Investigations Lesson 6 focused on the natural phenomena known as the "greenhouse effect." Investigations Lesson 7 dealt with the role the atmosphere plays in temperature regulation-the greenhouse effect. This lesson helps students distinguish between the "greenhouse effect" and the "enhanced greenhouse effect," the latter being due to the human production of greenhouse gases. It is important to note that there are two ways humans can increase the amounts of greenhouse gases. First, the processes that increase gases can be accelerated and second, the processses that remove gases (the sinks) can be decreased. Human production of CO2 currently is responsible for the majority of the enhanced greenhouse effect. Investigations Lesson 9 focuses on alternative conceptions many students have surrounding ozone depletion and the greenhouse effect. Investigations Lesson 10 is a laboratory exercise on CO2.
This lesson helps learners:
The concept map shows model relationships among concepts this lesson seeks to develop. Concepts introduced in this lesson are bolded on that concept map and concepts from other lessons are in plain text (not bolded).
Human-produced greenhouse gases and their natural and human-produced sources are outlined below. The greenhouse gas "water vapor" is not included in this presentation or in the instructional procedures. However, it does originate from natural sources as well as human activity (e.g., irrigation). Water vapor has been excluded from the instruction in this lesson because a study of its contribution to the enhanced greenhouse effect also involves clouds and yet warrants considerable study by scientists. A brief presentation on water vapor and clouds follows.
Water Vapor and Clouds - Scientists presently are uncertain about the degree to which water vapor enhances the greenhouse effect, and the contribution of water vapor is intertwined with other issues, such as increases in other greenhouse gases and the role clouds play in the greenhouse effect. For example, as human activity contributes more carbon dioxide and other greenhouse gases to the atmosphere, the atmosphere likely will warm. In turn, a warmer atmosphere results in more evaporation, and probably, an increase in the amount of water vapor in the atmosphere. More water vapor in the atmosphere leads to more absorption of infrared energy and also can lead to more clouds. Clouds, due to their water vapor content, absorb infrared energy emitted by earth: Through this phenomena, clouds enhance the greenhouse effect. However, clouds also reflect back to space some of the incoming solar radiant energy, and thereby decrease the amount of visible light that is absorbed and subsequently re-emited as infrared energy by earth's surface. The latter phenomena would lessen, not enhance the greenhouse effect. Put another way, the reflection of incoming solar radiation by clouds contributes a "negative feedback" to the enhanced greenhouse effect. The role of clouds in the enhanced greenhouse effect depends also on the type of clouds, where they are located, and other factors, which scientists need to learn more about. However, there is no doubt that clouds are a very important consideration in forcasting the probability and severity of global warming.
Human. The largest source of CO2 is combustion of fossil fuels (power plants, factories, automobiles). The second largest source is the conversion of forested areas to pasture and agricultural use. CO2 is released through the burning and decay of cleared vegetation and soil organic matter, and deforestation removes a CO2 sink or uptake. CO2 is also released during the manufacture of cement.
Human. Sources include rice paddies, municipal and industrial landfills, domesticated ruminants (e.g., cattle, sheep), incomplete combustion during biomass burning, and fossil fuel production (natural gas wells, coal mining).
Human. Sources include combustion of fossil fuels (automobile exhaust), use of nitrogenous fertilizers, and biomass burning in the tropics.
Human. Although ozone in the lower atmosphere does not come directly from motor vehicles, they are major sources of ozone precursors throughout the industrialized world. Increased ozone contributions in the troposphere are attributed to photochemical reactions involving primary pollutants (CO, NO, SO2, and hydrocarbons) emitted as a consequence of the incomplete combustion of fossil fuels from industrial plants, homes, and most importantly from motor vehicles. Biomass burning (destruction of the rain forests) also contributes precursors to ozone.
Human. CFCs are human-made products released when using aerosol propellants, refrigerants, coolants, and foam-blowing agents. Aerosols (e.g., spray cans) in the U.S. have not contained CFCs since 1979. The remainder of these background notes, as well as the notes and resource materials listed for Investigation Lesson 3, provide additional information on CFCs and the phase out of these substances through the use of CFC substitutes (e.g., HCFCs and HFCs).
In the chart that follows, natural levels of these gases in the atmosphere are compared to levels as a result of human activity. Note also the atmospheric lifetime (residence time), the annual rate of increase, and the relative contribution of each gas to the human-produced (anthropogenic) greenhouse effect. Carbon dioxide accounts for greater than half of the warming potential caused by human activity. Methane accounts for approximately 15% of the contribution to the human-produced greenhouse effect but molecule for molecule, CH4 traps heat 20-30 times more efficiently than CO2. Nitrous oxide is a long lasting gas which eventually reaches the stratosphere and helps destroy the ozone layer. CFCs are the most powerful of the greenhouse gases and are increasing at the fastest rate (one CFC molecule has about 20,000 times the heat-trapping power of a CO2 molecule). Tropospheric O3 has the shortest residence time but is increasing at a rate second only to CFCs.
As described in Investigation Lesson 3, CFCs also destroy ozone in the ozone layer: Each CFC molecule can destroy 10,000 or more O3 molecules in the stratosphere. However, to be clear: The thinning of the ozone layer does not enhance the greenhouse effect or cause global warming. The latter is important to point out , as the idea that the "ozone hole" causes global warming, or that CFCs cause global warming by destroying the ozone layer, are misconceptions commonly held by students. However, the resulting increase in UV rays at ground level is biologically damaging and has become an international concern. Because of this, recently all of the countries (including the United States) who are major developers and users of CFCs, have signed an agreement to phase out these substances and have been rapidly reducing their use of CFCs by employing CFC substitutes. Given this big reduction in CFC use, how is it, then, that CFCs are increasing in our atmosphere at a rate faster than any of the other greenhouse gases (as note above)?
One reason is that CFCs have a very long atmospheric lifetime: Because they stay in the atmosphere from 60 to 100 years, a reduction in use will not reduce decrease the amount in the atmosphere for many year. Another more important reason was introduced in Investigation Lesson 3: CFCs are still escaping into the atmosphere from products in junkyards and landfills that contain CFCs, e.g., refrigerators, freezers, and some types of foam products.
CFC substitutes include HCFCs and HFCs. Employing these substitutes as opposed to CFCs is certainly a step in the right direction, as they have considerably less or no ozone destroying capacity and a shorter atmospheric lifetime. However, some of these substitutes, such as HCFCs and HFCs, still have a powerful heat-trapping capacity, i.e. absorb infrared energy emitted by earth. Therefore, some of these substitutes are considered to be human-produced greenhouse gases that do enhance the greenhouse effect and can contribute to global warming.
| CO2 | CH4 | N2O | CFCS | O3 | |
| <---------------------------ppb--------------------------> | |||||
| (parts per billion) | |||||
| Natural level in atmosphere:(Pre-industrial levels in parts per billion [ppb] by volume at surface) | 280,000 | 790 | 288 | 0 | 10 |
| Level in atmosphere as result of human activity: (Data of 1991; ppb) | 353,000 | 1720 | 310 | 76* | 20-40 |
| Atmospheric Lifetime | 50-200 yr | 10 yr | 150 yr | 60-100 yr | wks-mnths |
| Present Annual Rate of Increase | 0.5% | 0.9% | 0.3% | 4% | 0.5-2.0% |
| Contribution to enhanced greehouse effect: | 60% | 15% | 5% | 12% | 8%** |
*Refers only to the sum of the amount of two CFCs: CFC-11 and CFC-12. Collectively, the amount of all CFCs as of 1991 was approximately 85 ppb.
**Although this report gives a specific quantity for the contribution of tropospheric ozone to the greenhouse effect, other reports give a range (e.g.. 5-10%) or do not give a specific quantity. In reference to the latter, the IPCC (Inter-governmental Panel for Climate Change) report of 1992 recognizes the contribution of tropospheric ozone to the greenhouse effect, but does not feel that data at present is sufficient to quantify this.
Students in the expert groups for the various human-produced greenhouse gases should be called upon as experts in future lessons to give specific data as needed about the different gases. The expert students should become an important information resource for the class.
Expert:______________________________
SOURCES:
NATURAL LEVEL IN ATMOSPHERE:
LEVEL IN ATMOSPHERE AS A RESULT OF HUMAN ACTIVITY:
ATMOSPHERIC LIFETIME:
PRESENT ANNUAL RATE OF INCREASE:
CONTRIBUTION TO ENHANCED GREENHOUSE EFFECT:
OTHER INFORMATION:
| Group Members | |
| 1. _______________________ | 3. _______________________ |
| 2. _______________________ | 4. _______________________ |
| CO2 | N2O | CFCs | O3 | |
| SOURCES | . | . | . | . |
| NATURAL LEVEL IN ATMOSPHERE | . | . | . | . |
| LEVEL IN ATMOSPHERE AS A RESULT OF HUMAN ACTIVITY: | . | . | . | . |
| ATMOSPHERIC LIFETIME: | . | . | . | . |
| PRESENT ANNUAL REATE OF INCREASE: | . | . | . | . |
| CONTRIBUTION TO ENHANCED GREENHOUSE EFFECT: | . | . | . | . |
(Adapted from Crane, R. and Borza, K. (1993). Global Warming.
University Park, PA: The Pennsylvania State University.)
Human-produced greenhouse gases include carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons. They are produced both naturally and by human activity and these sources will be examined.
Human Source. The amount of CO2 in the atmosphere remained constant for millions of years until the 1800s when the Industrial Revolution began. Large amounts of CO2 began to be produced from the burning of fossil fuels upsetting the natural balance. The largest human source of CO2 in the atmosphere is the burning of fossil fuels from vehicles or factories. The second largest source of CO2 is land-use change from forest to agriculture. CO2 is released through the burning and decay of plants and the loss of forest represents the loss of a sink or uptake for CO2. Much more CO2 is added to the atmosphere than is removed resulting in a net increase of CO2.
Human Source. Rice paddies and municipal and industrial landfills produce methane. Other sources include domesticated ruminants (cows, sheep), incomplete combustion during biomass burning in the tropics, fossil fuel production, leaks from natural gas pipelines, and coal mining.
Human Source. Three main sources of N2O are the combustion of fossil fuels, use of nitrogen-containing fertilizers, and biomass burning in the tropics.
Human Source. CFCs are human-made products released when using aerosol propellants, refrigerants, coolants, and foam-blowing agents. AEROSOLS (e.g., SPRAY CANS) IN THE U.S. HAVE NOT CONTAINED CFCs SINCE 1979. The remainder of these background notes, as well as the notes and resource materials listed for Investigation Lesson 3, provide additional information on CFCs and the phase out of these substances through the use of CFC substitutes (e.g., HCFCs and HFCs).
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