Earth's energy budget
Earth's energy budget or Earth's radiation balance describes the net flow of energy into Earth in the form of shortwave radiation and the outgoing infrared longwave radiation into space. Thus, the energy fluxes are important to understand climate change, defined by changes in Earth's energy balance.
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, known as Earth's heat engine, coupled processes which constantly even out solar heating imbalances through evaporation of surface water, convection, rainfall, winds, and ocean circulation, when distributing heat around the globe. When incoming solar energy is balanced by an equal flow of heat to space, Earth is in radiative equilibrium and global temperatures become relatively stable.
Disturbances of Earth's radiative equilibrium, such as the rise of heat-trapping gases, change global temperatures in response, because of the greenhouse effect, since energy radiated back to space is in part absorbed by greenhouse gas molecules. However, Earth's energy balance and heat fluxes depend on many factors, such as the atmospheric chemistry 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 (slow response) of the oceans and the cryosphere to react to the new energy budget. The net heat flux is buffered primarily in the ocean's heat content, until a new equilibrium state is established between incoming and outgoing radiative forcing and climate response.
Incoming radiant energy (shortwave)
The total amount of energy received by Earth's atmosphere is normally measured in watts (a joule per second) and determined by the solar constant. Earth's incoming solar radiation depends on day-night cycles and the angle at which sun rays strike, thus calculated by its cross section and distribution on the planet's surface, calculated with 4·π·RE2, in sum one-fourth the solar constant (approximately 340 W/m², plus or minus 2 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 about 240 W/m² of solar energy input to the Earth's energy budget.
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.
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.
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 (infrared) radiation, the result is an energy imbalance, resulting in net heat added to the planet (if the incoming flux is larger than the outgoing). Earth's energy imbalance measurements provided by Argo floats detected accumulation of ocean heat content (OHC) in the recent decade. The estimated imbalance is 0.58 ± 0.15 W/m².
Several satellites have been launched into Earth's orbit that 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.
Today the NASA satellite instruments, provided by CERES, part of the NASA's Earth Observing System (EOS), are especially designed to measure both solar-reflected and Earth-emitted radiation from the top of the atmosphere (TOA) to the Earth's surface.
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 radiate freely to space.
Because greenhouse gas molecules radiate infrared energy 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.
A change in the incident or radiated portion of the energy budget is referred to as a radiative forcing.
Climate forcings and global warming
Changes in Earth's climate system that affect the energy which enters or leaves the system alters Earth's radiative equilibrium, and thus can force temperatures to rise or fall, are called climate forcings. 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 large volcanic eruptions that inject light-reflecting particles as high as the stratosphere. Man-made forcings include particle pollution (aerosols), which 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 a much larger net human-made climate forcing.
Today, anthropogenic perturbations in greenhouse gas concentration are responsible for a positive radiative forcing which reduces the net longwave radiation loss out to space, hence the radiative equilibrium is disturbed. It has been suggested to reduce atmospheric CO2 content to about 350 ppm, in order to stop further global warming. The data also show that climate forcing by human-made aerosols is larger than usually assumed, hence more global aerosol monitoring would improve people's understanding of interpretation of recent climate change.
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|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