Modeling the future carbon sink: Land‐use and climate change may offset CO₂ fertilization in the United States

Societal Impact Statement The terrestrial carbon sink currently consumes about a third of the CO 2 released to the atmosphere by fossil fuel emissions and land‐use change. In light of future carbon emissions, it is important to understand how that carbon sink will change in order to better determine how much CO 2 remains in the atmosphere. This study uses a modeling approach to compare a middle‐of‐the‐road (SSP245) versus rocky road (SSP370) scenario for the conterminous United States. While both scenarios result in a carbon sink by the end of the century, it is larger in SSP245, due to less deforestation and more moderate climate. Summary This modeling study explores how elevated atmosphere CO 2 , climate warming, and land use and land cover change (LULCC) will affect the direction and magnitude of the terrestrial carbon sink during the 21st century for the conterminous United States. Initial conditions are averaged and condensed for each plant functional type (PFT) from a historical run from 1750 to 2014 with the Terrestrial Ecosystems Model (TEM). Future experiments consider the effect of climate change, LULCC, and the CO 2 fertilization effect (CFE) from 2015 to 2099 using the SSP245 and SSP370 scenarios from the National Center for Atmospheric Research Community Earth System Model (NCAR CESM2) model. The resultant effect on accumulated net carbon exchange (NCE) is 12.2 PgC for SSP245 versus 2.4 PgC for SSP370. Separating out the effects of climate, LULCC, and CFE results in a negative effect from both climate (−1.6 vs. −14.6 PgC) and LULCC (−5.6 vs. −15.4 PgC) and a positive effect from CFE (8 vs. 17.1 PgC) for SSP245 versus SSP370. The effect on water dynamics is that the SSP370 climate results in more evapotranspiration (ET) with less soil moisture and runoff, while the LULCC effect of SSP370 results in lower ET with more soil moisture and runoff. Both scenarios produce lower ET with elevated CO 2 , and more soil moisture and runoff. The SSP245 scenario has more afforestation and less deforestation than the SSP370 scenario, providing more potential carbon offsets for the voluntary carbon market. This study highlights the carbon and water benefits of a more sustainable LULCC scenario.