EarthLabs > Climate and the Biosphere > Lab 2: Climate and Earth's Energy Balance > 2B: Following the Energy Flow

Climate and Earth's Energy Balance

Part B: Following the Energy Flow

Solar power drives Earth's climate. Energy from the sun heats Earth's surface, warms the atmosphere, provides energy for photosynthesis, causes evaporation, drives the weather and water cycles, and powers the ocean currents. In the astronaut photograph at right, taken from the International Space Station, you can see the sun setting through the atmosphere.

When we look up at the sky from the ground, the atmosphere seems to go on forever, but in reality it is extremely thin when compared to the diameter of Earth. To get a sense of the thickness of the troposphere and stratosphere, two important layers of the atmosphere, try this simple exercise. Use a compass to draw a circle with a radius of 127 mm. This circle represents the Earth and the inner-most atmosphere. The 1 mm line, that your pencil draws, represents the average thickness of the the first two layers of the atmosphere: the troposphere, the region of weather, and the stratosphere, which protects us from most of the Sun's harmful ultraviolet (UV) radiation. As you work through the these labs keep this relative scale in mind.

In the example below, the line represents the thickness of the atmosphere to the top of the stratosphere (@50 km above the surface). Ninety-nine percent of the mass of the atmosphere's gases are within 32 km of Earth's surface, in these two layers. The troposphere alone contains 75-80 percent of the mass of the atmosphere. The outer edge of the 1 mm line would be 128 mm from the center of the arc (Earth radius = 6371 km). In the picture below, pixels are used as a measure of distance. To get a feel for how "thin" the atmosphere is, you might want to try this activity outdoors, using a scale of meters.

Radiation is the transfer of energy by electromagnetic waves, which are invisible. You have probably seen a heat lamp warming food in a cafeteria; the heat lamp is using one type of long-wave electromagnetic radiation, infraredinfrared radiation: the long wave, electromagnetic radiation of radiant heat emitted by all hot objects. On the electromagnetic spectrum, it can be found between microwave radiation and visible light. light waves, to heat the food. Energy is transferred from the sun to Earth via electromagnetic waves, or radiation. Most of the energy that passes through the upper atmosphere and reaches Earth's surface is in two forms, visible and infrared light. The majority of this light is in the visible spectrum. As sunlight enters the Earth system one of two different things can happen: it can either be absorbed or reflected. Once energy has been absorbed by the Earth system, it is transformed and transferred. Eventually, after multiple transfers, this radiation is emitted back to space, keeping our planet in an energy equilibrium.

All matter is made of particles, such as atoms and molecules. These particles are always in motion; this motion is known as kinetic energy. The thermal energy of a unit of matter is the total kinetic energy of all the particles in a given volume, which we measure as temperature. The transfer of energy from one region to another is called heat. This transfer of energy can take place by three processes: radiation, conduction, and convection. Thermal energy, or heat, always moves from things that are warmer (have more energy) to things that are cooler (have less energy). For example, when you touch an ice cube with your warm hand, the energy is transferred from your hand to the ice cube, causing it to melt.

In this lab, you will examine the complex energy pathways and balance that helps to keep our planet within an ideal temperature range.

An overview of the energy pathway

To begin, read through the text in the interactive graphic, below, to get a sense of how the solar energy moves through the system. On the interactive, you will need to click the forward and back arrows to move through the five steps in this simplified pathway. To play the interactive a second time, click the "start over" button at the end of the slides. Note: the representations of the processes are not drawn to scale.

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Step through the process

global energy balance 250
Next, in order to gain a deeper understanding of Earth's radiation balance, use the following interactive to step through how the energy is moving from the sun to Earth and back out to space. Read the text and study the graphics in this more complex interactive, Global Energy Balance (Click the link to open the interactive in a new window. Note: this interactive is only available on a flash-enabled device.)

On the interactive, you will need to click on the text in the images to gather the details needed to answer the questions about the Global Energy Balance. To access the interactive, you can click the link or the image, left, to view the interactive. Use your Back button to return to this page when you are done viewing the interactive.

After studying the interactive, answer the Checking In questions listed below about the Global Energy Balance.

Checking In

  • Sunlight provides energy in what form?
  • Why is it called "energy balance"?
    Equal amounts of energy enter and exit the system.
  • Describe three places that radiation is absorbed.
    Water vapor, clouds, and land absorb incoming solar radiation.
  • List the surfaces that reflect solar radiation.
    Water and clouds are two examples of materials that reflect solar radiation.
  • How does radiation get back out of the system? How is it emitted, and how is it transformed? Give several examples.
    Latent heat is an example of how solar radiation is transformed from electromagnetic radiation to the kinetic energy of water molecules.
    Sensible heat, can be detected as heat by thermometers. Temperature change is an indicator that solar radiation has been absorbed by a surface. As air is heated by surfaces or solar radiation, it triggers convection currents, sometimes called thermals.
    Other absorbed solar radiation is emitted from surfaces as infrared radiation and then eventually moves back out into space via the atmosphere.

Become an energy accountant

Now that you have worked through the Global Energy Balance interactive, review the annual Earth's Energy Balance diagram pictured below.

In order to simplify your accounting, you will break the process of energy flow into three parts. Use the diagrams and text below to guide your steps. While the process is continuous, and not step-by-step, this activity will help you to separate out the details and create an energy "account."

Before you start you will need to gather 100 pennies, paper squares, poker chips, Lego, or small cubes to help you with your accounting. You will also need 3 colored pencils: red, blue, and orange. Once you have the needed supplies, download and print this Energy Balance recording sheet (Acrobat (PDF) 1MB Dec3 18) and a copy of the Energy Balance Instructions (Acrobat (PDF) 2.6MB Mar29 13) to read as you work through the lab.

Once you have gathered your materials, you will read a section of the printed instructions and then move pennies representing the energy from one location to the next.

Overview of Energy Pathways
Begin this activity by gaining an overview of the energy pathways. Using the graphic shown above, identify the incoming solar radiation. On your printed version of the graphic, color the incoming radiation blue. Next, color the arrows representing outgoing radiation red, and the latent and sensible heat arrows orange. You have now separated the incoming and outgoing radiation.

Part 1. Incoming Solar Radiation

Solar energy, in the form of radiation, is constantly moving through space; bathing our planet and its atmosphere. The radiation that arrives at the top of the atmosphere is either reflected or absorbed.

  1. Read through the first five slides in the PDF that you downloaded (above).
  2. Start with 100 objects (i.e., pennies). Separate them into five columns on a piece of paper as follows. These pennies represent 100 percent of the solar energy coming from the sun, or 100 units. Stack the pennies according to what happens to each unit of energy as it travels through the atmosphere on its way to Earth's surface, as is diagrammed above.

    23 units – reflected by the clouds and atmosphere
    7 units – reflected by the Earth's surface
    19 units – absorbed by the atmosphere (ozone, aerosols, dust)
    4 units – absorbed by clouds
    47 units – absorbed by the Earth surfaces (primarily ocean)

  3. Next, add up and record the total units in your student notebook.
  4. Total the pennies that were reflected; you should have 30.
  5. Total the pennies that were absorbed; you should have 70. These pennies represent the amount of radiation that has entered the Earth's energy system. Some of this energy is now in the atmosphere (23 units) while the rest has been absorbed by the Earth (specifically, the hydrosphere, biosphere, and lithosphere - 47 units).
    Part 1 results

Part 2. Surface Energy Budget

In Part 1, you saw that about 30 percent of incoming sunlight is reflected back to space by particles in the atmosphere or bright ground surfaces, which leaves about 70 percent to be absorbed by the atmosphere (23 percent) and Earth's surface (47 percent) including the ocean. For the energy budget at Earth's surface to balance, processes on the surface must transfer and transform the 47 percent of incoming solar energy that the ocean and land surfaces absorbed back into the atmosphere and eventually space. Energy leaves the surface through three key processes: evaporation, convection, and emission of thermal infrared (IR) radiation.

  1. Read the next three slides (Part 2) of the PDF that you downloaded (above).
  2. Transfer the 47 pennies, which represent the absorbed energy in the Earth system, to a new piece of paper. This energy, which has been absorbed by the surface of the Earth, will now be transferred back to the atmosphere via several processes. To represent this, stack the pennies in four new columns as follows.

    24 units – latent heat: energy that is used in evaporation, transpiration, and condensation
    5 units – sensible heat: energy that drives convection
    12 units – emitted from Earth directly back to space
    6 units – net radiation amount absorbed by atmosphere

    This is the long-wave radiation that is emitted from Earth's surface to the atmosphere (116), minus the energy that is directly transferred to space (12) combined with that which re-radiated back to Earth by the atmosphere (98). The equation would be: [116-(12+98)]= 6

  3. Record these numbers in your student notebook.
    Part 2 results

Part 3. The Atmosphere's Energy Budget
The third step of the process moves the energy from the atmosphere back to space, through the following processes.
  1. Read the next two slides (Part 3) of the PDF that you downloaded (above).
  2. Collect the 19 and 4 pennies, which were absorbed by the atmosphere and clouds.
  3. Collect the 24 and 5 pennies that were transferred to the atmosphere via latent and sensible heat.
  4. Collect the 6 pennies that remained in the atmosphere.
  5. Move these 58 pennies to two remaining locations in the following amounts:
    49 units – emitted by the atmosphere
    9 units – emitted by clouds
  6. Total the three boxes on the top-right of the sheet. These are units of long-wave radiation transferred by the atmosphere back into space.
  7. Record these numbers on your piece of paper as a bar chart or histogram. Add up the total number of pennies that you have on your paper.
    Part 3 results

    When you are done, answer the Checking In questions, below.

Checking In

  • What is your total? Are there any pennies left over? Where are they, and what do they represent?
    All of the pennies should now be back in space in order for your global energy budget to balance.
  • What do you think would happen if you changed the amount of energy that was reflected by the atmosphere or Earth's surface? Can you think of an example when this might happen?
    Some types of clouds reflect sunlight, other particles, such as the ash from volcanoes, are also highly reflective to solar energy. Increases in the reflectivity of the Earth take place when the continents and oceans are covered with ice and snow. Either less incoming, or less absorbed radiation, would cause the system to cool.

Return to the Energy Flow interactive, above. Review the interactive one more time with the energy accountant steps in mind. Then answer the Stop and Think questions below.

Stop and Think

1. Now that you have worked through the Earth's energy balance, discuss how changes in Earth's surface characteristics and / or atmospheric composition could contribute to global warming or cooling.

2. Complete the following phrases and add one of your own:
  • More radiation = _______ warming
  • Less reflection = _______ warming
  • More absorption = ________ warming
  • ______________ = ________________

How do we know what we know?

Measuring Earth's radiation balance is an enormous and important task! How can we accurately and simultaneously know how much energy is coming into the Earth system, being reflected by clouds, and being emitted back to space? To gain a global understanding of this balance scientists use instruments on satellites. The following video explains how the Clouds and the Earth's Radiant Energy System (CERES) sensors on NASA's Aqua and Terra satellites measure Earth's Energy balance.

Optional Extension

The CERES mission homepage contains more information about how NASA science missions are measuring Earth's energy balance. Additional background information, datasets, and details about Earth's Energy Balance can be found at the following links:
NWS Jetstream - The Earth-Atmosphere Energy Balance website has additional explanations, diagrams and a short explanation of how cloud cover can contribute to warmer night time temperatures.
The units on the diagrams in this exercise are in terms of percentages of the incoming 342 watts per meter2 of solar energy. These percentages may not be exactly the same in every diagram, as there is some variation in scientists' explanation about how much energy is in each part of the system.