Radiation budget of the earth

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Change in solar exposure over time

The earth's radiation budget is the most important component of the earth's energy budget . Over the sub-area of the radiation balance , the various household sizes are calculated arithmetically in an equation, while the radiation balance also describes them and shows their interrelationships.

Radiation balance

after Kiehl and Trenberth (2009)

The incident solar radiation is (predominantly) short-wave, which is why this formula is also referred to as short-wave radiation balance ( ): ${\ displaystyle Q _ {\ mathrm {k}}}$

{\ displaystyle Q _ {\ mathrm {k}} = GR = D + HR = (1-a) G}

With

${\ displaystyle G}$ = Global radiation
${\ displaystyle D}$ = direct radiation
${\ displaystyle H}$ = diffuse radiation (sky radiation)
${\ displaystyle R}$ = Reflected radiation (influence of the ozone layer etc.)
${\ displaystyle a}$ = Albedo

The earth's surface emits thermal radiation ( infrared ). Since this radiation is long-wave, this formula is also known as the long-wave radiation balance ( ): ${\ displaystyle Q _ {\ mathrm {l}}}$

{\ displaystyle Q _ {\ mathrm {l}} = A _ {\ mathrm {E}} = A _ {\ mathrm {O}} -A _ {\ mathrm {G}}}

With

${\ displaystyle A _ {\ mathrm {E}}}$ = effective charisma
${\ displaystyle A _ {\ mathrm {O}}}$ = Radiation of the earth's surface (terrestrial radiation)
${\ displaystyle A _ {\ mathrm {G}}}$ = Counter radiation (influence of atmospheric gases , aerosols and clouds )

From the two formulas for the radiation absorption and the radiation output, i.e. for profit and loss, it can now be determined how much is available in total ( total radiation balance ( ), net radiation ): ${\ displaystyle Q}$

{\ displaystyle Q = Q _ {\ mathrm {k}} -Q _ {\ mathrm {l}} = GR-A _ {\ mathrm {E}}}

incident short-wave solar radiation	342 watts per m ²
reflected solar radiation	107 watts per m ²
emitted long-wave radiation	235 watts per m ²
Balance (effective energy - "input")	= ± 0 watts per m ²

Global radiation balance value

The effective energy balance is almost zero, because it has to level off at a value in the long term, as long as the astrophysical framework conditions are stable ( first law of thermodynamics over closed systems), and therefore - on a geological scale - results in a largely stable climate (global average temperature ). That it is not exactly zero is essential to climate change in general, and to current global warming specifically .

The energy of the total solar radiation falling on the earth above the atmosphere is approx. 341.3 W / m². This value is calculated from the solar constant , which is approx. 1367 W / m² on average over time, and also takes into account that the surface of the earth is mathematically exposed to solar radiation for only 1/4 of a day due to its spherical shape and rotation.

Radiation budget

30% (101.9 W / m²) of the solar radiation hitting the earth's atmosphere is absorbed by greenhouse gases (here in particular ozone) in the stratosphere, through cloud cover, atmospheric oxygen and the ground (here mainly from snow and water) Space reflects, which corresponds to an albedo of 0.30. The remaining 239.4 W / m² are absorbed in different ways: around 20% from the atmosphere and 50% from the earth's surface, where it is converted into heat. According to the rules, this heat is given back to the air envelope via heat conduction through thermal radiation and convection . If this energy were to be radiated unhindered into space and if no further solar radiation were added at the same time, the calculated mean temperature of the earth's surface would be −18 ° C, while it is estimated to be approx. +14.8 ° C.

The difference of 32.8 ° C mainly explains the greenhouse effect . The so-called greenhouse gases in the atmosphere (especially water vapor and carbon dioxide ) absorb the outgoing long-wave heat radiation from the earth and re-emit it in all directions, including towards the earth's surface. As a result, only part of the radiant energy radiated from the earth's surface is immediately returned to space, so that the reflection from the atmosphere weakens the cooling of the earth's surface.

These numbers only apply to the earth as a whole. Locally and regionally, the conditions depend on numerous factors:

from the albedo of the earth's surface - (e.g. snow 40–90%, desert 20–45%, forest 5–20%)
on the angle of incidence of the sun's rays mentioned above and the duration of their action
of cloud cover and humidity
heat transport through wind , air stratification, ocean currents, etc.
of proximity to the water
of exposure and altitude (negative temperature gradient in the troposphere)

Some of these factors can be modeled, but this does not apply to all factors, such as congestion effects on mountains or irregular movement of low pressure areas . In order to make good forecasts , meteorology not only requires enormous computing power but also a worldwide dense grid of measured values across all layers of air, which in practice has its limits.

Web links

Individual evidence

↑ See graphic in: NATIONAL AERONAUTICS AND SPACE ADMINISTRATION. In: The Earth Observer. November – December 2006. Volume 18, Issue 6. P. 38 (PDF file; 7.6 MB).
↑ Veerabhadran Ramanathan : Cloud-Radiative Forcing and Climate: Results from the Earth Radiation Budget Experiment . In: Science . 243, No. 4887, 1989, pp. 57-63. bibcode : 1989Sci ... 243 ... 57R . doi : 10.1126 / science.243.4887.57 . PMID 17780422 .

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