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The Sun, like any hot body, emits electromagnetic radiation towards the exterior. Given its surface temperature of 5 800 K (approx. 5 500 °C), this radiation is made up in the following manner :
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The ultraviolet range (wavelengths below 300 nm) : 10 %
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The visible range (wavelengths from 300 nm to 700 nm) : 40 %
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The infrared (wavelengths over 700 nm) : 50 %
This radiation mix reaches the outer layers of the Earth’s atmosphere. A third of the total is reflected back into space by the Earth-Atmosphere unit as a whole (the Albedo phenomenon). Part of the ultraviolet radiation is absorbed by stratospheric ozone, whilst the infrared radiation is captured by the greenhouse gases (water vapour, carbon dioxide…) at the edge of the troposphere.
Therefore, only half of the initial solar radiation arrives on the surface of the Earth. 25 % of this radiation is absorbed by the oceans, 21 % is absorbed by the land and only 0,2 % by plants.
The land surface heated by the solar radiation re-emits infrared radiation, known as telluric infrared radiation, towards the atmosphere. It is this infrared radiation which is trapped by the greenhouse gases in the upper troposphere : the main gases involved are water vapour (H2O), carbon dioxide (CO2) and methane (CH4).
Indeed, the symmetric two-atom molecules like H2, N2, O2… are poor infrared absorbers, whereas the asymmetric molecules, such as H20, CO2 and CH4 are a lot more effective as absorbers of infrared radiation. These molecules then re-emit infrared radiation, contributing to the heating of the surface and the atmosphere.
The mechanics of the greenhouse effect (see picture 1)
© Rémi Picard / rue des écoles
The greenhouse effect exists in nature. Without it, the mean temperature at the Earth’s surface would be -18°C. In reality it is 15 °C.
The greenhouse gases are in the minority in the atmosphere. They are gases which are not very toxic and inert or slightly acid. But, it is thanks to them that the mean temperature at the Earth’s surface is +15 °C, and not -18 °C. The water vapour is at 20°C and the CO2 at 10 °C within this overall difference.
The Earth’s natural radiation balance (see picture 2)
© Total / Véronique BENE /
Viva Design
The Earth’s radiation balance represents the difference between the solar radiation arriving on the planet and that which is radiated outwards. This energy can be radiated directly back into space, essentially by the clouds and the land surface, or be absorbed and then re-emitted as heat. When the radiation balance is in equilibrium, the mean temperature of the planet is stable.
Units : Watts per sq m
Taking into account the emission of infrared radiation by the land, it is also clear that the temperature on the surface is higher than at altitude and that the temperature of the troposphere continues to decrease as one climbs towards the tropopause – the boundary region between the troposphere and the stratosphere - since this layer contains the majority of the mass of the atmosphere and, hence, of the greenhouse gases.
The fact that the mean temperature of the Earth has remained stable over a period of millions of years, indicates that part of the infrared radiation is emitted back into space and hence that the Earth-atmosphere unit, viewed as a whole, absorbs as much solar energy as it radiates back.
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