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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 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.[2] Research to quantify changes in these amounts is required to accurately assess global warming.[3]

Incoming, top-of-atmosphere (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 metre) 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. 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 become relatively stable.

Disturbances of Earth's radiative equilibrium, such as an increase of greenhouse gases, change global temperatures in response.[4] 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.[5]

Energy budget

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 area 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).[1][6] 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.[1]

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.[7] 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.[8]

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.[9]

Longwave radiation

Main article: Outgoing longwave radiation

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.[1] 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.[3]

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) in the recent decade. The estimated imbalance was measured during a deep solar minimum of 2005-2010 at 0.58 ± 0.15 W/m².[10] Updated research has estimated the surface energy imbalance to be 0.60 ± 0.17 W/m²[11]

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.[12]

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.[13]

Natural greenhouse effect

refer to caption and image description
Diagram showing the energy budget of Earth's atmosphere, which includes the greenhouse effect
See also: 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.[14]

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 micrometres) 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)[15]

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.[14]

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.[14]

Climate sensitivity

Main article: Radiative forcing

A change in the incident or radiated portion of the energy budget is referred to as a radiative forcing.

The climate sensitivity is defined as the steady state change in the equilibrium temperature as a result of changes in the energy budget.

Climate forcings and global warming

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]

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.[15]

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.[10]

See also

References

  1. ^ a b c d "The NASA Earth's Energy Budget Poster". NASA.
  2. ^ IPCC AR5 WG1 Glossary 2013 "energy budget (of the earth)"
  3. ^ a b "An update on Earth's energy balance in light of the latest global observations" (PDF). Nature Geoscience. 23 September 2012. Bibcode:2012NatGe...5..691S. doi:10.1038/NGEO1580. {{cite journal}}: Cite journal requires |journal= (help); Unknown parameter |authors= ignored (help)
  4. ^ Lindsey, Rebecca (2009). "Climate and Earth's Energy Budget". NASA Earth Observatory. {{cite journal}}: Cite has empty unknown parameter: |coauthors= (help)
  5. ^ M, Previdi; et al. (2013). "Climate sensitivity in the Anthropocene". Royal Meteorological Society. Bibcode:2013QJRMS.139.1121P. doi:10.1002/qj.2165. {{cite journal}}: Cite has empty unknown parameter: |coauthors= (help)
  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.{{cite book}}: CS1 maint: multiple names: authors list (link)
  7. ^ Davies, J. H., & Davies, D. R. (2010). Earth's surface heat flux. Solid Earth, 1(1), 5–24.
  8. ^ Archer, D. (2012). Global Warming: Understanding the Forecast. ISBN 978-0-470-94341-0.
  9. ^ Connolley, William M. (18 May 2003). "William M. Connolley's page about Fourier 1827: MEMOIRE sur les temperatures du globe terrestre et des espaces planetaires". William M. Connolley. Retrieved 5 July 2010.
  10. ^ a b c James Hansen, Makiko Sato, Pushker Kharecha and Karina von Schuckmann (January 2012). "Earth's Energy Imbalance". NASA. {{cite journal}}: Cite journal requires |journal= (help)CS1 maint: multiple names: authors list (link)
  11. ^ http://www.nature.com/ngeo/journal/v5/n10/full/ngeo1580.html
  12. ^ Effect of the Sun's Energy on the Ocean and Atmosphere (1997)
  13. ^ 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.
  14. ^ a b c Edited quote from public-domain source: Lindsey, R. (14 January 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
  15. ^ a b "NASA: Climate Forcings and Global Warming". 14 January 2009.
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