Radiation balance on Earth

Apart from a small amount of energy that comes from inside the Earth, the energy that feeds the climate system of our planet comes mainly from the Sun. In fact the Earth receives the radiant energy of the Sun (i.e. transported by electromagnetic radiation),  about half of which is visible light, a small part is ultraviolet light, and the remaining part is infrared light.
The solar radiation that hits the Earth’s surface in one hour is equal to approximately 342 w/m2; out of which  only  235 w/m2 are actually absorbed by the Earth’s surface, while the remaining 107 are immediately reflected into space. The percentage of total incident radiation  reflected from the Earth’s surface is known as albedo. The Earth’s albedo, therefore, is equal to 30% (342/107=30%). Out of these 107 w/m2, 77 are reflected by the clouds, by gases and by the micro-particles that are present in the atmosphere (aerosol), while the remaining 30 w/m2 return to the atmosphere as they are reflected by light-coloured surfaces present on the Earth, consisting prevalently of glaciers, snow and deserts. Snow has a very high albedo, equal to 0.9, which means that 90% of the radiation that hits the snow is reflected.
The energy that is not reflected into space, equal to 235 w/m2, is absorbed by the Earth’s surface and by the atmosphere, and is re-emitted in the form of infrared radiations (heat). The atmosphere, which consists prevalently of nitrogen and oxygen, that are transparent to infrared thermal radiation, let these radiations escape into space. However there are some gases (known as greenhouse gases) that absorb thermal radiation and prevent their dispersion,  and this causes a warming of the atmosphere. This natural physical phenomenon, called greenhouse effect is very important for life on Earth because it allows the Earth’s surface to have an average temperature of 14°C instead of -18°C which would be the case without greenhouse gases in the atmosphere.

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From the Multimedia section

  • air

    The greenhouse effect

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    ecosystems

    Deforestation

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    air

    Albedo

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    Future scenarios of an increase in the temperature

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    Icebergs, ice packs and glaciers

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    Mean sea level rise foreseen by IPCC

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    Desertification

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    Life-cycle of different gases in the atmosphere

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    air

    Save the air

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  • air

    The greenhouse effect

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    ecosystems

    Deforestation

    Look
  • air

    Albedo

    Look

    air

    Future scenarios of an increase in the temperature

    Look
  • air

    Icebergs, ice packs and glaciers

    Look

    air

    Mean sea level rise foreseen by IPCC

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