Assessment of land energy uptake in the industrial period from observational and model products

Land-air interaction occurs at the ground surface at a wide range of time scales in the form of mass, momentum, and energy exchange. As a result of the water and heat fluxes, the land acts as a water and mostly energy storehouse for the climate system. Last estimates based on multimodel comparisons quantify the land contribution to terrestrial energy budget at about 2 % in the last four decades, whilst other studies based on borehole temperature profiles (BTPs) scale it up to be a 6 %. This uncertainty makes it necessary to explore other data sources to determine the land energy uptake, mostly under the increasing energy imbalance due to the ongoing anthropogenic-induced climate change.

State-of-the-art land surface models (LSMs) resolve a subsurface whose bottom boundary condition placement (BBCP) is not deep enough to correctly represent its thermal structure. This results both in a constrained capability to store energy, and an overestimation of temperature variability and industrial trends with increasing depth. A 2000-year-long forced simulation using a version of the Max Planck Institute (MPI) Earth System Model (ESM), MPI-ESM, including a very deep version of the LSM (BBCP at 1417 m), allows for assessing the behavior of subsurface temperatures and heat storage at long term scales, with a particular focus on the land response to the last century global warming. The analysis also allows for extending the assessment to CMIP6 historical simulations and climate reanalysis data.

Preliminary results show the energy uptaken by the MPI-ESM simulation with a deep version of the LSM is well above the range of values provided by CMIP6 model-based estimates and much closer to the observations. This underlines the great importance of BBCP depth in correctly representing the role of the land component in the terrestrial energy budget.