Was 2011 Perth's hottest year ever?


Contrary to media reports, climate records show 2011 was not the hottest year ever in Perth. The climate records also raise questions about the accuracy of the city's temperature trend for the past 114 years.


Most media outlets have reported since mid-December that 2011 was the hottest year ever recorded in Perth. The claim is correct in terms of average maxima recorded in the suburb of Mt Lawley since 1994 but incorrect in terms of mean temperatures and hot days at other recording locations for the Perth temperature record since 1897.

The Australian Bureau of Meteorology states that Perth temperatures spiked in the 1970s and have continued to climb each year, particularly over the past decade and consistent with climate change. The BoM also states that Perth didn't record a below average monthly temperature after November 2010.

Perth had 10mm above the annual average rainfall in 2011 and showery, mild conditions throughout winter, spring and into summer, with a burst of heat in late November and at the beginning of December hitting 37.6C.

With many Perth residents remarking on the overcast, comfortable weather and lack of "scorchers" during 2011, it's worth examining the claim that it was the hottest year ever recorded in Perth.


Perth temperature history

Perth's temperature history began in 1897 atop Mt Eliza (elevation 61m) with the establishment by the Bureau of Meteorology of weather station 9034 (Perth Regional Office).

In 1963 the station was moved about 300m and in 1967 it was relocated about two kilometres to inner city Wellington St, East Perth (elevation 19m).

Station 9034 closed in 1992 and the Stevenson Screen thermometer was moved four kilometres north to establish the Perth Metro station in Mt Lawley (9225, elevation 24.9m), with the 9034 historic record adjusted for baseline equivalence for both the Perth Metro and Perth Airport stations.

The 9034 temperature chart is:


perth temperature history


The 1970s spike is apparent and it's noticeable that Perth temperatures increased by about .7C when the Stevenson Screen was moved from the top of Mt Eliza in West Perth to Wellington St in East Perth. In a sense, it's Perth's very own climate hockey stick.

Is a few kilometres really that important in the same weather and does West Perth around Kings Park have different temperatures than East Perth in the central city?

The only known direct temperature comparison between West Perth and East Perth is within 11 year mean temperature records from 1897 to 1907, during which time Mt Eliza recorded 63.9F (17.72C) and the Perth Gardens (elevation unknown but at Supreme Court Gardens near the Swan River and about the same elevation as Wellington St) recorded 64.9F (18.27C).

That's almost 0.6C warmer at the lower elevation in the same city and the same weather at the same time, a similar shift to the Wellington St increase in the chart above.

It is assumed the BoM has made appropriate adjustments to counter the effect of the two kilometre relocation, although how this adjustment corrects anomalies is unclear.


Mt Lawley

Perth Metro in Mt Lawley (9225) became the source of Perth's daily weather records as of 1994, covering the city's past two decades of below average rainfall during which almost all of Perth's record maxima were broken.

Below is a chart of the three different stations that have recorded the temperatures of "Perth" since 1897:


perth temperature records


Thanks to Perth Metro's colder inland minima getting colder (low rainfall and little nighttime cloud cover), Perth's mean temperature has been stable and even dropped a tiny bit from 1994 to 2010.

The difference between Mt Eliza/Wellington St (9034) and Mt Lawley (9225) is illustrated by BoM trendline comparisons of minima and maxima from 1897 to 1911 at these two locations:


perth minimums history


perth maximums history


If station relocations have not altered Perth's temperature history, it must be accepted that in the 18 years to 2011, Perth's mean minimum dropped by 0.6C and mean maximum increased by 1.3C due to climate change.

Average maxima have been getting hotter over recent years at station 9225 but media reports would be more accurate if they read: "METEOROLOGISTS say 2011 will be Mt Lawley's hottest year in history - the suburb's third consecutive hottest year since records began in 1994."

Mt Lawley is not Wellington St is not Mt Eliza and the coincidence of different temperature plateaus and trends suggests location influences average minima and maxima.

Below are the mean temperatures from all three Perth recording locations from 1897 to 2011:


perth record temperature history


Perth's 1970s hockey stick indicates the two kilometre relocation from Mt Eliza to inner city Wellington St had an influence on Perth's temperature trend. If the sharp increase is attributed to climate change instead of location, it must be accepted that the subsequent reduction in mean temperatures was also due to climate change and not the four kilometre relocation to Mt Lawley.

2011 was a hot year mostly because of a very hot 2010/11 summer, particularly February, and the onset of above average rainfall after May creating nighttime cloud cover that traps heat and increases the overnight minima.

Based on the trendline evidence, it can confidently be stated that 2011 was the hottest year recorded in Perth (at Mt Lawley) since 1994, with the relevance of comparisons uncertain before that year.

The location influence on Perth's historic temperature trends is acknowledged by the BoM in footnotes to the Perth Metro Daily Weather Observations where it states that "Significant variations can be experienced across the Perth metropolitan area on individual days. On average the inner city area is warmer overnight than the current observation sites. The current official Perth observation site is Perth Metro, however in the past Perth Regional Office and Perth Gardens have been official Perth observation sites."

Note: the accuracy of minima trends at Perth Metro in 1995/96 is questionable as the BoM has published every day from March 31, 1995, to December 2, 1996, as rounded temperatures without a decimal place ... i.e. 10.0C, 11.0C, 12.0C, 13.0C, etc ... with the exception of July 1996.

According to the BoM, this is because Automatic Weather Stations were originally programmed to report in an international code limited to reporting to the nearest degree. High precision data from the AWS was monitored in the Perth Regional Office and manual intervention forced some data to be sent to the database with its full precision. However, from 31 March 1995 to 1 December 1996, the less precise data was automatically forwarded to the climate database, with some exceptions, and thereafter the software was altered to allow automatic transmission of full precision temperatures. The bureau states it is important to note that minimum temperature data to the nearest degree was not wrong, but simply less precise than the raw data.

This is presumably an error in the BoM web database and in the BoM CSV download files for all years of data, and it isn't known how this 19 months of incorrect data distorts Perth's temperature record.


Perth Airport

The High Quality temperature readings incorporated by global temperature bureaux from Perth Airport (9021) include 9034 data (Perth Regional - Mt Eliza/Wellington St) from before 1992 adjusted to be equivalent with Perth Airport after 1993:


perth airport temperature history


Like Mt Lawley, there was a plateau shift in temperatures at Perth Airport after 9034 was moved to Wellington St from 1967 and a small decline since 1994 when Wellington St closed.


Was 2011 the hottest ever?

The 2011 mean maximum at Perth Metro was 25.7C, the highest ever recorded.

This compares to an average 24.55C since 1994 and an average 23.3C since 1897 excluding Perth Metro 1994> (average 23.93C since 1897 including Perth Metro).

The 2011 mean minimum at Perth Metro was 14.0C.

This compares to an average 12.61C since 1994 and an average 13.3C since 1897 excluding Perth Metro 1994> (average 12.96C since 1897 including Perth Metro).

Equal or higher mean annual minima were recorded at Wellington St in 1991, 1989, 1988, 1987, 1985, 1983, 1982, 1980, 1979, 1978, 1977, 1976, 1972, 1970, 1967 and at Mt Eliza in 1963 and 1915.

The Perth Metro mean temperature in 2011 was 19.85C.

Below are the minimum and maximum records for 1978 in the Perth Regional 9034 record (Wellington St).


perth historic temperatures


The mean annual maximum was 25.2C and the mean annual minimum was 14.7C / 2 = 19.95C.

Rounded, that gives 2011 a mean annual temperature of 19.9C and 1978 a mean annual temperature of 20C.

In 1988, Perth Regional Office 9034 (Wellington St) recorded a mean minimum of 14.9C and a mean maximum of 24.8 / 2 = 19.85C, or 19.9C when rounded - the same as 2011.

The hottest year in Mt Lawley since 1994 doesn't exceed the hottest years in Wellington St, which because of its location had cooler maxima and warmer minima from 1967 to 1992 than at Perth Metro since 1994.

It's also worth noting that there were five days in 1978 exceeding 40C and three such days in 1988. In 2011 there was one.


What's hot?

Although there is no international definition, the Bureau of Meteorology describes temperatures above 40C as "very hot" and temperatures on WA's coast above 32C as "hot".

Throughout 2011 there was only one day which snuck above 40C at Perth Metro - 40.4C on Jan 29. It's not uncommon in Perth to have just one day in a year above 40C but it's interesting looking back at previous years and the number of days that exceeded 40C.

The average number of days per year with maxima above 40C at Mt Eliza (9034) from 1897 to 1967 was 1.41. In 2011, Perth Metro had less "very hot" days above 40C than the annual average since 1897 of two per year.

If station relocation anomalies have been correctly adjusted by the BoM, the trends indicate that over the past 17 years Perth residents have been enduring about five more "hot" days each year than in the '70s and '80s, and less than half the number of "hot" sticky nights:



Because Mt Lawley has cooler inland minima than Wellington St, there is a sharp fall in the annual number of 22C+ minima days. The number of days per year above 22C min or 32C max in Perth have trends similar to mean temperatures, with both jumping sharply since 2009 but not matching the 1978 record.

In November 2009 the BoM repaired a Daily Weather Observations database bug on its website that saw most WA temperatures for August 2009 adjusted up by as much as .5C, with all averaged temperatures since that November correction being higher than before. The bug correction may have unfortunately coincided with a significant warming of Perth's minima and maxima that began in November 2009.


Consecutive hottest years?

The media reports that 2011 was Perth's third consecutive hottest year since records began.

2010 BoM annual records state that "Perth Metro's annual mean daily maximum temperature in 2010 was 25.3C, which was the warmest year on record since records commenced in 1897", while "Perth Metro's annual mean daily minimum temperature in 2010 was 12.4C, which was 0.3C below normal"

The mean annual maximum in 2010 was 25.3C and the mean annual minimum was 12.4C / 2 = 18.85C.

The mean annual maximum in 2009 was 25C and the mean annual minimum was 12.7C / 2 = 18.85C.

All previous and alternative temperature records for Perth show there were 41 instances including 29 different years with a mean annual temperature equal to or exceeding 18.85C.

It might be claimed that 2011, 2010 and 2009 had the three consecutive hottest maxima at 25.7C, 25.3C and 25C, except that the average maximum at Wellington St in 1978 was 25.2C. By comparison, the Perth Airport (9021) raw maxima equalled or exceeded 25C in 17 different years before 2009, and equalled or exceeded 25.3C in six different years before 2009.

However, it is accurate that 2011 and 2010 had the consecutive hottest mean maxima recorded at any of the three Perth locations used for temperature observations since 1897.


WA overall

The Bureau of Meteorology's Monthly Weather Review is based on observations from all available years of data at all WA stations with a temperature record longer than 30 years.

Below are WA's monthly readings for 2011:

Jan - 29.7C - 1C above average
Feb - 27.9C - 0.9C below average
Mar - 25.3C - 1.3C below average
Apr - 23.1C - 0.1C below average
May - 18C - 0.9C below average
Jun - 15.5C - 0.4C below average
Jul - 15.9C - 1C above average
Aug - 17.6C - 1.1C above average
Sep - 19.6C - 0.1C above average
Oct - 23.4C - 0.2C above average
Nov - 25.3C - 0.6C below average
Dec - 27.9C - 0.1C below average


The BoM summary of 2011 for WA shows that across the state as a whole, the annual mean maximum was 0.2C below the long-term average while the annual mean minimum was 0.2C above the long-term average. In other words, the temperature in WA in 2011 was absolutely normal compared with the 1961-1990 baseline mean.

January 2012 across WA had the fifth coolest mean temperature on record.


Sea temperatures

Similar claims of climate change have been made about Perth's unseasonal December 2011 thunderstorm rain (the wettest December day in 65 years) being caused by sea temperatures rising 1C in the past 50 years.

The BoM's SEAFRAME monitoring of sea air temperatures since 1991 shows that the three WA locations of Esperance, Hillarys and Broome have recorded no temperature change for 20 years.


Misleading public impression

The public is informed that Perth broke historic temperature records in 2011 and endured the hottest year ever due to climate change. The theme is maintained with reports of the hottest December maximum ever recorded in Western Australia.

However, the climate records suggest a misleading public impression has been created that Perth and WA have never been hotter. There is no doubt 2011 was a very warm year in Perth thanks to hot, dry summer maxima combining with warm, wet winter/spring minima. However, it wasn't the hottest year ever according to the disjointed temperature records.

Fortunately, The Australian newspaper reported on New Year's Eve that 2011 was Perth's second hottest year on record, although it was actually the equal second hottest on record and the illusion was maintained that these temperatures were recorded at the same location. Most other media have branded 2011 as the hottest year ever in Perth and are unlikely to correct their errors in print or on air.

A week after its incorrect forecast that 2011 would be Perth's hottest year ever, The West Australian newspaper published a poorly worded story on its website on December 30 maintaining that "Perth has sweltered through its hottest year on record".


Bellwether

Perth and WA have been cited as the bellwether ("guinea pigs", "coalface") of global warming. If Perth's historic temperature trend is considered accurate (?) and the BoM's WA temperature trend similarly accurate, the city's mean temperature was flat from 1994 to 2010, 1978 was the hottest mean year ever and WA had below-average temperatures in 2011.

Perth's temperature trend over the past century is similar to global trends, including a flat trend since the 1990s. However, the relocation of Stevenson Screen recording stations around Perth, coinciding with different temperature plateaux, raises questions about the validity of trends since 1897 and claims of record hot years that precede 1994.

For more detail about the temperature recording history of "Perth", see Perth Temperature History.


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