Emulating absent land processes reduces carbon budgets in CMIP6 models

The Transient Climate Response to cumulative Emissions (TCRE) is the cornerstone of estimates of carbon emissions budgets to future warming targets, characterising the near-linear relationship between the increase in global mean temperature and cumulative emissions of CO 2 . TCRE is derived from highly sophisticated Earth System Models (ESMs) - some, however, lack important processes known to have a large impact on TCRE. Here, we use multiple configurations of UKESM, a full-complexity ESM, to estimate how TCRE of 11 ESMs that participated in Coupled Model Intercomparison Project 6 would change if each included six terrestrial carbon cycle related processes present in UKESM, such as nitrogen limitation of photosynthesis and fire-vegetation interactions. We find that accounting for processes not included in the models increases their TCRE by an average of 21.4% implying a reduction of 16.0% ± 0.8% in the carbon budget for the transition from 1.5 °C to 2 °C.