Forestation in CMIP6: wide model spread in tree cover and land carbon uptake

Forestation is expected to play an significant role as a terrestrial carbon dioxide removal technology in low-emission scenarios by storing carbon in the biosphere, thereby changing the physical properties of the land surface. To represent land use change, including afforestation and reforestation (AR), Earth system models (ESM) that contribute to the Coupled Model Intercomparison Project Phase 6 (CMIP6) draw on common projected land use data from Integrated Assessment Models. The extent and spatial distribution of AR differ substantially between the CMIP6 models ranging from −197 to 363 Mha of tree cover change by 2100 in the low-emission scenario SSP1-2.6 that has the highest AR among all future scenarios. The variability in simulated tree cover distributions, in combination with different representations of the carbon cycle, causes a high uncertainty in future land carbon uptake. Here, we disentangle the input information used to represent AR by CMIP6 models and differences in the carbon uptake process to explain the variable simulated potential of carbon sequestration due to AR. We provide recommendations on how AR might be implemented more consistently in future model intercomparison studies, especially regarding more consistent tree cover input (e.g. aligning present-day tree cover, consistent transition between land use and land cover types), and carbon-cycle-related processes (e.g. nitrogen cycle and disturbances). Adoption of these recommendations would increase the relevance of ESMs in terms of providing more accurate estimates of future land carbon uptake through AR.