Carbon In the Atmosphere

Part A: CO2- It’s a Gas!

View the image below. On which planet would you like to live?


With with a partner or group, compare the atmospheres of Mars, Earth, and Venus in the image above and then use the following questions to guide your discussion.

  • On which planet would it be possible for you to live? Why?
  • Which planet would have a greater diversity of life (biodiversity)? Why?
  • What relationship, if any, do you see between the amounts of carbon dioxide and the temperature in these three atmospheres?
  • You have probably heard about the “greenhouse effect” in previous science classes or in the media. Based on your current understanding of the greenhouse effect, which planet do you think has the strongest greenhouse effect? Which has the weakest? Why?

The Greenhouse Effect

Greenhouse gases regulate the temperature of Earth’s lower atmosphere via the greenhouse effect

Scientists now know the comfortable climate we enjoy today on Earth is due to a natural greenhouse effect natural phenomenon that warms the temperature of Earth’s surface and lower atmosphere because greenhouse gases absorb and emit infrared radiation that would otherwise escape to outer space. Some of this emitted infrared is returned to Earth’s surface regulated by greenhouse gases atmospheric gases that warm the temperature of Earth’s lower atmosphere by absorbing and emitting infrared radiation that would otherwise escape to outer space; includes carbon dioxide, methane, water vapor, ozone, nitrous oxide and CFCs.. Carbon dioxide (CO2) and methane (CH4) are two powerful greenhouse gases produced by the carbon cycle.

In this section, you will learn how the carbon cycle regulates Earth’s climate through the greenhouse effect. Without a greenhouse effect, Earth’s climate would be cold like Mars, with an average surface temperature surface of about -15 degrees Celsius (5 degrees Fahrenheit). With a temperature so cold, all water on Earth would freeze and life as we know it would not exist. With a very strong greenhouse effect, Earth’s climate could be more like that of Venus where temperatures are around 420 degrees Celsius (788 degrees Fahrenheit). Most living organisms we are familiar with could not exist in a climate this hot. Examine the image of Earth’s greenhouse effect pictured on the right and then watch the NASA video below. Make note of how each of the following contributes to Earth’s greenhouse effect:

  • solar shortwave radiationenergy radiated from the Sun mainly in the form of visible light, with small amounts of ultraviolet and infrared radiation; solar radiation is usually referred to as shortwave radiation while infrared radiation is referred to as longwave radiation.
  • infrared longwave radiation (IR) lies between the visible and microwave portions of the electromagnetic spectrum; “near infrared” light is closest in wavelength to visible light and “far infrared” is closer to the microwave region of the electromagnetic spectrum; far infrared waves are thermal which we feel as heat.
  • greenhouse gases
  • clouds

NOTE: If the video does not load, click Greenhouse Effect


With a partner or the class, discuss the following:

  • Describe how the greenhouse gases CO2 and H2O contribute to Earth’s greenhouse effect.
  • What if no infrared radiation was re-emitted back to Earth’s surface by greenhouse gases? Do you think Earth’s climate would be colder, warmer or the same? Explain why you think so.

Earth’s lower atmosphere (the troposphere) is comprised of greenhouse gases and non-greenhouse gases in different concentrations

As you can see in the pie graph pictured on the right, the lower atmosphere is made mostly of nitrogen(N2) and oxygen(O2) gas molecules. While both nitrogen and oxygen are important in supporting life on Earth, they are not greenhouse gases. Greenhouse gases such as carbon dioxide and water vapor comprise a very small part of the lower atmosphere and are found only in trace amounts.

Consider the table below and then answer the Checking In questions that follow.

To see molecular images of greenhouse gas molecules click on Greenhouse Gas Concentrations

Checking In

Greenhouse gases absorb and re-emit infrared photons

Why do some gases in the atmosphere absorb infrared photons very small packets of energy associated with different wavelengths of electromagnetic radiation; photons associated with specific wavelengths and frequencies of electromagnetic radiation can be absorbed by molecules with matching frequencies. whereas others do not? Nitrogen (N2) and oxygen (O2) molecules do not absorb infrared photons even though they make up more than 90% of Earth’s atmosphere. Conversely, CO2molecules comprise only 0.0397% of the atmosphere yet are strong absorbers of infrared photons. Why? It turns out that structure of a greenhouse gas molecule determines its ability to absorb and re-emit infrared photons. The physics of absorbing and re-emiting infrared photons creates the greenhouse effect.

In the next two videos, you will investigate the molecular structure of greenhouse gas molecules and the simple physics of absorbing and re-emiting infrared photons.

  1. First, watch the video animation “How do greenhouse gases actually work?” by Minute Earth and Kurzgesagt
  2. Then, watch geoscientist Scott Denning using his own personal dancing style to illustrate how greenhouse gas molecules absorb infrared radiation and make the Earth warmer. NOTE: You can pause and rerun sections of the videos as needed.
  3. As you view the two videos, make note of the following:
    • How the molecular structure of a greenhouse gas is related to its ability to absorb infrared radiation.
    • Why N2and O2 cannot absorb infrared photons.
    • When a greenhouse gas molecule absorbs an infrared photon, what happens next?
    • How absorbing and re-emitting infrared photons keeps the Earth warm
  4. When you finish, share your notes from the videos with your partner or group.
  5. Answer the Checking In and Stop and Think questions below.

Checking In

Stop and Think

1: Explain why carbon dioxide, methane and water molecules are greenhouse gases whereas nitrogen and oxygen are not. Try it in words or even your own dance!

Climate models can be used to predict the effect of CO2 concentration on global temperature

Ready to extend your knowledge and try your hand at modeling? Visit UCAR’s Very Simple Climate Model page to view the interactive and set up some experiments.

  1. First, explore the interactive using the preset CO2 emissions rate and time step size. Click Start Over to change the variables and investigate the relationship between CO2 and temperature.
  2. In the year 2000, 7.79 Gigatons of CO2 was released into the atmosphere. Discover what might happen to temperature if we increase our rate of emissions. Decide how much CO2 will be released into the atmosphere each year and set the CO2 emissions rate.
  3. When you have chosen your settings, click the Step Forward button (to the left of the Play button) to see how temperature and CO2 change in 5-year increments. Click the Step Forward button until the graph has filled to the year 2100.
  4. When you have finished exploring answer the Checking In questions below.

You are watching: It’s a Gas!. Info created by GBee English Center selection and synthesis along with other related topics.