Thursday, 24 January 2013

EARTH ENERGY BUDGET



BBQ time? Think again..
The Sun is the source of energy for the Earth system. This energy reaches the Earth primarily in the form of visible light, although it also includes some infrared energy (heat), ultraviolet energy, and other wavelengths of the electromagnetic spectrum. Taking into account night and day and the seasons, on average about 340 Watts of energy enter every square meter of the Earth system. This is slightly less than the energy that six 60 Watt light bulbs would produce, again, for every square meter of the Earth.

As it reaches the Earth system, some of the sunlight is reflected back to space by clouds and the atmosphere (particularly dust particles or aerosols in the atmosphere). A little more sunlight is reflected to space from the Earth surface, particularly from bright regions such as snow- and ice-covered areas. In total, about 30% of sunlight is reflected directly back to space. This percentage is called the albedo.

About 70% of the sunlight is absorbed by the Earth system (atmosphere and surface) and heats it up. The elements of the Earth system (surface, atmosphere, clouds) emit infrared radiation according to their temperature, following the Planck function. Cold objects emit less energy; warm objects emit more. This infrared radiation is emitted in all directions.

One net effect of all the infrared emission is that an amount of heat energy equivalent to ~70% of the incoming sunlight leaves the Earth system and goes back into space. This is because the Earth system constantly tends toward equilibrium between the energy that reaches the Earth from the Sun and the energy that is emitted to space. Scientists refer to this process as Earth's "radiation budget”, and it happens because the system tends toward equilibrium.

Another net effect of the infrared emission is that about 340 Watts of infrared energy is directed back to the surface from the atmosphere. This is called the greenhouse effect, and is due mainly to water vapor in the atmosphere. Carbon dioxide, methane and other infrared-absorbing gases enhance this effect. Without an atmosphere, the Earth would have an average temperature of -18 °C, too cold for life as we know it.


At the surface, two additional heat transfer mechanisms operate to balance the system, in addition to the radiation transfer: 1) convection and conduction in the form of thermals (which create weather), and 2) a change of state of water through evapotranspiration (which also feeds weather).

Just like a family budget for finances, the energy budget of the Earth should be balanced. In equation form:

Energy In = Energy Out

This balance can be considered at several levels in the Earth system:

At the top of the atmosphere, the energy coming in from the Sun is balanced by sunlight reflected back to space and the net infrared emission from the Earth. The equation is:

Sunlight In = Sunlight reflected from clouds/atmosphere + Sunlight reflected from surface + IR emission

At the Earth’s surface, absorbed sunlight is balanced by the net IR emission and the conduction/convection and evapotranspiration. The equation is:

Sunlight absorbed + IR back radiation (greenhouse effect) = IR emission + Thermals + Evapotranspiration

The most complicated balance is in the atmosphere, where absorbed sunlight and energy absorbed from the
surface are balanced by the net infrared emission. The equation is:

Sunlight absorbed + IR absorbed + Thermals + Evapotranspiration = IR emitted to space + IR emitted to ground

These balance equations are for an equilibrium state of the Earth. Equilibrium would be expected for a planet that has spent a long time in a stable solar system, but sometimes changes occur that take the system out of balance. For example, the ice ages occurred because of long-term changes in Earth’s orbit around the Sun, which resulted in a change to the “Sunlight In” term. Over time, reflected sunlight and IR emission changed to balance the first equation. The result was a colder surface and major glacial advances.

So the next time you are thinking of starting that giant BBQ in the yard, think about the balance. May the force be with you and may you bring balance to it.

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