Driest hottest year ever, or driest and hottest for selected records?

Mick Keogh, Australian Farm Institute, 25/01/2016


Almost without fail when discussions turn to the weather, media commentators or scientists will report that the particular weather phenomena they are talking about is the most extreme that has ever been observed or recorded, but the frequency of these reports (and even a basic understanding of statistics) leads to some serious questioning about the validity of these claims.

In early December, the Weather Bureau confirmed that Tasmania had its driest spring on record in 2015.  In support of that claim, reference was made to the fact that weather records for Cape Bruny lighthouse and Bushy Park go back more than 140 years, and the amount of rainfall recorded in Tasmania in the spring of 2015 was the lowest ever recorded over that period. To compare current rainfall with all records for the past 140 years in making the ‘driest on record’ claim seems to be pretty reasonable and few would argue with that.

But difficulties arise in situations where a claim is made, but it is not clear what the particular climate or weather event is being compared with. Take the example of the Bureau’s Annual Climate Statement for 2015. The first few sentences of the Overview section of the statement were as follows;

2015 was Australia’s fifth-warmest year on record (national observations commence in 1910). Above average temperatures were persistent throughout the year, with several periods of record warmth. The Australian area-averaged mean temperature for 2015 was 0.83 °C above the 1961–1990 average. Maximum temperatures were 0.96 °C above average, and minimum temperatures were 0.69 °C above average; both the sixth-warmest on record respectively. 

Looking at recent years more broadly, eight of Australia’s ten warmest years on record have occurred since 2002. The 10-year mean temperature for 2006–2015 was the second highest on record at 0.53 °C above average (and just behind 2005–2014). Only one year in the past ten was cooler than average: 2011.

What is obvious from a careful reading of the text is that comparisons are being made with several different baseline temperature references, one being the record dating back to 1910 when records commenced, and the second being the average of the period from 1961-1990. Leaving aside arguments about issues like which weather stations are included and the ‘heat island’ question, it certainly seems appropriate to compare current year average temperatures with the long-term record and make a statement about how the 2015 average temperature compared with those 115 years of records.
However, the very next sentence then makes a claim about how much higher the 2015 average temperature was than the average temperature recorded over the period from 1961-1990, a different reference period. And it is not clear from the text included on the accompanying map whether the temperature deciles being referenced in that map are based on 1961-1990 temperatures, or the entire temperature record.
A quick look at the plot of Australian average annual temperatures (based on data published by the BOM) reproduced below, highlights why it is important to be clear about the baseline being used in any comparison.
The graph shows that temperatures over the period from 1961-1990 were more frequently lower (the red line is a five year rolling average) than the 100+ year average, or than was the case from 1910 to 1930, or subsequent to the year 2000. This leads to suspicion that the 1961-1990 reference period is being used to make the 2015 average temperature increase sound more significant that it would be based on a comparison with the long-term average temperature record.
The BOM may well have legitimate reasons for using the 1961-1990 period as a reference period, but the fact that it not clear what reference period is being used – especially in maps and graphs or in claims about ‘highest’ or ‘driest’ years – leads to questions about the legitimacy of the claims being made. It is also evident that the media does not bother with the fine detail of what benchmark is being used for comparison data.
A little more clarity and consistency, especially when it comes to maps and graphs, might help to remove a lot of the confusion and uncertainty around this contentious issue.
This article first appeared on the Australian Farm Institute website and is reprinted here with permission from the author. To view the original article click here


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  1. deb newell, 28/01/2016

    Hi every one above, it is me again. You do realize that ‘The Bush’ can do its own ‘pop-up’ on this issue of temperatures. All you need is an acknowledged temperature gauge and someone to record the temperature range maybe 4 times across the day. I am sure there is some technology that will do this for you. Talk to everyone you know, spread this around Australia ( both urban and outback)and we can create our own temperature data. This is not rocket science although BOM (who couldn’t predict flood levels up the Brisbane River) would like us to think they are very clever and not swayed current cant. I know that this article might be dead and buried by now, but this could be a Beef Central project!

  2. deb newell, 26/01/2016

    The most sinister of wording confusion is at play here care of BOM. PLEASE NOTE THE STATEMENT ” THE AUSTRALIAN AREA-AVERAGED MEAN TEMPERATURE FOR 2015…” which is followed by the data on other Australian temperature ‘averages’ as a direct comparison. In 2015 BOM ‘area-averaged’ or homogenised/ amortised temperatures recorded across regions. In some of these regions there was a significant discrepancy in temperature (think Ipswich versus Brisbane on-the-bay recordings) that was largely nullified by this sleight of hand. Previously all temperatures from weather stations ( and yes the ‘heat island’ effect of urbanisation is ignored) were added and divided by the number of temperature inputs to get an ‘average’. Now there has been a preceding ‘area-averaging’ – often with selective regional borders – that brings in a whole new set of numbers that can’t be compared with the old ones. Scientists that alter empirical base data should be expelled and vilified. In a world of climate science now based on ‘modelling’ feeding in to the modelling systems incorrect data from the outset should make all further predictions null and void.

  3. Geoff Haack, 26/01/2016

    City temps will naturally rise marginally as more and more tar & cement, concrete & iron roofs, increased vehicle use, cooking etc etc radiate more heat as populations rise.
    When the inland is dry & droughted likewise the radiated heat will increase and move by wind, air currents etc over other parts of the country. Pretty basic stuff really. The other 2 issues that refute so much of what we are constantly told are 1 If ocean levels are rising how is the coral on the Barrier Reef going to suffer from bleaching when it will be (apparently) covered by up to an extra metre of water. Not to mention coral thrives in the tropics and should expand with warmer water temps surely. 2 Apparantly melting ice caps will cause this increase in sea levels. Freeze water it expands – melt ice and the area required is less – so how are the ocean levels going to rise I ask you?

  4. Dixie Nott, 25/01/2016

    The only statistical manipulation understood by BOM is the calculation of an “average”. The cherry picking of data makes them look irrelevant.

  5. charles nason, 25/01/2016

    And the 100 year record is just a pinprick in the historical record
    Remember it was global cooling in the 1960-70’s which was the bugi-man then , now it is global warming
    Tessa Vance has analysed salt levels in the ice cores ( for the last 100 years ) in the Antarctica and correlated this with rainfall in Eastern Australia ( best is with Miles Qld ) and concluded that the last 100 years has been unusually dry , similar to the 1000-1200 AD period . NIPCC articles also support this conclusion for Europe
    The Kimberley’s had a 1500 year mega drought about 5000 years ago
    I suspect the ocean conveyer belt which transfers heat around the world broke down
    Hugh Lunn had an article a few years ago ( after the recent Brisbane floods ) where he reported that the 1974 Brisbane flood was overdue as there were 4-5 floods in the 1800’s which were equal or higher than the 1974 flood .
    There was a 300 dry period about the late Bronze age in the Mediterranean area where civilization broke down
    I would suggest that the temperature record is far too short to reliably predict any trend , assuming it has not been homogenised also

  6. Richard Golden, 25/01/2016

    Right on, Mick! It is far and away too tough to decipher as it was written, and especially looking at the graph the 1961-1990 comparison period just raise questions about hunting for data to support a prejudiced (dictionary meaning) position.
    The old saying, “If the facts won’t support the theory, perhaps it isn’t worthy of support” looks to apply to the quoted material above.

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