Exploring the impact of overshoot-and-drawdown scenario on land carbon cycle feedbacks

We used the U.S. Department of Energy Exascale Earth System Model (E3SM) to explore land carbon sinks and their changes over space and time over the 21st century, under strong mitigation scenarios with significant overshoot and drawdown of atmospheric CO2 concentrations. We examined the influence of changing CO2 concentration on land carbon sink fertilization, and the interactions of these dynamics with land use and land cover change. We used the GCAM multisector dynamics model to estimate future scenarios, and evaluated consistency of climate feedbacks assumed by GCAM and those estimated prognostically with E3SM. We found that global land carbon uptake responds to drawdown concentration scenario with a variety of time lags related to carbon storage in woody pools and slowly cycling soil organic matter pools. Nutrient dynamics played an important role in controlling the land carbon-concentration feedback. As CO2 concentrations begin to fall during drawdown, plant gross primary production and plant demand for mineral nitrogen decline. This tends to alleviate nitrogen limitation in systems that had previously been experiencing high limitation under rising atmospheric CO2. Net land uptake is nearly neutral through the 21^st century under an overshoot and drawdown scenario.