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Amazonia as a carbon source linked to deforestation and climate change

Wetlands are the largest global source of atmospheric methane (CH), a potent greenhouse gas. However, methane emission inventories from the Amazon floodplain, the largest natural geographic source of CH in the tropics, consistently underestimate the atmospheric burden of CH determined via remote sensing and inversion modelling, pointing to a major gap in our understanding of the contribution of these ecosystems to CH emissions. Here we report CH fluxes from the stems of 2,357 individual Amazonian floodplain trees from 13 locations across the central Amazon basin. We find that escape of soil gas through wetland trees is the dominant source of regional CH emissions. Methane fluxes from Amazon tree stems were up to 200 times larger than emissions reported for temperate wet forests and tropical peat swamp forests, representing the largest non-ebullitive wetland fluxes observed. Emissions from trees had an average stable carbon isotope value (δC) of -66.2 ± 6.4 per mil, consistent with a soil biogenic origin. We estimate that floodplain trees emit 15.1 ± 1.8 to 21.2 ± 2.5 teragrams of CH a year, in addition to the 20.5 ± 5.3 teragrams a year emitted regionally from other sources. Furthermore, we provide a 'top-down' regional estimate of CH emissions of 42.7 ± 5.6 teragrams of CH a year for the Amazon basin, based on regular vertical lower-troposphere CH profiles covering the period 2010-2013. We find close agreement between our 'top-down' and combined 'bottom-up' estimates, indicating that large CH emissions from trees adapted to permanent or seasonal inundation can account for the emission source that is required to close the Amazon CH budget. Our findings demonstrate the importance of tree stem surfaces in mediating approximately half of all wetland CH emissions in the Amazon floodplain, a region that represents up to one-third of the global wetland CH source when trees are combined with other emission sources.

From 2000 until January 2010 vertical profiles were collected above eastern Amazonia to help determine regional‐scale (∼105–106 km2) fluxes of carbon cycle‐related greenhouse gases. Samples were collected aboard light aircraft between the surface and 4.3 km and a column integration technique was used to determine the CO2 flux. Measured CO2 profiles were differenced from the CO2 background determined from measurements in the tropical Atlantic. The observed annual flux between the coast and measurement sites was 0.40 ± 0.27 gC m−2 d−1 (90% confidence interval using a bootstrap analysis). The wet season (January–June) mean flux was 0.44 ± 0.38 gC m−2 d−1 (positive fluxes defined as a source to the atmosphere) and the dry season mean flux was 0.35 ± 0.17 gC m−2 d−1 (July–December). The observed flux variability is high, principally in the wet season. The influence of biomass burning has been removed using co‐measured CO, and revealed the presence of a significant dry season sink. The annual mean vegetation flux, after the biomass burning correction, was 0.02 ± 0.27 gC m−2 d−1, and a clear sink was observed between August and November of −0.70 ± 0.21 gC m−2 d−1 where for all of the dry season it was −0.24 ± 0.17 gC m−2 d−1.

51Understanding tropical rainforest carbon exchange and its response to heat and 52 drought is critical for quantifying the effects of climate change on tropical ecosystems, 53including global climate-carbon feedbacks. Of particular importance for the global 54 carbon budget is net biome exchange of CO2 with the atmosphere (NBE), which 55 represents non-fire carbon fluxes into and out of biomass and soils. Sub-annual and sub-56Basin Amazon NBE estimates have relied heavily on process-based biosphere models, 57 despite lack of model agreement with plot-scale observations. We present a new analysis 58 of airborne measurements that reveals monthly, regional-scale (~1 -8 x 10 6 km 2 ) NBE 59variations. We develop a regional atmospheric CO2 inversion that provides the first 60 analysis of geographic and temporal variability in Amazon biosphere-atmosphere carbon 61 exchange and that is minimally influenced by biosphere model-based first guesses of 62 seasonal and annual-mean fluxes. We find little evidence for a clear seasonal cycle in 63Amazon NBE but do find NBE sensitivity to aberrations from long-term mean climate. In 64 particular, we observe increased NBE (more carbon emitted to the atmosphere) 65 associated with heat and drought in 2010, and correlations between wet season NBE and 66 precipitation (negative correlation) and temperature (positive correlation). In the eastern 67 Amazon, pulses of increased NBE persisted through 2011, suggesting legacy effects of 68 2010 heat and drought. We also identify regional differences in post-drought NBE that 69 appear related to long-term water availability. We examine satellite proxies and find 70 evidence for higher gross primary productivity (GPP) during a pulse of increased carbon 71

We present an assessment of methane (CH 4 ) atmospheric concentrations over the Amazon Basin for 2010 and 2011 using a 3-D atmospheric chemical transport model, two wetland emission models, and new observations made during biweekly flights made over four locations within the basin. We attempt to constrain basin-wide CH 4 emissions using the observations, and since 2010 was an unusually dry year, we assess the effect of this drought on Amazonian methane emissions. We find that South American emissions contribute up to 150 ppb to concentrations at the sites, mainly originating from within the basin. Our atmospheric model simulations agree reasonably well with measurements at three of the locations (0.28 ≤ r 2 ≤ 0.63, mean bias ≤ 9.5 ppb). Attempts to improve the simulated background CH 4 concentration through analysis of simulated and observed sulphur hexafluoride concentrations do not improve the model performance, however. Through minimisation of seasonal biases between the simulated and observed atmospheric concentrations, we scale our prior emission inventories to derive total basin-wide methane emissions of 36.5-41.1 Tg(CH 4 )/yr in 2010 and 31.6-38.8 Tg(CH 4 )/yr in 2011. These totals suggest that the Amazon contributes significantly (up to 7%) to global CH 4 emissions. Our analysis indicates that factors other than precipitation, such as temperature variations or tree mortality, may have affected microbial emission rates. However, given the uncertainty of our emission estimates, we cannot say definitively whether the noncombustion emissions from the region were different in 2010 and 2011, despite contrasting meteorological conditions between the two years.

The Amazon Basin is an important region for global CH4 emissions. It hosts the largest area of humid tropical forests, and around 20% of this area is seasonally flooded. In a warming climate it is possible that CH4 emissions from the Amazon will increase both as a result of increased temperatures and precipitation. To examine if there are indications of first signs of such changes we present here a 13 year (2000–2013) record of regularly measured vertical CH4 mole fraction profiles above the eastern Brazilian Amazon, sensitive to fluxes from the region upwind of Santarém (SAN), between SAN and the Atlantic coast. Using a simple mass balance approach, we find substantial CH4 emissions with an annual average flux of 52.8 ± 6.8 mg CH4 m−2 d−1 over an area of approximately 1 × 106 km2. Fluxes are highest in two periods of the year: in the beginning of the wet season and during the dry season. Using a CO:CH4 emission factor estimated from the profile data, we estimated a contribution of biomass burning of around 15% to the total flux in the dry season, indicating that biogenic emissions dominate the CH4 flux. This 13 year record shows that CH4 emissions upwind of SAN varied over the years, with highest emissions in 2008 (around 25% higher than in 2007), mainly during the wet season, representing 19% of the observed global increase in this year.

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