- JoNova - https://joannenova.com.au -

Australian summer maximums “warmed” by 200%

Posted By Joanne Nova On October 6, 2014 @ 2:51 pm In Global Warming | Comments Disabled

Which causes more summer heatwaves: carbon dioxide or Bureau of Meteorology (BOM) adjustments?

Ken Stewart has analyzed the adjustments used to create the all-new ACORN wonder dataset and compared them with another BOM dataset called AWAP, and finds, extraordinarily, that the trend in average summer maximums has been tripled by adjustments that the BOM imply are neutral.

Since summer maxima are the ones used to generate the most headlines in Australia, I ask again if the Bureau of Meteorology is a scientific agency or a PR group?  Increasing the trend in summer maxima would produce more headlines of hottest ever month, season, heatwave, and weekend.

In this graph Stewart splits the data into months, and compares the trends in maxima in the AWAP and ACORN datasets, across the entire nation. We see that most of the adjustments happen to data from the hottest months of the year, October to March. Even though the measured maxima in February and March are possibly cooler now than they were in the early 1900s, they have been adjusted to show warming trends.

When was the last time you heard the BOM tell you that their “hottest ever” February record depended on adjusting down the past hotter records?

Ken Stewart points out that adjustments grossly exaggerate monthly and seasonal warming, and that anyone analyzing national data trends quickly gets into 2 degrees of quicksand. He asks: What was the national summer maximum in 1926?  AWAP says 35.9C.  Acorn says 33.5C.  Which dataset is to be believed?

We might be worried about “two degrees of warming” but people living in 1926 got two degrees of cooling some 88 years after the fact.

Three ways to not graph the adjustments that matter

The BOM created a new “adjustments” page which appeared a few weeks ago, after questions from skeptics Jennifer Marohasy, David Stockwell, Bill Johnston, and Ken Stewart were raised in The Australian newspaper.

The BOM show the next graph to convince us that their adjustments are neutral. But there are several problems. The graph  does not show the effect that adjustments have on seasons, or on maxima and minima. The BOM claim the AWAP data is “unadjusted”, but their CAWCR Technical Report No. 050 calls it “partially homogenized”, which in most scientist’s books is a rather un-raw and adjusted state.

The graph as usual starts in 1910, so ignores the hot historic records from 1880-1900. This older data is more difficult to combine with new data, but some of it was recorded on modern equipment. Yet the BOM shows no curiosity or interest in these historical records, as we’ve seen at Bourke, Rutherglen, Hay, Omeo, Bendigo, Alice Springs, Melbourne, Carnarvon and NewcastleBOM adjustments change cooling trends to warming trends because stations “might” have moved!

How honest are the BOM being about the impact of their adjustments with a graph like this?

The BOM use “mean temperatures” from two different adjusted data sets to claim that their adjustments don’t matter.

BOM ACORN Adjustments

Comparison between adjusted and unadjusted temperatures

Both adjusted and unadjusted temperatures show that Australia’s climate has warmed since 1910. Since 1955 adjusted and unadjusted data are virtually identical. It is also during this time that most of the warming has occurred in Australia.

The graph below shows temperature trends since 1910 from the unadjusted temperatures from more than 700 locations (AWAP), together with those that have been carefully curated, quality controlled and corrected for artificially induced biases at 112 locations (ACORN-SAT).

Note that “mean” temperatures lose a lot of the data that matters about our climate. The maxima and minima can both be more extreme, yet the mean can remain the same. What matters to the man in the street are the daily maxes and min’s — not the “average” which is a mere half way point between the peaks of the day. It doesn’t represent the true average temperature of the day because it doesn’t take into account the hours of hot or cold temperatures under the daily curve. “Mean trends” however could be useful if you’ve made adjustments to the maxes and mins, but want to make it look as though they don’t matter. Just sayin’…

Table of percentage changes to Trends in AWAP versus ACORN

(Click to expand)


On Stewart’s site there is discussion about the value of quadratic trends versus linear trends, but the main point is that the BOM use quadratic trends, so Stewart copied their approach. As far as I’m concerned, all trends from either AWAP or ACORN are hopelessly compromised. Both are area-weighted, gridded data, neither are “unadjusted”, and there are far too many anomalies, and adjustments that can’t be justified with documentation. These are statistical creations.

If we want to reduce temperatures across Australia in the next 100 years, the cheapest and most effective way to do it is with adjustments not windmills. Solving our temperature problems might be a simple as setting up a new BOM.

VN:F [1.9.22_1171]
Rating: 9.4/10 (102 votes cast)

Article printed from JoNova: https://joannenova.com.au

URL to article: https://joannenova.com.au/2014/10/australian-summer-maximums-warmed-by-200/

Copyright © 2008 JoNova. All rights reserved.