The Amplification of Australian Heatwave Characteristics in a Large Single-Model Ensemble.

Abstract:

Heatwaves are a common feature of Australia’s climate, however their frequency has increased since the mid-20^th Century, with trends accelerating since the 1970s. Future projections from climate models indicate that such trends will continue throughout the 21^st Century as anthropogenic forcing on the climate system rises. Increasing trends in heatwave intensity and duration over Australia will also result from rises in anthropogenic forcing.

While overall trends in heatwaves are responsive to anthropogenic forcing, a common question often asked by policy makers is “did climate change cause this individual heatwave?” This is an inherently difficult question to answer since any one event is caused by an interaction of meteorological systems, climate variability phenomena, and the background climate state. What can be answered, however, is “did climate change cause and increase in the likelihood of this heatwave?” Employing the method of Fraction of Attributable Risk (FAR) is pertinent in answering such a question.

In this study, we employ the FAR method on a 21-member ensemble of the Community Earth System model, a fully coupled model where each realization differs only in their initial conditions. This allows for a comprehensive sampling of internal climate variability that could plausibly arise under the given forcing. Heatwaves are measured using a predefined framework, accounting for heatwave intensity, frequency, duration, timing and spatial extent. The control run is used to estimate the influence of natural variability only, and we make use of the 21-member historical (1955-2005) and RCP8.5 (2006-2100) ensembles to estimate previous, current, and future FAR values and associated return periods.

Our results explore FAR analyses on each separate heatwave characteristic at the continental scale, and for southeast Australia. We find that 4 out of 5 heatwave characteristics are already larger than expected from internal variability alone, with FAR values rising considerably throughout the 21^st century.