A Comparison of Regional Climate Projections with a Range of Climate Sensitivities

To investigate the extent to which differences in regional model projections can be explained by differences in the warming rates of their driving models, we compare projections of temperature and precipitation over the UK from two regional climate ensembles-the EuroCORDEX multi-model ensemble and UKCP18 perturbed parameter ensemble-along with projections produced by the "parent" GCMs from which boundary conditions were taken. We evaluate the ensembles in terms of their representation of recent climate, then compare the changes simulated between 1981-2010 and 2050-2079. While both ensembles exhibit seasonal biases with similar magnitudes and spatial patterns during the evaluation period, the UKCP18 ensemble exhibits a somewhat stronger change signal in future simulations, due to a combination of higher climate sensitivity of the driving models, variations in the forcings applied, and-in the regional simulations-the inclusion of time-varying aerosols. In order to reconcile the two sets of projections, we compare two periods corresponding to fixed global warming levels in the driving models, to constrain the variability within and between the ensembles which can be ascribed to differing rates of global warming: the discrepancy between the ensembles is greatly reduced, although some differences in the local response remain, with the UKCP18 runs slightly warmer and drier than the EuroCORDEX runs, particularly in summer. We also highlight potential pitfalls of comparing warming levels with a reference time period, due to uncertainty about the warming that has already occurred in the driving models prior to the reference period. We compare temperature and precipitation over the UK from two different collections (known as "ensembles") of climate model runs: the EuroCORDEX ensemble, consisting of simulations from many combinations of global- and regional-scale models; and the UKCP18 regional ensemble, which uses a single pair of models, but adjusts the model parameters for each run. Both ensembles perform well in the current climate, but future changes in the UKCP18 ensemble are generally larger by 2050-2079 than those in the EuroCORDEX ensemble. This is largely because the UKCP18 global models warm more quickly in response to the greenhouse gases in the atmosphere, and use slightly higher concentrations of greenhouse gases. To understand the differences between the two ensembles that cannot be explained by differences in the rate of global warming, we also look at changes as the models warm from 1 to 2 degrees C globally above levels in the early 20th century. This reduces the discrepancy between the ensembles, although some differences remain: the UKCP18 ensemble remains slightly warmer and drier than EuroCORDEX, particularly in summer. We highlight issues that arise when comparing simulations at a given warming level against simulations in a fixed decade, due to uncertainty about how much warming has already occurred. The UKCP and EuroCORDEX regional model ensembles have similar biases, but project very different future climate over the UKThese differences are driven largely by differences in the climate sensitivity of the GCMs used to force the regional modelsComparing projections after a specified degree of warming, rather than in specified decades, reduces but does not resolve these differences