Friday, 27 November 2009

NIWA defends it adjustment of data

NIWA have released a statement that the data that shows a warming trend in New Zealand over 100 years was adjusted.
NIWA’s analysis of measured temperatures uses internationally accepted techniques, including making adjustments for changes such as movement of measurement sites.
Though the paper (and my post yesterday) suggest adjustment was the likely explanation. However the graph and the surrounding paragraph fail to mention the data is adjusted. I read significant numbers of scientific papers and they are always referencing the raw and the adjusted data labelling both. There are statistical issues with some of these papers but this is not one of them.

NIWA go on to say,
Such site differences are significant and must be accounted for when analysing long-term changes in temperature. The Climate Science Coalition has not done this.

NIWA climate scientists have previously explained to members of the Coalition why such corrections must be made. NIWA’s Chief Climate Scientist, Dr David Wratt, says he’s very disappointed that the Coalition continue to ignore such advice and therefore to present misleading analyses.
Unfortunately this comment fails to identify and thus address the issue which is: "why" is not the question the Coaliltion is asking; it is "what" and "how". What is the adjustment? and how have you done it? Treadgold (an author of the paper) writes,
We cannot account for adjustments, because we don’t know what they are. We ask only to know the adjustments that have been made, in detail, for all seven stations, and why.
Transparency demands that the specific reasons for data adjustment be given.
  • What stations have been adjusted?
  • When were they adjusted?
  • Is the adjustment stepwise or a trend?
  • Is there overlap of data when stations are shifted?
  • Does the overlapped data show good correlation?
  • Have adjustments been modified in subsequent years? Why?
  • What is the computer code that applies the adjustment?
This sort of information allows others to review the legitimacy of such decisions. And various groups can argue for and against these reasons and the weighing various reasons should be given.

Why the secrecy? The refusal to be open with data and theories is looked upon with suspicion, and rightly so.

Gareth Renowden writes a post explaining why adjustments are made to the data. The excessive rhetoric notwithstanding, the argument is plausible. But it still leaves questions unanswered. While the Wellington station may just be used an example, what of the other 6 stations? Wellington may show a rise after adjustment, but this will be diluted when averaged across all the station unless they all showed a rise. It they did what is the explanation for them.

Though I am not fully convinced with NIWA's explanation. The Airport and Kelburn temperatures seem well correlated, with Kelburn cooler being at a higher altitude. And Thorndon and Airport are both at the same elevation (sea level). But there is no correlation established between Thorndon and the other 2 locations.



Elevation is not the sole determiner of temperature. There may be other considerations that make Thorndon and the Airport different temperatures. If so, then the adjustment down of the Thorndon data may be excessive. It should be easy to set up further measurements at Thorndon currently and see how they correlate to Kelburn and the Airport. If they all correlate well then we can establish a more accurate correction factor for the pre-1930 Thorndon data.

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