Role of Earth system processes in the Transient Climate Response to cumulative Emissions

Estimates of remaining carbon emissions budgets to limit global warming to 1.5°C or 2°C rely on the near-linear relationship between the change in global mean temperature and total CO₂ emitted since the pre-industrial era. This relationship is known as the Transient Climate Response to cumulative Emissions (TCRE). Previous estimates of TCRE are derived from Earth System Models (ESMs) which are known to lack key processes that affect warming and therefore diagnosed CO₂ emissions. Here we use the UK Earth System Model to quantify, for the first time, the impact on TCRE of including six Earth system processes in isolation (results in parenthesis): fire-vegetation interactions (TCRE increased 14.6%); nitrogen limitation of vegetation (+9.7%); diffuse radiation effects on vegetation (+8.5%); changes in vegetation distribution (-1.5%); climate impacts from wetland methane emissions (+5.1%) and from biogenic volatile organic compounds (-1.4%). From these results we recalculate the TCRE of 11 ESMs of the 6th Coupled Model Intercomparison Project (CMIP6) as though each included all six processes. Averaged over the 11 models, TCRE increased by 23.7%, reducing by 19% the associated remaining carbon budget to both 1.5°C and 2°C.