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The laws of mass and energy conservation apply to planet Earth as to any physical system. For the Earth, almost all energy exchanges with its cosmic environment are made up of radiative exchanges: radiation received from the Sun (in the visible light, near-infrared and ultraviolet wavelength ranges) and heat radiation emitted from the Earth (in the middle infrared). The radiative balance of the Earth can be understood as a global balance of energy, close to zero since the planet is not very far from equilibrium. The balance of energy gains and losses is written as follows:<\/p>\n\n\n\n
\u201cNet global balance = absorbed solar radiation + internal heat production \u2013 thermal infrared radiation emitted to space\u201d<\/strong><\/p>\n\n\n\n It should also be noted that the heat production from natural radioactivity inside the Earth is very low (about 0.1 watt per square meter) compared to the radiative flux absorbed or emitted by the planet (239 W \u2022 m\u20132).\u00a0The production of non-renewable energy by human activities is negligible, even if it produces heat islands in large cities.<\/p>\n\n\n\n Solar radiation is close to 1,365 W \u2022 m\u20132 on a plane facing the Sun at the average distance between the Sun and the Earth (1 astronomical unit or AU).\u00a0But the earth is round and you have to divide this value by four ), are converted into heat on the planet\u2019s surface and in the atmosphere. Specialists speak of the overall balance of \u201cshort wave\u201d radiation because the main wavelengths of solar radiation range from 0.2 micrometers (\u03bcm), in the ultraviolet, to the range between 0.4 and 0.9 \u03bcm for visible light, and up to 4 \u03bcm in the near infrared. To maintain a rough balance and a global radiative balance close to zero, the planet must \u201cget rid\u201d of this heat from solar origin that is constantly arriving. The only possibility for the Earth is to radiate into space. This medium infrared radiation, also called thermal, is mainly emitted at wavelengths between 3 and 60 \u03bcm. The overall \u201cshort wave\u201d balance of +239 W \u2022 m\u20132 corresponds to a global \u201clong wave\u201d balance close to \u2013239 W \u2022 m\u20132. The arrows represent energy flows in proportion to their intensities.\u00a0In the long-wave field, radiation emitted from the surface (396 W \u2022 m\u20132) is partly transmitted directly to the top of the atmosphere without being absorbed in transit (40 W \u2022 m\u20132).\u00a0The rest is absorbed by the atmosphere and clouds.\u00a0Part of the \u201clong wave\u201d radiation is thus emitted into space by the atmosphere (169 W \u2022 m\u20132) and by clouds (30 W \u2022 m\u20132).<\/p>\n\n\n\n This \u201clong wave\u201d balance practically balances the \u201cshort wave\u201d radiation balance, which results from the difference between what is received from the Sun (341 W \u2022 m\u20132) and what is reflected (102 W \u2022 m\u20132).\u00a0In the end, the balance between these two balances can be written: 40+169+30=341-102=239.<\/p>\n\n\n\n The deviations to the equilibrium of the radiative balance of the Earth <\/strong><\/p>\n\n\n\n At present, the planet is not in equilibrium even if it is averaging over the year and the overall radiative balance is almost certainly not zero.\u00a0The main cause of this imbalance is not the variation of the solar constant.\u00a0This varies by only 0.1% with the 11-year solar activity cycle, a fluctuation of 0.24 W \u2022 m\u20132 when considering the solar radiation converted into heat (0.1% of 239 W \u2022 m\u20132). Only when the planet reaches a new equilibrium will the global radiative balance be close to zero again. This new equilibrium can only be achieved if the composition of the atmosphere stabilizes and, as it contains more CO2, the average temperature on the surface of the Earth will therefore be higher. On the other hand, the \u201clong wave\u201d balance remains negative at night as well as during the day, in winter and summer.\u00a0Temperatures on the Earth\u2019s surface and in the atmosphere never drop to absolute zero (-273\u00b0C) due to the retention of heat below the surface and the transport of heat by movements of the atmosphere and oceans.\u00a0Thermal infrared radiation towards space is particularly strong over deserts in summer. The net radiative balance varies greatly with latitudes and seasons: it is positive in summer and negative in winter. Although it is not very cold in France in winter, when the regional net radiative balance approaches \u2013100 W \u2022 m\u20132, it is because atmospheric and oceanic movements bring heat from more southern regions where the net radiative balance is positive. And in summer, our mid-latitudes export more heat to the Arctic than they do in winter. <\/p>\n\n\n\n The symbols have the following meanings: Source : Kandel, R. et Viollier, M., 2010 : Observation of the Earth\u2019s Radiation Budget from Space. Comptes Rendus G\u00e9osciences, 342, 286-300.<\/p>\n\n\n\n https:\/\/www.sciencedirect.com\/science\/article\/pii\/S1631071310000167<\/a><\/p>\n\n\n\n https:\/\/cleanet.org\/details\/files\/28510.html<\/a><\/p>\n\n\n\n![\"Detailed](\"https:\/\/ci3.googleusercontent.com\/meips\/ADKq_NYzUCoKZfBG2yC2VhAVruIXPIOSQbLyb9lnCy9P1tLzuOk3NV5l45O-Q9nq0qBTN20lIZlKGGhcaGJmHA3KhYkWsXyoRoWjKRxY6hxhwgtvwIzyAmcZWsUDawG-iO_ZmvNck6TrASugVJZGrh8Ebipi5iLYfiGhXTsl=s0-d-e1-ft#https:\/\/img.freepik.com\/free-vector\/detailed-colourful-earth-structure-infographic_23-2148394268.jpg\")
The elements of the global radiative balance of the Earth<\/strong>
It is more informative to examine the elements of this global balance, that is to study in detail the balance of radiation exchanges of different types between the planet, the Sun and space according to place and time.\u00a0After all, you can balance your financial assets year over year to see if you have more or less savings or debt, but it\u2019s still important to know where your money comes from and what expenses you\u2019ve spent it on.
It should be noted that the following explanations relate to the radiative balance at the \u201ctop\u201d of the atmosphere, which can be located, by convention, at 30 km, since very little infrared or solar radiation is emitted or reflected above this altitude.\u00a0The atmosphere extends well over 30 km, with no net limit, but for the lifetime of satellites this limit is approximately between 500 and 800 km.<\/p>\n\n\n\n![\"Earth](\"https:\/\/ci3.googleusercontent.com\/meips\/ADKq_NauLweIgjEXH7eu4utWIdmOeSMFlSqNRBbre8rJRKqz5dZFi6j_BhhjAmBLMbnT_vPe5ZIdAD6fXvR7kELEz497iU6qYG0oidlWa_2CdWMC7vRfaJTx3qhPp7hbhxfXnJOzBcNBPWs=s0-d-e1-ft#https:\/\/img.freepik.com\/free-vector\/earth-structure-infographic_23-2148403889.jpg\")
The balance of solar radiation<\/strong><\/p>\n\n\n\n
This value must be divided by two first time, because only half of the earth is illuminated by the sun, and a second time by two because of the inclination of the sun\u2019s rays with respect to the earth\u2019s surface.<\/p>\n\n\n\n![\"Earth](\"https:\/\/ci3.googleusercontent.com\/meips\/ADKq_NbXFnyctggxTY1YQFbjlztk_06E2HkKDSHH0jOYWDnQzL86ytjF7Xsyh3wwKxiAZagTjNjJTogRX9DDGIkZ4d-s-BGPdsURfrSatlwOLzXLzueBnY_4vywgcPgIMRP-0sdT6xFIvLE=s0-d-e1-ft#https:\/\/img.freepik.com\/free-vector\/earth-structure-infographic_23-2148403629.jpg\")
To obtain the mean incident flux at the top of the atmosphere near 341 W \u2022 m\u20132.<\/strong>
Also, because the Earth\u2019s orbit is slightly elliptical, the solar flux at the top of the atmosphere is greater when our planet is closer to the Sun, which happens in January.\u00a0This flux is weaker in July when the Earth is furthest from the Sun.
Of course, the inclination of the axis of rotation of the Earth means that the solar flux received in the winter hemisphere is less than that received in the summer hemisphere, for each of these two cases.
On average, the planet reflects and emits about 30% of the solar flux incident to space.\u00a0The remaining 70% of solar flux, or 239 W \u2022 m\u20132 absorbed (239.4 W \u2022 m\u20132 according to Trenberth et al., 2009<\/p>\n\n\n\n![\"Detailed](\"https:\/\/ci3.googleusercontent.com\/meips\/ADKq_NYWyPzYMMnxKwdcM-3k04n05v9cZUrMLp3u2T_qyzf5cCyZbNrYeKnr4tNp3fm99mq3lOzqeTnvAHKU_cVGS-73uq94vfr00Arp2_Tsz9bkADDiPOyqIbNXVgYpyXnxap23TFtCGgQp3jCaSNA4naEtlIj_03fBDn0=s0-d-e1-ft#https:\/\/img.freepik.com\/free-vector\/detailed-colorful-earth-structure-infographic_23-2148394269.jpg\")
Trenberth, Kevin E. et al., 2009: Earth\u2019s global energy budget.\u00a0Bulletin of the American Meteorological Society, 90 (3), 311-323.<\/strong><\/p>\n\n\n\n
Short wave balance = incident solar radiation \u2013 reflected solar radiation
The balance of thermal infrared radiation<\/strong><\/p>\n\n\n\n
Long wave (LW) balance = infrared thermal radiation emitted to space.Based on Trenberth et al. (2009).\u00a0Graphic adaptation: Elsa Godet.\u00a0Excerpt from: Jeandel C. and R. Mosseri (ed.), Le Climat \u00e0 d\u00e9couvert, Paris, CNRS \u00c9ditions, 2011, p. 47.<\/p>\n\n\n\n![\"Earth](\"https:\/\/ci3.googleusercontent.com\/meips\/ADKq_NZ56cVuwTwlUgjaD1dQ4uR6S-ALS_uAGmYmXCsw0s4lBcFF3N4OEZ8aEkQBtnkalvvfsI2YML9XnfWSnPYt41xuP1lJSxALYo6gEMzYu07LzwPb9S1pRDbq2zKjWnxgFxQ8XYGUNGc=s0-d-e1-ft#https:\/\/img.freepik.com\/free-vector\/earth-structure-infographic_23-2148405994.jpg\")
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Sometimes, as a result of powerful volcanic eruptions, the planet reflects a little more solar radiation and absorbs a little less.\u00a0But above all, over the past few decades, human activities have changed the composition of the atmosphere, particularly with increasing emissions of carbon dioxide (CO2).\u00a0These emissions cause an increase in the absorption of infrared radiation into the atmosphere, reinforcing the greenhouse effect.
As the continental surfaces and ocean surface layers warm, infrared heat radiation escaping into space is decreasing.\u00a0It is estimated that this decrease should be close to 1 W \u2022 m\u20132 even if it is still difficult to measure with confidence.
The overall \u201clong wave\u201d balance increases from \u2013239.4 W \u2022 m\u20132 to \u2013238.5 W \u2022 m\u20132 and the overall radiative balance increases by about 1 W \u2022 m\u20132, according to Trenberth et al. 2009.<\/p>\n\n\n\n
Structure of the radiative balance of the Earth in space and time<\/strong><\/p>\n\n\n\n
<\/strong>Let\u2019s take a closer look at the elements that make up the overall radiative balance of the Earth, starting with the top of the atmosphere. This is particularly important because measurements \u2013 mainly from space and taken by different artificial satellites \u2013 are carried out at different points in the world and at different times, in the cycle of seasons and the day-night cycle.
The solar balance (or \u00abshort wave\u00bb balance) is necessarily zero at night (which includes the polar night for months): nothing happens and nothing is reflected. During the day, especially in the tropics and in temperate or subpolar areas in summer, this balance is positive and can be strong where there are no clouds or ice to reflect the Sun. Measurements indicate that clouds account for half of the 102 W \u2022 m\u20132 solar reflected into space.<\/p>\n\n\n\n
But clouds \u2013 especially high and cold clouds \u2013 also block infrared radiation to space.\u00a0In tropical regions, thermal infrared radiation is strong where there are no clouds, or only low clouds, but it is very weak above the thick and high clouds of the intertropical front.\u00a0The measurements indicate that, overall, without clouds, the \u201clong wave\u201d balance would be more negative than 30 W \u2022 m\u20132 and, because of the decrease in reflected solar radiation (50 W \u2022 m\u20132), the planetary radiative balance would then be +20 W \u2022 m\u20132 (the difference between the gain in solar radiation of 50 W \u2022 m\u20132 and the loss in infrared of 30 W \u2022 m\u20132).\u00a0Without clouds, the planet would be at a much higher temperature.<\/p>\n\n\n\n
All of the above refers to the elements of the radiative balance at the \u00abtop\u00bb of the atmosphere, therefore to the exchanges between the Sun, the Earth and space. We can and must also study the elements of the radiative balance on the surface of the Earth. But be careful! In addition to radiative exchanges, the surface also exchanges heat with the atmosphere through other mechanisms.
The surface receives descending heat radiation from the atmosphere (an essential aspect of the natural greenhouse effect) while the \u00ablong wave\u00bb radiation coming from cosmic space is practically nil. The net radiative balance at the surface, on average, on the globe and over 24 h, is generally positive and, in the absence of an imbalance in the planet\u2019s energy balance, it is exactly balanced by non-radiative heat losses to the atmosphere (Sensitive heat flux and latent heat flux).
Under current conditions, the energy balance on the Earth\u2019s surface also increases by about 1 W \u2022 m\u20132, according to Trenberth et al., 2009.<\/p>\n\n\n\n![\"\"](\"https:\/\/save-us.life\/wp-content\/uploads\/2024\/08\/image-2024-08-23T102547.942.png\")
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<\/strong>
Top of the atmosphere (TOA):
SW (Short Wave): solar radiation (\u201cshort wave\u201d);
LW (Long Wave): thermal infrared radiation (\u201clong waves\u201d);
LE (Latent Evapotranspiration): latent heat flux from the surface to the atmosphere;
H (Heat): Sensitive heat flux.
The planetary averages of the elements of the radiative balance of the Earth at the top of the atmosphere (TOA) are, from left to right: the incident solar radiant flux, the reflected solar flux and the emitted long wave thermal flux. On the surface of the Earth are represented, from left to right: the solar flux descending, the solar flux ascending (reflected), the non-radiative flux of latent heat (LE) and sensitive heat (H), the long-wave flux descending, and the emission of the long-wave flux ascending. All these flows are in the range of 10 to 350 W \u2022 m-2 (except for the solar flux at night, zero). The mean flux from the interior of the Earth to its surface (about 0.1 W \u2022 m-2) is not shown.<\/p>\n\n\n\n