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ACORN 2.4 influence on Australian temperature trends

(With ACORN 2.5 addendum)


In September 2023 the Bureau of Meteorology again rewrote Australia's temperature history with the release of ACORN 2.4, an update to its homogenised ACORN 2.3 dataset released in December 2021, introducing adjustments that have further distorted the rate of climate warming over the previous 112 years.

Adjusted ACORN 2.4 temperatures currently show 1.29C warming from 1910-1919 to 2013-2022, compared to 0.74C warming with original, unadjusted RAW temperatures.

ACORN is the Australian Climate Observation Reference Network of 112 weather stations across Australia, 104 of which the BoM uses to estimate national/state/territory monthly and annual temperatures since 1910. The rewritten ACORN 2 dataset of 2018 updated the ACORN 1 dataset released in 2011, and immediately increased Australia's per decade rate of mean temperature warming by 23%.

Each ACORN weather station has individual characteristics in terms of site moves, equipment and location or infrastructure changes over the decades since 1910 that warrant temperature adjustment, but the BoM does not adjust for variables such as urban/airport heat islands, 1972 metrication, the introduction of automatic weather stations mostly in the 1990s, an estimated 72.3% artificial warming influence of deteriorating thermometer screens, and site changes either overlooked or misinterpreted (see BomWatch).

ACORN 2.4 extends from 1910 to 2022 and introduces new or reassessed adjustments at 23 of the 112 ACORN stations, and at its long term temperature record page the BoM states that Analysis has shown the changed adjustments in ACORN-SAT 2.4 have had a negligible impact on estimated long-term warming trend in Australia.

In fact, ACORN 2.4 slightly warmed Australia's average temperatures in the first half of the 20th century when compared to ACORN 2.3, although still with significant cooling of unadjusted RAW temperatures.

When ACORN 2.4 absolute daily temperatures are measured, rather than 1961-90 anomalies, it is apparent that ACORN 2.4 has reduced average maxima warming by 0.04C and increased average minima warming by 0.01C from 1910-1919 to 2010-2022 at the 104 non-urban ACORN stations used by the BoM to calculate Australia's national temperature trends. ACORN 2.4 cooled 1910-1919 minima by a further 0.02C compared to ACORN 2.3 at the 104 stations.

Below are charts and tables for minima, maxima and rainfall at the 61 ACORN stations observing temperatures in 1910, unlike all other ACORN stations that opened in following years or decades, and thus provide datasets with the longest timespan (albeit many shifted their location from, for example, a post office to an airport).

australia acorn 2.4 minimum temperatures

104 ACORN v2.3, 2.4 and RAW stations minima Excel spreadsheets

australia acorn 2.4 maximum temperatures

104 ACORN v2.3, 2.4 and RAW stations maxima Excel spreadsheets

In its climate data frequently asked questions (click question 26), the Bureau of Meteorology claims to have made new adjustments in ACORN 2.4 to four different stations (Snowtown, Cape Borda, Wyalong and Wilcannia) and claims to have reassessed new reference data for 11 other stations (Port Hedland, Tennant Creek, Oodnadatta, Palmerville, Georgetown, Charters Towers, Birdsville, Scone, Canberra, Deniliquin, Sale).

This totals 15 stations.

However, ACORN 2.4 has adjusted ACORN 2.3 at 23 different weather stations (Adelaide, Birdsville, Bourke, Broome, Canberra, Cape Borda, Charters Towers, Gabo Island, Georgetown, Mildura, Nowra, Oodnadatta, Palmerville, Port Hedland, Port Macquarie, Rutherglen, Scone, Snowtown, Sydney, Tennant Creek, Walgett, Wilcannia, Wyalong).

Station menu

For specific details of each of the 112 ACORN stations you can either scroll down or click your station of choice from the following list to see charts and tables (with changes from ACORN 2.3 to ACORN 2.4 shown in italics), each followed by a link to return to this list.

Adelaide, Albany, Alice Springs, Amberley, Barcaldine, Bathurst, Birdsville, Boulia, Bourke, Bridgetown, Brisbane, Broome, Bundaberg, Burketown, Butlers Gorge, Cabramurra, Cairns, Camooweal, Canberra, Cape Borda, Cape Bruny, Cape Leeuwin, Cape Moreton, Cape Otway, Carnarvon, Ceduna, Charleville, Charters Towers, Cobar, Coffs Harbour, Cunderdin, Dalwallinu, Darwin, Deniliquin, Eddystone Point, Esperance, Eucla, Forrest, Gabo Island, Gayndah, Georgetown, Geraldton, Giles, Grove, Gunnedah, Halls Creek, Hobart, Horn Island, Inverell, Kalgoorlie, Kalumburu, Karijini North, Katanning, Kerang, Kyancutta, Launceston, Laverton, Learmonth, Longreach, Low Head, Mackay, Marble Bar, Marree, Meekatharra, Melbourne, Merredin, Mildura, Miles, Morawa, Moree, Moruya Heads, Mount Gambier, Nhill, Normanton, Nowra, Nuriootpa, Oodnadatta, Orbost, Palmerville, Perth, Point Perpendicular, Port Hedland, Port Lincoln, Port Macquarie, Rabbit Flat, Richmond (NSW), Richmond (Qld), Robe, Rockhampton, Rutherglen, Sale, Scone, Snowtown, St George, Sydney, Tarcoola, Tennant Creek, Thargomindah, Tibooburra, Townsville, Victoria River Downs, Wagga Wagga, Walgett, Wandering, Weipa, Wilcannia, Williamtown, Wilsons Promontory, Woomera, Wyalong, Yamba

Average of all 112 stations

Wilcannia

The most significant update of ACORN 2.3 was the addition of Wilcannia's temperature data from 1910 to 1956 in ACORN 2.4, with maximums prior to 1950 at this weather station being cooled by -0.43C and minimums prior to 1921 being cooled by -0.87C.

These adjustments are indicative of the homogenisation that has cooled Australia's temperature history and its failure to account for all elements that influence the accuracy of historic temperature observations.

Prior to metrication of Australia's temperature observations in September 1972, most observations were recorded as rounded x.0F with a majority of observers considering decimal readings on their Fahrenheit thermometers unimportant and instead recording a full number such as 64F or 86F and considering this more accurate than rounding up if they could not clearly see the decimal position of the alcohol or mercury.

Three dataset comparisons by the BoM confirmed an anomalous and probably artificial 0.1C increase in Australian temperatures at the time of 1972 metrication but, with odd reasoning, these findings were dismissed and not included in ACORN homogenisation on the basis that there may have been natural warming due to record national cloud cover and rainfall in the early to mid 1970s.

Metrication influence on Australian temperature trends, analyses all ACORN stations including Wilcannia's historic temperatures when benchmarked against Extreme Temperature Events in Australia, a 2001 PhD thesis by ACORN-SAT architect Blair Trewin from the Bureau of Meteorology.

His study estimated that among 94 of the 112 ACORN-SAT stations, the average proportion of Fahrenheit temperatures rounded by observers every day from 1957 to 1971 was 51.5%.

Independent analysis by this website estimates the same 94 stations had a rounding average of 51.3% from 1957 to 1971, the similarity to 51.5% suggesting concurrence between analyses.

The analysis by Trewin estimated that 99.7% of all temperatures at Wilcannia were rounded .0F from 1957 to 1971, and this website's independent analysis also calculated the proportion to be 99.7%.

The likely artificial cooling influence of rounded Fahrenheit temperatures at Wilcannia is ignored in the station's update within ACORN 2.4, which instead further cools the original observations by -0.43C (max <1950) and -0.87C (min <1921).

Tabulated below is a comparison between the estimates of Trewin and independent analysis of the 94 ACORN stations, and on the right the annual 0.F rounding percentage at Wilcannia since 1910.

wilcannia temperature rounding

Variable adjustments

ACORN 2.4 anomalies, which are the difference compared to 1961-90 averages, show the maximum anomaly averaging -0.396C in 1910-1919 and +1.053C in 2010-2022, suggesting 1.449C warming of maxima between those periods. ACORN 2.4 anomalies show minimum averages at -0.518C in 1910-1919 and +0.678C in 2010-2022, suggesting 1.196C warming of minima between those periods.

ACORN 2.3 maxima averaged 24.75C in 1910-1939 and ACORN 2.4 averaged 24.80C, a warming of 0.05C. ACORN 2.3 minima averaged 12.44C in 1910-1939 and ACORN 2.4 averaged 12.42C, a cooling of 0.02C.

The graphics below help to illustrate the timing and amount of difference between ACORN 2.3 and ACORN 2.4 absolute average annual temperatures at the 104 non-urban ACORN stations within the timeline of 1910 to 2021 :

ACORN 2.3 australia acorn 2.4 temperatures

Although ACORN 2.4 in total makes little difference to ACORN 2.3 absolute temperatures or anomalies when compared to 1961-90 averages, it is worth noting the ACORN bias toward cooling historic trends by comparing absolute temperatures in RAW, ACORN 2.4 and the original ACORN 1 homogenised dataset released in 2011 which adjusted temperatures from 1910 to 2017 :

acorn and raw temperature differences

The ACORN updates from version 1 to version 2.4 have added 0.44C mean temperature warming if comparing 1910-1919 with 2010-2019 (final year of ACORN 1).

It is worth noting that these results accord with separate analysis on this website showing that Australia's mean temperature has been cooling for well over a decade prior to 2024.

Accuracy of ACORN adjustments

Apart from variations between the two ACORN datasets, different stations have had their original historic observation adjusted either up down, with averages showing a bias to cooling in the early 1900s.

Apart from 1972 metrication, the introduction of automatic weather stations and site changes either overlooked or misinterpreted, another variable that may influence warming since 1910 is rainfall trends.

The chart below shows that the 61 ACORN stations that were observing temperatures in 1910 had significantly more rainfall than the 21 other ACORN stations that started observing temperatures in years or decades later but which were nevertheless recording rainfall measurements in 1910 (the remaining ACORN station were not recording rainfall levels in 1910).

acorn weather station rainfall

As 61 stations v 21 stations is not an even comparison, the chart below compares the 61 ACORN stations that were observing temperatures in 1910 with 61 other stations, both ACORN and non-ACORN, that were widely distributed across Australia and recording rainfall measurements in 1910, showing a lesser difference but still suggesting that ACORN stations in the first half of the 20th century had above average rainfall which has since declined (possibly due to a disproportionate number of such stations located in the south-east quadrant of Australia).

This above average but thereafter declining rainfall is likely to have caused some natural cooling in the early 1900s which has since dissipated, resulting in cooler averages in the first half of the ACORN timeline and potentially distorting temperature trends attributed to artificial warming caused by carbon dioxide.

acorn and non-acorn weather station rainfall

Alternatively, the 61 ACORN stations observing temperatures in 1910 can be compared with the BoM's national average annual rainfall estimates from all weather stations across Australia, charted below.

acorn and national weather station rainfall

Individual stations

Minima and maxima at each of the 112 ACORN stations are charted and tabulated below as decadal averages for ACORN 2.3, ACORN 2.4 and RAW absolute temperatures.

For specific details of each of the 112 ACORN stations you can either scroll down or click your station of choice from the following list to see charts and tables (with changes from ACORN 2.3 to ACORN 2.4 shown in italics), each followed by a link to return to this list.

Adelaide, Albany, Alice Springs, Amberley, Barcaldine, Bathurst, Birdsville, Boulia, Bourke, Bridgetown, Brisbane, Broome, Bundaberg, Burketown, Butlers Gorge, Cabramurra, Cairns, Camooweal, Canberra, Cape Borda, Cape Bruny, Cape Leeuwin, Cape Moreton, Cape Otway, Carnarvon, Ceduna, Charleville, Charters Towers, Cobar, Coffs Harbour, Cunderdin, Dalwallinu, Darwin, Deniliquin, Eddystone Point, Esperance, Eucla, Forrest, Gabo Island, Gayndah, Georgetown, Geraldton, Giles, Grove, Gunnedah, Halls Creek, Hobart, Horn Island, Inverell, Kalgoorlie, Kalumburu, Karijini North, Katanning, Kerang, Kyancutta, Launceston, Laverton, Learmonth, Longreach, Low Head, Mackay, Marble Bar, Marree, Meekatharra, Melbourne, Merredin, Mildura, Miles, Morawa, Moree, Moruya Heads, Mount Gambier, Nhill, Normanton, Nowra, Nuriootpa, Oodnadatta, Orbost, Palmerville, Perth, Point Perpendicular, Port Hedland, Port Lincoln, Port Macquarie, Rabbit Flat, Richmond (NSW), Richmond (Qld), Robe, Rockhampton, Rutherglen, Sale, Scone, Snowtown, St George, Sydney, Tarcoola, Tennant Creek, Thargomindah, Tibooburra, Townsville, Victoria River Downs, Wagga Wagga, Walgett, Wandering, Weipa, Wilcannia, Williamtown, Wilsons Promontory, Woomera, Wyalong, Yamba

Average of all 112 stations

Adelaide

acorn temperatures adelaide

ACORN station list

Albany

acorn temperatures albany

ACORN station list

Alice Springs

acorn temperatures alice springs

ACORN station list

Amberley

acorn temperatures amberley

ACORN station list

Barcaldine

acorn temperatures barcaldine

ACORN station list

Bathurst

acorn temperatures bathurst

ACORN station list

Birdsville

acorn temperatures birdsville

ACORN station list

Boulia

acorn temperatures boulia

ACORN station list

Bourke

acorn temperatures bourke

ACORN station list

Bridgetown

acorn temperatures bridgetown

ACORN station list

Brisbane

acorn temperatures brisbane

ACORN station list

Broome

acorn temperatures broome

ACORN station list

Bundaberg

acorn temperatures bundaberg

ACORN station list

Burketown

acorn temperatures burketown

ACORN station list

Butlers Gorge

acorn temperatures butlers gorge

ACORN station list

Cabramurra

acorn temperatures cabramurra

ACORN station list

Cairns

acorn temperatures cairns

ACORN station list

Camooweal

acorn temperatures camooweal

ACORN station list

Canberra

acorn temperatures canberra

ACORN station list

Cape Borda

acorn temperatures cape borda

ACORN station list

Cape Bruny

acorn temperatures cape bruny

ACORN station list

Cape Leeuwin

acorn temperatures cape leeuwin

ACORN station list

Cape Moreton

acorn temperatures cape moreton

ACORN station list

Cape Otway

acorn temperatures cape otway

ACORN station list

Carnarvon

acorn temperatures carnarvon

ACORN station list

Ceduna

acorn temperatures ceduna

ACORN station list

Charleville

acorn temperatures charleville

ACORN station list

Charters Towers

acorn temperatures charters towers

ACORN station list

Cobar

acorn temperatures cobar

ACORN station list

Coffs Harbour

acorn temperatures coffs harbour

ACORN station list

Cunderdin

acorn temperatures cunderdin

ACORN station list

Dalwallinu

acorn temperatures dalwallinu

ACORN station list

Darwin

acorn temperatures darwin

ACORN station list

Deniliquin

acorn temperatures deniliquin

ACORN station list

Dubbo

acorn temperatures dubbo

ACORN station list

Eddystone Point

acorn temperatures eddystone point

ACORN station list

Esperance

acorn temperatures esperance

ACORN station list

Eucla

acorn temperatures eucla

ACORN station list

Forrest

acorn temperatures forrest

ACORN station list

Gabo Island

acorn temperatures gabo island

ACORN station list

Gayndah

acorn temperatures gayndah

ACORN station list

Georgetown

acorn temperatures georgetown

ACORN station list

Geraldton

acorn temperatures geraldton

ACORN station list

Giles

acorn temperatures giles

ACORN station list

Grove

acorn temperatures grove

ACORN station list

Gunnedah

acorn temperatures gunnedah

ACORN station list

Halls Creek

acorn temperatures halls creek

ACORN station list

Hobart

acorn temperatures hobart

ACORN station list

Horn Island

acorn temperatures horn island

ACORN station list

Inverell

acorn temperatures inverell

ACORN station list

Kalgoorlie

acorn temperatures kalgoorlie

ACORN station list

Kalumburu

acorn temperatures kalumburu

ACORN station list

Karijini North

acorn temperatures karijini

ACORN station list

Katanning

acorn temperatures katanning

ACORN station list

Kerang

acorn temperatures kerang

ACORN station list

Kyancutta

acorn temperatures kyancutta

ACORN station list

Launceston

acorn temperatures launceston

ACORN station list

Laverton

acorn temperatures laverton

ACORN station list

Learmonth

acorn temperatures learmonth

ACORN station list

Longreach

acorn temperatures longreach

ACORN station list

Low Head

acorn temperatures low head

ACORN station list

Mackay

acorn temperatures mackay

ACORN station list

Marble Bar

acorn temperatures marble bar

ACORN station list

Marree

acorn temperatures marree

ACORN station list

Meekatharra

acorn temperatures meekatharra

ACORN station list

Melbourne

acorn temperatures melbourne

ACORN station list

Merredin

acorn temperatures merredin

ACORN station list

Mildura

acorn temperatures mildura

ACORN station list

Miles

acorn temperatures miles

ACORN station list

Morawa

acorn temperatures morawa

ACORN station list

Moree

acorn temperatures moree

ACORN station list

Moruya Heads

acorn temperatures moruya heads

ACORN station list

Mount Gambier

acorn temperatures mount gambier

ACORN station list

Nhill

acorn temperatures nhill

ACORN station list

Normanton

acorn temperatures normanton

ACORN station list

Nowra

acorn temperatures nowra

ACORN station list

Nuriootpa

acorn temperatures nuriootpa

ACORN station list

Oodnadatta

acorn temperatures oodnadata

ACORN station list

Orbost

acorn temperatures orbost

ACORN station list

Palmerville

acorn temperatures palmerville

ACORN station list

Perth

acorn temperatures perth

ACORN station list

Point Perpendicular

acorn temperatures point perpendicular

ACORN station list

Port Hedland

acorn temperatures port hedland

ACORN station list

Port Lincoln

acorn temperatures port lincoln

ACORN station list

Port Macquarie

acorn temperatures port macquarie

ACORN station list

Rabbit Flat

acorn temperatures rabbit flat

ACORN station list

Richmond New South Wales

acorn temperatures richmond new south wales

ACORN station list

Richmond Queensland

acorn temperatures richmond queensland

ACORN station list

Robe

acorn temperatures robe

ACORN station list

Rockhampton

acorn temperatures rockhampton

ACORN station list

Rutherglen

acorn temperatures rutherglen

ACORN station list

Sale

acorn temperatures sale

ACORN station list

Scone

acorn temperatures scone

ACORN station list

Snowtown

acorn temperatures snowtown

ACORN station list

St George

acorn temperatures st george

ACORN station list

Sydney

acorn temperatures sydney

ACORN station list

Tarcoola

acorn temperatures tarcoola

ACORN station list

Tennant Creek

acorn temperatures tennant creek

ACORN station list

Thargomindah

acorn temperatures thargomindah

ACORN station list

Tibooburra

acorn temperatures tibooburra

ACORN station list

Townsville

acorn temperatures townsville

ACORN station list

Victoria River Downs

acorn temperatures victoria river downs

ACORN station list

Wagga Wagga

acorn temperatures wagga wagga

ACORN station list

Walgett

acorn temperatures walgett

ACORN station list

Wandering

acorn temperatures wandering

ACORN station list

Weipa

acorn temperatures weipa

ACORN station list

Wilcannia

acorn temperatures wilcannia

ACORN station list

Williamtown

acorn temperatures williamtown

ACORN station list

Wilsons Promontory

acorn temperatures wilsons promontory

ACORN station list

Woomera

acorn temperatures woomera

ACORN station list

Wyalong

acorn temperatures wyalong

ACORN station list

Yamba

acorn temperatures yamba

ACORN station list

Average of all 112 stations

acorn temperatures all 112 stations

ACORN station list




Addendum

ACORN 2.5 influence on Australian temperature trends

In August 2024 the Bureau of Meteorology again rewrote Australia's temperature history with the release of ACORN 2.5, an update to its homogenised ACORN 2.4 dataset released in September 2023, introducing further adjustments that have on average warmed the early 20th century temperatures at 18 weather stations but also further warmed more recent temperatures at those locations since then.

As detailed at the bureau's Australia's climate data FAQ (see final question), 10 stations have new statistical adjustments due to non-climatological influences not previously detected, and eight stations adjusted in ACORN 2.4 have been reassessed - again due to non-climatological influences.

All new adjustments are to maxima for every year of the 10 affected stations prior to either 2019 or 2020, and most reassassments have been to both maximum and minimum temperatures at the eight affected stations in all years prior to 2016 and 2020, dependent upon the location.

Nine of the newly adjusted stations had their histories warmed (Cairns, Cape Bruny, Cobar, Grove, Low Head, Mackay, Mt Gambier, Nowra, Richmond Qld) and one station had its pre-2020 annual maxima cooled (Canberra, where most politicians would be unaware the nation's capital has warmed 0.37C more than previously thought since 1914).

Among the ACORN 2.5 reassessments of ACORN 2.4 at nine weather stations (Birdsville, Cape Borda, Cobar which has been both reassessed and newly adjusted, Mildura, Port Hedland, Scone, Snowtown, Tennant Creek, Wyalong), two have had their historic maxima reassessed and seven have had their historic minima reassessed, while Snowtown has had both its minima and maxima reassessed.

Early 20th century RAW minimum temperatures at the nine newly adjusted weather stations that were operational in 1910 highlight the influence of ACORN adjustments to Australia's historic temperatures, and all tables below illustrate how the nation's average temperatures have plateaued or cooled from 2012 to 2024 (see here)

The charts below illustrate the average minima and maxima at the nine affected stations that were all operational in 1910, followed by all 18 affected stations that were operational as of 1965 (when Scone began observations in the ACORN datasets).

minimum temperatures since 1910 in acorn 2.5

maximum temperatures since 1910 in acorn 2.5

minimum temperatures since 1965 in acorn 2.5

maximum temperatures since 1965 in acorn 2.5




Climate researcher Ken Stewart has analysed how temperature trends in different Australian regions vary from 0.1C per decade to 0.3C per decade from 1910 to 2020.



The data collation and research on this page were kindly assisted by Queensland programmer and climate researcher Patrick Lloyd.



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