NZ temperature record – a brief historyRichard Treadgold | February 6, 2011
Official temperature records have been maintained in New Zealand since shortly after European settlement began in 1840. Throughout the ensuing 150 years, mean temperature levels appeared to remain stable. But NIWA (the responsible Government agency) has recently questioned the historical record, suggesting a long-term warming trend may have been hidden in the data.
Publications from 18681, 19202 19603 and contemporary records4 indicate that mean temperatures in New Zealand cities have not significantly changed since records began (Tables 1,2,3). Degrees Fahrenheit in the early figures have been converted to degrees Celsius.
Early recordTable 1 — Comparison of mean temperatures for New Zealand cities before 1868 and 1971-2000.1,4
|STATION||Years of Data||BEFORE 1868 (NZ Institute)||Years of Data||1971-2000 (NIWA)|
*NIWA Mean is the 1971-2000 ‘Normal’, representing current average temperature levels for New Zealand as a whole.
Nine-Station Series 1920
In 1920 D.C. Bates2, then Chief Meteorologist, published figures for the mean temperature of nine towns in New Zealand. These are compared with current NIWA figures2 in Table 2.Table 2 — Comparison between mean temperatures for New Zealand cities before 1920 and 1971-20002,4
|STATION||Years of Data before 1920||Mean Temperature ºC (Bates)||Years of data 1971–2000||Mean Temperature ºC (NIWA)|
It is notable that the averages for six of the nine stations currently remain within one-tenth of a degree of the average temperatures recorded a century earlier—as does the overall average.
24-Station Series 1960Table 3 — Comparison between mean temperatures for New Zealand cities before 19603 and 1971-20004
|STATION||Years of Data||1960 Met Service||Years of Data||1971-2000 NIWA|
It is notable that the averages for more than half of the stations currently remain within 0.2°C of the average temperatures recorded a half-century earlier.
In the late 1970s, a senior meteorologist with the New Zealand Meteorology Service, JWD Hessell, undertook an exhaustive study of climate records, culminating in publication of “Apparent Trends Of Mean Temperature In New Zealand Since 1930”5. He found no important change in annual mean temperature during 1930-80 except in the case of stations affected by changes in shelter, screenage and/or urbanisation.
Seven-station Series (7SS) 1992
In the 1970s, Salinger6-8 introduced the suggestion that certain “corrections” should be made for site changes at the seven longest-established weather stations. His method was based on a comparison with “neighbour” stations, but his calculations are no longer available. The Salinger adjustments would produce a positive upwards trend of approximately one degree Celsius per century.
NIWA later recruited Salinger and adopted his temperature chart in 1992 as the basis of the official New Zealand temperature record. The Seven-station Series (7SS) has since been converted into ‘temperature anomalies’ by subtraction of each annual figure from that location’s NIWA Mean. Figure 1 shows the 7SS chart to 20099.
Unadjusted Seven-station Series 2009
In November 2009, Treadgold10 published a graph of the temperature anomalies for the seven stations depicted in the NIWA official database of historical measurements—excluding the Salinger adjustments. This showed no temperature trend during 1850-2000.
NIWA revision (NZT7) 2010
Recently the mean annual temperature anomaly record in respect of the seven stations has been extensively revised9. The revised record (referred to by NIWA as NZT7) is shown in Figure 1 (here Figure 3).
The Report9 on the revision has 169 pages and many diagrams.
Table 4 summarises the Review results. The note which was included below it shows that all seven stations did not apply until 1913. The “trend” should therefore be 0.97ºC per yr.
Summary of review results7
* “This is the trend of the seven-station composite series (Figure 2), not the average of the 7 individual trends (which have different starting years). It is coincidental that the previous and revised trends agree exactly to the second decimal place. For example, had we chosen the period 1913-2009, the trends would be 0.95 °C/century (Previous) and 0.97 °C/century (Revised).”
The Report notes that “the 95% confidence intervals on the trends is approximately ±0.3 °C/century”. This high level of uncertainty reflects the difficulty of forcing a trend line on to a curve which does not exhibit any constant trend throughout its length. On further analysis, the graph in Figure 2 reveals three distinct trends.
The Report emphasises that, to date of publication, the error margins have been measured only in respect of the slope of the trend-line.
“It does not include any consideration of uncertainty about each adjustment separately. Further research is under way to quantify how the accumulating adjustments influence the uncertainty in the trend estimates. The individual station documents show the trends separately for each composite.”8
It seems inherently unlikely that the average New Zealand temperature a century ago should now prove to be almost 1.00°C cooler than all the contemporary records suggest. This provides a challenge for the NZT7, which can only be met by robust statistical support.
No useful assessment can be made of the proposed adjustments until the statistical uncertainties are known. Meantime, the NZT7 has replaced the 7SS on NIWA’s website—at least provisionally.
1. New Zealand Institute Transactions and Proceedings 1868.
2. Bates, D.C. “Climate and Meteorology of New Zealand” (1920), Wellington, M.F. Marks, 992.
3. New Zealand Meteorological Service “Summaries of Climatological Observations at New Zealand Stations” (1960), Wellington.
4. National Institute of Water and Atmospheric Research Ltd (NIWA) (2010). “New Zealand Mean Monthly Air Temperatures” http://www.niwa.co.nz/education-and-training/schools/resources/climate/meanairtemp
5. Hessell, JWD “Apparent Trends Of Mean Temperature In New Zealand Since 1930” (1980), New Zealand Journal of Science, 23, 2-9.
6. Salinger, M.J. (1981) Site Assessments on Climatological Stations. Appendix C in:
“New Zealand Climate: The instrumental record”. Thesis submitted for the degree of Doctor of Philosophy at the Victoria University of Wellington, January 1981.
7. Salinger M.J. (1982) “On the suggestion of post 1950 warming over New Zealand”. New Zealand Journal of Science 25, 77-86.
8. Salinger, M.J. (1991), “Greenhouse New Zealand,” Square One Press, Dunedin, New Zealand, 104 pages.
9. NIWA (2010). 7 station series review: http://www.niwa.co.nz/news-and-publications/news/all/7-station-series-review
10. “Are We Feeling Warmer Yet” (2009), The New Zealand Climate Science Coalition www.climateconversation.wordshine.co.nz/docs/awfw/are-we-feeling-warmer-yet.htm
11. Leyland, B. (2010) (to be published). ACKNOWLEDGEMENT: Much of the material in this paper was researched and collated by Dr Vincent Gray.
6th February 2011.