Understanding changes in heat waves are imperative due to the wide range of sectors these events impact. Such sectors include human health, infrastructure, ecological and agricultural systems. By introducing a novel framework that realizes the subjective nature of heat waves, Perkins and Alexander (2013) demonstrated that the frequency of heat waves have been increasing across much of the Australian continent. However, can we expect a physical climate model to replicate these changes? This is an important question since physical models are heavily relied on to help understand future changes in climate, including extreme events such as heat waves.

Using the 21-member CESM global climate model, this talk demonstrates that one model can produce a range of historical simulations for changes in Australian heat waves. Each member of the model ensemble differs only in their initial conditions, yet this results in quite different simulations of changes in various heat wave characteristics are obtained. However, we argue that since observed trends falls within the range that is produced by the CESM members, there is no evidence to suggest that the model does not have the ability to replicate true observed changes.

Moreover, the model ensemble average demonstrates more significant increases in heat waves than what is observed. It is argued that this is due to the higher signal the ensemble average possesses over the observations (and each individual ensemble member), and that this trend underlies noise caused by natural variability. Implications for investigating and interpreting future projections of heat waves are addressed, as well as the importance of furthering our understanding of the role of natural variability.