Current land models accurately reflect the observed global net land carbon sink. While all models agree that increased atmospheric CO2 and nitrogen deposition boost carbon stocks, this is partially offset by climate change and land-use/land-cover change (LULCC). However, there's disagreement on the sink's partitioning between vegetation and soil, with models differing even on the direction of carbon stock change over the past 60 years. This uncertainty stems from plant productivity, allocation, and turnover responses to atmospheric CO2 (and to a lesser extent LULCC), and soil responses to LULCC (and to a lesser extent climate). Differences in turnover account for about 70% of model variation in both vegetation and soil carbon changes. Further examination of internal plant and soil cycling is needed to reduce uncertainties.

Michael O’Sullivan, Pierre Friedlingstein, Stephen Sitch, Peter Anthoni, Almut Arneth, Vivek K. Arora, Vladislav Bastrikov, Christine Delire, Daniel S. Goll, Atul Jain, Etsushi Kato, Daniel Kennedy, Jürgen Knauer, Sebastian Lienert, Danica Lombardozzi, Patrick C. McGuire, Joe R. Melton, Julia E. M. S. Nabel, Julia Pongratz, Benjamin Poulter, Roland Séférian, Hanqin Tian, Nicolas Vuichard, Anthony P. Walker, Wenping Yuan, Xu Yue, Sönke Zaehle