Lesson 3: Carbon in the Past

Introduction

In the last activity, you saw that CO2 can enter and exit water. The activities were a good model for carbon moving between the ocean and air in the carbon cycle. Now think about the geologic past. Do you think there were times when more CO2 was leaving than entering the ocean? How might you, or a geoscientist, be able to investigate CO2 in Earth's past?

Ice Cores

Here's a hint — think of the atmosphere too. If CO2 is leaving the oceans, it's probably going into the atmosphere. Or, if CO2 is entering the ocean, it's probably coming from the atmosphere. So, how could you get samples of that prehistoric air?

Ice Bubbles

You're in luck. Geoscientists have a clever way to get tiny samples of "old" air. Can you think of a natural material on Earth that captures a small sample of air as it forms? It's not obvious, but there's an earth material that does this. It's snow!

3a. Ice Cores — CO2 Records

When snow falls, the flakes stack atop one another. Air is trapped between the snow crystals, like the model your teacher showed you. Some places on Earth are cold all year and most of the snow doesn't melt. It piles-up, layer-upon-layer. The best examples are near Earth's poles, on the ice sheets of Greenland and Antarctica. There, the climate is cold all year, mostly below freezing. As snow gets deeper, it hardens to glacier ice. The ice traps air from when the snow fell. Watch the first 7 minutes of a video showing ice layers, and how cores tell about CO2 in earth's past.

Because your computer is not online, go to the video folder and open Wais_CO2_Icelayers.mov in your computer's media player.

The data in the graphs below are from the Vostok, Antarctica ice core. The blue line shows the amount of CO2 in air bubbles in that ancient ice. The age of the ice (x-axis) goes back to 420,000 years ago. Other parts of the line (green, light blue, black) are parts of the CO2 record that you'll explore in Lesson 5. The core is 3.6 kilometers (over 2 miles) long. This distance is like walking eight times around the track that surround some football fields. That's deep! In fact, it took several years to extract this long core, piece by piece.

The y-axis on the graph tells the amount of CO2 in the air. The units are parts per million. You can think of this like the length of a track again. For a quarter mile track around a football field, levels of CO2 at 300 ppm are about 5 inches to one time around.

Carbon Dioxide Variations

Explore the record of CO2 in Earth's atmosphere from this graph. Working with a partner, answer the questions in your notebook.

  1. What are the highest levels of CO2 in the atmosphere in the blue line of the ice core? This goes back 420,000 years.
  2. What are the lowest levels of CO2 in air during the past 420,000 years?
  3. Do you see patterns in the data? If so, what are they?
  4. When CO2 was increasing in the air, where do you think that it was coming from?

Why would geologists study CO2 in the atmosphere? Perhaps you've learned that CO2 is related to Earth's climate. The ice core you explored also contains a record of the temperature around the ice sheet. From that core, you can investigate how CO2 and temperature may be related.

3b. Ice Cores — Temperature Records

Ice Core

Scientists measure past temperatures from the ice around the air bubbles. The data are for the exact, same pieces of ice that is next to the bubble with the CO2 data. You know that Antarctica is always cold. Still, the data show that temperatures have varied the past 420,000 years.

Look at the graph below. The diagram shows 2 dependent variables from the ice core. The upper y-axis shows the CO2 level of air. The lower y-axis shows how temperature has varied over time. The independent variable of time (x-axis) is the same for each line, but note that the record stops before the industrial revolution began in the late 1700's. it looks close, but the graph doesn't quite go to today. You'll also see a 0°C reference line (dotted). This is the average temperature for the past 10,000 years. The curve goes either below or slightly above that average.

Vostok Ice Core

Get a large handout of the graph from your teacher for the next steps. Working with your partner, explore this temperature history in the questions below.

  1. Were most of the past 420,000 years warmer than the average (represented by the 0°C line) or colder than that average?
  2. How do the patterns in the temperature and CO2 content of the atmosphere relate?
  3. Continue with these steps about glacial and interglacial periods. .
    1. In the past, times of a cold climate on Earth are called glacial periods. Circle these and label them on your graph.
    2. How many glacial periods do you count?
    3. Times of relatively warm global climate are called interglacial periods. Circle these and label them on your graph.
    4. How many interglacial periods do you count?
    5. Complete the following sentence in your notebook:
      Currently, the climate of earth matches a(n) ________________ period.

In the next section, you'll think more about how glacial and interglacial periods might have affected plants on land. That's a key part of the carbon cycle. But also note two key things:

The reason for glacial and interglacial cycles is a "hot" topic for earth scientists. It is due to changes in the amount of sunlight to Earth. These are caused from small variations in the shape of Earth's orbit around Sun. Seemingly small variations can grow to a big climate effect. To see the result on for Earth's climate, watch an animation of the last ten glacial/interglacial cycles. In particular, watch the slow build-up of ice to glacials, then the rapid melting after the glacials.

Because your computer is not online, go to the video folder and open Ron_Pleistocene.mov in your computer's media player.

3c. Vegetation Patterns

You have been exploring records of CO2 and temperature from air. You also saw that CO2 can enter or exit water. Yet from the carbon cycle game, carbon can also enter and exit plants. So, how about if you test this idea:

During the ice ages, when less CO2 was in the air, was more carbon being stored in plants?

Test this idea by comparing the amount of land plants at glacial and interglacial times. If CO2 in air is decreasing, while photosynthesis and land plants are increasing, that is consistent with carbon moving from the air and into plants.

Present Potential Vegetation

Look at the global map of vegetation from before the industrial revolution. It's the point "1,000 years ago" on the graph above. The green area shows closed forests at that time (>70% coverage by trees). The yellow shows areas with extreme desert (<2% vegetation cover).

  1. How many extreme deserts do you see? About how many regions of large forest do you see?
  2. Use the graph above to estimate the CO2 concentration in the atmosphere.
Last Glacial Maximum

Now, look at the map from 18,000 years ago during the last glacial period. Large areas of North America and Europe were covered with ice sheets.

  1. How does the area of closed forest at 18,000 years ago compare with today?
  2. How does the area of extreme desert compare with today?
  3. If the carbon in the atmosphere did not go into vegetation during the last ice age, where could it have gone?

3d. Summary

Summarize your findings in the following table. Three main reservoirs for carbon are listed in the table below. Use arrows (up or down) to show how the factors change between glacial and interglacial periods. Check your answers with your classmates.

In the next activity, you'll learn more about current climate and how that relates to the carbon cycle and you.

Reservoir Glacial Periods Interglacial Periods
Carbon in the atmosphere
Carbon in the vegetation
Carbon in the ocean