Process-dependence of Carbon-cycle feedback parameters in UKESM

We use the UK’s full complexity Earth System Model, UKESM, to examine the impact of five Earth system processes in isolation on the climate-carbon cycle feedback parameters. A control configuration of the model, and five experimental configurations which differ from the control by the inclusion or exclusion of one process, have been used to perform the standard idealized experiments necessary to diagnose the carbon-concentration and carbon-climate feedback parameters, Beta and Gamma respectively. By comparison with the control, the impact of each process on Beta and Gamma can be examined. The processes under consideration are: nitrogen limitation of land carbon uptake; dynamic vegetation; fire-vegetation interactions; interactive wetland methane emissions; and interactive emissions of biogenic volatile organic compounds with interactive diffuse fraction of photosynthetically active radiation. Using the same simulations we examine the impact of each process on the Transient Climate Response to cumulative Emissions (TCRE) evaluated at twice the pre-industrial atmospheric CO2 concentration. By comparing the TCRE of each experimental configuration with the control, we evaluate the influence of each process on the TCRE. We use this to derive a new value of the TCRE for eleven CMIP6 ESMs, estimating what it might be if each ESM were to include all five of the processes under consideration.