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1Greenhouse Gases and Aerosols

R.T. W ATSO N, H. RO DH E, H. OESCH GER, U. SIEGENTHA LERContributors:M. Andreae; R. C harlson; R. Cicerone; J. Coakley; R. Derwent; J. Elkins;F. Fehsenfeld;P. Fraser; R. Gammon; H. Grassl; R. Harriss; M. Heimann;R. Houghton; V. Kirchhoff; G. Kohlm aier; S. Lai; P. Liss; J. Logan; R. Luxm oore;L. Merlivat; K. Minami; G. Pearman; S. Penkett; D. Raynaud; E. Sanhueza; P. Simon;W. Su; B. Svensson; A. Thompson; P. Vitousek; A. Watson; M. Whitfield; P. Winkler;S. Wofsy.


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CONTENTSExecutive Summary

1.1 Introduction1.2 Carbon Dioxide

1 2 1 The Cycle of Carbon in Nature12 11 The role of the atmosphere12 12 The role of the ocean12 13 The role of terrestrial vegetation and soils1 2 2 Anthropogenic Perturbations1 2 2 1 Historical fossil fuel input12 2 2 Historical land use changes

1 2 3 Long-Term A tmosphenc Carbon Dioxide Van ations1 2 4 The Contemporary Record of Carbon Dioxide -

Observations and Interpretation12 4 1 The carbon dioxide increase from pre industnal

period12 4 2 Uptake by the ocean12 4 3 Redistribution of anthropogenic carbon dioxide12 4 4 Seasonal variations12 4 5 Interannual v ariations12 4 6 Temporal vanations of carbon isotopes

1 2 5 Evidence that the Contemporary Carbon DioxideIncrease is Anthropogenic

1 2 6 Sensitivity Analyses for Future Carbon DioxideConcentrations

1 2 7 Feedbacks from Climate C hange into the C arbonDioxide Cycle

1 2 7 1 Oceanic feedback effects12 7 11 Ocean temperature12 7 12 Ocean circulation12 7 13 Gas exchange rates12 7 14 Modification of oceanic biogeochemical

cycling12 7 15 UV-B radiation

12 7 2 Terrestnal biosphenc feedbacks12 7 2 1 Carbon dioxide fertilization12 7 2 2 Eutrophication and toxification12 7 2 3 Temperature12 7 2 4 Water12 7 2 5 Change in geographical distribution of

vegetation types12 7 2 6 UV-B radiation

2 8 Conclusions1.3 Methane

1 3 1 Atmosphenc Distribution of Methane13 11 Palaeo atmospheric record of methane13 12 Contemporary record of methane

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1010101011111112131414141414151616161616161616161717171717

1 3.1 3 Isotopic composition of methane1 3 2 Sinks of Methane1 3 3 Sources of Methane

1 3 3 1 Natural wetlands1 3 3 2 Rice paddies1 3 3 3 Biomass burning13 3 4 Entenc fermentation (animals)1 3 3 5 Termites13 3 6 Landfills13 3 7 Oceans and freshwaters1 3 3 8 Coal mining1 3 3 9 Gas dnlh ng, venting and transmission

1 3 4 Feedbacks from Climate Change into theMethane Cycle

1 3 4 1 Tropical methane sources13 4 2 High latitude methane sources

1 3 5 Conclusions1.4 Halocarbons

1 4 1 Atmospheric Distribution of Halocarbons1 4 2 Sinks for H alocarbons1 4 3 Sources of Halocarbons1 4 4 Future Atmosp henc Concentration of H alocarbons1 4 5 Conclusions

1.5 Nitrous Oxide1 5 1 Atmospheric Distnbution of Nitrous Oxide1 5 2 Sinks for Nitrous Oxide1 5 3 Sources of Nitrous Oxide

1 5 3 1 Oceans15 3 2 Soils15 3 3 Combustion15 3 4 Biomass burning15 3 5 Fertilizer / ground water

1 5 4 Conclusions1.6 Stratospheric Ozone

1 6 1 Straosphenc Ozone T rends16 11 Total column ozone trends16 12 Changes in the vertical distribution of ozone

1 6 2 Future Changes1.7 Tropospher ic Ozone and Rela ted Trace Gases(Carbon Monoxide , Non-Methane Hydroca rbons , and

19192020202121212121212121222222232324242424252525252526262627272727272828

Reactive Nitrogen Oxides) 2818 17 1 Troposph eric Ozone 2818 17 11 Atmosph eric distribution 2818 17 12T rends 2919 17 13 Relationships between ozone and its precursors 29


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4 Greenhouse Gases and Aerosols 1

1.7.2 Carbon Mo noxide 301.7.2.1 Atmospheric distribution of carbon monox ide 301.7.2.2 Sources and sinks for carbon monoxide 30

1.7.3 Reactive Nitrogen Oxide s 301.7.3.1 Atmosph eric distribution of nitrogen oxides 301.7.3.2 Sources and sinks of nitrogen oxides 30

1.7.4 Non-M ethane Hydrocarbons 311.7.4.1 Atmospheric distribution of non-methane

hydrocarbons 311.7.4.2 Sources and sinks for non-methane

hydrocarbons 311.7.5 Feedbacks Between C limate and the Methane /

Non-Methane Hydrocarbon / Carbon Monoxide /Oxides of Nitrogen / Tropospheric Ozone System 31

1.7.6 Con clusion s 311.8 Aerosol Particles 31

1.8.1 Concentrations and Trends of Aerosol P articlesin the Troposphere 31

1.8.2 The Atmosph eric Sulphur Budget 321.8.3 Aeroso l Particles in the Stratos phere 331.8.4 Con clusio ns 33References 34


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EXECUTIVE SUMMARYThe Earth's climate is dependent upon the radiative balance of theatmosphere, which in turn depends upon the input of solarradiation and the atmospheric abundances ot ladiatively activetrace gases (1 e , greenhouse gases), clouds and aerosols

Since the industrial revolution the atmospheric concentrationsof several greenhouse gases, i e , carbon dioxide (CO 2) methane(CH4), chlorofluorocarbons (CFCs), nitrous oxide (N2O), andtropospheric ozone (O3), have been increasing primarily due tohuman activities Several ol these gieenh ouse gases have longatmospheric lifetimes, decades to centuries, which means thattheir atmospheric concentrations respond slowly to changes inemission rates In additio n theie is evid ence that theconcentrations of troposphenc aerosols have increased at leastregionally

Carbon DioxideThe atmospheric C 0 2 concentration a t ' W ppm \ in 1990 is nowabout 25% greater than the pre industiial (1750 1800) value ofabout 280 ppmv, and higher than at any tune in at least the last160,000 years Carbon dio xide is currently rising at about 1 8ppmv (0 5%) per year due to anthro pog enic em ission sAnthropogenic emissions ol CC n are estimated to be 5 70 5 GtC (in 1987) due to fossil fuel burn ing , plus 0 6 2 5 Gt C (in1980) due to deforestation The atmospheric in uea se during thepast decade corresponds to (488)% ol the total emissions duringthe same period with the remamdei being taken up by the oceansand land Indirect evidence suggests that the land and oceanssequester C O2 in roughly equal piop ortion s though themechanisms are not all well undeisto od The time taken loratmospheric CO2 to adjust to changes in sources 01 sinks is otorder 50 200 years, determined mainly by the slow exchange ofcarbon between surface watcis and deepei layeis ot the oceanConsequently CO 2 emitted into the atmosph ere today willinfluence the atmospheric concentiation ot CO2 tor centuries intothe future Three models have been used to estim ate that even ifanthropogenic emissions of CO2 could be kept constant at presentday rates, atmospheric CO2 would increase to 415 - 480 ppmv bythe year 2050, and to 460 - 560 ppmv by the year 2100 In orderto stabilize concentration s at present day levels an imm ediatereduction in global anthropogenic emissions by 60 80 perceniwould be necessary

MethaneCurrent atmospheric C H4 con centration , at 1 72 ppim is nowmore than double the pre-industnal (1750 1800) value ot about

0 8 ppmv, and is increasing at a rate of about 0 015 ppmv (0 9%)per year The major sink for CH 4, reaction with hydroxyl (OH)radicals in the troposphere, results in a relatively shortatmo spheric lifetime of about 10 years Hum an activities such asrice cultivation, domestic ruminant rearing, biomass burning, coalmining, and natural gas venting have increased the input of CH4into the atmosphere, which combined with a possible decrease inthe concentration ot troposph eric OH yields the observed rise inglobal CH 4 However the quantitative importance of each ot thefactors contributing to the observed increase is not well known atpresent In order to stabilize conc entratio ns at present day levelsan immediate reduction in global anthropogenic emissions by 1520 percent would be necessary

Chlorof luorocarbonsThe current atmospheric concentrations of the anthropogenicallyproduced halocarbons CC I3F (CFC 11) CC I2F2 (CFC 12)C2CI1F3 (CFC 1H) and CCI4 (carbon tetrachloride) are about280 pptv 484 pptv 60 pptv and 146 pptv respectively Over thepast few decades their conc entrat ions, except tor CC I4 haveincreased more rapidly (on a percentage basis) than the othergieenhouse g ases, currently at rates ot at least 4% per year Thefully halogenated CFCs and CCI4 are primarily removed byphotolysis in the stratosphere, and have atmospheric lifetimes inexcess of 50 years Future emissions w ill, most likely, beeliminated or significantly lower than todays because of currentinternational negotia t ions to strengthen regula tions onchlorofluorocarbons Howev er, the atmospheric concentrations otCFC s 11 12 and 1 H will still be significant O 0 - 40% ot current)lor at least the next centuiy because ol their long atmosphenclifetimes

Nitrous OxideThe current atmospheric N2O concentration, at ^ 10 ppbv, is nowabout 8% greater than in the pre-industnal era, and is increasing ala rate of about 0 8 ppbv (0 25%) per year The m ajor sink foiN2O, photolysis in the stratosphere, results in a relatively longatm osp heric lifetime ot about 150 years It is difficult toquantitatively account tor the source of the current increase in theatmospheric concentiation of N2O but it is thought to be due tohuman activities Recent data suggest thai the total annual flu\ otN2O from combustion and biomass burning is much less thanpieviously believed Agricu ltural practices may stimu lateemissions ot N 2O from soils and play a major role In order tostabi'ize concentrations at present day levels an imm ediate


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6 Gi eenhouse Gases and Aei osols 1reduction of 70 80% of the additional flux of N 2 0 that hasoccurred since the pre industrial era would be necessary0 ) n eO/one is an effective greenhouse gas especially in the middle andupper troposphere and lower stratosphere Its concen tration in thetropo sphe re is highly variable because of its short lifetime It isphotochemically produced in-situ through a series of complexreactions involving carbon monoxide (CO), CH4, non-methanehydrocarbons (NMHC), and nitrogen oxide radicals (NO x ), andalso transported down ward from the stratosphere The limitedobservational data support positive trends of about 1% per yearfor O3 below 8 km in the northern hemisphere (consistent withpositive trends in several of the precursor gases, especially N O x ,CH 4 and CO) but probably close to zero trend in the southernhemisph ere There is also evidence that O3 has decreased by afew percent globally in the lower stratosphere (below 25 km)within the last decade Unfortunately, there are no reliable long-term data near the tropopause

Aerosol particlesAerosol particles have a lifetime of at most a few weeks in thetroposphere and occur in highly variable concentrations A largeproportion of the particles that influence cloud processes and theradiative balance is derived from gaseou s sulphur emissions Dueto fossil fuel combustion, these emissions have more than doubledglobally, causing a large increase in the concentration of aerosolsulphate especially over and around the industrialized regions ofEurope and North America Future concentrations of aerosolsulphate will vary in proportion to changes in anthropogenicemis sions Aerosol particles derived from natural (biologic al)emissions may contribute to climate feedback processes During afew years following major volcanic eruptions the concentrationsof natural aerosol particles in the stratosphere can be greatlyenhanced


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1 Gi eenhouse Gases and Aeiosols 7

1.1 IntroductionThe Earth s climate is dependent upon the radiative balanceot the atmosphere, which in turn depends upon the input ofsolar radiation and the atmospheric abundances ofradiatively active trace gases (l e , green hou se gase s),clouds and aerosols Con sequen tly, it is essential to gain anunderstanding of how each ol these climate forcingagents varies natura lly, and how som e of them might beinfluenced by human activities

The chemical composition of the Earth s atmosphere ischanging, largely due to human activ ities (Table 1 1) A irtrapped in Antarctic and Greenland ice shows that therehave been major increases in the concentrations ofradiatively active gases such as carbon dioxide (CO2),methane (CH 4), and nitrous oxide (N2O ) since thebeginning of the industrial revolution In additionindustrially-produced chlorofluorocarbons (CFCs) are nowpresent in the atmosphere in significant concentrations, andthere is evidence that the concentrations of troposphenc O3and aerosols have increased at least regionally

Atmospheric measurements indicate that in many cases therates of chang e have increased in recent decades Many olthe greenhouse gases have long atmospheric life-times,decades to centuries, which implies that their atmosphericconcentrations respond slowly to changes in emission rates

The effectiveness of a greenhouse gas in influencing theEarth s radiative budget is dependent upon its atmosphericconcentration and its ability to absorb outgoing long-waveterrestrial radiation Tro po sph enc water vapour is thesingle most important greenhouse gas, but its atmosphericconcentration is not significantly influenced by directanthrop ogenic emissions Of the greenho use gases that aredirectly alfected by human activities, CO2 has the largestradiative effect, followed by the CFCs, CH4, troposphencO 3, and N2O Although the present rate of increase in theatmo sphe ric conc entration of C O2 is about a factor of70,000 times greater than that of CCI3F (CFC-11) andCCI2F2 (CFC-12) combined, and a factor of about 120times greater than that of CH4, its contribution to changesin the radiative forcing during the decade of the 1980s was

Table 1.1 Summaiy of Key Gi eenhouse Gases Influenced by Human Activities 1

Parameter C 0 2 CH 4 CFC-11 CFC-12 N2O

Pre-industnal atmospheric 280 ppmv^ 0 8 ppmvconcentration (1750-1800) 288 ppbvJ

Current atmospheric concentration 353 ppmv 1 72 ppmv 280 pptv '(1990)3 484pptv 310ppbv

Current rate of annual atmospheric 1 8 ppmv 0 015 ppmv 9 5 pptv 17 pptv 0 8 ppbvaccumulation (0 5%) (0 9%) (4%) (4%) (0 25%)Atmospheric lifetime'* (years) (50 200) 10 65 130 150

1 Ozone has not been included in the table because of lack of precise data2 ppmv = parts per million by volume, ppbv = parts per billion by volume,

pptv = parts per trillion by volume3 The current (1990) concentrations have been estimated based upon an extrapolation of measurements reported for

earlier years, assuming that the recent trends remained approximately constant4 For each gas in the table, except CO2 the lifetime is defined here as the ratio of the atmospheric content to the total

rale ol removal This lime scale also chaiactcn/es the rate ot adjustment of the atmospheric concentrations if theemission rates are changed abruptly COT IS a special case since it has no real sinks but is merely circulated betweenvarious reservoirs (atmosphere ocean biota) The lifetime of CCb given in the table is a rough indication of thetime it would take for the CCb concentiation to adjust to changes in the emissions (see section I 2 1 for furtherdetails)


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8 Greenhouse Gases and Aei osols 1about 55%, compared to 17% for CFCs (11 and 12), and15 % for CH4 (see Section 2) Other CFCs and N2Oaccounted for about 8%, and 5%, respectively, of thechanges in the radiative forcing Wh ile the contributionfrom troposphenc O3 may be important, it has not beenquantified because the observational data is inadequate todeterm ine its trend This pattern arises becau se ofdifferences in the efficiencies of the gases to absorbterrestrial radiation

Aerosol particles play an important role in the climatesystem because of their direct interaction (absorption andscattering) with solar and terrestrial radiation, as well asthrough their influence on cloud processes and thereby,indirectly, on radiative fluxes

There is a clear need to document the historical record ofthe atmospheric concentrations of greenhouse gases andaerosols, as well as to understand the physical, chemical,geological, biological and social processes responsible forthe observed changes A quantitative understanding of theatmospheric concentrations of these gases requiresknow ledge of the cycling and distribu tion of carbo n,nitrogen and other key nutrients within and between theatmosphere, terrestrial ecosystems, oceans and sediments,and the influence of human actions on these cyclesWithout knowledge of the processes responsible for theobserved past and present changes in the atmosphericconcentrations of greenhouse gases and aerosols it will notbe possible to predict with confidence future changes inatmospheric composition, nor therefore the resultingchanges in the radiative forcing of the atmosphere

1.2 Carbon Dioxide1.2.1 The Cycle of Carbon in NatureCarbon in the form of CO 2, carbo nates , organ iccom pou nds, etc is cycled between various reservo irs,atmosphere, oceans, land biota and marine biota, and, ongeological time scales, also sediments and rocks (Figure1 1, for more detailed reviews see Sundqu ist, 1985 Bolin,1981. 1986, Trabalk a, 1985, Siege nthaler, 1986) Thelargest natural exchange fluxes occur between theatmosphere and the terrestrial biota and between theatmosp here and the surface water of the oceans Bycomparison, the net inputs into the atmosphere from fossilfuel combustion and deforestation are much smaller, butare large enough to modify the natural balance

The turnover time of CO2 in the atmosphere, measuredas the ratio of the content to the fluxes through it, is about 4years This m eans that on average it takes only a few yearsbetorc a CO2 molecule in the atmosphere is taken up byplants or dissolved in the ocean This short time scale mustnot be confused with the time it takes tor the atmosphericCO2 level to adjust to a new equilibrium if sources or sinkschange This adjustment time, corresponding to the lifetime

Delorestat onAtm ospher e 750 + 3/year

Land Biota550

Soil and Detr i tus Yi';'*, 1500

Fig ure 1.1: Global carbon reservoirs and fluxes The numbersapply for the present-day situation and represent typical literaturevalues Fluxes, e g between atmosphere and surface ocean, aregross annual exchanges Numbers underlined indicate net annualC 0 2 accumulation due to human action Units are gigatons ofcarbon (GtC, lGt = 109 metric tons = 1012kg) for reservoir sizesand GtC y r ' for fluxes More details and discussions are foundin several reviews (Sundquist, 1985, Trabalka, 1985, Bolin, 1986Siegenthaler, 1986)

in Table 1 1, is of the order of 50 - 200 years, determinedmainly by the slow exchange of carbon between surfacewaters and the deep ocean The adjustment time isimportant for the discussions on global warming potential,cf Section 2 2 7

Because of its complex cycle, the decay of excess CO2in the atmosphere does not follow a simple exponentialcurve, and therefore a single time scale cannot be given tocharacterize the whole adjustment process toward a newequilibrium The two curves in Figure 1 2, which representsimulations of a pulse input of CO2 into the atmosphereusing atmosp here-ocean mod els (a box model and aGeneral Circulation Model (GCM)), clearly show that theinitial response (governed mainly by the uptake of CO2 byocean surface waters) is much more rapid than the laterresponse (influenced by the slow exchange between surfacewaters and deeper layers of the oceans) For examp le, thefirst reduction by 50 percent occurs within some 50 years,whereas the reduction by another 50 percent (to 25 percentof the initial value) requires approximately another 250years The concentratio n will actually never return to itsoriginal value, but reach a new equilibrium level, about 15percent of the total amount of CO2 emitted will remain inthe atmosphere


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1 Gi eenhouse Gases and Aerosols 9\ 1 1 0^09108-07-06-05-04-03-n?-

\\

^ ^ ^

100 120Year

140 160 1S0 200

360350340330320310300290280270

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o a


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10 Gi eenhouse G ases and Aei osols 1very substantially without the biological pump ("deadocean ) the pre-in dus tnal CO2 level would have beenhigher than the observed value ol 280 ppmv, at perhaps 450ppmv (Wenk, 1985, Bacastow and Maier-Reimer, 1990)Alterations in the marine biota due to climatic change couldtherelore have a substantial effect on CO2 levels in thefuture Note, however, that the "biological pum p" does nothelp to sequester anthropogenic CO2 (see Section 12 4 2)12 11 The 1 ole of tei 1 esti la l 1 eqetation and sodsThe most important processes in the exchange of carbonare those of photos ynth esis, autotro phic res piration (1 e ,CO 2 production by the plants) and heterotroph ic (1 e ,essentially microbial) respiration converting the organicmaterial back into CCb mainly in soils (c f Section 10 for adetailed discussion) Net primary production (NPP) is thenet annual uptake of CCb by the vegetation, NPP is equalto the gioss uptake (gross primary production, GPP) minusautotiop hic respiration In an unperturbed w orld, NP P anddecomposition by heterotrophic respiration are approximately balanced on an annual basis, iormation of soils andpeat corresponds to a (relatively small) excess of NPP

The carbon balance can be changed considerably by thedirect impact ot human activities (land use changes,particularly deforestation), by climate changes, and byother chang es in the environ men t, e g , atm osphericcomposition Since the pools and fluxes are large (NPP 50-60 GtC per year, GPP 90 120 GtC per year, Houghton etal 1985b) any perturbations can have a significant effecton the atmospheric concentration of CO21.2.2 Anthropogenic PerturbationsThe concentrations of CO2 in the atmosphere are primarilyaffected by two anthro poge nic processes release of CO 2from fossil fuel combustion, and changes in land use suchas deforestation12 2 1 Histontalfossd fuel inputThe global input of CO2 to the atmosphere from fossil fuelcombustion, plus minor industrial sources like cementproduction, has shown an exponential increase since 1860(about 4% per year), with majoi interruptions during thetwo world wars and the economic cnsis in the thirties(Figure 1 5) Following the 'oil crisis ot 1973, the rate ofincrease of the CO2 emissions fust decreased toapproxim ately 2% per year, and after 1979 the globalemissions remained almost constant at a level of 5 3 GtCper year until 1985, when they started to rise again,reaching 5 7 GtC per year in 1987 (Figure 1 5) Thecumulative release of CO2 from fossil fuel use and cementmanufacturing from 1850 to 1987 is estimated at 200 GtC 10%(Marland, 1989)

Ninety five percent of the industrial CO2 emissions arefrom the Northern Hemisphere, dominated by industiial

::

/ ' */*+/*

/""". ' , 1 , 1 ,

_ - * < " * "/. / 'y

V *^:'-

1860 1880 1900 1920 1940 1960 1980 2000Year

Figure 1.5: Global annual emissions of CO2 from fossil fuelcombustion and cement manufacturing, expressed in GtC y r '(Rotty and M arland, 1986, Marland, 1989) The average rate otincrease in emissions between 1860 and 1910 and between 1950and 1970 is about 4% per year

countries, where annual releases reach up to about 5 tC percapita (Rotty and Marland , 1986) In con trast, CO2emission rates in most developing countries he between 0 2and 0 6 tC per capita per year How ever, the relative rate ofincrease of the CO2 emissions is much larger in thedeveloping countries (~ 6% per year), showing almost noslowing do wn after 1973 in contrast to Western Eu rope andNorth America where the rate of increase decreased fromabout 3% per year (1945-72) to less than 1% per year(1973-84)12 2 2 Hist01 ical land use changesTh e vegetation and soils of unman aged forests hold 20 to100 times more carbon per unit area than agriculturalsystems The amount of carbon released to the atmospherecompared to that accumulated on land as a result of landuse change depends on the amounts of carbon held inbiomass and soils, rates of oxidation of wood products(either rapidly through burning or more slowly throughdecay), rates of decay of organic matter in soils, and ratesof regrowth of forests following harvest or abandonment ofagricultural land The heterogen eity ot terrestrial ecosystems makes estimation of global inventories and fluxesdifficult

The total release of carbon to the atmosphere fromchanges in land use, primarily deforestation, between 1850and 1985 has been estim ated to be abou t 115 GtC(Houghton and Skole, 1990), with an error limit ol about35 GtC The com pon ents of the flux to the atmosphereare (1) burning associated w ith land use chan ge, (2) decay


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7 Gi eenhouse Gases and Aei osoli 11of biomass on site (roots, stum ps, slash, twigs etc ), (3)oxidation of wood products removed from site (paper,lumber, waste etc ), (4) oxidation of soil carbon, minus (5)regrowth of trees and redevelopment of soil organic matterfollowing harvest Althou gh the greatest releases of carbonin the nineteenth and early twentieth centuries were fromlands in the temperate zone (maximum 0 5 GtC per year),the major source of carbon during the past several decadeshas been from deforestation in the tropics, with asignificant increase occurring since 1950 Over the entire135 yr period, the release from tropical regio ns is estimatedto have been 2-3 times greater than the release from middleand high latitudes Estim ates of the flux in 1980 rang e from0 6 to 2 5 GtC (Hou ghton et al , 1985a, 1987, 1988,Detwiler and Hall, 1988) virtu ally all of this flux is fromthe tropics The few regions for which data exist suggestthat the annual flux is higher now than it was in 19801.2.3 Long-Term Atmospheric C arbon Dioxide VariationsThe most reliable information on past atmospheric CO2concentrations is obtained by the analysis of polar icecores The process of air occlusion lasts from about 10 upto 1000 years, depe nding on local co nditions (e g ,precipitation rate), so that an air sample in old ice reflectsthe atmospheric composition averaged over a corresponding time interval

Measurements on samples representing the last glacialmaximum (18,000 yr before present) fiom ice cores fromGreenland and Ant arcti ca (Neftel et al , 1982 1988Delmas et al , 1980) showed CO2 concentrations of 180-200 ppmv 1 e , about 70 percent of the pre-industnal valueAnalyses on the ice cores from Vostok, Antarctica, haveprovided new data on natural variations of CO2, covering afull glacial interglacial cycle (Figure 1 6, Barnola et al1987) Over the who le period there is a remark ablecorrelation between polar temperature, as deduced fromdeuterium data, and the CO2 pio hl e The glacial-mterglacial shifts ol CO2 concentrations must have beenlinked to large-scale changes in the circulation of the oceanand in the whole interplay of biological, chemical andphysical piocesses, but the detailed mechanisms are not yetvery clear The CO2 va riations were large enoug h topotentially contribute, via the greenhouse eflect, to asubstantial (although not the major) part ol the glacial-interglacial climate change (Hansen et al , 1984, BioccohandManabc 1987)

ke coie studies on Greenland ice indicate that during thelast glaua tion CO2 concentration shifts of the order of 50ppmv may have occurred within less than 100 years(Staufler ct al , 1984), paiallcl to abrupt, drastic climaticevents (temperatu ic chan ges of the order of 5 C) The serapid CO2 changes have not yet been identified in ice coicsfrom Antaictica (possibly due to long occlusion times.

ATCDepth (m)

1000 1500

C Oppmv300280260 I-24 0220 -200180

Age (kyrBP)

Figu re 1.6: CO2 concentrations (bottom) and estimatedtemperature changes (top) during the past 160,000 years, asdetermined on the ice core from Vostok, Antarctica (Barnola etal 1987) Temperature changes were estimated based on themeasured deuterium concentrations

Neftel et al , 1988), therefore, it is not yet clear if they arereal or represent artefacts in the ice record1.2.4 The Contemporary Record of Carbon Dioxide -

Observations and Interpretation124 1 The cat bon dioxide uulease fiom pie-industiial

pe i todRelatively detailed CO2 data have been obtained for thelast millen nium from An tarctic ice cores (Neftel et al ,1985a, Fn edli et al , 1986, S iegen thaler et al , 1988,Raynau d and Barnola, 1985, Pearman et al , 1986) Theyindicate that during the period 1000 to 1800, theatmospheric concentration was between 270 and 290 ppmvThe relative constancy seems surprrsmg in view of the factthat the atmosphere exchanges about 30 percent of its CO2with the oceans and biota each year This indicates that thesensitivity of atmospheric CO2 levels to minor climaticchanges such as the Little Ice Age (lasting from the end ofthe 16th to the middle of the 19th century), when globalmean temperatures probably d ecreased by about 1C, issmall


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12 Gwenhouse Gases and Aeiosols IA piecise reconstruction of the CO? increase during the

past two centunes has been obtained from an ice core fromSiple Station, Antaictica (Figure 1 3, Ncftel et al , 1985a,Fuedli et a l, 1986) These tesults indicate that CO2 staitedto use around 1800 and had already increased by about 15ppmv by 1900 Precise direct atmospheric measurementsstarted in 1958, when the level was about 315 ppmv and therate ol increase 0 6 ppmv per year The present atm osphericCO2 level has reached 353 ppmv, and the mean growth ratehas now reached about 1 8 ppmv per year (Figure 1 4,Keeling et al, 1989a)12 4 2 Uptake b\ the oceanThe ocean is an important reservoir for taking upanthropogenic CO2 The relative increase of dissolvedinorganic carbon (total CO2) in ocean water is smaller thanin the atmosphere (only 2-3 percent until now see below)Precise measurements of dissolved inorganic carbon can bemade with present analytical tools How ever, an accuratedetermination of the trend in dissolved inorganic carbon isdifficult because of its variability in time and spaceHence, lepeated transects and time series will be requiredto assess the total oceanic CO2 uptake with good precision

The net flux of CO2 into (or out of) the ocean is given bythe pioduct ot a gas transfer coefficient and Ap C0 2 (theCO2 paitial pressure difference between ocean andatmosphere) The gas tiansfer coefficient increases withmcicasing wind speed and also depends on watertempc iatuic Therefore, the net flux into the ocean can beestimated liom a know ledge of the atmo spheric CO2concen tration pCC b in surface water (for which the dataaic still sparse), the global distribution of wind speeds oveithe ocean as well as the relation between wind speed andgas tiansfei coefficient (which is known to 30% only)There have been several estimates of the global net uptakeot CO2 by the oceans using observations (e g Enting andPearman 1982 1987) The most lecent estimate yields 1 6GtC per year (Tans et al , 1990) the error ot this estimateis, according to the authors, not easy to estimate

Estimates of oceanic C Cb uptake in the past and in thetutuie lcqu ne m odels of the global carbon cycle that takeinto account air-sea gas exchan ge aqueo us carbo natechemistiy and the tiansport from the surface to deep oceanlayers The aqueous carbonate chemistr> in sea wateropeiates in a mode that if the atmospheric CO2con centratio n increase s by e g 10% then the con-centiation ol dissolved inorganic carbon in sea waterincreases by only about 1% at equ ilibnu m Therefo re, theocean is not such a powerlul sink foi anthropogenic CO2 asmight seem at Inst when comparing the relative sizes of theleseivoirs (Figuie 1 1)

The late at which anthropogenic C O2 is tiansported fiomthe suilace to deeper ocean layeis is determined by the iateof watei exc hang e in the vertical It is known horn

measurements ot the radioactive isotope l 4 C that onaverage it takes hundreds to about one thousand years forwatei at the surface to penetrate to well below the mixedlayer ot the majoi oceans (e g , Bioeckci and Peng, 1982)Thus, in most oceanic regions only the top seveial hundredmetres ot the oceans have at piesent taken up significantamou nts of anthropogenic CO 2 An exception is the NorthAtlantic Ocean wheie bomb-produced tritium has beenobserved even near the bottom of the sea, indicating theactive formation ol new deep water

The lain ol biogenic detntal particles, which is importantfor the natural caibon cycle, does not significantlycontribute to a sequestenng ol excess CO2, since themarine biota do not directly respond to the CO2 increaseTheir activity is contioiled by other factors, such as light,temp erature and limiting nutrien ts (e g , nitrog en,phosphorus, silicon) Thus only the input ot fertilizers(phosphate, nitrate) into the ocean through human activitiesmay lead to an additional sedimentation of organic carbonin the ocean, different authors have estimated the size ofthis additional sink at between 0 04 and 0 3 GtC per year(see Baes et al 1985) It seems thus justified to estimatethe fossil fuel CO2 uptake to date considering thebiological flux to be constant as long as climatic changesdue to increasing greenhouse gases, or natural causes, donot modify the marine biotic processes Althou gh thisappears a icasonable assumption for the past and presentsituation, it may well not be so in the future

The carbon cycle models used to date to simulate theatmosphere-ocean system have often been highly simplified, consisting of a few well-mixed or diffusivereservoirs (boxes) (e g , Oeschger et al , 1975, Broecker etal , 1980, Bolin, 1981, Enting and Pearm an, 1987,Siegen thalei, 1983) Even though these box mod els arehighly simplified they are a powerful means for identifyingthe importance of the diflerent processes that determine theflux of CO2 into the ocean (e g , Broecker and Peng, 1982,Peng and Broecker 1985) The results of these mod els areconsidered to be reasonable because, as long as the oceancirculation is not changing, the models need only simulatethe transport of excess CO2 from the atmosphere into theocea n, but not the actual dyna mic s of the ocean In thesimple models, the oceanic transport mech anisms e g,formation ol deep water are param eterized The transportparam eters (e g , eddy d iffusivity) are determ ined fromobservations of transient tracers that are analogues to theflux of anthro pog enic CO 2 into the ocean If a modelreproduces correctly the observed distribution of, e g ,bomb produced 14c, then it might be expected to simulatereasona bly the flux ol CO 2 into the ocean A 1-D box-dilfusion mod el yields an ocean ic u ptake of 2 4 GtC perycai on ave iag c foi th e decade 1980 - 1989, and anoutciop-diflusion model (both described by Siegenthaler,1983) 3 6 GtC per year The lattei mod el mo st probably


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/ Gi eenhouse Gases and Act o\ol\ 13overpredicts the flux into the ocean, because it includes aninfinitely last exchange between high-latitude surfacewaters and the deep ocean

However, it is obviously desirable to use 3-dimensional(3-D) general circulation models of the oceans for thispurpose At this time , only a few mod elling gro ups havestarted to do this One 3-D model (M aier-Reim er andHasselmann, 1987) gives a similar CO2 uptake as a 1-Dbox-diffusion model of Sieg entha ler (1983) as illustratedby the model respo nse to a pulse input of CO 2 (Figure1 21 In a recent revised v ersion of this mod el (M aier-Reimer et al , personal communication) the ocean takes upless CO2 about 1 2 GtC per year on average for the decade1980 1989 The GFDL 3-D ocean model (Sarmiento etal, 1990) has an oceanic uptake of 1 9 GtC per year for thesame period 3-D ocean mo dels and especially co upledatmosphere-ocean models arc the only means to study in arealistic way the feedback effects that climate change mayhave on atmospheric CCb via alteration of the oceancirculation (cf Section 12 7 1) Ho wever mod els need tobe constrained by more data than are presently available

The oceanic uptake of CCb for the decade 1980 1989,as estimated based on carbon models (e g Siegenthalerand Oesch ger 1987, M aier Reim er et al person alcommunication, 1990, Go udn aan, 1989 Sarmiento et al ,1990) is in the range 2 0+0 8 GtC per year

12 4 ? Redistiibulion of antluopo^enu cmbon dioxideDuring the period 1850 to 1986, 19520 GtC were releasedby fossil fuel burning and 117+3*5 GtC by deforestation andchanges in land use, adding up to a cumulative input of31240GtC

Atmospheric CO2 increased from about 288 ppmv to348 ppmv during this period cone spon ding to (4116)% ofthe cumulative input This percentage is sometim es calledthe airborne fraction , but that term should not be misunderstood all CO2 anthropo genic and non -anthio -pogenic is continuo usly being exchanged betw een atmosphere ocean and biosph ere C onv ention ally an airbornefraction icferring to the fossil fuel inpu t only has oftenbeen quoted because only the emission s due to fossil fuelburning aic known with good precision Howcvei this maybe misleading since the atm osp hcn c increase is a responseto the total emissio ns We therefore prefer the definitionbased on the latter The an born e fraction tor the p eriod1980 - 1989 (see calculation below) conesponds to(48=8)7? ol the cumulative input

In model simulations of the past CO2 incre ase usingestimated emis sion s from fossil fuels and defo iestatio n ithas gencially been found that the simulated increase islarger than that actually o bsei vcd An estimate I01 thedecade 1980 1989 is

Emissions from fossil fuels into the atmosphere GtC/yr(Figure 15) 5 4+0 SEmissions from deforestation and land use I 61 0Accumulation in the atmosphere 3 40 2Uptake by the ocean 2 0+0 8Net imbalance I 61 4

The result from this budget and from other studies is thatthe estimated emissions exceed the sum of atmosphericincrease plus model calculated oceanic uptake by asignificant amount The question therefore arises wheth eran important mechanism has been overlooked All attemptsto identify such a missing sink in the ocean havehowev er failed so far A possible exception is that a naturalfluctuation in the oceanic carbon system could have causeda decreasing atmospheric baseline concentration in the pastfew decades, this does not appear likely in view ol therelative constancy of the pre-industnal CCb concentrationThere are possible processes on land, which could accountfor the missing CCb (but it has not been possible to veritythem) They include the stimulation of vegetative growthby increasing CCb levels (the CO2 fertilization effect) thepossible enhanced productivity of vegetation undci warmercon diti on s and the direct effect of fertilizatio n fromagricultural fertilizers and from nitrogenous releases intothe atmo sphe re It has been estimated that increasedfertilization by nitrogenous releases could account loi asequ estering of up to a max imu m of 1 GtC per year interrestrial ecosystems (Melillo private communication1990) In addition, changed forest managem ent practicesmay also result in an increase in the amount ol caibonstored in northern mid latitude forests The extent to whichmid-latitude terrestrial systems can sequester caibon beforebecom ing saturated and ineffective is unkn own As midlatitude terrestrial systems become close to saluiation andhence ineffective in sequestering caibo n this would allowmore of the CO2 to remain in the atmosphere

A technique for establishing the global distnbution olsurface sources and sinks has been to take globalobservations ol atmospheric CO2 concentiation andisotopic composition and to invert these by means olatmospheric transport models to deduce spatial andtempo ral patterns ol surface fluxes (Pcarman et al 1983Pearman and Hyson 1986, Keeling and Heimann 1986)The observed inter-hemispher ic CO2 concentra t iondifference (currently ab out 3 ppm v) is sma ller than onewould expect given that nearly all fossil releases occur inthe Noithern Hem isphere The results of this appio achsuggest that there is an unexpectedly large sink in theNorthern H emisph ere equiv alent to more than hall ol thefossil fuel CO2 lelease (Enting and Mans bnd ge 1989T an se ta l 1990 Keeling et al 1989b) Fuithcrmoie it hasbeen concluded that the oceanic uptake compatible withocean ic and atm osp heric CO 2 data and with a 3dimensional atmospheric transport model is at most 1 GtC


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1 Gicenhouse Gases and Aeiosols 15sinks For the sake of illustration, several schematicscenarios are shown in Figures 1 7 and 1 8 Those of Figure1 7 are based on p rescribed total CO2 emission rates after1990, for those in Figure 1 8 atmospheric concentrationsafter 1990 were prescribed and the corresponding emissionrates were calculated to fit these concen tration s A box-diffusion model of the global cycle was used for thesesimulations (Enting and Pearman, 1982, 1987), with anoceanic eddy diffusivity of 5350 m 2 year ' and an air-seagas exchange rate corresponding to an exchan ge coefficientof 0 12 year"1 The calculations assume no biosphenc-climate feedbacks, and also assume that after 1990 the netbiosphenc input of CO2 is zero, 1 e , the input of C O2 fromtropical deforestation is balanced by uptake of CCb byterrestrial ecosystems

In case a (all em ission s sto pped Figure 1 7), theatmospheric concentration declines, but only slowly (from351 ppmv in 1990 to 331 ppmv in 2050 and 324 ppmv in2100), because the penetration of man-made CO2 to deeperocean layers takes a long time Even if the emissions werereduced by 2% per year lrom 1990 on (case b), atm ospheric CO2 would continue to increase tor several decadesCase c (constant emiss ion rate after 1990) gives C O2 levelsof about 450 ppmv in 2050 and 520 ppmv in 2100 Aconstant relative growth rate of 2% per year (case d) wouldyield 575 ppmv in 2050 and 1330 ppmv in 2100Comparison of cases b, c and d clearly shows that measuresto reduce emissions will result in slowing down the rate ofatmospheric CO2 growth

Cases b and c, in comparison to b and c, schematicallyillustrate the effect of reducing emissions in 2010 instead ofin 1990

If an (arbitrary) threshold ol 420 ppmv 1 e , 50% abovepre-industnal, is not to be exceeded (case e, Figure 1 8),then CO2 production rates should slowly decline, reachingabout 50% of their present value by 2050 and 30% by2100 In order to keep the con centratio n at the presentlevel (case f) emi ssion s w ould have to be reduceddrastically to 30% of present immediately and to less than20% by 2050The iesults of scenario calculations with a 3-D ocean-atmosphere model (Maier-Reimer and Hasselmann, 1987,Maier Reimer et al , personal com mu nicat ion, 1990 -revised model) give higher concentrations than thoseshown in Figure 1 7 obtained w ith a box-diffusion mo del,for instance, about 480 ppmv in the year 2050 and about560 ppmv in the year 2100 for Scenario C, compared toabout 450 ppmv and 520 ppmv On the other hand ,calculations with a box model that includes a biosphencCO2 sink (Goudriaan, 1989) yields somewhat lowerconcentrations than shown in Figure 1 7, for instance about415 ppmv in the year 2050 and 460 ppmv in the year 2100for Scenario C

600

1950 2000 w 2050Year 2100

Figure 1.7: Future atmospheric CO2 concentrations as simulatedby means of a box-diffusion carbon cycle model (Enting andPearman, 1982, 1987) for the following scenarios (a) - (d)anthropogenic CO2 production rate p prescribed after 1990 asfollows (a) p = 0, (b) p decreasing by 2% per year, (c) p =constant, (d) p increasing at 2% per year Scenarios (b ) and (c ) pgrows by 2% per year from 1990-2010, then decreases by 2% peryear (b) or is constant (c ) Before 1990, the concentrations arethose observed (cf Figure 1 3), and the production rate wascalculated to fit the observed concentrations

OOoa>J:a.oEe lates A change in the global windpattern could mlluence the gas transfer Irom theatmo spheie to the sea surface Carbon cycle mod els showthat the net CO2 uptake by the global ocean is not sensitiveto the gas transfer coefficients (because it is controlledmainly by vertical mixing, not by gas exchange, Oeschgeret al 197=5 B ro et k er et al 1980 Sarmien to et al 1990)so this eflett would probably be of minor influence12 7 14 Modifu ation of oceanic bioi>iochemicalc \c lint! The iain ot dead 01 game partitles tonesponds toa continuous export flux ol tarbon (and nutncnls) out ofthe ocean surtace whith undei non-peituibed tond itions isbalanted b> an equal upward tianspoil ot dissolved caibon

(and dissolved nutrients) by water motion In polar regionsand stiong upwelling /ones, where productivity is notlimited by nitrogen 01 phosphorus, the balance couldbecome disturbed consequent on variations in oceandynam ics (t I Settio n 12 7 12 ), so as to influenceatmo spheric CO2 As a result of th m ate change, thedistribution ot marine ecosystems and species compositioncould change, which could affect p C 0 2 in surface watersIt is not possible at present to predict the direction andmagnitude of such effects

Waiming of the oceans might lead to accelerateddecomposition ot dissolved organic carbon, converting itinto CO2 and thus amplify the atmo sphe ric increase(Brewer, peisonal communication, 1990)1 2 7 1 S UV B ladiation A 1 eduction in stratospheric O3would increase the intensity of UV-B radiation at theEa rth s surface This might have negative effects on themarine biota due to a decrease of marine productivity andthus on the biological caibon p um p This could lead to anincrease in the concentration of CO2 in surface waters andconsequently in the atmosphere12 7 2 Tei 1 esti lal biosphei ic feedbat ksThe lollowing are probable feedback effects on theterrestrial biosphere atmospheric carbon system12721 Caibo n dioxide feitilization Short-term experiments under controlled conditions with crops and otherannuals, as well as with a lew perennials, show an increasein the rates of photosynthesis and growth in most plantsunder elevated levels ot CO2 (Strain and Cure, 1985) Ifelevated levels of CO2 increase the productivity of naturalecosystems, more carbon may be stored in woody tissue orsoil organic matter Such a storage of carbon w ill withdrawcarbon from the atmosphere and serve as a negativeleedback on the CO2 increase Of particular importan ce isthe response of forests (Luxmoore et al , 1986), given thatforests conduct about 2/3 of global photosynthesis (50% ofthis cycles annually through leaves, while 50% is stored inwood y tissue) Ho wev er, it is not clear whethe r theincreases in photosynthesis and growth will persist formore than a few giowing seasons, whether they will occurat all in natuial ecosystems and to what degree they willresult in an increased storage of carbon in terrestrialecos) stems12 7 2 2 Eutiophication and toufuation The increasedavailability of nutrients such as nitrate and phosphate fromagricultural fertilizers and from combustion of fossil fuelsmay stimulate the growth of plants It has been estimatedthat the effect of cutrophication, both on land and in theocean s, could be as large as 1 GtC p er year (Mclillo,pi lvate comm unication 1990) How ever, it should be noted


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1 Gieenhouse Gases and Aei osols 17that the greater availability of nutrients has often beenassociated with increasing levels of acid precipitation andair pollution, which have been associated with a reductionin the growth ol tcnestnal biota1272 3 Tempeiatuie Under non-tropical conditions,photosynthesis and respiration by plants and by microbesboth tend to increase with increasing temperature, butrespiration is the more sensitive process, so that a warmingof global air temperature is likely to result in an initiallyincreased release of carbon to the atmo sphe re Estim atesindicate that the additional flux might be significantperhaps as large as one or a few GtC per year (Woodwell,1983, Kohlmaier, 1988, Lashol, 1989, Houghton andWoodwell, 1989) This temperature-enhanced respirationwould be a positive feedback on global wanning

12724 Watci Changes in soil water may affect carbonfixation and storage Increased m oisture can be expected tostimulate plant growth in dry ecosystems and to increasethe storage ot carbon in tundra peat There is a possibilitythat stresses biought about by climatic change may bealleviated by increased levels of atmospheric CO2 Atpresent however, it is not possible to predict reliably citherthe geographical distribution of changes in soil water or thenet eflect of these changes on caibon fluxes and storage indifferent eco systems Chan ges in climate are generallybelieved to be more important than changes in theatmospheric concentration of CCn in affecting ecosystemprocesses (c f Section 10 )12 7 2 5 Change in qeoqiaplutal disliibntion of \ dictationt\pes In response to environmental change, the structureand location of vegetation types may ch ange If the rate ofchange is slow, plant distributions m ay adjust II, howev erthe rate of change is fast, large areas of toiests might not beable to adapt rapidly enough, and hence be negativelyaffected with a subse qu ent release of CO ? to theatmosphere12 7 2 6 UV-B ladiation A reduction in stratospheric O3would increase the intensity ot UV-B radiation at theEarth s >urtace Increased UV -B may h ave a detrimentaleffect on many land biota, including crops (Teiamura,1983), thus affecting the strength of the biosphenc sink olCO2 over land1.2.8 ConclusionsThe atmo spheric CO 2 concentratio n is now about353ppmv 25% higher than the pre industrial (1750-1 800 )value and higher than at any time in at least the last160,000 yeais This use , currently amou nting to about 1 8ppmv per year, is beyond any doubt due to humanactivities Anthropogenic emissions of CO2 were 5 70 5

GtC due to fossil fuel burning in 1987, plus 0 6 to 2 5 GtCdue to deforestation (estima te for 1980) During the lastdecade (1980 - 1989) about 48% of the anthiopogemcemissions have stayed in the atmosphere, the remainder hasbeen taken up by the oceans and possibly by landecosystems Our qualitative knowledg e of the globalcarbon cycle is, in view of the complexity ol this cycle,relatively good How ever, the current quan titative estimatesof sources and of sinks of CO 2 do not balance theatmospheric increase is less rapid than expected fromcarbon cycle models (in which CO2 fertilization orenvironmental responses of the biosphere are not included)This, and model analyses ol the inter-hemispheric CO2giadient, indicate that the Northern Hemisphere terrestrialecosystem s may act as a significant sink of carbon Such asink has, how ever, not been directly identified Tosumm arize the total annual input of anthrop ogen ic CO2 iscurrently (1980-1989) about 7 01 1 GtC assuming acentral value for the input of CO2 from trop icaldeforestation, the annual uptake by the oceans is estimated(based on the box mode ls, GC Ms and Tans et al 1990) tobe about 2 0 + 1 0 GtC, and the annual a tmospher icaccumulation is about 3 40 2 GtC Thu s, the annualsequestering by the terrestrial biosphere should be about1 61 5 GtC Wh ile several m echan isms have beensuggested that could sequester carbon in terrestnalecosystems, it is difficult to account for the total requiredsink Th erefore, it appe ars likely that, (1) the uptake of CO 2by the oceans is und erestim ated (11) there are im portantunidentified piocesses in terrestrial ecosystems that cansequ ester CO 2, and/or (111) the amoun t ol CO 2 releasedfrom tropical deforestation is at the low end of cuirentestimates

If the land biota piesently act as a sink of carbon du e to afertilization effect, then they might become saturated withrespect to this feitilization at some time in the Iuture Thismeans that we cannot assume that the tenestnal sinkwhich may be active currently, will continue to existunchanged through the next century

In order to avoid a continued rapid growth of CO2 in theatmo spheie severe reductions on emission s will benecessary The time taken for atmo spheric CO2 to adjustto changes in sources or sinks is ot the order of 50-200years, determined mainly by the slow exchange of carbonbetween surface wateis and deeper layers ot the oceanEven if all anthropogenic emissions of CO2 weie haltedthe atmospheric concentration would decline only slowly,and it would not approach its pie-industrial level for manyhundieds ot years Thu s, any reductions in emissions willonly become fully elfective after a time of the order of acentury 01 more Based on some model estimates whichneglect the feedbacks discussed earlier the atmosp hencconcentiation in the year 2050 would be between 530 - 600ppm v foi a cons tant relative gio wth ol the annual


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/ < S ' Gi eenhouie Gases and Aeioiols I

anthiopogenic emissions by 2% pei year , and between 415- 480 pp mv ( increas ing to 460 - 560 ppm v by the year2100 ) lor a cons tant anthi opo gen ic emiss ion ra te a t the1990 level In ordei not to exceed 420 ppmv (50 % abo vepie- indust r ia l ) , annual anthropogenic emissions would haveto be reduced cont inuously to about 50% of thei r presentvalue by the year 2050 In order to stabilize con cen tratio nsa t p re sen t day concen t ra tions 0 5 3 p pmv ) , an imme dia t ereduc t ion in g loba l an th ropoge n ic emiss ion s by 60-80pei cent wo uld be necessary The size of the est im atedI educt ion depends on the carbon cycle model used

Duung the mil lennium preceding the anthropogenic CO2giowth, the concent ia t ion was re la t ively constant near 280ppm v, wi th a vai labi l i ty of less than + 10 ppm v Thi sindicates that the sensitivity ot atmospheric CO2 levels tominoi c l imat ic changes such as the Li t t le Ice Age, whereg loba l mean t empera tu re s p robab ly dec reased by abou t1C, is within this range Ho we ver, the anticip ated clim aticand environmental changes may soon become large enoughto act back on the oceanic and teirestnal carbon cycle in amore substant ia l way A close interact ion betwe en c l imatevanat ions and the carbon cycle i s indicated by the glacia l -mterglacia l CO 2 varia t ions The ice-core record shows thatCO2 concent ia t ions during the coldest par t of the lastglac ia t ion were about 30% lower than during the past10 000 yeais The glacia l mlerglacia l CO 2 vanat ions wereprobably due to changes in ocean c i rcula t ion and marinebiological ac t ivi ty , and were corre la ted to varia t ions inglobal c l imate Ther e is som e (not fully c lear) evi den cehorn ice cores that rapid chang es ol C O 2, ca 50 ppm vwithin about a century, occurred during and a t the end olthe ice age

If global tcmpeia tures increase , this could change thenatuia l I luxes of carbon, thus having feedback effects onatmo sphe ric CO 2 Som e ot the ident i f ied feedbacks arepotent ia l ly large and could signif icant ly inl luence futureC O 2 levels They are di ff icul t to quant i fy, but i t seem slikely that there would be a net positive feedback, 1 e , theywil l enhance the man -ma de increase On the longer term,the possibi l i ty ol unexpected large changes in the mechanisms of the carbon cycle due to a human-induced changein climate cannot be excluded

1 .3 Me thaneMeth ane is a chem ical ly and radia t ivcly ac t ive t race gasthat is produ ced from a wide va riety of an aer obi c (1 e ,oxygen defic ient) processes and is pr imari ly removed byreact ion wi th hydroxyl radicals (OH) in the t roposphereOxidation of CH4 by OH in the stratosphere is a significantsou rce o t s t r a tos phe r i c w a te r (H 2O ) where it i s animportant gicenhouse gas

ATCDepth (m)

1000 1500

CH 4(ppbv)700

600

-500

400

30080Age (kyr BP ) 160

Fi gu re 1.9: Methane concentrations (bottom) and estimatedtemperature changes (top) during the past 160,000 years asdetermined on the ice core from Vostok, Antarctica (Chappelaz etal 1990) Tem perature changes were estimated based on themeasured deuterium con centrations

/ 3.7 Atmospheric Distribution of Methane13 11 Palaeo-atmosphei u 1 ec 01 d of methaneThere are good data on the a tmospheric concentra t ion ofCH 4 (Figure 1 9) f rom An tarc t ic and Greenlan d ice coresfor the period between 10,000 and 160,000 years ago(Ra yna ud e t a l 1988 , Stauffer e t a l , 1988, Cra ig andCh ou , 1982 , Ch app e l l az e t a l , 1990) The min imu mconcen tra t ion during the last glac ia l per iods (about 20,000and 150,000 years ago) was around 0 15 ppm v, and roserapidly, in phase wi th the observed temperature increases,t o a b o u t 0 6 5 p p m v d u u n g t h e g l a c i a l - i n t e r g l a c i a lt r ans i t i ons (abou t 15 ,000 and 130 ,000 yea rs ago) Theatmospheric concentra t ions of CH4 decreased rapidly, pr iorto , and during the last deglacia t ion period about 10,00011,000 years ago ( the Younger Dryas period when therew e r e a b r u p t te m p e r a t u r e d e c r e a s e s in G r e e n l a n d a n dnorthern Europe) , and increased lapidly thereafter

Beca use ot the brit tle nature of the ice cor es, data on thea tmosphenc concen t i a t ions o f CH4 a t e i ehab lc on ly duungthe last 2 ,000 years ot the H oloce ne perio d ( last 10 000years)


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1 G>eenhouse Gases and Aeiosols 1913 12 Contempoi ai y i ec oi d of methaneIce core data (Figure 1 10) indicate that the atmo sphe ricconcentrations of CH4 averaged around 0 8 ppmv betweentwo hundred and two thousand years ago, increasing to 0 9ppmv one hundred years ago (Craig and Chou, 1982,Rasmussen and Khahl, 1984, Stauffer et al, 1985, Pearmanand Fraser, 1988, Pearma n et al , 1986, Et hen dg e et al ,1988) Since then, the atmo spheric concen tration of CH4has increased smoothly to present levels, highly correlatedwith global hum an populatio n Ana lysis of infrared solarspectra has shown that the atmospheric concentration ofCH4 has increased by about 30% over the last 40 years(Rinsland et al , 1985, Zander et al, 1990)

Atmospheric concentrations of CH4 have been measureddirectly since 1978 when the globally averaged value was1 51 ppmv (e g , Rasmussen and K hahl, 1981, Blake andRowland, 1988) Cu rrently the value is 1 72 ppm v,corresponding to an atmospheric reservoir of about 4900Tg (1 Tg = lO 1^ g) dnc} n 1S increasing at a rate of 14 to 17ppbv per year (40 to 48 Tg per year), 1 e , 0 8 to 1 0% peryear (Blake and Row land 1988, Steele et a l , 1987) Theatmospheric co ncen tration of CH 4 in the NorthernHemisphere is 1 76 ppmv compared to 1 68 ppmv in theSouthern Hemisphere (Figure 1 11) The magnitude of theseasonal variab ility varies with latitude (Steele et al 1987,Fraser et al 1984), being co ntrolled by the temporalvariability in source strengths and atmospheric concentration of OH radicals

13 13 Isotopic c omposition of methaneMethane is prod uced Irom diflerent sources withdistinctive proportions of carbon , 2 C '^C and 1 4 C, andhydrogen isotopes H, D (2H) and T (^H) Similarly therates ol processes that destroy CH4 depend upon itsisotopic com position Co nsequ ently the CH4 budget canbe constrained by knowledge oi the isotopic composition otatmospheric CH 4, the extent ot isotopic fractionationdunng removal, and the isotopic signaluies ol CH4 fromdifferent sou rces Recent work to elucidate the sources otCH4 has proceeded through an analysis of carbon isotopicsignatures (Cicerone and Orem land 1988 Wah len et al1989, Lowe et al , 1988 and iclerence s therein) Oneexample of this is an analy sis ot ' 4 C data which suggeststhat about 100 Tg CH 4 pei ycai m ay an se from fossilsources (Cicerone and Oiem land 1988, Wahlen et al1989) Such a distinctio n is poss ible becau se CH4 Iromfossil sources is ^^CAret while that tiom other souices hasessentially the ^C concentiation ol modem caibon1.3.2 Sinks of MethaneThe majoi sink loi atmospheric CH4 is reaction with OH inthe tioposph eie the OH concentiatio n being contiolled bya complex set ol leactions involving CH4 CO NM HCNOx and tropos phen c O^ (discussed in Section I 7 S/e

~ 1600 -

t 1200 -

Io 800 -1600 1700 1800Year 1900

Figu re 1.10: Atmospheric methane variations in the past fewcenturies measured from air in dated ice cores (Ethendge et al1988 Pearman and Fraser 1990)

Figure 1.11: The global distribution, seasonality and trend ofmethane from the GMCC network (Steele et al, 1987 andunpublished data)

1977, Crutzen 1987) Based on the reaction rate coefficientbetween CH4 and OH, and the estimated troposphencdistribution ol OH an atmosph eric lifetime for CH4 olbetween 8 and 11 8 years has been estimated (Pn nn et al1987) This estimate is supported by the fact that models olglobal OH are tested by analyses of the budgets forC H ^C C h (Logan et al 1981, Fraser et al 1986a Pnn n etal 1987) and 1 4 C O (Appendix to WM O 1989b) T hereaction between CH4 and OH currently represents a sinkol 400 to 600 Tg ol CH4 p er year The efficiency ol thissink may however have decieased dunng the last centunbecause the atmospheric concentiation ot OH in thetioposp heie may have decieased hence the litetime ol CH4would have increased in iesp on se to increas ingconcentrations o! CO NMHC and CH4 (S/e 1977)


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20 Gi eenhouse Gases and Aei osoh 1Table I 2 Estimated Sow ces and Sinks of Methane

Annua l Re lease (Tg CH4 ) Range (Tg CH 4 )

SourceNatural Wetlands (bogs, swam ps, tundra, etc)Rice PaddiesEnteric Fermentation (animals)Gas D rilling, venting, transmissionBiomass BurningTermitesLandfillsCoal MiningOceansFreshwatersCH4 Hydrate Destabilization

1151108045404040351055

1 0 0 - 2 0 025 - 17065 - 1002 5 - 502 0 - 801 0 - 1002 0 - 701 9 - 505 - 201- 250 - 1 0 0

SinkRemoval by soilsReaction with OH in the atmosphere

30500

1 5 - 4 5400 - 600

Atmospheric Increase 44 4 0 - 48

Soi ls may represen t a r emova l m echanism for CH 4 Themagnitude of this s ink has been est imated ( this assessment)to be 10+15 Tg CH4 per year f rom the work of Harr iss e ta l 1982 and Seller and Co nrad, 19871.3.3 Sources of MethaneM ethane i s p rod uced f rom a wide va r ie ty of anae rob ics o u r c e s ( C ic e r o n e a n d O r e ml a n d , 1 9 8 8 ) T w o ma inpa th wa ys to r CH 4 prod uc t ion have been iden t i f ied (1)leduction ol CO2 with hydrogen, fa t ty ac ids or a lcohols ashyd ioge n d ono rs, or (11) transm ethy la tion of acetic ac id orme th y l a l c o h o l by CH 4 - p r o d u c i n g b a c t e r i a T a b l e 1 2summaii/es identif ied sources of CH4 with ranges of l ikelyannual emissio ns The tota l annual CH4 source must equalthe a tmosphenc sink ol about 500 (400 to 600) Tg CH4 peryear the possible soil s ink of about 10 (15 to 45) Tg C H4per year , and the annual growth of 40 to 48 Tg CH4 in thea tmo sphere The sum of the pre sen t bes t e s t ima tes o f thesizes of the individual s ources identif ied in Tab le 1 2 equal525 Tg CH 4 per year I t shou ld be noted that the new estda ta lo r r ice paddies , b iomass burn ing , and coa l min ingsources suggest that the values may be even less than thoseof Tab le I 2, possibly indicating a miss ing source of CH 4,or an overestimate of the sink for CH4

13 31 Natui al wetlandsSigni f ican t p ro gress has been ma de in quan t i fy ing themagni tude of the source of CH4 f rom na tura l we t lands(Sven sson and R ossw a l l , 1984 , Seba che r et a l , 1986 ,Wha len and Reeburgh , 1988 , Moore and Knowles , 1987 ,M athew s and Fung , 198 7, Harr iss e t a l , 1985, C nll e t a l ,1 9 8 8 , A n d r o n o v a , 1 9 9 0 , H a r r i s s a n d S e b a c h e r , 1 9 8 1 ,Bu rke e t al , 1988, Harr i ss e t al , 198 8, Asel m an n andCrutz en, 1989) Recen t data supp ort ear l ier est im ates of aglobal f lux of 110 - 115 Tg CH4 per year , but reverses thele la t ive impor tance o t t rop ica l and h igh la t i tude sys tems(Bartle t t e t a l , 1990) Th e data base , which is s t i ll quitel imited (no data f rom Asia) , suggests 55 Tg CH4 per year(previou sly 12 Tg CH 4 per year) f rom tropic al w etlan ds,and 39 Tg CH4 per year (previously 63 Tg CH4 per year)f rom h igh la t i tude we t land s S in ce CH 4 is p rod ucedthrough b io log ica l p rocesses unde r anae robic condi t ions ,any factors a lfecting the physical , chemical or biologicalcharacter ist ics of soils could affect CH4 emission ra tes

13 3 2 Rue paddiesRic e p a d d i e s a r e a n imp o r t a n t s o u r c e o f CH 4 w i thestim ates ol the globa lly avera ged f lux rang ing from 25 -170 Tg CH 4 pei yeai (Neu e and Scha rpen seel , 1984, Yagiand M inam i , 1990 , Ho lzapfe l -Ps choin and Se l le r , 1986 ,Cicero ne and Shcttc i , 1 981, Cice rone e t a l , 1983) The f luxof CH 4 f rom n ee padd ies is c r i t ica l ly depen den t upon


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I Gi eenhouse Gases and Aei owls 21several factors including (1) agn cul tuia l practices (e g ,fertilization, water management, density of rice plants,double cropping systems, application ol manuie or ricestraw), (n) soil / paddy characteristics (soil type, acidity,redox potential, temperature, nutrient availability, substrate, profile of anaerobic environment), and (in) time ofseason One difficulty in obta ining accu rate estim ates isthat almost 90 % of the world s harvested area of ricepaddies is in Asia, and of this about 60% are in China andIndia from which no detailed data are available The annualproduction of rice since 1940 has approximately doubled asa result of double cropping practices and an increased areaof cultivation It is likely that CH 4 e mi ssio ns haveincreased proportion ally as wellI 3 1 3 Biomass bwmm>Biomass burning in tropical and sub tropical regions isthought to be a significant source of atmosp heric C H4, withestimates of global emission rates ranging from 20 to 80 TgCH4 per year (Andreae et al , 1988, Bingemer and Crutzen,1987, Crutzen et al , 1979, Crutzen et al , 1985, Crutzen1989, Greenberg et al , 1984 Steven s et al 1990 Quay etal , 1990) Improved estimates require an enh ancedunderstanding of (1) CH 4 emissio n factors, (11) the am oun t,by type, of vegetation burnt each year on an area basis and(111) type of burn ing (sm ou lde nn g vs flaming) Cu rrentestimates indicate that over the last century the rate otforest clearing by burn ing has incieased (c f Section122 2)13 3 4 Entcnc feimentation (animals)Methane emissions lrom enteric fermentation in ruminantanimals includin g all cattle , sheep and wild anim als isestimated to provide an atmospheric source ot 65 - 100 TgCH4 per year (Crutzen et al , 1986 Lcrner et al 1988)Methane emissions depend upon animal populations aswell as the amo unt and typ e of food It is difficult toestimate the change in this source over the last centuryaccurately because the significant increase in the number ofcattle and sheep has been partially offset by decreases inthe populations of elephants and North American bisonOne estimate suggests that the magnitude of this souice hasincreased from 21 Tg CH 4 per year in 1890 to 78 Tg CH 4per year in 1983 (Crutzen et al , 1986)13 3 5 TeimitesThere is a large range in the magnitude of the estimatedfluxes of CH4 from termites, 10 - 100 Tg CH4 per year(Cicerone and Oremland, 1988, Zimmerman et al , 1982,Rasmussen and Kh ahl, 1983, Seilci et al 1984 Fiasci etal 1986b) T he values are based on the results otlaboiatory exp enm ent s applied to estima tes of globaltermite populations and the amount of biomass consumedby teimites both of which aie un ceita in, and field

exp erime nts It is imp ortant to determ ine whethe r theglobal termite population is currently increasing, andwhether it is likely to lespond to changes in climate13 36 LandfillsThe anaerobic decay of organic wastes in landfills may be asignificant anthropogenic source of atmospheric CH4, 20 -70 Tg CH4 per year Ho wev er, several factors need to bestudied in order to quantify the magnitude of this sourcemore precisely, including amounts, trends, and types ofwaste materials, and landfill practices (Bingemer andCrutzen, 1987)13 3 7 Oceans andfteshwatei sOceans and freshwaters are thought to be a minor source ofatmospheric CH4 The estimated flux of CH4 from theocea ns is based on a limited data set taken in the late1960 s / early 1970 s when the atmospheric concentration ofCH 4 was about 20% lower They showed that the openoceans were only slightly supersaturated in CH4 withrespect to its partial pressure in the atmosph ere There areinadequate recent data from either the open oceans orcoastal waters to reduce the uncertainty in these estimates(Cicerone and Oremland, 1988)13 3 8 Coal mininqMethane is released to the atmosphere from coal mineventilation, and degassing from coal during transport to anend-use site A recent unpublished study estimated the fluxof CH4 from coal mining, on a country basis, for the toptwenty coal producing countries, and deduced a globalminimum emission of 19 Tg CH4 per year Global CH4fluxes tiom coal mining have been estimated to range from10 - 50 Tg CH4 per year (Cicerone and Oremland 1988,ICF, 1990, and recent unpublished studies by others)13 3 9 Gas diilhnq \tntinq and tiansnnssionMethane is the major compon ent of natural gas henceleakage from pipelines and venting from oil and gas wellscould represent a significant source of atmospheric CH4(Cicerone and Oremland, 1988) The global flux from thesesources is estimated based on limited data of questiona blereliability, to range from 25 - 50 Tg CH4 per year1.3.4 Feedbacks from Climate Change into the Methane

CycleFuture atmospheric concentrations of CH4 will depend onchanges in the strengths of either the sources or sinkswhich are dependent upon social, economic, and politicaland also environmental factors and in particular changes inclimate Methane emission s from w etlands are particularlysensitive to temperature and soil moisture and hence futureclimatic changes could significantly change the 1 luxes ofCH4 from both natuial wetlands and rice paddies
http://tntinq/http://tntinq/


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? ? Greenhouse Gases and Aerosols 1Tropospheric OH, which provides the atmospheric sink forCH4, is dependent upon a number of factors, including theintensity of UV-B radiation, and the ambient concentrations of H2O, CO, CH4, reactive nitrogen oxides, andtropospheric O3 (See Section 1.7) (Crutzen, 1987; Isaksenand Hov, 1987; Thom pson and Cicerone, 198 6).1.3.4.1 Tropical methane sourcesThe major sources of CH4 in tropical regions (naturalwetlands and rice paddies) are quite sensitive to variationsin soil moisture. Consequently, changes in soil moisture,which would result from changes in temperature andprecipitation, could significantly alter the magnitude ofthese large sources of atmospheric CH4. Increased soilmoisture would result in larger fluxes, whereas a decreasein soil moisture would result in smaller fluxes.

1.3.4.2 High latitude methane sourcesMethane fluxes from the relatively flat tundra regionswould be sensitive to changes of only a few centimetres inthe level of the water table, with flooded soils producing afactor of 100 more CH4 than dry soils. Similarly, emissionsof CH4 are significantly larger at warmer temperatures, dueto accelerated microbiological decomposition of organicmaterial in the near-surface soils (Whalen and Reeburgh,1988; Crill et al.. 1988). Consequently, an increase in soilmoisture and temperatures in high latitude wetlands wouldresult in enhanced CH4 emissions, whereas wanner dryer

soils might have decreased CH4 emissions.Higher temperatures could also increase the fluxes of

CH4 at high northern latitudes from; (i) CH4 trapped inpermafrost, (ii) decomposable organic matter frozen in thepermafrost, and (iii) decomposition of CH4 hydrates(Cicerone and Oremland, 1988; Kvenvolden, 1988; Nisbet,1989). Quantifying the magnitudes of these positivefeedbacks is difficult . Time-scales for thawing thepermafrost, located between a few centimetres to metresbelow the surface, could be decades to centuries, while thetime for warming the CH4 hydrates could be even longer,although one study (Kvenvolden, 1988) estimated that theflux of CH4 from hydrate decomposition could reach 100Tg CH4 per year within a century.1.3.5 ConclusionsCurrent atmospheric CH4 concentrations, at 1.72 ppmv, arenow more than double the pre-industrial value (1750-1800)of about 0.8 ppmv, and are increasing at a rate of 0.9% peryear. The ice core record shows that CH4 concentrationswere about 0.35 ppmv during glacial periods, and increasedin phase with temperature during glacial-interglacialtransitions. The current atmosp heric concentration of CH4is greater than at any time during the last 160,000 years.

Reaction with OH in the troposphere, the major sink forCH4, results in a relatively short atmospheric lifetime of102 years. The short lifetime of CH4 implies thatatmospheric concentrations will respond quite rapidly, in

Table 13 Halocarbon Concentrations and Trends (1990) f

Haloca rbon

CCI3FCCI2F2CCIF3C2CI3F3C2CI2F4C2CIF5CCI4CHCIF2CH3CICH3CCI3CBrClF2CBrF3CH3Br

itCFC-11)((CFC-12)CFC-13)CCFC-113)(CFC-114)(CFC-115)tfHCFC-22)

(halon 1211)(halon 1301)

Mixing Ra t iopptv

28 048456015514612260 01581.72.010-15

Annual Rate of Increasepptv

9.516.54-5

2.076.00.20.3

%

4410

1.5741215

Life t imeYears

651304009020 040 050151.57251101.5

t There are a few min or differences b etween the lifetimes reported in this table and the equiv alent table in W M O 1989 b.These differences are well within the uncertainty limits. The 1990 mixing ratios have been estimated based upon anextrapolation of measurem ents reported in 1987 or 1988, assuming that the recent trends remained approximatelyconstant.


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1 Gi eenhouse Gases and Aerosols 23

400

i I rCape Gr im

C F C 1 2

1978 1980 1982 1984Year

1986 1988

Figure 1.12: Halocarbon concentrations measured at Cape Grim, Tasmania during the period 1978-1989 (Fraser and Derek, 1989,and unpublished data)

comparison to the longer lived gases such as CO2, N2O,and CFCs, to changes in emissions In order to stabilizeconcentrations at present day levels, an immediatereduction in global man-made emissions by 15-20 percentwould be necessary (this and other scientific sensitivityanalyses are discussed in the Annex) Global co ncentrations of OH are dependent upon the intensity of UV-B radiation, and the concentrations of gases such as h bO ,CO, CH4, NO x , NM HC , and O3 and may have declinedduring the twentieth century due to changes in theatmospheric concentrations ol these gases

The individual sources of atmospheric CH4 have beenqualitatively identified, but there are significant uncertainties in the magn itude of their strengths Humanactivities such as rice cultivation, rearing of domesticruminants, biomass burning, coal mining, and natural gasventing have increase d the inp ut oi CH 4 into theatmosphere, and these combined with an apparent decreasein the concentration of troposphenc OH, yields theobserved rise in global CH4 How ever, the quantitativeimportance of each of the factors contributing to theobserved increase is not well known at present

Several potential feedbacks exist between climate changeand CH4 emissions, in both tropical and high latitudewetland sources In particular, an increase in high latitudetemperatures could result in a significant release of CH4from the melting of permafrost and decomposition of CH4hydrates

1.4 HalocarbonsHalocarbons containing chlorine and bromine have beenshown to deplete O3 in the stratosphe re In addition , it hasbeen recognized that they are important greenhouse gasesTheir sources, sinks, atmospheric distributions, and role inperturbing stratospheric O3 and the Earth's radiativebalance have been reviewed in detail (WMO 1985, 1989a,1989b) Many gov ernm ents, recognizing the harmfuleffects of halocarbons on the environment, signed the

Montreal Protocol on Substances that Deplete the OzoneLa ye r' (UN EP 1987) in 1987 to limit the produ ction andconsumption of a number of fully halogenated CFCs andhalons The control measures of the Mon treal Protocolfreeze the production and consum ption of C FCs 11. 12,111, 114, and 115 in developed countries at their 1986levels from the year 1990, a reduction to 80% of their 1986levels from the year 1993, with a further reduction to 50%of their 1986 levels from the year 1998 Dev elopin gcountries, with a per capita use of CFCs of less than 0 3 kgper capita, are allowed to increase their per capita use up tothis limit and can delay compliance with the controlmeasures by 10 years All major producing and consum ingdeveloped countries, and many developing countries, havesigned and ratified the Montreal Protocol1.4.1 Atmospheric Distribution of HalocarbonsThe mean atmospheric concentrations of the most abundantradiatively active halocarbons are shown in Table 1 3 Theatmospheric concentrations of the halocarbons are currentlyincreasing more rapidly on a global scale (on a percentage


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24 Gi eenlwuse Ga ses and Aei osols 1basis) than the other green hou se gases (Figure 1 12)The concentrations of the fully halogenated chlorofluorocarbons (CFCs), slightly greater in the northernhemisphere than in the southern hemisphere, are consistentwith the geographical distribution of releases (>90% fromthe industrialized nations), a 45N - 45S mixing time ofabout 1 year, and their very long atmospheric lifetimes/ .4.2 Sinks for HalocarbonsThere is no significant troposphenc removal mechanism forthe fully halogenated halocarbons such as CCI3F (CFC-11),CCI2F2 (CFC 12), C2CI3F3 (CFC 113), C2CI2F4 (CF C-114), C2CIF5 (CFC-115), carbon tetrachloride (CCI4), andhalon 1301 (CBrF3) They have long atmospheric lifetimesdecades to centuries, and are primarily removed byphotodissociation in the mid - upper stiatosphe re There iscurrently a significant imbalance between the sources andsinks giving rise to a rapid growth in atmosphericconcentrations To stabilize the atmosph eric concen tiationsol CFCs 11, 12 and 113 at current levels would requirereductions in emissions of approximately 7 0-75% , 75-8 5% ,and 85-95%, respectively (see Annex)

Non fully halog enated h aloca rbon s conta inin g ahydrogen a tom such as methyl chlor ide (CH3CI) ,methylchloroform (CH3CCI3), CHCIF2 (HC FC-22), and anumber of other HCFCs and HFCs being considered assubstitutes for the current CFC s (c f Section 1 4 4) arepnmarily removed in the troposphere by reaction with OHThese hydrogen containing species have atmosphericliletimes ranging from about one to forty years, muchshorter on average than the fully ha logen ated CF Cs Tostabilize the atmospheric concentrations of HCFC-22 atcurrent levels would require reductions in emissions otapproximately 40-50%

1.4.3 Sources of HalocarbonsMost halocarbons, with the notable exception of CH3CI,are exclusiv ely of industrial origin Haloca rbons are used asaerosol piop ellants (CFC s 11, 12, and 114), refrigerants(CFC s 12 and 114, and HCF C-22) loam blowing agen ts(CFCs 11 and 12) solvents (CFC -113 CH 3C CI 3, andCCI4 ), and fire retardants (halons 1211 and 1301) Currentemission fluxes are approximately CFC 11 350Gg/y CFC12 450 Gg/y CF C-113 150 Gg /y HCFC-22 140 Gg/y,others a re s ignificant ly smallc i The a tmo sph encconcentration of methyl chloride is about 0 6 ppbv, and ispnmailly released from the oceans and during biomassburning There is no evidence that the atm osph encconc cntiation of CH3CI is increasing Methyl bromide(C H 3 B r) is produ ced by oceanic algae and there isevidence that i ts atmosphenc concentiation has beenincieasing in recent times due to a significant anthropo genic source (Penkett et al 1985 W ofsy et al 1975)

1.4.4 Future Atmospheric Concentration of HalocarbonsFuture emissions of CFCs 11, 12, 113, 114, and 115 will begoverned by the Montreal Protocol on "Substances thatDeplete the Ozon e Layer as discussed in Section 1 4 Inaddition, international negotiations are currently in progressthat will likely (1) result in a complete global phase-out ofproduction of these chemicals by the year 2000, and (11)enact limitations on the emissions (via production andconsump tion controls) of CCI4, and CH 3CC I3 Howev er,even with a complete cessation of production of CFCs 11,12 and 113 in the year 2000 their atm osp hericconcentrations will still be significant for at least the nextcentury because of their long atmosp heric lifetimes Itshould be noted that emissions of these gases into theatmosphere will continue for a period of time afterproduction has ceased because of their uses as refrigerants,foam b lowing agents fire retardants, etc

A numb er of hydro l lu oroca rb ons (HFCs) andhydrochloiofluorocarbons (HCFCs) are being considered aspotential replacements for the long-lived CFCs (11, 12,113, 114, and 115) that aie regulated under the terms of theMontreal Protocol The HFCs and HCFCs primarily beingconsidered include HCFC 22, HCFC -123 (CHCI2CF 3),HCFC 124 (CHCIFCF3) , HFC 125 (CHF2CF3), HFC-134a (CH2FCF3), HCFC-141b (CH3CCI2F), HCFC-142b(CH 3C CIF 2) , HFC 143a (CH 3CF 3) , and HFC-152a(CH 3CH F2) The calculated atmospheric lifetimes of thesechemicals are controlled primarily by reaction withtroposph enc OH and range between about 1 and 40 yearsIt has been estimated (UN EP 1989) that a mix of HFCsand HCFCs will replace the CFCs currently in use at a rateof about 0 4 kg of substitute for every kg of CFCs currentlyproduced, with an annual growth rate of about 3%Because of their shorter lifetimes, and expected rates ofsubstitution and emissions growth rates, the atmosphericconcentrations of HFCs and HCFCs will be much lower forthe next several decades than if CFCs had continued to beused, even at current rates How ever, continued use,accompanied by growth in the emission rates of HFCs andHCFCs for more than several decades would result inatmospheric concentrations that would be radiativelyimportant1.4.5 ConclusionsThe atmospheric concentrations of the industrially-produced ha loca rbons , p r imar i ly C CI3F, CC I2F2,C2CI3F3, and CCI4 are about 280 pptv, 484 pptv, 60 pptv,and 146 pptv, respectively Over the past few decades theirconcentrations (except CCI4) have increased more rapidly(on a percentage basis) than the other greenhouse gases,curre ntly at rates ol at least 4% per yea r Th e fullyhalogenated CFCs and CCI4 arc primarily removed byphotolysis in the stratosphere and have atmosphericlifetimes in excess ol 50 years


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/ Gi eenhouse Gases and Aei osols 25Most ha loca rbons , wi th the no tab le except ion of me thyl

ch lor ide , a re exc lus ive ly an thropogenic and the i r sources( so lven ts , r e f r ige ran ts , foam b lowin g agen ts , and ae roso lpropellants) are well understood.

To s tab i l ize , and then reduce , the cur ren t a tmosphe r icconcen tra tions of the fully halo gen ated C FC s (e .g , 11, 12a n d 1 1 3 ) w o u ld r e q u i r e a p p r o x im a te r e d u c t i o n s ine mi s s io n s of 7 0 - 7 5 % , 7 5 - 8 5 % , a nd 8 5 - 9 5 % , r e s p e c t i v e ly .Future emiss ions of CFCs and CCI4 wi l l , mos t l ike ly , bee l imina ted or be s ign i f ican t ly lower than today ' s becauseth e s t r i n g e n c y , s c o p e , a n d t im in g o l i n t e r n a t i o n a lregula t ions on ch lor ine and bromine conta in ing chemica ls ,(i.e., the Montreal Protocol on Substances that Deple te theOzone Laye r ) a re cur ren t ly be ing renegot ia ted . However ,the a tmos phe r ic con cent ra t ions o l CF Cs 11 , 12 and 113will still be significan t (30 - 4 0 % of curre nt) for at least thenext century because of their long a tmospheric l ife t imes.

3 2 03103 0 0

I 310S 300coa 310

300 -ooO 310300

310300

Pt Bar row Alaska

Niwot R idge Co lo rad o

Mauna Loa

Samoa

South Po le- ^ A - ^ V ^ ^ -

77 78 79 80 81 82 83 84T i m e ( ye a r ) 85 86 87 88

1.5 Ni t rous OxideNitrous oxide is a chemically and radia tively active tracegas tha t i s p rodu ced f rom a wide va r ie ty of b io log ica lsources in soils and water and is pr imarily removed in thestra tosphere by photolysis and reaction with e lectronicallyexcited oxygen a toms.

1.5.1 Atmospheric Distribution of Nitrous OxideThe mean a tmo sphe r ic conc ent ra t io n of N2 O in 1990 i sabout 310 ppbv, corresponding to a reservoir of about 1500TgN, and increasing a t a ra te of 0 2 - 0.3% per year (Figure1 13 , W e i s s , 1 9 8 1 ;P n n n e t a l , 1 9 90 ; Ro b in s o n e t a l ., 1 9 88 ;Elk ins and Ro ssen , 1989 , Ras mu ssen and Kha l i l , 1 986) .This obse rved ra te o f inc rease r epresen ts an a tmo sphe r icgrowth ra te o f about 3 to 4 .5 Tg N pe r yea r . T heatmospheric concentra tion of N2O is higher in the NorthernHemisphe re than in the Southe rn Hemisphe re by about 1ppbv Ice co ie mea surem ents show tha t the pre - ind us tna lvalue of N2O was re la t ively stable a t about 285 ppbv formost of the past 2000 years, and star ted to increase aroundthe year 1700 (Figu re 1 14, Pea rm an e t a l , 1986, Khali land Rasmussen , 1988b , E t hen dg e e t al , 1988; Za rd in i e ta l , 1989) F ig ure 1 .14 sho ws tha t the a tm os ph e r i cconcentra tions of N2O may have decreased by a few ppbvduring the period of the "Little Ice Age"

Fig ure 1.13: Atmo spheric measurements of nitrous oxide fromthe NOAA/GMCC network (Elkins and Rossen, 1989)

3 5 0

Q.D.Co 300ooO 2 7 5

25 0

Khali l and Rasmussen (1988 b)0 Ethendge, Pearman and de Silva (1988) Zardini , Raynaud, Scharffe and Seller (1989)

500 1000 1500Date of sample (Year AD) 2000

Fig ure 1.14: Nitrous oxide measurem ents from ice-core samples

sys tems a re cons ide red to be sma l l (E lk ins e t a l . , 1978 ,Blackmer and Bremner , 1976) .

1.5.2 Sinks for Nitrous OxideT h e m a j o r a t m o s p h e r i c l o s s p r o c e s s f o r N 2 O i sphotochemica l decompos i t ion in the s t r a tosphe re , and i scalcula ted to be 10 3 Tg N pe r year (Table 1 4) Nitro usoxide has an a tm osp heric l ife t ime ol about 150 years Th eo b s e iv e d u i t e of g r o w th r e p r e s e n t s a 3 0 % imb a l a n c ebe tween the so urce s and s ink s (H ao c t al , 1987 )Troposphenc sinks such as surface loss in aquatic and soil

7.5.3 Sources of Nitrous Oxide15 3 1 OceansThe oceans a re a s ign i f ican t , bu t no t dominant source ofN2O (McElroy and Wofsy , 1986) Based on measurem entso l the concent ra t ion grad ien ts be tween the a tmosphe re andsur face wa te r s (But le r e t a l . , 1990 , and NO AA G M C Cunpubl ished da ta ) , and on e s t ima tes o f the gas exchangecoe f f ic ien t , the cur ren t e s t ima te of the magni tude of the


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26 Gi eenhouse Gases and Aei asols 1Table 1 4 Estimated Sow c es and Sinks ofNiti ous O xide

Range(TgN per year)

SourceOceans 14 - 2 6Soils (tropical forests) 2 2 - 3 7

(temperate forests) 0 7 - 1 5Combustion 0 1 - 0 3Biomass burning 0 02 - 0 2Fertilizer (including ground-water) 0 01 - 2 2TOTAL 4 4 10 5SinkRemoval by soils ">Photolysis in the stratosphere 7 -1 3Atmospheric Increase 3 4 5

ocean source ranges from 1 4 - 2 6 Tg N per year,significantly lower than earlier esti ma tes (Elk ins et al ,1978, Cohen and Gordon, 1979, Ch ne et al , 1987) Anaccurate determination of the

ipcc far wg i chapter 01

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