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TIPS:
Targeted Interventions for Proficiency in Science |
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Earth Science
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| 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 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).
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 Australian Greenhouse Office National Oceanic and Atmospheric Administration 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
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.
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).
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
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.
Performance Benchmark E.12.A.3 Students understand the role of the atmosphere in Earth’s greenhouse effect. E/S 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
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? Water Vapor Rules the Greenhouse System
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
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 Performance Benchmark E.12.A.3 Students understand the role of the atmosphere in Earth’s greenhouse effect. E/S Sample Test Questions 1. The greenhouse effect is: 2. Carbon dioxide is best known of the greenhouse
gases, but___________ is the most abundant. 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. 4. Two scientists in the 1800s first recognized
the possibility of the greenhouse effect as a means of warming Earth.
These two are: 5. Based on the accompanying graphic, after carbon dioxide, the largest source of greenhouse gases produced by the United States in 2001 was:
a. HFCs 6. As solar radiation reaches Earth: 7. By studying the chart below, we can tell:
Students understand the role of the atmosphere in Earth’s greenhouse effect. E/S Answers to Sample Test Questions Performance Benchmark E.12.A.3 Students understand the role of the atmosphere in Earth’s greenhouse effect. E/S 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 Greenhouse Effect
2. National Center for Atmospheric Research What is a Greenhouse? What Factors Impact a Greenhouse?
3. Michigan Reach Out! The Greenhouse Effect
4. The Global Warming Unit from the Michigan
State University College of Education Global Warming Unit Homepage Lesson 2: The Greenhouse Effect Debate
5. National Geographic XPEDITIONS Climate and CO2: Analyzing their Relationship
6. Global Atmospheric Change Global Atmospheric Change |
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