# Earth's Energy Balance

# Earth's Energy Balance

Light from the Sun warms our planet. Earth radiates heat out into the frigid vacuum of space. There is a balance between this warming and cooling that determines the temperature of our home world. Using some fairly straightforward math, we can calculate the temperature of Earth.

This simple interactive allows you to try some "what if" scenarios with our planet's energy balance.

- The slider on the right controls Earth's "
**albedo**" or reflectivity. Bright surfaces like snow have a high albedo. Forested regions are darker. How would Earth's temperature change if the albedo was higher or lower? Overall, Earth's average albedo is about 31%. - The slider on the left controls the
**brightness of the Sun**. How would Earth's temperature change if the Sun was brighter or dimmer? - This model has one extreme simplification - it assumes Earth has no atmosphere! That makes for simpler math, but is clearly unrealistic. Earth's atmosphere acts as a blanket, making our planet habitable. Without the heat-trapping greenhouse gases in our atmosphere, Earth would be a frozen ball of ice!

## More Things to Try

- Use the "
**Pick a surface...**" popup menu under the albedo slider to cover Earth with ice or forests or even asphalt and see how the albedo of that surface type influences global temperature. - Click the "
**Show Math**" button to display the calculations used in this simulation. The equations dynamically update as you change the albedo and brightness values. Note that the equations use units of**watts per square meter**(not % Sun) for**brightness**and**kelvins**(not °C or °F) for**temperature**, no matter what you have the scales for those two variables set to. A detailed explanation of the math behind the simulation is available on our**Calculating Planetary Energy Balance & Temperature**web page. - You can alter the Sun's brightness to explore the
**Faint Young Sun Paradox**(by setting brightness to 70% of the Sun's current value), or see how the temperature of an Earth-like planet would vary if it was orbiting a dimmer or brighter star. Use the popup menu beneath the brightness slider to switch between settings in terms of the**Sun's current brightness (percent)**or in units of**watts per square meter**. - Use the popup menu beneath the
**thermometer**to switch between Celsius, Fahrenheit and Kelvin temperature scales. - Click the buttons to find out more about
**albedo**or the**Sun's brightness**.

This interactive simulation relies on two basic scientific concepts: conservation of energy and the Stefan-Boltzmann law. As sunlight streams from the Sun to Earth, it carries energy to our planet. As the sunlight warms Earth, Earth in turn sheds excess heat as infrared energy emitted into space. At a certain temperature, the amount of energy leaving Earth exactly balances the amount of incoming energy from sunlight. Physicists call this the law of conservation of energy. You can think of it as "energy in = energy out" for short. The Stefan-Boltzmann law describes how much energy is emitted by an object with a known temperature. Combining the two laws provides the mathematics behind this interactive model. You can learn lots more details about the math at our **Calculating Planetary Energy Balance & Temperature** web page.

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