Resource: Carbon Cycle Diagram

Print Background Essay

Not long ago, few people thought much about the role carbon dioxide plays in Earth's atmosphere. Today, with each new mention of melting ice caps, rising sea levels, and higher than average temperatures, we hear more than ever about a connection between human activities, carbon dioxide, and global warming. What is the connection between this gas and a potential global catastrophe?

Before looking at carbon dioxide's role in global warming, it is important to recognize the role this naturally occurring gas plays in maintaining our planet's temperature. Earth's relatively warm surface temperature is the result of an atmospheric process called the greenhouse effect. Among the molecules that blanket our planet are some gases that absorb heat. They act much like the glass of a greenhouse, letting sunlight in while preventing heat from escaping. For this reason they are called greenhouse gases. The most important naturally occurring greenhouse gases are water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Carbon dioxide is the greenhouse gas climate experts are most concerned about because the increase in human-related CO₂ emissions since the industrial revolution is now linked to the current global warming trend.

As this resource illustrates, carbon cycles through the environment and often changes its molecular form. Some types of carbon molecules remain unchanged, or sequestered, in the environment for long periods of time. The diagram shows these figures in black. For example, more than 1,500 billion metric tons (1650 billion tons) of carbon (GtC [G = giga = billion]) are stored in the soil, while more than 38,000 GtC (41,890 billion tons) is stored in the deep ocean.

Other forms of carbon are in greater flux. The diagram represents these with purple arrows and much smaller numbers. For example, about 90 GtC (99 billion tons) moves from the surface of the ocean to the atmosphere as CO₂, while a comparable amount, about 92 GtC (101 billion tons), moves back into the ocean surface waters through the photosynthetic activities of phytoplankton. Scientists have identified similarly balanced carbon transfer processes occurring in many areas of the carbon cycle. However, other carbon transfer processes in the cycle appear to be out of balance. For example, human activities are responsible for the release of roughly 7.1 GtC (7.8 billion tons) per year. Of this, approximately 3.2 GtC (3.5 billion tons) remains in the atmosphere and another 2.0 GtC (2.3 billion tons) diffuses into the oceans. Scientists aren't sure what happens to the remaining 1.9 GtC (2.0 billion tons), and consequently this figure remains the focus of great concern and study.

Currently we are extracting and burning stored forms of carbon, such as fossil fuels, and pumping carbon dioxide into the atmosphere at rates that appear to exceed the rate at which natural processes remove CO₂ from the atmosphere. The net increase of carbon dioxide is thought to be responsible for global warming through the greenhouse effect.

To learn more about the role CO₂ plays in Earth's temperature, check out Global Warming: The Physics of the Greenhouse Effect and Global Warming: Carbon Dioxide and the Greenhouse Effect.

To learn more about how many common human activities might contribute to global warming, check out Your Carbon Diet.

Print Questions for Discussion

• In what part of the ecosystem is the most stored carbon found? Can you suggest reasons why this might be?
• In what ways do you produce carbon dioxide during your daily activities?
• Look around your classroom. Discuss how many objects you see that contain carbon or produce carbon dioxide.
• What fuels are made of carbon compounds? How can the carbon dioxide produced when these fuels are "burned" influence climate?
• What might be the consequences of human activities that increase the amount of CO₂?