Trends and Drivers of Terrestrial Sources and Sinks of Carbon Dioxide: An Overview of the TRENDY Project

Abstract

Abstract The terrestrial biosphere plays a major role in the global carbon cycle, and there is a recognized need for regularly updated estimates of land_atmosphere exchange at regional and global scales. An international ensemble of Dynamic Global Vegetation Models (DGVMs), known as the �Trends and drivers of the regional scale terrestrial sources and sinks of carbon dioxide� (TRENDY) project, quantifies land biophysical exchange processes and biogeochemistry cycles in support of the annual Global Carbon Budget assessments and the REgional Carbon Cycle Assessment and Processes, phase 2 project. DGVMs use a common protocol and set of driving data sets. A set of factorial simulations allows attribution of spatio_temporal changes in land surface processes to three primary global change drivers: changes in atmospheric CO 2 , climate change and variability, and Land Use and Land Cover Changes (LULCC). Here, we describe the TRENDY project, benchmark DGVM performance using remote_sensing and other observational data, and present results for the contemporary period. Simulation results show a large global carbon sink in natural vegetation over 2012�2021, attributed to the CO 2 fertilization effect (3.8 � 0.8 PgC/yr) and climate (_0.58 � 0.54 PgC/yr). Forests and semi_arid ecosystems contribute approximately equally to the mean and trend in the natural land sink, and semi_arid ecosystems continue to dominate interannual variability. The natural sink is offset by net emissions from LULCC (_1.6 � 0.5 PgC/yr), with a net land sink of 1.7 � 0.6 PgC/yr. Despite the largest gross fluxes being in the tropics, the largest net land_atmosphere exchange is simulated in the extratropical regions.