar latitudes, two
significant influences on climate. Many have called for changes in manufacturing processes and
consumer habits to minimize humankind’s impact on global climate. Others have argued that
Earth’s climate is so complex that present scientific research capabilities are not sophisticated
enough to determine to what extent human activity affects global climate. The problem is complicated: large-scale, natural climate variations make it difficult to precisely know the impact of
human activity on our climate. We are faced with many unanswered questions; does human
activity adversely impact our climate? If it does, what would be the most appropriate and effective way to minimize or perhaps reverse that impact?
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THE COMPLEXITY OF CLIMATE
Earth’s climate results from the interaction of many global systems. The oceans affect Earth’s
climate by serving as a heat reservoir and by transporting heat energy around the globe. Winds
drive ocean currents and so affect global heat flow. Changes in cloud cover influence the amount
of solar energy over the land and oceans. Small variations in the tilt of Earth’s axis and the shape
of Earth’s orbit around the sun also affect where and how much heat reaches the ground. The
concentration of gases such as CO2 (carbon dioxide), CH4 (methane), and water (H2O) vapor
affect how much heat Earth’s atmosphere can retain.
All of the above factors are interrelated. Changes in one factor can alter another factor, which,
in turn, can lead to changes in yet another factor. In some cases, a feedback loop exists among two
or more climate factors, i.e., a change in system A leads to a change in system B, which in turn
causes another change in system A, and so on. Some feedback loops can self-regulate. For example, when a home thermostat is set to a given temperature, a heating or cooling system operates until the air temperature reaches the desired temperature. When the air temperature changes,
the heating or cooling system once again activates to bring the temperature back to the set point.
Feedback loops can also run out of control. A common example of this kind of feedback
occurs when a microphone is placed too close to the speaker that is amplifying its signal. The
resulting high-pitched noise is due to the speaker feeding its sound into the microphone, which
sends a signal to the speaker, which feeds more sound into the microphone, etc. The signal
quickly becomes too much for the speaker to handle and results in an ear-piercing squeal.
Feedback is an example of a non-linear effect. The consequences of nonlinear effects are often
difficult to predict. This is because sometimes a very small perturbation in one part of a system
may lead to large changes elsewhere. For example, a small increase in the CO2 concentration in
Earth’s atmosphere could lead to a large increase in atmospheric temperature, since small amounts
of CO2 trap heat very efficiently. Some climate changes are probably self-regulating, like a thermostat, but others are probably not, like the example of the microphone getting too close to the
speaker.
Because many systems within our climate interact--and small perturbations can lead to big
changes later--it is extremely difficult for scientists to accurately predict long-term trends in climate. In the microphone example, it’s difficult to know exactly how close the microphone can
get to the speaker before feedback occurs. Similarly for climate, it’s difficult to predict how
much of a change in one of the many climate parameters might have dramatic consequences.
To try to predict climate trends, scientists create mathematical models: systems of equations
that describe how given climate components, such as global temperature, wind and ocean current patterns, polar ice cap extent, etc., will change over time. The individual equations in the
climate model must be mathematically linked together, because each climate factor typically has
some influence over several others, and is itself influenced by other components.
Scientists use climate model equations to understand and predict how changes in various
systems affect our overall climate. For example, scientists believe that an increase in atmospheric
2
Figure 1. Greenland ice-core temperature data. This 160,000-year temperature record from a Greenland
ice core shows that temperatures over the past 10,000 years have been warm and stable. This time period
coincides with the development of human civilization. However, temperatures were not so temperate in
earlier times. As can be seen in the above plot, frequent, sudden, temperature changes (ranging from -55°F
up to 5°F) were very common. Other evidence shows that the pattern of fluctuations from this Greenland
core sample are typical of the whole planet.
CO2 could eventually lead to increased temperatures. What they don’t know is how much of an
increase in CO2 is required to cause significant or irreversible change to Earth’s environment.
They are also unsure of whether other mechanisms--such as absorption of CO2 by plants on land
and in the oceans--might naturally compensate for significant increases in CO2 (similar to our
thermostat example).
Moreover, it is clear that Earth’s climate does change naturally over time, sometimes to an
extent that humans might consider drastic. The Ice Ages are an example of this. By measuring
levels of CO2 and deuterium from ice cores drilled in Greenland, Siberia, and Antarctica, scientists have found that Earth has had two major Ice Ages in the past 160,000 years [Fig. 1]. These
and other data support the conclusion that Earth has experienced extreme natural shifts in climate.
3
NATURAL VERSUS MAN-MADE CHANGES
Human population and industrial activity have grown to the point where they may have a
global impact on climate. Sorting out climate changes caused by natural factors from those attributed to human activity is one of the most difficult issues in global change research.
Emissions from factories, oil refineries, automobiles, and other technologies can travel hundreds of miles through the atmosphere. Some car exhaust products (particulates) from Los Angeles find their way to the Grand Canyon in Arizona, and add to the haze that often obscures its
natural beauty.
Deforestation is another way
that human activity can potentially impact climate. As human
population grows, more forest
land is cleared for housing, farming, and ranching. This has become a particular problem in
South America’s Amazon rain
forest [Fig. 2]. This rain forest
provides a significant amount of
oxygen for our planet. So, it is
not clear to what extent this deforestation might affect climate.
In addition to human activity,
natural forces, both periodic and
unexpected, have significant effects on climate. Volcanic eruptions can alter climate by throwing particulates and gases into
the upper atmosphere. Mount
Pinatubo’s 1991 eruption probably contributed to the slight Figure 2. Deforestation in the Amazon rain forest. This view of
drops in average global tempera- deforestation in Rondonia, far western Brazil, is taken from the
ture during the following years. Space Shuttle. The patterns of deforestation in this part of the AmaParticulates from the eruption zon River Basin are usually aligned adjacent to highways, secondrose high into the stratosphere, ary roads, and streams because of ease of access and for transportawhere they remained for over tion.
one year, partially scattering sunlight and reducing the amount of solar radiation reaching Earth’s surface. For similar reasons,
some human produced particulates and chemicals put into the atmosphere may also cause some
drops in regional and possibly global temperatures.
Moreover, the Sun’s energy output is not unchanging. Recent measurements have indicated
that the solar constant has risen slightly in recent decades. This may in part account for the increase in average global temperature.
4
OZONE AND CFCS
The thinning of Earth’s ozone (O3) layer has been tied to human actions with a high degree of
confidence. As a result, many nations have begun to phase out the production of chemicals
proven by scientists to deplete the ozone.
Much of our atmospheric ozone occurs in the stratosphere, roughly 10 to 20 miles above Earth’s
surface. This is known as the ozone layer. Ozone filters out (absorbs) much of the Sun’s harmful
ultraviolet radiation. A significantly thinner ozone layer would allow strong ultraviolet radiation to reach Earth’s surface. This would cause increased skin cancers, susceptibility to disease
for humans, and could kill many smaller organisms.
Figure 3. TOMS data showing ozone decrease. This data from the Total Ozone Mapping Spectrometer
(TOMS) that flew on board the NIMBUS 7 satellite, graphically shows the decline in ozone over the south
pole (the ozone hole which is actually not completely lacking in ozone as the name implies, but is significantly reduced in ozone) between October 1979 and October 1994. These false color images indicate
amounts of ozone by using various colors. Green is the average amount, blue is less, and purple is still less.
During this time period, ozone concentrations fell by more than 50%.
5
In recent decades, data from ground stations, balloons, and satellites have measured reductions within the ozone layer [Fig. 3]. It has been particularly evident over the south and north
poles and during certain seasons.
The chemicals that have been strongly implicated in the thinning of Earth’s ozone layer are
called chlorofluorocarbons (CFCs). CFCs include several chemicals containing Chlorine, Fluorine,
and Carbon atoms. CFCs have been
used primarily in refrigerants, aerosol spray cans, and fast food and
other containers. Chlorine atoms
(Cl) and chlorine monoxide radicals
(ClO) are very effective ozone-destroying catalysts. CFCs are very
stable in our lower atmosphere, so
rather than being broken down they
are eventually transported to the
stratosphere. There, the Sun’s ultraviolet light breaks CFCs apart.
This frees chlorine, and can eventually lead to ozone destruction.
Figure 4 shows satellite data which
have helped to confirm the connection between CFCs and ozone
depletion.
In addition to health effects,
thinning of the ozone layer affects
several climate-related processes. It
allows more radiation to reach
Earth’s surface which may reduce
the amount of oceanic plankton.
This is important because phytoplankton help to absorb much of
our atmospheric carbon dioxide.
Moreover, by changing atmospheric heating, reduced ozone
could affect global wind, weather,
and climate patterns.
THE GREENHOUSE EFFECT
Figure 4. UARS ozone and ClO data. These Upper Atmospheric
Research Satellite (UARS) data from the Microwave Limb
Sounder (MLS) instrument are from Sept. 21, 1991 (top) and
Sept. 20, 1992 (bottom). They show a correlation between the
concentrations and locations of chlorine monoxide (ClO) and
ozone (O3). These and similar data are evidence for the effect of
CFCs on ozone concentrations, as ClO is proposed to be part of
the ozone loss reaction that results from CFCs.
The story of CFCs and ozone is a story of triumph: science discovered that certain human
activities were damaging our atmosphere, and countries took action to stop it. More complex
processes, however, are more difficult to directly tie to human activity. One of the better known
examples is the greenhouse effect.
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A greenhouse is warmer than the outside air because visible light travels through its transparent glass walls and warms the interior. The warmed interior gases re-radiate some of the
absorbed heat in the form of infrared radiation. The glass walls of the greenhouse, however, block
infrared radiation, trap it inside, and further raise the interior temperature. This also occurs
inside a car parked in direct sunlight, causing the inside to become hotter than the outside temperature.
Certain gases, known as greenhouse gases, in Earth’s atmosphere transmit visible light but
block infrared radiation. By trapping some of Earth’s infrared emission, the atmosphere acts as
a kind of blanket, helping to keep our planet warm [Fig. 5]. Without these so-called greenhouse
gases, Earth’s surface would be much colder. In fact, the average global temperature would
probably be below the freezing point of water.
On the other hand, too much of a good thing can lead to another problem: extreme heating of
Earth’s surface. Comparing Earth to the planet Venus demonstrates this. The atmosphere of
Figure 5. Energy and the Greenhouse Effect. This drawing shows schematically what happens to
the amount of radiation energy coming in from the Sun. Numbers are percentages of the incoming
solar radiation affected by various processes. Notice that greenhouse gases absorb both the radiation
directly from the Sun (mostly visible light) and those emitted from the ground (mostly infrared), and
then emit energy themselves in the infrared. Effectively, the greenhouse gases are trapping heat,
warming Earth.
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Venus contains greenhouse gases (CO2 in particular) that allow sunlight in but absorb significant
amounts of infrared radiation emitted by the surface. There is so much CO 2 in the Venusian
atmosphere that a runaway greenhouse effect has occurred there. As the Venusian atmosphere
heated, it became hot enough to emit significant quantities of infrared radiation. Some of this
radiation was absorbed by the ground and re-radiated, heating the atmosphere further still. As
the atmosphere of Venus thickened, it was able to trap and retain even more heat. As a result, the
atmosphere and surface of Venus are extremely hot, approximately 460ºC (855ºF)!
Greenhouse gases in Earth’s atmosphere have been increasing over the last several decades,
likely in part because of human activity. Cows and some other farm animals, as well as marshes
and swamp lands, emit significant quantities of methane (CH4), another greenhouse gas. An
emission product of fossil fuel burning is CO2, perhaps the most widely known greenhouse gas.
The level of CO2 in Earth’s atmosphere has been increasing in recent years, and so has our average global temperature [Fig. 6]. But this correlation does not mean that the two are necessarily
connected. It may be that both CO2 and global average temperature are rising for separate reasons.
It is safe to assume that Earth will never have a runaway greenhouse effect like Venus. However, a rise of just a few degrees in our average global temperature could have dramatic effects
on climate. Growing seasons, for example, would be affected, becoming
longer in some regions and shorter in
others. Increases in
average global temperatures could also
cause significant
changes in sea level.
Some
computer
models predict that
about 3ºC of warming would occur if
atmospheric CO 2
doubles.
This
would cause a sea
level rise of approximately 1 meter, leaving about 3% of
Earth’s land surface
(inhabited by about
20% of the world’s
people) under wa- Figure 6. Levels of CO and average global temperature have both risen over
2
ter.
the last 100 years. It is not yet clear to what extent the burning of fossil fuels
may be contributing to the increase in CO2 or the rise in global temperatures.
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CONCLUSION
It is clear that human activity has an impact on Earth’s environment. It is also clear that the
mean global temperature has risen slowly over the past century by about 1ºC. What is not clear
is how much of this increase--if any--is the result of human industry and pollution and how
much--if any--is the result of natural causes. It will probably be some time before a consensus
about this is reached among scientists. What does this mean for developing countries struggling
to raise their living standards through industrialization or for government leaders who have to
make decisions about environmental regulations? Given the potentially devastating consequences
of significant global change, it may be best to be cautious. Taking small but significant steps
toward reducing emissions of greenhouses gases now could forestall a future environmental
crisis. It is worth remembering that Earth is the only planet known to be capable of sustaining
human life. If Earth’s climate is irreversibly damaged, there may be no place else we can go.
VOCABULARY
catalyst
deuterium
greenhouse effect
mathematical models
ozone layer
plankton
stratosphere
chlorofluorocarbons
feedback loop
greenhouse gas
non-linear
particulates
runaway greenhouse
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deforestation
global warming
infrared radiation
ozone
phytoplankton
solar constant