Unit 3: Carbon in the Future & You

3.2: Testing Forcings

  1. Four Forcings
  2. Reflect and Connect

Focus Question:

What factors influence Earth's temperature?


Volcanic Eruption

In Lesson 3.1, you looked at Earth's temperature over the past 120 years. For you, that global view may be a new way to think about air temperature. At your home, you probably feel differences in temperature between day and night, or between seasons. For the Earth, scientists average out those kinds of differences between locations by using thousands of stations all over the Earth.

In this lesson, you will test four influences on that temperature record. These were at the end of Lesson 3.1: El Niño/La Niña cycles, volcanic eruptions, solar cycles, and human influences. The climate model you will use in Carbon Connections is a real climate model, similar to the ones used by scientists at NASA, the Naval Research Laboratory, and other scientific organizations. Using the model, you will get some specific answers for the focus question. In particular, you will learn in Lesson 3.2 that:

  • Some climate forcings lead to a change in Earth's average temperature.
  • Some climate forcings lead to pattern of cycles in Earth's average temperature, but no net change.
  • Scientific models help you understand relationships in the Earth system.

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A. Four Forcings

Scientists have shown that Earth's temperature responds to the four forcings described below. You will get to test each of these forcings in this activity. You might wonder about other forcings, but the four below are the best predictors of changes in Earth's temperature. The Climate Model will help you test the influence of each forcing on Earth's temperature.

Forcing: El Niño/La Niña Cycles

Perhaps you have heard someone say, "We are getting extra rain because of El Niño." Or maybe they've said, "rainfall is a lot less." You might have heard that El Niño events are in the tropics. So, besides rainfall, do El Niño events influence global temperatures?

El Nino and La Nina

The cause of El Niño/ La Niña cycles is not well understood. Sea surface temperatures (SSTs) go between warmer than average (reds), and cooler than average (blue). (a) An El Niño event in January, 1998, in the eastern Pacific Ocean. (b) A La Niña event in June 1998. The warm SSTs can quickly change to cool SSTs only months later.

But first, what is El Niño? It is a period of high sea surface temperatures (SSTs) in the Pacific Ocean along the equator. El Niño is Spanish for "the boy," referring to the Christ child, because in some years the warm waters occur around Christmas. El Niño can be strong along the Pacific Coast of South America, changing surface winds back toward the continent. This reduces upwelling of nutrient-rich waters to the photic zone. The photic zone is the upper layer of the ocean where photosynthesis occurs. (For more, click on the El Niño Animation below.) You saw in Unit 2 how nutrient-rich waters are part of the marine food web and the carbon cycle. Typically, the west of South America is one area of very high marine productivity.

A La Niña (Spanish for "the girl") event is the opposite of an El Niño event. La Niña events are periods of colder ocean temperatures in the same region. During a La Niña, upwelling cold waters move to the ocean surface off the coast of South America. This increases productivity in the marine ecosystem, and benefits fisheries and food production. Together, El Niño and La Niña form repeating cycles, as shown in the graph.


Explore More: El Niño/La Niña Cycles

El Niño/La Niña Cycles (hide)

Several pieces of evidence show that El Niño/La Niña cycles influence the temperature of the atmosphere. First, air above the oceans in the tropics is heated by the warm ocean water. When surface waters are warmer, it leads to extra heating of the air above. Scientists can measure that extra heating of the surface waters and air. (To see the distribution of hot water, click on the El Niño Animation.) Second, El Niño/La Niña cycles affect climate and the carbon cycle globally. For example, rainfall patterns can be strongly altered in parts of the United States. Third, climate data show that Earth's temperature responds about 5-8 months after the peaks or valleys in the El Niño/La Niña cycles. This is the lag time for the affect to spread from the equator to the entire atmosphere.

Climate Model & El Niño/La Niña Cycles

Climate Model

Empirical Climate Model LAUNCH »

Do these steps to investigate the influence of El Niño/La Niña cycles on Earth's temperature.

  1. Open the Climate Model. To test only El Niño/La Niña cycles, use the slide bar to set the other three forcings equal to zero.
  2. Adjust the size of the El Niño/La Niña cycles with the slide bar. When you do this, you are increasing or decreasing the effect of the forcing relative to the observed temperature record. Answer the following questions:
    1. In what years was El Niño the largest?
    2. In what years was La Niña the largest?
    3. Do El Niño events and temperature correlate? What is your evidence?
  3. Look at the red, vertical bar next to the graph. It shows how much of the temperature (Ta) can be explained by the El Niño/La Niña cycles. What is the highest value you can get?
  4. Write a claim about the relationship between El Niño/La Niña cycles and global temperatures.

Forcing: Volcanic Aerosols

Aerosols are very small particulates in air. They stay suspended in the air, and include droplets of liquid or fine pieces of dust. Erupting volcanos eject this mixture into the air.

But not all volcanic eruptions are the same. Some eruptive clouds do not go above the troposphere. The troposphere is the lower part of the atmosphere, from Earth's surface to 12-15 km in altitude. To have the largest effect on Earth's temperature, the eruptive cloud needs to go higher.

Volcanic aerosols have to be ejected into the stratosphere. The stratosphere is the layer of the atmosphere above the troposphere. In the stratosphere, volcanic aerosols reflect some of the Sun's energy back to space. The reflected energy never enters the Earth system, and thus cannot contribute to the temperature of the atmosphere. The amount of reflected energy is the greatest when the aerosols in the stratosphere are along the equator, which is where most of the solar radiation enters Earth.

For example, Mount Pinatubo, a volcano in the Philippines, began to awaken in May 1991. The Philippines is a country in Asia along the equator. A massive eruption in June 1991 sent a cloud of volcanic aerosols into the stratosphere. Images from a NASA satellite show the stratosphere before the eruption (April 1991), and then after. Initially, the volcanic aerosols were mostly along the equator (June). Then they spread into the northern and southern hemispheres (August-September).


Philippines Particle Animation

Watch the animation of mixing of volcanic aerosols in the atmosphere after the Mount Pinatubo eruption. For more about volcanic aerosols and climate, click on Volcanic Eruptions.

Climate Model & Volcanic Aerosols

Climate Model

Empirical Climate Model LAUNCH »

Use the climate model to test how volcanic aerosols in the stratosphere correlate with the global temperatures.

  1. Open the climate model. Adjust the slide bars so that you are able to test only volcanic aerosols.
  2. What years had volcanic aerosol events? Do they lead to warming or cooling?
  3. The early 1980s saw two large volcanic eruptions: Mount St. Helens in Washington State (May 1980) and El Chichón in southern Mexico (March-April 1982). It takes volcanic aerosols 4-8 months to spread in the stratosphere. From the climate model, which eruption had a larger impact on global temperature? What was the effect?
  4. Look at the vertical bar next to the graph. Move the slide bar to adjust the size of the volcanic aerosols. What is the highest value you can get for the influence of volcanic aerosols on Earth's temperature?
  5. A student wrote the claim below about volcanic eruptions and global temperatures. What do you think of the claim? Use the climate model to explain your answer.

    "Volcanic explosions can have a large impact on climate. They lead to increases in global temperature because of the heat released to the atmosphere during the volcanic eruption."

Explore More: Volcanic Eruptions

Volcanic Eruptions — Severe Stratospheric Sulfates (hide)

Human history shows that volcanic eruptions affect Earth's climate. For example, the 1815 eruption of Tambora in Indonesia saw global impacts. New England had snow in the summer, while Europe saw crop failure and famine. For several years, particles in the air caused blazing-red sunsets and sunrises. Similar events occurred again when Krakatoa (Indonesia) erupted in 1883. Scientists estimate that global temperatures in the mid-1880s decreased about 1.0-1.2°C after this massive event.

So, do volcanoes merely correlate, or are they a cause of the temperature decrease? Both! They correlate with temperature and are cause of temperature change. There are several reasons for this.

First, the temperature decrease comes after the large eruption by 5-10 months. You cannot predict when the eruptions will occur; so, when there is an eruption, scientists can test whether cooling follows. Second, scientists have tested the reason for the forcing. Sulfur gases from volcanos are largely of sulfur dioxide (SO2). These molecules react with water to form sulfate (SO4)-2 particles in the stratosphere. Volcanic ash in the stratosphere quickly settles out, but the sulfate can remain in the atmosphere for more than a year. The sulfate (SO4)-2 particles reflect and scatter solar energy back to space. That energy never enters the troposphere.

The graph below shows the record of volcanic particles in the stratosphere over the past 160 years. These data have been important in testing the relationship between volcanic aerosols and Earth's temperature.

sever stratopsheric sulfates

Forcing: Solar Cycles


Almost all of the energy to Earth's surface comes from the Sun. Greenhouse gases help to trap some of that thermal energy in the atmosphere. In fact, without greenhouse gases, Earth would probably be too cold for life. But does the amount of energy from the sun ever change or vary? Imagine if you had a dimmer switch that could brighten, and then dim, the Sun. You can then imagine how changing the energy from the Sun could affect Earth's temperature.

Scientists have investigated sunspots to see if they affect the amount of energy coming to Earth. Sunspots are dark, cooler areas on the Sun's surface. They can also produce solar flares and massive eruptions that can cause power outages and failures in satellites and telecommunications. Because of this, sunspots are studied closely by scientists and even the U.S. military. Click on the videos to zoom in on a sunspot, or see a dazzling, active region around a sunspot.

Sunspot Cycle Record, 1950-2010

Scientists have seen that the number of sunspots increase and decrease in a clear pattern. Sunspots have an 11-year cycle. Researchers from NASA, NOAA, and European groups have found a connection between the number of sunspots and the amount of solar radiation to Earth. When the sunspots are at a peak, solar radiation is at a peak too. Solar radiation is lowest at the valleys in the sunspot cycle. You can think of the sunspot curve like a dimmer switch that controls the energy output of a light bulb. Watch "Comparison of Solar Activity" to compare the brightness and radiation between a peak (1999) and a valley in the cycle (1996).

Climate Model & Solar Cycles

Climate Model

Empirical Climate Model LAUNCH »

Use these steps with the climate model to test how solar cycles relate to changes in Earth's climate.

  1. Return to the model. Adjust the variables so that everything is zero, except solar cycles.
  2. From 1979 through 2010, you see three peaks. Check that these match cycles in the sunspot graph above.
  3. Now, adjust the size of solar cycles, El Niño/La Niña cycles, and volcanic aerosols to "1." These three are Earth's natural forcings.
    1. Which of these show the most cooling? How much more?
    2. Which show the most warming? How much more?
    3. Look at the vertical bar next to the graph. How close to the best-fit model do you get with just the natural forcings?
    4. What happens if you double the size of the natural forcings? Do this by moving the slide-bars from 1 to 2.
  4. The temperature response from solar cycles has an up-and-down pattern. That explains some variability in temperature. In contrast, do the solar cycles lead to any overall change in temperature from 1979 through 2010? Test this question with the climate model, and then complete the blanks below.

    Solar cycles lead to _______________________ variability in Earth's mean temperature.
    Solar cycles lead to _______________________ overall change in Earth's mean temperature.

Explore More: Why More Sunspots Mean More Radiation

Why More Sunspots Mean More Radiation (hide)

Scientists have shown that the number of sunspots correlate with increased solar radiation to Earth. But if sunspots are cooler regions, you might expect the opposite — more sunspots mean less total radiation. There is a reason why this is not the case.

Look at the images of the Sun from late March 2001. The first one has several trails of sunspots. The second is also late March 2001, but with a different telescope lens. It shows large, bright patches joining around the sunspots. These are called faculae. When the number of sunspots increase, the size and number of faculae increase even faster. The two effects of facular brightening and sunspot darkening are shown in the graph below. The net effect is greater solar radiation at peaks in the sunspot cycle. The brightening effect of the faculae is greater than the dimming effect of the sunspots.

Image A Image B

Sunspots and faculae. (a) Image of sunspots on March 29, 2001. (b) An image on March 28, 2001, shows more large, bright areas (faculae) than sunspots. The "brightening" from faculae is greater than the "darkening" from sunspots.

You can find many other incredible videos of sunspots, faculae, and solar eruptions at the website for the Solar and Heliospheric Observatory (SOHO).




SORCE. The SORCE satellite closely watches the Sun. It measures the total solar radiation coming to Earth from the Sun.

Forcing: Human Factors


You have explored three natural forcings on Earth's temperature. But what about humans? In Unit 2, you saw that using fossil fuels increases CO2 in the atmosphere; in Unit 1, you learned that CO2 is a greenhouse gas. More greenhouse gases lead to more warming of the atmosphere. At the same time, fossil fuels are vital to you and your community for transportation and electricity. Humans can't suddenly stop using fossil fuels. The climate model lets you investigate the period from 1979-2011. With those data, you can test the influence humans have had. This will help you make more informed decisions for the future.

Human factors on climate are also called anthropogenic forcings. Forcings from the Sun have one influence on temperature — the radiant energy to Earth. In contrast, human influences on climate have several parts. The main factor is CO2 added to the atmosphere from using fossil fuels. As you saw in Unit 2, this differs from CO2 that is a part of the breathing biosphere.

The second largest source of CO2 to the atmosphere is from humans is making cement, a vital part of concrete. Concrete is a probably a part of the building where you are sitting right now, as well as the roads, sidewalks, and street curbs in your community.

The third largest source of CO2 to the atmosphere is how humans use land. This includes harvesting wood, as well as using land for agriculture and producing food.

But some human activities also remove CO2 from the atmosphere. These are included in the model you will use below. For example, when forests grow or farm fields are not used, CO2 moves from air to the plants and soils. Similarly, new technologies are exploring how to "capture" CO2 when your electrical energy is generated — so the CO2 doesn't go to the atmosphere. Other new technologies can generate electrical energy by not using fossil fuels. You may have heard of these. These energy sources include nuclear, hydroelectric, solar, wind, wave, and geothermal energy.

Climate Model & Human Factors

Climate Model

Empirical Climate Model LAUNCH »

Your focus question for this lesson was, "What factors influence Earth's temperature?" Follow the steps below to investigate whether humans have a role.

  1. Return to the climate model. Adjust the forcings to test only the human influences.
  2. Look at the data from 1979 through 2010.
    1. What pattern do you see?
    2. Do the data have more variability or less variability than the natural forcings? (In other words, is the data more spiky or less spiky than the other forcings?)
    3. Look at the vertical bar next to the graph. How close to the best-fit do you get with only the human forcings?
  3. You used an interactive in Lesson 2.3 for CO2 at Mauna Loa. How does the line for the human influence in that model compare with the human influence in the climate model?

    (Remember that you can turn off photosynthesis and respiration in the Mauna Loa model by clicking on "R&P=0".)
  4. Complete these sentences about how human factors influence the temperature of Earth.

    Human factors lead to _______________________ variability in Earth's mean temperature.
    Human factors lead to _______________________ overall change in Earth's mean temperature.


You can download the data used in the Climate Model. The best detail is from 1979 through 2010. It includes El Niño/La Niña cycles, volcanic events, solar cycles, and human factors. By using X-Y graphs, you can compare different data sets. For example, you can test how the data for El Niño/La Niña cycles or volcanic aerosols compare with the temperature data. Or, you could graph El Niño/La Niña cycles against solar cycles to see if these are correlated. You just might find something interesting!

Download Climate Model Data »

B. Reflect and Connect

  1. You used a sophisticated climate model today. It is used by NASA scientists and other scientists. Answer these questions about the climate model.
    1. How would you describe to a family member what you investigated in class today?
    2. Imagine that you and your family were looking at the Carbon Connections website. What would you show them?

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Next: 3.3 Future Forcings