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Heat Budget of Planet Earth

Reveals what happens to the solar energy that enters Earth's atmosphere and how that affects the heat budget.

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Heat Budget of Planet Earth

How does heat on Earth resemble a household budget?

The heat left on Earth is heat in minus heat out. If more energy comes into the system than goes out of the system, the planet warms. If less energy goes into the system than goes out of the system, the planet cools. Replace the word "money" for "heat" and "on Earth" to "in your bank account" and you describe a household budget. Of course, Earth's heat budget is a lot more complex than a simple household budget.

Heat at Earth’s Surface

About half of the solar radiation that strikes the top of the atmosphere is filtered out before it reaches the ground. This energy can be absorbed by atmospheric gases, reflected by clouds, or scattered. Scattering occurs when a light wave strikes a particle and bounces off in some other direction.

About 3% of the energy that strikes the ground is reflected back into the atmosphere. The rest is absorbed by rocks, soil, and water and then radiated back into the air as heat. These infrared wavelengths can only be seen by infrared sensors.

The basics of Earth's annual heat budget are described in this video: http://www.youtube.com/watch?v=mjj2i3hNQF0 (5:40).

The Heat Budget

Because solar energy continually enters Earth’s atmosphere and ground surface, is the planet getting hotter? The answer is no (although the next section contains an exception), because energy from Earth escapes into space through the top of the atmosphere. If the amount that exits is equal to the amount that comes in, then average global temperature stays the same. This means that the planet’s heat budget is in balance. What happens if more energy comes in than goes out? If more energy goes out than comes in?

To say that the Earth’s heat budget is balanced ignores an important point. The amount of incoming solar energy is different at different latitudes. Where do you think the most solar energy ends up and why? Where does the least solar energy end up and why? See the Table below .

The Amount of Incoming Solar Energy
Day Length Sun Angle Solar Radiation Albedo
Equatorial Region Nearly the same all year High High Low
Polar Regions Night 6 months Low Low High

Note: Colder temperatures mean more ice and snow cover the ground, making albedo relatively high.

This animation shows the average surface temperature across the planet as it changes through the year: Monthly Mean Temperatures at http://upload.wikimedia.org/wikipedia/commons/b/b3/MonthlyMeanT.gif .

The difference in solar energy received at different latitudes drives atmospheric circulation.

Summary

• Incoming solar radiation is absorbed by atmospheric gases, reflected by clouds, or scattered.
• Much of the radiation that strikes the ground is radiated back into the atmosphere as heat.
• More solar radiation strikes the Equator than the poles.

Practice

Use this resource to answer the questions that follow.

1. What does CERES measure?

2. What does the acronym CERES stand for?

3. What is the ideal radiation budget?

4. How much of the Sun's radiation is reflected?

5. How much energy does the ocean absorb?

6. What are scientists finding with CERES?

7. Why is the Earth warming?

8. What is a carbon footprint?

Review

1. If the Sun suddenly started to emit more energy, what would happen to Earth's heat budget and the planet's temperature?

2. If more greenhouse gases were added to the atmosphere, what would happen to Earth's heat budget and the planet's temperature?

3. What happens to sunlight that strikes the ground?