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Inconsistent Australian climate warming


Analysis of Australian maximum temperatures shows significantly different rates of climate warming at different weather stations since 1910.

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, 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.

The analysis below compares the 30 weather stations with the least warming and the 29 with the most warming from 1910-1969 to 1970-2019 and from 1910-1929 to 2000-2019.

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 bureau defines a hot day as above 35C and a very hot day as above 40C.

Least warming station counts

The analysis charts below show the 30 stations with the least warming expressed as the decadal annual average 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.

These ACORN stations are Boulia, Bourke, Bridgetown, Broome, Bundaberg, Cairns, Cape Moreton, Charleville, Charters Towers, Cobar, Darwin, Deniliquin, Gayndah, Halls Creek, Inverell, Kalgoorlie, Katanning, Launceston, Longreach, Mackay, Marble Bar, Mildura, Miles, Moree, Normanton, Palmerville, Sale, Tennant Creek, Tibooburra and Wagga Wagga.

The charts also show decadal average rainfall at the 30 low-warming ACORN weather stations (1910-1969 : 709.1mm > 1970-2019 : 721.9mm - up 1.8%).

sum maximum temperatures at coolest stations

Most warming station counts

The analysis charts below show the 29 stations with the most warming expressed as the decadal annual average 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.

These ACORN stations are Adelaide, Albany, Alice Springs, Bathurst, Burketown, Cape Leeuwin, Cape Otway, Carnarvon, Eddystone Point, Esperance, Gabo Island, Georgetown, Geraldton, Kerang, Low Head, Marree, Melbourne, Moruya Heads, Mount Gambier, Perth, Port Lincoln, Richmond (Qld), Robe, Snowtown, Sydney, Walgett, Wandering, Wilson Promontory and Yamba.

The charts also show decadal average rainfall at the 29 high-warming ACORN weather stations (1910-1969 : 693.5mm > 1970-2019 : 683.5mm - down 1.4%).

sum maximum temperatures at warmest stations

Least warming station Tmax averages

The analysis charts below show the 30 stations with the least warming expressed as the decadal annual average maximum temperature (Tmax) 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.

average maximum temperatures at coolest stations

Most warming station Tmax averages

The analysis charts below show the 29 stations with the most warming expressed as the decadal annual average maximum temperature (Tmax) 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.

average maximum temperatures at warmest stations

The four preceding charts can be tabulated, showing the average changes in the number and temperature of each 5C category at each of the 30 stations with the least warming and the 29 with the most warming when comparing 1910-1969 with 1970-2019:

australia decadal maximum temperature count and averages

Analysis : 1910-1969 to 1970-2019

When based on original RAW temperatures and comparing 1910-1969 with 1970-2019, Australians living near the 30 weather stations with the least warming (Boulia, Bourke, Bridgetown, Broome, Bundaberg, Cairns, Cape Moreton, Charleville, Charters Towers, Cobar, Darwin, Deniliquin, Gayndah, Halls Creek, Inverell, Kalgoorlie, Katanning, Launceston, Longreach, Mackay, Marble Bar, Mildura, Miles, Moree, Normanton, Palmerville, Sale, Tennant Creek, Tibooburra and Wagga Wagga) have experienced the following changes:

  • Uncomfortably cold 5C-10C maximum : 0.05 more days per year and 0.17C warmer with 12.8mm (1.8%) more rainfall
  • Uncomfortably chilly 10C-15C maximum : 0.55 fewer days per year and 0.16C warmer with 12.8mm (1.8%) more rainfall
  • Comfortably cool 15C-20C maximum : 0.32 more days per year and 0.15C warmer with 12.8mm (1.8%) more rainfall
  • Comfortable 20C-25C maximum : 0.26 fewer days per year and 0.12C warmer with 12.8mm (1.8%) more rainfall
  • Comfortably warm 25C-30C maximum : 0.71 more days per year and 0.10C warmer with 12.8mm (1.8%) more rainfall
  • Comfortably very warm 30C-35C maximum : 4.36 more days per year and 0.09C warmer with 12.8mm (1.8%) more rainfall
  • Uncomfortably hot 35C-40C maximum : 0.60 fewer days per year and 0.05C warmer with 12.8mm (1.8%) more rainfall
  • Uncomfortably very hot 40C-45C maximum : 1.03 fewer days per year and 0.06C warmer with 12.8mm (1.8%) more rainfall
  • Uncomfortably extremely hot 45C-50C maximum : 0.08 fewer days per year and 0.03C cooler with 12.8mm (1.8%) more rainfall

When based on original RAW temperatures and comparing 1910-1969 with 1970-2019, Australians living near the 29 weather stations with the most warming (Adelaide, Albany, Alice Springs, Bathurst, Burketown, Cape Leeuwin, Cape Otway, Carnarvon, Eddystone Point, Esperance, Gabo Island, Georgetown, Geraldton, Kerang, Low Head, Marree, Melbourne, Moruya Heads, Mount Gambier, Perth, Port Lincoln, Richmond (Qld), Robe, Snowtown, Sydney, Walgett, Wandering, Wilson Promontory and Yamba) have experienced the following changes:

  • Uncomfortably cold 5C-10C maximum : 0.63 fewer days per year and 0.10C warmer with 10mm (1.4%) less rainfall
  • Uncomfortably chilly 10C-15C maximum : 8.62 fewer days per year and 0.20C warmer with 10mm (1.4%) less rainfall
  • Comfortably cool 15C-20C maximum : 3.15 fewer days per year and 0.11C warmer with 10mm (1.4%) less rainfall
  • Comfortable 20C-25C maximum : 2.77 more days per year and 0.12C warmer with 10mm (1.4%) less rainfall
  • Comfortably warm 25C-30C maximum : 3.22 more days per year and 0.09C warmer with 10mm (1.4%) less rainfall
  • Comfortably very warm 30C-35C maximum : 3.04 more days per year and 0.08C warmer with 10mm (1.4%) less rainfall
  • Uncomfortably hot 35C-40C maximum : 3.08 more days per year and 0.08C warmer with 10mm (1.4%) less rainfall
  • Uncomfortably very hot 40C-45C maximum : 1.47 more days per year and 0.17C warmer with 10mm (1.4%) less rainfall
  • Uncomfortably extremely hot 45C-50C maximum : 0.14 more days per year and 0.11C warmer with 10mm (1.4%) less rainfall

Analysis : 1910-1929 to 2000-2019

When based on original RAW temperatures and comparing 1910-1929 with 2000-2019, Australians living near the 30 weather stations with the least warming (Boulia, Bourke, Bridgetown, Broome, Bundaberg, Cairns, Cape Moreton, Charleville, Charters Towers, Cobar, Darwin, Deniliquin, Gayndah, Halls Creek, Inverell, Kalgoorlie, Katanning, Launceston, Longreach, Mackay, Marble Bar, Mildura, Miles, Moree, Normanton, Palmerville, Sale, Tennant Creek, Tibooburra and Wagga Wagga) have experienced the following changes:

  • Uncomfortably cold 5C-10C maximum : 0.05 more days per year and 0.35C warmer with 24.1mm (3.4%) less rainfall
  • Uncomfortably chilly 10C-15C maximum : 1.94 fewer days per year and 0.20C warmer with 24.1mm (3.4%) less rainfall
  • Comfortably cool 15C-20C maximum : 1.30 fewer days per year and 0.16C warmer with 24.1mm (3.4%) less rainfall
  • Comfortable 20C-25C maximum : 1.37 fewer days per year and 0.15C warmer with 24.1mm (3.4%) less rainfall
  • Comfortably warm 25C-30C maximum : 0.36 more days per year and 0.13C warmer with 24.1mm (3.4%) less rainfall
  • Comfortably very warm 30C-35C maximum : 6.36 more days per year and 0.09C warmer with 24.1mm (3.4%) less rainfall
  • Uncomfortably hot 35C-40C maximum : 0.79 more days per year and 0.08C warmer with 24.1mm (3.4%) less rainfall
  • Uncomfortably very hot 40C-45C maximum : 0.68 more days per year and 0.15C warmer with 24.1mm (3.4%) less rainfall
  • Uncomfortably extremely hot 45C-50C maximum : 0.08 more days per year and 0.01C warmer with 24.1mm (3.4%) less rainfall

When based on original RAW temperatures and comparing 1910-1929 with 2000-2019, Australians living near the 29 weather stations with the most warming (Adelaide, Albany, Alice Springs, Bathurst, Burketown, Cape Leeuwin, Cape Otway, Carnarvon, Eddystone Point, Esperance, Gabo Island, Georgetown, Geraldton, Kerang, Low Head, Marree, Melbourne, Moruya Heads, Mount Gambier, Perth, Port Lincoln, Richmond (Qld), Robe, Snowtown, Sydney, Walgett, Wandering, Wilson Promontory and Yamba) have experienced the following changes:

  • Uncomfortably cold 5C-10C maximum : 1.17 fewer days per year and 0.17C warmer with 47.1mm (6.9%) less rainfall
  • Uncomfortably chilly 10C-15C maximum : 12.09 fewer days per year and 0.29C warmer with 47.1mm (6.9%) less rainfall
  • Comfortably cool 15C-20C maximum : 7.10 fewer days per year and 0.13C warmer with 47.1mm (6.9%) less rainfall
  • Comfortable 20C-25C maximum : 4.20 more days per year and 0.14C warmer with 47.1mm (6.9%) less rainfall
  • Comfortably warm 25C-30C maximum : 4.19 more days per year and 0.10C warmer with 47.1mm (6.9%) less rainfall
  • Comfortably very warm 30C-35C maximum : 4.41 more days per year and 0.12C warmer with 47.1mm (6.9%) less rainfall
  • Uncomfortably hot 35C-40C maximum : 5.79 more days per year and 0.05C warmer with 47.1mm (6.9%) less rainfall
  • Uncomfortably very hot 40C-45C maximum : 2.28 more days per year and 0.38C warmer with 47.1mm (6.9%) less rainfall
  • Uncomfortably extremely hot 45C-50C maximum : 0.30 more days per year and 0.22C warmer with 47.1mm (6.9%) less rainfall

Among the 30 locations with the least warming, 17 are in northern Australia (Boulia, Broome, Bundaberg, Cairns, Cape Moreton, Charleville, Charters Towers, Darwin, Gayndah, Halls Creek, Longreach, Mackay, Marble Bar, Miles, Normanton, Palmerville and Tennant Creek) and the other 13 in southern Australia (Bourke, Bridgetown, Cobar, Deniliquin, Inverell, Kalgoorlie, Katanning, Launceston, Mildura, Moree, Sale, Tibooburra and Wagga Wagga).

Among the 29 locations with the most warming, five are in northern Australia (Alice Springs, Burketown, Carnarvon, Georgetown and Richmond (Qld)) and the other 24 in southern Australia (Adelaide, Albany, Bathurst, Cape Leeuwin, Cape Otway, Eddystone Point, Esperance, Gabo Island, Geraldton, Kerang, Low Head, Marree, Melbourne, Moruya Heads, Mount Gambier, Perth, Port Lincoln, Robe, Snowtown, Sydney, Walgett, Wandering, Wilson Promontory and Yamba).

This suggests increased rainfall in northern Australia since the 1970s has contributed to respective warming trends among weather stations with the least and most.

The rainfall influence is supported by the fact that from 1910-1969 to 1970-2019 the 30 stations with least warming had 0.06 fewer days with a maximum below 20C, while the 29 stations with most warming had 4.13 fewer days with a maximum below 20C.

From 1910-1929 to 2000-2019 the 30 stations with least warming had 1.06 fewer days with a maximum below 20C, while the 29 stations with most warming had 6.79 fewer days with a maximum below 20C.

Spotlight : 1910-1969 to 1970-2019

From 1910-1969 to 1970-2019, the 30 stations with least warming had an average 1.60 more days with a warm maximum between 20C and 35C, while the 29 stations with most warming had an average 3.01 more days with a warm maximum between 20C and 35C.

From 1910-1969 to 1970-2019, the 30 stations with least warming had an average 0.57 fewer days with a hot maximum above 35C, while the 29 stations with most warming had an average 1.56 more days with a hot maximum above 35C.

Albeit influenced by a greater number of days falling into a higher 5C category from 1910-1969 to 1970-2019 (e.g. 24.9C days more frequently 25C or more), there isn't a significant difference in the Tmax of those categories below 35C. Above 35C, the 30 stations with the least warming had an average 0.02C increased Tmax while the 29 stations with the most warming had an average 0.12C increased Tmax.

From 1910-1969 to 1970-2019, the only 5C category in which the 30 stations with the least warming had a frequency increase greater than among the 29 stations with the most warming was 30C to 35C (4.36 v 3.04), and the preceding and following 5C categories suggests this may be due to fewer days with a higher maximum temperature due to increased rainfall (e.g. 30.0C days more frequently 29.9C or less).

From 1910-1969 to 1970-2019, the 30 stations with the least warming had an average 0.25 fewer uncomfortably cold 5C to 15C maximum temperature days per year, an average 1.28 more comfortably warm days per year from 15C to 35C, and an average 0.57 fewer uncomfortably hot days per year above 35C.

From 1910-1969 to 1970-2019, the 29 stations with the most warming had an average 4.62 fewer uncomfortably cold 5C to 15C maximum temperature days per year, an average 1.47 more comfortably warm days per year from 15C to 35C, and an average 1.56 more uncomfortably hot days per year above 35C.

Spotlight : 1910-1929 to 2000-2019

From 1910-1929 to 2000-2019, the 30 stations with least warming had an average 1.78 more days with a warm maximum between 20C and 35C, while the 29 stations with most warming had an average 4.27 more days with a warm maximum between 25C and 35C.

From 1910-1929 to 2000-2019, the 30 stations with least warming had a cumulative 0.52 more days with a hot maximum above 35C, while the 29 stations with most warming had an average 2.79 more days with a hot maximum above 35C.

Albeit influenced by a greater number of days falling into a higher 5C category from 1910-1969 to 1970-2019 (e.g. 24.9C days more frequently 25C or more), the averaged Tmax of those categories warmed an average 0.18C among the 30 lower-warming stations and an average 0.16C among the 29 high-warming stations. Above 35C, the 30 stations with the least warming had an average 0.08C increased Tmax while the 29 stations with the most warming had an average 0.21C increased Tmax.

From 1910-1929 to 2000-2019, the only 5C category in which the 30 stations with the least warming had a frequency increase greater than among the 29 stations with the most warming was 30C to 35C (6.36 v 4.41), and the preceding and following 5C categories suggests this may be due to fewer days with a higher maximum temperature due to increased rainfall (e.g. 30.0C days more frequently 29.9C or less).

From 1910-1929 to 2000-2019, the 30 stations with the least warming had an average 0.95 fewer uncomfortably cold 5C to 15C maximum temperature days per year, an average 1.01 more comfortably warm days per year from 15C to 35C, and an average 0.52 more uncomfortably hot days per year above 35C.

From 1910-1929 to 2000-2019, the 29 stations with the most warming had an average 6.63 fewer uncomfortably cold 5C to 15C maximum temperature days per year, an average 1.43 more comfortably warm days per year from 15C to 35C, and an average 2.79 more uncomfortably hot days per year above 35C.

Discussion

The average maximum temperature at the 30 ACORN stations with the least warming from 1910-1969 to 1970-2019 (RAW 27.35C > 27.42C; ACORN 2.1 26.91C > 27.43C) can be compared with the 29 stations with the most warming (RAW 22.36C > 23.10C; ACORN 2.1 22.43C > 23.08C).

The 30 ACORN stations with the least warming are on average in warmer local climates than are the 29 stations with the most warming. The least warming stations had an average RAW maximum temperature increase of 0.07C from 1910-1969 to 1970-2019 and the most warming stations had an average RAW maximum temperature increase of 0.74C.

The least warming stations had an average ACORN 2.1 maximum temperature increase of 0.52C from 1910-1969 to 1970-2019 and the most warming stations had an average ACORN 2.1 maximum temperature increase of 0.61C.

The average maximum temperature at the 30 ACORN stations with the least warming from 1910-1929 to 2000-2019 (RAW 27.47C > 27.83C; ACORN 2.1 26.94C > 27.85C) can be compared with the 29 stations with the most warming (RAW 22.42C > 23.53C; ACORN 2.1 22.55C > 23.52C).

The least warming stations had an average RAW maximum temperature increase of 0.11C from 1910-1929 to 2000-2019 and the most warming stations had an average RAW maximum temperature increase of 1.11C.

The least warming stations had an average ACORN 2.1 maximum temperature increase of 0.91C from 1910-1929 to 2000-2019 and the most warming stations had an average ACORN 2.1 maximum temperature increase of 0.97C.

This suggests Australians in cooler climates have experienced the most warming while Australians in hotter climates have experienced the least warming.

Another way to consider the different rates of warming is by comparing the average temperature in the 10 years before and the 10 years after an automatic weather station started operating at each of the ACORN stations :

minimum temperatures before and after automatic weather stations

maximum temperatures before and after automatic weather stations

Conclusion

There is a correlation between rainfall and maximum temperature trends at ACORN long-term weather stations with the most and least warming since 1910.

However, it cannot be confirmed if the significant differences between the least and most warming ACORN stations are due to rainfall or other variables such as instruments, screens and relocations of individual weather stations which may or may not have been accurately compensated in the ACORN temperature adjustment process.

From 1910-1969 to 1970-2019, ACORN has warmed maximum temperatures increases at the 30 least-warming stations by 0.45C and cooled maximum temperature increases at the 29 most-warming stations by 0.13C

From 1910-1929 to 2000-2019, ACORN has warmed maximum temperatures increases at the 30 least-warming stations by 0.80C and cooled maximum temperature increases at the 29 most-warming stations by 0.14C

As in the analysis of national, northern and southern ACORN weather stations, the increased frequency of days with a maximum between 20C and 35C suggests a more temperate climate.

However, this analysis suggests an inconsistent rate of climate warming and temperate climate shift in different Australians regions.




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