From Wikipedia, the free encyclopedia Jump to: navigation, search A schematic representation of the energy exchanges between the Earth's surface, the

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 is approximately equal to the sum of the losses.

Note on accompanying images: These graphics depict only net energy transfer. There is no attempt to depict the role of greenhouse gases and the exchange that occurs between the Earth's surface and the atmosphere or any other exchanges।

The energy budget

Incoming energy

The total flux of energy entering the Earth's atmosphere is estimated at 174 petawatts. This flux 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 1,366 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
  • 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.
• 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].

There are other minor sources of energy that are usually ignored in these calculations: accretion of interplanetary dust and solar wind, light from distant stars, the thermal radiation of space. Although these are now known to be negligibly small, this was not always obvious: Joseph Fourier initially thought radiation from deep space was significant when he discussed the earth's energy budget in a paper often cited as the first on the greenhouse effect [2].

[edit] Outgoing energy

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.

So 30% of the incident energy is reflected, consisting of:

• 6% reflected from the atmosphere
• 20% reflected from clouds
• 4% reflected from the ground (including land, water and ice)

Earth's longwave thermal radiation intensity, from clouds, atmosphere and ground

The remaining 70% of the incident energy is absorbed:

• 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, called latent heat flux
  • 7% transferred back into the atmosphere by heated rising air, called Sensible heat flux
  • 6% radiated directly into space
  • 15% transferred into the atmosphere by radiation, then reradiated into space
• 19% absorbed by the atmosphere and clouds, including:
  • 16% reradiated back into space
  • 3% transferred to clouds, from where it is radiated back into space

When the Earth is at thermal equilibrium, the same 70% that is absorbed is reradiated:

• 64% by the clouds and atmosphere
• 6% by the ground

Anthropogenic modification

Emissions of greenhouse gases, and other factors such as land-use changes, modify the energy budget slightly but significantly. The Intergovernmental Panel on Climate Change (IPCC) provides an estimate of this forcing, insofar as it is known [3]. 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. This is less than 1% of the solar input, but contributes to the observed increase in atmospheric and oceanic temperature.