The Earth can be considered as a physical system with an energy budget that includes all gains of incoming energy and all losses of outgoing energy. The planet is approximately in equilibrium, so the sum of the gains should be approximately equal to the sum of the losses.
Note: although the term "energy budget" is widely used, the flow of energy in and out of the Earth is actually measured in units of power (watts), not units of energy (joules). Therefore, "power budget" would be a more accurate term.
The power budget
Incoming power
The total flux of power entering the Earth's atmosphere is estimated at 174 petawatts. This consists of:
- solar radiation (99.985%, or nearly 174 petawatts; or about 341.5 W/m²)
- This is equal to the product of the solar constant, about 1366 watts per square metre, and the area of the Earth's disc as seen from the Sun, about 1.28 × 1014 square metres. The figure of 341.5 W/m² is then the global average value.
- geothermal energy (0.013%, or about 23 terawatts; or about 0.18 W/m²)
- This is produced by stored heat and heat produced by radioactive decay leaking out of the Earth's interior.
- tidal energy (0.002%, or about 3 terawatts; or about 0.02 W/m²)
- This is produced by the interaction of the Earth's mass with the gravitational fields of other bodies such as the Moon and Sun.
- Waste heat from fossil fuel consumption is about 13 terawattts of 0.025 Wm² [1].
Note that the solar constant varies (by approximately 0.1% over a solar cycle); and is not known absolutely to within better than about one watt per square metre. Hence the geothermal and tidal contributions are less than the uncertainty in the solar power.
Outgoing power
The average albedo (reflectivity) of the Earth is about 0.3, which means that 30% of the incident solar energy is reflected back into space, while 70% is absorbed by the Earth and reradiated as infrared. The planet's albedo varies from month to month, but 0.3 is the average figure. The contributions from geothermal and tidal power sources are so small that they are omitted from the following calculations.
The 30% reflected energy consists of:
- 6% reflected from the atmosphere
- 20% reflected from clouds
- 4% reflected from the ground (including land, water and ice)
All of the 70% absorbed energy is eventually reradiated:
- 64% by the clouds and atmosphere
- 6% by the ground
The same 70% of absorbed energy can be split this way:
- 51% absorbed by land and water, then emerging in the following ways:
- 23% transferred back into the atmosphere as latent heat by the evaporation of water
- 7% transferred back into the atmosphere by heated rising air
- 6% radiated directly into space
- 15% transferred into the atmosphere by radiation, then reradiated into space
and
- 19% absorbed by the atmosphere, including:
- 16% reradiated back into space
- 3% transferred to clouds, from where it is radiated back into space
See also
References