Earth's atmosphere from space, showing a blue layer at thestratosphere, above the clouds of thetroposphere. TheMoon is visible as a crescent in the background.[1]
Theatmosphere of Earth consists of a layer of mixedgas (commonly referred to asair) that is retained bygravity, surrounding theEarth'ssurface. It contains variable quantities of suspendedaerosols andparticulates that createweather features such asclouds andhazes. The atmosphere serves as a protective buffer between the Earth's surface andouter space. It shields the surface from mostmeteoroids and ultravioletsolar radiation, reducesdiurnal temperature variation – the temperature extremes betweenday andnight, and keeps it warm through heat retention via thegreenhouse effect. The atmosphere redistributes heat and moisture among different regions viaair currents, and provides thechemical andclimate conditions that allowlife to exist andevolve on Earth.
Composition of Earth's atmosphere by molecular count, excluding water vapor. Lower pie represents trace gases that together compose about 0.0434% of the atmosphere.[5][6][7]
The three major constituents of Earth's atmosphere arenitrogen,oxygen, andargon. Water vapor accounts for roughly 0.25% of the atmosphere by mass. In the lower atmosphere, the concentration of water vapor (a greenhouse gas) varies significantly from around 10 ppm by mole fraction in the coldest portions of the atmosphere to as much as 5% by mole fraction in hot, humid air masses, and concentrations of other atmospheric gases are typically quoted in terms of dry air (without water vapor).[8]: 8 The remaining gases are often referred to as trace gases,[9] among which are othergreenhouse gases, principally carbon dioxide, methane, nitrous oxide, and ozone. Besides argon, othernoble gases,neon,helium,krypton, andxenon are also present. Filtered air includes trace amounts of many otherchemical compounds.[10]
Many substances of natural origin may be present in locally and seasonally variable small amounts asaerosols in an unfiltered air sample, includingdust of mineral and organic composition,pollen andspores,sea spray, andvolcanic ash.[11] Various industrialpollutants also may be present as gases or aerosols, such aschlorine (elemental or in compounds),[12]fluorine compounds,[13] and elementalmercury vapor.[14] Sulfur compounds such ashydrogen sulfide andsulfur dioxide (SO2) may be derived from natural sources or from industrial air pollution.[11][15]
The volume fraction of the main constituents of the Earth's atmosphere as a function of height, based on theMSIS-E-90 atmospheric model. The model only works above 85 km.
The total ppm above adds up to more than 1 million (currently 83.43 above it) due toexperimental error. Notes (A) In the atmosphere the pressure is low enough for the ideal gas laws to be correct within 1%. Therefore, the mole fraction is very close to thevolume fraction.[16]: 4 (B) ppm:parts per million by molecular count (C) The concentration of CO2 has beenincreasing in recent decades, as has that ofCH4. (D) Water vapor is about 0.25%by mass over full atmosphere (E) Water vapor varies significantly locally[8]
The averagemolecular weight of dry air, which can be used to calculate densities or to convert between mole fraction andmass fraction, is about 28.946[17] or 28.964[18][5]: 257 g/mol. This is decreased when the air is humid.
Up to an altitude of around 100 km (62 mi), atmosphericturbulence mixes the component gases so that their relative concentrations remain the same. There exists a transition zone from roughly 80 to 120 km (50 to 75 mi) where this turbulent mixing gradually yields tomolecular diffusion. The latter process forms theheterosphere where the relative concentration of lighter gases increase with altitude.[19]
Stratification
A prism cross-section of the Earth's atmosphere. The strata are drawn to scale, but individual features are not.[20]: 322
In general, air pressure and density decrease with altitude in the atmosphere. However, temperature has a more complicated profile with altitude and may remain relatively constant or even increase with altitude in some regions (see thetemperature section).[21] Because the general pattern of the temperature/altitude profile, orlapse rate, is constant and measurable by means of instrumentedballoon soundings, the temperature behavior provides a useful metric to distinguish atmospheric layers. Thisatmospheric stratification divides the Earth's atmosphere into five main layers with these typical altitude ranges:[22][23]
The exosphere is the outermost layer of Earth's atmosphere (though it is so tenuous that some scientists consider it to be part of interplanetary space rather than part of the atmosphere). It extends from thethermopause (also known as the "exobase") at the top of thethermosphere to a poorly defined boundary with thesolar wind andinterplanetary medium. The altitude of the exobase varies from about 500 kilometres (310 mi; 1,600,000 ft) to about 1,000 kilometres (620 mi) in times of higher incoming solar radiation.[27]
The upper limit varies depending on the definition. Various authorities consider it to end at about 10,000 kilometres (6,200 mi)[28] or about 190,000 kilometres (120,000 mi)—about halfway to the moon, where the influence of Earth's gravity is about the same asradiation pressure from sunlight.[27] Thegeocorona visible in the far ultraviolet (caused by neutral hydrogen) extends to at least 100,000 kilometres (62,000 mi).[27]
This layer is mainly composed of extremely low densities of hydrogen, with limited amounts of helium, carbon dioxide, andnascent oxygen closer to the exobase.[29] The atoms and molecules are so far apart that they can travel hundreds of kilometres without colliding with one another.[21]: 14–4 Thus, the exosphere no longer behaves like a gas, and the particles constantlyescape into space. These free-moving particles followballistictrajectories and may migrate in and out of themagnetosphere or the solar wind. Every second, the Earth loses about 3 kg of hydrogen, 50 g of helium, and much smaller amounts of other constituents.[30]
The thermosphere is the second-highest layer of Earth's atmosphere. It extends from the mesopause (which separates it from the mesosphere) at an altitude of about 80 km (50 mi) up to thethermopause at an altitude range of 500–1000 km (310–620 mi). The height of the thermopause varies considerably due to changes in solar activity.[25] The passage of the dusk and dawnsolar terminator creates background density perturbations up to a factor of two through this layer, forming a dominant feature in this region.[32] Because the thermopause lies at the lower boundary of the exosphere, it is also referred to as theexobase. Overlapping the thermosphere, from 50 to 600 kilometres (31 to 373 mi) above Earth's surface, is theionosphere – a region of enhancedplasma density.[33][34]
The temperature of the thermosphere gradually increases with height and can rise as high as 1500 °C (2700 °F), though the gas molecules are so far apart that itstemperature in the usual sense is not very meaningful. This temperature increase is caused by absorption ofionizingUV andX-ray emission from the Sun.[34][35] The air is so rarefied that an individual molecule (ofoxygen, for example) travels an average of 1 kilometre (0.62 mi; 3300 ft) between collisions with other molecules.[36] Although the thermosphere has a high proportion of molecules with high energy, it would not feel hot to a human in direct contact, because its density is too low to conduct a significant amount of energy to or from the skin.[34]
This layer is completely cloudless and free of water vapor. However, non-hydrometeorological phenomena such as theaurora borealis andaurora australis are occasionally seen in the thermosphere at an altitude of around 100 km (62 mi).[37] The colors of the aurora are linked to the properties of the atmosphere at the altitude they occur. The most common is the green aurora, which comes from atomic oxygen in the1S state, and occurs at altitudes from 120 to 400 km (75 to 250 mi).[38] TheInternational Space Station orbits in the thermosphere, between 370 and 460 km (230 and 290 mi).[39] It is this layer where many of the satellites orbiting the Earth are present.[31]
The mesosphere is the third highest layer of Earth's atmosphere, occupying the region above the stratosphere and below the thermosphere. It extends from the stratopause at an altitude of about 50 km (31 mi) to the mesopause at 80–85 km (50–53 mi) above sea level.[34] Temperatures drop with increasing altitude to themesopause that marks the top of this middle layer of the atmosphere. It is the coldest place on Earth and has an average temperature around −85 °C (−120 °F; 190 K).[40][41] Because the atmosphere absorbs sound waves at a rate that is proportional to the square of thefrequency, audible sounds from the ground do not reach the mesosphere. Infrasonic waves can reach this altitude, but they are difficult to emit at a high power level.[42]
Just below the mesopause, the air is so cold that even the very scarce water vapor at this altitude can condense into polar-mesosphericnoctilucent clouds of ice particles. These are the highest clouds in the atmosphere and may be visible to the naked eye if sunlight reflects off them about an hour or two after sunset or similarly before sunrise. They are most readily visible when the Sun is around 4 to 16 degrees below the horizon.[43]
In 1960,Joseph Kittinger set a record by parachuting from a gondola at 31.3 km (19.4 mi)
The stratosphere is the second-lowest layer of Earth's atmosphere. It lies above the troposphere and is separated from it by thetropopause. This layer extends from the top of the troposphere at roughly 12 km (7.5 mi) above Earth's surface to thestratopause at an altitude of about 50 to 55 km (31 to 34 mi).[22] 99% of the total mass of the atmosphere lies below 30 km (19 mi),[47] and the atmospheric pressure at the top of the stratosphere is roughly 1/1000 thepressure at sea level.[48] It contains theozone layer, which is the part of Earth's atmosphere that contains relatively high concentrations of that gas.[49]
The stratosphere defines a layer in which temperatures rise with increasing altitude. This rise in temperature is caused by the absorption ofultraviolet radiation (UV) from the Sun by the ozone layer, which restricts turbulence and mixing. Although the temperature may be −80 °C (−110 °F; 190 K) at the tropopause, the top of the stratosphere is much warmer, and may be just below 0 °C.[50][49] This layer is unique to the Earth; neither Mars nor Venus have a stratosphere because of low abundances of oxygen in their atmospheres.[51]
The stratospheric temperature profile creates very stable atmospheric conditions, so the stratosphere lacks the weather-producing air turbulence that is so prevalent in the troposphere. Consequently, the stratosphere is almost completely free of clouds and other forms of weather.[49] However, polar stratospheric ornacreous clouds are occasionally seen in the lower part of this layer of the atmosphere where the air is coldest.[52] The stratosphere is the highest layer that can be accessed byjet-powered aircraft.[53]
Earth from orbit, showing sunlight filtered into a reddish hue by the troposphere, while casting shadows from differentcloud types. Thestratosphere forms a thin band ofblue scattered sunlight along the horizon.
The troposphere is the lowest layer of Earth's atmosphere. It extends from Earth's surface to an average height of about 12 km (7.5 mi), although thisaltitude varies from about 9 km (5.6 mi) at thegeographic poles to 17 km (11 mi) at theEquator,[26] with some variation due to weather. The troposphere is bounded above by thetropopause, a boundary marked in most places by atemperature inversion (i.e. a layer of relatively warm air above a colder one), and in others by a zone that isisothermal with height.[54][55]
Although variations do occur, the temperature usually declines with increasing altitude in the troposphere because the troposphere is mostly heated through energy transfer from the surface. Thus, the lowest part of the troposphere (i.e. Earth's surface) is typically the warmest section of the troposphere. This promotes vertical mixing (hence, the origin of its name in the Greek word τρόπος,tropos, meaning "turn").[56] The troposphere contains roughly 80% of themass of Earth's atmosphere.[57] The troposphere is denser than all its overlying layers because a larger atmospheric weight sits on top of the troposphere and causes it to be more severely compressed. Fifty percent of the total mass of the atmosphere is located in the lower 5.5 km (3.4 mi) of the troposphere.[47]
Nearly all atmospheric water vapor or moisture is found in the troposphere, so it is the layer where most of Earth's weather takes place. The ability of the atmosphere to retain water decreases as the temperature declines, so 90% of the water vapor is held in the lower part of the troposphere.[58] It has basically all the weather-associated cloud genus types generated by active wind circulation, although very tall cumulonimbus thunder clouds can penetrate the tropopause from below and rise into the lower part of the stratosphere.[59] Most conventionalaviation activity takes place in the troposphere, and it is the only layer accessible bypropeller-driven aircraft.[53]Contrails are formed from jet engine water emission at altitudes where the atmospheric temperature is about −53 °C (−63 °F); typically around 7.7 km (4.8 mi) for modern engines.[60]
Other layers
Within the five principal layers above, which are largely determined by temperature, several secondary layers may be distinguished by other properties:
Theozone layer is contained within the stratosphere. In this layerozone reaches a peak concentration of 15 parts per million at an altitude of 32 km (20 mi), which is much higher than in the lower atmosphere but still very small compared to the main components of the atmosphere.[61] It is mainly located in the lower portion of the stratosphere from about 15–35 km (9.3–21.7 mi),[5]: 260 though the thickness varies seasonally and geographically. About 90% of the ozone in Earth's atmosphere is contained in the stratosphere.[62]
Theionosphere is a region of the atmosphere that is ionized by solar radiation. It plays a significant role inauroras,airglow, andspace weather phenomenon.[63][64] During daytime hours, it stretches from 50 to 1,000 km (31 to 621 mi) and includes the mesosphere, thermosphere, and parts of the exosphere. However, ionization in the mesosphere largely ceases during the night.[65] The ionosphere forms the inner edge of theplasmasphere – the innermagnetosphere.[66] It has practical importance because it influences, for example, radio propagation on Earth.[67]
Thehomosphere andheterosphere are defined by whether the atmospheric gases are well mixed. The surface-based homosphere includes the troposphere, stratosphere, mesosphere, and the lowest part of the thermosphere, where the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence.[68] This relatively homogeneous layer ends at theturbopause found at about 100 km (62 mi; 330,000 ft),[19] the veryedge of space itself as accepted by theFAI, which places it about 20 km (12 mi; 66,000 ft) above the mesopause.
Above this altitude lies the heterosphere, which includes the exosphere and most of the thermosphere. Here, the chemical composition varies with altitude. This is because thedistance that particles can move without colliding with one another is large compared with the size of motions that cause mixing. This allows the gases to stratify by molecular weight,[19] with the heavier ones, such as oxygen and nitrogen, present only near the bottom of the heterosphere. The upper part of the heterosphere is composed almost completely of hydrogen, the lightest element.[69]
Theplanetary boundary layer is the part of the troposphere that is closest to Earth's surface and is directly affected by it, mainly throughturbulent diffusion. During the day the planetary boundary layer usually is well-mixed, whereas at night it becomes stably stratified with weak or intermittent mixing. The depth of the planetary boundary layer ranges from as little as about 100 metres (330 ft) on clear, calm nights to 1,000–1,500 m (3,300–4,900 ft) or more during the afternoon.[70]
Thebarosphere is the region of the atmosphere where thebarometric law applies. It ranges from the ground to the thermopause. Above this altitude, the velocity distribution isnon-Maxwellian due to high velocity atoms and molecules being able to escape the atmosphere.[71]
The average temperature of the atmosphere at Earth's surface is 14 °C (57 °F; 287 K)[72] or 15 °C (59 °F; 288 K),[73] depending on the reference.[74][75][76]
The average atmospheric pressure at sea level is defined by theInternational Standard Atmosphere as 101325pascals (760.00 Torr; 14.6959 psi; 760.00 mmHg).[5]: 257 This is sometimes referred to as a unit ofstandard atmospheres (atm). Total atmospheric mass is 5.1480×1018 kg (1.13494×1019 lb),[78] about 2.5% less than would be inferred from the average sea-level pressure and Earth's area of 51007.2 megahectares,[5]: 240 this portion being displaced by Earth's mountainous terrain. Atmospheric pressure is the total weight of the air above unit area at the point where the pressure is measured. Thus air pressure varies with location andweather.
Air pressuredecreases exponentially with altitude at a rate that depends on the air temperature. The rate of decrease is determined by a temperature-dependent parameter called thescale height: for each increase in altitude by this height, the pressure decreases by a factor ofe (the base ofnatural logarithms, approximately 2.718). For Earth, this value is typically5.5 to 6 km for altitudes up to around 80 km (50 mi).[79] However, the atmosphere is more accurately modeled with a customized equation for each layer that takes gradients of temperature, molecular composition, solar radiation and gravity into account. At heights over 100 km, the atmosphere is not well mixed, so each chemical species has its own scale height. At altitudes of200 to 300 km, the combined scale height is20 to 30 km.[79]
The mass of Earth's atmosphere is distributed approximately as follows:[80]
50% is below 5.6 km (18,000 ft)
90% is below 16 km (52,000 ft)
99.99997% is below 100 km (62 mi; 330,000 ft), theKármán line. By international convention, this marks the beginning of space where human travelers are consideredastronauts.
By comparison, the summit ofMount Everest is at 8,848 m (29,029 ft); commercialairliners typically cruise between 9 and 12 km (30,000 and 38,000 ft),[81] where the lower density and temperature of the air improve fuel economy;weather balloons reach about 35 km (115,000 ft);[82] and the highestX-15 flight in 1963 reached 108.0 km (354,300 ft).
Even above the Kármán line, significant atmospheric effects such asauroras still occur.[37]Meteors begin to glow in this region,[34] though the larger ones may not burn up until they penetrate more deeply. The various layers of Earth'sionosphere, important toHF radio propagation, begin below 100 km and extend beyond 500 km. By comparison, theInternational Space Station typically orbit at 370–460 km,[39] within theF-layer of the ionosphere,[5]: 271 where they encounter enoughatmospheric drag to require reboosts every few months, otherwiseorbital decay will occur, resulting in a return to Earth.[39] Depending on solar activity, satellites can experience noticeable atmospheric drag at altitudes as high as 600–800 km.[83]
Starting at sea level, the temperature decreases with altitude until reaching thestratosphere at around 11 km. Above, the temperature stabilizes over a large vertical distance. Starting above about 20 km, the temperature increases with height, due to heating within the ozone layer caused by the capture of significantultraviolet radiation from theSun by themolecular oxygen and ozone gas in this region. A second region of increasing temperature with altitude occurs at very high altitudes, in the aptly-namedthermosphere above 90 km.[34]
During the night, the ground radiates more energy than it gains from the atmosphere. As energy is conducted from the nearby atmosphere to the cooler ground, it creates atemperature inversion where the local temperature increases with altitude up to around 1,000 m.[84]
Because in anideal gas of constant composition thespeed of sound depends only on temperature and not on pressure or density, the speed of sound in the atmosphere with altitude takes on the form of the complicated temperature profile (see illustration to the right), and does not mirror altitudinal changes in density or pressure.[85] For example, at sea level the speed of sound is 340 m/s. At the average temperature of the stratosphere, −60 °C, the speed of sound decreases to 290 m/s.[86]
Temperature and mass density against altitude from theNRLMSISE-00standard atmosphere model (the eight dotted lines in each "decade" are at the eight cubes 8, 27, 64, ..., 729)
The density of air at sea level is about 1.29 kg/m3 (1.29 g/L, 0.00129 g/cm3).[5]: 257 Density is not measured directly but is calculated from measurements of temperature, pressure and humidity using the equation of state for air (a form of theideal gas law). Atmospheric density decreases as the altitude increases. This variation can be approximately modeled using thebarometric formula.[87] More sophisticated models are used to predict the orbital decay of satellites.[88]
The average mass of the atmosphere is about 5 quadrillion (5×1015)tonnes or 1/1,200,000 the mass of Earth. According to the AmericanNational Center for Atmospheric Research, "The total mean mass of the atmosphere is 5.1480×1018 kg with an annual range due to water vapor of 1.2 or 1.5×1015 kg, depending on whether surface pressure or water vapor data are used; somewhat smaller than the previous estimate. The mean mass of water vapor is estimated as 1.27×1016 kg and the dry air mass as 5.1352 ±0.0003×1018 kg."[89]
The relative absorption, emission and reflection of solar radiation by the atmosphere, clouds and surface
Solarradiation (or sunlight) is the energy Earth receives from theSun. Earth also emits radiation back into space, but at longer wavelengths that humans cannot see. As energy propagates through the atmosphere, it is impacted by the process ofradiative transfer. That is, some of the incoming and emitted radiation is subject toabsorption, emission, andscattering by the atmosphere. Another portion of the incident energy is reflected,[90][91] with the two most important atmospheric reflectors beingdust and clouds. Depending on the properties of theaerosol, clouds can reflect up to 70% of the incident radiation. Globally, clouds reflect 20% of the incoming energy, contributing two thirds of the planet's totalalbedo.[92] In May 2017, glints of light, seen as twinkling from an orbiting satellite a million miles away, were found to bereflected light fromice crystals in the troposphere.[93][94]
When light passes through Earth's atmosphere,photons interact with it throughscattering. If the light does not interact with the atmosphere, it is calleddirect radiation and is what you see if you were to look directly at the Sun.Indirect radiation is light that has been scattered in the atmosphere. For example, on anovercast day when you cannot see your shadow, there is no direct radiation reaching you, it has all been scattered. As another example, due to a phenomenon calledRayleigh scattering, shorter (blue) wavelengths scatter more easily than longer (red) wavelengths. This is why the sky looks blue; you are seeing scattered blue light. This is also why sunsets are red. Because the Sun is close to the horizon, the Sun's rays pass through more atmosphere than normal before reaching your eye. Much of the blue light has been scattered out, leaving the red light in a sunset.[95]
Rough plot of Earth's atmospherictransmittance (or opacity) to various wavelengths of electromagnetic radiation, includingvisible light
Different molecules absorb different wavelengths of radiation. For example, O2 and O3 absorb almost all radiation with wavelengths shorter than 300nanometres.[96] Water (H2O) absorbs at many wavelengths above 700 nm.[97] When a molecule absorbs a photon, it increases the energy of the molecule. This heats the atmosphere, but the atmosphere also cools by emitting radiation, as discussed below. Inastronomical spectroscopy, the absorption of specific frequencies by the atmosphere is referred to astelluric contamination.[98]
The combinedabsorption spectra of the gases in the atmosphere leave "windows" of lowopacity, allowing the transmission of only certain bands of light. Theoptical window runs from around 300 nm (ultraviolet-C) up into the range humans can see, thevisible spectrum (commonly called light), at roughly 400–700 nm and continues to theinfrared to around 1100 nm. There are alsoinfrared andradio windows that transmit some infrared andradio waves at longer wavelengths. For example, the radio window runs from about one centimetre to about eleven-metre waves.[99]
Emission is the opposite of absorption, it is when an object emits radiation. Objects tend to emit amounts and wavelengths of radiation depending on their "black body" emission curves, therefore hotter objects tend to emit more radiation, with shorterwavelengths. Colder objects emit less radiation, with longer wavelengths. For example, the Sun is approximately 6,000 K (5,730 °C; 10,340 °F), its radiation peaks near 500 nm, and is visible to the human eye. Earth is approximately 290 K (17 °C; 62 °F), so its radiation peaks near 10,000 nm, and is much too long to be visible to humans.[100]
Because of its temperature, the atmosphere emits infrared radiation. For example, on clear nights Earth's surface cools down faster than on cloudy nights. This is because clouds (H2O) are strong absorbers and emitters of infrared radiation.[101] This is also why it becomes colder at night at higher elevations.
Thegreenhouse effect is directly related to this absorption and emission effect. Some gases in the atmosphere absorb and emit infrared radiation, but do not interact in this manner with sunlight in the visible spectrum. Common examples of these are CO2 and H2O.[102] Without greenhouse gases in the atmosphere, the average temperature ofEarth's surface would be a frozen −18 °C (0 °F), rather than the present comfortable average of 15 °C (59 °F).[103]
Therefractive index of air is close to, but just greater than, 1.[104] Systematic variations in the refractive index can lead to the bending of light rays over long optical paths. One example is that, under some circumstances, observers on board ships can see other vessels just over thehorizon because light is refracted in the same direction as thecurvature of Earth's surface.[105]
The refractive index of air depends on temperature,[106] giving rise to refraction effects when the temperature gradient is large. An example of such effects is themirage.[107]
An idealised view of three pairs of large circulation cells
Atmospheric circulation is the large-scale movement of air through the troposphere, and the means (withocean circulation) by which heat is distributed around Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant because it is determined by Earth's rotation rate and the difference in solar radiation between the equator and poles. Theaxial tilt of the planet means the location of maximum heat is continually changing, resulting inseasonal variations. The uneven distribution of land and water further breaks up the flow of air.[108]
The flow of air around the planet is divided into three mainconvection cells by latitude. Around the equator, theHadley cell is driven by the rising flow of air along theequator. In the upper atmosphere, this air flows toward the poles. At mid latitudes, this circulation is reversed, with ground air flowing toward the poles with theFerrel cell. Finally, in the high latitudes is thePolar cell, where air again rises and flows toward the poles.[108]
The interface between these cells is responsible forjet streams. These are narrow, fast moving bands that flow from west to east and typically form at an elevation of around 9,100 m (30,000 ft). Jet streams can shift around depending on conditions. They are strongest in winter, when the boundaries between hot and cold air are the most pronounced.[109] In the middle latitudes, it isinstabilities in the jet streams that are responsible for movingweather systems.[110]
As with theoceans, the Earth's atmosphere is subject to waves and tidal forces. These are triggered by non-uniform heating by the Sun, and by the dailysolar cycle, respectively. Wave-like behavior can occur on a variety of scales, from smallergravity waves that transfermomentum into the higher atmospheric layers, to much largerplanetary waves, orRossby waves.Atmospheric tides are periodic oscillations of the troposphere and stratosphere that transport energy to the upper atmosphere.[111]
About 3.4 billion years ago, nitrogen formed the major component of the then-stable "second atmosphere". The influence of theevolution of life has to be taken into account rather soon in the history of the atmosphere because hints ofearliest life forms appeared as early as 3.5 billion years ago.[114] How Earth at that time maintained a climate warm enough for liquid water and life, if the early Sun put out 30% lower solar radiance than today, is a puzzle known as the "faint young Sun paradox".[115]
The geological record however shows a continuous relatively warm surface during the complete earlytemperature record of Earth – with the exception of one cold glacial phase about 2.4 billion years ago. In the lateNeoarchean, an oxygen-containing atmosphere began to develop, apparently due to a billion years ofcyanobacterialphotosynthesis (known as theGreat Oxygenation Event),[116] which have been found asstromatolitefossils from 2.7 billion years ago. The early basic carbon isotopy (isotope ratio proportions) strongly suggests conditions similar to the current, and that the fundamental features of thecarbon cycle became established as early as 4 billion years ago.[117]
The constant re-arrangement ofcontinents byplate tectonics influences the long-term evolution of the atmosphere by transferring carbon dioxide to and from large continentalcarbonate stores. Free oxygen did not exist in the atmosphere until about 2.4 billion years ago during theGreat Oxygenation Event[119] and its appearance is indicated by the end ofbanded iron formations (which signals the depletion ofsubstrates that can react with oxygen to produceferric deposits) during the earlyProterozoic eon.[120]
Before this time, any oxygen produced by cyanobacterial photosynthesis would be readily removed by theoxidation ofreducing substances on the Earth's surface, notablyferrous iron,sulfur andatmospheric methane. Free oxygen molecules did not start to accumulate in the atmosphere until the rate of production of oxygen began to exceed the availability of reductant materials that removed oxygen. This point signifies a shift from areducing atmosphere to anoxidizing atmosphere.[121] O2 showed major variations during the Proterozoic, includinga billion-year period ofeuxinia, until reaching a steady state of more than 15% by the end of thePrecambrian.[122]
The amount of oxygen in the atmosphere has fluctuated over the last 600 million years, reaching a peak of about 35% around 280 million years ago during theCarboniferous period, significantly higher than today's 21%.[126] Two main processes govern changes in the atmosphere: theevolution ofplants and their increasing role incarbon fixation, and the consumption of oxygen by rapidly diversifyinganimalfaunae and also by plants forphotorespiration and their own metabolic needs at night. Breakdown ofpyrite andvolcanic eruptions release sulfur into the atmosphere, which reacts and hence reduces oxygen in the atmosphere.[127] However, volcanic eruptions also release carbon dioxide,[128] which can fuel oxygenic photosynthesis byterrestrial andaquatic plants. The cause of the variation of the amount of oxygen in the atmosphere is not precisely understood. Periods with more oxygen in the atmosphere were often associated with more rapid development of animals.[119]
Animation shows the buildup of tropospheric CO2 in the Northern Hemisphere with a maximum around May. The maximum in the vegetation cycle follows in the late summer. Following the peak in vegetation, the drawdown of atmospheric CO2 due to photosynthesis is apparent, particularly over theboreal forests.
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