Assessing terrestrial carbon fluxes and stocks in South America and its major biomes using CMIP6 Earth System Models

Abstract. South America plays a significant role in the global carbon cycle, with its ecosystems storing substantial amounts of carbon in vegetation and soils. This study analyses the behaviour of components of the carbon cycle and stocks in the whole continent and two of its major biomes, the Amazon and the Savannas, using a set of 18 Earth System Models (ESMs) from the Coupled Model Intercomparison Project Phase 6 (CMIP6). We discuss the variability of the model simulations throughout the 20th and first 20 years of the 21st centuries. Results show that South America accounts for 25–30 % of the global Gross Primary Productivity (GPP), 21–28 % of the global Net Primary Productivity (NPP), 17–50 % of the global Net Ecosystem Productivity (NEP), and 15–30 % of the global Net Biome Productivity (NBP), and also contributes significantly to global autotrophic (Ra) and heterotrophic (Rh) respiration. The temporal evolution of NBP in South America indicates a combination of the values estimated for the Amazon and the Savannas, with most models showing a small decreasing trend in the 20th century, likely dominated by emissions from land use change, and shifting to positive values after 1990, likely driven by an increasing productivity in response of atmospheric CO₂ fertilization. Comparing the Amazon and Savannas, apart from the magnitude of the fluxes, we see similarities in both ecosystems responses when widespread dry years occur, with higher NBP and GPP in wet years and higher Rh and disturbances in dry years. These results highlight the vulnerability of South America to climate change, with the potential for parts of the continent to shift from carbon sinks to sources under widespread droughts.