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New paper finds no significant 20th century warming for NZ

New paper finds no significant 20th century warming for New Zealand

A Reanalysis of Long-Term Surface Air Temperature Trends in New Zealand * by C.R. de Freitas, M.O. Dedekind and B.E. Brill.

A research paper on the homogenisation of the temperature record in New Zealand, reducing the current official rate of 0.9°C per century to 0.3°C, has just been published in the international scientific journal Environmental Modeling & Assessment.

The paper addresses the values of the data adjustments required during 100 years of the Seven-station Series, which is recognised as being representative of New Zealand as a whole. It also considers corrections to station data contaminated by vegetation growth, urbanisation and other factors.

The New Zealand historical temperature trend has not been addressed in the scientific literature since the first Seven Station Series was published by MJ Salinger in 1980. At about the same time, a paper by JWD Hessell called in question the quality of the New Zealand historical weather data used in the series.

The new paper builds on both viewpoints by applying modern techniques to correct sub-optimal raw data and to recalculate the 1980 adjustments. The method used for recalculations was that described in the leading New Zealand paper, Rhoades & Salinger (1993).

Lead author Chris de Freitas commented: “Regional and national temperature trends are widely used for a large number of societal design and planning purposes and it is important that they should be as reliable as modern methods allow”.

He added “New Zealand provides one of the longest continuous climate series in the Pacific Ocean as well as one of the longest in the Southern Hemisphere. This means our trends are of ongoing interest to a wide audience of scientists.”

The paper finds that New Zealand warmed over the 20th century at the rate of 0.3°C per century, which allowing for accepted margins of error means that there has been no significant warming.

The de Freitas et al paper has already attracted international attention. At the widely read website,, Dr Craig Idso's review says, inter alia:
"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.”


Detecting trends in climate is important in assessments of global change based on regional long-term data. Equally important is the reliability of the results that are widely used as a major input for a large number of societal design and planning purposes. New Zealand provides a rare long temperature time series in the Southern Hemisphere, and it is one of the longest continuous climate series available in the Southern Hemisphere Pacific. It is therefore important that this temperature dataset meets the highest quality control standards. New Zealand’s national record for the period 1909 to 2009 is analysed and the data homogenized. Current New Zealand century-long climatology based on 1981 methods produces a trend of 0.91 °C per century. Our analysis, which uses updated measurement techniques and corrects for shelter-contaminated data, produces a trend of 0.28 °C per century.


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