Incoming & Outgoing Incoming & Outgoing of Energy of the Earthof Energy of the Earth

The Earth’s Energy BalanceThe Earth’s Energy Balance

The Earth's The Earth's average temperatureaverage temperature remains fairly remains fairly constantconstant from year to year. from year to year.

Therefore, the Earth must be Therefore, the Earth must be releasing into spacreleasing into space the same amount of energy that it receives froe the same amount of energy that it receives from the sunm the sun. .

If this did not occur, the atmosphere would measIf this did not occur, the atmosphere would measurably warm or cool, depending on the amount ourably warm or cool, depending on the amount of heat lost to space.f heat lost to space.

Incoming Solar RadiationIncoming Solar Radiation

The sun radiates mostly in the visible band, but The sun radiates mostly in the visible band, but also in the ultraviolet (shorter wavelength). also in the ultraviolet (shorter wavelength).

When averaged globally and annually, only 50% When averaged globally and annually, only 50% of the solar radiation striking the earth and its of the solar radiation striking the earth and its atmosphere is absorbed at the surface.atmosphere is absorbed at the surface.

The atmosphere absorbs 20% of incoming solar The atmosphere absorbs 20% of incoming solar radiation and the remaining 30% is reflected radiation and the remaining 30% is reflected back into space.back into space.

Incoming Solar RadiationIncoming Solar RadiationThe The shortest wavelengthsshortest wavelengths of solar radiation (thos of solar radiation (those most dangerous to life) are e most dangerous to life) are absorbed by molecabsorbed by molecules in the upper and middle atmosphereules in the upper and middle atmosphere. .

In particular, In particular, ozoneozone selectively absorbs ultraviolet selectively absorbs ultraviolet radiationradiation while while allowing visible radiation to pass tallowing visible radiation to pass throughhrough relatively unhindered. relatively unhindered.

ReflectionReflection significantly affects the solar radiation tsignificantly affects the solar radiation that reaches the ground, as the sun's rays could bhat reaches the ground, as the sun's rays could be reflected off of air molecules (termed e reflected off of air molecules (termed scatterinscatteringg), clouds or the ground itself.), clouds or the ground itself.

Incoming Solar RadiationIncoming Solar Radiation

Light-colored or shiny objects reflect more radiatiLight-colored or shiny objects reflect more radiation than dark objects. Energy that is reflected caon than dark objects. Energy that is reflected cannot be absorbed or transmitted through an objennot be absorbed or transmitted through an object.ct.

Different surfaces have different Different surfaces have different albedos. albedos. (Refer to T.B. for details, p.207-208)(Refer to T.B. for details, p.207-208)

Average Reflectivity of Surfaces Average Reflectivity of Surfaces to Solar Radiationto Solar Radiation

Surface typeSurface type % Reflected% Reflected AlbedoAlbedo

Fresh snowFresh snow ~ 90 %~ 90 % ~ 0.9~ 0.9Thick cloudsThick clouds ~ 80 %~ 80 % ~ 0.8~ 0.8Thin cloudsThin clouds ~ 40 %~ 40 % ~ 0.4~ 0.4

IceIce ~ 35 %~ 35 % ~ 0.35~ 0.35SoilSoil ~ 20 %~ 20 % ~ 0.2~ 0.2

ForestForest ~ 5 %~ 5 % ~ 0.05~ 0.05Ocean (high sun angle)Ocean (high sun angle) ~ 5 %~ 5 % ~ 0.05~ 0.05

Ocean (low sun angle)Ocean (low sun angle) ~ 95 %~ 95 % ~ 0.95~ 0.95

Outgoing Terrestrial RadiationOutgoing Terrestrial RadiationThe earth’s surface, atmosphere, and clouds emThe earth’s surface, atmosphere, and clouds emit radiation in the it radiation in the infrared bandinfrared band and and near-infrared near-infrared band.band.

Outgoing infrared (IR) radiation from the earth's Outgoing infrared (IR) radiation from the earth's surface (also called surface (also called terrestrial radiationterrestrial radiation) is ) is selectselectively absorbedively absorbed by certain molecules, particularly by certain molecules, particularly water vapor and carbon dioxidewater vapor and carbon dioxide. .

Gases which absorb IR radiation are termed collGases which absorb IR radiation are termed collectively as “ectively as “greenhouse gasesgreenhouse gases”.”.

Outgoing Terrestrial RadiationOutgoing Terrestrial Radiation

Water vapor and carbon dioxide emit Water vapor and carbon dioxide emit infrared infrared (IR) radiation(IR) radiation. . Infrared radiation from greenhouse gases in the Infrared radiation from greenhouse gases in the atmosphere is atmosphere is emitted in all directions, including emitted in all directions, including back to the earth's surface. back to the earth's surface. It is this re-emission to the earth's surface that It is this re-emission to the earth's surface that maintains a higher temperaturemaintains a higher temperature on our planet on our planet than what would be possible without the than what would be possible without the atmosphere.atmosphere.

Outgoing Terrestrial RadiationOutgoing Terrestrial Radiation

Condensed water is also an efficient absorber aCondensed water is also an efficient absorber and emitter of nd emitter of IRIR radiation. radiation. Thus, clouds act in a manner similar to greenhouThus, clouds act in a manner similar to greenhouse gases. se gases.

Global Energy BalanceGlobal Energy Balance

Since the earth-atmosphere system is neither wSince the earth-atmosphere system is neither warming up nor cooling down, there must be a balarming up nor cooling down, there must be a balance between the incoming insolation and outgoiance between the incoming insolation and outgoing terrestrial radiation.ng terrestrial radiation.In the entire In the entire Earth-atmosphere systemEarth-atmosphere system, , the amothe amount of radiation entering the system must equal tunt of radiation entering the system must equal to the amount leavingo the amount leaving, or the system would conti, or the system would continually heat or cool. nually heat or cool. However, not all of this energy is radiative energHowever, not all of this energy is radiative energy; some isy; some is sensible and latent heatsensible and latent heat..

Global Energy BalanceGlobal Energy Balance

If we consider If we consider the atmosphere alonethe atmosphere alone, we fi, we find that the atmosphere experiences nd that the atmosphere experiences radiatiradiativeve cooling cooling. The atmosphere is kept from a . The atmosphere is kept from a net cooling by the addition of energy by net cooling by the addition of energy by latentlatent and and sensible heatingsensible heating..The atmosphere has The atmosphere has a warming effecta warming effect on on Earth's Earth's surfacesurface -- the "atmospheric greenh -- the "atmospheric greenhouse effect". ouse effect".

Global Energy BalanceGlobal Energy Balance

If Earth had no atmosphere, the If Earth had no atmosphere, the globally averageglobally averaged surface temperatured surface temperature would be would be

-18 degrees Celsius-18 degrees Celsius. . Because Earth does have an atmosphere, the Because Earth does have an atmosphere, the aaverage surface temperatureverage surface temperature actually is 15 degreactually is 15 degrees Celsiuses Celsius. .

Global Energy BalanceGlobal Energy BalanceThe atmosphere acts as The atmosphere acts as a greenhousea greenhouse because of ga because of gases that ses that selectivelyselectively allow solar radiation to pass throuallow solar radiation to pass through but absorb and then re-emit terrestrial radiationgh but absorb and then re-emit terrestrial radiation. . These gases are collectively called "These gases are collectively called "greenhouse gasgreenhouse gaseses" and " and include water vapor, carbon dioxide, ozone, molecular oxygen, methane and nitrous oxide. These gases are These gases are selectiveselective as to which wavelengths tas to which wavelengths they will absorbhey will absorb. For example, ozone absorbs shortwa. For example, ozone absorbs shortwave ultraviolet radiation whereas water vapor absorbs ive ultraviolet radiation whereas water vapor absorbs infrared radiation more readily.nfrared radiation more readily.

Global Energy BalanceGlobal Energy BalanceMost of the sun's radiation that passes through the Most of the sun's radiation that passes through the atmosphere to hit the earth is in the visible part of tatmosphere to hit the earth is in the visible part of the spectrum. he spectrum.

Most of the earth's radiation that Most of the earth's radiation that escapes the atmoescapes the atmosphere is in sphere is in the infrared bandthe infrared band between between 8 microns 8 microns and 11 micronsand 11 microns. This region of the spectrum is calle. This region of the spectrum is called the "d the "atmospheric windowatmospheric window". ".

Global Energy BudgetGlobal Energy Budget1. Atmospheric Subsystem1. Atmospheric Subsystem- radiation budget: deficit- radiation budget: deficit- heat budget: surplus (from ground)- heat budget: surplus (from ground)

2. The Earth / Ground Subsystem2. The Earth / Ground Subsystem- radiation budget: surplus- radiation budget: surplus- heat budget: deficit (to atmosphere)- heat budget: deficit (to atmosphere)

* * Radiation budget and heat budget result in a balanced Radiation budget and heat budget result in a balanced energy budget in both the ground / earth system.energy budget in both the ground / earth system.

(For details, please refer to the WS p.3-5)(For details, please refer to the WS p.3-5)* Work out also the radiation budget involved.* Work out also the radiation budget involved.

Next: Energy Budget Next: Energy Budget Vs Vs

Heat TransferHeat Transfer