A cold reception for carbon dioxide tax

The Gillard Government picked an ironic time to announce its new carbon dioxide tax aimed at preventing climate warming. On Carbon Sunday (July 10, 2011), the West Australian capital city of Perth was enduring its coldest 11 day spell of maximum temperatures since records began.

Well, since records began at the Perth Metro (9225) weather station in the suburb of Mt Lawley, which recorded its first temperatures in 1994 to become the official gauge of the city's weather.

From July 1 to July 11, 2011, the Perth Metro daily maxima were 17.2, 16.7, 14.3, 15.4, 16.4, 16.4, 15.3, 16.6, 16.6, 14 and 13C. This produces a mean maximum of 15.63C over 11 days - the coldest daytime temperature spell of this duration since the station's records began in 1994.

The local media had no interest in Perth's cold weather but websites such as weatherzone and even the Sydney Morning Herald on the other side of Australia noticed that the chill was setting in.

The Australian Bureau of Meteorology has noted that Australia has experienced its coldest autumn since at least 1950 for mean temperatures with an Australian average of 20.9C. This was 1.15C below the historical average and 0.2C below the previous coolest autumn in 1960.

The BoM also reports that despite above average temperatures in the South West Land Division during May 2011, across Western Australia as a whole the mean temperature was 0.9C below normal and it was the 11th coolest May on record.

Weather station locations

Before 1994, the official Perth weather station was Perth Regional Office (9034) in Wellington St (elevation 19 metres), central city and roughly four kilometres south of Perth Metro (9225) station in Mt Lawley (elevation 25 metres).

However, before 1967 and since 1897, Perth Regional Office (9034) was situated about two kilometres west of Wellington St on Mt Eliza where Dumas House now stands (elevation 61 metres).

The BoM and the media like to compare the temperature histories of these three separate locations as though they are the same location, regardless of different elevations, UHI within the city and proximity to the broad waters of the Swan River.

Perth's temperature records actually began in 1994 (Perth Metro 9225 in Mt Lawley), since when the location of the thermometer has been static and historically relevant. The mean temperature at Perth Metro hasn't changed since 1994.

Study the charts and you'll see that Mt Lawley has hotter daytime maxima and considerably colder nighttime minima than either Wellington St or Mt Eliza.

Both minima and maxima shifted immediately when Perth's official weather station was relocated from Wellington St to Mt Lawley in 1994, probably because of UHI, cooling sea and river breezes, and inland Mt Lawley not having a nearby body of water to trap heat and warm the night air.

Different locations = different temperatures

A comparison of BoM monthly mean minimum temperatures shows the difference between Mt Eliza/Wellington St (9334: 1897-1992) and Mt Lawley (9225: 1994-2011) is most pronounced during winter:

And with mean monthly maxima, Mt Lawley is uniformly hotter throughout the year:

Since Perth's record "hot" weather spells recorded at Mt Lawley are incorrectly compared with records at the two other locations (Wellington St and Mt Eliza) prior to 1994, critics will insist the same nonsensical comparison is made for claims of record cold spells. So ...

The daily temperature records of all three locations (Perth Regional and Perth Metro) show the following spells of 11 consecutive days with an average mean maximum below 15.63C:

Perth Metro Mt Lawley 1994-2011

2011 / Jul 1 - Jul 11: average mean maximum 15.63C

Perth Regional Wellington St 1968-1993

1986 / Jul 14 - Jul 24 average mean maximum 15.51
1972 / Jul 30 - Aug 9 - average mean maximum 15.5

Perth Regional Mt Eliza 1897-1967

Colder 11 day spells in 1966, 1956, 1955, 1951, 1945, 1939, 1932 and the frequency of 11 day spells below 15.63C gradually increasing back to 1897.

It should be noted in the tables above that the 11 day mean maximum cold spell of 15.63C in July 2011 was 2.67C below the long-term July mean of 18.3C for Perth Metro in Mt Lawley. However, the 1986 and 1972 cold spells of 15.51C and 15.5C were just 1.9C and 1.89C below the long-term July mean of 17.4C for Perth Regional Office before 1994.

This greater difference suggests the 11 day cold spell in 2011 was a bigger temperature anomaly than were the cold spells at Perth Regional Office, particularly considering the mean maximum temperatures in the Wellington St 11 day cold spells of 1986 and 1972 were only marginally less than in July 2011.

So is the decreasing frequency of colder 11 day spells indicative of a warming climate, or is it UHI and the change in station locations? There are some clues if you look at a chart of the winter (Jun, Jul, Aug) mean minima from 1897 to 2010 ...

The winter mean mimimum was fairly stable from the beginning of the 1900s till the late 1960s when Perth Regional Office moved from Mt Eliza to Wellington St. At that point it increased by almost a degree and was then stable until 1994 when Perth's official temperatures started being recorded at Perth Metro in Mt Lawley. At that point it dropped sharply and has since been falling.

For a bit of fun, It's worth charting the entire Perth summer mean maximum and winter mean minimum, averaged together from 1897 to 2010 ...

Again the shifts in temperature coincide with movement of the BoM weather stations, although there was a significant temperature increase in the early 1900s - a bit too early to blame carbon dioxide.

How about the winter mean minimum at the static 9225 Perth Metro site in Mt Lawley since 1994?

Yes, Perth winters have been getting colder (in Mt Lawley, anyway). How about the summer mean maximum at Perth Metro since 1994?

The summer maximum has edged up a bit on the trendline, but if you ignore the last two years (hot due to low rainfall and less cloud cover) it's been flat.

How about an average mean of the summer maxima and winter minima records at Perth Metro since 1994?

It's fallen by about .2C. The reduction obviously occurs because of the falling winter minima, not the rising summer maxima.

Why compare or average winter minimum and summer maximum temperatures? Because they're the seasons of "extreme" weather when people turn on their air conditioning, fans and heaters to stay comfortable. Perth's mediterranean climate is usually close to body temperature during spring and autumn, when electricity isn't an essential service to survive.

The intent of the carbon dioxide tax is to discourage electricity use through higher pricing so as to benefit all by preventing warming temperatures, yet its launch occurred during a record cold spell that is symptomatic of Perth's cooling climate over the past 16 years - as recorded by the city's official weather station in Mt Lawley.

When electricity prices go up as promised under the tax, it will make it more expensive to stay cool in summer and warm in winter - an additional burden for the poor and potentially lethal in winter.

Perth Temperature History

Average Temperature Trends Across Western Australia

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