Performance Benchmark E.12.A.3

Students understand the role of the atmosphere in Earth’s greenhouse effect. E/S

The greenhouse effect is an increase in the atmospheric temperature caused by water vapor, carbon dioxide and other greenhouse gases that absorb and retain heat radiation which would otherwise escape from Earth. Jean-Baptiste Fourier, a French scientist, speculated as to the possibility of the greenhouse effect in 1827. Seven decades later, in 1896, a Swedish chemist named Svante Arrhenius observed, as a result of the Industrial Revolution, that more carbon dioxide was being released into the atmosphere. He believed that carbon dioxide levels would rise as industry grew. Arrhenius predicted that this increase would eventually cause a rise in the temperature of Earth.

Contrary to what many believe, the greenhouse effect is not a bad thing. Without the greenhouse effect, Earth’s average global temperature would range from around 21C to 33C cooler than it is today, which would be far too cold for the flora and fauna living on the planet. What is of concern is the “enhanced greenhouse effect,” a phenomenon in which increasing atmospheric concentrations of greenhouse gases might lead to excessive global warming. For more on the enhanced greenhouse effect, go to http://www.science.org.au/nova/016/016key.htm

For an overview of concepts associated with the greenhouse effect, go to http://www.ucar.edu/learn/1_3_1.htm

The temperature of Earth and its atmosphere is influenced by the atmosphere in a number of ways. Radiant energy from the sun encounters the atmosphere. Part of the radiant energy is reflected away, part is absorbed by the atmosphere, and part passes all the way through the atmosphere and reaches Earth’s surface (Figure 1).

Figure 1: Interactions of Solar Radiant Energy with Earth’s Atmosphere (from National Center for Atmospheric Research, http://www.ucar.edu/learn/1_3_1.htm) See Benchmark E.12.A.1 for a more complete explanation of the Sun as the source of Earth’s Energy.

Clouds (and their constituent water vapor) might either reflect or absorb solar radiation. Generally speaking, clouds situated high in the atmosphere absorb some radiation and warm the atmosphere. Clouds lower in the atmosphere, near Earth’s surface, are more reflective and have a cooling effect. Overall, clouds seem to have a slight cooling effect on atmospheric temperature.

Another family of gases which cause cooling in the atmosphere is comprised of anthropogenic (man-made) sulfur compounds. Sulfur dioxide and various sulfate gases released as ground- level pollution from automotive and industrial emissions block solar radiation. The result is a cooling effect. These sulfate aerosols have a residence time of only about a week, so they do not accumulate in the atmosphere over time the way other gases can. Thus, their effect is contingent upon their continued release in pollution.

Most people are concerned about the concentration of greenhouse gasses in the atmosphere. In too great a concentrations, the gasses might absorb much of the infrared radiation that is radiated from Earth’s surface, and lead to a dangerous increase in global temperatures. The following websites do a nice job of discussing greenhouse gases and how they affect atmospheric temperature.

PhysicalGeography.Net: Introduction to the Atmosphere http://www.physicalgeography.net/fundamentals/7h.html

Australian Greenhouse Office http://www.greenhouse.gov.au/education/factsheets/what.html

National Oceanic and Atmospheric Administration http://lwf.ncdc.noaa.gov/oa/climate/gases.html#cd

The greenhouse gases which are of greatest importance are: water vapor, carbon dioxide, methane, nitrous oxide, chlorofluorocarbons and halocarbons. Water vapor is the most abundant greenhouse gas, and is very efficient in absorbing infrared radiation, but its atmospheric concentration is highly variable. The rapid turnover of water vapor in the lower atmosphere, via the water cycle, prevents it from accumulating over time. Water vapor has a limited effect over land, but the high concentration of water vapor over warm oceans, such as the Pacific Ocean, can lead to what has been termed a “supergreenhouse effect” and a very significant increase in the temperature of the ocean surface as well as the air above it. This effect can contribute to greater storm intensity.

Carbon dioxide is the gas most often thought of during discussions on the greenhouse effect. Levels of carbon dioxide, in modern times, began increasing with the Industrial Revolution. Figure 2 “ Trends in Atmospheric Concentrations…” depicts the trend in atmospheric carbon dioxide concentration over the past 250 years. Carbon dioxide concentrations have been obtained both from measurements taken from ice cores as well as directly from the atmosphere. Clearly, carbon dioxide levels have significantly increased.

Figure 2. Trends in Atmospheric Concentrations and Anthropogenic Emissions of Carbon Dioxide (http://www.eia.doe.gov/oiaf/1605/ggccebro/chapter1.html)

There is debate over what this increase means. Some scientists are convinced further increases will inevitably lead to a disastrous increase in global temperature, with ice caps melting and sea levels rising. Other scientists point out that modern levels of atmospheric carbon dioxide are far lower than levels during our geologic past (Figure 3). The highest concentrations of CO2 during the previous 600 million years of history occurred during the Cambrian Period (nearly 7000 ppm -- almost 19 times higher than the 360 ppm of today). Scientists who argue there are many more factors to consider than just carbon dioxide point out there was an ice age during Late Ordovician while CO2 concentrations (at 4400 ppm) were around 12 times higher than today. If high levels of carbon dioxide must lead to global warming, Earth should have been exceedingly hot. Instead, global temperatures were cooler than today. Figure 3. Global Temperature and Atmospheric CO2 over Geologic Time (from Climate and the Carboniferous Period, http://www.clearlight.com/~mhieb/WVFossils/Carboniferous_climate.html)

Methane (CH4) is added to the atmosphere via microbial fermentation in wetlands, emissions from coal mines, gas pipelines and oil wells, additionally Methane is produced in the stomachs of ruminant animals, such as cattle, and is produced in large quantities by termite colonies. While methane exists at far lower concentrations (1.65 ppm) than does CO2, it is far more effective at absorbing infrared radiation than is carbon dioxide, and its concentration must be taken seriously (Figure 4).

Figure 4. Concentrations of Greenhouse Gases (from Hyperphysics, http://hyperphysics.phy- astr.gsu.edu/hbase/thermo/grnhse.html#c3) Nitrous oxide (N2O) is added to the atmosphere through the burning of biomass, the use of chemical fertilizers, and to a small extent from the combustion of fossil fuels. Nitrous oxide has a lengthy residence time, almost 120 years. Because of this, it tends to accumulate over time, causing problems in both the troposphere, where it contributes to warming, and in the stratosphere, where it contributes to the destruction of ozone.

Chlorofluorocarbons and halocarbons are found at levels from 10 ppt to 20 ppt, but, as shown in Figure 4, are around 10,000 times more effective at absorbing infrared radiation than carbon dioxide. These are introduced to the atmosphere strictly by human activity. They are used as refrigerants, solvents and fire retardants. We have begun to use a less damaging form of chlorofluorocarbon in domestic and automotive air conditioners in an attempt to ameliorate the damaging effects to our atmosphere.

For greater detail on the various greenhouse gases, visit “Greenhouse Gases and Society” at, http://www.umich.edu/~gs265/society/greenhouse.htm.

The greenhouse gases act in concert to warm Earth. As the incoming solar radiation reaches Earth, the shorter wavelength radiation (such as visible light and ultraviolet light) is able to pass through the atmosphere more easily than can the longer wavelength radiation, such as infrared radiation, microwaves and radiowaves (Figure 5). Greenhouse gases in the upper atmosphere absorb some of the infrared radiation, but much of the infrared coming from the sun is reflected back out to space.

Figure 5. A Comparison of Wavelengths of Energy from Solar Radiation (from National Center for Atmospheric Research, http://www.ucar.edu/learn/1_3_1.htm)

Some of the shorter wavelength radiation which reaches Earth’s surface is absorbed, a process which converts it into infrared radiation. This warms the surface, which in turn radiates the infrared energy outward and into the atmosphere. Greenhouse gases in the atmosphere also absorb this radiation, further increasing the temperature of the atmosphere, and re-radiating the infrared back to the Earth. Some scientists liken this to having “trapped” the infrared radiation by an atmosphere which acts as a thermal blanket.

The greenhouse gases in Earth’s atmosphere currently block and absorb only a portion of infrared (heat) radiation (Figure 6). The question is what concentration of greenhouse gases will cause excessive absorption of infrared radiation. It is a question, currently, without a definitive answer, as there is not consensus across the scientific community.

Figure 6. The Greenhouse Effect (from Saskatchewan Interactive, http://interactive.usask.ca/ski/media/drawings/agriculture/greenhouse.jpg) Performance Benchmark E.12.A.3

Common misconceptions associated with this benchmark:

1. Students incorrectly believe the greenhouse effect is a bad thing.

Students confuse “greenhouse effect” with “global warming”. Often, when students are first taught about the greenhouse effect, they are also told of the dangerous consequences of global warming. The example of Venus is presented to emphasize the extreme of what could result when concentrations of greenhouse gases get so high that they trap virtually all the infrared radiation which was emitted after surface absorption of shorter wavelength radiation. As a result, students believe we need to stop the greenhouse effect when what we really need to do is address global warming. The National Oceanic and Atmospheric Administration has an FAQ website on global warming which addresses this issue under the topic of “What is the greenhouse effect, and is it affecting our climate?”

Global Warming – Frequently Asked Questions http://lwf.ncdc.noaa.gov/oa/climate/globalwarming.html#Q1

2. Students mistakenly believe carbon dioxide is the only greenhouse gas and/or that it is the most important greenhouse gas.

When students are first taught about the greenhouse effect, carbon dioxide is very often the only greenhouse gas mentioned. Students need to learn that carbon dioxide is one of several greenhouse gases, is not the most abundant of those gases, and is not even the most efficient at absorbing infrared radiation. The accompanying websites address the various greenhouse gases and their importance.

Greenhouse? What’s that? http://www.greenhouse.gov.au/education/factsheets/what.html

Water Vapor Rules the Greenhouse System http://www.geocraft.com/WVFossils/greenhouse_data.html

3. Students incorrectly believe all infrared radiation is trapped by the atmosphere.

When students are taught that the Earth’s atmosphere acts like a blanket which traps heat near Earth’s surface, they do not understand the use of metaphor and believe that this “blanket” is able to hold all the infrared heat caused by absorption of shorter wavelengths of light on Earth’s surface. They do not see that only a fraction of the infrared radiation is “trapped” while the rest escapes to space. The following website links to an image which compares the energy of the solar radiation which enters the Earth system to the energy which leaves this system. United Nations Environmental Program Maps and Graphics http://www.grida.no/climate/vitalafrica/english/09.htm

4. Students incorrectly believe any increase in the greenhouse effect must lead to a dramatic increase in Earth’s global temperature.

Students are of the impression that increases in the greenhouse effect lead to large increases in surface and atmospheric temperatures over the short term. Earth’s temperature has risen 0.6C in the past 140 years as a result of greenhouse gases increasing global warming. Computer models suggest that doubling the present concentration of greenhouse gases would warm Earth’s average temperature between 1.5C to 4.5C. This is not a huge increase in temperature, but it would significantly alter Earth’s climate. By means of comparison, Earth’s last ice age, 18,000 years ago, occurred when Earth’s average temperature was just 5C cooler than it is today. The National Oceanic and Atmospheric Administration global warming FAQ website addresses this issue under the topics of “Are greenhouse gases increasing?” and “Is the climate warming?”

Global Warming – Frequently Asked Questions http://www.ncdc.noaa.gov/oa/climate/globalwarming.html#Q2 Performance Benchmark E.12.A.3

Sample Test Questions

1. The greenhouse effect is:

a. an undesired atmospheric phenomenon which should be eliminated b. directly responsible for the global cooling which resulted in our last ice age c. a recent atmospheric event due solely to the actions of humans burning fossil fuels d. an increase in atmospheric temperature due to gases that absorb and retain heat radiation

2. Carbon dioxide is best known of the greenhouse gases, but______is the most abundant.

a. chlorofluorocarbon b. nitrous oxide c. water vapor d. methane

3. In comparing the ability of greenhouse gases to absorb infrared radiation, ______is most effective and can absorb close to 10,000 times more infrared radiation than the others.

a. carbon dioxide b. chlorofluorcarbon c. methane d. nitrous oxide

4. Two scientists in the 1800s first recognized the possibility of the greenhouse effect as a means of warming Earth. These two are:

a. Fourier and Arrhenius b. Lewis and Clark c. Watson and Crick d. Newton and Einstein 5. Based on the accompanying graphic, after carbon dioxide, the largest source of greenhouse gases produced by the United States in 2001 was:

Figure reference: http://www.eia.doe.gov/oiaf/1605/ggccebro/images/New%20Fig%203.gif

a. HFCs b. Nitrous Oxide c. Methane d. SF6

6. As solar radiation reaches Earth:

a. only the longest wavelengths of radiation are able to punch through the atmosphere b. shorter wavelength radiation reaching the surface is absorbed and changed to infrared c. infrared radiation reacts with ozone and becomes short-wavelength ultraviolet radiation d. we find having a high albedo decreases the amount of radiation reflected back to space 7. By studying the chart below, we can tell:

a. in most years, emissions of greenhouse gases increased b. the rate of increase in greenhouse gas emissions has risen steadily c. people stopped burning fossil fuels in 1991 and 2001 d. it is impossible to decrease quantities of greenhouse gases being emitted

Figure reference: http://www.epa.gov/climatechange/emissions/downloads06/06Trends.pdf Performance Benchmark E.12.A.3

Answers to Sample Test Questions 1. (d) 2. (c) 3. (b) 4. (a) 5. (c) 6. (b) 7. (a) Performance Benchmark E.12.A.3

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. United States Environmental Protection Agency The United States EPA has a website which discusses the greenhouse effect, then lets students interact with an animated tutorial to help them understand the concepts involved.

Greenhouse Effect http://www.epa.gov/climatechange/kids/greenhouse.html

2. National Center for Atmospheric Research The National Center for Atmospheric Research provides a couple of lessons to help students understand about the greenhouse effect and what factors affect it.

What is a Greenhouse? http://www.ucar.edu/learn/1_3_2_12t.htm

What Factors Impact a Greenhouse? http://www.ucar.edu/learn/1_3_2_13t.htm

3. Michigan Reach Out! Michigan Reach Out! has a very comprehensive page providing lessons and activities to guide students as they study about the greenhouse effect and global warming.

The Greenhouse Effect http://www.reachoutmichigan.org/funexperiments/agesubject/lessons/greenhouse.html

4. The Global Warming Unit from the Michigan State University College of Education Many activities and lessons are present, with Lessons 2 & 3 being most appropriate for our topic.

Global Warming Unit Homepage http://commtechlab.msu.edu/SITES/letsnet/noframes/subjects/science/b5u1.html#plans

Lesson 2: The Greenhouse Effect Debate http://commtechlab.msu.edu/SITES/letsnet/noframes/subjects/science/b5u1l2.html

Lesson 3: The Greenhouse Effect Visualizer http://commtechlab.msu.edu/Sites/letsnet/noframes/subjects/science/b5u1l3.html 5. National Geographic XPEDITIONS National Geographic XPEDITIONS has produced a very in-depth lesson in which students explore the relationship between atmospheric carbon dioxide and climate. This is a lengthy lesson which would take several days.

Climate and CO2: Analyzing their Relationship http://www.nationalgeographic.com/xpeditions/lessons/07/g912/co2.html

6. Global Atmospheric Change The College of Education at Penn State University has created a thorough website entitled Global Atmospheric Change. Many of their lessons are appropriate to our topic. Once at the homepage of the site, scroll down and view the selections under Investigations Lessons.

Global Atmospheric Change http://www.ed.psu.edu/ci/Papers/STS/toc.html