The Amazon Carbon Balance: An Evaluation of Methods and Results

A comparison is made between two strategies to characterise the Amazonian carbon budget: (i) the ‘bottom-up’ approach using plot data and remote sensing and (ii) the ‘top-down’ approach using aircraft-based measurements in the planetary boundary layer. Plot data provide insights into processes which aircraft data cannot provide, but upscaling plot data to the entire basin involves many assumptions and uncertainties: it is necessary to estimate the separate fluxes of carbon to represent rates of deforestation, degradation, timber harvests, and other terms and then add them together. As each term is uncertain, the overall uncertainty is considerable. We estimate the carbon budget to be near to zero, the deforestation fluxes being roughly balanced by the growth of old and successional forests in a normal year. Drought may tip the balance to make the basin a strong source of CO2. Aircraft flights have provided a more direct measurement over the Amazon basin area. Flights need to be made several times a year, and the profiles may be compared with concentrations measured in the incoming air from the Atlantic Ocean. Such measurements demonstrate that the vegetation itself is a sink in a normal year, but the basin is near neutral over 12 months because of carbon losses through burning. Overall, the results from aircraft profiles are not very different from the upscaled estimate.