Disentangling the uncertainties in regional projections for Australia

Understanding, quantifying and visualising projected ranges of future regional climate change is important for informing robust climate change impact assessments. Here, we examine projections of Australian sub-continental regionally averaged surface air temperature and precipitation in the Sixth Coupled Model Intercomparison Project (CMIP6) global and Coordinated Regional climate Downscaling Experiment (CORDEX)-Australasia regional model ensembles and illustrate the relative sources of uncertainty from emissions scenarios, models and internal climate variability. As expected, the uncertainty in temperature change for all regions by the end of the century is predominantly determined by the emissions scenario. Here, we examine a low and high emissions scenario, bookending a range of plausible cases. In contrast, the uncertainty in precipitation changes towards the end of the 21st Century is largely related to model-to-model differences, in particular owing to the differences between global models, with regional models contributing a smaller, but still significant, source of uncertainty. Regional models can significantly alter precipitation projections; however, we find few cases of consistency across the regional models. Decadal variability is an important contributing factor for precipitation uncertainty for the entire 21st Century. Large changes in interannual precipitation variability are projected by some climate models by the end of the 21st Century, and these changes tend to be well correlated to mean precipitation changes. Robust responses to climate change must account for all of these dimensions in a structured way.