Millennial-Scale Land–Climate Interactions Using a Deepened CLM5 Lower Boundary: Permafrost Evolution, Vegetation Dynamics, and Coupled Feedback in CESM2

Permafrost evolution and subsurface thermal dynamics play a key role in the climate system, yet their representation in Earth System Models (ESMs) remains constrained by shallow soil configurations. This study investigates millennial-scale land–climate interactions using simulations with the Land Surface Model CESM2/CLM5 employing a modified deep lower boundary extending to 500 m, alongside the standard 43 m configuration. Simulations span 500–2014 CE and are driven by boundary conditions obtained from two NorESM1-F PARCIM experiments over the same time interval. These experiments follow PMIP last-millennium protocols and include prescribed variations in solar irradiance, volcanic forcing, and greenhouse gas concentrations, while anthropogenic aerosol and land cover are held fixed at pre-industrial conditions. Two solar forcing reconstructions are used, representing low and high solar variability, enabling assessment of how land model depth and natural external forcing shape subsurface thermal states, permafrost extent, active-layer thickness, and soil carbon evolution on centennial to millennial timescales. Simulations using the deep land configuration exhibit reduced variability in the simulated permafrost area during the pre-industrial period (500–1850 CE) relative to the standard shallow configuration. Evaluation against observational and reanalysis-based datasets over 1997–2021 indicates closer agreement for the high solar variability forcing than for the low variability forcing with respect to permafrost extent and subsurface thermal conditions. High-latitude vegetation responses are further explored using modified CLM-FATES configurations. Finally, selected CAM–CLM coupled simulations are used to assess land–atmosphere feedback and to examine how differences between deep and shallow land states propagate into future projections under SSP scenarios from 2015 to 2100.