Assessment of land energy uptake in the industrial period from observation and simulation-based products

Under increased warming from ongoing anthropogenic climate change, the land acts as an energy sink for the climate system, interacting with the atmosphere at a wide range of time scales. Based on CMIP multi-model comparisons, the latest estimates of the global energy budget quantify the land contribution to be 2% in the last six decades, whereas other studies based on borehole temperature profiles scale it up to 5%. This discrepancy is suspected to stem from state-of-the-art CMIP land surface models using a shallow zero flux bottom boundary condition placement (BBCP) that severely constrains land energy storage by halting ground heat flux penetration at the BBCP depth and biasing subsurface thermal structure. A 2000-year-long (past2k) forced simulation using a version of the Max Planck Institute (MPI) Earth System Model (ESM) with a deep BBCP (1417 m) was performed to assess the behavior of subsurface temperature and energy storage at long-term scales. Results show that land energy uptake is 4 times higher in a coupled MPI-ESM simulation with a deep version of the land component compared to standard shallow (~10m) simulations. These estimates are well above those provided by CMIP6 models and are much closer to observations, underlining the importance of BBCP-depth in correctly representing the role of the land component in the global energy budget. The results of the analysis of the past2k simulation also allow for deriving reliable estimates of land energy uptake from other observational and reanalysis products as well as providing corrected estimates for the shallow LSM CMIP6 historical and scenario simulations. Land energy uptake estimates rendered from this new approach are much closer to previous BTP-based estimates and agree with the value derived from MPI-ESM deep simulation.