Earth's energy budget

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A schematic representation of the energy exchanges between the Earth's surface, the Earth's atmosphere, and outer space. The recycling of energy between the Earth's surface and the atmosphere is characteristic of the greenhouse effect. Note that the total energy entering each level is equal to the energy leaving that level as should be expected for a system in balance.

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.

1 The power budget
- 1.1 Incoming power
- 1.2 Outgoing power
2 Anthropogenic modification
3 See also
4 References

[edit] The power budget

[edit] Incoming power

The total flux of power entering the Earth's atmosphere is estimated at 174 petawatts. This consists of:

solar radiation (99.978%, or nearly 174 petawatts; or about 340 W m^-2)
- 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, averaged over the Earth's surface, which is four times larger. The solar flux averaged over just the sunlit half of the earth's surface is about 680 W m^-2
geothermal energy (0.013%, or about 23 terawatts; or about 0.045 W m^-2)
- 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 0.0059 W m^-2)
- 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 (about 0.007%, or about 13 terawatts; or about 0.025 W m^-2) [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.

[edit] 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. It also varies very strongly spatially: ice sheets have a high albedo, oceans low. 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

[edit] Anthropogenic modification

Emission of greenhouse gases, and other factors (such as land-use changes), modify the energy budget slightly but significantly. The IPCC provides an estimate of these forcing, insofar as they are known [2]. The largest and best known are from the well-mixed greenhouse gases (CO₂, CH₄, halocarbons, etc.), totalling an increase in forcing of 2.4 W m^-2 relative to 1750. These are less than 1% of the solar input, but contributes to the observed increase in atmospheric and oceanic temperature.