Lesson 4: Climate Forcings



In Lesson 3, you saw from Antarctica a relationship between the amount of CO2 in air and its temperature. The curves for CO2 and temperature increased and decreased together. In this lesson, you'll investigate the causes for those changes. In particular, you'll see that

4a. A Forcing Brainstorm

You've probably been forced to do things, such as by a parent or older sister. How did you respond? Did you respond right away, or did you get around to it later?

  1. Get together with two teammates for 3 minutes. Think creatively to fill columns in the table below. Get ready to share your best ideas with the class. The table shows a student example. The columns are:
    • Forcings: any event or thing in your community that causes something else.
    • Response: what is the result of that event or thing in your community
    • Lag: How long after the event or thing does it takes for the result to occur.
Forcing Response Lag
Mom tells me, "Clean your room!" I clean my room 3 days later
you continue here...

4b. Forcings in Systems

Earth's climate changes depending on what's happening in the Earth system. As you just showed, a forcing is an input that causes a change in a system. From the forcing, the system has a response, or an output. Think of heating a kettle of water on the stove. The input, or forcing, is thermal energy from the stove. The top graph shows when the stove is "on" or "off." The response or output is the temperature change of water in the kettle. A lag is the time it takes for the response to start occurring. It's not the time for the full response to occur.

Forcing and Response

Answer two questions about the forcing and response in this kettle system:

  1. Look at the two graphs. How does water temperature compare to turning to stove on and off?
  2. Imagine a rock heating all day in the sun on the Colorado Plateau. After sunset, how long do you think it would take to cool to air temperature? Sketch your answer with a graph of the rock's temperature with time.

The atmosphere also responds to forcings. The graph below shows an example from a weather station at a high desert grassland right here on the Colorado Plateau. The station measures air temperature and sunlight (solar radiation). Discuss the questions below with your partner and write your "response" in your notebook.

Sun on Rocks

Sun on rocks. Photo courtesy of Joëlle Clark

  1. Look at the graph.
    1. What variable is on the x-axis? How long is the time shown?
    2. What variable is on the left y-axis? Which curve does it go with?
    3. What variable is on the right y-axis? Which curve does it go with?
  2. Complete these sentences:
    The peaks in the graph are about in the month(s) of ______________
    The valleys in the graph are about in the month(s) of ______________

    I think the forcing in the system is _________________
    I think the response of the system is _________________________
  3. Look again carefully. Can you see whether the response is right away, or after a lag? Either way, what is your evidence?

4c. Greenhouse Gases


Maybe you've heard of the greenhouse effect. The greenhouse effect is the warming of the Earth's atmosphere. This effect is similar to the warming in a greenhouse with a ceiling. Sunlight comes in, but the ceiling keeps the warmed air inside the greenhouse. This is vital for Earth. Without a natural greenhouse effect, Earth would be about 33°C (60°F) colder. Without greenhouse warming, most of earth's water would be ice. Earth would be too cold for most plants and animals.

The greenhouse effect is due to certain gases in the atmosphere called greenhouse gases. Greenhouse gases are molecules that absorb the energy given off by Earth's surface or the atmosphere itself. That energy originally comes from solar radiation that entered the Earth system. The greenhouse gases cause the atmosphere to trap energy that would otherwise leave Earth and go to space.

Greenhouse Effect

Greenhouse gases include water (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3). Look at the atomic diagrams for these gases. They have a central atom about which the other atoms vibrate and rotate. In contrast, most of the atmosphere is nitrogen (N2) and oxygen (O2) gas. These do not have a central atom. They do not absorb the energy given off by the Earth's surface or the atmosphere, so they don't contribute significantly to warming of the atmosphere.

Water vapor contributes the most of any greenhouse gas toward the greenhouse effect on Earth. Scientists estimate that about 75% of greenhouse warming is due to H2O vapor. Water vapor is part of the water cycle. When the water vapor in the air increases to a point, H2O leaves the air as rain. Thus, the H2O content of air doesn't increase beyond certain limits.

Carbon dioxide (CO2) is the second most-important greenhouse gas. Overall, CO2 contributes about 20 percent of the greenhouse warming. Yet unlike H2O, humans are adding CO2 to air. They do this mostly by using of fossil fuels (coal, petroleum, and natural gas). Fossil fuels are the remains plants and animals that existed millions of years ago. The carbon in ancient plants and animals was trapped in the earth and over time became fossil fuels. Unlike H2O vapor, CO2 cannot just "rain" out of the sky when CO2 gets to a certain level. The main way that CO2 is removed from the atmosphere is by the carbon cycle. Currently, however, the carbon cycle can only remove about half of the CO2 added by humans. Thus the level of CO2 is increasing.

Greenhouse Gases

The other greenhouse gases lead to a small amount of greenhouse warming, about 5% of the total. Putting it all together, you should note a few key things shown in the figure below:

Greenhouse Effect

Maybe you didn't quite "get" all the ideas in the reading above. To help you, your teacher will do a demonstration. It gets a question you might have: Why would one kind of gas absorb energy, while another, similar kind of gas would not absorb the energy? The kind of energy you'll use is from a microwave! These emit a kind of radiation that you can't see — like the energy radiated from Earth to the atmosphere.

  1. Watch what happens to different kinds of fluids that look the same. Which fluids absorb the microwave radiation, and which don't?

Climate Effect: Fire
Wildfires in the southwest have become more frequent and intense over the past decades, and evidence points to recent warming as part of the cause. Because of a century of suppressing wildfires, small trees fill the forests, and provide a pathway for fire to go from the ground to the tops of the trees. With warming and drying, the situation will only worsen, threatening wildlife and people, as the forests of the Southwest burn.

4d. Summary — A Climate Model

You're seeing that your climate system is complex! There are basic forcings and responses, but when you mix it all, it's hard sometimes to see what will happen. Scientists explore complex systems with models. Models can represent physical things, like the plastic airplanes you buy in the hobby store. Or, other kinds of models can show forcings and responses in systems, such as the climate system.

You'll use a real climate model to summarize this lesson. It has been developed by NASA to explore forcings and responses of Earth's climate. Models like this let you study real situations, or test what could happen if the system gets pushed past normal.

The model results show a spike experiment. The forcing is sudden. For example, what if the CO2 levels suddenly increased from the low values of an ice age, to the high values of an interglacial? This is a "spike" in CO2 from about 180 to 280 ppm. The interactive lets you model spikes that are more or less than this. Do the questions below with a partner, and discuss with your class.

  1. Look at the model:
    1. What are units for the x-axis?
    2. What year does the spike in CO2 occur?
    3. What response variables are on the y-axis? What color are they?
  2. Look at the response of temperature (red)..
    1. What shape does this response have? Does it look like the forcing, or spike?
    2. How long does it take for temperature to "catch-up" and be stable?
  3. Look at two other model responses, ice cover (green) and water vapor (blue).
    1. With a CO2 spike, does ice cover increase or decrease?
    2. With a CO2 spike, does water vapor in air increase or decrease?
  4. Could changes in greenhouse gases in the atmosphere affect the other "spheres" (biosphere, cryosphere, geosphere, hydrosphere)? Use the model results to discuss this with your class.