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• carbon dioxide, 7:9#

• methane, ":7#

• o;one, *:/#

The mechanism that produces this difference between the actual surface temperature and theeffective temperature is due to the atmosphere and is 'nown as the greenhouse effect."*+ 2ithoutthe greenhouse effect, the Earth%s temperature would be about 630 <C (!." <=) .""+">+ The surfacetemperature would be ** <C (>/.9 <=) below Earth%s actual surface temperature of approximately3" <C (>/. <=)."9+

 Atmospheric carbon dioxide and the carbon cycle 

 ?tmospheric carbon dioxide plays an integral role in the Earth%s carbon cycle whereby carbondioxide is removed from the atmosphere by some natural processes and added bac' to theatmosphere by other natural processes. There are two broad carbon cycles on earth8 the fastcarbon cycle and the slow carbon cycle. The fast carbon cycle refers to movements of carbonbetween the environment and living things in the biosphere whereas the slow carbon cycleinvolves the movement of carbon between the atmosphere, oceans, soil, roc's and volcanism.@oth carbon cycles are intrinsically interconnected and atmospheric gaseous carbon dioxidefacilitates the carbon cycle.

Aatural sources of atmospheric carbon dioxide include volcanic outgassing, the combustion of organic matter ,wildfires and the respiration processes of living aerobicorganisms. 5anmade sources of carbon dioxide include the burning of  fossil fuels forheating, power generation and transport, as well as some industrial processes such as cementma'ing. &t is also produced by various microorganisms from fermentation and cellularrespiration.4lants, algae and cyanobacteria convert carbon dioxide to carbohydrates by aprocess called photosynthesis. They gain the energy needed for this reaction from absorption ofsunlight by chlorophyll and other pigments. Oxygen, produced as a byproduct of photosynthesis,is released into the atmosphere and subse1uently used for respirationby heterotrophic organisms and other plants, forming a cycle.

5ost sources of CO emissions are natural, and are balanced to various degrees by naturalCO sin's. =or example, the natural decay of organic material in forests and grasslands and theaction of forest fires results in the release of about "*7 gigatonnes of carbon dioxide every year,while new growth entirely counteracts this effect, absorbing ">! gigatonnes per year."/+ ?lthough

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the initial carbon dioxide in the atmosphere of the young Earth was produced by volcanic activity,modern volcanic activity releases only 3*! to *! megatonnes of carbon dioxide each year."0+ These natural sources are nearly balanced by natural sin's, physical and biological processeswhich remove carbon dioxide from the atmosphere. =or example, some is directly removed fromthe atmosphere by land plants for photosynthesis and it is soluble in water forming carbonic acid.There is a large natural flux of CO into and out of the biosphere and oceans."7+ &n the pre

industrial era these fluxes were largely in balance. Currently about >/# of humanemitted CO isremoved by the biosphere and oceans.>!+>3+ =rom preindustrial era to !3!, the terrestrialbiosphere represented a net source of atmospheric CO prior to 37"!, switching subse1uently toa net sin'.>3+The ratio of the increase in atmospheric CO to emitted CO is 'nown as the airbornefraction (Beeling et al., 377>) this varies for shortterm averages and is typically about "># overlonger (> year) periods.>3+ Estimated carbon in global terrestrial vegetation increased fromapproximately /"! billion tons in 373! to /0! billion tons in 377!. >+

Atmospheric carbon dioxide and photosynthesis8

4hotosynthesis changes sunlight into chemical energy, splits water to liberate O , and fixes CO into sugar.

Carbon dioxide in the Earth%s atmosphere is essential to life and to the present planetarybiosphere. Over the course of Earth%s geologic history CO concentrations have played a role inbiological evolution. The first photosynthetic organisms probably evolved early in the evolutionaryhistory of life and most li'ely used reducing agents such as hydrogen or  hydrogen sulfide assources of electrons, rather than water .>*+ Cyanobacteria appeared later, and the excess oxygenthey produced contributed to the oxygen catastrophe,>"+ which rendered the evolution of complexlife possible. &n recent geologic times, low CO  concentrations below 9!! parts per million mighthave been the stimulus that favored the evolution of  C" plants which increased greatly inabundance between / and > million years ago over plants that use the less efficient C* metabolic

pathway.7+  ?t current atmospheric pressures photosynthesis shuts down when atmosphericCO concentrations fall below 3>! ppm and !! ppm although some microbes can extract carbon

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from the air at much lower concentrations.>>+>9+Today, the average rate of energy capture byphotosynthesis globally is approximately 3*! terawatts,>/+>0+>7+which is about six times larger thanthe current power consumption of human civili;ation.9!+ 4hotosynthetic organisms also convertaround 3!!:33> thousand million metric tonnes of carbon into biomass per year.93+9+

4hotosynthetic organisms are photoautotrophs which means that they are able

to synthesi;e food directly from CO and water using energy from light. Dowever, not allorganisms that use light as a source of energy carry out photosynthesis,since  photoheterotrophs use organic compounds, rather than CO, as a source of carbon.9*+ &nplants, algae and cyanobacteria, photosynthesis releases oxygen. This is called oxygenic photosynthesis. ?lthough there are some differences between oxygenic photosynthesisin plants, algae, and cyanobacteria, the overall process is 1uite similar in these organisms.Dowever, there are some types of bacteria that carry out anoxygenic photosynthesis, whichconsumes CO but does not release oxygen.

Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation isan endothermic redox reaction, so photosynthesis needs to supply both a source of energy todrive this process, and the electrons needed to convert CO into a carbohydrate. This addition ofthe electrons is a reduction reaction. &n general outline and in effect, photosynthesis is the

opposite of cellular respiration, in which glucose and other compounds are oxidi;ed to produceCO and water, and to release exothermic chemical energy to drive the organism%s metabolism.Dowever, the two processes ta'e place through a different se1uence of chemical reactions andin different cellular compartments.

5ost organisms that utili;e photosynthesis to produce oxygen use visible light to do so, althoughat least three use shortwave infrared or, more specifically, farred radiation.9"+

Atmospheric carbon dioxide and the oceanic carbon cycle

 ?irsea exchange of CO

2hen CO dissolves, it reacts with water to form a balance of ionic and nonionic chemicalspecies8 dissolved free carbon dioxide (CO(a1)), carbonic acid (DCO*), bicarbonate (DCO6*)and carbonate (CO6*). The ratio of these species depends on factors suchasseawater  temperature and al'alinity (as shown in a @-errum plot). These different formsof dissolved inorganic carbon are transferred from an ocean%s surface to its interior by theocean%s solubility pump.

The Earth%s oceans contain a large amount of CO in the form of bicarbonate and carbonate ions much more than the amount in the atmosphere. The bicarbonate is produced in reactionsbetween roc', water, and carbon dioxide. One example is the dissolution of calcium carbonate8

CaCO* F CO F DO ⇌ CaFF DCO6*

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Geactions li'e this tend to buffer changes in atmospheric CO . ince the right side of thereaction produces an acidic compound, adding CO on the left side decreases the pD of seawater, a process which has been termed ocean acidification (pD of the ocean becomes moreacidic although the pD value remains in the al'aline range). Geactions between CO andnoncarbonate roc's also add bicarbonate to the seas. This can later undergo the reverse ofthe above reaction to form carbonate roc's, releasing half of the bicarbonate as CO. Over

hundreds of millions of years, this has produced huge 1uantities of carbonate roc's.

Hltimately, most of the CO emitted by human activities will dissolve in the ocean 9>+ however,the rate at which the ocean will ta'e it up in the future is less certain. Even if e1uilibrium isreached, including dissolution of carbonate minerals, the increased concentration ofbicarbonate and decreased or unchanged concentration of carbonate ion will give rise to ahigher concentration of unioni;ed carbonic acid and dissolved CO. This, along with highertemperatures, would mean a higher e1uilibrium concentration of CO in the air.

Weathering 

Carbon dioxide and the other atmospheric gases dissolve in surface waters.

Dissolved gases are in equilibrium with the gas in the atmosphere. Carbon dioxidereacts with water in solution to form the weak acid, carbonic acid. Carbonic acid

disassociates into hydrogen ions and bicarbonate ions. The hydrogen ions andwater react with most common minerals (silicates and carbonates altering the

minerals. The products of weathering are predominantly clays (a group of silicateminerals and soluble ions such as calcium, iron, sodium, and potassium.

!icarbonate ions also remain in solution" a remnant of the carbonic acid that wasused to weather the rocks.

 

Carbonate Rocks

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#. Carbon dioxide is removed from the atmosphere by dissolving in water andforming carbonic acid

C$% & '%$ ) '%C$* (carbonic acid

%. Carbonic acid is used to weather rocks, yielding bicarbonate ions, other ions,

and clays

'%C$* & '%$ & silicate minerals ) 'C$* & cations (Ca&&, +e&&, a&, etc. & clays

*. Calcium carbonate is precipitated from calcium and bicarbonate ions in seawater 

 by marine organisms like coral

Ca&& & %'C$* ) CaC$* & C$% & '%$

the carbon is now stored on the seafloor in layers of limestone 

 Metamorphism of Carbonates

-ome of this carbon is returned to the atmosphere via metamorphism of limestone

at depth in subduction ones or in orogenic belts

CaC$* & -i$% ) C$% & Ca-i$*

followed by outgassing at the volcanic arc.

The Carbon Cycle

The primary source of carbon/C$% is outgassing from the 0arth1s interior at

midocean ridges, hotspot volcanoes, and subductionrelated volcanic arcs. 2uch of the C$% released at subduction ones is derived from the metamorphism of

carbonate rocks subducting with the ocean crust. 2uch of the overall outgassingC$%, expecially as midocean ridges and hotpot volcanoes, was stored in the mantle

when the 0arth formed. -ome of the outgassed carbon remains as C$% in theatmosphere, some is dissolved in the oceans, some carbon is held as biomass in

living or dead and decaying organisms, and some is bound in carbonate rocks.Carbon is removed into long term storage by burial of sedimentary strata,

especially coal and black shales that store organic carbon from undecayed biomassand carbonate rocks like limestone (calcium carbonate.

 

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 Photosynthesis

3lants and photosynthetic algae and bacteria use energy from sunlight to combinecarbon dioxide (C4% from the atmosphere with water ('%$ to form carbohydrates.

These carbohydrates store energy. $xygen ($% is a byproduct that is released intothe atmosphere. This process is known as photosynthesis.

carbon dioxide & water & sunlight ) carbohydrate & oxygen

C$% & '%$ & sunlight ) C'%$ & $%

 

 Respiration

3lants (and photosynthetic algae and bacteria then use some of the stored

carbohydrates as an energy source to carry out their life functions. -ome of thecarbohydrates remain as biomass (the bulk of the plant, etc.. Consumers such as

animals, fungi, and bacteria get their energy from this excess biomass either whileliving or dead and decaying. $xygen from the atmosphere is combined with

carbohydrates to liberate the stored energy. 5ater and carbon dioxide are byproducts.

oxygen & carbohydrate ) energy & water & carbohydrate

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$% & C'%$ ) energy & '%$ & C$%

 otice that photosynthesis and respiration are essentially the opposite of one

another. 3hotosynthesis removes C$% from the atmosphere and replaces it with $%.6espiration takes $% from the atmosphere and replaces it with C$%. 'owever, these

 processes are not in balance. ot all organic matter is oxidied. -ome is buried insedimentary rocks. The result is that over geologic time, there has been more

oxygen put into the atmosphere and carbon dioxide removed by photosynthesisthan the reverse.

 

The Greenhouse Effect

2ost of the sun1s energy that falls on the 0arth1s surface is in the visible light portion of the electromagnetic spectrum. This is in large part because the 0arth1s

atmosphere is transparent to these wavelengths (we all know that with afunctioning oone layer, the higher frequencies like ultraviolet are mostly screened

out. 3art of the sunlight is reflected back into space, depending on the albedo orreflectivity of the surface. 3art of the sunlight is changed into infrared (lower

frequency than visible light. 5hile the dominant gases of the atmosphere(nitrogen and oxygen are transparent to infrared, the socalled greenhouse gasses,

 primarily water vapor ('%$, carbon dioxide, and methane (C'7, absorb theinfrared radiation. They collect this heat energy and hold it in the atmosphere.5hile we worry about possible global warming from the additional C$% we put

into the atmosphere by burning fossil fuels, if there was no C$% in the atmospherethe global climate would be significantly cooler.

 

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The Climate Buffer

!ecause of the role of C$% in climate, feedbacks in the carbon cycle act to maintainglobal temperatures within certain bounds so that the climate never gets too hot or

too cold to support life on 0arth. The process is a largescale example of8eChatelier1s 3rinciple. This chemical principle states that if a reaction at

equilibrium is perturbed by the addition or removal of a product or reactant, thereaction will ad9ust so as to attempt to bring that chemical species back to its

original concentration. +or example, as carbonic acid is removed from solution by

weathering of rocks, the reaction will ad9ust by producing more carbonic acid. :ndsince the dissolved C$% is in equilibrium with atmospheric C$%, more C$% isremoved from the atmosphere to replace that removed from solution by

weathering.

 some examples:

;f C$% concentration increases in the atmosphere because of an increased rate of

outgassing, global temperature will rise. 6ising temperature and more dissolvedC$% will lead to increased weathering of crustal rocks as a result of faster reaction

rates (temperature effect and greater acidity. 0nhanced weathering will use up theexcess C$% thereby cooling the climate.

;f global temperature cools as a result of some astronomical forcing ortectonic/ocean circulation effect, the lower temperatures will result in lower rates

of chemical weathering. Decreased weathering means less C$% being drawn fromthe atmosphere by weathering reactions, leaving more C$% in the atmosphere to

increase temperatures.

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;f more rocks become available for rapid weathering as a result of mountain upliftthe enhanced weathering will draw down atmospheric C$% and decrease global

temperatures. !ut the decreased temperatures will slow reaction rates, therebyusing less C$%, thus allowing temperatures to moderate.

 

Negative role of Atmospheric carbon dioxide and global warming

The recent phenomenon of global warming has been attributed primarily to increasingatmospheric carbon dioxide concentrations in Earth%s atmosphere. 2hile CO  absorption andrelease is always happening as a result of natural processes, the recent rise in CO  levels inthe atmosphere is 'nown to be mainly due to human activity. 90+ Gesearchers 'now this bothby calculating the amount released based on various national statistics, and by examiningthe ratio of various carbon isotopes in the atmosphere,90+ as the burning of longburied fossilfuels releases CO containing carbon of different isotopic ratios to those of living plants,enabling them to distinguish between natural and humancaused contributions toCO concentration.

@urning fossil fuels such as coal and petroleum is the leading cause ofincreased anthropogenic COdeforestation is the second ma-or cause.

This addition, about *# of annual natural emissions, as of 377/, is sufficient to exceed thebalancing effect of sin's./"+ ?s a result, carbon dioxide has gradually accumulated in theatmosphere, and as of !3*, its concentration is almost "*# above preindustriallevels. Iarious techni1ues have been proposed for removing excess carbon dioxide from theatmosphere in carbon dioxide sin's.

Carbon dioxide has uni1ue longterm effects on climate change that are largely JirreversibleJfor one thousand years after emissions stop (;ero further emissions) even though carbondioxide tends toward e1uilibrium with the ocean on a scale of 3!! years. The greenhousegases methane and nitrous oxide do not persist over time in the same way as carbon

dioxide. Even if human carbon dioxide emissions were to completely cease, atmospherictemperatures are not expected to decrease significantly in the short term.

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 Although carbon dioxide is present in very small proportions on the earth, it performs very useful

functions. Some important functions of C2 are described below!

"i# Carbon dioxide is used up by plants to prepare their food through photosynthesis. $hus,

atmospheric carbon dioxide provides us food through plants.

"ii# Carbon dioxide dissolves an ocean water to form sedimentary carbonate roc%s such as

marble.

"iii# Carbon dioxide gas can absorb infrared radiation. $his property of carbon dioxide has helped

in maintaining temperature on the earth atomic a moderate level. Earth&s moderate temperature

is necessary for the existence of life on this planet.

owever, if the concentration of C( in the air increases then the temperature of the earth&satmosphere would also rise due to the greenhouse effect. $his rise in temperature would

adversely affect the plant and the animal life only the earth.