Natural hazards in Australia: heatwaves.

Heatwaves have disastrous impacts on many different systems, including human health, infrastructure, and natural ecosystems. Australia is no stranger to these impacts, with over 370 fatalities attributed to the 2009 Victorian heatwave, and 3500 flying foxes killed when temperatures soared to 42oC over New South Wales in 2002. Marine heatwaves (prolonged periods of anomalously warm oceans) also have significant impacts. The infamous “Ningaloo-Niño” in 2011 led to a “tropicalization” of fish communities and severely damaged the distribution of temperate seaweeds and fish, which may never fully recover. Recent research efforts have concentrated on furthering our understanding of both terrestrial and marine heatwaves. However, a comprehensive understanding of these events also requires a comprehensive understanding of the underpinning physical mechanisms and their interactions. This presentation reviews the current state of the scientific knowledge of marine and terrestrial heatwaves, with particular emphasis for the Australian region. Based on this knowledge, recommendations are given for the concentration of future research, both short and long term, and how the Australian science community, as well as the global science community, will benefit from this. The supporting paper for this presentation is part of the OZewex special edition on Australian hazards. Specifically, the review finds that while unified metrics for measuring terrestrial heatwaves is yet to be determined, increases in intensity, frequency and duration are detectable since at least the 1950s. Moreover, projections from the Coupled Model Intercomparison Project phase 5 (CMIP5) global climate model ensemble and the dynamically-downscaled NSW/ACT Regional Climate Modelling (NARCliM) simulations project larger increases of all heatwave characteristics in the future. While there are spatial similarities between the two types of simulations, future changes in heatwave projections by NARCliM are generally of smaller magnitude. A similarly comprehensive analysis on future projections of marine heatwaves is yet to be constructed. Moreover, an extensive research effort has been undertaken in understanding how the land surface and atmosphere couples together in priming local terrestrial heatwave conditions, as well as their interactions and feedbacks during particular events. For a similar analysis to be undertaken over Australia, various atmospheric modeling simulations at high spatial resolution are imperative. Such experiments may be run, for example, with a range of prescribed soil moisture levels to investigate how dry it must be for a heatwave to occur, persist and intensify. Another recommendation of this review includes the employment of coupled climate model simulations that are run both with and without greenhouse gas forcings in order to ascertain the human signal behind observed trends in Australian terrestrial heatwaves. This method, called formal optimal fingerprinting, has been successfully applied regional trends in other extremes, and can be readily applied to numerous models part of the CMIP5 archive.