Atmospheric CO 2 inversion reveals the Amazon as a minor carbon source caused by fire emissions, with forest uptake offsetting about half of these emissions

• Published in: Atmospheric chemistry and physics Vol. 23; no. 17; pp. 9685 - 9723
• Main Authors: Basso, Luana S., Wilson, Chris, Chipperfield, Martyn P., Tejada, Graciela, Cassol, Henrique L. G., Arai, Egídio, Williams, Mathew, Smallman, T. Luke, Peters, Wouter, Naus, Stijn, Miller, John B., Gloor, Manuel
• Format: Journal Article
• Language: English
• Published: 01-09-2023
• ISSN: 1680-7324; 1680-7324
• DOI: 10.5194/acp-23-9685-2023

Tropical forests such as the Amazonian rainforests play an important role for climate, are large carbon stores and are a treasure of biodiversity. Amazonian forests have been exposed to large-scale deforestation and degradation for many decades. Deforestation declined between 2005 and 2012 but more recently has again increased with similar rates as in 2007–2008. The resulting forest fragments are exposed to substantially elevated temperatures in an already warming world. These temperature and land cover changes are expected to affect the forests, and an important diagnostic of their health and sensitivity to climate variation is
their carbon balance. In a recent study based on CO2 atmospheric
vertical profile observations between 2010 and 2018, and an air column
budgeting technique used to estimate fluxes, we reported the Amazon region
as a carbon source to the atmosphere, mainly due to fire emissions. Instead
of an air column budgeting technique, we use an inverse of the global
atmospheric transport model, TOMCAT, to assimilate CO2 observations
from Amazon vertical profiles and global flask measurements. We thus
estimate inter- and intra-annual variability in the carbon fluxes, trends
over time and controls for the period of 2010–2018. This is the longest period covered by a Bayesian inversion of these atmospheric CO2 profile observations to date. Our analyses indicate that the Amazon is a small net source of carbon to the atmosphere (mean
2010–2018 = 0.13 ± 0.17 Pg C yr−1, where 0.17 is the 1σ uncertainty), with the majority of
the emissions coming from the eastern region (77 % of total Amazon
emissions). Fire is the primary driver of the Amazonian source (0.26 ± 0.13 Pg C yr−1), while forest carbon uptake removes around half of the fire emissions to the atmosphere (−0.13 ± 0.20 Pg C yr−1). The largest net carbon sink was observed in the western-central Amazon region (72 % of the fire emissions). We find larger carbon emissions during the
extreme drought years (such as 2010, 2015 and 2016), correlated with
increases in temperature, cumulative water deficit and burned area. Despite
the increase in total carbon emissions during drought years, we do not
observe a significant trend over time in our carbon total, fire and net
biome exchange estimates between 2010 and 2018. Our analysis thus cannot
provide clear evidence for a weakening of the carbon uptake by Amazonian
tropical forests.