Earth's energy budget
The Earth can be considered as a physical system with an energy budget. The shortwave radiation net flow of energy into Earth and the longwave radiation out to Space determine the Earth’s energy budget. The Earth equilibrium sensitivity describes a steady state energy budget. Today anthropogenic perturbations are responsible for a positive radiative forcing which reduces the net longwave radiation loss out to Space, hence the radiative equilibrium is disturbed and Earth's energy budget changes, which doesn't occur instantaneously due to the slow response/inertia of the cryosphere to react to the new energy budget. The net heat flux is buffered primarily in the ocean, until a new equilibrium state is established between in- and outgoing radiative forcing and climate response. The surface and the atmosphere simultaneously radiate heat back to space.
The energy budget
- solar radiation (99.97%, or nearly 173 petawatts)
- This is equal to the product of the solar constant, about 1,366 watts per square meter, and the area of the Earth's disc as seen from the Sun, about 1.28 × 1014 square meters, averaged over the Earth's surface, which is four times larger. (That is, the area of a disc with the Earth's diameter, which is effectively the target for solar energy, is 1/4 the area of the entire surface of the Earth.) The solar flux averaged over just the sunlit half of the Earth's surface is about 680 W m−2
- This is the incident energy. The energy actually absorbed by the earth is lower by a factor of the albedo; this is discussed in the next section.
- 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 meter. Hence geothermal, tidal, and waste heat contributions are less uncertain than solar power.
- energy from Earth's internal heat (0.025%; or about 44 to 47 terawatts)
- 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)
- 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). The total energy used by commercial energy sources from 1880 to 2000 (including fossil fuels and nuclear) is calculated to be 17.3x1021 joules.
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.
The average albedo (reflectivity) of the Earth is about 0.3, which means that 30% of the incident solar energy is reflected into space, while 70% is absorbed by the Earth and reradiated as infrared. The planet's albedo varies from month to month and place to place, 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.
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)
The remaining 70% of the incident energy is absorbed:
- 51% is absorbed by land and water, and then emerges in the following ways:
- 19% is absorbed by the atmosphere (16% by the air, 3% by clouds).
The Earth and its atmosphere are also radiant energy sources themselves. The atmosphere absorbs 90% of the energy radiated by the Earth, and radiates its own energy, 50% back towards the ground and 50% into space.
When the Earth is at thermal equilibrium, the absorbed and radiated energy are equal: 70% of the incident solar energy = 50% of the atmosphere's radiation + 11% of the land+water radiation + 9% of the cloud's radiation.
Natural greenhouse effect
 The major atmospheric gases (oxygen and nitrogen) are transparent to incoming sunlight, and are also transparent to outgoing thermal infrared. However, water vapor, carbon dioxide, methane, and other trace gases are opaque to many wavelengths of thermal infrared energy. The Earth's surface radiates the net equivalent of 17 percent of incoming solar energy as thermal infrared. However, the amount that directly escapes to space is only about 12 percent of incoming solar energy. The remaining fraction—a net 5-6 percent of incoming solar energy—is transferred to the atmosphere when greenhouse gas molecules absorb thermal infrared energy radiated by the surface.
 When greenhouse gas molecules absorb thermal infrared energy, their temperature rises. Like coals from a fire that are warm but not glowing, greenhouse gases then radiate an increased amount of thermal infrared energy in all directions. Heat radiated upward continues to encounter greenhouse gas molecules; those molecules absorb the heat, their temperature rises, and the amount of heat they radiate increases. At an altitude of roughly 5-6 kilometers, the concentration of greenhouse gases in the overlying atmosphere is so small that heat can radiate freely to space.
 Because greenhouse gas molecules radiate heat in all directions, some of it spreads downward and ultimately comes back into contact with the Earth’s surface, where it is absorbed. The temperature of the surface becomes warmer than it would be if it were heated only by direct solar heating. This supplemental heating of the Earth’s surface by the atmosphere is the natural greenhouse effect.
Greenhouse warming of the Earth's surface
 The natural greenhouse effect raises the Earth’s surface temperature to about 15 degrees Celsius on average—more than 30 degrees warmer than it would be if it didn’t have an atmosphere. Earth maintains a balance between incoming solar energy in the form of ultraviolet and visible light that it absorbs and the amount of heat in the form of infrared radiation that it redirects back toward space. Variables that cause this energy balance to change and affect the global average temperature are called forcings because they force the temperature up or down. Forcings include changes in the sun’s brightness and other influences that operate on roughly 11-year and longer length cycles; aerosols and particulate matter originating from the oceans, volcanic eruptions, and man-made air pollution such as heat-trapping CO2.
 The amount of energy a surface radiates always increases faster than its temperature rises. The amount of heat a surface radiates is proportional to the fourth power of its temperature (the Stefan-Boltzmann law). As solar heating and “back radiation” from the atmosphere raise the surface temperature, the surface simultaneously releases an increasing amount of heat—equivalent to about 117 percent of incoming solar energy. The net upward heat flow from the Earth's surface is equivalent to 17 percent of incoming sunlight (117 percent up minus 100 percent down).
 Some of the heat escapes directly to space, and the rest is transferred to higher and higher levels of the atmosphere, until the energy leaving the top of the atmosphere matches the amount of incoming solar energy. This includes solar radiation reflected from the Earth and outgoing heat radiation. Because the maximum possible amount of incoming sunlight is fixed by the solar constant (which depends only on Earth’s distance from the Sun and very small variations during the solar cycle), the natural greenhouse effect does not cause a runaway increase in surface temperature on Earth.
- Data to produce this graphic was taken from a NASA publication.
- M, Previdi et al. (2013). "Climate sensitivity in the Anthropocene". Royal Meteorological Society.
- Lindsey, Rebecca (2009). "Climate and Earth’s Energy Budget". NASA Earth Observatory.
- Pollack, H.N.; S. J. Hurter, and J. R. Johnson (1993). "Heat Flow from the Earth's Interior: Analysis of the Global Data Set". Rev. Geophys. 30 (3). pp. 267–280
- J. H. Davies and D. R. Davies, "Earth’s Surface heat flux," Solid Earth, 1, 5–24 (2010), available in pdf form here (accessed 8 October 2010)
- Nordell, Bo; Bruno Gervet. Global energy accumulation and net heat emission. Retrieved 2009-12-23.
- Connolley, William M. (18 May 2003). "William M. Connolley's page about Fourier 1827: MEMO IRE sur les temperatures du globe terrestre et des espaces planetaires". William M. Connolley. Retrieved 5 July 2010.
- Edited quote from public-domain source: Lindsey, R. (January 14, 2009), The Atmosphere’s Energy Budget (page 6), in: Climate and Earth’s Energy Budget: Feature Articles, Earth Observatory, part of the EOS Project Science Office, located at NASA Goddard Space Flight Center
- Edited quote from public-domain source: Lindsey, R. (January 14, 2009), Earth’s Energy Budget (page 4), in: Climate and Earth’s Energy Budget: Feature Articles, Earth Observatory, part of the EOS Project Science Office, located at NASA Goddard Space Flight Center
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