Green House Effect Unit 1

8/3/2019 Green House Effect Unit 1

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The greenhouse effect is a process by which thermal radiation from a planetarysurface is absorbed by atmospheric greenhouse gases, and is re-radiated in alldirections. Since part of this re-radiation is back towards the surface, energy istransferred to the surface and the lower atmosphere, As a result, the temperaturethere is higher than it would be if direct heating by solar radiation were the onlywarming mechanism.Solar radiation at the high frequencies of visible light passes through theatmosphere to warm the planetary surface, which then emits this energy at thelower frequencies of infrared thermal radiation. lnfrared radiation is absorbed bygreenhouse gases, which in turn re-radiate much of the energy to the surfaceand lower atmosphere. The mechanism is named after the effect of solarradiation passing through glass and warming a greenhouse, but the way it retainsheat is fundamentally different as a greenhouse works by reducing airflow,isolating the warm air inside the structure so that heat is not lost by convection.The greenhouse effect was discovered by French mathematician JosephFourier in 1824, first reliably experimented on by British physicist John Tyndall in1858, and first reported quantitatively by Swedish scientis Svante Arrhenius in1 896.lf an ideal thermally conductive blackbody was the same distance from the Sunas the Earth is, it would have a temperature of about 5.3 'C. However, since theEarth reflects about 30%(or 28o/o\ of the incoming sunlight, the planet's effectivetemperature (the temperature of a blackbody that would emit the same amount ofradiation) is about -18 or -19 'c,about 33"c below the actual surfacetemperature of about 14 "C or 15 "C.The mechanism that produces thisdifference between the actual surface temperature and the effective temperatureis due to the atmosphere and is known as the greenhouse effect.Earth's natural greenhouse effect makes life as we know it possible. However,human activities, primarily the burning of fossil fuels and clearing of forests, havegreatly intensified the natural greenhouse effect, causing global warming.


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The Earth r"""iu", energy from the sun in the form UV, visible, and nearlR radiation, most of which passes through the atmosphere without beingabsorbed. Of the total amount of energy available at the top of the atmosphere(TOA), about 50% is absorbed at the Earth's surface. Because it is warm, thesurface radiates far lR thermal radiation that consists of wavelengths that arepredominantly much longer than the wavelengths that were absorbed. Most ofthis thermal radiation is absorbed by the atmosphere and re-radiated bothupwards and downwards; that radiated downwards is absorbed by the Earth'ssurface. This trapping of long-wavelength thermal radiation leads to a higherequilibrium temperature than if the atmosphere were absent.This highly simplified picture of the basic mechanism needs to be qualified in anumber of ways, none of which affect the fundamental process.

The incoming radiation from the Sun is mostly in the form of visible lightand nearby wavelengths, largely in the range 024 um, corresponding to theSun's radiative temperature of 6,000 K. Almost half the radiation is in the formof "visible" light, which our eyes are adapted to use.About 50% of the Sun's energy is absorbed at the Earth's surface and therest is reflected or absorbed by the atmosphere. The reflection of light backinto space-largely by clouds-does not much affect the basic mechanism;this light, effectively, is lost to the system.The absorbed energy warms the surface. Simple presentations of thegreenhouse effect, such as the idealized greenhouse model, show this heatbeing lost as thermal radiation. The reality is more complex: the atmosphere


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near the surface is largely opaque to thermal radiation (with importantexceptions for "window" bands), and most heat loss from the surface isby sensible heat and latent heat transport. Radiative energy losses becomeincreasingly important higher in the atmosphere largely because of thedecreasing concentration of water vapor, an important greenhouse gas. lt ismore realistic to think of the greenhouse effect as applying to a "surface" inthe midtroposphere, which is effectively coupled to the surface by a lapserate.

Within the region where radiative effects are important the descriptiongiven by the idealized greenhouse model becomes realistic: The surface ofthe Earth, warmed to a temperature around 255 K, radiates long-wavelength, infrared heat in the range 4-100 pm. At these wavelengths,greenhouse gases that were largely transparent to incoming solar radiationare more absorbent.Each layer of atmosphere with greenhouses gasesabsorbs some of the heat being radiated upwards from lower layers. Tomaintain its own equilibrium, it re-radiates the absorbed heat in all directions,both upwards and downwards. This results in more warmth below, while stillradiating enough heat back out into deep space from the upper layers tomaintain overall thermal equilibrium. lncreasing the concentration of thegases increases the amount of absorption and re-radiation, and therebyfurther warms the layers and ultimately the surface below.

Greenhouse gases-including most diatomic gases with two differentatoms (such as carbon monoxide, CO) and all gases with three or moreatoms-are able to absorb and emit infrared radiation. Though more than99% of the dry atmosphere is lR transparent (because the mainconstituents-Nz, 02, ?od Ar-are not able to directly absorb or emit infraredradiation), intermolecular collisions cause the energy absorbed and emittedby the greenhouse gases to be shared with the other, non-lR-active, gases.The simple picture assumes equilibrium. ln the real world there isthe diurnal cycle as well as seasonal cycles and weather. Solar heating onlyapplies during daytime. During the night, the atmosphere cools somewhat,but not greatly, because its emissivity is low, and during the day the


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atmosphere warms. Diurrnal temperature changes decrease with height in tre50ler Eadi*ri*n $P'eatrum

atmosphere.

By their percentage contribution to the greenhouse effect on Earth the four majorgases are:

water vapor, 36-700/ocarbon dioxide, 9-26%methane, 4-9o/oozone, 3-7%

COz is produced by fossil fuel burning and other activities such as cementproduction and tropical deforestation. Measurements of COz from the Mauna Loaobservatory show that concentrations have increased from about 313 ppm in1g60 to about 389 ppm in 2010. The current observed amount of COzexceedsthe geological record maxima (-300 ppm) from ice core data. The effect ofcombustion-produced carbon dioxide on the global climate, a special case of thegreenhouse effect first described in 1896 by Svante Arrhenius, has also beencalled the Callendar effect.Because it is a greenhouse gas, elevated coz levels contribute toadditional absorption and emission of thermal infrared in the atmosphere, whichproduce net warming. According to the latest Assessment Report from

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the lntergovernmental Panel on Climate Change, "mosf of the observed increasein globally averaged temperatures since the mid-2lth century is very tikely due tothe observed increase in anthropogenic greenhouse gas concentration.,,

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1. "NASA Earth Fact Sheet". Nssdc,gsfc.nasa.gov. Retriev ed 2o1o-10_15.2. "lntroduction to Atmospheric chemistry, by Daniel J. Jacob, princetonUniversity press, 1ggg. chapter 7, ,'The Greenhouse Effect,,,,.Acmg.seas. harvard.edu. Retrieved 201 0-1 O_1 5.3' "Solar Radiation and the Earth's Energy Balance". Eesc.columbia.edu.Retrieved 2010-10-1S.lntergovernmental panel on climate change Fourth Assessment Report.chapter 1: Historical overview of climate change science page 97The elusive "absolute surface air temperature,', see Glss discussion

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Green House Effect Unit 1