# Earth's Albedo and the Sun's Brightness Affect Climate

The temperature of our planet is affected by the amount of energy coming to Earth from the Sun and amount of energy reflected away from Earth and out into space. Try the simulation below to explore how the Sun's brightness and the Earth's albedo affect global climate.

• Albedo: Light surfaces like snow, have a higher albedo, reflecting more energy away. Dark surfaces, like forests, have a low albedo, reflecting less energy away. Overall, Earth's average albedo is about 31%.
• Brightness of the Sun. The brightness of the Sun can change in small amounts over years to centuries and can change large amounts over millions to billions of years. The Sun has become less bright since 1900, according to NASA.

This simple model lets you see the effects of albedo and the Sun's brightness as if Earth didn't have an atmosphere.  The atmosphere warms Earth an average of 33-35 Kelvin (33-35°C).  This warming from the atmosphere is the difference between a freezing planet and a habitable one.

## Questions to explore:

• How would Earth's temperature change if the albedo was higher or lower?
• How would Earth's temperature change if the Sun was brighter or dimmer?
• Today climate is warming because of extra greenhouse gases. As warmer temperatures melt ice, how will that change the albedo and temperature?

## 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 which scales you are using.
• 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.

## How this simulation works

• This interactive simulation relies on the conservation of energy and the Stefan-Boltzmann law.
• Sunlight provides energy to our planet, and Earth absorbs and emits energy back to 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. ("energy in = energy out").
• 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 more details at our Calculating Planetary Energy Balance & Temperature web page. The model equation that describes the relation between energy and temperature is below:

Energy(to, from Earth) = e 𝝈 Temperature4

where

• Energy is measured in W m-2. For this exercise, the energy incoming to Earth (directly related to the Sun’s brightness) equals the energy outgoing from Earth, equivalent to 1361 W m-2 for a 100% brightness value.
• e = 1-albedo, or emissivity, where emissivity is an estimate of how much energy an object can absorb and release
• 𝝈 = Stefan Boltzmann constant = 5.67x10-8 W m-2 K-4
• Temperature is measured in Kelvin

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 any heat-trapping greenhouse gases in our atmosphere, Earth would be a frozen ball of ice. For 100% Sun brightness and an albedo of 31%, Earth’s temperature as calculated from the simulation is 253 K, or -20°C. In reality, the atmosphere’s effect is powerful and warms our planet by an average of 33-35 K (33-35°C), increasing the globally averaged temperature from -20°C to a much more habitable 15°C.  Excess greenhouse gases in our atmosphere today are the main reason that Earth is heating up