Earth's energy budget

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Earth's climate is largely determined by the planet's energy budget, i.e, the balance of incoming and outgoing radiation. It is measured by satellites and shown in W/m2.[1]

Earth's energy budget or Earth's radiation balance, describes the status of shortwave radiation net flow of energy into Earth and the longwave radiation out to Space.[2] Climate change is defined by the state and changes in Earth's energy budget.[3]

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 (Ocean heat content), until a new equilibrium state is established between in- and outgoing radiative forcing and climate response.[4]

The energy budget[edit]

Incoming, top-of-atmophere (TOA) shortwave flux radiation, shows energy received from the sun (Jan 26-27, 2012).
Outgoing, longwave flux radiation at the top-of-atmosphere (Jan 26-27, 2012). Heat energy radiated from Earth (in watts per square meter) is shown in shades of yellow, red, blue and white. The brightest-yellow areas are the hottest and are emitting the most energy out to space, while the dark blue areas and the bright white clouds are much colder, emitting the least energy.

Received radiation is unevenly distributed over the planet, because the Sun heats equatorial regions more than polar regions. Earth’s heat engine, are the coupled processes of the atmosphere and hydrosphere to even out solar heating imbalances through evaporation of surface water, convection, rainfall, winds, and ocean circulation.[5] When the amount of the solar energy reaching Earth equals the thermal energy amount being radiated out, the radiative forcings are in a state of equilibrium balance.

Incoming energy/radiation (shortwave)[edit]

The total amount of energy received by Earth's atmosphere is normally measured in watts and determined by the solar constant. Earth 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 planets surface, calculated with 4·π·RE2, in sum one-fourth the solar constant (approximately 340 W/m2, plus or minus 2 W/m2).[1][6]

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.[7]

Outgoing energy/radiation (longwave)[edit]

From the ~340 W/m2 solar radiation received, ~77 W/m2 is reflected back to Space, by clouds and the atmosphere and ~23 W/m2 is reflected by the surface albedo. About 0.6 W/m2, is absorbed by the upper layers of the planet. Outgoing infrared radiation is radiated by the planet surface layers (Land and ocean), or transported via, evapotranspiration (84.4 W/m2, the latent heat) or conduction/convection (18.4 W/m2) processes.[1]

Earth's internal heat budget[edit]

Earth's internal heat budget, is estimated to be 47 terawatts.[8] However, the heat energy coming from Earth's interior is only 0.03% of Earth's total energy budget at the surface, which is dominated by 173,000 terawatts of incoming solar radiation.[9]

Earth's energy imbalance[edit]

Expected Earth energy imbalance for three choices of aerosol climate forcing. Measured imbalance, close to 0.6 W/m2, implies that aerosol forcing is close to -1.6 W/m2. (Credit: NASA/GISS)[10]

The term Earth's Energy Imbalance describes, measurements provided by Argo floats which detected accumulation of ocean heat content (OHC) in the recent decade. Thus, the observed planetary energy gain during the recent solar minimum shows that solar forcing of climate, although significant, is overwhelmed by a much larger net human-made climate forcing. The estimated imbalance is 0.58± 0.15 W/m2. It has been suggested to reduce atmospheric CO2 content to about 350 ppm, in order to stop further global warming. The data also shows that climate forcing by human-made aerosols is larger than usually assumed, hence more global aerosol monitoring would improve our understanding of interpretation of recent climate change.[10]

Research[edit]

Several satellites have been launched into Earth's orbit that indirectly measure the energy absorbed and radiated by Earth, and by inference the energy stored. 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.[11]

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.[12]

Natural greenhouse effect[edit]

refer to caption and image description
Diagram showing the energy budget of Earth's atmosphere, which includes the greenhouse effect.

[13] 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.

[13] 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.

[13] 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.

Climate forcings and global warming[edit]

Atmospheric gases only absorb some wavelengths of energy but are transparent to others. The absorption patterns of water vapor (blue peaks) and carbon dioxide (pink peaks) overlap in some wavelengths. Carbon dioxide is not as strong a greenhouse gas as water vapor, but it absorbs energy in wavelengths (12-15 micrometers) that water vapor does not, partially closing the “window” through which heat radiated by the surface would normally escape to space. (Illustration NASA, Robert Rohde)[14]

NASA explains climate forcings: Changes in Earth’s climate system that affect the energy which enters or leaves the system alters Earth’s radiative equilibrium, thus can force temperatures to rise or fall, and 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. Manmade 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 or weaken the original forcing. The loss of ice at the poles, which makes them less reflective, is an example of a feedback.[14]

See also[edit]

References[edit]

  1. ^ a b c "The NASA Earth's Energy Budget Poster". NASA. 
  2. ^ Chiacchio, Marc; Solmon, Fabien; Giorgi, Filippo; Stackhouse, Paul, Jr. (April 2013). The global energy budget with a regional climate model over Europe. Copernicus. Bibcode:2013EGUGA..15.6581C. 
  3. ^ Graeme L. Stephens, Juilin Li, Martin Wild, Carol Anne Clayson, Norman Loeb, Seiji Kato, Tristan L’Ecuyer, Paul W. Stackhouse Jr, Matthew Lebsock and Timothy Andrews (September 23, 2012). An update on Earth’s energy balance in light of the latest global observations. Nature Geoscience. doi:10.1038/NGEO1580. 
  4. ^ M, Previdi et al. (2013). "Climate sensitivity in the Anthropocene". Royal Meteorological Society. doi:10.1002/qj.2165. 
  5. ^ Lindsey, Rebecca (2009). "Climate and Earth’s Energy Budget". NASA Earth Observatory. 
  6. ^ Wild, Martin; Folini, Doris; Schär, Christoph; Loeb, Norman; Dutton, Ellsworth; König-Langlo, Gert (2013). The Earth's radiation balance and its representation in CMIP5 models. Copernicus. Bibcode:2013EGUGA..15.1286W. 
  7. ^ 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. 
  8. ^ Davies, J. H., & Davies, D. R. (2010). Earth's surface heat flux. Solid Earth, 1(1), 5–24.
  9. ^ Archer, D. (2012). Global Warming: Understanding the Forecast. ISBN 978-0-470-94341-0. 
  10. ^ a b James Hansen, Makiko Sato, Pushker Kharecha and Karina von Schuckmann (January 2012). Earth's Energy Imbalance. NASA. 
  11. ^ Effect of the Sun's Energy on the Ocean and Atmosphere (1997)
  12. ^ B.A. Wielicki, et al. (1996). "Mission to Planet Earth: Role of Clouds and Radiation in Climate". Bull. Amer. Meteorol. Soc. 77 (5): 853–868. Bibcode:1996BAMS...77..853W. doi:10.1175/1520-0477(1996)077<0853:CATERE>2.0.CO;2. 
  13. ^ a b c 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 
  14. ^ a b "NASA: Climate Forcings and Global Warming". January 14, 2009. 

External links[edit]