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How ACORN changed the climate in eastern Australia


Bureau of Meteorology adjustments to historic temperature observations have turned a reduction in the number of very hot days (40C+) across Australia since 1910 into an increase in very hot days (see No more extreme hot days in Australia than 100 years ago).

But what about eastern Australia where most of our population live with all the associated infrastructure, and where drought and bushfires in 2019/2020 have caused many people to claim it’s evidence that the science of climate warming is settled?

The BoM provides annual and monthly data for specified rainfall regions, including eastern Australia which encompasses all of Queensland, NSW, Victoria and Tasmania.

Within those four states there are 36 stations in the Australian Climate Observation Reference Network (ACORN) that were open and recording temperatures in 1910 (Bathurst, Boulia, Bourke, Bundaberg, Burketown, Cairns, Cape Moreton, Cape Otway, Charleville, Charters Towers, Cobar, Deniliquin, Eddystone Point, Gabo Island, Gayndah, Georgetown, Inverell, Kerang, Launceston, Longreach, Low Head, Mackay, Melbourne, Mildura, Miles, Moruya Heads, Normanton, Palmerville, Port Macquarie, Richmond Qld, Sydney, Tibooburra, Wagga Wagga, Walgett, Wilsons Promontory, Yamba).

These 36 are among the total 112, including 104 non-urban, ACORN stations across Australia used by the bureau to calculate annual temperatures since 1910 through homogenisation that adjusts the original RAW observations. The ACORN homogenisation mostly cools average temperatures in the first half of the 20th century, resulting in bureau estimates that mean temperatures have increased by a little over 1C since 1910.

ACORN was originally introduced in 2011. Without any bureau announcement, ACORN 2 replaced ACORN 1 at the beginning of 2019 and the new method of homogenisation increased Australia’s per decade mean temperature warming trend since 1910 by 23%.

ACORN goes nuts with daily temperatures

ACORN 1 and ACORN 2 algorithms both illogically adjust historic daily temperature observations.

Overall, ACORN 2 adjustments further cooled ACORN 1 temperatures in the early 1900s which, in turn, had significantly cooled the RAW recordings. This can be graphically illustrated using only the 36 long-term weather stations across eastern Australia that were open in 1910 to calculate the average number of very hot days (40C+) that were recorded each year.

Apart from ensuring an identical timeline among all 36 long-term stations, this avoids the warming bias caused by the other eastern Australia stations that opened after 1910 and up to 1965, measurements showing that, on average, they’re in hotter locations that more often have 40C+ days.

The animated chart below shows the annual average number of very hot days at the 36 eastern Australia weather stations in the RAW, ACORN 1 and ACORN 2 daily temperature datasets …

very hot days in 36 eastern australia ACORN weather stations

So it appears that the original RAW daily observations at the 36 long-term stations, all recorded in Stevenson screens, show very hot days occurred more frequently before the 1970s when climate change is said to have started to influence temperatures.

ACORN 1 significantly reduced these annual averages in the early years, and ACORN 2 has created an even greater increase in the number of very hot days across eastern Australia.

Rainfall

The 36 long-term stations compared above in RAW, ACORN 1 and ACORN 2 are all within the BoM’s climatologically distinct “eastern Australia” rainfall region.

So what about the rainfall? The chart below demonstrates that annual rainfall, which invariably requires cloud cover on the day and often on surrounding days, has a clear correlation with the yearly number of very hot day.

The average rainfall data in November, December, January, February, March and April each year (the months when 40C+ days occur) can be compared with how many very hot days (defined by the bureau as 40C+) occurred annually at the 36 ACORN stations that were open in 1910 within the eastern Australia rainfall region.

“Unprecedented” drought and widespread bushfires starting in September 2019 have been portrayed as examples of climate change. It might be assumed that rainfall levels have never been so low and “extreme” very hot days have never so frequently scorched the landscape of eastern Australia.

But when the average rainfall at all bureau rainfall stations in eastern Australia from November to April each year is compared with annual very hot days at the 36 ACORN stations, it seems the opposite trends have happened since 1910 …

very hot days and rainfall in 36 eastern australia ACORN weather stations

RAW very hot day (40C+) annual averages:

1910-1963 12.02
1964-2017 10.65

ACORN 1 very hot day (40C+) annual averages:

1910-1963 10.68
1964-2017 10.84

ACORN 2 very hot day (40C+) annual averages:

1910-1963 9.66
1964-2017 10.08

Rainfall November to April averages:

1910-1963 69.17mm
1964-2017 72.26mm

The frequency of original RAW very hot days mostly correlates with cloudy rainfall averages and their trends since 1910, not with C02 or climate warming. ACORN adjustments reduce the original correlation between cloudy rainfall days and the number of very hot days that occur in wet or dry years.

It should be noted that the average annual frequency of very hot days is different to the average annual maximum temperature recorded each year in eastern Australia. Analysis of annual maxima at all 36 long-term ACORN weather stations in Queensland, NSW, Victoria and Tasmania shows averages from 1910-1963 to 1964-2017 increased 0.3C in RAW and 0.5C in ACORN 2.

Also, average November to April eastern Australia annual rainfall in 2013-2017 was 62.9mm, 11% down on the 1910-2017 average of 70.7mm (albeit better than the 1920s 10 year average of 62.6mm, 1926-1935 at 62.1mm, the 1930s 10 year average of 61.9mm, and the 1960s 10 year average of 62.8mm).

Most of the increase in rainfall and reduction in very hot days has been experienced in northern Australia. In south-east Australia, rainfall increased only slightly and average annual very hot days increased from 3.42 in 1910-1963 to 3.90 in 1964-2017.

Droughts and bushfires

Drought conditions in the centre of eastern Australia during 2015-2019 have been severe, and associated low rainfall winters have created a dry landscape sparking early bushfires over a wide area.

However, conditions aren’t unprecedented as there have been drier hot season conditions since 1910, and the original RAW temperature observations suggest our grandparents sweltered through more very hot days of 40C+.

Rainfall levels in the drought areas of NSW and Queensland aren’t necessarily the same as the rest of eastern Australia. Inland areas with reduced rainfall in recent years have had an increase in very hot days and average annual maxima.

There are 20 long-term ACORN weather stations (open in 1910) located in areas of southern Queensland and throughout NSW that in 2019 were declared drought affected, and where bushfires have caused damage since August/September 2019 (weather stations are Bathurst, Boulia, Bourke, Bundaberg, Cape Moreton, Charleville, Cobar, Deniliquin, Gayndah, Inverell, Longreach, Miles, Moree, Moruya Heads, Port Macquarie, Sydney, Tibooburra, Wagga Wagga, Walgett, Yamba).

In four year blocks, the records of these 20 stations show they received average November to April rainfall of:

61.7mm in 2015-2018
60.3mm in 2005-2008
60.0mm in 2004-2007
59.6mm in 2003-2006
57.5mm in 1979-1982
60.4mm in 1965-1968
56.6mm in 1943-1946
59.6mm in 1942-1945
61.3mm in 1919-1922
55.2mm in 1912-1915

The chart below shows a fairly strong correlation between Nov-Apr rainfall and the frequency of very hot days at the 20 long-term stations within the drought affected areas of southern Queensland and all of NSW, with November to April rainfall increasing in the mid 1900s but a decline since the 1970s.

However, the annual frequency of very hots days has declined by comparison with the early 1900s, possibly due to a slight increase in rainfall and associated cloudy days during the hot season (1910-1945 - 68.0mm / 1983-2018 - 71.1mm).

ACORN adjustments have reduced the average annual number of very hot days in the first half of the 1910-2017 temperature record by 29.2% :

very hot days and rainfall in 20 drought affected ACORN weather stations

RAW very hot day (40C+) annual averages:

1910-1963 14.61
1964-2017 12.48

ACORN 1 very hot day (40C+) annual averages:

1910-1963 12.20
1964-2017 13.21

ACORN 2 very hot day (40C+) annual averages:

1910-1963 10.34
1964-2017 11.78

Rainfall November to April averages:

1910-1963 73.1mm
1964-2017 72.6mm

From 1910-1964 to 1964-2017, the frequency of very hot days at the 20 ACORN station has declined by 14.6% based on original RAW recordings, but increased by 13.9% when adjusted by ACORN 2.

Average maxima

The frequency of very hot days (40C+) doesn’t necessarily reflect the average maximum daytime temperature in any given year.

Below compares annual average rainfall (mm) with annual average maxima within the ACORN 1, ACORN 2 and RAW datasets for the 20 long-term stations in drought affected regions of eastern Australia :

hot days and rainfall in 20 drought affected ACORN weather stations

RAW annual maximum averages:

1910-1963 25.35
1964-2017 25.57
(0.22C warmer)

ACORN 1 annual maximum averages:

1910-1963 25.24
1964-2017 25.66
(0.42C warmer)

ACORN 2 annual maximum averages:

1910-1963 25.22
1964-2017 25.63
(0.41C warmer)

Annual rainfall averages:

1910-1963 737.0mm
1964-2017 717.6mm

Based on the 20 long-term ACORN stations within drought affected regions of eastern Australia, there has been a 2.6% reduction in annual rainfall when comparing the first and second halves of 1910-2018.

From 1910-1963 to 1964-2017, average maximum temperatures have increased 0.22C in RAW and 0.41C in ACORN 2. Comparing 30 year climate periods, the average maximum RAW temperature was 25.43C in 1910-1939 and 25.84C in 1989-2018, up 0.41C.

The coolest average annual maximum at the 20 stations since 1910 was 23.91C in 1956, which also had the second highest annual rainfall of 1106.0mm. The warmest average annual maximum at the 20 stations since 1910 was 27.0C in 2018, which had the fifth lowest annual rainfall of 512.7mm  (1915 - 432.1mm; 1957 - 490.5mm; 2002 - 470.2mm; 2014 - 506.9mm). The lowest annual rainfall since 1910 was 432.1mm in 1915 with an average maximum of 26.3C.

The severity of drought and associated bushfires in southern Queensland and throughout NSW in 2019/20 is due to climate conditions that are extreme but not unprecedented. At the 20 long-term ACORN weather stations within these drought areas, an average annual 600.1mm of rain fell in the five years of 2014-2018. Average annual rainfall was 589.6mm in the five years of 2002-2006.

Farmers in the eastern Australia drought regions are struggling mostly because of a decline in winter rainfall, coupled with an approximate 11mm decline in average Nov-Apr rainfall over the past five years causing record hot season temperatures.

Dry vegetation after five years of low rainfall is fuel for bushfires. However, it is nevertheless worth noting reductions in firebreaks and increased fuel accumulation in recent decades, and that an estimated 85% of bushfires are nowadays lit by humans, which was a smaller problem in earlier years because there were fewer people, less infrastructure and arson was not as frequent.

The 2019/2020 bushfire crisis is exacerbated by delayed monsoonal rainfall arriving in northern Australia and spilling into eastern Australia, coupled with warm temperatures above Antarctica which traditionally suppresses rainfall in southern Queensland and NSW.

The drought since 2015 might be attributed to climate warming (aka CO2) but there is also evidence it is due to shifting rainfall patterns that can be expected to cycle back to normal with a commensurate reduction in average maxima and very hot days, as they have cycled since 1910.

The question is the timing of that drought-breaking shift influenced by otherwise increasing rainfall levels and reduced very hot day frequency in northern and eastern Australia, with the eastern seaboard enjoying heavy rainfall in January and February 2020.

A Study of Meteorological Conditions Associated with Bush and Grass Fires and Fire Protection Strategy in Australia, published by the BoM in 1947, provides a comprehensive analysis of the timing, frequency, extent and cause of bushfires since the 1800s.

Temperature and rainfall data for the 20 long-term ACORN stations within drought affected areas of southern Queensland and throughout NSW can be downloaded.

Weather station examples

Charleville is a long-term ACORN weather station centrally located in the southern Queensland region that was fully drought declared in 2019 :

very hot days and rainfall in charleville

RAW very hot day (40C+) annual averages:

1910-1963 14.4
1964-2017 7.5

ACORN 1 very hot day (40C+) annual averages:

1910-1963 6.0
1964-2017 8.4

ACORN 2 very hot day (40C+) annual averages:

1910-1963 5.2
1964-2017 7.9

Rainfall November to April averages:

1910-1963 56.62mm
1964-2017 50.70mm

The abrupt change in RAW very hot days at Charleville coincides with the site's location switching from the post office to the airport, about two kilometres away, so the chart’s data source changed in 1952.

The reduction in very hot days, as originally observed in RAW, also coincides with a significant rainfall increase at this time (49.7mm per year in 1932-1951, 59.1mm per year in 1953-1972), posing questions as to whether a two kilometre site relocation rather than natural variation changed both rainfall and temperatures levels, and why ACORN adjustments ignore the rainfall (cloudy day) data that correlates with very hot day frequency.

Moree is a long-term ACORN weather station centrally located in the northern NSW region declared to be suffering intense drought in 2019 :

very hot days and rainfall in Moree

RAW very hot day (40C+) annual averages:

1910-1963 7.5
1964-2017 3.3

ACORN 1 very hot day (40C+) annual averages:

1910-1963 0.6
1964-2017 3.2

ACORN 2 very hot day (40C+) annual averages:

1910-1963 3.5
1964-2017 3.2

Rainfall November to April averages:

1910-1963 55.13mm
1964-2017 60.09mm

Moree experienced a similar rainfall increase as Charleville (47.7mm per year in 1932-1951, 63.1mm per year in 1953-1972) and a similar decline in RAW very hot days. The Moree chart data source changes from the Moree Post Office to the Moree Comparison site, about six kilometres away, in 1965.

The enormous ACORN-adjusted reduction of very hots days at Charleville and Moree in the early 1900s are typical of national maximum and minimum annual averages that were cooled during that period by ACORN 1 and ACORN 2.

It might be assumed the historic cooling is based on evidence that most thermometers were recording hotter temperatures than actually happened, despite all stations using standardised recording equipment in Stevenson screens.

Seasonal and annual rainfall/cloud levels appear to be meaningless in the homogenised ACORN adjustments that calculate daily historic temperatures.

Rainfall and temperature observations

Although well below average since 2015, rainfall patterns across eastern Australia (Queensland, NSW, Victoria and Tasmania) have not decreased since 1910, particularly in the north-east, but there has been a slight decline in the central drought areas.

ACORN 2 cooling adjustments to original very hot day observations have been negligible since the late 1990s, coinciding with the national installation of automatic weather stations that log one second electronic recordings of brief hot air instead of the delayed response of the liquid thermometers they replaced.

All observations since 1910 have been in Stevenson screens, but most automatic weather stations are in smaller Stevenson screens that have greater heat conduction than their larger screen predecessors.

Note: The charts on this page are not produced by the Bureau of Meteorology but are based on the daily temperature and rainfall data provided by the bureau in its different datasets. Animation charts are free for use on any platform.




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