Implications of biosphere variability for future emission budgets and carbon dioxide removal

The biosphere’s first-order response to changing Earth system conditions shifts under future emission pathways. One can anticipate such low-order responses, like rebounding carbon stocks once emissions diminish when forecasting emission budgets. However, the impact of changes in second- and higher-order biosphere variability on emission pathways and the prospective large-scale artificial carbon dioxide removal (CDR) remains unclear. An example of such higher-order responses is the vulnerability of land carbon uptake to more extreme climate forcing. In addition, implementing CDR has notable implications for land use, exerting an influence on spatial and temporal biosphere variability. Thus, constraining the interplay between the biosphere’s variability and the emission pathway could inform future emission accounting.

Here, we leverage state-of-the-art Earth system model simulations to investigate the magnitude and pathway-dependency of interactions between the terrestrial biosphere’s variability and the emission pathway. We characterize biosphere variability under different emission scenarios and with varying degrees of representing CO₂ removal. The emission- and concentration-driven simulations cover pathways that reach Paris targets without and with temperature overshoot. CDR is either implicitly represented in emission and land use scenarios or explicitly simulated in the model’s land component to match the respective socio-economic pathway.

To understand the structure of modeled biosphere variability under the different pathways and test the consistency of the joint model system, we investigate regional events like a vegetation expansion event in the Northern Sahara. Here, the objective is to examine the interplay between terrestrial carbon fluxes and CDR utilization in detail on a smaller scale, later expanding to the global level. The following research focuses on identifying and quantifying shifts in biosphere variability over time, their interplay with CDR measures, and their effects on global carbon stocks. We test the model’s sensitivity to design choices (how and where CDR is represented) and pathway (emission target, timing, and duration of temperature overshoot). To this end, we aim to infer the margin of error biosphere variability could cause in emission accounting. Our results will help to evaluate the significance of varying biospheric carbon fluxes for future emission stock-taking in the context of CDR.