Comparing Future Scenarios of Carbon Emissions/Land Use and Land Cover Change on the Carbon Sink for the Conterminous U.S.

The precursor study used constant land use and land cover change (LULCC) for the future to demonstrate the importance of accounting for historical land-use legacy.  The age distribution of trees in the forest depends upon the history of past disturbances, while the nutrients in the soil depend up past LULC.  Thus establishing the correct initial state of the vegetation and soil is crucial to model accurately the effect of biogeochemical cycling with environmental change in the future.  This study starts with initial conditions averaged and condensed for each plant functional type (PFT) from a historical run from 1750 to 2014 using the Land Use Harmonization dataset (LUH2) of land use transitions with the Terrestrial Ecosystems Model (TEM) for the conterminous U.S.  LULCC is treated using a cohort approach, in which a separate cohort occurs every year there is a land use transition, thereby ensuring proper age structure of forests and regrowth with the correct soil nutrients. Future experiments consider the effect of LULCC with and without CO₂ fertilization from 2015 to 2099 using the SSP370 and SSP245 scenarios from the NCAR CESM2 model, resulting in eight experiments.    

The SSP245 scenario accumulates 603 ppm CO₂ vs 867 ppm for SSP370 by 2099, and only half a degree (^oC) cooler over the conterminous U.S., with slightly more precipitation.  The SSP370 results in less forest, shrublands, urban areas, and timber harvest, but more grasslands, croplands, and pasturelands than the SSP245.  The resultant effect of the LULCC on carbon by 2099 (accounting for product decomposition and conversion fluxes) is more net accumulated carbon in the land (6.3 vs 2.4 PgC) in SSP245, while without LULCC there is more accumulated carbon (22.6 vs 18.1 PgC) in SSP370, which is consistent with the higher CO₂ fertilization effect.  There is more soil moisture and runoff for SSP245 both with and without LULCC.  The effect of climate alone (without CO₂ or LULCC) is more accumulated carbon (9.4 vs 4.8 PgC) with the SSP245 climate, while the CO₂ effect is much stronger in SSP370 (17.8 vs 8.7 PgC), and the effect of LULCC reduces carbon more in SPP370 by -20.1 vs -11.8 PgC.  Therefore the effect of the loss of forest in particular for SS370 contributes to its greater carbon loss, while the larger CO₂ levels are an offsetting factor, and the less extreme climate of SSP245 is favorable for that scenario.  LULCC has the largest effect on carbon accumulation, resulting in a loss, while both CO₂ and climate contribute to net carbon uptake by the land.