Earth System Models are not capturing present-day tropical forest carbon dynamics

Published: 23-10-2020| Version 1 | DOI: 10.17632/frhkj7vxbs.1
Alexander Koch


Tropical forests play a key role in absorbing carbon from the atmosphere into the land surface. Recent analyses of 565 long-term forest inventory plots across Africa and Amazonia show that structurally intact tropical forest are a large carbon sink, but that this sink has recently saturated and is projected to be in long-term decline. Here we compare these results with estimates from the two most recent generations of Earth System Models, CMIP5 (19 models) and CMIP6 (17 models). While CMIP5 and CMIP6 are of similar skill, they do not reproduce the observed 1985-2014 carbon dynamics. The pan-tropical net sink from inventory data is 0.99 Pg C yr-1 (95% CI 0.7-1.3) between 2000-2010, the best sampled decade, double the CMIP6 multimodel-mean of 0.45 Pg C yr-1 (95% CI 0.35-0.55) over the same decade. The observed saturating and declining sink is not captured by the models, which show modest increases in sink strength. The future pan-tropical net sink from the statistical model decreases by 0.23 Pg C per decade (95% CI 0.09-0.39) until the 2030s, while CMIP6 multimodel-means project an increasing carbon sink under all scenarios (0.01-0.03 Pg C per decade; 95% CI 0.00-0.06) except the low-CO2 scenario (-0.02 Pg C per decade; 95% CI -0.01--0.03). CMIP models reproduce the response of carbon gains from tree growth to environmental drivers, but the modelling of carbon losses from tree mortality does not correspond well to the inventory data. Reason for the model-observation differences is the treatment of mortality in models. This repository contains the R code to reproduce all CMIP figures.