Lab 4: The Carbon Cycle (Part 1)

Section 1

You learned in the troposphere lab that carbon dioxide (CO₂) makes up about 0.04% of the atmosphere. You will see in later labs just how important this relatively small concentration of CO₂ is with expect to Earth’s energy balance, but for now we need to focus on the cycle in which atmospheric CO₂ is involved. As you have hopefully deduced by looking at the title of the lab, the name of the cycle is the carbon cycle. Carbon is constantly being moved around the Earth through the processes that make up the Carbon Cycle. This lab is concerned with the different processes comprising the carbon cycle, but its importance goes beyond just becoming familiar with the various ways that carbon is transformed and transported around the globe. That is because the lab also is focused on how human activity can influence those processes – and potentially influence the weather and climate on the planet.

By the end of this lab, you should be able to answer the following research questions:

How can carbon be transferred between the atmosphere and Earth’s other spheres?

How would you describe changes in fossil-fuel carbon emissions from 1959 to 2014?

How would you best describe general changes in the uptake of carbon by the atmosphere, oceans, and terrestrial biosphere from 1959 to 2014?

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Entering with the right mindset
Throughout this lab you will be asked to answer some questions. Those questions will come in three different varieties:

Fact-based question →This will be a question with a rather clear-cut answer. That answer will be based on information (1) presented by your instructor, (2) found in background sections, or (3) determined by you from data, graphs, pictures, etc. There is more of an expectation of you providing a certain answer for a question of this type as compared to questions of the other types.

Synthesis-based question → This will be a question that will require you to pull together ideas from different places in order to give a complete answer. There is still an expectation that your answer will match up to a certain response, but you should feel comfortable in expressing your understanding of how these different ideas fit together.

Hypothesis-based question → This will be a question which will require you to stretch your mind little bit. A question like this will ask you to speculate about why something is the way it is, for instance. There is not one certain answer to a question of this type. This is a more open- ended question where we will be more interested in the ideas that you propose and the justification (‘I think this because . . .’) that you provide.

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Section 2

One of the key ideas you should have taken from the Carbon Cycle Prezi, provided you have seen it, is that in any matter cycle, the substance at the heart of that cycle (e.g., water, nitrogen, and carbon) is converted to different physical and chemical forms and moved from one part of the Earth to another. As you move through the Carbon Cycle lab, you will see how carbon moves through the different ‘spheres’ into which scientists divide the planet: the biosphere (global set of ecosystems where living things are found), the lithosphere (the rocky upper layers of the Earth), the hydrosphere (combination of all sources of water on under and over the Earth’s surface), and the atmosphere (layers of gases above the Earth). In the biosphere, carbon is found in the form of organic compounds trapped in living organisms and in the soil; in the lithosphere, it is held in carbonate rocks and other materials like coal; and in the hydrosphere it dissolves in the water to form carbonic acid. Practically all the carbon in the Earth’s atmosphere exists in the form of carbon dioxide (CO₂) and methane (CH₄).

The picture below shows the global carbon budget for 2010. This is a way of tabulating the amount of carbon (in billions of tons) that is released by carbon sources and absorbed by carbon sinks. Ideally, the Earth would have a ‘balanced spreadsheet’ where those two amounts were equal; as you examine the picture you should consider whether this was the case in 2010.

Q1: Identify two sources of carbon in the picture. Identify two sinks.

Q2: Where did the 9 petagrams of carbon emitted into the atmosphere by anthropogenic activities in 2010 end up going?

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Section 3

So let’s follow a carbon atom through one portion of the carbon cycle. Look at the picture below that focuses on that portion: the exchange of carbon between the atmosphere and the biosphere.

As noted above, most of the carbon atoms in the atmosphere are nestled in between two oxygen atoms in the form of CO₂. Green plants can take in that CO₂, combine it with water (H₂O) and make carbohydrates (literally, hydrated carbon) through the process of photosynthesis. It is through that process that plants grow and gain mass. While it may be hard to imagine that plants get their mass from water and an invisible gas in the air, the time-lapse photograph of wheat-grass growing may make that more believable.

Q3: If that wheatgrass were out in nature (instead of in a videographer’s studio), what would happen to the carbon captured in the wheatgrass once it died?

Now, there are a lot of ways that this carbon in the carbohydrates can be released back into the atmosphere. Both plants and the animals who eat them can break the carbohydrates (mostly the sugar glucose) back down into water and carbon dioxide, getting some useful energy out of the process, and releasing the CO₂ through respiration. Also, fungi and bacteria can break down the carbon compounds in dead plants and animals and convert the carbon to CO₂if oxygen is present. Finally, combustion (which is really the same process as respiration, except that it involves burning fuels instead of foods) can oxidize the organic (carbon-containing) materials in plants back to CO₂. One form of combustion that you ran across in a previous lab (The Troposphere) was slash and burn, which is used to clear out large areas of forest for agriculture and is thus a major cause of deforestation. The image to the left below, which you saw in the previous lab, shows the slash-and-burn process in action. The the NASA-created time series of deforestation in Rondônia in western Brazil shows the massive changes in the landscape caused by deforestaton. When clicking on the image to the right below, you will be able to visualize in Google Earth the deforestation in Rondônia.

Q4: Green plants both photosynthesize (causing them to act as a carbon sink) and respire (causing them to act as a carbon source); based on the carbon cycle picture to the right above, are these plants a net sink or source of carbon?

Q5: People have concerns about the use of slash and burn partly because it affects the carbon cycle in multiple ways. What are two of those ways?

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Section 4

Next, let’s focus on the other half of the larger carbon cycle: the exchange of carbon between the atmosphere and the hydrosphere. Take a look at the picture below that focuses on the processes involved in this exchange.

It should make sense that, since oceans cover 70% of the Earth’s surface, we would want to concentrate on how carbon moves between the atmosphere and the oceans. You can see from the picture above that some of the same processes – photosynthesis and respiration – are part of the exchange between these two parts of the global ecosystem. There is another mechanism by which the oceans can act as both a carbon sink and a carbon source: the water in the ocean can absorb carbon dioxide (leading to acidification of the ocean water) and, like a soda going flat, release it through the agitation caused by ocean currents.

Q6: Overall, is there a greater amount of carbon exchanged between the atmosphere and the biosphere, or between the atmosphere and the hydrosphere?

Q7: How did the amount of carbon absorbed by the hydrosphere compare to the amount carbon released by the hydrosphere in 2010?

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Section 5

There is one last portion of the carbon cycle on which we want to focus. Before we do so, though, it is important for us to remember that the carbon cycle represents a global system, so even though we have looked at the cycle in manageable portions, we have to remember that the processes in each portion are interrelated and any change in one process produces changes in the others.

All right, so with that reminder in place, we can direct our attention to how carbon is exchanged between the lithosphere and the atmosphere. In the Prezi for this lab, we saw one video (the original full-length NASA video is provided below) that talked about “young fast carbon”. This is the kind of carbon that is found in living plants among other things, and we call it that because it can easily move into and out of the carbon cycle. The video also talked about “old slow carbon”, which is the kind found in fossil fuels. It is labeled this way because this kind of carbon has been trapped in a form (natural gas, oil, and coal) that makes it more unlikely that the carbon will enter the carbon cycle. Well, more unlikely until humans started to pull it out of the Earth and burn it for electricity production. When that combustion of fossil fuels takes place, carbon with hydrogen atoms attached to it (hydrocarbons) is transformed to carbon with oxygen attached to it (carbon monoxide and carbon dioxide). The image on the left below shows the percentage of carbon dioxide from the combustion of various fossil fuels. The image in the middle is of Plant Bowen, a coal-burning power plant near Cartersville, GA; it should be familiar to you as the background of the Prezi. Finally, the image on the right is a schematic of Plant Scherer which is southeast of Atlanta.

Q8: Which form of fossil fuel did you expect would have contributed the most to carbon dioxide production? Which form actually contributes the most?

Q9: In the Plant Bowen picture, the cooling towers are highlighted. Many people incorrectly link the presence of cooling towers to the presence of nuclear power plants, but all large-scale power plants have them. What is it that is coming out of the cooling towers? (Hint: This has an important relationship to the matter cycle you considered in the Troposphere lab.)

Humanity’s thirst for cheap sources of energy has led us to search for new places from which to obtain fossil fuels. One of those is shale formations which are found throughout the United States and other countires. Watch the PBS video below about the extraction of oil and natural gas from these shale formations, particular Marcellus shale, then answer Q10 and Q11.

Q10: What is meant by “fracking”?

Q11: What are the benefits – and the risks – with getting natural gas and oil from shale through the fracking process?

Before moving on to the next section of this lab, it is important to put together the separate pieces of the carbon cycle that were discussed above. To that end, take a look at the picture of the 2010 global carbon budget that is found below and that also opened this section.

Notice that the anthropogenic (human as source) emissions of carbon is greater than the uptake of carbon by the biosphere and hydrosphere. Man’s activities such as fossil-fuel combustion and deforestation released nine petagrams (where a petagram is 1,000,000,000,000,000 or 10¹⁵ grams) of carbon to the atmosphere in 2010. [Approximately 90% of those anthropogenic carbon emissions came from burning fossil fuels.] Out of those 9 petagrams, three were taken up by plants for use in photosynthesis; two of the 9 petagrams were absorbed by the ocean. If you do some tough math, you realize that that left 4 petagrams unaccounted for – i.e. there was a net addition of 4 petagrams of carbon in the form of carbon dioxide to the atmosphere. Another way of saying that is that the amount of carbon in the lithosphere (in the form of fossil fuels) is decreasing, and the amount of carbon in the atmosphere (in the form of CO₂), biosphere and hydrosphere is increasing. Something for us to think about.

Q12: What do you think would be the effect on life on this planet if the kind of net release of carbon described above for 2010 were to continue for the next 20 years?

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Section 6

The carbon cycle graphic you explored in Section 2 only shows data from 2010. As has been in the case in previous labs, we are interested in changes over time – in this case, changes in the global carbon carbon budget. Researchers have been able to estimate the following for each year from 1959 to the present: (1) carbon emissions from fossil-fuel combustion and cement production, (2) carbon emissions from land-use change (i.e. converting forest to agriculture, as in slash-and-burn farming), (3) uptake of carbon by the atmosphere, (4) uptake of carbon by the oceans, and (5) uptake of carbon by the terrestrial biosphere. The graphic below shows changes in those five processes from 1959 to 2010. Anthropogenic emissions of carbon (fossil-fuel combustion, cement production, and land-use change) have negative values since those processes represent a loss of carbon from a particular sphere (i.e., the burning of fossil fuels releases carbon from the lithosphere, which is acting as a carbon source). A positive value indicates that a sphere (e.g., atmosphere, hydrosphere, and biosphere) has gained carbon (i.e. that sphere is acting as a carbon sink). The atmospheric, ocean, and terrestrial biosphere can have positive or negative values since they can act as sources or sinks.

The graphic below shows the amount of carbon released from the main sources and taken in by the main sinks from 1959 to 2014. As you examine it, you should be thinking about the relationship between the amount of carbon released by the sources and absorbed by the sinks over the 55 years shown, as well as the trends in those processes over that time frame.

Q13: What has been the general trend in the sizes of sources and sinks from 1959-2014?

Q14: For each year, what is the relationship between the magnitude of carbon emissions and the magnitude of carbon uptake?

Now, we are going to look at the trends in that same data a little more closely through the power of Microsoft® Excel. Click CarbonEmissions_1959-2014 to open the Excel file of interest. This file contains annual emissions of carbon (in petagrams) from 1959 to 2014 from fossil-fuel combustion and land-use change (primarily slash and burn). Do the following to create a graph from the data in that file:

Select cells in rows 4 through 60 of columns A, B, and C.
Under the Insert tab, select Line and then choose the first 2-D line.
The resulting chart shows annual emissions of carbon fossil-fuel combustion and land-use change

Q15: How does the trend in fossil-fuel emissions differ from the trend in land-use change emissions?

Q16: For the source that had the largest increase over those years, what do you think was responsible for that increase?

The Excel data allowed us to look at trends in emissions and uptake of carbon from various sources and sinks over time. We would like to zoom in on one of those sources – fossil fuel combustion – and try to pinpoint places where the emissions from that source changed the most over time. Maps of fossil-fuel carbon emissions have been developed by the Carbon Dioxide Information Analysis Center for all years from 1781 to 2010. Click Carbon Emissions 1950-2010 to open the file in Google™ Earth see how carbon emissions have changed from 1950 to 2010. Look at country-level CO₂ emissions in 2014 to answer Q18.

Q17: From what regions/countries was most of the carbonemitted in 1950?

Q18: What country had the most emissions in 2014?

Next, let’s focus on the changes in the carbon sinks over time. Click CarbonSinks_1959-2014 to open the appropriate Microsoft® Excel file. This file contains annual uptake of carbon (in petagrams) by the atmosphere, oceans, and terrestrial biosphere from 1959 to 2014.

Select cells in rows 4 through 60 of columns A, B, C, and D.
Under the Insert tab, select Line and then choose the first 2-D line.
The resulting graph shows annual uptake of carbon by the atmosphere, oceans, and terrestrial biosphere.
Since you will need the graph to answer the last question on this page keep it open after you complete the questions below.

Q19: What has been the general trend in the amount of carbon taken up by the atmosphere, oceans, and terrestrial biosphere over the last 50+ years?

Q20: How did carbon uptake during 2005-2014 differ from carbon uptake during 1959-1968? Where has more and more of the carbon been going?

You should have noticed an anomaly (piece of data that did not fit the general pattern) around 1992.

Q21: What was that anomaly? Provide some reasonable hypothesis for what might have caused the unusual data collected in that year?

You learned in Section 2 of this lab that photosynthesis is a carbon sink in the terrestrial biosphere. That means that anything that increases the amount of photosynthesis taking place will increase the amount of carbon uptake by the biosphere. Photosynthesis requires light in the visible part of the electromagnetic spectrum to provide the energy for converting water and carbon dioxide into carbohydrates (see picture on left below of the EM spectrum). As a result, anything that increases the amount of visible light available to plants will increase the level of photosynthesis and increase carbon uptake from this process.

Volcanic eruptions release sulfate aerosols into the atmosphere (see the picture of the 1991 Mount Pinatubo eruption below that shows this happening). Now, at first, it would seem that those aerosols would be likely to decrease the amount of sunlight reaching the Earth as they would reflect that light up and away from the Earth. However, recent research (see the thumbnails of two examples on the right below) has shown that increasing the amount of sulfate aerosols can actually increase the amount of solar radiation available for photosynthesis. This is because the solar radiation (in the form of visible light) that does reach the surface is more diffuse than normal. That means that it is coming from many directions, rather than being a direct beam from the Sun. As a result, the radiation hits more leaves thereby causing an increase in photosynthesis.

Now, look back at the graph you produced in Microsoft® Excel from CarbonSinks_1959-2014 and reconsider the question from above:

Q22: Why was the uptake of carbon in 1992 so much larger for the terrestrial biosphere than for the atmosphere and oceans (i.e. why did that anomaly in the 1992 data occur)? Does this match the prediction in Q21?

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Section 7

Write responses of one to two sentences for each of the following research questions of the lab.

Q23: How can carbon be transferred between the atmosphere and Earth’s other spheres?

Q24: How would you describe changes in fossil-fuel carbon emissions from 1959 to 2014?

Q25: How would you best describe general changes in the uptake of carbon by the atmosphere, oceans, and terrestrial biosphere from 1959 to 2014?