Content Benchmark E.12.A.2
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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.

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_billion_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.

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.

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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.

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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)

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)

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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/08b920b96bfe4
c5df374660405f0899,0/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

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