Using a last 2k baseline to derive a first comprehensive assessment of industrial era land heat uptake

Abstract

The anthropogenically-intensified greenhouse effect causes a radiative imbalance at the top of the atmosphere. This in turn leads to an energy surplus of the Earth system, with the ocean component absorbing the greatest part and the land the second largest. The latest observational estimates based on borehole temperature profiles quantify the land contribution to the terrestrial energy surplus to be 6 % in the last five decades, whereas studies based on state-of-the-art climate models scale it down to 2 %. This underestimation stems from land surface models (LSMs) having a too shallow representation of the subsurface, which severely constrains the land heat uptake simulated by Earth System Models (ESMs). A forced simulation of the last 2000 years with the Max Planck Institute ESM (MPI-ESM) using a deep LSM captures about 4 times more heat than the standard shallow MPI-ESM simulations in the historical period, well above the estimates provided by other ESMs. However, deepening the MPI-ESM LSM does not affect the simulated temperature at the ground surface. As a consequence, it is shown that the land heat uptake values of ESMs with shallow LSM components can be corrected considering their simulated surface temperatures and propagating them with a standalone heat conduction forward model. This result is extended to all available ground surface temperature sources, such as observational data, reanalyses, and the latest generation of ESMs. This new approach yields values of 10-16 ZJ for 1971-2018, which are in close agreement with the values derived from the MPI-ESM deep simulation (12 ZJ), and relatively close to the latest borehole-based estimates (ca. 18 ZJ).