Background on the Atmosphere - Week 1
In order for my students to grasp the far-reaching effects of pollutants, they must understand the atmosphere and its properties. Our state science curriculum includes a unit in eighth grade on meteorology that should cover these topics, but from talking with freshman students, these things are not well understood. Meteorology is a great topic to teach to students also, since it is really relevant to their lives, and it lends itself well to many engaging laboratory based experiences.
The Earth was formed some five billion years ago, and was probably so hot that it did not retain any of the primitive atmosphere it had to begin with as indicated by the current absence of noble gases. The first atmosphere most likely consisted of hydrogen, helium, methane, ammonia, nitrogen, neon and a small amount of argon. Scientists assume that volcanoes five billion years ago emitted the same gases as today consisting of nitrogen, water vapor, and carbon dioxide. The gases were expelled from the Earth's interior by a process known as outgassing. Since nitrogen is not very chemically active, the expelled nitrogen filled the atmosphere. The large amount of water vapor expelled resulted in the formation of clouds which in turn, produced rain. This rain accumulated on the Earth's surface in river, lake and ocean basins. Luckily for us these large water reservoirs acted as large sinks for the carbon dioxide. Also, on the continental portions of the early Earth the calcium silicates (CaSiO 3) contained combined with the carbon dioxide to form quartz (SiO 2) and calcium carbonates (CaCO 3) binding up even more of the carbon dioxide into solids on the surface and kept CO 2 from filling the atmosphere. Due to the distance from the Sun, the Earth was cool enough for this sequestering of carbon dioxide in these two processes to operate. To give you a perspective as to the significance of the distance lets look at our neighbor Venus. The primitive atmosphere of both planets was the same but; the temperature of Venus without an atmosphere was a warm 20 oC, too hot for the efficient formation of carbonates so almost all of the carbon dioxide expelled from the interior of the planet accumulated in the atmosphere. Thus 90-95% of Venus's atmosphere today is made up of carbon dioxide, and with this type of atmosphere the surface of Venus is approximately 400 oC, which melts lead, destroys all familiar life, and is essentially a dead planet compared to the Earth which has a surface temperature of approximately 10 oC and flourishes with biodiversity.
While oxygen makes up 21% of the current atmosphere of the Earth there was only a trace in the atmosphere when life originated on Earth. Single celled bacteria were the first life forms and did not need oxygen to live. Oxygen first appeared on the planet when these bacteria developed the ability to split water molecules apart by harnessing the energy of sunlight, making them the first photosynthetic organisms. They produced enough oxygen that accumulated over geologic time. With these processes acting simultaneously they have produced the delicate balance of 78% nitrogen and 21% oxygen we observe today. There are other gases present in the atmosphere. The permanent gases, concentrations remain relatively constant, are oxygen and nitrogen, as well as neon and argon. These gases are not involved in the global warming process and thus we need not spend a lot of time on them with the students. The reason why these gases are not greenhouse gases (GHGs) is because their molecular structure is transparent both to incoming solar radiation as well as outgoing infrared radiation. The variable gases, however, will need to be discussed in detail with the students. These gases are the main greenhouse gases, and include water vapor (approximately 0-4%), carbon dioxide (approximately 0.035%), methane (approximately 0.0002%), and ozone (approximately 0.000004%). What makes these greenhouse gases is that their molecular structure is made up of three or more atoms, making them transparent to incoming solar radiation and opaque to outgoing infrared radiation so they trap in the transmitted infrared radiation in the atmosphere heating it up and upsetting the natural balance. A good activity is to have students construct three-dimensional models of these molecules in order to see their heat trapping ability. [1]
The lowest layer of the atmosphere where the right mixture of gases can support life is a very tiny layer of our atmosphere, approximately ten miles up from the Earth's surface, called the troposphere. Within this small layer almost all of our weather and climate is created. The moisture available to the planet is encompassed in this small layer, as well as the balance of all the gases that have been added to our atmosphere through the human experience. To give the students perspective you can quote Carl Sagan "If you had a globe covered with a coat of varnish, the thickness of that varnish would be the thickness of the atmosphere compared to the Earth itself." Once they visualize that, they should have the realization of how small and vulnerable our immediate atmosphere is. There are many other upper layers that make up the outer atmosphere but again they do not play a lead role in global warming. Hopefully they have had this information in prior grades; in Delaware it is part of the middle school science curriculum. One misconception you should address with your students when going over the structure of the atmosphere is the stratosphere and ozone. In the stratosphere, oxygen atoms (O) are formed through the photolysis of oxygen (O 2) by harmful ultraviolet (UV) light. The products of this lysis form ozone molecules (O 3). Ozone is in turn photolyzed to regenerate more oxygen atoms and oxygen by absorbing harmful UV light, thus protecting the biota on Earth. Many will tell you that global warming is caused by the ozone hole and that is simply not true. The two issues are apples and oranges; two totally different issues. You can however discuss the fact that the ozone hole is now closing up due to governmental legislation eliminating the uses of chlorofluorocarbons (cfcs), and with the newly developed chemicals we have reversed this environmental problem giving us hope that our efforts to combat global warming may make a difference if we act now. Have students read an article about the shrinking of the ozone hole due to these regulations that were imposed on the chemicals that caused the problem. [2]
The climate system is a very complex system driven by the energy of the Sun, involving the oceans, the atmosphere, various Earth surfaces, and plant and animal life. There are so many factors that affect climate and the weather that it is very hard to make long-term predictions. Substantial uncertainties are present in predictions over more that a three-day period. One thing you must discuss with your students is the difference between weather and climate. They are not one in the same! Simply, weather is what changes from hour to hour, day to day, and month to month. Climate describes how the weather tends to be in a certain place over the long term; you may want to term it "average weather". It is really important to understand the difference between weather and climate to understand the issue of global warming. I use an example when talking with students to help them distinguish this difference. If on a day in July you are asked, what has the weather been like in Delaware? You may answer, "It was hot and muggy yesterday, but today it is clear and cool." On the other hand, if you were asked what the climate is like during the summer in Delaware you would be correct to answer, "in the summer it is hot and muggy." They must realize that even though a day happens to be clear and cool, that does not mean the climate has changed. When climate does change, it usually changes relatively slowly and this change is very slight, changing a fraction of a degree over period of tens of years.
Factors that determine the climate of an area are numerous. One of the major factors is the proximity to the ocean. Ocean currents can increase or reduce temperatures depending upon their origin. Coastal areas are cooler and wetter than inland areas. There are a number of reasons for this. One is that clouds form here more readily since the warm air from the land meets with the cool air from the ocean. This phenomenon will change depending upon the ocean temperature during the different seasons of the year, relative to the adjacent continental temperature. Generally, the more inland an area is, the less moisture and thus the hotter it is since the moisture from the oceans evaporates before it reaches the center of the continent. The direction of the prevailing winds also makes a difference in the climate since if they move from the ocean to the continent they may carry substantial amounts of moisture; on the other hand if they are moving from the center of the continent they may move drier air. Altitude plays a major role in the determination of the climate of an area. Mountains are a great example (not in Delaware since we are as flat as it gets…) since mountains receive more rainfall than the low lying areas because the temperature of the top of the mountains is lower and closer to the dew point than the temperature at sea level. The higher the place above sea level, the colder it will be since in the troposphere as altitude increases, air becomes thinner and is less able to absorb and retain heat. That is why you see snow on the tops of mountains all year round although global warming is altering that. Proximity to the equator is a large determinant of the climate since the equatorial countries receive more solar energy than anywhere else on Earth due to its position in relation to the Sun. The equatorial regions receive full Sun incidence, whereas in the higher latitudes the same amount of energy is spread out over a wider area due to the tilt of the Earth. Have students determine the factors that affect their local climate. They can look at maps to see the prevailing winds, determine their height above sea level, and their proximity to the ocean. Students need to access information online to determine the local climate of their area and relate how this climate is derived.
Air pressure creates the constant changing skies and sets winds in motion. Air pressure is the force air exerts on everything it touches. Air moves at variable speeds and the speed responds to pressure differences that depend on temperature. Warm the air up and the speed increases, cool it off and it slows down. The impacts of the billions of bouncing air molecules are what determine the pressure. Air pressure depends upon the sheer number of air molecules in a given space and how fast they are moving. Winds are the result of the action of various forces that are happening simultaneously. The main force that starts winds in motion is air pressure. Higher pressure pushes air toward lower pressure. The greater the difference in pressure, the faster the wind moves. There are other forces that help to determine how strong the wind will be and which way they will blow. Pressure gradient force is the key factor in the strength of winds. This force is affected by the size of the pressure difference between high and low as well as the distance between them. The closer together the high and low pressure areas are and the larger the difference in pressure, the stronger the winds. Friction is a big factor in velocity of the winds. Obviously, the nearer to the surface and the more buildings, trees, and objects it rubs up against the slower the winds. There is much less friction over water than land which is a key factor in hurricane formation. And lastly the wind will not move in a straight line due to the Coriolis Effect. This phenomenon is due to the spinning of the Earth, which is why winds curve when they blow. Using the information from the Wikipedia article on atmospheric circulation, have students chart winds to determine their origins and the temperature effects they will have on the climate. [3]
Albedo is Latin for white and is an essential concept to address with the students. Albedo is the reflectivity of surfaces. Albedo is expressed by a number between zero and one. An albedo of one would indicate the highest reflectivity possible like a perfect mirror. On the other hand an albedo of zero would indicate a perfect absorber of radiation which scientists call a black body. But obviously the surfaces of Earth will fall in between this range. This is a key aspect to greenhouse warming. Common Earth surfaces such as glacial ice reflect most of the solar energy and have an albedo of approximately 0.8, whereas the oceans have a much lower albedo and absorb the energy of the Sun with an albedo of 0.2. The albedo of the Planet Earth is 0.4, which equates to 40% of all the incident solar radiation being reflected by the Earth and 60% being absorbed. There is much speculation as to the amount of change in albedo produced by the increasing melting ice around the globe. There are many labs that model this concept that you would be able to do with your students. A very simple one would be just using colored paper. [4]
Water vapor is a natural greenhouse gas, and accounts for the largest portion of the greenhouse effect. Increasing temperature will increase vaporization and the amount of water vapor in the troposphere which will magnify the effect of the greenhouse gases. On the other hand, the increase in water vapor may increase the formation of clouds, and more clouds, will increase the albedo thus reducing the amount of solar radiation that is absorbed. This could be such a factor as to reverse the warming and actually cause cooling. The lesson canned cloud in the unit addresses cloud formation in a learning cycle approach.
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