Lesson 7: Testing Forcings


Volcanic Eruption

In Lesson 6, you looked at Earth's temperature over the past 120 years. A key point was that climate stations around Earth give a single, global monthly temperature. In this lesson, you'll use a climate model to test four factors that influence Earth's temperature: El Niño/La Niña cycles, volcanic eruptions, solar cycles, and human influences. This climate model is similar to one used by scientists at NASA, the Naval Research Laboratory, and other research organizations.

Climate models help you see that certain forcings lead to specific effects in the climate. The global scale may seem too big to compare to the Colorado Plateau. However, certain regional effects are part of the global response. As you'll see, several of these are part of the climate and the ecology of the Colorado Plateau. In fact, some of the climate events compelled people to move from settlements like Chaco Canyon or Mesa Verde to other regions.

7a. Four Forcings

Scientists have shown that Earth's temperature responds to the four forcings below. There are other factors, but the four below are the best predictors. As you test each, you can think about how they relate to where you live.

Forcing #1: El Niño/La Nina Cycles
Perhaps you've heard someone say, "We will have more snow this winter because of El Niño." That is likely where you live on the Colorado Plateau. You may also have heard that El Niño events begin in the tropics. But the broader question is: do El Niño/La Niña cycles influence global temperatures?

 El Niño/ La Niña cycles

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 in June, 1998. The warm SSTs can quickly change to cool SSTs only months later.

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" because in some years, the warm waters occur at Christmas. A strong El Niño can change surface winds and reduce the 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 El Niño Animation.) This is a key part of the marine food web and the carbon cycle. The changes in winds may also result in more precipitation for the Southwestern U.S.

A La Niña event is the opposite of an El Niño. La Niña events are periods of colder ocean temperatures in the same region (figure). During a La Niña, upwelling brings cold waters to the ocean surface off the coast of South America. This increases productivity in the marine ecosystem, and helps fisheries and food production. On the Colorado Plateau, La Niña events tend to result in drier conditions. This was the case in much of 2010 and 2011. El Niño and La Niña cycle back-and-forth about every 4-7 years. But you can see in the graph that the pattern isn't regular.

Because your computer is not online, go to the video folder and open ElNino.mov in your computer's media player.
 El Niño/ La Niña cycles

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.
    1. What years are El Niño the largest?
    2. What years are La Niña the largest?
    3. Do El Niño events and temperature correlate? What is your evidence?
  3. Look at the vertical bar next to the graph. It shows how much of the temperature (Ta) can be explained by the factor(s) you are testing. What's the highest value you can get for El Niño/La Niña cycles.
  4. Write a claim about the relationship between El Niño/La Niña cycles and global temperatures.

Forcing #2: Volcanic Aerosols
Aerosols are very small particulates in air. They include fine droplets of liquid, ash and dust that can stay aloft in the atmosphere for months. Erupting volcanos are a source of aerosols.

Because your computer is not online, go to the video folder and open Volcano.mov in your computer's media player.

But not all volcanic eruptions are the same. Some eruptive clouds don't go above the troposphere. The troposphere is the lower part of the atmosphere, from Earth's surface to 12-15 km (7-9 miles) in altitude. To impact Earth's temperature, the eruptive cloud needs to go higher. There are two reasons why.

First, volcanic aerosols have to be ejected into the stratosphere. This 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. Second, for the largest effect, the aerosols need to be near the equator where incoming solar energy is the greatest.

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 1880's decreased about 1.0-1.2°C after this massive event

Take an example from Mount Pinatubo in the Philippines, a country along the equator in Asia. 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).

Watch the following animation of volcanic aerosols mixing in the atmosphere after the Mount Pinatubo eruption.

SAGE 11 1020 nm Optical Depth
Because your computer is not online, go to the video folder and open particles.mov in your computer's media player.

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

  1. Open the climate model. Adjust the slide bars so all the forcings are at zero, except volcanic events.
  2. What years had volcanic aerosol events? Do these events lead to warming or cooling?
  3. The early 1980s saw two large volcanic eruptions:
    • Mount St. Helens in Washington State (May, 1980)
    • 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?

  1. Look at the vertical bar next to the graph. What's the highest value you can get for the influence of volcanic aerosols?
  2. A student wrote the claim below about volcanic eruptions and global temperatures. What do you think of her claim?

    "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."

Forcing #3: Solar Cycles
Almost all of the Earth's surface energy comes from the Sun. Greenhouse gases then help trap some of that thermal energy in the atmosphere. In fact, without greenhouse gases, Earth would be too cold for life as we know it. But does the amount of energy from the sun ever change or vary? That could affect Earth's temperature.

The amount of energy that Earth gets from the Sun changes with the number of sunspots. Sunspots are dark, cooler areas on the Sun's surface. Sunspots can also produce solar flares and massive eruptions that can cause power outages and failures in satellites and telecommunications. You can see why sunspots are studied closely by scientists and the U.S. military!

Because your computer is not online, go to the video folder and open Sunspot_Closeup.mov in your computer's media player.
Because your computer is not online, go to the video folder and open Dazzling_Solar_Eruption.mov in your computer's media player.

Scientists observe that the number of sunspots increase and decrease in a clear 11-year cycle. Researchers 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 also at a peak. 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).

Because your computer is not online, go to the video folder and open Comparison_Solar_Activity.mov in your computer's media player.

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

  1. Return to the model. Adjust the slide bars so all the forcings are at zero, except solar cycles.
  2. From 1979 through 2010, you see three peaks. Check that these match cycles in the sunspot graph above.
  3. 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, 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.
  4. 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?
    2. Which show the most warming? How much?
    3. Look at the red bar next to the graph. How close to the best-fit model do you get with just the natural forcings?
    4. What if you double the size of the natural forcings? Do this by moving the slide-bars from 1 to 2.

Forcing #4: Human Factors

But what about humans — do they influence Earth's climate? If yes, then you can also be part of a solution. You learned how using fossil fuels affects CO2 levels in the atmosphere. At the same time, fossil fuels are important for transportation and generating electricity.

Human influences on climate are called anthropogenic forcings. The main human forcing is an increase in greenhouse gases like CO2. Some CO2 is natural, but the "extra" CO2 added to the atmosphere is from humans. Humans impact the level of greenhouse gases in other ways too. For example, CO2 is given off when cement is made. Or, patterns of land use can affect the amount of carbon that can be stored in that reservoir. When forests or farm fields grow, CO2 moves from the air to plants and soils. When trees and plants are removed, they can't store carbon.

Follow the steps below to investigate the influence of humans on Earth's temperature.

  1. Return to the model. Adjust the forcings to only test the human part.
  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, are the data more or less spiky than the others?
    3. Look at the 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 4 for CO2 at Mauna Loa. How does the line for the human contribution 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 _______________________ change in Earth's mean temperature.

You may wish to Download the data in the climate model, and plot it yourself. The period of highest detail is from 1979 through 2010. It includes El Niño/ La Niña cycles, volcanic events, solar cycles, and human factors. These data form the basis of the climate model that you have used.

Climate Effect: Sand and Dust Storms
Rapid warming in the Southwest leads to the hot, dry, and windy conditions that create dust storms. More and larger dust storms have occurred recently, along with some of the hottest years on record. Dust storms erode the landscape, and bury roads, houses and buildings, causing extensive damage.

7b. Effects
In these lessons you have seen how carbon cycles though biological and physical systems on Earth. You've seen that increases in greenhouse gases cause temperatures to increase globally. You have worked with some natural forcings that cause climate to change. You have also seen firsthand the role of human forcing in the climate system.

So what are the effects of this additional greenhouse warming, especially on the Colorado Plateau? You have seen some. They include increased likelihood of drought. This leads to greater risk of wildfires, more sand and dust storms, and even insect damage to forests.

In the next two lessons, you will learn what you and others can do to make a positive difference. What you have learned is not meant to burden you; it is meant to inspire you. Then you can move forward, with knowledge, to improve your home the Earth.