Initial Publication Date: July 7, 2011 | Fix Links: July 9, 2024 (see revision history: 2 events)

Carbon In the Atmosphere

Part A: CO₂- It's a Gas!

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

Discuss

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?

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:

NOTE: If the video does not load, click here

Discuss

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.

Show show me how to interpret the data in this table

Parts Per Million describes the concentration of one type of atmospheric gas to the concentration of other gases in the atmosphere. For example, carbon dioxide has been expressed as 397 ppm. This means that for every million molecules in the atmosphere, there are approximately 397 molecules of carbon dioxide. NOTE: The concentration of CO2 continues to rise. Check this site to get the current global concentration of CO2 in ppm: NASA Vital Signs of the Planet

Average Residence Times describes the approximate amount of time that different types of atmospheric gases spend in the atmosphere before chemically decaying or moving to another reservoir. A greenhouse gas with a long residence time has greater potential to build up to higher concentrations. This would in turn lead to more infrared being absorbed and a stronger greenhouse effect.

Variability over Time and Spatial Scales describes how the concentration of an atmospheric gas varies over time and space. For example, concentrations of nitrogen and oxygen remain fairly constant around the globe. In contrast, the concentration of CO₂ varies over both time and space. For example, in the northern hemisphere (a large hemispheric spatial scale), the concentration of CO₂ varies from season to season. H₂O vapor in the atmosphere is highly variable because it is part of the water cycle. Some days and regions are dry whereas others have quite a bit of rain.

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

Checking In

Which greenhouse gas resides in the atmosphere for longer periods of time? Choose all that apply.
methane

water vapor

carbon dioxide

ozone

nitrous oxide

CFCs
Which greenhouse gases in the table are carbon compounds? Choose all that apply.
methane

water

carbon dioxide

ozone

CFCS
Which greenhouse gas is most abundant in the atmosphere?
methane

carbon dioxide

ozone

nitrous oxide

water vapor

CFCs
Which greenhouse gas has the greatest potential to build up larger concentrations over time?
methane

water

carbon dioxide

ozone

water

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.

First, watch the video animation "How do greenhouse gases actually work?" by Minute Earth and Kurzgesagt
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.
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
When you finish, share your notes from the videos with your partner or group.
Answer the Checking In and Stop and Think questions below.

Video 1: How do greenhouse gases work? If the video does not play, you can watch it here

Video 2: Why greenhouse gases make the Earth warmer. If the video does not play, you can watch it here

Checking In

Which of the following statements are true about gas molecules in the atmosphere? Choose all that apply.
Lopsided gas molecules with three or more atoms can absorb infrared photons

Non-GHG molecules vibrate more than GHG molecules when absorbing infrared photons

GHG molecules vibrate when they absorb infrared photons, become excited, then re-emit infrared photon when they de-excite

Infrared radiation is often referred to as thermal (heat) radiation.
When GHG molecules re-emit infrared photons, where can these re-emitted photons go? Choose all that apply.
to outer space

to other GHG molecules

to Earth's surface

to nitrogen and oxygen molecules
Which of the following contributes to the greenhouse effect? Choose all that apply
nitrogen and oxygen absorb and re-emit infrared photons

infrared photons re-emitted from GHGs back to Earth's surface

infrared photons re-emitted to other GHG molecules

delay of infrared thermal energy escaping to space
If we added more greenhouse gas molecules to the atmosphere, what might we expect to happen? Choose all that apply.
More infrared photons would be re-emitted back to Earth's surface.

Less infrared photons would be absorbed by greenhouse gases.

More infrared photons would be delayed from leaving the atmosphere.

The atmosphere would eventually become cooler.

The atmosphere would eventually become warmer.

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.

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.
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.
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.
When you have finished exploring answer the Checking In questions below.
Show me more about the graph

What does the graph mean?
Blue dots (and blue y-axis scale) indicate the emissions of CO₂ in the atmosphere each year. This is measured in Gigtatons of CO₂ (GtC) per year. In the year 2000, we released 7.79 Gigatons of CO₂ into the atmosphere.

Black triangles (and black y-axis scale) show how much carbon dioxide has built up in the atmosphere over time. This is measured in parts per million by volume (ppmv). The actual amount was around 369.52 ppmv in the year 2000.

Red squares (and red y-axis scale) shows average global temperature in degree Celsius. For reference, this value was around 14.4° C in the year 2000. In this simple model, temperature is based entirely on the atmospheric CO₂ concentration

Checking In

What happens to the average global temperature as you increase the concentration of CO₂ in the atmosphere?
Show me
How do the slope of the temperature and CO₂ concentration lines change as you increase the emission rates?
Show me