Objective: The purpose of this activity is to analyze the Earth’s energy budget in greater detail. A mathematical analysis will be done to examine how the Earth’s energy budget is balanced.
Materials Required: Everything that you need for this activity is included in the on-screen components.
Time Required: approximately 30 minutes
The Earth’s energy budget describes the relationship between incoming energy and outgoing energy. On average and over the long term, the Earth is in a state of equilibrium. This means that the Earth has a balanced energy budget with the amount of energy coming in equalled by the amount of energy going out. This balance of “Energy In = Energy Out” should be true at all levels in the Earth system.
The diagram to the right highlights the Earth’s annual energy balance. It is different from the energy budget diagram in lesson B4 in two important respects:
At the top of the atmosphere, the energy coming in from the sun is balanced by the energy reflected back into space and the net outgoing longwave radiation. The equation describing this balance is:
Incoming Solar Radiation = Reflected Solar Radiation + Outgoing Longwave Radiation
L.S. (left side of equation) | R.S. (right side of equation) |
Incoming Solar Radiation = 342 W/m^{2} | Reflected Solar Radiation + Outgoing Longwave Radiation = 107 W/m^{2} + 235 W/m^{2} = 342 W/m^{2} |
L.S. = R.S.
The energy balance at the Earth’s surface is more complicated because it also involves heating from the greenhouse effect.
All incoming energy (i.e. Energy In) at the Earth’s surface in the energy balance diagram is represented by arrows going into the Earth’s surface, while all outgoing energy (i.e. Energy Out) is represented by arrows leaving the Earth’s surface. The complete equation describing this energy balance (i.e. Energy In = Energy Out) at the Earth’s surface is:
Energy Absorbed by Surface + Back Radiation (from Greenhouse Effect)
= Surface Radiation + Evapotranspiration + Thermals
L.S. (left side of equation) | R.S. (right side of equation) |
Energy Absorbed by Surface + Back Radiation (from greenhouse effect) = 168 W/m^{2} + 324 W/m^{2} = 492 W/m^{2} |
Surface Radiation + Evapotranspiration + Thermals = 390 W/m^{2} + 78 W/m^{2} + 24 W/m^{2} = 492 W/m^{2} |
L.S. = R.S.
The most complicated relationship exists in the atmosphere, in part due to the greenhouse effect. Energy from sunlight absorbed by the atmosphere and energy from the Earth’s surface must be balanced by outgoing energy emitted from the atmosphere. The complete equation describing this energy balance in the atmosphere is given by:
Sunlight Absorbed by Atmosphere + Thermals + Evapotranspiration + Surface Radiation from Earth (into atmosphere) = Energy Emitted Back to Ground (i.e. Back Radiation) + Energy Emitted by Clouds + Energy Emitted by Atmosphere
L.S. (left side of equation) | R.S. (right side of equation) |
Sunlight Absorbed by Atmosphere + Thermals + Evapotranspiration + Surface Radiation from Earth (into atmosphere) = 67 W/m^{2} + 24 W/m^{2} + 78 W/m^{2} + 350 W/m^{2} = 519 W/m^{2} |
Energy Emitted Back to Ground (i.e. Back Radiation) + Energy Emitted by Clouds + Energy Emitted by Atmosphere = 324 W/m^{2} + 30 W/m^{2} + 165 W/m^{2} = 519 W/m^{2} |
L.S. = R.S.