Lab 4: The Carbon Cycle (Part 2)

Section 1

When last we left our hero the carbon atom, he was being wrested away from his comfortable hibernation in a piece of coal (or pool of oil or bubble of natural gas), combusted in a power plant (or car engine), turned into carbon dioxide, and sent flying off into the atmosphere.

Part 1 of this lab was really designed to take us through the different parts of the carbon cycle and look at how carbon moved between the various spheres of the Earth. We were also interested in the extent to which the movement between sinks and sources was balanced – as the Earth’s global carbon budget should be. All of this focused on the issue of carbon emission and uptake globally; in this lab, we take a look at this issue more locally.


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

  • How and why have atmospheric CO2 concentrations changed over the past 50 to 60 years? 

  • How do atmospheric CO2 concentrations vary across the globe?

  • How has the uptake of CO2 affected the oceans and terrestrial biosphere?


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.


 Section 2

We discovered on Day 1 of the lab that human activities – mostly fossil-fuel combustion – adds enormous amounts of carbon to the atmosphere every year. Nearly all of that carbon is in the form of CO2. For example, there was a net addition of approximately 4 petagrams of carbon to the atmosphere each year from 2009-2011; the picture below shows this (as part of the global carbon budget) from 2010.  That translates into approximately 15 petagrams of CO2.  Since a petagram is 1,000,000,000,000,000 grams, that amounts to 15,000,000,000,000,000 grams or 1,102,313,061 tons of CO2 during that four years – or a lot of carbon dioxide.

The figure below shows the growth of atmospheric carbon (i.e. the net increase in carbon) and changes in atmospheric CO2 concentrations from 1959-2010.  Click CO2_Stations to open the file in Google™ Earth see the locations of the sites (Barrow, Mauna Loa, Cape Matatula, and South Pole) where CO2 has been measured over the past several decades.  Be sure to zoom in on each station to see the actual observatory where CO2 measurements are made.

Q1: How would you describe the change in atmospheric CO2 concentrations over the past several decades?

Q2: At the end of the last lab, we were looking at a graph of carbon uptake and saw an unusual value (an anomaly) for the year 1992. Likewise, in the graph above, there is an unusual value for 1992 compared to the years around it. What could explain this minimal growth in atmospheric carbon in 1992?

Q3: Why do the two locations in the Northern Hemisphere consistently have higher concentrations than the two locations in the Southern Hemisphere? Referring back to CO2Emissions_1950-2010 can help you answer this question.

Click CO2_2010 to open the file in Google™ Earth and see changes in monthly CO2 concentrations in 2010. These data were derived from two sensors (the Atmospheric Infrared Sounder and the Advanced Microwave Sounding Unit-A) on-board NASA’s Aqua satellite.

Q4: What is the range (i.e. smallest and largest) in the values of the CO2 concentrations? Compared with what you saw in the Troposphere lab for water vapor and ozone, would you say this a relatively large or relatively small range in the concentration of a gas?

Q5: Based on what have seen in the lab so far, would you say that CO2 is a well-mixed gas or a short-lived gas?  We will revisit these terms in Lab 7 (Temperature Changes over the Past Millennium).

In the Google™ Earth file turn the globe so that you can see both North and South America at the same time. Replay the animation in order to answer the question below.

Q6: How do CO2 concentrations vary between the Northern Hemisphere and Southern Hemisphere?  Why are concentrations higher in one hemisphere compared to the other?


Section 3

In the Troposphere Lab, we learned about the Clean Air Act in the United States in which different sets of regulations controlling various pollutants in the air were passed in 1970, 1977, and 1990. As part of understanding the impact of that Act, you looked at the effect on the levels of PM10, NOx, and SOs emissions, and you should have noticed a decrease in pollutant emissions.  Regarding the sources of the pollutants, you learned the following in Lab 3 (The Troposphere): (1) industrial processes (e.g., smokestacks on factories) has been largest source of PM10; (2) motor vehicles has been the largest source of NOx; and (3) electricity production (i.e., power plants) has been the largest source of SO2.

Q7: What effect did the Clean Air Act have on pollutant emissions in the United States?

Q8: What effect do you think the Clean Air Act had on CO2 emissions in the United States? 

Click SO2&CO2_1970-2010 to open the file in Microsoft® Excel.  This file contains annual emissions of SO2 and fossil-fuel CO2 in the United States from 1970 to 2010.

  • Select cells in rows 1 through 42 of columns A, B, and C.
  • Under the Insert tab, select Line and then choose the first 2-D line.
  • Right-click on the red line (CO2 emissions) and put that on the Secondary Axis (under Format Data Series …)
  • The resulting graph shows annual emissions of SO2 and CO2 in the United States.

Q9: What was the general trend in CO2 emissions from 1970 to 2010?

Q10: Why do the trends in SOand CO2 emissions differ?

So far in Part 2 of the Carbon Cycle lab, we have been looking globally at CO2 emissions. Now we want to start to zoom in on the local situation by first looking across the U.S. as a whole, then focusing on the southeastern part of the country, and finally, concentrating on the Atlanta Metro area and surrounding counties.

We determined in Day 1 of the lab that carbon emissions have been much higher in the eastern half of the United States compared to the western half.  Let’s try to account for this difference. Click CO2_Emissions_2010 to open the file in Google™ Earth and see emissions of CO2 from fossil-fuel combustion in 2010. Click on Power_Plants to open the file in Google™ Earth and see locations of the power plants with the largest emissions of CO2 in 2002. The size of an orange circle is proportional to the emissions of CO2 from a facility.

Q11: Where are the CO2 emissions the highest? How does that compare to where we found the SO2  emissions to be the highest in the Troposphere lab?

Q12: Why are CO2 emissions so much higher in that part of the country?

All right, so what is the situation more locally? Click CarbonEmissions_Counties and Atlanta_MSA to open the files in Google™ Earth to get a feeling for the major sources of CO2 within the Atlanta Metropolitan Statistical Area (MSA; the white boundary in Atlanta_MSA outlines that area). These data come from the Vulcan Project, which is a NASA/DOE funded to quantify North American fossil-fuel CO2 emissions. The red counties have the most emissions. The pie charts shows the percentages of emissions from various activities. Click on the pie charts to see emissions data.

Q13: Look at Cobb, Fulton, DeKalb, and Gwinnett counties; what is the major source of CO2 emissions in those counties?

Q14: Which county in the Atlanta MSA has the highest CO2 emissions? What is it that is different about the major source of emissions in this county than the ones you looked at in Q13? What particular facility is the reason for this difference?

So far in the Day 2 lab, we have examined CO2 emissions at various scales (e.g., global, regional, national, and local) and CO2 concentrations in the middle troposphere – at remote locations and across the globe. We also measured the CO2 concentration at Hurt Park in downtown Atlanta in the Troposphere lab.  Answer the following question based on the knowledge you have gained to this point.

Q15: Do you think CO2 concentrations in rural parts of the eastern United States are generally higher or lower than CO2 concentrations at remote locations, such as Barrow, Mauna Loa, Cape Matatula, and the South Pole? Why?

A good example of a rural location in the eastern United States is Beech Island, South Carolina. CO2 is measured at 305 meters above the surface at Beech Island atop a tall tower; therefore, CO2 concentrations are not contaminated by local sources (e.g., an idling truck). Click BeechIsland to view the location of Beech Island in the southeastern United States.  Zoom in on the red bullseye to see the tall tower on which CO2 measurements are made.  Click MonthlyCO2_2010 to open the file in Microsoft® Excel.  This file contains monthly CO2 concentrations at Beech Island as well at Barrow, Mauna Loa, Cape Matatula, and the South Pole in 2010. Do the following to create a graph from the data in that file:

  • Select cells in rows 1 through 13 of columns A, B, C, D, E, and F.
  • Under the Insert tab, select Line and then choose the first 2-D line.
  • The resulting graph shows average monthly concentrations of CO2 at Beech Island, Barrow, Mauna Loa, Cape Matatula, and the South Pole during 2010.

Q16: Why did Beech Island have the highest CO2 concentrations among the five stations for a majority of the months?

Q17: Why were CO2 concentrations at Beech Island, and especially at Barrow, lower during the summer months than the winter months? You will learn more about the reason behind this in Section 4.

There are very few monitors in urban areas, but we are fortunate to have one of those monitors on the campus of GSU. The Department of Geosciences operates the monitor. Click CO2_Monitoring_Station to see a picture of the equipment that was involved in making accurate and precise measurements of CO2 concentrations on campus.  The CO2 analyzer measured the ambient CO2 concentration every 10 seconds. You are going to compare CO2 concentrations in downtown Atlanta with concentrations at Beech Island.

Click HourlyCO2_December2010 to open the file in Microsoft® Excel. This file contains the average concentration for each day at downtown Atlanta and Beech Island during December 2010. Do the following to create a graph from the data in that file:

  • Select cells in rows 1 through 25 of columns A, B, and C.
  • Under the Insert tab, select Line and then choose the first 2-D line.
  • The resulting graph shows average concentration for each of the day at downtown Atlanta and Beech Island during December 2010.

Q18: What is the major source of CO2 emissions in downtown Atlanta? What do you notice in the pattern of data that provides a clue to help answer this question?


Section 4

In Day 1 of this lab, we focused on the carbon cycle in terms of sources and sinks. In the first few parts of this lab, the emphasis has been on human activity – mostly the burning of fossil fuels – as an additional source of carbon that gets emitted into the atmosphere. As with any system in nature, the other parts of the system can respond to such a stress on the system’s equilibrium. For example, one response from the parts of the carbon cycle system to the increased CO2 levels from fossil fuel combustion could be increased absorption of CO2 by green plants to drive photosynthesis.

Given the importance of this process, let’s turn our attention to the terrestrial biosphere as a carbon sink. As shown top picture below, the terrestrial biosphere absorbs approximately three petagrams of carbon annually. When you see the two words “terrestrial biosphere” you should think of forests. There are three general types of forests across the globe: (1) tropical forests, which are predominantly evergreen broadleaf forests, (2) temperate forests, which have varying types of trees as a result of yearly variations in the temperature, and (3) boreal forests, which are predominantly evergreen needleleaf forests. Click MODIS Land Cover to open the file in Google™ Earth and including location of these different types of forests. [The image was derived from data from the MODIS (MODerate resolution Imaging Spectroradiometer) instrument on NASA’s Terra satellite (see the bottom picture below).]

Q19: Identify one place across the globe where there is a (1) tropical forest, (2) temperate forest, and (3) boreal forest?

  1. Tropical forest:
  2. Temperate forest:
  3. Boreal forest:



Q20: Which of the three forests do you think is the largest carbon sink and why?

In Part 1 of this lab, you learned that green plants within the terrestrial biosphere can act both as a carbon sink – as they turn CO2 and water into carbohydrates through photosynthesis – and as a carbon source – as they respire CO2 produced by the metabolism of carbohydrates when the plants need energy. What is really important to know is how much more carbon is taken up by photosynthesis than released through respiration.  This is known as net primary production (NPP).  Positive values of NPP indicate that more carbon was absorbed by photosynthesis than was released by respiration (i.e., that the terrestrial biosphere was acting as a carbon sink); negative values indicate the opposite situation.

NPP changes over time and space. We can explore these changes for one year by clicking on NPP_2010 to open this file in Google™ Earth and watching the animation of the variation of the NPP values for different places over that year. The data come from NASA’s MODIS instrument. Values range from near 0 grams of carbon per square meter per day to 6.5 grams per square meter per day.

Q21: Which forests have large positive values of NPP during each month (i.e., what forests have a green color year-round)?

Q22: Why do these forests have little seasonal differences in NPP?

Focus on North America by typing “North America” in the “Fly to” box.

Q23:  Which of the three types of forests in North America had the highest NPP values during the summer months?  Why did this occur during the summer and not during the winter?

After viewing NPP values for each month of 2010, you should now revisit your response to Question 17.

Q24: Which of the three forests do you think is the largest carbon sink and why?

Scientists are still trying to figure out which of the three forest types is the largest carbon sink.  The three images below provide recent information on the terrestrial biosphere carbon sink.  The first two images are the first pages from two articles from the journal Science, while the third image is a portion of Table 1 from the second Science article.

Q25:Which of the three forests appears to be the largest carbon sink?




Section 5

As with the terrestrial biosphere, the oceans (the largest component of the hydrosphere) can function as either a carbon source or a carbon sink. Look at the image below as a reminder of the different processes by which the ocean can act as a carbon sink and a carbon source.

Typically, the oceans take up approximately two petagrams of carbon each year. More specifically, the oceans have taken up half of all the CO2 emitted by human activities since pre-industrial times. With the increased production of CO2 by humans over the past two centuries, more and more carbon has been added to the oceans. When CO2 enters the ocean, a complex series of reactions occur in which the carbon dioxide is transformed into carbonic acid (by reacting with the water), the carbonic acid breaks apart into hydrogen ions and bicarbonate ions (which are found in baking soda), and some percentage of the hydrogen ions combine with carbonate ions (which are found in washing soda) to form additional bicarbonate ions. Why it is not important to remember all the parts of that complex series of reactions, it is valuable to understand one of its effects: the removal of carbonate ions that it makes it more difficult for marine organisms (known as calcifiers) to grow shells.

Q26: What other effects do you think anthropogenic CO2 emissions are having on marine organisms besides the impact on calcifiers described above?

As a result of data collected in Hawaii, we are better able to understand the effects of the increasing amounts of CO2 that the oceans are absorbing. Scientists working on the Hawaii Ocean Time-series (HOT) program have been making repeated observations of the hydrography, chemistry, and biology of the water column at a station north of Oahu, Hawaii U.S.A. since October 1988.  As you have seen earlier in this lab, measurements of atmospheric CO2 concentrations at Mauna Loa, Hawaii have been made since 1958. Click HOT_MaunaLoa to open the file in Google™ Earth and see the location of the HOT measurements relative to the Mauna Loa station.  The image below shows changes in atmospheric CO2 concentrations and the amount of dissolved CO2 in the middle of the Pacific Ocean (i.e. Hawaii).

Q27: What is the general relationship between atmospheric CO2 concentrations and the amount of CO2 dissolved in seawater?

As noted above, changes in the amount of CO2 in seawater also will affect the pH of the water as as well as the availability of carbonate ions.  The image on the left below shows changes in seawater pH and the concentration of carbonate ions near Hawaii.  The same pH diagram you saw in Lab 3 is shown again below.

Q28: What effect did the increased absorption of CO2 by the oceans have on the pH? Did the pH level ever get to the point where the ocean water was acidic?

There is one last issue to consider before leaving the topic of the hydrosphere as sink in particular and the carbon cycle in general. Some people have suggested boosting the oceans capacity to act as a sink by (1) seeding the ocean with iron and (2) adding lime to seawater. It remains to be seen whether these are effective forms of carbon capture. Even if they are effective, the question remains …

Q29: Do you think that we should be exploring means like this to increase the ocean’s capacity to absorb CO2? Why or why not?


Section 6

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

Q30: How and why have atmospheric CO2 concentrations changed over the past 50 to 60 years?

Q31: How do atmospheric CO2 concentrations vary across the globe?

Q32: How has the uptake of CO2 affected the oceans and terrestrial biosphere?






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