Three-dimensional Learning Progression Understanding how the Keeling Curve represents patterns in the Earth’s atmosphere. The Keeling Curve is often presented as easily interpretable evidence that the concentration of CO2 in the Earth’s atmosphere is increasing, but our research shows that interpreting this graph presents many challenges for students. In particular: The variable measured—concentration of CO2 in parts per million—is not easy for students to understand. It is not at all clear to students how measurements of CO2 concentration on a mountain in Hawaii might be related to CO2 concentrations in other parts of the world. The Pumphandle Video, introduced to students in Lesson 2, shows the complex relationships among measures of CO2 concentration taken at different locations on Earth. (See below for a description of the Pumphandle Video.) Explaining patterns of change in CO2 concentrations. We assume that students studying this Unit will be familiar with carbon-transforming processes (photosynthesis, cellular respiration, combustion, digestion, biosynthesis) in individual plants and possibly animals and decomposers. In this Lesson they consider how these processes affect carbon pools on a global scale. There are two patterns evident in the Keeling Curve: an annual cycle caused primarily by changing rates of photosynthesis in the Northern Hemisphere and a long-term increase caused primarily by burning of fossil fuels and land-use changes that release carbon from biomass or soil carbon into the atmosphere. This lesson focuses on helping students use pool-and-flux models to explain those patterns. There are many fluxes that move carbon into or out of the atmosphere, but most of those are balanced by other fluxes. The flux from fossil fuel combustion, in particular, is not balanced: it moves carbon permanently from the fossil fuel pool into the atmosphere. However, most students rely on simpler heuristics or rules of thumb rather than pool-and-flux models to explain patterns of change, including the good vs.bad heuristic and the correlation heuristic. Good vs. bad heuristic They use an informal frame that describes things that happen to the environment as good (e.g., less pollution) or bad (e.g., using fossil fuels). For instance, here is a reason that one student gave for cutting fossil fuel use: “If it cuts down and maintain a low level use, the air will clear up and it will be good for animals and humans to breath clean air.” Students using this heuristic also connect bad actions to bad outcomes: “[b]ecause I think we’ve reached a point where we’ve done too much damage to earth, personally. And I don’t think we can come back from that.” Correlation heuristic: These students often applied the correlation heuristic, conflating changes in flux (slope of the graphed line) with changes in pool size (value on the Y-axis). The following written response reflects this type of thinking: “fossil fuels help to produce CO2 so if we cut it in half it would decrease.” Note how this student used “it” twice in the same sentence, perhaps without recognizing that each “it” had a different meaning: …if we cut it (CO2 emissions—the flux arrow) in half, …it (CO2 concentration—a measure of the size of the atmospheric CO2 pool) would decrease. Predicting patterns of change in CO2 concentrations. The good vs. bad heuristic and the correlation heuristic can be useful for many purposes, helping us to identify environmentally responsible actions and processes that cause climate change. However, these approaches often lead to spurious quantitative reasoning, such as when students conflate a change in flux with a change in pool size: Cutting CO2 emissions in half does NOT decrease CO2 concentration in the atmosphere; it merely makes the concentration go more slowly. So in order to make accurate predictions, students must use quantitative reasoning to balance all the CO2 fluxes into and out of the atmosphere. Activities 4.3, 4.4, and 4.5 engage them in using the balance of fluxes to make predictions about changes in pool sizes in increasingly sophisticated ways. Key Ideas and Practices for Each Activity Activity 4.1 Introduces students to the two patterns in the Keeling Curve (the short-term seasonal fluctuation and the long-term trend) and asks them to document their initial ideas about these two key patterns. Because increasing atmospheric CO2 is the driver of all other Earth Systems explored in this unit, we spend more time on this pattern than others. In previous lessons they collected evidence that shows that this phenomenon is happening; in this activity they go one step further and explain why they think this is happening. Because this is an initial ideas stage, students should not be penalized for incorrect ideas. Activity 4.2 takes a step back from the regular progression of the unit to examine fossil fuels. In the organismal units, this would be the equivalent of the “foundational knowledge” activity. Students examine fossil fuels in three different ways. The first is through introduction to the Carbon Pools Question, where they examine the different Carbon Pools in the unit. They then zoom into the fossil fuels pool specifically to learn about (a) the molecular structure of fossil fuels, and (b) how fossil fuels were formed. This provides the foundational information for understanding why fossil fuels burn (they are constructed from organic molecules). Activity 4.3 uses a hands-on activity (the Tiny World Modeling game) to help students figure out explanations for both the annual cycle and the long-term trend in the Keeling Curve. The Tiny World has three carbon pools: (a) atmospheric CO2, (b) organic carbon in living systems and soils, and (c) fossil fuels. Carbon atoms move among these pools in three carbon fluxes associated with carbon-transforming processes that students have studied before: Photosynthesis moves carbon atoms from the atmosphere to organic matter in plants, animals, and soil. Cellular respiration moves carbon atoms from organic matter in plants, animals, and soil to the atmosphere Combustion moves carbon atoms from the fossil fuels pool to the atmosphere. In playing the game students see how the balance among fluxes determines changes in pools, and how seasonal variations in fluxes and combustion of fossil fuels leads to patterns like those in the Keeling Curve. In Activity 4.4 students use a computer model to make quantitative predictions about effects of changes in pools and fluxes. The computer model has the same pools and fluxes as the Tiny World model (Activity 4.3), but the sizes of pools and fluxes are based on current data. Students can control the size and timing of changes in fluxes and see projections of the long-term effects (50 years) for those changes. Activity 4.5 is an optional (two turtle) activity that dives into the seasonal fluctuation of carbon dioxide in the atmosphere pool and into other pools and fluxes in the global system. Students identify photosynthesis as the specific flux driving seasonal variations in CO2 concentrations. They view videos with animations of data to show how variations in sunlight in the hemispheres drive different yearly patterns of concentration. They also use a global carbon cycling diagram to discuss other pools and fluxes (associated with oceans and land use change) and make predictions about the effects of decreasing use of fossil fuels. These explanations and predictions are the most complex in the unit. More information on the Pumphandle Video.[1] This video provides a visual representation of the data that has been collected of carbon dioxide concentrations in the atmosphere and how it has changed over time. Tell students that when they are watching the video, to see if the same seasonal and upward trends that are present in the Keeling Curve are present in other places on the planet where data about carbon dioxide are collected. Watch the video, which is 3 minutes and 35 seconds long. 0:00-0:30—Point out to students the various pieces of information in this image. Each colorful dot represents a location on the planet where data about atmospheric carbon dioxide has been collected over time. The data are aligned by latitude, with the right side of the graph showing the most northern points. The bright red dot represents Mauna Loa. The bright blue dot represents the South Pole in Antarctica. They can see where these locations are on the small map on the right side of the image. 0:30-1:00—Point out that the graph on the right side of the images is charting data from both Mauna Loa (northern hemisphere) and also Antarctica (southern hemisphere). Ask them if they notice the difference between the data from the northern and southern hemispheres. What might be the reason for the larger flux in the northern hemisphere, and the smaller flux in the southern hemisphere? (There is more land and more plants in the middle of the Northern Hemisphere compared to the South Pole.) Point out the small circle on the right side of the image that shows the time of year. 1:00 – Around this time in the video, students will notice that some of the CO2 levels recorded at different places on the planet fluctuate much more dramatically than at Mauna Loa. Ask them why they think this might be happening. Point out that the Mauna Loa data does not fluctuate as much because the readings are taken on top of a mountain that is surrounded by ocean, so the signals from the plants and human emissions (both releasing and taking in CO2) do not impact the reading as dramatically. 1:15 – Pause the video and ask students to notice during which months of the year the CO2 levels are highest in the northern hemisphere. Ask them to make connections to what they see on the screen and their image of the Keeling Curve. Point out that this is the seasonal cycle. Then, ask students to notice that the line on the left is continuously rising. Ask them which line on the Keeling Curve image on their worksheet corresponds with this increase. Point out that this is the upward trend. 2:00 – Remind students that in addition to having data about CO2 in the atmosphere from the past 60 years from Keeling’s and others’ experiments, we also have information about how much carbon was in the atmosphere from many years in the past. We get these data from studying carbon isotopes in the ice cores in Antarctica. 3:10 – With the long-term carbon dioxide levels on display in the graph, ask the students what they think this graph is showing. Point out that although the levels of carbon dioxide on the planet have fluctuated over time, in millions of years they have never reached the levels that they are today: nearly 400 ppm [1] This video is a nice opportunity to point out that even though there are short-term variations in the temperature and CO2 levels, that the overall trend is still increasing. Students may ask “If global warming is happening why was it so cold this winter?” Even as the global temperatures continue to increase, we will still see unusually cold winters and even summers. These short-term cold periods (e.g., one season or month) are due to local weather, short-term changes in the movement of polar winds, and ocean circulations, and do not reflect the overall warming trend. However, it is predicted that climate change might make some of these local, extreme weather events more severe over a long-time scale. Content Boundaries and Extensions The primary focus of this lesson is on understanding how pools and fluxes, particularly the unbalanced flux from combustion of fossil fuels, Activities 4.1 through 4.4 focus on terrestrial carbon pools and hands-on or computer models. Activity 4.5, focusing on a graphic model and including fluxes into and out of the oceans, is an optional extension. Key Carbon-Transforming Processes: Combustion, Photosynthesis, Cellular Respiration
