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Will the Real New Zealand Temperature Trend Please Stand Up!

Paper Reviewed
de Freitas, C.R., Dedekind, M.O. and Brill, B.E. 2014. A realanysis of long-term surface air temperature trends in New Zealand. Environmental Modeling and Assessment: 10.1007/s10666-014-9429-z.

In introducing their recent study of the subject, deFreitas et al. (2014) write that "a homogenized New Zealand national temperature record has only once appeared in the literature," citing the one produced by Salinger (1980). They report, however, that it was based on a measurement technique that was significantly improved by its author and a colleague (Rhoades and Salinger, 1993) over a decade later. And although they state that "applying that improvement could have a significant effect on trends," they indicate that such an improved trend for New Zealand "has never previously been published."

The three New Zealand researchers thus set out to fill this void by applying the measurement technique described by Rhoads and Salinger to data for the period 1909-2009. And they did it, in their words, "exactly as they [Rhoads and Salinger] describe, without adjusting it in any way," although they say they corrected for "the contamination of raw data identified in the refereed literature (Hessell, 1980)" and for "shelter-contaminated data." So what was the final result?

De Freitas et al. report that, whereas the previous analysis yielded a trend of 0.91 ± 0.30°C per century, their analysis - which used updated measurement techniques and corrects for shelter-contaminated data - produces a trend of only 0.28 ± 0.29°C per century, which is a heck of a lot less than what had previously been believed to have been the case.

The significance of de Freitas et al.'s work is two-fold. First, the authors report that the old, contaminated data with the inflated warming trend has been "widely used as inputs for societal design and planning purposes" all across New Zealand. Second, de Freitas et al. note these data are "extensively used in hindcast verifications for regional and local models." However, as the saying goes, "garbage in equals garbage out." Therefore, at best, the corrected New Zealand temperature trend, which is three times smaller than the uncorrected version, calls into question all results, findings, conclusions, and policies built upon or derived from the old contaminated data record. And at worst, it invalidates them.

Given the great importance of starting with the proper baseline, one would hope that with so much at stake in terms of economics, personal freedoms, and governance, much greater care and scrutiny would be applied to ensuring the quality and reliability of near-surface air temperature records. But obviously, such has not been the case for New Zealand. And it begs the question as to where else temperature records might be less than par.

Hessell, J.W.D. 1980. Apparent trends of mean temperature in New Zealand since 1930. New Zealand Journal of Science 23: 1-9.

Rhoades, D.A. and Salinger, M.J. 1993. Adjustment of temperature and rainfall records for site changes. International Journal of Climatology 13: 899-913.

Salinger, M.J. 1980. The New Zealand temperature series. Climate Monitor 9: 112-118.

Posted 28 October 2014