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IDS 102Trace Gases in the Atmosphere

“The Greenhouse Effect”Imagine that you have a light source and some way to detect the intensity of light at various distances. If you increase the distance of the detector from the light bulb, the intensity of the light decreases dramatically. From lab measurements of this type and knowledge of the energy radiating from the Sun, we can predict the average temperature of the surface of various planets.

As you may know, Venus is the second planet from the Sun, while the Earth is the third planet out from the Sun. Since Venus is closer to the Sun, it is reasonable that the average surface temperature of Venus should be higher than the average surface temperature of Earth. If we do some calculations we find that the temperature of Venus’s atmosphere should be around 67C. This compares to the average temperature of Earth’s atmosphere at about 15C. This sounds logical, but the temperature at the surface of Venus is about 460C! The purpose of this module is to help you understand how the greenhouse effect has made the Earth a place in which life can exist, while on Venus, the same process produces temperatures hot enough to melt lead at the surface.

The atmosphere of Venus is about 96% carbon dioxide (CO2). This gas has a major role in creating what we term a “greenhouse effect.” Before we discuss the role of CO2 in both the Earth’s and Venus’s atmosphere, we need to understand more about electromagnetic radiation, waves, reflection, transmission, and most importantly about absorption.

An aside: What is even more amazing is that about 80% of the solar radiation arriving at Venus is reflected back into space. (For comparison, about 29% of the energy arriving at the Earth is reflected back into space.) The reason for this large reflectance value for Venus is the density of the atmosphere of Venus. The atmosphere of Venus is 90 times as dense as the Earth’s atmosphere. (Is the air pressure on Venus higher or lower than on Earth?)

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WAVES, LIGHT, and the ELECTROMAGNETIC SPECTRUMWhat is a wave? For our purposes, we will think of a wave as something that travels from one place to another. The shape of a wave is usually something like alternating bumps and valleys: first a bump, then a valley, then a bump, and so on. Here is a cartoon of a wave going by on the surface of a body of water, moving to the right.

As you may have noticed, there is a hummingbird hovering just above the surface of the water in the middle of one of the "valleys" (between two bumps).

If our hummingbird continues to hover in that specific place in space (so that it does not move), what will happen to it as the wave moves? (Think about it! This may seem obvious, but it is an important point about the behavior of waves.)

You probably concluded that the hummingbird would get wet. Very good. Now for the important point about waves…

Two students are arguing. Student #1 says that waves move back and forth in a zigzag type motion. Student #2 says that unless something gets in the way waves move (for the most part) in straight lines. "The shape of the wave," Student #2 says, "is not the same as the direction that the wave is going." Based on your ideas about the wave above, which student do you and your hummingbird agree with? Did that wave move in a straight line or in a zigzag?

Among the other things that waves do, they carry energy. Our poor hummingbird was smacked with the energy of a passing water wave.

Look at the two waves below. Imagine that you saw the surface of the sea on a day when it appeared like wave #1 and on a day when it appeared like wave #2. On which day would you say the sea had more energy? (assume waves are the same height)

Hopefully by now you have concluded that waves travel in straight lines, not zigzags, and that waves in which the crests are close together seem to carry more energy than long

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waves. If you are having trouble believing either of these things, you are not alone. They are two of the most important and most misunderstood properties of waves.

Light WavesTo understand the greenhouse effect on Earth, we must first understand the concepts of emission, absorption, and reflection. To understand these topics we will use light. Light travels in waves. Remember, this does not mean that light travels along a zigzag path. It means that light travels in "packages" that are shaped like waves (we call them waveforms).We usually think of a wave as something that goes by in lumps and bumps. First one bump goes by, then another, and then another. We don't notice light going by that way because the bumps are so small and they go by so fast. When a wave of orange light reaches our eyes, for example, there are half a million bumps crammed into every foot, and it only takes a nanosecond (one billionth of a second) for those half a million waves to go by. Still, it is precisely the size of those waves, so small that we can fit half a million of them into one foot (or more precisely 1.5 million into each meter) that tells our eyes that we are looking at orange light and not blue light (2 million waves per meter) or red light (1.3 million waves per meter).Imagine you have two lights that are equally bright, a blue light and a red light. The blue light packs 2 million waves into each meter. The red light only gets 1.3 million waves into each meter.

We call the length of a wave (take a guess) the wavelength. Which one has longer waves, the blue light or the red light? Explain your reasoning.

Which one carries more energy, the blue light or the red light? Explain your reasoning.

It turns out that the blue light and the red light move with the same speed (three hundred million meters per second). We call the number of waves that pass each second the frequency. Which one sends more waves past your eye per second? Explain your reasoning.

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The visible light spectrumWhen we compare blue light to red light we see that blue light has a shorter wavelength, higher frequency, and carries more energy for the same amount of brightness (red light has a Longer wavelength, Lower frequency, and Less energy – the “L”s go together). Still, what’s the fun of knowing that if you don’t understand color? It turns out that most of us have eyes that detect three colors of light: Red, Green, and Blue. Some people detect fewer colors (they have partial color blindness) but nobody detects more.* Every other color you have perceived in your life has been a mixture of those three colors of light. Every color on a computer monitor is a combination of red, green, and blue dots.ACTIVITY #1: Open Microsoft Word to a new document page and look at the white page on the screen with a magnifier. See all of the pretty red, green, and blue dots? Cool, huh?ACTIVITY #2Find a computer and go the following web site (this site is also on our “links” page on the IDS web site):http://mc2.cchem.berkeley.edu/Java/emission/Java Classes/emission.html

What happens when you have red and green at the maximum intensity?’

What happens when you have green and blue at the maximum intensity?

What happens when you have red and blue at the maximum intensity?

What happens if you have all three colors at the maximum intensity?

What combination produces orange?

ACTIVITY #2B: Somewhere around the room find a “light box” that emits all three colors of light. Don’t pick up the light box; they fall apart easily. Move the mirrors around to make different mixtures of red, green, and blue light (if you want to block one of the colors of light, try putting a hand or a sheet of paper in front of it).

What color do you see when you mix red and green light? What color do you see when you mix green and blue light? What color do you see when you mix red and blue light?

* Technical detail: we can still see a single wavelength of light, even if it has a wavelength somewhere between green and red. When we see that wavelength, it triggers the receptors in our brain for both green and red, but not as strongly as if we saw only green or only red light. The curious thing is that we can't distinguish between a yellow light that is all one wavelength, and a mixture of red and green light that appears to be the same shade of yellow. We also see violet light even though it has a shorter wavelength than blue light. Our eyes are not very sensitive to violet light, however, and violet light has to be very bright for us to perceive it as being equally bright with, say, green light.

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What color do you see when you mix red, green, and blue light?

We say that red , green and blue are the primary colors of light. When we see all three colors mixed equally, our eyes perceive that as “white light,” so you can think of white light as an equal mixture of red, blue, and green.

Before you go to the next web page, imagine that you have some white light. If you could absorb all of the blue light from it, what color would remain?

Before you go to the next web page, imagine you have some white light. If you could absorb all of the red light from the white light, what color would remain?

ACTIVITY #3:

Next go to the following site and check your answers:http://mc2.cchem.berkeley.edu/Java/absorption/Java Classes/absorption.html

What is a definition for absorption?

ACTIVITY #4: Go to the next web page:http://mc2.cchem.berkeley.edu/Java/single/Java Classes/single.htmlBy playing with the controls on this web site, create a definition for a filter. How is an optical filter different than something like a “water filter”?

If you are looking through a red filter at a white object, what color will it appear? If you are looking at a yellow object through a red filter what color will it appear? If you look at a blue object through a red filter, what will you see?

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In Summary: Absorption, reflection, and transmissionWhen light encounters a substance, there are three things that can happen, and sometimes they all happen at once.

1. The light can be reflected which means that it bounces off. It changes direction, but aside from that it is pretty much unchanged. A mirror is very smooth and it reflects light all in the same direction. A piece of sandpaper is rough and it scatters light in all directions. Most objects are somewhere in between. Most of the objects we see in our everyday world reflect light to our eyes—that is why we see the objects. (Some people have the misconception that in a totally dark room, your eyes will eventually adjust so that you can see objects in the room. This is not true! If there is no light to reflect off an object, we would not see the object!)

2. Light can be absorbed which means that the energy in the light is absorbed by the substance. Something that absorbs some colors (or wavelengths) of visible light is called a pigment and it is what we use to make paint. When light is absorbed, the light is gone but the energy remains in the substance in another form. (Hint of things to come: the energy usually comes back out!)

3. Light can be transmitted which means that it passes through the substance. A window is clear because visible light is transmitted. Stained glass appears brightly colored because some colors (or wavelengths) are absorbed and others are transmitted. Something that transmits some wavelengths but not others is called a filter.

Check your understanding with the following questions:

Imagine that white light were to hit a substance that absorbed all of the blue light so that a mixture of red and green light was reflected. Read that sentence again and ask questions if you don’t understand. When your eye detects the red and green light that is reflected, what color would your eye see? What color would you say this substance is?

Imagine that white light were to hit a substance that absorbed all of the green light so that a mixture of red and blue light was reflected. When your eye detects the red and blue light that is reflected, what color would your eye see? What color would you say this substance is?

A substance that absorbs some colors and reflects others is called a pigment. We say that the three primary colors of pigment (or paint) are yellow, cyan, and magenta. (In primary school you probably learned that the primary colors of paint were red, blue, and green, but you never could get that cool magenta or turquoise color, could you?)

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ACTIVITY #5: Find some colored paper (pigments) and filters (translucent plastic). You should have magenta, yellow, and cyan sheets of paper and at least a red and blue filter. (Our cyan paper is not truly cyan, but it is close!.)

White light is hitting each of your sheets of paper. Think of which two colors are reflected by each of them:

Cyan Magenta Yellow

The red filter only lets red light through. How will the three sheets of paper appear through the red filter? Make a prediction and then place the three sheets of paper so that they are overlapping but you can see all of them. Place the red filter over them and record your observations. Do you understand why you see what you see?

The blue filter only lets blue light through. Which two sheets of paper will look the same through the blue filter? How will the other one appear? Make a prediction and then repeat the experiment with the blue filter. Record your observations.

If you understood the previous sections on visible light, you have understood a great deal. Light does not just come in one wavelength (color). There is a whole spectrum of colors. The spectrum is the complete collection of all possible wavelengths. When we separate all of the different wavelengths that are hidden in white light, we see the spectrum as a rainbow.ACTIVITY #6: Put on a pair of "rainbow glasses" and try not to look silly. The rainbow glasses contain diffraction gratings, which separate white light into a spectrum the same way that prisms do.

1. Look at a white light (use an incandescent light - not a fluorescent light) through the glasses. You should see lots of rainbows stretched out in many directions. Ask your instructor to increase or reduce the energy that the light is producing.

When the energy is increased, what happens to the brightness of the light?

The total amount of light increases when the brightness is increased. Now think about the fraction of the light that appears as different colors. When

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the energy is increased, what happens to the relative amount of blue light? What happens to the relative amount of red light?

The incandescent bulb produces light simply because it is hot.

2. Some other light sources (fluorescent bulbs, neon lights, sodium lights) produce light through specific atomic changes. Look at a neon light, sodium light, or other chemical gas light through the glasses.

What do you notice about the spectrum produced by neon light, sodium light, or other chemical gas light?

How would you describe the difference between the spectrums produced by the incandescent bulb and the chemical gas bulb? The spectrum of the incandescent bulb appears to be more… more what?

Think about rainbows that you have seen in the sky. These are the spectrum of the Sun. What does this tell you about the spectrum of the Sun? Is the spectrum more similar to the incandescent bulb or the gas bulb?

We are progressing toward an understanding of the Earth’s “greenhouse effect”. The process involves the transmission, absorption and emission of energy in the form of waves. Please let us know if you do not understand these properties.

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from : http://www.lbl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.htmlElectromagnetic wavesMost of the waves we are familiar with, such as waves in water and sound waves, require a medium (or substance) to travel through. However, light is part of a spectrum of electromagnetic radiation that will travel through a vacuum (no substance). Electric and magnetic "fields" can carry waves the same way the surface of a body of water can. Light travels along as "bumps and valleys" of electrical "pushes and pulls." Honest.When this property of light was discovered, it immediately raised a question. We see electromagnetic waves (EM waves) with wavelengths between 450 nanometers (blue) and 700 nanometers (red). We call them light. Are there EM waves with longer wavelengths? Shorter wavelengths? The physics of electricity suggested that there would be, but we could not see them.We now know that there are EM waves with wavelengths thousands of times shorter than blue light (and thus energy thousands of times greater than light). There are EM waves with wavelengths longer than light, too. Our eyes don't detect them, but they are important in nature and we use them in technology.ACTIVITY #7: Using the EM chart above rank order the list of wave types in order (1 means largest wavelength) from those types that have the longest wavelengths to those that have the shortest wavelengths microwavesradio wavesgamma rayshard (used to study rocks) x-rayssoft (medical) x-raysinfraredultravioletvisible light

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From the same list as the previous question, rank the types of energy according to the amount of energy carried by each type of wave in the space below (1 means the greatest energy):

Recall that waves that have a short wavelength have the highest energy. This is the reason that there are limits to amount of x-rays a person should be exposed to during a certain period of time (this is mostly an issue for x-ray technicians rather than the patients). Fortunately for us, the Sun does not produce a lot of gamma rays and x-rays. Most of the gamma rays and x-rays that come to the Earth from elsewhere in the universe are absorbed in the far upper atmosphere (above the troposphere). The small amount of high energy EM radiation reaching the Earth is a good thing because otherwise life on Earth as we know it would not be possible.

A short review: As electromagnetic radiation from the Sun arrives at the Earth, what are the three

things that can happen to this energy?

Recall that energy can be reflected, absorbed and/or transmitted. The climates of the Earth and Venus are dependent on the amount of reflection, absorption, and transmission of the Sun’s energy. Let’s study these ideas a little more….We use the term albedo to describe the amount of radiation that the Earth reflects back into space. On the first page of this module you read that about 29% of the Sun’s energy reaching the Earth is reflected back into space. (Imagine if no energy were reflected, it would have been difficult for the astronauts on the moon to see the Earth!)The table below is the albedo values for different types of earth surface:

Earth Surface Type Average Albedo

Forests 15%

Agricultural land 20%

Deserts 28%

Snow and ice cover 80%

Ocean (<70 º latitude) 3.8%

Ocean (>70 º latitude) 9.2%

Clouds 50%

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As you might expect, snow and ice reflect a lot of incoming radiation, while forests do not reflect nearly as much radiation. Agricultural land reflects more radiation than forests, so what effect have people had on the Earth’s albedo by cutting forests and growing crops in the same location?

Ice and snow reflect more radiation than ocean water. If large ice sheets melt and there is more ocean water surface, how will the albedo of the Earth change?

If there is an increase in the Earth’s albedo, what will happen to the temperature of the Earth?

What happens to the 71% of the energy that does not reflect back into space?

One of the types of radiation you labeled under the Electromagnetic Waves section above was ultraviolet radiation (or UV). UV radiation is often called ultraviolet “light” even though we can’t see it. For the sake of accuracy, we should try to use the word “light” to describe only what we can see. Despite what you may have seen with "black lights" that are commercially available, you cannot see ultraviolet waves. The violet light that we see coming from "black lights" is light with a wavelength that is not quite short enough to really be ultraviolet. A black light makes ultraviolet waves as well, but you can't see them.

Look back at your table of different kinds of EM waves (p. 9) with different energies. Do you think a molecule could absorb some UV waves and then give off gamma rays or x-rays? Explain your reasoning. (Hint: if Keith gave you a dollar could you turn around and give Bob a million dollars?—Bob likes the idea!)

Could a molecule absorb some UV waves and then give off visible light or infrared energy? Explain your reasoning. (Think about that dollar.)

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Activity #8Ask one of your instructors for help with one of the UV sources. CAUTION! These are high intensity UV sources and they are very different from ordinary black lights. Do not look directly into them when they are in use!

Shine a UV source on a dull patch of the wall (not a shiny surface and not a "bleached" surface such as a piece of paper or your socks). Do you think you can see ultraviolet waves? (Could this just be a very dim ordinary light?)

Shine a UV source on one of the wondrous and very cool rocks of science. Does it look the same as the wall? What do you suppose is happening to the UV radiation that is being absorbed by the rock?

If we had a source of infrared radiation, could we shine the IR on the rock and get the same result? Why? Why not?

When infrared radiation (IR) shines on our skin, we feel it as heat. Heat can move from one place to another by conduction, by convection, or by radiation. When heat travels by radiation it is traveling in the form of infrared radiation.* When we feel "the warmth of the Sunshine" we are feeling infrared waves that reach the Earth after traveling a hundred million miles. Another example is when we feel warm from a campfire even when the air around us is cold. Whenever we feel heat radiated by anything, we are feeling infrared waves. Any object that is warmer than its surroundings will radiate IR. Any object that is cooler than its surroundings will absorb more IR than it radiates.

Recall during fall quarter that we used the special thermometer to measure our skin temperature and we found that some of us were warmer than others. How did this thermometer measure our temperature without touching our skin?

Actually, the atmosphere absorbs or reflects much of the infrared radiation that reaches the Earth, so much of the infrared radiation that heads our way does not

* Technical note: Some textbook authors separate infrared waves from short wavelength radio waves, in which case one has to say that radiated heat travels as infrared and/or radio waves. For the purposes of studying weather and climate, it is sufficient to call both of these forms by the name "infrared.” Still, you may be interested to know that every time you cook something in a "microwave" oven, you are heating it up with very intense radio waves.

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make it down to the surface of the Earth. If the atmosphere absorbs the IR, what happens to the atmosphere?

An important idea most of the "solar energy" that reaches the surface of the Earth from the Sun is in the form of VISIBLE LIGHT. (Infrared trails just behind.) Remember, visible light is higher in energy than infrared.

Key question: When objects on the surface of the Earth absorb visible light, can they turn around and give off ultraviolet waves? X-rays? (HINT: Some objects can give off gamma waves even if they don't absorb anything, but what we are really asking here is whether the absorption of some visible light would cause something to be able to give off x-rays or UV waves.)

When objects on the surface of the Earth absorb visible light, can they turn around and give off infrared waves? Explain your reasoning.

In the summer you may have noticed that when you touch a dark colored object, the object feels very hot. If very little IR reaches the Earth’s surface, why is the object hot?

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A Really Important Idea :If 71% of the Sun’s energy is not reflected back into space, it is mostly absorbed by the Earth’s surface. Objects on the Earth’s surface re-emit that radiation in the form of IR. We call this “black body radiation”. When the IR is re-emitted by the Earth, gases in the atmosphere absorb most of that IR and the atmosphere becomes warmer.

If your car has been parked for a couple of hours with the windows closed you will find that when you get into your car that the temperature inside the car is higher than the outside temperature. Why is the inside of your car warmer than the outside air?

Gardeners use greenhouses to provide a warmer, lighted environment for plants to grow. Draw a picture of how a greenhouse works. Be specific about the types of energy and the energy conversions!

In what ways is the Earth similar and different than a greenhouse?

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The gases in the atmosphere act like the glass in a greenhouse, keeping the infrared radiation from escaping, so the temperature inside increases. The specific gases that absorb IR are carbon dioxide, methane, water vapor, and chorofluorocarbons (cfc’s such as “Freon”). The first three of these gases are natural gases, but the fourth is a human compound that was used primarily in air conditioners and refrigerators until the last few years. Our Atmosphere:With each breath, we inhale and exhale atmospheric gases that are primarily nitrogen and oxygen. However, the atmosphere has not always been so hospitable to humans. The evolution of the atmosphere is tied directly to the evolution of life on the planet and is directly tied to the subject of our interdisciplinary sequence: climate and global change.

We find it very difficult to say exactly what the Earth’s early atmosphere was like because we have little or no direct evidence for it. In situations like this we try to form a “model” which explains what we see (recall the idea of models from fall quarter?) and then we examine the evidence to see if it agrees with our model. If the evidence indicates that the model is incorrect we may throw out the idea completely or revise the model to fit the new data.

From a variety of sources, the Earth seems to be about 4600 million years old (4.6 billion). The oldest rocks we know of on the Earth’s surface are about 4000 million years old. Most of these rocks have been severely deformed over time so their stories are more difficult to read. Some of the most interesting rocks in the world were formed at about 2500 million years ago in a shallow sedimentary basin. These rocks are today alternations of iron oxide and a silica mineral called chert.

The sedimentary materials deposited today may have trace amounts of iron oxide in them, but these iron oxide and chert layers, termed banded iron formations, clearly show that iron oxides where deposited directly from water. If we run water over a rusty piece of iron, the water will absorb (dissolve) very little of the iron oxide from the rust. These 2.5 billion year old rocks clearly show that iron oxide was present in the water so it must have been dissolved in a manner different than what we see today.

If you take the same rusty piece of iron described above and run water over the surface with no oxygen present, the iron oxide will be dissolved in the water readily. So, the fact that we see these layers of iron oxides in rock layers, tells us that there was very little oxygen in the water or the atmosphere at that time.

Where did the oxygen come from and what types of gases were in the atmosphere in the early history of the Earth and what happened to them? (rhetorical question)

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If we look at the gaseous planets in the outer part of the solar system, we see that they have abundant hydrogen and hydrogen-rich gases, such as methane and ammonia. Although there is no direct evidence of this, researchers believe that the earliest atmosphere of the Earth was rich in the hydrogen gases. Based on evidence in the oldest rocks and the geological ideas about the formation of the Earth that the Earth was hotter early on and has cooled significantly over time. The high temperature of the Earth and the lack of a magnetic field caused the gases to be driven away from the Earth by the solar wind (ions emitted by the Sun). It sounds a bit like science fiction, doesn’t it! But there is very little to counter this model and it seems to make sense!

If these early gases were lost by the Earth, how did we get the nitrogen and oxygen atmosphere we have today? Another result of this heat release in the Earth was abundant volcanism. Besides the rocky materials that erupt from volcanoes, significant amount of gas is also expelled. Most of the gases emitted from a volcano today are water vapor (80-90%), carbon dioxide (~10%), nitrogen (1-2%), and some other gases we term trace gases.

After the Earth cooled, the water vapor condensed to form the oceans and lakes, some of the carbon dioxide was deposited in the oceans as limestone, and the nitrogen began to increase in percentage because nitrogen gas is not very reactive. However, the abundance of oxygen in today’s atmosphere is not as high as the emissions from volcanoes, so oxygen must have become abundant by other means.

Some of the water emitted early in the Earth’s history probably dissociated (broke into H2 and O2) from radiation from the Sun. Before the Earth’s magnetic field developed, more radiation from the sum could strike the Earth. This energy was sufficient to break the bonds between the hydrogen and oxygen in the water vapor. The free hydrogen was probably lost to space because hydrogen’s mass is so small that the Earth gravity is not sufficient to keep it in the atmosphere. This dissociation process may account for a small amount of the oxygen in our atmosphere but this period of time was not sufficient to develop today’s abundance of oxygen.

About 2.2 billion years ago plants began to photosynthesize enough to change the atmosphere. Imagine that for about the first half of the Earth’s history that the atmosphere would have been toxic to us. Only since photosynthesis developed has there been enough oxygen in the atmosphere for animals to exist.

If you enroll in IDS 103, you will learn more about photosynthesis and cellular respiration. Most people know that the plants and animals of Earth are dependent on our atmosphere, but many people are surprised to find out that the atmosphere is also dependent on photosynthesis and respiration.

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Nitrogen and oxygen are the major gases of the atmosphere. All the remaining gases are “trace gases” including carbon dioxide and the other greenhouse gases. Some of these trace gases play a very important role in modifying the Earth’s climate. Carbon dioxide is released in cellular respiration in plants and animals.

The greater the amount of carbon dioxide and the other greenhouse gases in the atmosphere, the greater the absorption effect of the blackbody IR. The graph on the next page is from measurements of carbon dioxide in the atmosphere from March, 1958 to December, 1998 at the Mauna Loa observatory on Hawai’i. This site is thought to be the least polluted air on the Earth because there is no source of air pollution upwind for thousands of miles from Mauna Loa.

Describe the pattern created by the data:

Why does the carbon dioxide concentration increase and decrease each year?

If you average the carbon dioxide values on an annual basis, what would the overall trend of the values be?

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Most scientists have said that this increase in CO2 is due to human emissions and the destruction of the rain forests. It is possible that some of the variation could be due to natural variations in carbon dioxide. To fully understand the issues related to greenhouse gases and global change, we need to examine the past to see if CO2 levels have changed over time and if so, to what extent.

In the same way we studied past climates with oxygen isotopes, we can use ice cores for Antarctica to understand past variations in trace gases. As snow falls it traps small amounts of air in the spaces between the snowflakes. Some of this gas forms small bubbles as the snow metamorphoses into glacial ice. These bubbles become fossil atmospheres providing us with a means to examine the gas content of the atmosphere thousands of years in the past. Of primary interest to those studying global change and our present atmosphere, is the abundance of carbon dioxide and methane, the primary natural greenhouse gases. The graph below is the carbon dioxide concentration from an Antarctica ice core through the last approximately 400,000 years:

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The graph below is of methane (CH4) from the same ice cores in Antarctica.

What similarities and differences do you observe between the two graphs? (be specific)

There have been natural variations in both the carbon dioxide and methane concentrations through time as reported from this ice core. It is tempting to relate the fluctuations in climate we observed from the oxygen isotope data to variations in these trace gases. However, as we have mentioned several times in class, it is clear that “correlation does not mean causation”. In other words, just because carbon dioxide goes up during a warm period, does not mean that carbon dioxide necessarily caused the warming, although it is consistent with our model!

In conclusion, we have learned that climate may be altered when the albedo of the Earth is changed and that the climate of both the Earth and Venus is heavily influenced by the absorption of re-emitted infrared radiation by atmospheric gases. If the Earth did not have the greenhouse effect, our climate would be about 60F colder and the Earth would not be hospitable for life. However, human activities such as burning fossil fuels and cutting forests, may create many undesirable environmental changes if the temperature increases as predicted by the climate models.

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Another related but separate issue: The ozone “hole”:The stratosphere contains lots of ozone (a molecule made of three oxygen atoms bound together). Ozone absorbs ultraviolet waves. The "ozone layer" (within the stratosphere) absorbs most of the UV waves that reach the Earth from the Sun, but not all of them. If you hang out in the sunshine without sunscreen, the UV radiation can give you a sunburn (and may cause cancer, so take care of yourself!).What happens to the UV radiation after it is absorbed by the ozone? Excellent question!

The ozone in the stratosphere absorbs a lot of UV radiation, but it does not glow with lots of pretty colors like the “amazing rocks” of science. What do you suppose happens to the energy in the UV waves that are absorbed in the ozone in the stratosphere? (Memory test: what happens to the temperature as we move up through the stratosphere?)

Previously we mentioned cfc’s (chlorofluorcarbons). These gases were used in air conditioners and refrigerators. Unless you have a new car and new appliances, you probably have cfc’s in your air conditioner and refrigerator. These gases may leak from the appliance or car and go into the atmosphere. One important characteristic of cfc’s is that they do not react with many other gases except ozone. When the cfc’s get into the stratosphere they react with the ozone and create areas of lower ozone concentration. These areas of lower ozone concentrations have been called an “ozone hole”. Although there is some ozone present in these areas, it is not in the concentration it had been before. For reasons that are beyond the scope of this class, most of these regions of lower concentration are near the poles. If the concentration of ozone is low, more UV radiation hits the Earth’s surface, including us. There are concerns that if these areas of low ozone concentration enlarge, many people at high latitudes will be exposed to damaging UV radiation.

So, cfc’s play a role in both the greenhouse issue and the ozone hole issue, but these two topics are not directly linked together.A graph from NASA indicates that the concentration of ozone over Antarctica has decreased dramatically since 1979. Hopefully, as people stop using cfc’s, we will see this area of low concentration stabilize.

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