Performance Benchmark E.12.A.2
Students know the composition of Earth’s atmosphere has changed in the past and is changing
today. I/S
Currently, Earth’s atmosphere consists primarily of just three gases. Argon, nitrogen and oxygen
make up just over 99.9% of our present day atmosphere (Table 1). At first blush, the atmosphere
appears to be of relatively constant composition. In fact, argon, nitrogen and oxygen, together
with helium, neon and krypton, are often referred to as the “non-variable” gases of our
atmosphere. Water vapor is the most variable component, ranging from 0.01% to 3% of the
atmosphere. Carbon dioxide (CO2) has slowly, but steadily, increased in abundance over the
past several decades (Figure 1), but still constitutes less than 0.04% of the atmosphere.
Table 1. Composition of the Atmosphere
Major Permanent Gases in the Atmosphere
Nitrogen
78.1 %
Oxygen
20.9%
Other Permanent Gases in the Atmosphere
Argon
0.9%
Neon
0.002%
Helium
0.005%
Krypton
0.001%
Hydrogen
0.00005%
Variable Gases in the Atmosphere
Water Vapor
0 to 4%
Carbon Dioxide
0.035%
Methane
0.0002%
Ozone
0.000004%
Figure 1. Historic atmospheric carbon dioxide concentrations of carbon dioxide at the
Mauna Loa Observatory (from http://www.aip.org/history/climate/co2.htm)
Matter cycles through the atmosphere just as it does through other natural reservoirs. Gases
might be added or removed during the biological processes of photosynthesis and respiration.
Minerals in Earth’s crust can chemically react with gases of the atmosphere (the oxidation of
iron-bearing minerals removes O2). Bodies of water dissolve certain atmospheric gases, such as
CO2, only to release them at some later time. Further, the dissociated ions in ocean waters can
react with atmospheric gases during the formation of chemically precipitated minerals, tying up
those gases in solid rock. Combustion of organic matter adds to atmospheric gases. Volcanic
eruptions and hydrothermal events can return to the atmosphere gases which have been stored for
eons.
Concern exists over the quantity of greenhouse gases (e.g., CO2) emitted into the atmosphere as
the result of burning fossil fuels. Also, significant quantities of methane (another strong
greenhouse gas) released into the atmosphere via livestock flatulence, decomposition of animal
waste, and melting of the permafrost in the artic regions, particularly in Siberia and Alaska.
To learn more about methane entering the atmosphere and atmospheric composition, go to
http://www.physicalgeography.net/fundamentals/7a.html
Geochemists attempt to estimate how long the various gases will remain in the atmosphere as
they cycle in and out of other parts of the environment. The concept of “residence time” has
been developed to address these studies. Residence time, by definition, is: the average length of
time a substance persists in a system; the capacity per rate of influx.
The residence time of nitrogen (N2) is about 44 million years; that of oxygen (O2) is 7 million
years. It is easy to see why we think of these gases as non-variable components of the
atmosphere. Conversely, the residence time of carbon dioxide (CO2) is 4 years while that of
methane (CH4) is 3.6 years. It does not take long to affect the atmospheric concentration of these
two gases.
The discussion on our evolving atmosphere is not complete without addressing the composition
of Earth’s earliest atmosphere. Evidence suggests our ancient atmosphere was chemically
reducing (as opposed to the oxidizing atmosphere of today). Hydrogen (H2) and helium (He)
were the most abundant gases in the universe at the time of Earth’s accretion, thus it is likely
they helped form an atmosphere. Their molecular masses were too low to be easily held by
Earth’s gravity and the majority of H2 and He were lost to space. Volcanic outgassing is
believed to have contributed water vapor, carbon dioxide, carbon monoxide, sulfur dioxide,
hydrogen sulfide, chlorine and nitrogen. As gases in the primordial atmosphere reacted,
ammonia and methane formed.
To learn more about Earth's early years: differentiation, water and early atmosphere, go to
http://www.globalchange.umich.edu/globalchange1/current/lectures/first_billion_years/first_billi
on_years.html
Over time, two processes were working to add oxygen to Earth’s atmosphere. Beginning over 4
billion years ago, during the Hadean, ultraviolet radiation caused the photochemical dissociation
of water vapor in the upper atmosphere. Hydrogen and oxygen were both released. Some of the
oxygen remained as O2, while other oxygen was converted to ozone (O3). Ultimately,
photochemical dissociation may have contributed as much as 2% of the oxygen in our current
atmosphere. Atmospheric oxygen increased as primitive photosynthetic organisms began filling
the warm oceans. Oxygen was produced as CO2 was consumed. Ancient cyanobacteria
produced so much O2 during the Proterozoic that by its end, around 545 million years ago, the
atmosphere became oxidizing, as evidenced by the prevalence of deposits of red, iron-rich
sediments.
Figure 2. Most of the early Earth gases were released through land volcanism (from
http://rst.gsfc.nasa.gov/Sect19/Sect19_2a.html)
Figure 3. The Earth's atmosphere for at least the first two billion years was very oxygen-poor and hence
reducing (from http://rst.gsfc.nasa.gov/Sect19/Sect19_2a.html)
To learn more about the evolution of Earth’s atmosphere, go to
http://www.geolor.com/geoteach/How_Did_Earths_Atmosphere_Evolve-geoteach.htm
As Earth matured and cooled, its rate of geothermal outgassing decreased. While Earth’s
atmosphere does still change, it does so at a slower pace than early in our history. Natural events
are still of consequence, and more recently, human activities have made significant changes to
the composition of the atmosphere. In the last 200 years, human combustion of wood and fossil
fuels has greatly increased the concentrations of CO2 and other greenhouse gases in the
atmosphere.
In the past 500,000 years, atmospheric concentrations of CO2 have been significantly greater
than current levels. Evidence from ice coring in Greenland and Antarctica shows a strong
correlation between greater concentrations of atmospheric CO2 and significantly higher
atmospheric temperatures.
To learn more about the evidence supporting Earth’s changing atmosphere, go to
http://www.niwascience.co.nz/pubs/wa/09-1/ice.htm.
Ice core data have provided a very compelling line of scientific evidence suggesting that humans
maybe causing global warming and climate change because their activities are increasing
atmospheric concentrations of CO2 and other greenhouse gases.
An excellent site discussing global climate change is found at
http://www.exploratorium.edu/climate/index.html.
Humans have also significantly altered the atmosphere through emissions of chlorofluorocarbons
(CFCs). During much of the 20th Century, CFCs were used to propel aerosols, and also in
refrigeration. Through these uses, CFCs were released to the atmosphere. While CFCs
themselves are not harmful, they have a very long residence time in the atmosphere and will
eventually migrate to the Earth’s stratosphere. In the stratosphere, CFCs react with O3 (ozone)
found naturally in this atmosphere layer. In particular, these reactions cause stratospheric O3
concentrations to be significantly reduced over the Earth’s poles, resulting in what are commonly
called “ozone holes.” Because stratospheric O3 absorbs ultraviolet light from space that is
harmful to biota, these ozone holes present a significant danger to humans, animals, and plants in
the arctic and antarctic regions. In 1993, worldwide production and use of CFCs was virtually
eliminated through international agreements. However, due to the long atmospheric residence
times of CFCs, the ozone hole phenomenon persists.
To learn more about stratospheric O3 depletion, go to
http://www.epa.gov/ozone/science/index.html.
Performance Benchmark E.12.A.2
Students know the composition of Earth’s atmosphere has changed in the past and is changing
today. I/S
Common misconceptions associated with this benchmark:
1. Students incorrectly believe Earth’s atmosphere has always been a constant
composition or that changes to Earth’s atmosphere occurred only during prehistoric
times.
Students may have been instructed previously that a specific compositional percentage of
nitrogen and oxygen exist in the Earth’s atmosphere and that this percentage has not varied
from the earliest to the most recent point of their education. This may lead students to
incorrectly believe that the composition of the atmosphere is unchanging.
Students may also understand Earth of the past was up to three times hotter than it is today.
They grasp the concept of early Earth as a much more geologically dynamic planet then than
now and most have learned how the original atmosphere formed during outgassing as
countless volcanoes erupted across Earth’s surface. But, their Earth seems a much less active
place, with volcanic activity relatively infrequent. If the atmosphere is to change at all, they
may incorrectly believe it will be far in the future as Earth cools even more.
A research article discussing student misconceptions about weather, including student
misconceptions of atmospheric composition is found at
http://www.csulb.edu/~lhenriqu/NARST2000.htm.
2. Students incorrectly think the Earth’s atmosphere is a reservoir so vast that it cannot
be significantly altered by natural events or by human industry.
Students correctly visualize the atmosphere as enveloping the Earth and extending hundreds
of kilometers towards space. It seems reasonable to them that pollution from an automobile
would no more alter the atmospheric composition than a 1 pound container of table salt
would affect an ocean’s salinity. Students have observed first hand the exhaust from a car, or
smoke from a stack, simply dissipating throughout the air with no noticeable effects. Even in
cities with early morning smog, they will see as the day warms and the winds blow, the smog
will disappear, its components distributed across the county, the state, and eventually across
the world.
An excellent site to learn about human impact on the atmosphere is found at
http://www.carbonfootprint.com.
3. Students incorrectly believe Earth’s atmosphere is of a strictly terrestrial origin.
Students have generally been taught that a little under 5 billion years ago, volcanic
outgassing began to create our atmosphere and subsequently filled our oceans. It is their
belief that all the gases in Earth’s atmosphere were originally trapped within magma, and
have been released over time, first rapidly and then now more slowly. The students do not
acknowledge evidence that our atmosphere has been added to by influx of cometary gases
contributed as comets burn up in the atmosphere.
To learn more about comets and the evolution of Earth’s atmosphere, go to
http://smallcomets.physics.uiowa.edu/faq.htmlx
4. Students confuse the enhanced greenhouse effect with issues regarding the ozone hole.
The enhanced greenhouse effect and ozone hole are often discussed simultaneously, and
therefore, students incorrectly think that these two result in global climate change. In fact, the
two phenomena are not closely related. The enhanced greenhouse effect is predicted to result
from increased emission of CO2 and other greenhouse gases due to human activity. This
enhanced greenhouse effect would increase global surface temperatures resulting in global
climate change. On the other hand, stratospheric O3 depletion does not result in significant
increases in global temperatures and climate change. Although ozone holes allow more
ultraviolet light from the Sun to reach the Earth’s surface, the amount of the Sun’s energy in
this frequency is so much smaller than that received in visible and infrared.
To learn more about this misconception, go to
http://www.gcrio.org/gwcc/misconceptions.html.
Performance Benchmark E.12.A.2
Students know the composition of Earth’s atmosphere has changed in the past and is changing
today. I/S
Sample Test Questions
1. By percentage, the three most abundant gases in Earth’s atmosphere are:
a. carbon dioxide, krypton, and neon
b. hydrogen, helium, and xenon
c. argon, nitrogen, and oxygen
d. sulfur dioxide, ammonia, and chlorine
2. The most variable gas in our atmosphere is:
a. oxygen
b. water vapor
c. carbon dioxide
d. nitrogen
3. The term “anthropogenic emissions” refers to greenhouse gases released into the atmosphere
as a result of human activities. Study the accompanying graph to determine which statement
is most accurate.
a. atmospheric concentrations of CO2 have steadily decreased as anthropogenic emissions
have steadily increased
b. before 1850, humans were releasing so much CO2 into Earth’s atmosphere that the
values won’t even fit on the scale of this graph
c. careful scrutiny of the graph reveals there is absolutely no correlation between
anthropogenic emissions and atmospheric concentrations of CO2
d. since the 1960s, atmospheric concentrations of CO2 have risen at a rate approximately
equal to that of anthropogenic emissions
4. Certain atmospheric gases are thought of as “non-variable” since their abundance remains
unchanged over vast periods of time. The two gases with the longest residence time in
Earth’s atmosphere are:
a. nitrogen and oxygen
b. carbon dioxide and methane
c. carbon monoxide and chlorine
d. argon and helium
5. Earth’s early atmosphere gained oxygen through photosynthesis and ____________.
a. cellular respiration
b. photochemical dissociation
c. reverse osmosis
d. nitrogen fixation
6. Of the following, which provides evidence that Earth’s atmosphere was altered from that of a
reducing atmosphere to an oxidizing atmosphere?
a. volcanic outgassing poured vast quantities of water vapor into the atmosphere
b. levels of carbon dioxide have steadily increased over the past several decades
c. sediments rich in oxidized iron have been layered in thick deposits across the Earth
d. the majority of hydrogen & helium has escaped Earth’s atmosphere into space
7. Volcanic outgassing was a major source for Earth’s early atmosphere. Another source which
contributed significant quantities of gases to the atmosphere was:
a. sublimation of Earth’s polar ice caps and thawing of arctic permafrost
b. widespread burning of the Earth’s forests
c. ejection of gaseous matter from a nearby supernova
d. comets impacting Earth, releasing gases as the comet vaporized
8. Which of the following are the two most abundant greenhouse gases in Earth’s atmosphere?
a. water vapor (H2O) and carbon dioxide (CO2)
b. carbon dioxide (CO2) and methane (CH4)
c. ozone (O3) and carbon dioxide (CO2)
d. nitrogen (N2) and oxygen (O2)
Performance Benchmark E.12.A.2
Students know the composition of Earth’s atmosphere has changed in the past and is changing
today. I/S
Answers to Sample Test Questions
1. (c)
2. (b)
3. (d)
4. (a)
5. (b)
6. (c)
7. (d)
8. (a)
Performance Benchmark E.12.A.2
Students know the composition of Earth’s atmosphere has changed in the past and is changing
today. I/S
Intervention Strategies and Resources
The following is a list of intervention strategies and resources that will facilitate student
understanding of this benchmark.
1. The Goldilocks Principle: A Model of Atmospheric Gases.
The National Center for Atmospheric Research developed a content site with lessons useful
to the understanding of Earth’s atmosphere. Activity 1, referenced below, specifically
addresses topics in Performance Benchmark E.12.A.2.
This content and lessons can be accessed at http://www.ucar.edu/learn/1_1_2_1t.htm
2. Ozone Information and Lessons
NOAA Research provides information and lessons on many aspects of Earth’s oceans and
atmosphere. This site addresses ozone concentrations, sources and importance.
To access this site, go to http://www.oar.noaa.gov/k12/html/ozone2.html
3. Student Investigations of the Atmosphere
Penn State’s College of Education has prepared a series of lessons regarding Earth’s
atmosphere. Lesson 1 deals with the structure, composition and function of the atmosphere.
Lesson 3 further addressed ozone depletion as a change in composition of the atmosphere.
 Lesson 1
http://www.ed.psu.edu/ci/Papers/STS/gac-3/in01.htm
 Lesson 3
http://www.ed.psu.edu/ci/Papers/STS/gac-3/in03.htm
4. WebQuest: The Nitrogen Cycle
ACCENT (Atmospheric Composition Change the European Network of Excellence) contains
lessons tracing nitrogen, in its various forms, through the atmosphere and biosphere.
The WebQuest is found at
http://www.atmosphere.mpg.de/enid/08b920b96bfe49c5df374660405f0899,0/A__Activities/
WebQuest_5m4.html
5. Atmosphere, Climate, and Environment Information Program Lessons
Encyclopedia of the Atmospheric Environment has a very comprehensive set of lessons on
all aspects of the atmosphere. Many of the lessons are offered at two levels of difficulty.
To access their lessons on the atmosphere, go to http://www.ace.mmu.ac.uk/eae/english.html
6. History and Composition of the Atmosphere Lessons
GirlTECH has created a set of lessons suitable for middle school and early high school
students to teach about the history and composition of the atmosphere.
These lessons are found at http://teachertech.rice.edu/Participants/louviere/atmos.html
7. Chemistry of the Atmosphere Activity
Chemistry NOW produced an activity examining how Earth’s present atmosphere might
have evolved from possible earlier atmospheres.
To download this activity, go to http://www.chemsoc.org/pdf/LearnNet/rsc/Atmos.pdf