Temperature
- Australia has warmed, on average, by 1.47 ± 0.24 °C since national records began in 1910.
- There has been an increase in extreme heat events associated with the warming.
Australia has warmed, on average, by 1.47 ± 0.24 °C since national records began in 1910, with most warming occurring since 1950. Every decade since 1950 has been warmer than preceding decades. The warming in Australia is consistent with global trends, with the degree of warming similar to the overall average across the world’s land areas.
Australia’s warmest year on record was 2019. The eight years from 2013–20 all rank among the 10 warmest years on record. The long-term warming trend means that most years are now warmer than almost any observed during the 20th century.
Warming is observed across Australia in all months with both day and night-time temperatures increasing. This shift is accompanied by an increased number of extreme nationally averaged daily heat events across all months, including a greater frequency of very hot days in summer. For example, 2019 experienced 41 extremely warm days, about triple the highest number in any year prior to 2000. Also in 2019, there were 33 days when national daily average maximum temperatures exceeded 39 °C, a larger number than seen in the 59 years from 1960–2018 combined. Increasing trends in extreme heat are observed at locations across all of Australia. Extreme heat has caused more deaths in Australia than any other natural hazard and has major impacts on ecosystems and infrastructure.
There has also been an increase in the frequency of months that are much warmer than usual. Very high monthly maximum temperatures that occurred nearly 2 per cent of the time in 1960–89 now (2007–21) occur over 11 per cent of the time. This is about a sixfold increase over the period. Very high monthly night-time temperatures, which are also a major contributor to heat stress, occurred nearly 2 per cent of the time in 1960–89 but now occur around 10 per cent of the time.
The frequencies of extremely cold days and nights have declined across Australia. An exception is for extremely cold nights in parts of south-east and south-west Australia, which have seen significant cool season drying, and hence more clear winter nights. This results in colder nights due to increased heat loss from the ground. The frequency of frost in these parts has been relatively unchanged since the 1980s.
Fire weather
- There has been an increase in extreme fire weather, and in the length of the fire season, across large parts of Australia since the 1950s. This has led to larger and more frequent fires, especially in southern Australia.
Climate change is contributing to changes in fire weather through its impacts on temperature and relative humidity, and the associated changes to fuel moisture content. Considerable year-to-year variability in fire weather also occurs. La Niña years—which occurred, for example, in 2010–11 and 2020–21—are associated with wet and cool climate anomalies and a lower number of days with high FFDI values through southern and eastern Australia. In northern and central Australia, increased rainfall can contribute to vegetation growth, which increases fuel loads when the vegetation dries out. High fuel loads are one of the key contributing factors to fire risk in northern and central regions.
Lightning that occurs without significant rainfall (known as ‘dry lightning’) is a major source of natural ignition for bushfires. Understanding changes to bushfire ignition in Australia, including the frequency of dry lightning, is a current area of active research.
There has also been an increase in the frequency of months that are much warmer than usual. Very high monthly maximum temperatures that occurred nearly 2 per cent of the time in 1960–89 now (2007–21) occur over 11 per cent of the time. This is about a sixfold increase over the period. Very high monthly night-time temperatures, which are also a major contributor to heat stress, occurred nearly 2 per cent of the time in 1960–89 but now occur around 10 per cent of the time.
The frequencies of extremely cold days and nights have declined across Australia. An exception is for extremely cold nights in parts of south-east and south-west Australia, which have seen significant cool season drying, and hence more clear winter nights. This results in colder nights due to increased heat loss from the ground. The frequency of frost in these parts has been relatively unchanged since the 1980s.
Rainfall
- There has been a decline of around 15 per cent in April to October rainfall in the south-west of Australia since 1970. Across the same region, May to July rainfall has seen the largest decrease, of around 19 per cent since 1970.
- In the south-east of Australia, there has been a decrease of around 10 per cent in April to October rainfall since the late 1990s.
- Rainfall has increased across most of northern Australia since the 1970s.
Australian rainfall is highly variable and is strongly influenced by drivers such as El Niño, La Niña, the Indian Ocean Dipole and the Southern Annular Mode. Despite this natural variability, long-term trends are evident in Australia’s rainfall records. There has been a shift towards drier conditions across the south-west and south-east, with more frequent years of below-average rainfall, especially for the cool season months of April to October. In 19 of the 22 years from 2000–21, April to October rainfall in southern Australia has been below the 1961–90 average. This is due to a combination of natural variability on decadal timescales and changes in large-scale circulation caused by an increase in greenhouse gas emissions.
The drying trend in southern Australia is most evident in the south-west and south-east of the country. The recent drying across these regions is the most sustained large-scale change in observed rainfall since widespread observations became available in the late 1880s. The trend is particularly strong for the period from May to July over south-west Western Australia, with rainfall since 1970 around 19 per cent less than the average from 1900–69.
Over the full April to October season the decline over the same period is around 15 per cent. Since 2000, this decline has increased to around 27 per cent, despite relatively high cool season rainfall during 2021. For the south-east of the continent, average April to October rainfall for the period 2000–21 was around 10 per cent lower than the 1900–99 period. The Millennium Drought was a major influence on this, affecting rainfall totals across the region from 1997–2009.
However, cool season rainfall totals are still 7 per cent below the 1900–99 average in the years since 2010. The only years with above-average cool season rainfall in south-east Australia in the past two decades (2010, 2016 and 2020) occurred during the La Niña events of 2010–12 and 2020–22, and the strong negative Indian Ocean Dipole event of 2016.
Rainfall in the cooler months (April to October) is particularly important for southern Australia, as it is the main growing season for many crops. It is also when peak streamflow occurs in most catchments in the region, as cool season rainfall is generally more effective than warm-season rainfall in generating runoff.
The declining trend in rainfall during this period is associated with a trend towards higher surface atmospheric pressure in the region and a shift in large-scale weather patterns, more highs, fewer lows and a reduction in the number of cold fronts that produce rainfall.
Conversely, northern Australia has been wetter in the 21st century than in the 20th century, across all seasons, especially in the north-west during the northern wet season (October to April). However, rainfall variability remains high, with the 2010s being not as far above average as the 2000s. In southern Queensland, there has been a trend towards lower rainfall throughout the year, particularly during the past decade. Warm season rainfall has also been below average in parts of Tasmania and the south-east and south-west coasts of mainland Australia in recent decades.
Heavy rainfall
- Heavy rainfall events are becoming more intense.
Observations show an increase in the intensity of heavy rainfall events in Australia that occur on timescales of less than a day.
The intensity of short‑duration (hourly) extreme rainfall events has increased by around 10 per cent or more in some regions and in recent decades, with larger increases typically observed in the north of the country. Short-duration extreme rainfall events (such as for hourly rainfall totals) are often associated with flash flooding, which brings increased risk to communities. This is particularly the case in urban environments, where the large amount of impervious ground cover (e.g. concrete) leads to increased flooding during heavy downpours.
Heavy rainfall events are typically caused by weather systems such as thunderstorms, cyclones and east coast lows. Daily rainfall totals associated with thunderstorms have increased since 1979, particularly in northern Australia. This is primarily due to an increase in the intensity of rainfall per storm.
Conversely, the number of low-pressure systems that can bring sustained heavy rainfall to highly populated parts of southern Australia has declined in recent decades. This has implications for recharging water storages and water resource management.
As the climate warms, the atmosphere can hold more water vapour than cooler air can. This relationship alone can increase moisture in the atmosphere by 7 per cent per degree of warming, all other things being equal. This can cause an increased likelihood of heavy rainfall events, even in parts of Australia where average rainfall is expected to decrease.
Increased atmospheric moisture can also provide more energy for some processes that generate extreme rainfall events, which can further increase the intensity of heavy rainfall due to global warming.
Streamflow
- 60 per cent of hydrologic reference stations around Australia show a declining trend in streamflow.
The observed long-term reduction in rainfall across many parts of southern Australia has led to reduced streamflow, although with considerable interannual variability. Declines in annual median streamflow have been observed in the South Australian Gulf, Tasmania, Pilbara Gascoyne, and Lake Eyre Basin drainage divisions. In each of these, around half of the gauges show a declining streamflow trend since 1975.
In the Murray–Darling Basin, nearly half of the long-term streamflow gauges show a declining trend since records began in 1970. This is more severe in the northern Basin where most gauges show a declining trend in streamflow. In the northern Basin catchments, where these trends are strongly evident, there are statistically significant declining trends in the headwaters, including the Namoi, Border Rivers, Condamine–Culgoa and Gwydir River catchments. In the Murrumbidgee, Lachlan, Goulburn and Loddon River catchments of the southern Basin, the majority of streamflow gauges also show declining trends. In the Darling River region, declining trends were observed at almost all gauges.
In the Tanami–Timor Sea Coast drainage division in northern Australia, which includes Darwin and much of the Northern Territory, there has been an increasing trend in annual median flows at about 51 per cent of the gauges since 1975. This is consistent with the observed increase in rainfall since the 1970s in the region.
More than 60 per cent of Australia’s hydrologic reference stations show a declining trend in streamflow, and more than 20 per cent show a statistically significant declining trend. Hydrologic reference stations are an indicator of long-term impacts from climate change on streamflow, as they are gauges in catchments with little disturbance from human activities and with at least a 30‑year record.
Changes in weather systems and climate drivers
Australia’s weather systems are changing. Southern Australia receives much of its rainfall during the cooler months of the year from low-pressure systems and cold fronts to the south of the subtropical high-pressure ridge. During recent decades, these systems have become less common over southern Australia, and are less likely to produce rainfall when they do occur, contributing to declines in cool season rainfall.
Mean sea level pressure is increasing over Australia, and there has been an increase in the number of high-pressure systems over southern Australia, which bring dry, clear weather and little rainfall. This increase in surface pressure across southern latitudes is a known climate system response to climate change.
There is large variability in the frequency of individual weather systems between months and years. However, many of these trends are consistent with simulations from climate models, which demonstrate that increased greenhouse levels lead to fewer low-pressure systems in southern Australia and a stronger subtropical ridge, but an increase in the intensity of heavy rainfall, including from thunderstorms.
Australia's climate is also strongly influenced from year to year by various broadscale climate drivers, such as the El Niño-Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD) and the Southern Annular Mode (SAM).
SAM shows a sustained trend towards more positive conditions from 1950 to the present day, particularly in summer. The level of ENSO activity over the past 50 years is higher, with more significant El Niño and La Niña events than in the years between 1920 and 1970.
However, there is no clear indication that recent activity levels are outside the long-term range of variability, with evidence of high levels of ENSO activity in the late 19th and early 20th centuries. There is low confidence in long-term trends in the IOD, particularly prior to the 1960s, although paleoclimate data indicate that the recent frequency of strong positive IOD events is high in the context of multi-century variability.
Tropical cyclones
- There has been a decrease in the number of tropical cyclones observed in the Australian region since at least 1982.
Tropical cyclone activity in Australia’s cyclone region varies substantially from year to year. This is partially due to the influence of large-scale climate drivers: the number of cyclones in our region generally declines with El Niño and increases with La Niña. Intense tropical cyclones can cause serious impacts associated with catastrophic winds, storm surges and extreme rainfall and flooding.
There has been a downward trend in the number of tropical cyclones observed in the Australian region since reliable satellite observations began in 1982. Additional non-satellite observations suggest there has also been a longer‑term reduction in the number of tropical cyclones since 1900. The trend in cyclone intensity in the Australian region is harder to quantify than cyclone frequency, due to uncertainties in estimating the intensity of individual cyclones and the relatively small number of intense cyclones.
Climate change, extreme rainfall and flood risk
Flooding is one of the major natural hazards facing Australia. Multiple factors contribute to the risk of flooding. The most important weather‑related factors include how extreme a rainfall event is and how wet catchments are prior to the rain event. In estuarine and coastal environments, tides and sea levels can also be important. Flood risk can also change over time as a result of changes in land use and land cover, and through changes in the extent to which streams in the catchment area are regulated.
Sustained heavy rainfall and associated flooding in much of Australia, particularly the east, is most common during La Niña, as illustrated by the multiple floods that occurred in eastern Australia in 2022. The 11 wettest years on record in eastern Australia were all influenced by La Niña. Many of eastern Australia’s most significant flood years, such as 1974, 2010−11 and 2021–22, have occurred during strong La Niña events, although significant flooding can sometimes occur in non-La Niña years. The impact of multiple flood events is particularly pronounced when La Niña events occur in multiple successive years, as occurred in 2020–22, and previously in periods such as 1954–56 and 1973–76.
Flood risk is influenced by rainfall at a range of timescales, depending on the catchment. Flash flooding is driven by intense rainfall in localised regions on timescales of minutes to hours, while river flooding in larger catchments responds to rainfall on timescales of days to weeks. While the observed trend in daily rainfall extremes is mixed across Australia, with some areas showing increases and others decreases, an increase in sub-daily extremes is now very clear over recent decades.
It is expected that extreme rainfall will increase in Australia with climate change. At particular locations, extreme rainfall changes in particular locations are also influenced by changes in weather systems, such as east coast lows and tropical cyclones (both of which are expected to become less frequent). This may lead to some regions experiencing trends in extreme rainfall that differ from typical values at national or global scale.
Climate change may also affect the drivers of multi-day rainfall extremes, such as atmospheric rivers for moisture transport, the behaviour of El Niño and La Niña, and persistent blocking highs (strong high-pressure systems that remain almost stationary for an extended period of time, blocking the eastward progression of weather systems across southern Australia) in the Tasman Sea, but the details of these effects are subject to ongoing research. Changes in extreme rainfall will not necessarily follow those in mean rainfall: regions in southern Australia, which are expected to see continued long-term drying, may still experience increases in extreme rainfall.
Snowfall
- Maximum snow depth, snow cover and number of snow days have decreased in Australian alpine regions since the late 1950s.
Downward trends in maximum snow depth have been observed for Australian alpine regions since the late 1950s, with the largest declines during spring and at lower altitudes. Downward trends in the temporal and spatial extent of snow cover have also been observed. The number of snowfall days has also decreased. Years with persistent heavy snow cover have become rare.
Snow depth is closely related to temperature, and the observed declines are associated with global warming trends. Decreasing trends in snow depth are greater over the corresponding period in the late-season month of September than during winter. Maximum snow depth remains highly variable and is strongly influenced by rare heavy snowfall days, which have no observed trends in frequency. Several heavy snowfall events contributed to average to high maximum snow depths in the years from 2017–19.