After completing the investigations in Chapter 4, you should

Transcription

1 chapter five The Bigger Picture: Global Climate After completing the investigations in Chapter 4, you should realize that your sense of what is normal weather at any particular time of the year may actually be very different from someone living elsewhere. Whether the days are cold or warm, rainy or dry, sunny or partly cloudy, you become used to the conditions you expect in your home place. But what if they started to change? If you pay attention to the news, you ve heard much talk about climate. Measurements show that climate change is happening, but there are different projections about what this means. As citizens, it s hard to know how to react to all these warnings. You may be asking questions such as: Are these warnings valid? Is climate change really going to affect me? Or simply, why should I care? The story in this chapter is somewhat different from the ones you ve encountered earlier in the course, because it poses questions not fully addressed in this chapter or even in this course: What is causing the climate to change, and what impact will this have on local communities during your lifetime? You will continue to follow the story of humanity s quest to understand climate change and its implications as you watch and read news accounts over the coming years. Instead of trying to provide answers, this course will help you develop the basic understandings necessary so that you can assess what you hear about climate change and understand what this might mean to you. This chapter covers the fundamental factors that cause energy to be absorbed and held in Earth s atmosphere, and can cause the atmosphere near Earth s surface to become warmer or cooler over time. In Chapter 6, you ll look back millions of years into Earth s history and investigate how these factors, coupled with long-term changes in Earth s surface features and its orbit, have caused the planet s climate to be very different in the past from the way it is today. 111

2 EDC Earth Science Unit 2 atmosphere and climate Consider Investigate Process Brainstorming Discuss the following with your partner and be prepared to share your ideas with the class. Don t worry if you don t know all the answers at this point. You will explore many of these questions in the next few chapters. 1. Describe what you currently know about climate change, based on what you ve heard in the media or from previous science classes. a. What evidence do you know of that supports the idea that global warming is happening? b. What have you heard about the potential impacts of climate change? c. Do you think climate change could affect you? If so, how? d. What are your ideas about what might be causing the global climate to warm? The true story below is about one community that is already feeling the effects of a warming climate. Read the story and answer the questions that follow in About the Reading. WHAT S THE STORY? Washing Away The residents of Kivalina, a tiny village in Alaska, are feeling threatened by the sea. Kivalina, a remote island 190 kilometers (120 miles) north of the Arctic Circle, as shown in Figure 5.1, is accessible only by plane or boat during the summer. The Inupiat residents have for generations lived their lives largely independently of other people and have relied on the ocean and land to sustain them. They hunt caribou on the tundra landscape and whales and fish at sea. They ve adapted their lives to the rhythms of the seasons the long, cold winters and short summers. Recently, the patterns in their local climate have been changing, and the future for Kivalina doesn t look good. 70º Arctic circle 60º 50º Kivalina Enock Adams, Jr., a resident of Kivalina, considers his words and speaks carefully as he describes their worsening situation: It s getting warmer, we know that. We re right in the middle of this and we know what s been happening. The sea ice is developing later, so that proves to us that there has been significant change in terms of the weather. Having to deal with it is very frightening. It s created a situation that we really have no control over. Each fall in Kivalina there are storms that blow mile winds off the ocean, and these strong winds create huge waves. Normally, a thick layer of figure 5.1 Kivalina, Alaska, is located in the arctic at a latitude of 67 49ʹ North. 112

3 chapter 5 The Bigger Picture: global climate slush has formed along the shore by the time the storm season arrives in late September early October. This slush buffers the effect of the powerful waves and protects the coastline from erosion. However, in recent years the slush hasn t been forming until December or January, after the storm season has already been underway for several months. During the fall months, the shore is not protected from the storm waves, and as a result, the soft sand that forms the foundation of this village has been disappearing into the sea (see Figure 5.2). Residents of Kivalina are wondering what is happening, why it is happening, and what the future holds for their families, friends, and neighbors. Should they move their village away from the ocean and disrupt the way of living they have known for generations? This is one of many stories of communities around the world affected by a changing climate. If you lived in one of these places and you wanted to know the answers, how would you find them? Why are these changes happening, and what can be done about it? 1 figure 5.2 A sandbagged seawall fights erosion from storm waves in Kivalina, Alaska. About the Reading Write your responses to the following questions in your notebook. Be prepared to discuss your answers with the class. 1. The villagers of Kivalina have experienced warmer temperatures recently. Why has this threatened their community? 2. Have you heard of other stories similar to this? Are you aware of any other communities that are feeling the effects of climate change? Describe what you know about this. 3. If you were a resident of Kivalina, what would your concerns be? What questions would you have? Climate is something you take for granted: Most who live in the northern United States and Canada expect to get snow in winter. People who live in the southwestern United States expect it to be hot and dry in summer. You don t even think about these expectations until the conditions you take for granted start to change. Then you want to know why it is happening and if the changes will last. Scientists have been concerned with these questions, particularly in recent decades, because signs of climate change have appeared. What causes Earth s climate to be the way it is now, and what can cause it to change? That s what you ll explore in this chapter. 113

4 EDC Earth Science Unit 2 atmosphere and climate Consider Investigate Process CHALLENGE What factors cause Earth s climate to be the way it is, and what can cause it to change? To the residents of Kivalina, the future of their village looks grim. The very existence of their island depends on certain climate conditions, and these seem to be changing. How does Earth s climate work, and what causes it to change? Scientists are working hard to find the answers to these challenging questions and so will you in this chapter. You ll play the role of a Kivalina resident and investigate the factors that cause climate to change. Then you ll gather with your community at a public meeting to discuss what is happening and what should be done. In this chapter, you ll explore factors that influence global climate and that can affect communities such as Kivalina. To gather the knowledge you ll need, investigate how Earth s atmosphere regulates the planet s surface temperature. Specifically, explore the following: How carbon dioxide (CO 2 ) and other greenhouses gases control the amount of heat energy absorbed by the atmosphere. How variations in the materials on Earth s surface and in the atmosphere control the amount of the Sun s energy that is reflected back into space. How CO 2 is added to and subtracted from the atmosphere. How negative and positive feedback loops can either stabilize Earth s climate or accelerate change. Then pull together what you have learned and prepare your recommendations. Gather Knowledge Begin by reading about how the Sun delivers energy to Earth and about the role played by the atmosphere in creating conditions on Earth s surface that are very different from a few miles away in space. As you read this, take notes about the path that light energy takes as it enters Earth s atmosphere. You ll use these notes to draw a diagram when you answer the questions in About the Reading at the end. 114

5 chapter 5 The Bigger Picture: global climate READING Following the Path of Light Energy The temperature of space outside Earth s atmosphere is 270 C ( 454 F). Yet the atmosphere near Earth s surface is a much more comfortable average temperature of 15 C (59 F). It seems remarkable that the atmosphere could be so effective in protecting Earthlings from the deep cold of space, especially because the atmosphere is very thin, as shown in the view of Earth from the International Space Station in Figure 5.3. In fact, compared to the diameter of the entire Earth, the atmosphere s thickness is like the peel on an apple. If you got in a car and drove straight upward at 97 kph (60 mph), you would reach outer space in just about an hour. Yet the atmosphere has so successfully regulated Earth s surface temperature that for nearly 4 billion years Earth has remained the only known planet to have supported life. How does the atmosphere regulate Earth s temperature? The Sun delivers energy to Earth in the form of light, or electromagnetic radiation. The atmosphere controls the amount of energy that is absorbed by Earth and warms it, and the amount that is radiated back into space. On average, scientists have determined that for every 100 units of light energy falling on Earth, approximately 25 units (25%) are reflected by clouds back into space. Another 25 units (25%) of this light energy is absorbed by the atmosphere. The remaining 50% percent travels to the surface of Earth. Approximately 5% percent of the light energy striking Earth s surface is reflected off the ground, polar ice, or ocean surface directly back into space. However, most of the light energy that reaches the surface (45% of the original 100 units) is absorbed into the water or ground. This energy absorbed by Earth s surface is key to the greenhouse effect, which you will learn about in Activity 1. The preceding description of what happens to light energy from the Sun is simplified. The percentages given are averages; they actually vary from one part of the globe to another and also vary through time. During Activities 1 and 2 that follow, you ll explore two of the major factors that influence how much of the Sun s light energy is absorbed by Earth s atmosphere (the greenhouse effect) and how much is reflected back into space (the albedo effect). Figure 5.3 Earth s atmosphere, although very thin, is what makes Earth habitable for living things. 115

6 EDC Earth Science Unit 2 atmosphere and climate About the Reading Record your responses to the following questions in your notebook. Be prepared to discuss your answers with the rest of the class. 1. Draw a diagram that shows what happens to the light energy that is transmitted to Earth from the Sun, based on the information in the reading. 2. The reading points out that the description of what happens to the light energy from the Sun is simplified and that the percentages of absorbed and reflected light actually vary from place to place on the globe. Based on what you know from previous chapters, explain how and why the amount of energy absorbed by the oceans at the equator varies from the amount of energy absorbed nearer the poles. 3. The reading points out that on average 5% percent of the light energy that reaches Earth from the Sun is reflected off Earth s surface back into space. Might the amount of light energy reflected vary from one region to another? How and why? In Activity 1, you ll explore how CO 2 and other greenhouse gases control the amount of heat energy that is retained near Earth s surface. ACTIVITY 1 The Greenhouse Effect Setting the Stage: What is the Greenhouse Effect? The composition of a planet s atmosphere can tell you much about that planet and what it is like on the surface. In fact, as scientists look for life on planets orbiting other stars, the first clues they will find will be in the composition of the alien planets atmosphere. Earth s oxygen and methane are present because life is here and continually replenishing the atmosphere with these gases. It turns out that oxygen isn t the only gas in Earth s atmosphere that s important to humans there are other gases that are just as crucial to the comfortable life you live on this planet. To understand what keeps Earth a comfortable temperature for Earthlings, you must focus on the light energy that is absorbed by Earth s surface. Light energy has a relatively short wavelength, which can pass easily (back and forth) through Earth s atmosphere. However, most of the light energy that is absorbed into Earth s surface is transformed into longer-wavelength heat energy (also called infrared radiation), which you can feel but not see. Have you ever tried to walk barefoot on black asphalt that has been in the bright sunlight for several hours on a summer day? The light energy from the Sun is absorbed by the asphalt and transformed into heat energy, which warms the surface, sometimes making it very hot. According to a basic law of physics, heat naturally flows from warmer areas to cooler areas. Therefore, the heat energy warming the surface of the 116

7 chapter 5 The Bigger Picture: global climate asphalt begins to rise into the air (you can feel this happening as you stand on the asphalt). This longer-wavelength heat energy rises into the atmosphere, and this time, instead of passing easily through the atmosphere and back into space, some of it is trapped. This is because certain gases in the atmosphere, primarily water and CO 2, allow shorter-wavelength light energy to pass through toward the surface easily but tend to trap some of the longer-wavelength heat energy and reradiate it back toward Earth. These heat-trapping gases in Earth s atmosphere are known as greenhouse gases, and without them, Earth would be too cold for life to survive. The amount of reradiated heat that is trapped by the atmosphere depends on the concentration of greenhouse gases in the atmosphere. As the concentration of these gases increases, more heat is trapped and the temperature of Earth s atmosphere rises. The warming that results when heat is trapped near Earth s surface by greenhouse gases is known as the greenhouse effect. During this activity, you will consider the composition of Earth s atmosphere, and compare it to Earth s closest twin in the solar system, Venus. Pre-Activity Discussion Discuss the following question with your classmates to help you prepare for the investigation. Record your answers in your notebook. 1. Modify the diagram you drew showing the pathway of light energy from the Sun to show what happens to the longer-wavelength heat energy that is reradiated from Earth s surface (you can t show percentages in this case, but you can use labeled arrows to show what is happening). Procedure 1. Working with your group, read the Atmosphere Component Cards carefully and share with your group what you know about each gas. 2. As a group, predict the relative amounts of these gases in Earth s atmosphere and arrange the components in order from highest to lowest percentage. Don t worry if you re not sure of your answers use your group s knowledge from previous classes or outside of school to make your best educated guess. 3. When your group has arranged the Atmosphere Component Cards in what you think is the proper order, show your teacher what you have done. Your teacher will give you a set of Data Cards. Use Card 1: Earth s Atmosphere to compare the actual measured percentages with your predicted order. Answer these questions in your notebook: a. How close was your educated guess to the actual order of the gases? Did anything about the order surprise you? b. Why do you think the abundance of some of the gases is not only shown as a percentage, but also in parts per million? Materials For each group of students set of 11 Atmosphere Component Cards set of 5 Greenhouse Gas Data Cards 117

8 EDC Earth Science Unit 2 atmosphere and climate Put aside your Atmosphere compenent Cards and use information from Greenhouse Effect Card 1 and Card 2: Composition of Venus s Atmosphere, to create a table and another type of illustration (such as a graph or diagram) that compares the atmospheric composition of Earth and Venus. Below your table and illustration, write a few sentences summarizing the similarities and differences between the atmospheres of Earth and Venus. 5. Using the information in Card 3: Greenhouse Gases, do the following: a. Add labels to the table and illustration to show which gases are greenhouse gases. b. How do greenhouse gases behave differently from other gases? What effect does this have on a planet? Write your answers in your notebook. 6. Study the information you ve gathered so far, and answer the following questions: a. Which greenhouse gas is found in the highest concentration in Earth s atmosphere? In Venus s atmosphere? b. The CO 2 abundance in Earth s atmosphere isn t as high as that of water vapor, which is also a greenhouse gas. However, CO 2 has a longer lifetime, so it lasts much longer in the atmosphere. Why do you think the lifetime of a greenhouse gas in the atmosphere might matter? 7. Next investigate how differences in the composition of Venus s and Earth s atmosphere have affected conditions on their surfaces. Venus is often called Earth s twin, because it is Earth s neighbor and is similar in diameter. However, the surface conditions are so different that humans would never consider setting up a colony there for one thing, the surface temperature of Venus is hot enough to melt lead! Why is it so different? Is it because Venus is closer to the Sun? Use the information on Card 4: Terrestrial Planet Comparison to investigate how the distance from the Sun relates to the surface temperatures of the terrestrial planets. Do this by drawing a graph to show if there is a clear and predictable relationship between distance from the Sun and surface temperature, and answer the following questions: a. Based on your graph, is distance from the Sun alone enough to explain the difference in surface temperatures between Earth and Venus? b. What, other than distance from the Sun, could be affecting the surface temperatures of the terrestrial planets? 8. In addition to distance from the Sun and surface temperature, Card 4: Terrestrial Planet Comparison shows more information about Earth and Venus. Use this information to answer the following: a. Surface pressure is the force per unit area exerted on the surface by the weight of the air in the atmosphere above. Two factors influence atmospheric pressure: the thickness of the atmosphere and the molecular weight of gases in the atmosphere. Describe what you think it would be like on Earth s surface if the air were 90 times heavier than on Venus. b. What questions do you have about the characteristics described in this Card? Do you understand what all of them mean and how they would

9 chapter 5 The Bigger Picture: global climate affect your experience on the surface of a planet? Do some additional research to better understand them. c. Do you see any other atmospheric characteristics that might affect the temperatures on the surface of the two planets? Explain your thinking. 9. Study the diagrams on Card 5: Energy Balance of Earth and Venus, also shown in Figure 5.4. The yellow arrows on these diagrams represent short-wave radiation and the red arrows represent long-wave (infrared) radiation. The thickness of the arrows shows the relative amount of radiation. Answer the following questions about Figure 5.4: a. Why is the arrow showing incoming solar radiation thicker on the Venus diagram than it is on the Earth diagram? b. How does the amount of sunlight reflected by clouds in the atmosphere compare between the two planets? c. How does the amount of sunlight striking the surface compare between the two planets? d. Venus s surface is very hot, so a large amount of infrared radiation is emitted, as you can see by the thickness of the arrow. How much of the infrared radiation emitted from Venus s surface makes it back into space? (Give an approximate proportion.) e. Approximately what proportion of the infrared radiation emitted from Earth s surface makes it back into space? f. Use the answers to d. and e., and your understanding of the greenhouse effect to explain how Venus s surface came to be so much hotter than Earth s. Figure 5.4 This diagram compares Earth's energy balance with Venus's energy balance. Although much less of the sunlight that strikes the top of Venus's atmosphere reaches the surface, Venus's surface is much hotter than Earth s, due to a runaway greenhouse effect. The yellow arrows represent short-wave solar radiation and the red arrows represent longwave infrared radiation. The thick ness of each arrow shows the relative amount of radiation (generalized, not to scale). EARTH incoming sunlight VENUS incoming sunlight reflected sunlight infrared radiation escaping to space reflected sunlight reflective cloud layers atmospheric carbon dioxide infrared radiation escaping to space atmospheric carbon dioxide planet surface sunlight striking surface infrared radiation emitted from surface reflective clouds reabsorbed infrared radiation planet surface sunlight striking surface infrared radiation emitted from surface reabsorbed infrared radiation 119

10 EDC Earth Science Unit 2 atmosphere and climate Analysis With your group, complete the following questions and record your answers in your notebook. Be prepared to share your answers with the rest of the class. 1. Explain in your own words: a. How can greenhouse gases in the atmosphere of a planet affect the temperature of the planet s surface? b. How could an increase in greenhouse gases in a planet s atmosphere cause the climate to change? The greenhouse effect is a key factor linked to global warming. As the concentration of CO 2 in the atmosphere increases, more heat is trapped near Earth s surface. Do rising CO 2 levels alone explain the changes in climate that are happening now and what will happen in the future? The operations of Earth s systems are not quite that simple. There are other factors that influence the amount of heat retained by Earth s atmosphere, and these factors must be considered as well. In the next activity, you ll look at how variations in Earth s surface materials can affect the amount of radiant energy from the Sun that is reflected back into space. Biogeology: Using Greenhouse Gases to Terraform Mars Some scientists have proposed that greenhouse gases could be used to warm the surface of Mars and make it habitable for humans and other Earth organisms. Solarpowered, greenhouse gas-producing factories could be placed on Mars. Additional factories could be used to simultaneously oxygenate the atmosphere. These machines would mimic photosynthesis by taking in carbon dioxide and releasing oxygen into the atmosphere. However, hundreds of these greenhouse gasproducing machines would be needed, and it would take an extremely long time to build up the atmosphere. The machines would also need to operate in perpetuity to maintain adequate levels of greenhouse gases in the atmosphere. Some scientists believe a quicker way to introduce greenhouse gases to Mars s atmosphere would be to hurl large, icy, ammonia-rich asteroids into Mars. This would release large amounts of greenhouse gases and water from beneath Mars s surface. To accomplish this, rocket engines would have to be somehow attached to asteroids from the outer solar system. At 4 km/sec, it would take the rockets 10 years to propel the asteroids to Mars. This would be an incredibly complex process, and the bombardment of the red planet by an asteroid would release energy equivalent to approximately 70,000 one-megaton bombs. Terraforming by asteroid smashing would delay human settlement of the planet for centuries. ACTIVITY 2 The Albedo Effect Setting the Stage: What Is the Albedo Effect? Another significant factor affecting Earth s temperature is the albedo effect. The albedo is the percentage of incoming light energy that is reflected rather than absorbed by a particular surface. In the diagram of Earth s energy balance you drew earlier in this chapter, you should have shown approximately 25% of the Sun s light energy being reflected back into space by the clouds and 5% being reflected back into space by Earth s surface. You know from your own experience that the cloudiness of the sky varies from day to day. This means that the albedo effect, or degree to which the Sun s radiant energy is reflected, varies as well. The effect of clouds is particularly complex because the water vapor within 120

11 the clouds also serves as a potent greenhouse gas. High, thin clouds tend to warm the planet by trapping heat energy and reradiating some of it back toward Earth, whereas low, thick clouds tend to reflect more of the Sun s energy back into space, having the reverse effect. The albedo of a particular surface is represented by a number from 0 to 1. A surface with an albedo of 1 reflects 100% of the light energy that strikes it and will appear white. A surface with an albedo of 0 absorbs 100% of the light energy that strikes it and will appear black. A surface with an albedo between these two extremes might be represented (for example) by the number 0.3, which means that it reflects 30% of the light that strikes it. Just as with the atmosphere, the surface of Earth varies in the degree to which it reflects light energy as well. The albedo is a key variable used by scientists in computer models used to forecast weather. Therefore, Earth s albedo is monitored on a daily basis by satellites. In this activity, you will develop hypotheses about the relative albedo of various Earth surface materials using images of Earth from space. Then, perform an experiment to prove that light-colored surfaces reflect the most light energy. chapter 5 The Bigger Picture: global climate Procedure Record all observations and answers in your notebook as you work. Part A: Studying Images of Earth 1. Study the satellite images of Earth in Figures , and answer the Think About It question that goes with each image. Think About It 1 Study Figure 5.5, an image of Earth taken by astronauts on the Apollo 17 mission in Based on what you see in this image, come up with a hypothesis about how the following three types of surfaces rank in terms of their albedo: clouds, land, and oceans. Make it clear which you think has the highest albedo (is the most reflective) and which has the lowest albedo (absorbs the most light energy). Record your hypothesis and the evidence that supports it in your notebook. Figure 5.5 View of Africa and Saudi Arabia from Apollo 17. This picture was taken by the astronauts as they left Earth s orbit for the moon. 121

12 EDC Earth Science Unit 2 atmosphere and climate Think About It 2 Study Figure 5.6, which shows a true- color albedo image of Earth taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA s Terra satellite. This is like the image of Earth taken by Apollo 17 astronauts in Figure 5.5; however, the clouds are removed and Earth s surface is unrolled to look flat. Based on this image, make a list of various types of Earth s surface materials (including the oceans and the polar areas) and rank them according to albedo (make it clear which has the highest albedo and which has the lowest). Figure 5.6 True color albedo image taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. Think About It 3 Figure 5.7 shows Earth s average albedo for March 2005 and includes the cloud albedo. You can see from the color bar at the bottom that the lighter colors on the map represent areas with higher albedo, and the darker blue colors represent areas with lower albedos. Describe the patterns that you see in this image, and write your hypotheses about what causes these patterns. Figure 5.7 Image showing Earth s average albedo for March 2005, measured by the Clouds and Earth s Radiant Energy System (CERES) instrument aboard NASA s Terra satellite. White shows areas where Earth reflected the highest percentage of light energy. 122

13 chapter 5 The Bigger Picture: global climate Look at the image carefully and describe it in as much detail as you can. Part B: Albedo Experiment Record all observations and answers in your notebook as you work. 1. Review the items you have available and discuss with your group how you might use these to compare the albedos of different colored materials. 2. Design an experiment to compare the albedos. Your experimental procedures should be clearly written and include: accurate descriptions/sketches of how the materials will be used instructions for collecting precise measurements a data table to record your measurements 3. Write the procedures in your notebook and obtain your teacher s approval before proceeding. 4. Perform your experiment and record your data. 5. Make a graph of your data and write a brief summary of what you can conclude from your data. 6. Complete the Analysis questions. Materials Part B for each group of students 2 30-ml graduated cups 2 thermometers 30 ml white sand 30 ml black sand timer (or access to a clock with a second hand) access to sunlight (or light from a 100-watt bulb) Analysis Complete the following questions and record the answers in your notebook. Be prepared to share your answers with the rest of the class. 1. Were any measurement errors possible in your experiment? Would you do your experiment differently if you did it again? 2. Thinking back to the satellite images in the first part of the activity: a. List the different types of surface materials found on Earth. Which have an albedo closer to 1? Closer to 0? b. Which types of surface materials vary in extent by season? How would you expect Earth s albedo to vary by season? 3. Describe one of the surface materials that could change in extent so that it covers more or less of Earth s surface over time. Referring to the light energy path diagram you drew, how would you expect this change to affect Earth s average temperature? 4. Another factor influencing albedo is the angle at which sunlight strikes the surface. When light strikes at a lower angle, more is reflected. How does this factor affect the albedo of different parts of the globe? 5. In the introductory story, residents of Kivalina have noticed that snow and ice cover the water near their island for shorter periods during the winter than in the past. Over time, how would you expect this reduction in snow and ice to affect the region s average albedo? How would you expect the change in albedo to affect the region s average temperature? 123

14 EDC Earth Science Unit 2 atmosphere and climate So now you know that the temperature near Earth s surface is related to whether energy is reflected into space or trapped by greenhouse gases such as CO 2. The more CO 2 in the atmosphere, the more heat will be trapped, and the more Earth s temperature will rise. So where does this CO 2 come from? Explore this in the next two activities. ACTIVITY 3 Moving Carbon Around Setting the Stage: Carbon in Earth s Systems The element carbon is one of the essential components of every cell. Did you know that the carbon in your body has been around since before Earth formed 4.6 billion years ago? In fact, it formed in the center of a star that lived and died before Earth s Sun was formed. Since the carbon in your body first formed, it has combined with other atoms and changed form many times. It has resided in various parts of Earth the soil, the rocks, the oceans, living things, and the atmosphere. These parts of Earth s systems that contain carbon are also known as carbon reservoirs. Table 5.1 shows the approximate amount of carbon that resides in each of Earth s major carbon reservoirs. Table 5.1: The Approximate Amount of Carbon in Earth s Major Carbon Reservoirs in Gigatons (1 gigaton = 1 billion tons) 2 Major Carbon Reservoir Approximate Amount of Carbon (gigatons) Vegetation 610 Soils 1,560 Atmosphere (preindustrial) 600 Ocean mixed layer* 1,000 Deep ocean 38,000 Sediments and rocks 66,000,000 Materials Parts A and C For each team of students: 1 calcium atom (green) 3 carbon atoms (black) 8 hydrogen atoms (white) 10 oxygen atoms (red) 20 covalent bonds (white tubes) Part B For each group of students: 1 bottle of limewater, (Ca(OH) 2 ) 1 wide mouth bottle with plug-style cap 1 small candle 1 foil square access to matches or lighter 124 * Ocean mixed layer: the shallower parts of the oceans where more mixing with air occurs. The carbon moves in and out of some of these reservoirs (such as vege tation) relatively quickly. On the other hand, carbon typically is stored in other reservoirs particularly rocks for very long periods of time. The term carbon flux refers to the amount of carbon that is transferred from one reservoir to another during a given time period. How does a carbon atom get from rocks to the atmosphere? From water to rocks? In this next activity, you ll investigate the processes that move carbon atoms around.

15 chapter 5 The Bigger Picture: global climate Procedure Record all observations and answers in your notebook as you work. Part A: Modeling Combustion 1. Use the structural diagrams in Figure 5.8 and the molecular modeling pieces to create one molecule of propane (C 3 H 8 ), a common fuel, and one molecule of oxygen gas (O 2 ). H H C H C H C H O O Figure 5.8 The molecular structure of propane and oxygen. H H H propane oxygen 2. Model the combustion of propane by: a. Making 4 more O 2 models then putting any unused modeling pieces away. b. Modeling the breaking of the reactant bonds by removing all the white bonds from the atoms of all six reactant molecules (one C 3 H 8 and five O 2 ). c. Modeling the formation of the products by reassembling all the bonds and atoms from the broken reactant molecules to make as many water (H 2 O) and carbon dioxide (CO 2 ) molecules as you can. 3. Count up the number of H 2 O and CO 2 molecules you made in Step 2 then complete the chemical equation below that shows the combustion of propane. Write the completed equation in your notebook. C 3 H 8 + 5O 2 CO 2 + H 2 O Part B: Producing and Sequestering CO 2 4. Work in groups of 4. Obtain the Part B materials from your teacher. 5. Place the bottle cap upside down on the table and fill it almost completely up with limewater. Holding the bottle upside down, push it down firmly onto the cap. Then flip the capped bottle right side up. Observe and describe the limewater. 6. Gently shake the bottle 10 times then place it upright on the table. Observe the limewater and record changes, if any, to its appearance. 7. Uncap the bottle. Place the cap on the table and then carefully pour the limewater back into the cap. Place the cap with limewater somewhere safe because you will use it again in Step Place the candle in the center of the foil square as shown in Figure Carefully light the candle then hold the bottle upside-down so that the bottle-mouth is 2 5 cm (1 2 inches) over the candle flame. Hold it there for about seconds to catch the combustion gases. flame candle foil square Figure 5.9 Setup for producing and sequestering CO 2 in Activity

16 EDC Earth Science Unit 2 atmosphere and climate 10. Keeping the bottle upside down, quickly, but carefully, move it over the cap filled with limewater and push the bottle down firmly onto the cap. 11. Gently blow the candle out then flip the capped bottle right side up, gently shake it 10 times, and place it upright on the table. Observe the contents and record your observations. 12. Put the bottle someplace safe so it can sit undisturbed while you complete Part C. Part C: Forming Limestone (working in pairs) 13. Use the structural diagrams in Figure 5.10 and the molecular modeling pieces to create one molecule of carbon dioxide (CO 2 ) and one molecule of calcium hydroxide (Ca(OH) 2 ). Put any unused modeling pieces away. O C O carbon dioxide H O Ca O H calcium hydroxide Figure 5.10 The molecular structure of carbon dioxide and calcium hydroxide. 14. Limestone rock is made of calcite molecules (CaCO 3 ). To model limestone formation, remove all the white bonds from between all the atoms of the CO 2 and Ca(OH) 2 molecules. Reassemble all the bonds and atoms from the broken reactant molecules into a calcite molecule (CaCO 3 ) and a water molecule (H 2 O). 15. Write down the chemical equation for the formation of limestone. 16. Observe the bottle of limewater again and record your observations. Analysis With your group, complete the following questions and record your answers in your notebook. Be prepared to share your answers with the rest of the class 1. The combustion of any hydrocarbon results in the production of the same two substances. Name these two substances. 2. What happens when CO 2 is mixed with limewater? Explain why you think this happens. 3. CO 2 dissolves readily in seawater, which contains abundant Ca 2+ ions. What do you think would happen to Earth s climate if a lot of CO 2 were to dissolve from the atmosphere into seawater? Explain why you think this would happen. Carbon dioxide is an important greenhouse gas, and its concentration in the atmosphere is an important driver of climate change. What are the factors that can cause CO 2 levels in the atmosphere to rise? What processes in Earth s systems subtract CO 2 from the atmosphere? What types of human activities affect CO 2 levels in the atmosphere? In Activity 4, explore these questions. 126

17 chapter 5 The Bigger Picture: global climate ACTIVITY 4 Calling All Carbons Setting the Stage: The Carbon Cycle The element carbon is critical to life on Earth. All living organisms contain many different and essential carbon-based molecules. Food is made from carbon-based molecules. Carbon dioxide (CO 2 ) is needed for plants to survive and if the atmosphere did not have any, the Earth would be much colder, perhaps too cold for living things to survive. However, too much CO 2 in the atmosphere could make the planet too hot for living things. Several Earth processes work together to cycle carbon from one carbon reservoir to another and to keep the amount in each reservoir stable. There are several major transfer processes that occur constantly that help carbon most often in the form of CO 2 move from reservoir to reservoir. These carbon reservoirs and transfer processes work together to create Earth s carbon cycle. Because the actual carbon cycle is so vast and complex (it includes every plant, animal, microbe, ocean lake, river, puddle, soil, sediment, volcanic eruption, etc.), it is often simplified to show only the most important reservoirs and processes, as shown in Figure Because the amount of CO 2 in the atmosphere is so critical to Earth s climate, you will focus on how CO 2 is added to and subtracted from the atmosphere in this activity. Some processes add CO 2 to the atmosphere and are called CO 2 sources; other processes remove CO 2 from the atmosphere and store it in various places called carbon sinks. This movement of carbon from one reservoir to another is called the carbon flux and when properly balanced, it keeps the amount of carbon in each place relatively constant. This helps maintain stable conditions in Earth s biosphere, hydrosphere, atmosphere, and geosphere. Figure 5.11 Simplified diagram of the carbon cycle, showing reservoirs in blue and carbon transfer processes in red. combustion of fossil fuels volcanism atmosphere chemical weathering photosynthesis combustion during forest fires animal respiration ocean diffusion plant respiration and decomposition rivers vegetation oceans soils fossil fuels rock burial of sediments and fossil fuel formation 127

18 EDC Earth Science Unit 2 atmosphere and climate Procedure Record all observations and answers in your notebook as you work. 1. With your group, study the information on all 8 Carbon Cards and make sure everyone in your group knows the name of each process and understands how the process works. You might do this by splitting up the cards and explaining the processes to each other. 2. Sort the 7 Carbon Transfer Process Cards into groups, based on whether each process is a CO 2 source or contributes to a carbon sink. Record your results. 3. Sort the cards within each group (sources and sinks), in order from highest annual carbon flux to lowest carbon flux. Record your results. 4. Summarize your findings by drawing a table in your notebook with seven rows beneath the headings shown in Table 5.2 below. Then fill in the information. Materials For each group of students 1 set of 8 Carbon Cards Table 5.2 Name of process How Process Works Source or Sink Annual Carbon Flux 5. Write answers to the Analysis questions in your notebook. Be prepared to discuss them with the class. Carbon Transfer Process: Chemical Weathering of Rocks net effect: removes CO 2 from atmosphere and stores it in the ocean carbon flux: 0.1 billion metric tons/year Rainwater (H 2 ) and carbon dioxide (CO 2 ) from the atmosphere combine in soil and rock crevices on the land to form a weak acid called carbonic acid (H 2 CO 3 ). This carbonic acid weathers and chemically breaks down the rocks, as shown in Figure 5.12, and rainwater washes away dissolved ions. These ions are carried by rivers to the sea where some of them (Si +4, Ca +2, and C0 2 3 ) are taken up in the growing shells of microorganisms (CaCO 3 and SiO 2 ). These shells settle to the bottom of the ocean when they die. The net effect of this entire multistep process is that CO 2 is removed from the atmosphere and stored in the shells of organisms. A generalized chemical equation for this process is CaSiO 3 + CO 2 CaCO 6 + SiO 2 rocks from the atmosphere calcite in shells quartz dissolved in seawater or incorporated into shells Figure 5.12 Weathered rocks. 128

19 chapter 5 The Bigger Picture: global climate Carbon Transfer Process: Volcanism Associated with Tectonic Plate Processes net effect: adds CO 2 to the atmosphere that was previously stored in rocks carbon flux: 0.2 billion metric tons/year Rocks of Earth s crust containing carbon are recycled by tectonic plate processes. This carbon is released in the form of CO 2 when the rocks are melted, and the gas separates out and is erupted from volcanoes, such as the one shown in Figure These volcanoes most commonly occur near plate boundaries, along subduction zones and seafloor spreading ridges. The net effect of this process is that carbon stored in rocks is released in the form of CO 2 to the atmosphere. Figure 5.13 Mount St. Helens. Carbon Transfer Process: Formation of Fossil Fuels net effect: CO 2 is removed from the atmosphere and incorporated into rocks carbon flux: < 1 billion metric tons/year Usually, when living things die, they decompose, releasing the carbon from which they are built to the atmosphere. In some (rare) cases, these organic remains are instead preserved in an oxygen-poor environment such as the bottom of certain ocean basins or in stagnant swamps such as the coal swamp in Figure These remains accumulate and, over millions of years, are buried and incorporated into rock. Heat and pressure transform the preserved organic material into oil, natural gas, and coal. Oil and natural gas form primarily from the remains of microscopic marine organisms. Coal forms from land plants. It is estimated that approximately 100 tons of ancient life is converted to 1 gallon of gasoline. The net effect is to store carbon in the rocks of Earth. Figure 5.14 Picture of a coal swamp. 129

20 EDC Earth Science Unit 2 atmosphere and climate Carbon Transfer Process: Combustion of Fossil Fuels and Other Organic Matter (such as wood) net effect: CO 2 that was stored in rocks is added to the atmosphere carbon flux: 5 billion metric tons/year When fossil fuels such as oil, natural gas, and coal are extracted from Earth and burned (combusted), carbon and hydrogen in the fuel reacts with oxygen from the air to form CO 2, water, and heat. The CO 2, water vapor, and other by-products are released to the air through smokestacks such as the ones at the coal-burning power plant in Figure The heat may be transformed into electricity and transported to people s homes and businesses for power, or may be transformed into mechanical energy in cars, trucks, or other vehicles. When wood or other organic material is burned, it also releases CO 2. The net effect of the combustion of fossil fuels and other organic materials is to release CO 2 into the atmosphere. Figure 5.15 Coal-burning power plant. Carbon Transfer Process: Photosynthesis net effect: removes CO 2 from the atmosphere and stores it in plants carbon flux: 102 billion metric tons/year Plants such as the one in Figure 5.16 remove CO 2 from the atmosphere and store it. Photosynthesis uses CO 2 from the atmosphere, energy from the Sun, and water to produce carbohydrates (sugar) and oxygen. Light energy from the Sun is stored in the chemical bonds of the sugar molecule. Some of these carbohydrates are incorporated into plant tissue, where they are stored. The net effect of this process is that CO 2 is removed from the atmosphere and stored in the tissue of plants. A generalized chemical equation for the process of photosynthesis is Figure 5.16: Plants use CO 2, water, and energy from the Sun during photosynthesis. 6CO 2 + 6H 2 O C 6 H 12 O 6 (sugar) + 6O 2 130

21 chapter 5 The Bigger Picture: global climate Carbon Transfer Process: Respiration and Decomposition net effect: removes carbon from plants and adds CO 2 to the atmosphere carbon flux: 100 billion metric tons/year Respiration (the reverse of photosynthesis) involves the breakdown of sugar using oxygen to obtain energy and building blocks to form new biomolecules. These reactions release CO 2, water, and energy as products and byproducts. Respiration occurs in both plants and animals, such as the dog in Figure Another important way that CO 2 is added to the atmosphere is decomposition. When organisms decompose, they are consumed mostly by bacteria and fungi, and CO 2 is released. The net effect of respiration and decomposition is carbon stored in carbohydrates within plants and animals is released into the atmosphere in the form of CO 2. A generalized chemical equation for the process of respiration is C 6 H 12 O 6 + 6O 2 6H 2 O + 6CO 2 Figure 5.17 This dog is breathing in oxygen and breathing out CO 2. Carbon Transfer Process: Diffusion of Carbon Dioxide Into and Out of the Ocean net effect: variable; depends on temperature of ocean carbon flux: 92 billion metric tons/year from atmosphere into ocean; 90 billion metric tons/year from ocean into atmosphere There is a certain amount of CO 2 that can be dissolved in seawater. That amount depends on the temperature of the water. Cold water can hold more CO 2 than warm water, so cold seawater tends to absorb CO 2 from the atmosphere. Warmer water has higher kinetic energy. This higher kinetic energy increases molecular motion, which breaks intermolecular bonds and causes gases to escape from the solution. Therefore, warm seawater tends to release CO 2 to the atmosphere. So the net effect of the oceans on atmospheric CO 2 varies with the ocean temperature, as shown in Figure Higher latitude oceans tend to remove more CO 2 from the atmosphere than they release (although as the water warms, this balance can change), and tropical oceans tend to release more CO 2 to the atmosphere than they take up. Figure 5.18 Sea surface temperature: The orange areas are the highest temperatures and the dark blue areas are the lowest. 131

22 EDC Earth Science Unit 2 atmosphere and climate Analysis Complete the following questions and record your answers in your notebook. Be prepared to share your answers with the rest of the class. 1. List Earth s major carbon reservoirs. 2. Describe a process that has the net effect of removing carbon from the atmosphere. 3. Describe a process that has the net effect of adding carbon to the atmosphere. 4. Describe a process that both adds carbon to and subtracts carbon from the atmosphere. 5. During the 19th century, much of the forested land area in the eastern United States was cleared for agriculture. What effect would you expect this to have on CO 2 levels in the atmosphere? Explain your thinking. 6. If a large volcanic eruption were to occur somewhere on Earth, how would you expect this to affect CO 2 levels in the atmosphere? 7. Compare the carbon flux rates of the various carbon transfer processes. Which processes absorb or release CO 2 at a lower rate, and which processes have a higher yearly flux rate? 8. Scientists have related the increased use of fossil fuels by humans to the warming of Earth s atmosphere that is occurring. Using data from the Carbon Cards and what you ve learned about the greenhouse effect, explain how increases in fossil-fuel use could cause the climate to warm. 9. Energy can also be produced from biomass, such as corn and sugar cane, and proponents of biofuels say this is a better alternative than fossil fuels. Based on what you know about the carbon cycle, what do you think of this idea? 10. Describe and explain other natural events and processes or human activities that could affect CO 2 levels in the atmosphere. By now you should understand that, although there s a clear link between CO 2 levels and Earth s temperature, there are many other factors that need to be considered. This next reading discusses how Earth s systems can interact to either stabilize the climate or accelerate climate change. 132