NIWA: NZ temperature rise clear

NZ temperature rise clear

NIWA rejects criticisms of its New Zealand temperature series.

1. Warming analysis is backed up by other observations

The warming of New Zealand over the 20th Century shown by NIWA’s analysis of long-term measurements from seven weather stations is backed up by data from other locations.

(a) measurements from ships

A paper published in 1995 identified an upward trend of about 0.7°C from 1900 to 1993 in night time minimum air temperatures measured from ships over the ocean surrounding New Zealand. That trend is similar to the trend from the seven-station land network over the same period. Also, sea surface temperatures measured from the same ships warmed by 0.6°C in that period.

This analysis has been further extended, and published in a 2003 paper by Folland et al.

References:

Folland, C.K. and Salinger, M.J., 1995: Surface temperature trends in New Zealand and the Surrounding Ocean, 1871 – 1993. International Journal of Climatology 15, 1195 – 1218.

Folland, C.K., Salinger, M.J., Jiang, N., Rayner, N.A., 2003: Trends and Variations in South Pacific Island and Ocean Surface Temperatures. Journal of Climate 16, 2859–2874.

(b) measurements from climate stations which have never been shifted

Dr Jim Salinger has identified from the NIWA climate archive a set of 11 stations with long records where there have been no significant site changes. When the annual temperatures from all of these sites are averaged to form a temperature series for New Zealand, the best-fit linear trend is a warming of 1°C from 1931 to 2008. We will be placing more information about this on the web later this week.

2. Ignoring major changes in site location will produce wrong results

We again reiterate that for the longer “seven station” time series, adjustments to account for significant site changes are necessary in order to provide a meaningful estimate of New Zealand temperature trends.

For example, in Wellington, early temperature measurements were made in Thorndon at three metres above sea level, but in 1928 the measurement site was moved to Kelburn at 125 metres above sea level. The Kelburn site is on average 0.8ºC cooler than Thorndon because of the extra height above sea level. So, the raw data need to be adjusted to ensure we are comparing apples with apples.

By the same token, if the climate station had been moved the other way – from Kelburn down to Thorndon – the raw data would then give too warm a reading and would also need to be adjusted.

NIWA’s analysis of measured temperatures uses internationally-accepted techniques to make adjustments for changes such as movement of measurements sites.

3. NZ Climate Science Coalition disingenuous

For more than two years, New Zealand Climate Science Coalition members have known of the need to adjust the “seven station” data. They have had access to:

• the raw data

• the adjusted data (anomalies)

• information needed to identify the adjustments made by Dr Salinger

• information needed to develop their own adjustments.

However the NZ Climate Science Coalition paper collated by Richard Treadgold (25 November 2009), and the media release issued by the Coalition on 26 November, were based on analysing the “seven station” data without any adjustments at all for site changes. This is why NIWA Chief Climate Scientist Dr David Wratt expressed his disappointment with the coalition statements, in NIWA’s media release of 26 November.

(a) Coalition told in 2006 of need to take account of site changes

NIWA advised NZ Climate Science Coalition member Dr Vincent Gray of the need to calculate appropriate adjustments to account for significant site changes in an email to him dated 19 July 2006, pointing out problems with graphs he had produced without any such adjustments. We advised him that over the period covered by his analysis the Hokitika site moved from the town centre, to the edge of town, then the airport. We also advised him there had been several site changes in Auckland and Dunedin, and told him about the 120 metre change in the height of the Wellington measurements.

(b) Methodology for adjusting data publicly available

The methodology for adjusting for site changes in the NZ temperature record was published in the peer-reviewed International Journal of Climatology in 1993. NIWA referred Dr Vincent Gray of the NZ Climate Science Coalition to this paper on 19 July 2006.

Reference:

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

(c) Unadjusted (raw) data publicly available

NIWA’s unadjusted climate data is available to anyone at no charge, through web access to the NIWA climate database. This has been the case since 1 July 2007.

(d) Adjusted series provided in 2006

NIWA provided Dr Salinger’s adjusted temperature series (anomalies compared with 1961–1990 averages) for each of the seven stations, to NZ Climate Science Coalition member Warwick Hughes on 19 July 2006. Information about changes to the seven station sites is documented in a publicly-available report published by the NZ Meteorological Service in 1992, and much of this information is also available from the metadata in the climate database.

Reference:

Fouhy, E., Coutts, L., McGann, R., Collen, B., Salinger, M.J., 1992: South Pacific Historical Climate Network Climate Station Histories. Part 2, New Zealand and Offshore Islands. NZ Meteorological Service, Wellington. ISBN 0-477-01583-2.

ENDS

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