Climate Change Lesson 2 : Earth's Energy Balance

1. Review basic types of radiation in the electromagnetic spectrum.

Show the NASA video Radiation from the Sun and Earth on the MEECS Climate Change Resource DVD. After the video, students fill in the blanks on the Radiation from the Sun and Earth student activity.

Using transparency master Electromagnetic Spectrum, have students discuss types of radiation (radio, microwaves, infrared, visible light, ultraviolet, x-rays, gamma rays).

Review main points:

• The shorter the wave length, the higher the energy level.

• Radio waves are long, low energy waves that pass through us without causing damage.

• Gamma rays are short much higher energy waves that will cause cell damage.

Relate the types of radiation to relative size of wavelengths (long, short) as on the transparency, the Electromagnetic Spectrum. Students may correct their answers on the Radiation from the Sun and Earth student activity sheet.

2. Explore the Earth’s Energy Balance.

Show the NASA video The Earth’s Energy

Balance found on the MEECS Climate Change

Resource DVD. Reinforce the main points:

• The sun and earth both emit electromagnetic

radiation.

−− How does electromagnetic radiation

travel? (in the form of electromagnetic

waves)

• Energy emitted by the sun is also referred to as solar, or short-wave, radiation. The Earth emits lower energy, longer wavelength radiation.

−− How much solar energy radiation is reflected by the earth? (about 30%)

−− How much is absorbed by the earth? (remaining 70%) The absorbed solar radiation heats up the earth.

• While the earth continually absorbs solar radiation, it will not simply continue to heat up.

• Radiative equilibrium tells us that the long-wave radiation emitted by the earth will (eventually) equal the amount of solar radiation that is absorbed by the earth.

−− How can we determine the temperature of the earth? (by knowing the amount of long-wave radiation it emits)

The balance between the amount of solar radiation absorbed, and the amount of longwave radiation emitted by the earth is known as radiative equilibrium.

Note: For an additional hands-on activity about this topic, see MEECS Climate Change Resource DVD, Investigating Earth’s Energy Balance.

3. Balancing the Earth’s Energy Budget.

The Earth’s energy budget describes the various kinds and amounts of energy that enter and leave the Earth system. It includes both radiative components (light and heat) that can be measured, and other components like conduction, convection, and evaporation which also transport heat from the Earth’s surface. On average, and over the long term, there is a balance at the top of the atmosphere. The amount of energy coming in (from the sun) is the same as the amount going out (from reflection of sunlight and from emission of infrared radiation).

There are two options for reinforcing the learning about the energy budget. The first option is more detailed.

Option 1: The Earth’s Energy Budget student activity sheets contain a step-by-step method of determining radiative equilibrium. Recommended for this option: Use the Earth’s Energy Budget PowerPoint and complete Parts 1-3 as a class (See Teacher Resource for guided notes). Earth’s Energy Budget: Part 4 – Human Activity is a summary; it could be completed individually to check for understanding. Encourage students to refer to activity sheets 1-3 for help.

Option 2: Show students the NASA’s Earth’s Energy Budget poster. This is a diagram showing the distribution of the energy in the Earth system. Taking into account night and day and the seasons, on average about 340 Watts of energy enter every square meter of the Earth system. The same image is displayed on the Balancing the Energy Budget by Percentage student handout; point out to the students that the amounts of energy shown on the poster as Watts now have been converted to percentages.

• There are three levels of this energy budget: the top of the atmosphere, the Earth’s surface, and the atmosphere. For each level, an equation can be balanced.

• Display the PowerPoint slide Balancing the Energy Budget by Percentage. Distribute the Balancing the Energy Budget student handout and student activity sheets. Have students work through the equations and answer the questions.

NOTE: A color copy of the Teacher Resource, Balancing the Energy Budget, can be found in the kit and on the MEECS Climate Change Resource DVD. The colors used on the arrows relate to the colors used in the equations to help explain the source and destinations of energy.

Equilibrium would be expected for a planet that has spent a long time in a stable solar system, but sometimes changes occur that take the system out of balance. For example, the ice ages occurred because of long-term changes in Earth’s orbit around the Sun, which resulted in a change to the “Sunlight In” term. Over time, reflected sunlight and IR emission changed to balance the first equation. The result was a colder surface and major glacial advances.

In more recent years, changes in the atmosphere have also caused unbalance in these equations, with another departure from equilibrium. According to data from satellite instruments and other data sources, the radiation budget at the top-of-atmosphere was not balanced during the five years from 2000-2005. Approximately 0.85 Watts of energy were added to the Earth system, on average, for each square meter of the Earth’s surface. A continued imbalance of the radiation budget would mean a change in Earth’s climate.

Ask: If the Earth is heating up slowly, will the radiative equilibrium change? Why or why not?

4. Tying it all together.

At least part of this change is due to human activities. Show Energy Budget Changes Since 1950 transparency or PowerPoint. Discuss these graphs with the following questions:

Figure a: (shows items that have caused changes in the energy budget since 1950)

1) What greenhouse gases are shown on the graph? (carbon dioxide (CO2), methane (CH4), halocarbons, nitrous oxide (N2O), stratospheric and tropospheric ozone (O3))

2) A natural change (labeled solar) is shown along the bottom of the graph. What probably caused this change? (variations in the Sun’s output)

3) This figure shows the cumulative effect of small changes. What is the result? (additional heat trapped continues to add up to a warmer Earth)

Figure b: (partitions the added energy based on observed changes from Figure a)

4) How do we know how much additional heat was absorbed? (we know how much of these gases were emitted)

5) A small amount of the energy has gone into the ocean – the part of the Earth that stores the most energy. What is the consequence? (warming the ocean)

6) Some energy has escaped Earth in the form of increased infrared radiative emission, and some was reflected to space by aerosols in the stratosphere. The remainder (white band) is inferred to have been reflected due to pollution and changing reflection of the land surface. What could cause the latter? (deforestation, loss of ice)