Earth’s Energy Budget
For introductory information see:
The energy budget
Earth’s Energy Budget”, Oklahoma Climatological Survey
The Earth Radiation Budget Experiment
Earth’s Energy Out of Balance: The Smoking Gun for Global Warming
Image: Earth’s Energy Budget, NASA
The radiation budget represents the balance between incoming energy from the Sun and outgoing thermal (longwave) and reflected (shortwave) energy from the Earth.
The total flux of energy entering the Earth’s atmosphere is estimated at 174 peta watts. (A Peta Watt is 10 exponential 15 joules per second). 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).
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.
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% of the incident energy reflected, consists of:
- 6% reflected from the atmosphere
- 20% reflected from clouds
- 4% reflected from the ground (including land, water and ice)
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
- 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
- 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
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. The largest and best known are from the well-mixed greenhouse gases (CO2, CH4, halocarbons, etc.), totaling 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.