Many of the most significant impacts – human, environmental and economic – of climate change occur as a result of extreme events. These include heatwaves, deluges, drought, fire, and storms and cyclones (see the Extreme events chapter).
Extreme events can have an especially high impact when subsequent events multiply the impact of initial events, a phenomenon referred to as ‘compounding’ or ‘cascading’ extremes. In some cases, certain types of events are inherently associated – for example, in a drought year, bushfire risk will be increased in forest areas but can be suppressed in arid regions because of a lack of fuel. In other cases, multiple events in rapid succession, even if not meteorologically related (e.g. a flood shortly after a bushfire), can have multiplicative impacts beyond those that would be expected from the individual events alone.
Extreme events affect all cultures. Some extreme events can force Indigenous people off their Country and into poverty. Sociopolitical factors place Indigenous people at further risk.
Extreme events are generally increasing in frequency and severity, with the clearest changes occurring in temperature extremes and fire weather.
Related to United Nations Sustainable Development Goal targets 11.5, 13.1, 13.2, 15.3
The occurrence of high-temperature extremes has increased on a wide range of measures. Heatwaves that occurred in 2018–19 and 2019–20 were of record-breaking intensity and area. The occurrence of low-temperature extremes has broadly decreased, but the frequency of frosts has stabilised since the 1980s in parts of inland south-western and south-eastern Australia, which have experienced substantial cool-season rainfall decline.
There is not yet a clear signal in the observed occurrence of most types of high-rainfall extremes, although there are indications of an increase in short-duration (less than a day) extremes (medium confidence). Extreme low rainfall has become more frequent in parts of south-western and south-eastern Australia that have experienced long-term declines in mean rainfall.
Related to United Nations Sustainable Development Goal targets 11.5, 13.1, 13.2, 15.3
The occurrence of dangerous fire weather, as indicated by the Forest Fire Danger Index, has increased over many parts of Australia during the past 50 years (high confidence). This has been particularly manifested as a lengthening of the fire season in forested areas of southern Australia.
The frequency of tropical cyclones in the Australian region has decreased since the 1980s (medium confidence). At a global scale, there is evidence of an increase in the proportion of tropical cyclones that are intense, and in the amount of rainfall associated with tropical cyclones, but no clear signal of these increases has yet emerged in the Australian region.
Links of individual extreme events to climate change
An increasingly active area of research since the 2016 state of the environment report has been the attribution of extreme events – that is, assessing the extent (if any) to which human-caused climate change has made a specific extreme event more or less likely.
The global literature shows that the risk of most observed extreme heat and cold events has been significantly altered by climate change. For example, some recent heat events would have been extremely unlikely without human influence on the climate system.
Human influence on other types of extreme events, such as extreme precipitation, has been harder to establish, although some recent extreme high-rainfall events have shown evidence of a human basis (for example, Kirchmeier-Young et al. (2021), Reed et al. (2021)). Most published studies to date on extreme precipitation events in Australia – for example, on the extremely wet cool season of 2016 in south-east Australia (Hope et al. 2018, King 2018) – have found that natural climate variability was the primary driver.
Several studies have been published since 2016 on various Australian events, including abnormal fire weather in Queensland in spring 2018 (Lewis et al. 2020) and in eastern Australia more generally in 2019−20 (van Oldenborgh et al. 2021), warm weather and drought in Tasmania in 2015−16, and a high frequency of spring frosts in southern Western Australia in 2016 (Grose et al. 2018, Grose et al. 2020). These studies found, in general, that climate change had greatly increased the probability of extreme high-temperature events and the temperature component of fire weather indices, but conclusions relating to drought were more uncertain. Climate change can also influence circulation changes that offset the overall warming signal, as occurred in the 2016 Western Australian frost event.
Attribution studies to date have mostly been published months or years after the event, although rapid attribution is being carried out for some event types with well-developed attribution methodologies (such as heatwaves). Several national meteorological services and associated organisations are investigating the attribution of events in near real-time, but no such service is currently operational.
Temperature extremes
In a warming climate, it is expected that the frequency of warm extremes will increase, and that of cold extremes will decrease. In general, temperature extremes in Australia are following this trend, but with some local variations.
Temperature extremes can be considered in different ways, including exceptional high or low values on a single day at a specific location, periods of persistent heat or cold over several days, and extreme heat or cold over a large area.
Temperature extremes can have health and wellbeing implications for human communities across Australia, especially in urban environments (see the Urban chapter). For Indigenous people, extreme temperatures can force them to migrate away from their traditional lands for long periods into an urban setting or to seek cooler climates. Temperature extremes place environmental change stresses on traditional knowledge, Country and biodiversity.
For example, the bogong moth is a traditional delicacy for some Indigenous groups in Australia, but the bogong moth population has been in steady decline since 1973 due to land clearing and fires. In 2017, a summer drought in the Western Plains region meant that juvenile larvae growing in the cracked clay of the desert soil were not able to get the nutrients they needed from plant matter. This event was a localised event in which the summer drought occurred in areas where the bogong moth is known to breed; the moths did not survive the summer drought and did not come back (Monash University 2021).
High temperatures
There is a broad trend towards a much higher frequency of very hot days on a national scale.
Extreme and extensive heatwaves occurred in many parts of Australia in both the 2018−19 and 2019−20 summers. The heat was particularly notable for the large area it covered, which led to unprecedented daily temperature highs averaged over Australia. Before the 2018−19 summer, there were only 4 days (2 in 1972−73 and 2 in 2012−13) on which the daily maximum temperature, averaged over Australia, exceeded 40 °C. There were 7 days with a national average above 40 °C in 2018−19 and 11 in December 2019, including 7 in a row. On 18 December 2019, the national average reached 41.88 °C, more than 1.5 °C above the pre-2019 record.
Over Australia as a whole, increases in temperature extremes roughly parallel increases in average summer temperatures. This contrasts with some parts of the world, such as Europe, where summer extremes are increasing faster than averages (partly driven by feedbacks from low soil moisture). In inland south-east Australia, summer extremes are generally increasing faster than averages, particularly since 2000. This is reflected in a dramatic increase in the occurrence of very high temperatures (Figure 10). For example, the average number of days per year on which the temperature has reached 45 °C somewhere in Victoria has increased from 0.3 in 1961−2000 to 2.0 in 2001−20, and 2.6 in 2011−20 (Figure 11). Conversely, in much of northern and central Australia, the highest extremes have increased more slowly than averages, although the number of hot days has increased substantially.
Low temperatures
The frequency of extreme low temperatures has decreased over most of Australia over the past 100 years. Some regions of inland south-eastern and south-western Australia, which have experienced decreasing cool-season rainfall in recent decades, have seen the frequency of cold nights (and, in susceptible areas, frosts) stabilise or increase since the 1980s, despite rising average temperatures (Pepler et al. 2018). This is driven by strengthening of the subtropical ridge, more nights with clear skies and, in some areas, feedbacks from lower soil moisture.
High-rainfall extremes
High-rainfall extremes are expected to increase globally in a warming climate, because a warmer atmosphere can hold more water. Extreme rainfall events are often localised and highly variable from year to year, increasing the difficulty of detecting a clear trend in observational data.
A clear overall upward trend in rainfall extremes has not yet emerged in Australian data (whereas it has in some other parts of the world, especially in the mid-latitudes of the Northern Hemisphere). An increased occurrence of daily and multiday rainfall extremes is projected to occur in northern and central Australia with 1.5 °C of global warming, extending to the remainder of the country if higher warming levels are reached (IPCC 2021).
However, an increase in certain types of rainfall extremes has been observed (BOM & CSIRO 2020):
- The intensity of short-duration (hourly) extreme rainfall events has increased by around 10% or more in some regions and in recent decades.
- Daily rainfall totals associated with thunderstorms have increased since 1979.
In both cases, the increases are most pronounced in northern Australia. In some parts of southern Australia where average rainfall has been declining, high-rainfall events have also become less frequent. However, it is not yet clear whether high rainfall is decreasing at similar rates to annual totals in these regions.
Heavy rainfall can lead to:
- increased soil run-off
- impacts on crops
- loss of livestock
- increased risk of landslides and natural hazards
- damage to infrastructure
- damage to cultural sites.
High-rainfall extremes resulting in flash floods can affect all constructions of the landscape, with devastating impacts on both urban settings and the natural environment. For Indigenous people’s Country, such events can occur at the ‘wrong’ time and therefore water Country when it is not ready to receive it. For inland waterway systems, the volume and velocity of extreme rainfall events often lead to accelerated erosion of riverbanks and over-bank flows, resulting in movement of sediment into foreign areas and loss of biodiversity in riparian areas. Extreme weather events can also lead to varying impacts on cultural heritage sites, such as midden sites, birthing sites and scarred trees. Other contributing factors on Country, both historically or recent, will also affect the level of recovery required after events. For example, an increase in extreme bushfire events in an area that has not recovered will increase the nutrient levels in systems, creating an unbalanced ecosystem for sustainable biodiversity in both freshwater and coastal regions.
Drought
Drought is an inherent part of the Australian climate and can be considered from various perspectives. A shortage of rainfall places stresses on the natural environment and agriculture, as well as limiting water availability for the environment, and for urban and rural uses.
Drought definitions
The recognition of drought as a natural part of the Australian climate is enshrined in the major government policy framework for managing drought: the National Drought Agreement between the national and state and territory governments, last renewed in 2018. A key underlying principle of this framework is that drought is not considered to be a natural disaster, but rather a normal part of Australian climatic variability. Policy focuses on supporting preparedness and risk management. At both the national and the state and territory levels, there has been a shift away from in-drought support, particularly transactional subsidies. A major focus of drought policy is to support risk management by farmers through measures such as tax concessions (e.g. income tax averaging and accelerated depreciation), concessional loans and the Farm Management Deposits Scheme. There is also support for on-farm water infrastructure. Time-limited financial support is still provided to farmers facing financial hardship through the Farm Household Allowance program.
Various indicators exist to define drought, depending on the application. The standard drought monitoring carried out by the Bureau of Meteorology uses rainfall only, but other drought indicators in use locally and internationally incorporate other indicators, such as evapotranspiration. For example, one definition seeks to incorporate ideas of how low rainfall affects the land: ‘The term drought refers to a period of time over which the water content of the soil is reduced by inadequate precipitation to an extent where plants suffer from a lack of water sufficient to disrupt normal life processes’ (Coder 1999).
Over long timescales, there is also the question of when long-term drought becomes a permanent change in the climate. To give one example, the average April–October rainfall in the south-west region of Western Australia from 2001 to 2020 was 478 millimetres (mm), but, in the 20th century, a total of 478 mm would have ranked 13th lowest out of 100 years. In other words, a ‘normal’ 21st century cool season would have been considered as being close to drought conditions had it happened in the 20th century. State of the climate 2020 (BOM & CSIRO 2020) recognises the role of climate change in rainfall reduction over southern Australia and along the Great Dividing Range.
Indigenous people in their traditional lands will view and experience drought depending on the knowledge resources available within their Country to read climate. Definitions that focus on rainfall ignore the impact of water management practices on the environment and culture. Within the Murray–Darling Basin, for example, water is prioritised for irrigation and consumptive purposes, and it is difficult to say whether low water flows in the Murray River are due to ‘drought’ or due to water that is held in storage dams for other purposes (MDBA 2021). For Indigenous people, discussions of drought divert people from the real issues around the legacy of harm to the Murray–Darling Basin system, and associated waterways and wetland systems, caused by disconnected flows, low flows, water trading and loss of Indigenous cultural knowledge (see the Inland water chapter). Environmental and cultural connections to water are an afterthought in planning and assessment, and there are no laws that require mandatory engagement and protection of Indigenous knowledge and law (unless required under cultural heritage laws) (see the Heritage chapter). Drought (low rainfall) is not solely responsible for the low flows in the Murray–Darling. Although drought is a contributing factor, Indigenous knowledge and the environment are being more impacted by in-channel flows and regulation of the Murray River, the use of the river and its banks as a commercial water highway, and a high-end piping system.
Drought events and trends
Some notable long-term droughts in eastern Australia include those of 1895−1903 (often known as the ‘federation drought’), 1937−46 and 1997−2009 (often known as the ‘millennium drought’). These droughts, particularly the federation and millennium droughts, were notable for the lack of any sustained wet periods, even though only a small number of individual years during the respective timespans were exceptionally dry. For example, in the Murray–Darling Basin, the 9 years from 2001 to 2009 all had below-average rainfall averaged over the Basin, but only 2006 (4th lowest) and 2002 (5th lowest) were in the 30 driest years of the last 120. In the millennium drought, this limited the opportunity for recharge of large water storages, such as those of the southern Murray–Darling Basin (among the regions with the most persistent rainfall deficits) and the urban supply systems of Sydney and Brisbane.
There have also been severe shorter-term droughts on roughly annual timescales; many, including those of 1914 and 1982, were associated with El Niño events. However, some have occurred in the absence of El Niño, such as the 1967 drought that brought Adelaide and Melbourne their driest years on record and contributed to the destructive bushfires of February 1967 in Hobart. Prolonged droughts have also affected other parts of Australia. Much of interior Australia, particularly outback South Australia and western Queensland, was very dry in the 1920s and 1930s, and central Australia also experienced persistent dry conditions from the late 1950s to the late 1960s.
More recently, severe drought affected many parts of eastern Australia from 2017 to 2019 (see case study: The 2017−19 Australian drought), extending to cover much of the continent in late 2018 and 2019, and easing over most areas during 2020.
Assessments of the impact of climate change on drought, both in Australia and internationally, are sensitive to the indicators used to assess drought (see Drought definitions). In particular, indicators that use rainfall only will give different outcomes in many regions from indicators that incorporate other variables such as evapotranspiration.
In Australia, an increased incidence of drought has been found in regions that have experienced long-term rainfall declines, such as south-west Western Australia. However, to date, there has been limited assessment of whether there have been changes in drought incidence in Australia over and above those that would be expected from changes in mean rainfall alone. Severe and prolonged droughts have occurred in the past decade in some regions (such as western Queensland) that show little evidence of declines in mean rainfall over the longer term, but there is little evidence as yet that the recent drought there represents a longer-term trend.
Fire weather
The frequency of occurrence of significant fire weather is a major element in determining bushfire risk. A number of factors contribute to fire weather: a lack of rainfall in the lead-up period, low humidity, strong winds and high temperatures – these all contribute to fire risk on the day but can also increase moisture stress on vegetation in the lead-up period.
A number of indices are used to assess fire weather. The most commonly used in Australia are the McArthur Forest Fire Danger Index (FFDI) (McArthur 1967, Noble et al. 1980) and the Grassland Fire Danger Index (GFDI). The FFDI is strongly associated with fire spread in forest and woodland areas. It combines temperature, wind speed, humidity and an index of fuel dryness, based on rainfall in the preceding period.
The FFDI aggregated over the fire season shows a significant increasing trend since the 1970s over most forested areas of south-eastern and south-western Australia (Clarke et al. 2012, Dowdy 2018). In general, this increase comes more from a lengthening of the fire season, particularly in spring, than from an intensification of the peak of the season.
The number of days with very high or above fire danger has also generally increased (Figure 13). The exceptional 2019−20 fire season in temperate Australia occurred during a period when numerous indicators of fire weather aggregated over the season were at record high levels. Megafires and Indigenous fire management are discussed in the Extreme events chapter (see the Extreme events chapter).
Cyclones
Tropical cyclones are a major natural hazard in coastal regions of Queensland, the Northern Territory, and tropical and subtropical parts of Western Australia. They have been responsible for some of Australia’s most damaging natural disasters, including the destruction of much of Darwin during cyclone Tracy in 1974. The coast is most exposed to damage from wind and storm surges, but heavy rains and flooding can extend well beyond the cyclone landfall point. Cyclone-related disruptions to the resources industry, especially in Western Australia, can also result in major economic losses.
On average, there are about 11 tropical cyclones per year in the broader Australian region (extending from 90°E to 160°E), of which about half either cross the Australian coast or approach it closely enough to have impacts on land. Tropical cyclones are relatively small features, with the zone of destructive winds typically less than 200 kilometres across, and hence only a small proportion of the coastline is directly affected in any given year. In general, tropical cyclones in the Australian region are more common during La Niña years and less common during El Niño years.
The number of tropical cyclones in the Australian region has declined since the 1980s. The 1970s were a very active period for tropical cyclone activity, attributable in part to numerous La Niña events. In the 10 seasons from 2010−11 to 2019−20, there was an average of 8.9 cyclones per year, about 20% below the 1981–2010 average, and there has not been an above-average season since 2005−06. It is likely that the proportion of cyclones globally reaching category 3 intensity or above has increased over the past 40 years, and there is some evidence of similar changes in the Australian region. Some international studies have also indicated an increase in the amount of rainfall associated with tropical cyclones, but no clear evidence of this has yet emerged in Australia.
Consistent and comprehensive records of tropical cyclones in the Australian region require satellite data, and so are only available from the 1970s for cyclone numbers and the 1980s for intensity. Data from well-populated parts of the Queensland coast indicate a decreasing number of severe tropical cyclone landfalls there over the 20th century (Callaghan & Power 2011).
Depending on the location of cyclones, Indigenous coastal communities and their traditional knowledge are at risk of loss (Griffith University & CSIRO 2014). Damage can also be done to cultural sites. For example, Indigenous coastal communities have buried their ancestors in sand dunes. Cyclones inflict mass erosion events that expose their ancestors to the elements; this is of major concern to Indigenous coastal communities.
Indigenous coastal communities use traditional knowledge to prepare for, mitigate, recover from and heal from disasters, including cyclones. This resilience is becoming even more important and evident in a changing climate. Indigenous people read Country to predict weather extremes such as cyclones. Natural indicators – including the behaviour of insects and birds, tree movement from changing winds, and species movement – can tell Indigenous people that a cyclone or extreme event is nearing and allow them to prepare. Mainstream science does not always look at the links and understanding that traditional stories provide, that can assist in making Country stronger.