Scientific records of the weather only span about a century, and so recordings of extreme events in the sense used by the Intergovernmental Panel on Climate Change (Seneviratne et al. 2012) – that is, weather or climate variables that fall near the ends of the range of observed values – are only occasionally made.
However, the frequency of record-breaking events is increasing, as are the scale and magnitude of such events (Steffen et al. 2013). Anthropogenic (human-caused) climate change is driving atmospheric and climatological change that imposes pressures on Australian environments (see the Climate chapter). Relatively minor shifts in climate averages may convert into major shifts in the occurrence of extreme events (Seneviratne et al. 2012). Growing concern about, and efforts to reduce the pressure of, climate change are important, but inertia in the global climate system will mean that some change is inevitable. Recognising these changes, policy and planning initiatives are preparing for the impacts of extreme events on ecosystems and communities by supporting adaptation, mitigation and resistance building.
Temperature-related extremes
Australia is experiencing increased duration and frequency of heatwaves, both on land and in the oceans (see the Climate chapter). The hottest days have become hotter. This is having direct consequences for people, agriculture and the environment, and indirect consequences through impacts on water availability, seasonal duration and vegetation drying.
In general, the number of frost days is decreasing, although not evenly or consistently. This will affect agriculture and native species.
Rainfall and drought
Parts of Australia have become drier in the past 50 years; in other areas, total rainfall has not decreased, but the rain is delivered in shorter, more intense falls (see the Climate chapter). Increasing intensity of rainfall may exceed the capacity of catchments or urban stormwater systems to absorb or channel the rainfall and exacerbate flood risk in a nonlinear fashion (Buckley et al. 2019). Increased rainfall intensity over shorter periods may also lead to a decrease in percolation and groundwater recharge, reduce inflows to water impoundments, and thus reduce water security.
Rainfall itself can be an extreme event. Rainwater ingress causes high levels of property damage – the water does not have to come from a flood.
Combined with longer, hotter periods between rainfall events, the reduced groundwater recharge associated with rainfall events of greater intensity and shorter duration is likely to increase the frequency, intensity, duration and spatial extent of drought.
Bushfires
Increases in drought and heatwaves, together with low humidity and greater wind speeds, will increase the probability of extreme fire weather days and provide conditions that are likely to result in damaging bushfires if ignition occurs. Much of Australia is already experiencing an increase in extreme fire weather days and an extension of the bushfire season (see the Climate chapter). Dry lightning, a major cause of ignition, is likely to increase in the warmer seasons in south-eastern Australia.
Sea level rise and coastal flooding
Sea level rise will result in more frequent inundation and erosion, with a higher impact than previously (see the Coasts chapter). Storm tides and intense rainfall inland both increase the potential impact of inundation.
Tropical cyclones and storms
Warmer oceans support higher-intensity tropical cyclones, although the total number of cyclones may decrease. Some studies suggest that cyclones may track further south more frequently than in the past, although with low modelling confidence. Storms and cyclones are predicted to deliver more intense rainfall (see the Climate chapter).