Modeling the combined environmental effects on net land carbon flux in the present and future scenarios
Anthropogenic activities such as fossil fuel production and Land Use and Land Use Changes (LULCC) have been contributing to the sources for the atmospheric carbon dioxide (CO2) in the global carbon cycle. The terrestrial ecosystem globally absorbs about 30% of the anthropogenic CO2 emissions as a sink. Land therefore consists of two counteracting CO2 fluxes, a source from LULCC and a sink from the terrestrial ecosystem. This net land carbon flux (e.g., Net Biome Production, NBP) is the most uncertain component of the Global Carbon Budget (GCB). Hence, it is important to understand the major drivers of sinks and sources of CO2 in order to reduce the uncertainty of GCB. This study quantifies the combined environmental effects (atmospheric CO2, nitrogen (N) deposition, climate change, and LULCC) on NBP and the relative contribution of individual environmental effects in the present decade (2009~2018) and the last decade (2090~2099) of the 21st century by using a global land surface model, Integrated Science Assessment Model (ISAM). The model is driven by historical observation data and future projection data from Scenario Model Intercomparison Project (ScenarioMIP) of Coupled Models Intercomparison Project Phase 6 (CMIP6). Two combinations of Shared Socioeconomic Pathways (SSP) and radiative forcing (SSP2-45 and SSP5-85) are used to force the model. SSP2-45 has radiative forcing of 4.5 W/m2 and Middle of the Road of SSP. SSP5-85 has radiative forcing of 8.5 W/m2 and Fossil Fueled Development of SSP. The modeled simulation results show that the land continuously acts as a net sink of CO2 in the future scenarios. The estimated combined environmental effects on the mean NBP for the 2090s under SSP2-45 and SSP5-85 are 0.75 PgC/yr and 3.47 PgC/yr, respectively from the value of 0.24 PgC/yr for the 2010s. The atmospheric CO2 + N deposition effect contributes to the net sink of CO2 for the present decade (3.8 PgC/yr) and the last decade of the future scenarios (SSP2-45: 3.79 PgC/yr and SSP585: 10.49 PgC/yr). The greatest sinks correspond to forest areas in the tropics, which show higher CO2 fertilization and lesser N limitations than the extra-tropics. The LULCC effect (-2.54 PgC/yr) dominates over the atmospheric CO2 + N deposition effect in the present decade. This leads to the net source of CO2 in some tropical and extra-tropical regions. However, in the 2090s, warmer climate under two future scenarios leads to more CO2 release (SSP2-45: -1.68 PgC/yr and SSP5-85: -5.64 PgC/yr) to the atmosphere than due to LULCC effect (SSP2-45: -1.35 PgC/yr and SSP5-85: -1.37 PgC/yr). Overall, the study emphasizes how the environmental effects drive the temporal and spatial variations of net land carbon flux through their compensation and dominance in the present and future scenarios.
Global carbon budget, net land carbon flux, Terrestrial carbon, Global land surface model, Net Biome Production, Net Ecosystem Production,
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign