Contributions of Greenhouse Gases and Anthropogenic Aerosols to Temperature Extremes over Australia

Abstract

This study investigates the impacts of greenhouse gases (GHG) and anthropogenic aerosols (AER) on changes in temperature extremes in Australia during the historical period (1950-2014). Using five large ensembles (LEs) from the Coupled Model Intercomparison Project phase 6 (CMIP6), we first analyze long-term changes. As expected, GHG dominate warming but with the signal partially offset by a more subtle AER-induced cooling. However, the AER cooling effect is not evident for cold extremes compared to hot extremes. Based on 40-yr running trends, we find that recent decades show faster warming of hot extremes, likely due to a more rapid increase under GHG combined with a decrease under AER. The AER influence is notable for trends centered in the mid-1980s in southern regions. In contrast, the role of GHG in extreme cold remains uncertain due to limited ensemble sizes. A notable AER cooling appears around 1988 in northern regions. In particular, aerosol optical depth, a proxy for AER strength, shows a decline beginning between 2006 and 2012, which is later than the global trend. Next, an event attribution assessment indicates that the contributions of the two forcings to the risk assessment (as assessed by risk ratio and magnitude shift) are similar to the trends across Australian regions. However, there can be large model differences in risk ratios. Overall, conclusions, particularly regarding AER and the impacts of internal variability, are sometimes less robust due to large intermodel differences and limited ensemble sizes, for which the associated findings should be interpreted with caution. Show more