Enhanced Methane Emissions during Amazonian Drought

12 Pages·2016·1.64 MB·English

RESEARCHARTICLE
Enhanced Methane Emissions during
Amazonian Drought by Biomass Burning
MakotoSaito*☯,Heon-SookKim☯,AkihikoIto,TatsuyaYokota,ShamilMaksyutov
CenterforGlobalEnvironmentalResearch,NationalInstituteforEnvironmentalStudies,16-2Onogawa,
Tsukuba,Ibaraki305-8506,Japan
☯Theseauthorscontributedequallytothiswork.
*[email protected]
Abstract
a11111
TheAmazonisasignificantsourceofatmosphericmethane,butlittleisknownaboutthe
sourceresponsetoincreasingdroughtseverityandfrequency.Weinvestigatedsatellite
observationsofatmosphericcolumn-averagedmethaneforthe2010droughtandsubse-
quent2011wetyearintheAmazonusinganatmosphericinversionscheme.Ouranalysis
indicatesanincreaseinatmosphericmethaneoverthesouthernAmazonregionduringthe
drought,representinganincreaseinannualemissionsrelativetothewetyear.Weattribute
OPENACCESS
theincreasetoemissionsfrombiomassburningdrivenbyintensedrought,combinedwith
Citation:SaitoM,KimH-S,ItoA,YokotaT, carbonmonoxideshowingseasonalvariationscorrespondingtomethanevariations.We
MaksyutovS(2016)EnhancedMethaneEmissions
showthatthereisprobablyastrongcorrespondencebetweendroughtandmethaneemis-
duringAmazonianDroughtbyBiomassBurning.
PLoSONE11(11):e0166039.doi:10.1371/journal. sionsintheAmazon.
pone.0166039
Editor:YiguoHong,CAS,CHINA
Received:April21,2016
Accepted:October21,2016 Introduction
Published:November16,2016 TheAmazonregionplaysanimportantroleintheglobalcarboncycleasalargeanddynamic
Copyright:©2016Saitoetal.Thisisanopen reservoiroforganiccarbonstoredinforestbiomassandsoil[1],andasasignificantsourceof
accessarticledistributedunderthetermsofthe atmosphericmethane(CH ),accountingfor*10%ofatmosphericcontributionsglobally[2].
4
CreativeCommonsAttributionLicense,which TheAmazonregionsufferedseveredroughtsin2005and2010,causedbyfactorsrelatedto
permitsunrestricteduse,distribution,and
higherthanaverageseasurfacetemperaturesinthetropicalNorthAtlantic,weakertrade
reproductioninanymedium,providedtheoriginal
winds,lowerthanaveragewatervaportransportintothesouthernAmazonregion,anda
authorandsourcearecredited.
weakeningofupwardatmosphericflowovertheregion[3].Thedroughteventsillustratethe
DataAvailabilityStatement:Allrelevantdataare
vulnerabilityofforeststoclimatevariationsandtheimpactofintensemoisturedeficitsoncan-
withinthepaperanditsSupportingInformation
opystructure,leadingtoasignificantnetlossofcarbon[4].Theriskandoccurrenceofhigh-
files.
intensityforestfiresisincreasedduringdroughts,thusacceleratingcarbonemissions[5].Car-
Funding:ThisstudywassupportedbyGrantA-
bondioxide(CO )emittedtotheatmosphereisbalancedinpartbysubsequentregrowthof
2
1202ofEnvironmentResearchandTechnology
vegetation,whereasCH releaseisnot.Climatemodelspredictthattheintensityandfrequency
DevelopmentFund,MinistryofEnvironmentJapan. 4
ofAmazoniandroughtswillincreaserapidlyinthe21stcentury[6].Therefore,analysesofthe
Thefundershadnoroleinstudydesign,data
collectionandanalysis,decisiontopublish,or 2005and2010droughtscouldprovideabetterunderstandingoftheroleofAmazonianforests
preparationofthemanuscript. intheglobalcarboncycleundernear-futureclimateconditions.
AmazonianwetlandsareaprimarysourceofatmosphericCH [7].TheAmazonianCH
CompetingInterests:Theauthorshavedeclared 4 4
thatnocompetinginterestsexist. budgetincludesmultiplesourcesandsinks(i.e.,riversanduplandsoils)whosecontributions
PLOSONE|DOI:10.1371/journal.pone.0166039 November16,2016 1/12
MethaneEmissionsduringAmazonianDrought
aresensitivetofeedbackfromdroughtconditions.Fieldmeasurementsunderexperimental
droughtconditionsprovideestimatesofthedegreeofsensitivityonsomesources.However,
significantgapsremaininourunderstandingofhowCH budgetsintheAmazonregionwill
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respondtodroughtevents,inpartbecauseobservationsrepresentativeoftheentireregionare
lacking,whicharerequiredtoreliablyassessregional-scaleCH budgets.
4
Here,weexaminetherelativeimportanceofdroughtontheCH budgetintheAmazon
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region,withafocusonmajoremissionsources.Weusedretrievedatmosphericcolumn-aver-
agedCH (XCH ;V02.21)levelsfromGreenhousegasesObservingSATellite(GOSAT)mea-
4 4
surements[8]for2010and2011;in2010,theAmazonexperiencedaseveredrought,whereas
2011wasrelativelywet[5].TheGOSATmeasurementsprovideuniformglobalcoverageand
thusshowastandardizeddistributionofCH concentrationsovertheAmazon(Fig1a),allow-
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inganassessmentofpotentialimpactsofAmazoniandroughtontheCH budget.
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Methods
UsingaBayesianinversionscheme,surfaceCH fluxestimatesfor43regionsgloballywere
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optimizedbymatchingXCH retrievalsandsurfaceobservations.Weexcludedthenorthern
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AmazonregionfromtheanalysesbecauseonlyafewXCH retrievalswereobtainedforthis
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region,owingtothepresenceofstationaryclouds(Fig1a).However,thesignatureofthe2010
droughtremainedstrongintheXCH retrievaldata,asthe2010droughtwasparticularly
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severeinthesouthernAmazonregion(Fig1b).
OverviewofGOSATmeasurements
TheGOSAThasbeenmonitoringatmosphericCO andCH concentrationsfromspacesince
2 4
itslaunchinJanuary2009[11].MeasurementsaretakenbyanonboardThermalAndNear-
infraredSensorforcarbonObservation(TANSO),consistingofaFourierTransformSpec-
trometer(FTS)andaCloudandAerosolImager(CAI)[12].TheTANSO-FTSobservesinten-
sitiesofsunlightreflectedoremittedfromtheEarth’ssurfaceandatmosphereinfourspectral
bands:0.758–0.775,1.56–1.72,1.92–2.08,and5.5–14.3μm.Thenadirfootprintdiameterof
theTANSO-FTSis10.5km.TheTANSO-CAIrecordsanimageoftheatmosphereandthe
Earth’ssurfaceinfourspectralbands,fromthenear-ultraviolettonear-infrared:0.370–0.390,
0.664–0.684,0.860–0.880,and1.56–1.65μm,atspatialresolutionsof0.5,0.5,0.5,and1.5km,
respectively.TheTANSO-CAIdataareusedforcloudscreeningoftheTANSO-FTSdata.We
usedXCH valuesfromshort-wavelengthinfraredradiancespectrafromTANSO-FTS
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bands1–3[8].
Atmosphericinversions
SurfaceCH fluxestimatesareoptimizedtodetectatmosphericCH concentrationsbymini-
4 4
mizingthedifferencebetweenobservedandmodeledvalues,J(s),usingaBayesianinversion:
1
JðsÞ¼ fðz(cid:0) HsÞTR(cid:0) 1ðz(cid:0) HsÞþðs(cid:0) s ÞTQ(cid:0) 1ðs(cid:0) s Þg ð1Þ
2 p p
wherezrepresentsanobservationvector;Hisamatrixofresponseateachmeasurementsite
toemissionsfromeachregion;sands arevectorsofestimatedandaprioriestimatedregional
p
fluxes,respectively;RandQareerrorcovariancematricesforthemodel–datamismatchanda
prioriflux,respectively;andTisthetransposeofamatrix.TheelementsofHwerecalculated
usingtheatmospherictracertransportmodelNIES-TM[13].Thevectoroftheposteriorflux
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MethaneEmissionsduringAmazonianDrought
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MethaneEmissionsduringAmazonianDrought
Fig1. (a)DistributionofaveragedconcentrationsofXCH (ppb)fromobservationaldataacquiredbyGOSAT
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forAugustandSeptember2010ata1˚×1˚gridresolution,with479and1582observedconcentrationsfor
thesouthernAmazonandcentralSouthAmericaregions,respectively.Thetworegionsanalyzedareshown
indarkgreen(southernAmazonregion)andlightgreen(centralSouthAmericaregion).(b)Distributionofthe
standardizedReconnaissanceDroughtIndex(RDI )inOctober2009–September2010calculatedusingthe
std
methodof[9]withmonthlyprecipitationandpotentialevapotranspirationdatafromtheClimateResearchUnit
(CRU),UniversityofEastAnglia,UnitedKingdom,using0.5˚×0.5˚meanmonthlyclimatologyCRUTS3.22
[10].BothXCH concentrationsandRDI valuesarecolorcoded.
4 std
doi:10.1371/journal.pone.0166039.g001
s0anditscovariancematrixQ0aregivenby
s0 ¼s þðHTR(cid:0) 1HþQ(cid:0) 1Þ(cid:0) 1HTR(cid:0) 1ðz(cid:0) HsÞ ð2Þ
p
Q0 ¼ðHTR(cid:0) 1HþQ(cid:0) 1Þ(cid:0) 1 ð3Þ
TheinversematricesinEqs(2)and(3)weresolvedusingLUfactorization.Weestimated
monthlyCH fluxesfor43regions,including42landregionsatasub-continentalscaleand
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oneglobaloceanregion(FigAinS1File).Adetaileddescriptionoftheinversionschemeis
givenby[14].
Observationsofsingle-shotXCH retrievalsandindividualground-basedobservations
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fromtheWorldDataCenterforGreenhouseGases(WDCGG)[15]andamonitoringproject
ofgreenhousegasesoverSiberia[16]wereintegratedintheinversion(FigBinS1File).Biases
inXCH retrievalswerecorrectedtolatitudinalandmonthlyground-basedobservationsusing
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asecond-orderpolynomialapproximation[17](themeanbiaswasassumedtobe-5.9ppb,
withastandarddeviationof12.6ppb(V02.XX)[8]).TheuncertaintyoftheXCH retrievals
4
wassetasthedifferencefromthepolynomial,withaminimumvalueof12ppb[18].The
uncertaintiesoftheground-basedobservationdataweregivenbythemeanresidualstandard
deviation(RSD)oftheGLOBALVIEW-CH 2009dataset[19],withminimumvaluesofthe
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flasksamplingandcontinuousmeasurementdatasetas6and20ppb,respectively.
TheinversionschemeconsistsoftheNIES-TM,amoduleforCH destructionprocesses
4
causedbyhydroxylfreeradicals(OH)[20],andafixed-lagKalmansmootherwitha4-month
lagwindow[21].TheNIES-TMwasimplementedusinga2.5˚×2.5˚horizontalgridresolu-
tionand32verticallevels,usingahybridsigma-isentropiccoordinatesystem.TheNIES-TM
wasdrivenusing6-hourlyclimateforcingsfromtheJapanMeteorologicalAgencyClimate
DataAssimilationSystem(JCDAS)[22].
Theapriorifluxeswerecomposedofthreegroupsofemissionsources:anthropogenic,nat-
ural,andbiomassburning(TableAinS1File).Theanthropogenicsourceswerebasedonthe
emissioninventoriesoftheEmissionDatabaseforGlobalAtmosphericResearch(EDGAR)
v4.2[23],andincludefuelcombustionandfugitiveemissionsfromfuels,industrialprocesses,
andproductuse,aswellasagriculturalsources,includinganimalwasteandentericfermenta-
tion,wastetreatment,andfossilfuelfires,butexcludingricepaddies.Naturalsourcesinclude
emissionsfromwetlands,ricepaddies,andsoilsourcesandsinks,whichwerebasedonthe
VegetationIntegrativeSImulatorforTracegases(VISIT)terrestrialbiospheremodel[24],and
alsoincludeemissionsfromtermites,whichwerebasedonthetracertransportmodelGISS
[25].BiomassburningwasbasedontheGlobalFireEmissionsDatabase(GFED)v3.1product
[26].Theapriorimonthlyfluxesofthesethreesourcegroupswerepreparedata1˚×1˚spatial
resolution,andeachgroupwasoptimizedforthe43regionsintheinversionscheme.Uncer-
taintiesintheapriorifluxesweresetto20%ofthevaluesfortheanthropogenicandbiomass
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MethaneEmissionsduringAmazonianDrought
burningsourcesand50%forthenaturalsources.Afterinversion,theoptimizedemission
sourcesinthefourregions(Numbers9,10,15and16inFigAinS1File)ofSouthAmerica
wereaggregatedovertwolargeregions:thesouthernAmazonandcentralSouthAmerica.
ResultsandDiscussion
ThedryseasoninthesouthernAmazonoccursbetweenJulyandOctober.Thelargestpositive
temperatureandthelargestnegativeprecipitationanomaliesbetween2010and2011occurred
inAugustandSeptember2010[5].TheXCH retrievalsforthisperiodshowstronglatitudinal
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gradients(Fig1a)duetodifferencesbetweenCH emissionratesintheNorthernandSouth-
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ernHemispheres.ThisagreeswithpreviousmeasurementsfromtheSCanningImaging
AbsorptionSpectroMeterforAtmosphericCHartographY(SCIAMACHY)satellite[27].As
confirmedbyinsitusurfacemeasurements[28],longitudinalgradientswerealsoobservedin
theXCH retrievals.InthesouthernAmazon,low-levelnortheasterlyairflowsweredomi-
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natedbyyear-roundtradewindinflowsacrossthenortheasterncoastofBrazil,flowingfrom
thetropicalNorthAtlanticregion[29].LevelsofCH inthesouthernAmazonweregenerally
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enhancedcomparedwiththoseinthecentralSouthAmericaregion,indicatingthecontribu-
tionsofupwindemissionsources.
Fig2showstherelativeeffectsofdroughtonatmosphericCH concentrations,bycompar-
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ingthevariabilityofXCH withthatofabackgroundregion.TheAtlanticareabetween
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0–20˚Sand20–50˚Wwasdeterminedasthebackgroundregionforthedominanttradewind.
AftercomparingXCH withthebackgroundobservationstoobtaintherelativeeffects
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(ΔXCH ),wefoundastatisticallysignificantdifference(P<0.01)betweentheXCH variabil-
4 4
ityforJuly–Octoberin2010and2011inboththesouthernAmazonandthecentralSouth
Americaregions.MonthlyaveragedΔXCH valuesincreasedfromJulytoSeptember2010in
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bothregions,withmedianΔXCH valuesof+14.8and+7.6ppbforthesouthernAmazonand
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thecentralSouthAmericaregions,respectively.Incontrast,during2011theΔXCH levels
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increasedinthesouthernAmazonregioninAugustanddecreasedinSeptember.Additionally,
inthecentralSouthAmericaregion,changesinΔXCH levelsduring2011werenotdetect-
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able.Theanalysissuggeststhatthe2010droughtenhancedCH emissionsinthesouthern
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Amazonregion.
TheinversemodelshowsthatdifferencesinXCH variabilitybetween2010and2011were
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closelyrelatedtoyear-to-yearchangesinemissionsfrombiomassburning(Fig3).Inthe
southernAmazonregion,estimatedCH emissionsfrombiomassburningduring2010were
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sixtimeshigherthanin2011(7.0±1.0versus1.1±0.2TgCH yr−1,respectively,oralterna-
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tivelygivenas1.46±0.21versus0.24±0.04gCH m−2yr−1,respectively).Thisresultedina
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22%increaseinannualCH fluxesin2010relativetothosein2011(26.6±7.5versus
4
21.8±6.4TgCH yr−1,respectively)despiteareductioninnaturalemissions(16.5±6.0versus
4
17.5±5.6TgCH yr−1,respectively)andnochangesinanthropogenicemissions(3.2±0.5Tg
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CH yr−1).EnhancedemissionsfrombiomassburningwerealsodetectedinthecentralSouth
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Americaregionin2010comparedwith2011(1.5±0.3verses0.5±0.1TgCH yr−1,respec-
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tively),butacomparableincreaseinannualCH fluxesin2010(43.9±11.2TgCH in2010
4 4
verses45.0±10.9in2011TgCH )wasnotdiscernablebecause2011includedincreasednatu-
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ralemissions.AnnualCH fluxesoverthenorthernAmazonregionin2010and2011were
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estimatedas23.9and23.0TgCH yr−1,respectively,forreference.In2010,theposteriorfluxes
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ledtoincreasesintotalemissionsof17%and9%inthesouthernAmazonandthecentral
SouthAmericaregions,respectively,comparedwiththeapriorifluxes(22.6±8.7and
40.0±13.6TgCH yr−1,respectively)(TableAinS1File).
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MethaneEmissionsduringAmazonianDrought
Fig2.TimeseriesofCH andCOconcentrationsfor2010and2011.(a)VariabilityinXCH
4 4
concentrations(ppb)inthesouthernAmazonregion(darkgreencircles),centralSouthAmericaregion(light
greencircles),andbackgroundregion(oceanareabetween0˚–20˚Sand20˚–50˚W;redline).Dailydataof
thebackgroundregionwereconstructedfromdiscreteXCH datausingaHummingwindowwithawindow-
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widthof80days.Thestandarderrors(onesigma)ofXCH dataaregivenbythegrayboundary.The
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discontinuityatthebackgroundregionforMay22–June20in2011reflectsalackofXCH data.(b)Monthly
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variabilityinXCH values(ppb)withthebackgroundsubtracted(ΔXCH)inthesouthernAmazonregion(dark
4 4
greenbox-and-whiskers)andthecentralSouthAmericaregion(lightgreenbox-and-whiskers).Upperand
lowervaluesoftheboxesaremonthlyquartilesofΔXCH ,barsthroughtheboxesaremedians,andwhiskers
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representtherangeofthe10thand90thpercentiles(dataarenotshownformonthsinwhichthenumberof
datapointswas<25).(c)Sameas(b),butshowingsurfaceCOconcentrations(ppb)fromtheMOPITTL3
product.
doi:10.1371/journal.pone.0166039.g002
InthesouthernAmazonregion,theCH emissionratefrombiomassburningin2010
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exceededpreviousestimatesfortheLegalAmazonregionbetween1992and1993of0.84g
CH m−2yr−1[30],andforthenorthernAmazonin2001–2005of0.20gCH m−2yr−1(assum-
4 4
ingthatfireoccursononethirdofthedaysduringthe4-month-longdryseason)[28],andare
comparabletolocal-scaleemissionsforprimaryAmazonianforestbetween1986and1992
(*2gCH m−2yr−1)[31].Recently[32]estimatedthebiomassburningemissioninthesouth-
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ernAmazonregionduring2010tobe6.8TgCH yr−1,usingaCH assimilationsystemCar-
4 4
bonTracker-CH .Additionally,[33]reportedthebiomassburningemissionoftheAmazon
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Basinduring2010and2011tobe5.4and0.5TgCH yr−1,respectively,basedontheGFED
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v3.1product.
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MethaneEmissionsduringAmazonianDrought
Fig3.TotalannualCH flux(upper)andrelativecontributionsofanthropogenic(magenta),natural
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(blue),andbiomass-burning(red)fluxes(lower)inthesouthernAmazonregion(left)andcentral
SouthAmericaregion(right)for2010and2011(inTgCH region−1yr−1).Errorbarsrepresentflux
4
uncertainties.
doi:10.1371/journal.pone.0166039.g003
TheprobabilitydistributionforXCH estimatedwiththeseposteriorfluxes(PST)for
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2010–2011wascomparedwithXCH retrievalsusingtheχ2goodness-of-fittest,aswellas
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thoseestimatedwithapriorifluxes(APR)andposteriorfluxesforinversionusingground-
basedobservationsonly(GB)(FigCinS1File).ThetestforXCH wasdividedinto14classes
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andyieldsχ2=151,643forXCH _APR,χ2=1051forXCH _GB,andχ2=451forXCH _PST.
4 4 4
Theestimateduncertaintyoftheaveragetotalemissionsfor2010–2011inthesouthernAma-
zonandthecentralSouthAmericaregionswas7.9and13.3TgCH yr−1forAPR,7.7(-2%for
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APR)and13.0TgCH yr−1(-2%)forGB,and7.0(-12%)and11.0TgCH yr−1(-16%)for
4 4
PST,respectively.Theseχ2statisticsanduncertaintyreductionsindicatethatthesatelliteand
ground-basedobservationsconstrainthetotalCH budgetmoretightlythantheground-
4
basedobservationsalone,duetotheirregionalandglobalcoverage.Theprecisionimprove-
mentoffluxestimatesprovidesconfidenceinourinterpretationofobservedXCH variability
4
relativetopreviousstudiesthatonlyusedground-basedobservations.Therefore,theimpor-
tanceofsatelliteobservationsforerrorreductionininversemodelingisconfirmed,especially
overregionswithfewerground-basedobservations,suchasthetropics[34].
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MethaneEmissionsduringAmazonianDrought
However,thevalueofχ2=451showsthattheXCH _PSTdistributionissignificantlydiffer-
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enttothatoftheXCH retrievals(P<0.01),likelyduetoacombinationoffactors.Thesam-
4
plingofsatelliteobservationsinthesouthernAmazonregionisrestrictedinthedryseason
andthespatialdistributiondiffersbetween2010and2011.Thesespatio-temporalsampling
biasesmaydisturbannualandinterannualbalancesbetweenthedryandwetseasonsinthe
posteriorfluxes[35].TheXCH retrievalsalsohaveahigherscatterthanground-basedobser-
4
vationsbecauseoftheinherentlimitationsofretrievalalgorithms[36]andthedetectionper-
formanceofsatelliteinstruments[12],leadingtoerrorsinposteriorfluxes.Errorsalsooccur
inthemodels,suchasuncertaintiesinapriorifluxesandintheatmospherictransportmodel.
Forinstance,theaprioritotalannualfluxesacrossthesouthernAmazonandcentralSouth
Americaregionsfor2010werehigherthanthoseofCarbonTracker-CH (14.1and36.3Tg
4
CH yr−1,respectively),althoughposteriorfluxesinCarbonTracker-CH wereclosertoour
4 4
results(19.9and45.8TgCH yr−1).Weassignedarelativelysmallmagnitudetothepriorflux
4
uncertaintyofbiomassburningemissionsinordertohandleprominentfireeventsinthe2010
dryseason.Thismeansthattheposteriorfluxofbiomassburninglargelyreliedonthegiven
priorflux.Thechoiceofpriorfluxuncertaintiesiscriticalforposteriorfluxese.g.,[37],and
previousstudieshaveinvestigatedmethodstocharacterizetheuncertaintye.g.,[38–40].How-
ever,statisticsrelatedtotheuncertaintyarenotfullyunderstoodandremainamajorchallenge
ininversionschemes.
TohelpinterpretdifferencesinXCH variabilitybetween2010and2011anddependence
4
ofposteriorfluxesonpriorones,werantheinversemodelwithidenticalpriorfluxesand
uncertainties(TableBinS1File).Insteadofpriorfluxeswithinterannualvariations,average
seasonalcycleofindividualemissionsourcesfortheperiodof2000–2004wereusedasidenti-
calpriorfluxesforboth2010and2011.Uncertaintiesintheidenticalpriorfluxesweresetto
20%ofthevaluesfortheanthropogenicsourcesand50%forthenaturalandbiomassburning
sourcesbasedonGOSATLevel4CH dataproductscheme.Theresultsshowenhancementof
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totalemissionsof1.4and2.6TgCH yr−1inthesouthernAmazonandcentralSouthAmerica
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regionsin2010comparedwith2011.Theenhancementismainlyrelatedtochangesinnatural
emissionsourcesinposteriorfluxesduetothelargestuncertaintyofpriorfluxesinthree
sourcegroups,andthereisnocleardifferenceinbiomassburningemissionsbetween2010
and2011.Althoughtheinversemodelwithidenticalpriorfluxesperformlesswellinthistest
asshowninsmalluncertaintyreductions,thetestrevealsthelimitationofourabilitytoquan-
tifybiomassburningemissionsduringAmazoniandroughtin2010.Astheseissuesassociated
witherrorsinposteriorfluxesaredifficulttotakeintoaccountandarekeylimitationsinthis
study,itshouldbeemphasizedthattheposteriorfluxescanvarywiththedistributionofsatel-
litesampling,theretrievalalgorithms,andthemodelsthatwereused.
Toestimatebiomassburningemissionswithadifferentway,weanalyzedcarbonmonoxide
(CO)concentrationsretrievedfromtheMeasurementsOfPollutionInTheTroposphere
(MOPITT)satelliteobservations[41](Fig2c),asCOservesasaproxyforCH emittedduring
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smoldering[42].MonthlyCOlevelsincreasedmarkedlyinthesouthernAmazonregionin
2010,peakingat290–312ppb(medianmonthlyvalues)inAugustandSeptember.Addition-
ally,seasonalvariationsinthetworegionscorrespondtotheΔXCH variationsforthetwo
4
years.Thesedatasupportthecorrelationbetweenhigh-intensityfiresandenhancedCH emis-
4
sionsbybiomassburninginthesouthernAmazonregionduringthe2010drought.Smoke
fromfiresintheregionwasalsodetectablebyvisualinspectionofcoarse-pixelGOSATcloud
imagerydata(FigDinS1File).
Globally,wetlandCH emissionsemergeastheprimarydriverofvariabilityinatmospheric
4
CH [43].Inwetlands,thewatertabledepthisakeydeterminantofemissionrates,alongwith
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temperatureandsubstrateavailability[44].Increasedlevelsofsolarradiationduringdroughts
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maypartlyincreaseCH emissionsfromwetlands,byincreasingtheamountofsubstrateavail-
4
ableformethanogenesisduetoAmazonianforestsgreen-up[45]andincreasingtemperature
inthesoilandwater.However,duringthe2010drought,lowwaterlevelscausedbyunusually
lowprecipitationresultedinasmallerfloodedarea[46],thuspromotingaerobicsoilcondi-
tionsandreducingCH production.Inourestimate,thereducedCH emissionsfromwet-
4 4
landscausedbythedroughtoccurredbeyondthedryseasoninthesouthernAmazonregion
(10.2±4.2and11.1±3.9TgCH for2010and2011respectively).Consequently,wetland
4
dynamicsisunlikelytoexplaintheenhancedCH emissionsin2010.Driersoilconditions
4
maystimulatetermiteactivitybypresentinggreatervolumesofdecayingrootbiomass,possi-
blyleadingtoincreasedCH emissionstotheatmosphere.However,acurrentestimateof
4
globalemissionsfromthissourceisonly*10TgCH yr−1,andtheemissionsinSouthAmer-
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icaaccountforabout30%ofthistotal[2,44].Thus,thiscontributionalonecannotexplainthe
increasedCH emissionsof*4.8TgCH yr−1inthesouthernAmazonregionin2010.
4 4
AnthropogenicemissionsareanotherimportantsourceofatmosphericCH .InBrazil,
4
whichconstitutesthegreaterpartofthesouthernAmazonregion,anthropogenicCH sources
4
aredominatedbyemissionsfromruminants,whichin2005accountedfor57%(11.5TgCH
4
yr−1)oftotalCH emissions[47].InBrazil,bothtotalandruminantemissionsdecreased
4
slightlyfrom2005to2010,by6%and13%,respectively,probablybecauseoftheglobaleco-
nomicrecession.Overall,anthropogenicemissionsappeartohaveaminorinfluenceonthe
CH budgetinthesouthernAmazonregion.
4
Summary
AlthoughnodefiniteconclusionsaboutCH emissionsarepossiblebecauseoftheuncertainty
4
inourestimates,ouranalysesraisethepossibilitythatseveredroughteventsin2010influenced
CH emissionsintheAmazon.Thisisapparentasenhancedbiomassburningrelatedtoland
4
clearingandpasturemaintenance[48].TheCH emittedfrombiomassburningaccumulates
4
asgreenhousegasesintheatmosphere,whichmayfurtherenhancetheseverityandfrequency
ofdrought(althoughdroughtcanalsocauseadeclineingreenhousegasemissionsfromtropi-
calsoils,viachangesinsoilredoxpotentialandnutrientavailability[49]).Thesatelliteobser-
vationsprovideasolutionformappingXCH distributionsovertheAmazonregion,allowing
4
improvementstoourknowledgeofthecarbonbudget.Improvementsindataacquirement
overcloud-coveredareascouldleadtofurtherconstraintsoftheCH budgetintheregion.
4
TheoveralleffectsofAmazoniandroughtongreenhousegasbudgetsarenotyetfullyunder-
stood.Furtherresearch,suchasmulti-siteinvestigationsoffireregimesandpreciseground-
basedobservationsofatmosphericCH emissions,arekeytoimprovingourunderstanding
4
thecontributionofAmazoniandroughttogreenhousegasemissions.
SupportingInformation
S1File.Thesupplementarymaterialsforthisstudy.
(PDF)
Acknowledgments
ThisstudywassupportedbyGrantA-1202ofEnvironmentResearchandTechnologyDevel-
opmentFund,MinistryofEnvironmentJapan.WethankI.Morino,O.Uchino,Y.Yoshida,
T.Machida,M.Sasakawa,theGOSATprojectbyNIES,CarbonTracker-CH ,CRU,EDGAR,
4
GFED,GLOBALVIEW,JCDAS,MOPITT,andWDGCCforprovidingdata.GOSATdatawas
PLOSONE|DOI:10.1371/journal.pone.0166039 November16,2016 9/12
MethaneEmissionsduringAmazonianDrought
obtainedfromtheGOSATUserInterfaceGatewayathttp://www.gosat.nies.go.jp/en/.Map-
toolusestheGenericMappingTools,version5.2.1(http://gmt.soest.hawaii.edu/).
AuthorContributions
Conceptualization:MS.
Formalanalysis:MSHSK.
Fundingacquisition:SMTY.
Investigation:MS.
Methodology:MSHSKAISM.
Projectadministration:SMTY.
Resources:SM.
Supervision:SMTY.
Validation:HSK.
Visualization:MS.
Writing–originaldraft:MS.
Writing–review&editing:MS.
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