Anthromes, CO2 and Terrestrial Carbon – Session 2 (part 2): CO2 and land use change as drivers of terrestrial carbon dynamics
Authors: Lapola David , Li Yue 
Published: March 2023

2.00 – 2.20: The Amazon as a showcase on the interplay between CO2 effects, LUC and carbon dynamics , David Lapola

2.20 – 2.40: Contributions of CO 2 -fertilization and deforestation to future climate and carbon storage change in the Amazon, Yue Li

The Amazon as a showcase on the interplay between CO2 effects, LUC and carbon dynamics

In this presentation I show the indissociable relation between land-use change, CO2 effects and carbon dynamics in the Amazon anthrome. I explore how uncertainties in the region’s major anthropogenic processes, namely deforestation, forest degradation, climate change and CO2 fertilization, are shaping the existence of the world’s largest tropical forest. Amazon deforestation rates have unexpectedly rebounded in the last 10 years, whereas forest degradation, although more uncertain, seems to affect a forest area and result in carbon emissions of the same magnitude as deforestation. On the other hand, climatic extremes and/or a saturation of CO2 fertilization effect have reduced the carbon sink strength of untouched Amazon forests in 30% since the 1990’s, with related uncertainties to be addressed in the upcoming AmazonFACE (Free-Air CO2 Enrichment) experiment in central Amazon. Altogether, these processes may be pushing the Amazon anthrome closer to the so-called tipping point, with potentially serious consequences for the global climate, socioeconomics, and biodiversity. Finally, I argue how uncertainties related to CO2 effects and land tenure may be decisive for afforestation, REDD+ and BECCS projects in the region.

Contributions of CO2-fertilization and deforestation to future climate and carbon storage change in the Amazon

Climate change is a major threat to the Amazon rainforests. Despite the hypothesis that future atmospheric CO2 increase fertilizes rainforest and the fact that human land use (e.g., deforestation) causes a net carbon source, carbon storage changes in the Amazon due to biophysical climate effects from plant physiological response to CO2 and deforestation are often neglected in carbon accounting systems. Here, we identify the contributions of atmospheric CO2 concentration and deforestation to future Amazonian climate and carbon storage change by combining Earth system model simulations and satellite-derived estimates of aboveground biomass. Forcings from CO2-fertilization and deforestation are found to linearly decrease annual precipitation and increase surface air temperature in the Amazon. We further build a simple machine learning model, by which climate changes by CO2-fertilization and deforestation are converted into values of vegetation carbon storage. Due to a larger biomass carbon sensitivity to precipitation (about 3.4 Mg C ha-1 / 100 mm yr-1), future carbon storage in the Amazon is projected to decline due to increased water stress caused by the CO2-fertilization and deforestation. Our results highlight the importance of considering the indirect carbon impacts from climate change by CO2-fertilization and human land use for achieving a net-zero motivated future.