Australia not so much hotter but warmer and more temperate
The Bureau of Meteorology's homogenised Australian Climate Observation Reference Network (ACORN 2.1) comprises 112 weather stations, of which 59 have been in operation since 1910 with comparable daily maximum temperature observations. These stations are Adelaide, Albany, Alice Springs, Bathurst, Boulia, Bourke, Bridgetown, Broome, Bundaberg, Burketown, Cairns, Cape Leeuwin, Cape Moreton, Cape Otway, Carnarvon, Charleville, Charters Towers, Cobar, Darwin, Deniliquin, Eddystone Point, Esperance, Gabo Island, Gayndah, Georgetown, Geraldton, Halls Creek, Inverell, Kalgoorlie, Katanning, Kerang, Launceston, Longreach, Low Head, Mackay, Marble Bar, Marree, Melbourne, Mildura, Miles, Moree, Moruya Heads, Mount Gambier, Normanton, Palmerville, Perth, Port Lincoln, Richmond Qld, Robe, Sale, Snowtown, Sydney, Tennant Creek, Tibooburra, Wagga Wagga, Walgett, Wandering, Wilsons Promontory and Yamba. ACORN 2.1, which was implemented by the bureau in October 2020, updates the ACORN 1 and ACORN 2 datasets containing homogenised, or adjusted, original RAW daily temperatures. Data in the analysis below is sourced to original unadjusted temperates here and adjusted ACORN temperatures here. ACORN 2.1 creates anomalies compared to 1961-90 and these are used by the bureau to describe prevailing temperatures in Australia. Both ACORN anomalies and absolute temperatures generally cool the daily RAW observations at most stations prior to the 1980s. The analysis charts below show the decadal sum number of days at all 59 stations that were 5C or warmer, 10C or warmer, 15C or warmer, 20C or warmer, 25C or warmer, 30C or warmer, 35C or hotter, 40C or hotter, 45C or hotter, and 50C or hotter. The charts also show decadal average rainfall at the 59 ACORN weather stations (1910-1969 : 701.4mm > 1970-2019 : 702.8mm - up 0.2%).
The bureau defines a hot day as above 35C and a very hot day as above 40C. The charts above suggests that over the past 50 years, Australians have enjoyed a substantially greater annual number of warm days than previous generations and a comparatively small increase in the annual number of hot days. The increase in warm and hot days since the year 2000 correlates with a decline in rainfall and associated cloud cover at the 59 stations (1910-1999 : 711.1mm > 2000-2019 : 661.1mm - down 7.0%) The charts above can alternatively be expressed as the decadal average rather than sum number of days at all 59 stations that were 5C or warmer, 10C or warmer, 15C or warmer, 20C or warmer, 25C or warmer, 30C or warmer, 35C or hotter, 40C or hotter, 45C or hotter, and 50C or hotter, including data comparing the first and second halves of the 1910-2019 timespan.
The analysis charts below show the decadal average maximum temperature (Tmax C) of days at all 59 stations that were 5C or warmer, 10C or warmer, 15C or warmer, 20C or warmer, 25C or warmer, 30C or warmer, 35C or hotter, 40C or hotter, 45C or hotter, and 50C or hotter.
The charts below analyse the annual sum number of days during 1910-2019 when the maximum was between 5C and 10C, between 10C and 15C, between 15C and 20C, between 20C and 25C, between 25C and 30C, between 30C and 35C, between 35C and 40C, between 40C and 45C, and between 45C and 50C.
The line charts above can also be expressed as decadal averages:
The data above shows that if the 59 weather stations are considered indicative and using unadjusted RAW temperatures comparing 1910-1969 with 1970-2019, Australians have experienced an average 2.1 fewer days per year with the maximum from 5C to 20C, an average 2.3 more days per year with the maximum from 20C to 35C, and an average 0.5 days per year with the maximum above 35C. The average Tmax within each 5C maximum temperature category can also be expressed as decadal averages:
The two preceding charts can be tabulated, showing the average changes in the number and temperature of each 5C category at each of the 59 individual stations when comparing 1910-1969 with 1970-2019:
When based on original RAW temperatures and comparing 1910-1969 with 1970-2019, Australians living in the 59 cities and towns have experienced the following changes: When based on original RAW temperatures and comparing 1910-1929 with 2000-2019, Australians living in the 59 cities and towns have experienced the following changes: Northern Australia Among Australia's 59 ACORN weather stations with adequate maximum daily recordings from a start year of 1910, 19 are located in northern Australia. These are Alice Springs, Boulia, Broome, Bundaberg, Burketown, Cairns, Carnarvon, Charters Towers, Darwin, Gayndah, Georgetown, Halls Creek, Longreach, Mackay, Marble Bar, Normanton, Palmerville, Richmond (Qld) and Tennant Creek. Across northern Australia, BoM data show average annual rainfall increased 14.8% from 491.9mm in 1910-1969 to 564.6mm in 1970-2019. The charts below analyse the annual sum number of days during 1910-2019 in northern Australia when the maximum was between 5C and 10C, between 10C and 15C, between 15C and 20C, between 20C and 25C, between 25C and 30C, between 30C and 35C, between 35C and 40C, between 40C and 45C, and between 45C and 50C. The charts also show decadal average rainfall at the 19 ACORN weather stations (1910-1969 : 769.2mm > 1970-2019 : 810.7mm - up 5.4%).
The average Tmax within each 5C maximum temperature category can also be expressed as decadal averages:
The two preceding charts can be tabulated, showing the average changes in the number and temperature of each 5C category at each of the 19 individual stations in northern Australia when comparing 1910-1969 with 1970-2019:
When based on original RAW temperatures and comparing 1910-1969 with 1970-2019, Australians living in the 19 northern cities and towns have experienced the following changes: When based on original RAW temperatures and comparing 1910-1929 with 2000-2019, Australians living in the 19 northern cities and towns have experienced the following changes: Southern Australia Among Australia's 59 ACORN weather stations with adequate maximum daily recordings from a start year of 1910, 40 are located in southern Australia. These are Adelaide, Albany, Bathurst, Bourke, Bridgetown, Cape Leeuwin, Cape Moreton, Cape Otway, Charleville, Cobar, Deniliquin, Eddystone Point, Esperance, Gabo Island, Geraldton, Inverell, Kalgoorlie, Katanning, Kerang, Launceston, Low Head, Marree, Melbourne, Mildura, Miles, Moree, Moruya Heads, Mount Gambier, Perth, Port Lincoln, Robe, Sale, Snowtown, Sydney, Tibooburra, Wagga Wagga, Walgett, Wandering, Wilsons Promontory and Yamba. Across southern Australia, BoM data show average annual rainfall increased 5.0% from 373.8mm in 1910-1969 to 392.4mm in 1970-2019. Northern Australia had an average 118.1mm more annual rainfall than southern Australia in 1910-1969, and an average 172.2mm more annual rainfall in 1970-2019. The charts below analyse the annual sum number of days during 1910-2019 in southern Australia when the maximum was between 5C and 10C, between 10C and 15C, between 15C and 20C, between 20C and 25C, between 25C and 30C, between 30C and 35C, between 35C and 40C, between 40C and 45C, and between 45C and 50C. The charts also show decadal average rainfall at the 40 ACORN weather stations (1910-1969 : 669.1mm > 1970-2019 : 650.6mm - down 2.8%).
The average Tmax within each 5C maximum temperature category can also be expressed as decadal averages:
The two preceding charts can be tabulated, showing the average changes in the number and temperature of each 5C category at each of the 40 individual stations in southern Australia when comparing 1910-1969 with 1970-2019:
When based on original RAW temperatures and comparing 1910-1969 with 1970-2019, Australians living in the 40 southern cities and towns have experienced the following changes: When based on original RAW temperatures and comparing 1910-1929 with 2000-2019, Australians living in the 40 southern cities and towns have experienced the following changes: National analysis The frequency of 10C blocks of maximum temperatures can be tabulated to compare 19101-1969 with 1970-2019:
The average Tmax of 10C blocks of maximum temperatures can be tabulated to compare 19101-1969 with 1970-2019:
Most Australians enjoy an average 1.37 more comfortable maximum temperature days each year between 15C and 35C that are on average 0.11C warmer than in 1910-1969. They enjoy an average 2.41 fewer uncomfortably cold or chilly maximum temperature days each year between 5C and 15C that are an average 0.16C warmer than in 1910-1969. They endure an average 0.47 more uncomfortably hot maximum temperature days each year between 35C and 50C that are an average 0.09C warmer than in 1910-1969. Northern analysis Most Australians in northern Australia enjoy an average 3.7 fewer comfortable maximum temperature days each year between 15C and 35C that are on average 0.13C warmer than in 1910-1969. They enjoy an average 0.31 fewer uncomfortably cold or chilly maximum temperature days each year between 5C and 15C that are an average 0.13C warmer than in 1910-1969. They endure an average two more uncomfortably hot maximum temperature days each year between 35C and 50C that are an average 0.06C warmer than in 1910-1969. This includes an average 0.72 fewer days above 40C that are an average 0.07C warmer than in 1910-1969. Southern analysis Most Australians in southern Australia enjoy an average 9.8 more comfortable maximum temperature days each year between 15C and 35C that are on average 0.11C warmer than in 1910-1969. They enjoy an average 6.91 fewer uncomfortably cold or chilly maximum temperature days each year between 5C and 15C that are an average 0.16C warmer than in 1910-1969. They endure an average 2.1 more uncomfortably hot maximum temperature days each year between 35C and 50C that are an average 0.1C warmer than in 1910-1969. On average, southern Australia residents can expect to experience one more day each decade when the temperature exceeds 45C and that day will be 0.2C warmer than it used to be. Discussion The charts and table above show a decrease from 1910 to 2019 in the number of days when the maximum temperature was only between 5C and 20C, suggesting a warming climate. Increasing frequency is apparent in days with a maximum between 20C and 25C, between 25C and 30C, and between 30C and 35C. The biggest increase is an average 3.7 days each year between 30C and 35C, although the average of 32.15C is only 0.09C warmer than in 1910-1969 and would leave many Australian undecided if it was hot enough to go to the beach. The unadjusted original RAW daily maximum temperatures show an increased frequency of days above 35C across Australia, although this is only an average 1.43 days each year with maxima an average 0.1C warmer than in 1910-1969. The analysis above counts and averages all maximum daily temperatures since 1910, including summer, autumn, winter and spring. The increased frequency and warmth of maximum daily temperatures below 30C may be due to an increase during cooler seasons/months, with a reduction during warmer seasons/months. The original RAW maximum daily temperature variations may also be influenced by significant changes to Australia's rainfall patterns, with more national rainfall due to increased northern Australia rainfall that is greater than a significant decline in the south-west and a slight decline in the south-east since the 1970s. The frequency of days with a comfortable maximum between 20C and 35C has increased by an average 135 per year, or 2.3 days per year at each station, when comparing 1910-1969 with 1970-2019 across all 59 weather stations. These averages are applied to locations as far south as Launceston and as far north as Darwin, so there is a substantial variation in the temperature trends for each weather station. Automatic Weather Stations There is a consistent increase in the number and average maximum temperature of days since the year 2000. This is due to either a substantial increase in climate warming or automatic weather stations (AWS) progressively installed since the mid 1990s that use one second observations exaggerating the frequency and averages of daily maximum temperatures. All extreme maximum temperatures recorded progressively since the mid-1990s should be regarded as suspect due to the one second sensitivity of automatic weather stations, as well as the replacement of large Stevenson screens with small screens over the past 30 years. It is noteworthy and relevant in this AWS context that the frequency of maximum temperatures between 5C and 20C saw a downward shift from 1990 to 2019, while their frequency from 40C to 50C saw a significant upward shift during these years. This may be influenced by the AWS sensitivity to extreme temperatures when compared to liquid in glass thermometers that were previously observed manually (AWS first installed in the mid 1990s and now the dominant instrument). Conclusion The data suggests reduced rainfall cloud cover and AWS extreme sensitivity have influenced temperature trends since the year 2000. Other long-term factors influencing temperature trends include urban heating and 1972 metrication, with both ACORN 2.1 and RAW temperatures accepted at face value despite questions surrounding the accuracy of both original recordings and homogenised adjustments. The increased frequency of days with a maximum between 20C and 35C suggests a more temperate climate. It is more accurate to say that Australia's climate has become warmer rather than hotter since 1910. Click here for an Excel download containing the annual tallies and average annual maxima at all 59 stations, and the calculations used in the analysis above. 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