Earth's energy budget
Earth's energy budget accounts for how much energy comes into the Earth's climate system from the Sun, how much energy is lost to space and accounting for the remainder on Earth and its atmosphere. Quantifying changes in these amounts is required to accurately model climate.
Received radiation is unevenly distributed over the planet, because the Sun heats equatorial regions more than polar regions. Energy is absorbed by the atmosphere and hydrosphere and, in a process informally described as Earth's heat engine, the solar heating is distributed through evaporation of surface water, convection, rainfall, winds and ocean circulation. When incoming solar energy is balanced by an equal flow of heat to space, Earth is in radiative equilibrium and global temperatures stabilize.
Disturbances of Earth's radiative equilibrium, such as an increase of greenhouse gases, change global temperatures in response. However, Earth's energy balance and heat fluxes depend on many factors, such as atmospheric composition (mainly aerosols and greenhouse gases), the albedo (reflectivity) of surface properties, cloud cover and vegetation and land use patterns. Changes in surface temperature due to Earth's energy budget do not occur instantaneously, due to the inertia of the oceans and the cryosphere. The net heat flux is buffered primarily in the ocean's heat content, until a new equilibrium state is established between radiative forcings and climate response.
- 1 Energy budget
- 2 Measurement
- 3 Natural greenhouse effect
- 4 Climate sensitivity
- 5 See also
- 6 References
- 7 External links
In spite of the enormous transfer of energy into and from Earth, it maintains a constant temperature because, as a whole, there is no net gain or loss: Earth receives the same amount of energy via insolation (short-wave or ultraviolet radiation) as it emits via atmospheric and terrestrial radiation (long-wave or infrared radiation).
To quantify Earth's heat budget or heat balance, let the insolation received at the top of the atmosphere be 100 units, as shown in the accompanying illustration. Called the albedo of Earth, around 35 units are reflected back to space: 27 from the top of clouds, 2 from snow and ice-covered areas, and 6 by other parts of the atmosphere. The 65 remaining units are absorbed: 14 within the atmosphere and 51 by the Earth’s surface. These 51 units are radiated back in the form of terrestrial radiation: 17 directly radiated to space and 34 absorbed by the atmosphere (19 through latent heat of condensation, 9 via convection and turbulence, and 6 directly absorbed). The 48 units absorbed by the atmosphere (34 units from terrestrial radiation and 14 from insolation) are finally radiated back to space. These 65 units (17 from the ground and 48 from the atmosphere) balance the 65 units absorbed from the sun.
Incoming radiant energy (shortwave)
The total amount of energy received per second at the top of Earth's atmosphere (TOA) is measured in watts and is given by the solar constant times the cross-sectional area of the Earth. Because the surface of a sphere is four times the cross-sectional area of a sphere, the average TOA flux is one quarter of the solar constant and so is approximately 340 W/m². Since the absorption varies with location as well as with diurnal, seasonal and annual variations, numbers quoted are long-term averages, typically averaged from multiple satellite measurements.
Of the ~340 W/m² of solar radiation received by the Earth, an average of ~77 W/m² is reflected back to space by clouds and the atmosphere and ~23 W/m² is reflected by the surface albedo, leaving ~240 W/m² of solar energy input to the Earth's energy budget. This gives the earth a mean net albedo of 0.29.
Earth's internal heat and other small effects
The geothermal heat flux from the Earth's interior is estimated to be 47 terawatts. This comes to 0.087 watt/square metre, which represents only 0.027% of Earth's total energy budget at the surface, which is dominated by 173,000 terawatts of incoming solar radiation.
Other minor sources of energy are usually ignored in these calculations, including accretion of interplanetary dust and solar wind, light from distant stars and the thermal radiation of space. Earlier, Joseph Fourier had claimed that deep space radiation was significant in a paper often cited as the first on the greenhouse effect.
Longwave radiation is usually defined as outgoing infrared energy, leaving the planet. However, the atmosphere absorbs parts initially, or cloud cover can reflect radiation. Generally, heat energy is transported between the planet's surface layers (land and ocean) to the atmosphere, transported via evapotranspiration and latent heat fluxes or conduction/convection processes. Ultimately, energy is radiated in the form of longwave infrared radiation back into space.
Recent satellite observations indicate additional precipitation, which is sustained by increased energy leaving the surface through evaporation (the latent heat flux), offsetting increases in longwave flux to the surface.
Earth's energy imbalance
If the incoming energy flux is not equal to the outgoing thermal radiation, the result is an energy imbalance, resulting in net heat added to or lost by the planet (if the incoming flux is larger or smaller than the outgoing). Earth's energy imbalance measurements provided by Argo floats detected accumulation of ocean heat content (OHC). The estimated imbalance was measured during a deep solar minimum of 2005-2010 at 0.58 ± 0.15 W/m². Later research estimated the surface energy imbalance to be 0.60 ± 0.17 W/m².
Several satellites indirectly measure the energy absorbed and radiated by Earth and by inference the energy imbalance. The NASA Earth Radiation Budget Experiment (ERBE) project involves three such satellites: the Earth Radiation Budget Satellite (ERBS), launched October 1984; NOAA-9, launched December 1984; and NOAA-10, launched September 1986.
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 kilometres, the concentration of greenhouse gases in the overlying atmosphere is so small that heat can reach space.
Because greenhouse gas molecules radiate infrared energy in all directions, some of it spreads downward and ultimately returns to Earth's surface, where it is absorbed. Surface temperature is thus higher than it would be if it were heated only by direct solar heating. This supplemental heating is the natural greenhouse effect.
A change in the incident radiated portion of the energy budget is referred to as a radiative forcing.
Climate forcings and global warming
Climate forcings are energy balance changes that affect that cause temperatures to rise or fall. Natural climate forcings include changes in the Sun's brightness, Milankovitch cycles (small variations in the shape of Earth's orbit and its axis of rotation that occur over thousands of years) and volcanic eruptions that inject light-reflecting particles as high as the stratosphere. Man-made forcings include particle pollution (aerosols) that absorb and reflect incoming sunlight; deforestation, which changes how the surface reflects and absorbs sunlight; and the rising concentration of atmospheric carbon dioxide and other greenhouse gases, which decrease heat radiated to space.
A forcing can trigger feedbacks that intensify (positive feedback) or weaken (negative feedback) the original forcing. For example, loss of ice at the poles, which makes them less reflective, is an example of a positive feedback.
The observed planetary energy imbalance during the recent solar minimum shows that solar forcing of climate, although significant, is overwhelmed by anthropogenic climate forcing.
Eliminating the anthropological forcing would require that atmospheric CO2 content be reduced to about 350 ppm. The impact of anthropogenic aerosols has not been quantified, but individual aerosol types are thought to have substantial heating and cooling effects.
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- 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
- "NASA: Climate Forcings and Global Warming". January 14, 2009.
|Wikimedia Commons has media related to Earth's energy budget.|
- NASA: The Atmosphere's Energy Budget
- Clouds and Earth's Radiant Energy System (CERES)
- NASA/GEWEX Surface Radiation Budget (SRB) Project