Global energy budget changes from underestimated land heat uptake in CMIP6 models

Under current climate-change conditions, the energy imbalance at the top of the atmosphere results in an uptake of energy by the Earth system. Previous efforts have identified the magnitude and proportions of this energy excess and how it is distributed among the different components of the climate system. However, the bulk of the Earth System Models (ESMs) participating in CMIP5/6 deliver Earth energy inventory estimates that differ substantially from recent observations. Particularly for the land component, there is a significant underestimation of simulated continental energy uptake, which was hypothesized to be caused by too shallow land surface model components in current-generation ESMs. Support for the latter was given by previous modeling estimates based on analytical heat conduction models and standalone land surface model simulations. Here we use a suite of current-generation fully-coupled CMIP6 ESMs and a version of the MPI-ESM that includes a deep land model component, accommodating the required space for increased terrestrial energy storage. The simulations show that a sufficiently deep land model leads to more realistic subsurface energy storage - correlating with model depth rather than climate sensitivity, and an adjusted estimate of energy uptake ratios among the Earth system components compared to observational estimates. However, the impact of changes in the land energy budget from the perspective of the entire Earth system appears to have only a marginal influence due to its relatively small fraction of the Earth energy inventory.