Climate Change Part 2 - the Greenhouse Effect

IDS 102 Climate Change Part 2 - “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 67C. This compares to the average temperature of Earth’s atmosphere at about 15C. This sounds logical, but the temperature at the surface of Venus is about 460C! 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.” In order to discuss the role of CO2 in both the Earth’s and Venus’s atmosphere, we need to review concepts relating to electromagnetic radiation, such as reflection, transmission, and, most importantly, absorption. Recall that light from the Sun (or any energy) can be 1) reflected, 2) absorbed and/or 3) 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 review these ideas a little more….

 Draw a diagram and/or describe in your own word what each of the following terms means. Give an example of each process.

Transmission:

Absorption:

Reflection:

1 We use the term albedo to describe the amount of radiation that the Earth reflects back into space. On average, ~29% of the Sun’s energy reaching the Earth is reflected back into space.. 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%

 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?

2 One of the types of solar radiation is 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 (purple module) 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 we gave you a dollar could you turn around and give us a million dollars back?)

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

Activity #1/ Review of EM Spectrum Ask 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? Describe what 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 3 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.  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 make it down to the surface of the Earth. If the atmosphere absorbs the IR, what happens to the temperature of the atmosphere?

An important idea of the "solar energy" that reaches the surface of the Earth from the Sun, the most intense is in the form of VISIBLE LIGHT.

Infrared trails just behind. (The part of the energy spectrum occupied by infrared is much larger than the part occupied by visible light, so the total energy that we receive in the form of IR is actually greater. Still, our eyes detect the most intense radiation that we receive from the sun. Pretty cool, huh?) 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: 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.) Explain your answer

 Key Idea: 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?

4 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?

Note: 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.

Concept Review/ End of Module Questions

On February 2, 2007, the Intergovernmental Panel on Climate Change concluded that “Global warming is very likely caused by man, meaning more than 90 percent certain.” That's the strongest expression of certainty to date from the panel. They also announced that if nothing is done to change current emissions patterns of greenhouse gases, global temperature could increase as much as 11 degrees Fahrenheit by 2100. But if the world does get greenhouse gas emissions under control something scientists say they hope can be done the best estimate is about 3 degrees Fahrenheit. (http://www.ipcc.ch/). Consider how these changes will influence our planet in the end of module questions.

1) As global temperatures rise, what will happen to the size of the polar ice caps and mountain snow pack? Why? How will this change global sea-levels?

2) Given that the snow pack in the Cascades provide water to streams in the summertime and thus supplies much of the water that is used by cities in Western Washington and farmers in Eastern Washington, discuss how will increased temperature influence the fresh water supply in Washington State. Consider how the amount of snow that melts in the spring (the rate of spring melting) increase or decrease and how will this affect water levels in local rivers and the number of spring flooding events as well as how will the

5 summertime level of local rivers will change? How will our ability to generate hydroelectric power in the summertime be affected?

3) Tropical rainforests are disappearing at a rate of 10-20 million hectares (1 hectare = 2.5 acres) per year. What effect would removing the tropical rainforests have on CO2 concentration in the atmosphere and why?

4) Given that tropical storm intensity is related to ocean temperature, (warmer waters contribute to stronger storms), how would you expect the intensity of tropical storms to change with global warming?

5) In many places, the length of the growing season (= time that is frost free) is increasing as we see increased global warming? This might seem to be beneficial to farming, but so far we have not seen an increased crop yield. Suggest at least 2 negative effect of global warming that might contribute to decreased crop yields.

6) Below is a crude diagram of a greenhouse with a glass roof, which is transparent to visible light, but is NOT transparent to infrared radiation. Assume that the light source only emits visible light. Describe why the temperature inside the greenhouses is warmer than the outside temperatures by circling the correct type of radiation (visible light and/or infrared) and the process (absorption, emission, etc.)

Visible light / Infrared radiation is absorbed/ emitted /transmitted/ reflected by the Sun

Visible light / Infrared radiation is absorbed/ emitted /transmitted/ reflected by the inside surface of the glass

Visible light / Infrared radiation is then absorbed/ re- emitted /transmitted/ reflected by the floor of the greenhouse

Visible light / Infrared radiation is: absorbed/ emitted /transmitted/Light from reflected by the floor of the thegreenhouse Sun

7) Below is a crude diagram of the surface of the moon. The moon is light by light from the sun, but the moon does not have an atmosphere that will absorb either visible light or infrared light. Describe what happens to visible light from the sunSurface that hits of the the moon’s Moon surface and determine if the surface of the moon will be warm or cold.