Planetary Energy Balance

Planetary Energy Balance

Planetary Energy BalanceThis page contains two interactive simulations.

Click the Blackbody tab above to experiment with a blackbody radiator and learn how warm objects give off energy as electromagnetic radiation.

Click the Brightness & Albedo tab to investigate the theoretical temperature for an Earth-like planet. That simulation allows you to alter the brightness of the Sun and vary the albedo of the planet to see what effect those changes have on the planet's temperature.

These simple simulations illustrate how inhospitable our planet would be without the warming caused by the greenhouse effect. Our Greenhouse Effect Teaching Box has lots of info about the more complicated, and more realistic, case of a planet blanketed with an atmosphere containing greenhouse gases.

This simulation illustrates the relationship between the temperature and the amount of energy emitted by a blackbody radiator.

The flashlight shines visible light onto the spherical blackbody radiator. The energy carried by the light warms the sphere, which emits the heat in all directions as longwave infrared radiation.

Use the slider to change the brightness of the flashlight. What happens to the temperature of the sphere?

A blackbody radiator is a hypothetical physical object that absorbs all electromagnetic radiation which strikes it. Since light is a form of electromagnetic (EM) radiation, all light striking a blackbody radiator would be absorbed by the object, so the object would appear perfectly black. Hence the "blackbody" part of the name.

Law of Energy Conservation applied to a planetThe law of conservation of energy tells us that the amount of energy emitted by any isolated system or object must precisely balance the amount of energy absorbed by the object. In simpler language, this can be summarized as "energy in" = "energy out". A blackbody radiator must, therefore, give off exactly as much energy as it absorbs.

Any object with a temperature above absolute zero emits electromagnetic radiation. Most objects emit infrared (IR) "light". Objects that are very hot can have peak emissions in the visible light portion of the EM spectrum, which is why the burners on an electric stove or hotplate glow red when they are turned on high. Many stars, which are far hotter still, glow with higher-energy yellow or blue light, or sometimes even ultraviolet "light".

If light shines on a blackbody radiator, the object absorbs all of the energy from that light. The energy is converted to heat, warming the blackbody. Warm objects emit energy as EM radiation, usually as IR "light". Warmer objects emit more EM radiation than cooler ones. The blackbody warms in response to the incoming light until it is hot enough to radiate all of that energy away as infrared. At a certain temperature the outgoing emissions balance the incoming light; the blackbody radiator reaches a state of thermal equilibrium.

Although a perfect blackbody radiator is just a hypothetical "thought experiment", many actual objects behave in a way that is pretty close to a perfect blackbody radiator. Planets are enough like theoretical blackbody radiators that we can use this concept to approximate the temperature of a planet, including Earth.

Use the slider to change the albedo of your planet. What happens to the average temperature of your planet when you make its surface darker? lighter?

Can you raise the temperature of your planet above the freezing point of water just by changing the albedo?

© 2013 UCAR