Part III: Global Warming and Stoichiometry
Objectives and Strategies
In this section of the unit on global warming and stoichiometry, we will introduce the concept of moles within the context of the stage in the carbon cycle where CO 2 is removed from the atmosphere through the process of photosynthesis. The significance of an increase in atmospheric CO 2 will be discussed as it relates to global warming. Students will compare the quantities (moles and atoms) of CO 2 consumed through photosynthesis vs. the quantities released as a result driving their cars through the use of stoichiometry. The purpose of this section is to demonstrate each student's personal contribution to atmospheric carbon as a result of driving.
Although most scientists have reached a consensus, many students have heard mixed messages with respect to the validity of the claim that global warming is a reality. The best way to squelch these doubts is to allow the students to experience the role that data play in differentiating a scientific argument from a philosophical argument. The World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) united to form the Intergovernmental Panel on Climate Change (IPCC) to take a look at the current scientific data on the anthropogenic (human-related) impact on climate change. This global collaboration of scientists utilizes data available from peer reviewed scientific literature as the basis for their argument and has issued several conclusive reports with respect to the reality of global warming [14]. The rigor of peer review is an extremely important concept for students to understand, and these discussions should improve their ability to construct quality lab reports and prepare them to be critical consumers beyond their science class.
According to the data presented by the IPCC, global temperature is increasing at a rate that correlates with the increase in the concentration of atmospheric global warming gases [15]. However, as all scientists know, correlation does not necessarily indicate causation. In order to determine if the increase in these gases has caused the increase in atmospheric temperature, it is important to understand how energy is transferred to the Earth and how that energy impacts the chemistry of the global warming gases.
Energy from the sun can follow three different paths. It can be transmitted, absorbed, or reflected. Approximately 50% of the energy that reaches our atmosphere never reaches the surface of the Earth. It is either reflected or absorbed by the atmosphere. The radiant energy that reaches the surface is either absorbed by the Earth or radiated to the atmosphere. Of the energy that is radiated to the atmosphere, approximately 84% is absorbed by "special" gases in the atmosphere and reradiated back to the Earth. This is a natural process that keeps the Earth at a comfortable and relatively constant temperature. The problem arises when the concentration of these special gases increases such that more than 84% of the energy is reradiated back to the Earth. This causes an unbalance in the natural cycle of absorption and radiation and subsequently leads to an increase in temperature [16].
Not every gas has this special ability to absorb and radiate energy to the extent that the greenhouse gases can. The geometry of certain molecules allows them to transmit visible light, but absorb electromagnetic energy in the infrared portion of the spectrum. Ultraviolet radiation has sufficient energy to break bonds, while infrared supplies only enough energy sufficient for vibration. The tendency to vibrate is determined by the bond angles, positions of lone pairs, strength of the bonds and masses of the component atoms. If given sufficient energy to vibrate, the bonds will vibrate for a short while until they return to their unexcited state and reradiate the energy back to the Earth [16].
The most abundant greenhouse gas is water, but carbon dioxide, has received most of the attention with respect to global warming. The primary reason carbon dioxide is at the center of the discussion is because humans have not added a significant amount of water to the atmosphere. However, this does not mean that water is not a major player in the climate change caused by greenhouse gases. Increasing the temperature will increase the amount of water vapor in the air that will in turn compound the effects of increasing the concentration of the other greenhouse gases [18].
Some of the primary greenhouse gases (CO 2, N 2O, SO 2) have already been discussed in this unit in relation to their production as a result of driving an automobile. However, the contribution of internal combustion to the concentration of N 2O and SO 2 is minimal in comparison to other sources. It has been estimated that approximately 70% of anthropogenic nitrogen is a result of the agricultural industry [19], while the primary source of SO 2 is burning coal. In addition to the aforementioned compounds, methane, ozone and chlorofluorocarbons (CFCs) play a similar role in increasing the temperature of the Earth.
Carbon dioxide is generated by several natural processes, but the focus of this unit is how we can affect the anthropogenic contribution to the concentration of carbon dioxide. Carbon dioxide is captured by several natural sinks through the carbon cycle and normally the amount of CO 2 that is captured in the forests, soil, oceans and fossils is equivalent to the amount that is released through respiration and decomposition of rocks and living matter. When students begin to understand the chemistry behind global warming and the role that humans play in restoring the balance in the carbon cycle, they are empowered to take ownership over their personal contributions. In addition, I will emphasize the human impact through the use of images. There is very strong evidence that the temperature increases are melting glaciers, increasing the number of forest fires, increasing risk of disease, and increasing the risk of coastal communities to high intensity tropical cyclones (hurricanes, typhoons, etc) [20,21]. Al Gore's film, An Inconvenient Truth, presents some truly disturbing images that will undoubtedly engage students in this issue. For example, in one scene a polar bear struggles to regain its footing on a small piece of ice floating in the middle of the ocean. The use of these images will undoubtedly maintain student interest in global warming and provide the motivation to understand the accompanying chemistry.
A complementary method of engaging students in this issue is through the use of video games. Video games are a large part of youth culture and we often complain about the violence and negative influence that they can pass on to our children. In an effort to capture the influential nature of video games, there is currently a movement to develop dynamic, engaging, socially conscious and scientifically accurate video games. Microsoft is currently working on a global warming game for its Xbox 360 gaming system [17, 22].
As a hands-on experience, students will complete a lab to determine how much CO 2 a plant can consume through the process of photosynthesis [23]. Using a CO 2 probe, the students will measure the CO 2 consumption rate of a plant. They will then use stoichiometry to determine how much CO 2 they would emit as a result of driving their car and compare that to the amount of CO 2 that can be consumed by a plant (See Sample Activity 2).
Instructional Plan
This section of the unit should take about two weeks. It unit will begin with a viewing of the film An Inconvenient Truth. Next, students will measure how the concentration of CO 2 in a closed system decreases as a result of photosynthesis by a green plant. With these data as a basis, students will use stoichiometry to determine how many trees are needed to consume the CO 2 generated in driving a gasoline-fueled car for one year.
Comments: