Gatti et al. 2014

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Drought sensitivity of Amazonian carbon balance revealed by atmospheric measurements

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<ul><li><p>LETTERdoi:10.1038/nature12957</p><p>Drought sensitivity of Amazonian carbon balancerevealed by atmospheric measurementsL.V.Gatti1*,M.Gloor2*, J. B.Miller3,4*, C. E.Doughty5, Y.Malhi5, L. G.Domingues1, L. S. Basso1, A.Martinewski1, C. S. C. Correia1,V. F. Borges1, S. Freitas6, R. Braz6, L. O. Anderson5,7, H. Rocha8, J. Grace9, O. L. Phillips2 &amp; J. Lloyd10,11</p><p>Feedbacks between land carbonpools and climate provide oneof thelargest sources of uncertainty in our predictions of global climate1,2.Estimates of the sensitivity of the terrestrial carbon budget to cli-mate anomalies in the tropics and the identification of the mechan-isms responsible for feedback effects remainuncertain3,4.TheAmazonbasin stores a vast amount of carbon5, and has experienced increas-ingly higher temperatures and more frequent floods and droughtsover the past two decades6. Here we report seasonal and annualcarbon balances across the Amazon basin, based on carbon dioxideand carbon monoxide measurements for the anomalously dry andwet years 2010 and 2011, respectively. We find that the Amazonbasin lost 0.486 0.18 petagrams of carbon per year (PgC yr21)during the dry year but was carbon neutral (0.066 0.1 PgC yr21)during the wet year. Taking into account carbon losses from fire byusing carbon monoxide measurements, we derived the basin netbiome exchange (that is, the carbon flux between the non-burnedforest and the atmosphere) revealing that during the dry year, vege-tationwas carbonneutral.During thewet year, vegetationwas a netcarbon sink of 0.2560.14PgCyr21, which is roughly consistent withthemean long-termintact-forestbiomass sinkof0.3960.10PgCyr21</p><p>previously estimated fromforest censuses7.Observations fromAma-zonian forest plots suggest the suppression of photosynthesis dur-ing drought as the primary cause for the 2010 sink neutralization.Overall, our results suggest that moisture has an important role indetermining the Amazonian carbon balance. If the recent trend ofincreasing precipitation extremes persists6, the Amazonmay becomean increasing carbon source as a result of both emissions from firesand the suppression of net biome exchange by drought.To observe the state, changes and climate sensitivity of the Amazon</p><p>carbon pools we initiated a lower-troposphere greenhouse-gas sam-pling programmeover theAmazonbasin in 2010,measuring bi-weeklyvertical profiles of carbon dioxide (CO2), sulphur hexafluoride (SF6)and carbonmonoxide (CO) from just above the forest canopy to4.4 kmabove sea level (a.s.l.) at four locations spread across the basin (Fig. 1).Repeated measurements of the CO2 mole fraction in the low to mid-troposphere have the ability to constrain surfaceCO2 fluxes at regionalscales (about 105106 km2) including all knownandunknownprocesses.This is in contrast to small temporal8,9 and spatial10,11 scale atmosphericapproaches, which need substantial and difficult-to-verify assumptionsto scale up; it is also in contrast to basin-scale surface-based studies,which include only a subset of relevant processes3,12,13.Our selection of sites reflects the dominant mode of horizontal air</p><p>flow atmid- to low-troposphere altitudes across theAmazon basin, withair entering the basin from the equatorial Atlantic Ocean, sweeping</p><p>over the tropical forested region towards theAndes and turning south-wards and back to theAtlantic (Fig. 1). Air at the end-of-the-basin sitesTabatinga (TAB) andRioBranco (RBA) is thus exposed to carbon fluxesfrom a large fraction of the basins rainforest vegetation. Flux signatures</p><p>*These authors contributed equally to this work.</p><p>1Instituto de Pesquisas Energeticas e Nucleares (IPEN)Comissao Nacional de Energia Nuclear (CNEN)Atmospheric Chemistry Laboratory, 2242 Avenida Professor Lineu Prestes, Cidade Universitaria,Sao Paulo CEP 05508-000, Brazil. 2School of Geography, University of Leeds, Woodhouse Lane, Leeds LS9 2JT, UK. 3Global Monitoring Division, Earth System Research Laboratory, National Oceanic andAtmospheric Administration, 325Broadway, Boulder, Colorado 80305, USA. 4Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado 80309, USA.5Environmental Change Institute, School of Geography and the Environment, University of Oxford, South Parks Road, Oxford OX1 3QY, UK. 6Center for Weather Forecasts and Climate Studies, InstitutoNacional de Pesquisas Espaciais (INPE), Rodovia Dutra, km 39, Cachoeira Paulista CEP 12630-000, Brazil. 7Remote Sensing Division, INPE (National Institute for Space Research), 1758 Avenida dosAstronautas, Sao Jose dos Campos CEP 12227-010, Brazil. 8Departamento de Ciencias Atmosfericas/Instituto de Astronomia e Geofisica (IAG)/Universidade de Sao Paulo, 1226 Rua do Matao, CidadeUniversitaria, Sao Paulo CEP 05508-090, Brazil. 9Crew Building, The Kings Buildings, West Mains Road, Edinburgh EH9 3JN, UK. 10School of Tropical and Marine Biology and Centre for TerrestrialEnvironmental and Sustainability Sciences, James Cook University, Cairns 4870, Queensland, Australia. 11Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot SL5 7PY, Berkshire, UK.</p><p>15</p><p>80 70 60 50Longitude ()</p><p>Latit</p><p>ude </p><p>()</p><p>40 30 20</p><p>80</p><p>0.001 0.002 0.005 0.010</p><p>9 m s1Wind speed</p><p>0.020 0.050 0.100 0.200</p><p>70 60 50 40 30 20</p><p>10</p><p>5</p><p>0</p><p>5</p><p>10</p><p>15</p><p>20</p><p>25</p><p>15</p><p>10</p><p>5</p><p>0</p><p>5</p><p>10</p><p>15</p><p>20</p><p>25</p><p>Surface sensitivity (p.p.m. mol1 m2 s)</p><p>RPB</p><p>TABTABTAB</p><p>SANSANSAN</p><p>RBARBARBA ALFALFALF ASC</p><p>Figure 1 | Stations region of influence (footprint). The combinedsensitivity of all observed atmospheric CO2 concentrations to surface fluxes(that is, measurement footprints) is shown for the four sites TAB, RBA, SANandALF (solid black dots). Sensitivity is given in units of concentration (p.p.m.)per unit flux (mmolm22 s21). As seen in Extended Data Fig. 6a, footprintsfrom the four sites overlap substantially. Footprints are calculated at 0.5-degreeresolution using ensembles of stochastically generated back trajectories usingthe FLEXPART Lagrangian particle dispersion model and then calculatingthe residence times of these back trajectories in the 100m layer above thesurface. Values above 0.001 p.p.m.mmol21m22 s21 comprise 97% of the landsurface signal and values above 0.01 p.p.m.mmol21m22 s21 comprise 50%of the land surface signal; thus apparently small values are still importantbecause they occupy a large area. Black arrows represent average climatologicalwind speed and direction in June, July and August (from the National Centersfor Environmental Prediction (NCEP); http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.html) averaged between the surface and600mbar.Open symbols (RPB andASC) represent theNOAA tropical Atlanticsites used to define the background concentrations of CO2, CO and SF6 cominginto the Amazon basin. Solid green dots indicate the locations of forest plotclusters where long-term biomass gains and respiration have been observed.</p><p>7 6 | N A T U R E | V O L 5 0 6 | 6 F E B R U A R Y 2 0 1 4</p><p>Macmillan Publishers Limited. All rights reserved2014</p></li><li><p>in air at the other two sites, Alta Floresta (ALF) and Santarem (SAN)are not only from forests but also from savanna and agricultural land.Ourmeasurements represent the firstnetworkofongoing,well-calibratedCO2 measurements over a large stretch of tropical land. Such mea-surements are vital, because the near-absence of CO2 measurementssensitive to the tropical biosphere is the underlying cause of the largeuncertainties in net flux estimates for tropical regions obtained by inversemodelling of atmospheric CO2 (refs 14 and 15).Fortuitously, the two years of atmospheric observations reported here</p><p>are for an unusually dry year followed by a wet one (Fig. 2 and ExtendedData Fig. 1a, b). Our measurements thus document the sensitivity ofAmazon basin carbon pools to the effect of drought. The reasons forthe dry conditions in 2010 were twofold. For the first three months anEl Nino episode caused dry conditions in the north and centre of theAmazon basin, whereas during the second half of the year a positiveNorthAtlantic sea surface temperature anomaly locked the inter-tropicalconvergence zone (where the northeast and southeast trade winds con-verge) into a position that was more northerly than usual. This causedenhanced and prolonged dry conditions in the southern areas of theAmazon basin (Extended Data Fig. 1a, b). A simple diagnostic of thestress on vegetation exerted by the negative precipitation anomalies isthe climatological water deficit (CWD16; see Methods and Fig. 2), inwhich in 2010 large negative anomalies occurred for the northwesternbasin. This is consistent with river discharge records17. Lesser negativeanomalies in the northeastern basinwere caused by early-year negativeprecipitation anomalies and the centraleastern and southern parts ofthe Amazon basin (the arc of deforestation) had anomalies caused bylow precipitation during the third quarter of the year. Monthly meantemperatures (ExtendedData Fig. 1c, d) in 2010were higher than aver-age in everymonth, with especially large anomalies in February/Marchand August/September. These mirror the periods of greatest negativeprecipitationanomalies.Warmer thanaverage temperatures (with respectto the last three decades) were also observed for every month of 2011,but 2011 was also an unusually wet year (ExtendedData Fig. 1a, b). Asshownbelow, observed basin-wide carbon flux variations for 2010 and2011 reflect these temporal precipitation patterns.To isolate the contributionofAmazon terrestrial carbon sources and</p><p>sinks to the atmospheric CO2 profiles, we first subtract a scalar back-groundmole fraction fromeachof theobservedprofiles.This background</p><p>represents the composition of air entering the Amazon basin fromthe Atlantic and is estimated as a weighted average of CO2 at Ascen-sion Island (ASC) and Ragged Point, Barbados (RPB) using a linearmixing model based on ASC and RPB SF6 with weights determinedfrom SF6measured at the site1820 (Methods). SF6 is well suited for thispurpose (that is, to estimate the fractional contributions of Northernand SouthernHemispheric air entering the basin) because it has a largeinter-hemispheric difference (ExtendedData Figure 8) and virtually noAmazonian emissions21.Carbon sources and sinks reveal themselves in the referenced pro-</p><p>files DX5Xsite2Xbg as mole fraction enhancements and depletions,where X is the mole fraction of CO2 or CO, for site and background.The enhancements and depletions are generally confined to the low-ermost 2 kmor so of the profiles (Fig. 3). ForDCO2 (Fig. 3ad), there isa strong tendency towards surface enhancements during the dry sea-son, although both lower-troposphere depletions and enhancementscan be observed at any time of the year. Vertical profiles of DCO showvery large enhancements above the Atlantic background in the dryseason, persisting into the free troposphere (Fig. 3eh and ExtendedData Fig. 2). CO is a product of incomplete combustion and in theAmazon it reflects a contribution to CO2 enhancements from biomassburning. This is confirmed by calculated air-mass back-trajectoriesintersecting satellite-sensed fire hotspots (Extended Data Fig. 3) andby our observed CO:CO2 ratios, which are typical for those from trop-ical forest fires (Methods).From the profiles of DX we estimate fluxes by dividing them by the</p><p>air-mass travel time t from the coast to the sampling site and integ-rating from the surface (0 km above ground level, a.g.l.) to 4.4 km a.s.l.determined by air-mass back-trajectories calculated separately for eachof (typically) 12 air samples per profile1820 to obtain:</p><p>FX~4:4 km a:s:l:</p><p>z ~ 0 km (a:g:l:)</p><p>DXt(z)</p><p>dz 1</p><p>Using measured CO:CO2 emission ratios, rbbCO2:CO (refs 9 and 20), wefurther estimate the biomass burning contribution (FbbCO2 ) to the net car-bon flux using:</p><p>FbbCO2~rbbCO:CO2 FCO{F</p><p>bioCO</p><p>" # 2where FbioCO is the stable (background) value of FCO during the wetseason20, reflecting direct plant and soil CO emissions as well as pro-duction from rapid oxidation of biogenic volatile organic compounds22.The non-fire net biome exchange (NBE) flux FNBECO2 is then given by:</p><p>FNBECO2~FtotalCO2{F</p><p>bbCO2 3</p><p>Our flux calculations (Fig. 4 andTable 1) reveal basin-wide average totalfluxes of 0.196 0.07 gCm22 d21 in 2010 and0.026 0.04 gCm22 d1</p><p>in 2011. Riverine carbon outgassing13 is included in these fluxes but con-tributesminimally because the riverineorganic carbon loop is verynearlyclosedwithin theAmazon basin23, and fossil fuel emissions in the basinare negligibly small (,0.02 PgC yr21; seeMethods). Flux uncertaintiespresented in Fig. 4 andTable 1maybe underestimates because of lossesof surface signal above 4.4 km caused by convective processes not cap-tured by our extrapolation technique (Extended Data Table 1a). Ourimperfect knowledge of convection and the difficulty ofmeasuringCO2in the upper troposphere hamper quantification of these errors.Using a basin area of 6.773 106 km2 we calculate a source to the</p><p>atmosphere of 0.486 0.18 PgC in 2010. In contrast, 2011 displayed anapproximately neutral carbon balance (0.066 0.10 PgC yr21). In 2010,we calculate carbon losses due to fires of 0.516 0.12 PgC yr21, imply-ing a carbon-neutral residual (that is, approximately zeroNBE). On theotherhand, for 2011whenNBEwas20.256 0.14PgC yr21, the overallcarbon balance was neutral, because this was offset by fire-associatedlosses of roughly the same size (0.306 0.10 PgC yr21). The return of</p><p>CW</p><p>D (m</p><p>illim</p><p>etre</p><p>s pe</p><p>r mon</p><p>th)</p><p>250</p><p>150</p><p>50</p><p>0</p><p>Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec</p><p>1998201120102011</p><p>a</p><p>Num</p><p>ber o</p><p>f ES</p><p>A fi</p><p>re c</p><p>ount</p><p>s</p><p>0</p><p>1,000</p><p>3,000</p><p>Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec</p><p>19982011 mean20102011</p><p>b</p><p>Year</p><p>Figure 2 | Climatological water deficit. a, Basin-wide averages and standarddeviation of CWD, based on the Tropical Rainfall MeasuringMission28. b, Firecounts based on European Space Agency (ESA; http://due.esrin.esa.int/wfa/)fire count data29 for 2010, 2011 and 19982011, respectively.</p><p>LETTER RESEARCH</p><p>6 F E B R U A R Y 2 0 1 4 | V O L 5 0 6 | N A T U R E | 7 7</p><p>Macmillan Publishers Limited. All rights reserved2014</p></li><li><p>the unburned Amazonian vegetation to being a sink in 2011 seems tohave been driven primarily by precipitation, which changed from anega-tive anomaly in 2010 to a positive anomaly in 2011 (Extended DataFig. 1a, b). However, temperatures were higher than average for bothyears, reflecting a netwarming trend in recent decades (ExtendedDataFig. 1c, d).</p><p>A more detailed picture of the Amazonian carbon cycle response toclimate is revealed by the quarterly fluxes and by focusing first on RBA,TAB and ALF. For both years, during the first quarter of the year (thestart of the wet season), measurements indicate a net carbon sink, andduring the second and drier half of the year, measurements indicate anet source (Fig. 4a). However, during the second quarter of 2010 (in</p><p>TAB 2010 RBA 2010 ALF 2010 SAN 2010</p><p>Alti</p><p>tude</p><p> (km</p><p>)A</p><p>ltitu</p><p>de (k</p><p>m)</p><p>4</p><p>3</p><p>2</p><p>1</p><p>4</p><p>0 50 100</p><p>150</p><p>200</p><p>250</p><p>300 0 50 10</p><p>015</p><p>020</p><p>025</p><p>030</p><p>0 0 50 100</p><p>150</p><p>200</p><p>250</p><p>300 0 50 10</p><p>015</p><p>020</p><p>025</p><p>030</p><p>0</p><p>2 0CO2 (p.p.m.)</p><p>C...</p></li></ul>