Drought sensitivity of Amazonian carbon balance revealed by atmospheric measurements

Abstract

Feedbacks between land carbon pools and climate provide one of the largest sources of uncertainty in our predictions of global climate1,2. Estimates of the sensitivity of the terrestrial carbon budget to climate anomalies in the tropics and the identification of the mechanisms responsible for feedback effects remain uncertain3,4. The Amazon basin stores a vast amount of carbon5, and has experienced increasingly higher temperatures and more frequent floods and droughts over the past two decades6. Here we report seasonal and annual carbon balances across the Amazon basin, based on carbon dioxide and carbon monoxide measurements for the anomalously dry and wet years 2010 and 2011, respectively. We find that the Amazon basin lost 0.48 ± 0.18 petagrams of carbon per year (Pg C yr−1) during the dry year but was carbon neutral (0.06 ± 0.1 Pg C yr−1) during the wet year. Taking into account carbon losses from fire by using carbon monoxide measurements, we derived the basin net biome exchange (that is, the carbon flux between the non-burned forest and the atmosphere) revealing that during the dry year, vegetation was carbon neutral. During the wet year, vegetation was a net carbon sink of 0.25 ± 0.14 Pg C yr−1, which is roughly consistent with the mean long-term intact-forest biomass sink of 0.39 ± 0.10 Pg C yr−1 previously estimated from forest censuses7. Observations from Amazonian forest plots suggest the suppression of photosynthesis during drought as the primary cause for the 2010 sink neutralization. Overall, our results suggest that moisture has an important role in determining the Amazonian carbon balance. If the recent trend of increasing precipitation extremes persists6, the Amazon may become an increasing carbon source as a result of both emissions from fires and the suppression of net biome exchange by drought.

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Figure 1: Station’s region of influence (‘footprint’).
Figure 2: Climatological water deficit.
Figure 3: Surface flux signals in vertical profiles.
Figure 4: Flux estimates summary.

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Acknowledgements

We thank P. Tans and P. Bakwin, who had the foresight to initiate a long-term high-precision greenhouse gas measurement laboratory in Sao Paulo, and D. Wickland, the NASA programme manager who initially supported this effort. This work has been financed primarily by the UK Environmental Research Council (NERC) via the consortium grant ‘AMAZONICA’ NERC (NE/F005806/1) and also by the State of Sao Paulo Science Foundation (FAPESP) via the ‘Carbon Tracker’ project (08/58120-3), and the EU via the 7th grant framework GEOCARBON project (grant number agreement 283080). NASA, NOAA and IPEN made large contributions to the construction and maintenance of the GHG laboratory in Brazil. Intensive plot measurements were supported by NERC and the Moore Foundation via grants given to RAINFOR. L.G.D., L.S.B., C.S.S.C., V.F.B. and A.M. were supported by CNPq, CAPES, Fapesp and IPEN, and O.L.P. by an ERC Advanced Grant. We thank measurement analysts and scientists at NOAA for providing data, and the pilots who collected the air samples. Numerous people at NOAA, especially A. Crotwell, D. Guenther, C. Sweeney and K. Thoning, provided advice and technical support for air sampling and measurements in Brazil. E. Dlugokencky provided data from Ascension Island and Ragged Point in Barbados. We also thank D. Galbraith for help with the comprehensive forest census plot data and R. Brienen for comments. Finally, we acknowledge S. Denning for reviews of the manuscript.

Author information

L.V.G., M.G., J.B.M., J.L., H.R., O.L.P., Y.M. and J.G. conceived the basin-wide measurement programme and approach. M.G., J.B.M. and L.V.G. wrote the paper. C.E.D. and Y.M. analysed and contributed the data of the comprehensive biometric forests census plots. S.F., R.B., L.O.A., L.G.D. and L.S.B. helped with data analysis. V.F.B., C.S.C.C. and A.M. helped with greenhouse gas concentration analysis. All co-authors commented on the manuscript.

Correspondence to L. V. Gatti or M. Gloor or J. B. Miller.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Amazon climate anomalies in 2010 and 2011.

a, Monthly Southern Hemisphere Amazon basin precipitation from the Global Precipitation Climatology Project (2.5° × 2.5°) for the Southern Hemisphere Amazon basin (accessed from www.esrl.noaa.gov/psd/)44 The red line with diamond data points shows the monthly mean precipitation; the black solid line is the 1981–2010 mean and its standard deviation (dashed black lines) for each month. The grey solid line is the annual mean and its standard deviation (dashed grey lines) for 1981–2010 and the filled red circles are annual averages for 2010 and 2011. b, Precipitation anomalies in 2010 (left) and 2011 (right) calculated as the annual mean differences from the 1981–2010 averages. c, Monthly Southern Hemisphere Amazon basin temperature from the Global Historical Climatology Network version 2 and the Climate Anomaly Monitoring System (0.5° × 0.5°) for the Southern Hemisphere Amazon basin (accessed from www.esrl.noaa.gov/psd/)45. The red line with diamond data points shows the monthly mean temperature; the black solid line is the 1981–2010 mean and its standard deviation (dashed black lines) for each month. The grey solid line is the annual mean and its standard deviation (dashed grey lines) for 1981–2010 and the filled red circles are annual averages for 2010 and 2011. d, Temperature anomalies in 2010 (left) and 2011 (right) calculated as the annual mean differences from the 1981–2010 averages.

Extended Data Figure 2 CO concentrations in 2010 and 2011.

Data are grouped into above and below 1.5 km height above ground measurements for four sites. p.p.b., parts per billion.

Extended Data Figure 3 Air parcel paths to measurement sites.

Mean seven-day back-trajectories from measurement sites (from FLEXPART) during the 2010 dry season months and fire hotspots from ATSR-WFA, from the Data User Element of the European Space Agency29.

Extended Data Figure 4 Flux uncertainty statistics.

a, Sensitivity of flux estimates to profile extrapolation height (months and years are abbreviated below). Comparison of quarterly flux estimates calculated by mass balance of air column up to the top level of measurements (4.4 km a.s.l.), up to 10 km and 8 km a.s.l. during the dry and 12 km during the wet season. b, Distributions of annual net carbon flux estimates obtained with Monte Carlo uncertainty propagation (described above) and 68 and 95 percentile intervals of the mean.

Extended Data Figure 5 Comprehensive forest plot measurement results.

a, Plant carbon expenditure (NPP plus autotrophic respiration, an upper bound on gross primary production) for 14 1-hectare plots where all NPP and autotrophic respiration components are measured. Eight 1-hectare plots did not experience drought (blue line), six experienced drought, three in the dry lowlands (red line), and three in humid lowland regions ±standard error (black line). The black dashed line is the average seasonal value for 2009 (a typical year) repeated through 2010 and 2011. The hatched bar is the mean drought period for the six drought sites, based on CWD. b, Meteorology data from drought plots. Data from Skye instruments meteorology stations from January 2009 to December 2011 near the drought plots (black) for (top left) cumulative water deficit (millimetres per month) and (bottom left) air temperature (in °C). On the right, both plots are the anomalies for the same variable directly to its left with negative values representing a lower than average temperature or precipitation. The hatched bar highlights the approximate period of the 2010 drought in the region based on CWD anomaly. c, Intensive carbon balance forest census sites.

Extended Data Figure 6 Sensitivity of site atmospheric CO2 concentrations to surface fluxes.

a, Sensitivities calculated separately for the four sites (clockwise from the lower left) TAB, RBA, SAN and ALF, and for 2010 calculated with back-trajectory ensembles from the FLEXPART Lagrangian particle dispersion model. The star symbol represents the centroid of the footprint: that is, the point at which footprint contributions are equal to the north and south, and east and west. Note that there is significant overlap of footprints for the 2010 annual mean. b, As for a, but displaying only the tropical forest biome fraction.

Extended Data Figure 7 Geographical Summary for South America.

a, Land cover map of South America from remote sensing (MODIS, Moderate Resolution Imaging Spectroradiometer) obtained from http://modis-land.gsfc.nasa.gov/landcover.html (ref. 46). Black arrows represent average climatological wind speed and direction in June, July and August (from NCEP) averaged between the surface and 600 mbar. b, Population density in South America in the year 2005 (ref. 47).

Extended Data Figure 8 SF6 and Amazon background concentration calculation.

a, SF6 at RPB and ASC and the ‘ASC fraction’ (fASC). Data shown for all Amazonian sites. b, CO2 at RPB and ASC and background values estimated based on in situ SF6 concentrations. Small diamonds (RPB and ASC) represent flask pair averages and thin lines are smooth curve fits to the data33. Filled circles (SAN, ALF, TAB and RBA) represent scalar background values for each Amazonian site determined from the smooth curve fits to ASC and RPB and SF6 values according to equations (4) and (5).

Extended Data Table 1 Annual flux estimate sensitivity results
Extended Data Table 2 Basinwide annual total fluxes

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Gatti, L., Gloor, M., Miller, J. et al. Drought sensitivity of Amazonian carbon balance revealed by atmospheric measurements. Nature 506, 76–80 (2014). https://doi.org/10.1038/nature12957

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