Earth's Energy Budget or Can You Spare a Sun?

Tom Gates, Oklahoma State University, NASA Ames Research Center Office
Dale E. Peters, Urbana High School, Ijamsville, MD
Jeanne Seeley, Randolph High School, Randolph, NJ

Objectives: Students will:

Disciplines Encompassed:
Earth science
environmental science

Key Concepts:

energy budget
Equilibrium between the radiation received by, and the radiation emitted by Earth.
concept mapping
Visual representation of information that includes concepts and the relationship between concepts -- it can encourage overview or systemic thinking.
Cognitive Tasks:
Energy Budget Sources:
U Florida, Earth's radiation budget
U Florida, color illustration, Earth's energy budget
Key Terms:
The ratio of the outgoing solar radiation reflected by an object to the incoming solar radiation incident upon it. A measurement of reflectivity. Albedo is measured from 0-1. The more reflective a surface, the higher the albedo.
Any object with a temperature above absolute zero (0 Kelvin) emits some radiation. An object that is a perfect thermal radiator--that absorbs and reradiates all incident radiation with complete (perfect) efficiency--is called a blackbody.
greenhouse effect
Process by which changes in the chemistry of Earth's atmosphere may enhance the natural process that warms our planet. If the Earth's average temperatures change, some plant and animal species could be threatened with extinction.
Absolute zero is 0 on the Kelvin scale. On the centigrade scale absolute zero is -273.16 degrees. To convert centigrade to Kelvin, K = C + 273 or more accurately, K = C + 273.16
selective absorption
Specific substances impact the retention of radiant energy. The absorbing medium may emit radiation, but only after an energy conversion has occurred.
solar constant
The constant expressing the amount of solar radiation that reaches the top of the atmosphere when Earth is at its average distance from the Sun. The constant - 1370 W/m2 - is actually variable.
solar luminosity
Energy coming from the Sun as visible light, radio waves, heat, ultraviolet waves, and x-rays. These forms of energy make up the electromagnetic spectrum.
Concept maps are used to represent information visually. They can enable students to "see" the relationships that exist among specific information and encourage the understanding of complex information quickly. The use of concept maps should help students develop their powers of critical thinking. To develop their own maps, students will have to identify and rank concepts and select crosslinks, as well as possibly adding specific examples to their concept labels. There are four major formats for developing concept maps: spider, hierarchy, flowchart, and systems.

Life on this planet is supported by energy from the Sun. The Earth's energy budget is a balance between energy inputs and outputs. Earth's climatic system has an important role in maintaining the balance between the energy that reaches the Earth from the Sun and the energy going back into space from the Earth. Radiation received from the Sun is absorbed and scattered by molecules of gases, liquids, and solids in the atmosphere, by clouds, by the Earth's surface. Radiation that is absorbed causes the planet to heat up. Earth retransmits as much energy as it absorbs from the Sun via reflection and emission. However, the heat emitted by Earth is in the form of long wave radiation, rather than in the shorter wavelengths that it received. The air will absorb some of the radiation emitted by Earth, some of the energy is radiated back to Earth, and some radiated into space. See figure titled Earth's Energy Budget from .

The amount of solar energy that is reflected back to space is called the albedo. Earth's average albedo is .3, meaning about 30% of incoming solar energy is reflected back to space.

A potentially important effect in climate change is a variation in the solar irradiance reaching Earth. Scientists monitor solar variability and use models--mathematical representations--of the Earth system to understand how changes could affect climate. For example, an increase or decrease in global cloud cover could increase or decrease Earth's albedo, which would increase or decrease the amount of solar radiation reaching Earth.

Earth's energy balance is described by the following equation:


Oceans and forests reflect only small amounts of solar radiation, and thus have low albedos. Deserts and clouds have high albedoes because they reflect large portions of the Sun's energy. Clouds reflect large portions of the Sun's energy before it can reach and heat the Earth's surface, thus causing the Earth to be cooler. Clouds also trap some of the long wave radiation emitted by Earth and radiate it back to Earth's surface, causing the Earth to be warmer. Generally, low, thick clouds tend to reflect radiation--meaning less radiation reaches Earth's surface, resulting in cooler temperatures. High, thin clouds will reflect less radiation and allow more to reach the surface, warming Earth. Cloud surfaces can reflect up to 50 W/m2 of solar radiation.

The temperature, thickness, and types of particles making up a cloud will impact the cloud's albedo and its transmission of long wave radiation. The position of the continents, biological innovation (vegetation type and density), mountains, deforestation, and the rise and fall of sea levels also impact the albedo of Earth. If the albedo goes down, more energy is added to the system.

Albedo is influenced by dust and water in the atmosphere, and snow and ice. Albedo is also a function of the angle of incident. The larger the angle of incident the smaller the fraction of reflection.

The balance between the cooling and warming actions of global cloud cover is very close, but overall, cloud cover produces cooling on a global basis.
    Ultraviolet Radiation

    Low-energy ultraviolet light -- sometimes called long-wave UV -- penetrates to Earth's surface. This low-energy ultraviolet light cause Day-Glo paints to give off spectacular colors and white clothing to glow brightly when washed in detergents that contain fluorescent dyes (advertised as making clothes "whiter than white"). Most of the high-energy ultraviolet light -- sometimes called short-wave UV -- is blocked by the ozone layer in Earth's upper atmosphere. High energy ultraviolet light causes skin tanning. Extended exposure can lead to eye damage and skin cancer in light-skinned people. Skin cancer is rare in dark-skinned people.

    Although transparent to lower-energy ultraviolet light, glass blocks higher-energy ultraviolet light. Lotions that are advertised as Sun blockers also block higher energy ultraviolet light. When the glass is inserted between the lamp and the fluorescing material in this demonstration, the fluorescence diminishes or stops. Some materials fluoresce with lower energy ultraviolet waves as well as the higher energy waves, and any continued fluorescence is the result of lower energy waves.

    Ultraviolet light tells astronomers several things. For example, the local neighborhood of our Sun -- within 50 light years -- contains many thousands of low-mass stars that glow in the ultraviolet. When low-mass stars use up all their fuel, they begin to cool. Over billions of years, the internal heat left over from stellar fusion reactions radiates into space. This leftover heat contains a great deal of energy. These stars, called white dwarfs, radiate mostly ultraviolet light. Until astronomers could make observations with ultraviolet telescopes in space, they had very little information about this phase of a star's evolution.

    Visible Light

    Visible light, passing through a prism at a suitable angle, is dispersed into its component colors. This happens because of refraction. When visible light waves cross an interface between two media of different densities (such as from air into glass) at an angle other than 90 degrees, the light waves are bent (refracted). Different wavelengths of visible light are bent different amounts and this causes them to be dispersed into a continuum of colors. (See diagram)


    Diffraction gratings also disperse light. There are two main kinds of gratings. One transmits light directly. The other is a mirror-like reflection grating. In either case, diffraction gratings have thousands of tiny lines cut into their surfaces. In both kinds of gratings, the visible colors are created by constructive and deconstructive interference. Additional information on how diffraction gratings work is found in the Analytical Spectroscope activity and in many physics and physical science textbooks.