Abstract
de Freitas et al. (2015) (henceforth dFDB) report a trend of 0.28 °C per century over the period 1909–2009 for New Zealand land surface temperatures, from their reanalysis of a composite of seven long-term records. This is much lower than the warming trend of about 0.9 °C per century reported previously by other researchers and much smaller than trends estimated from independent sea surface temperature data from the surrounding region. We show these differences result primarily from the way inhomogeneities in temperature time series at individual stations due to site or instrument changes are identified and adjusted for in the dFDB paper. The adjustments reported in that paper are based on a method designed by one of us (Salinger), but use only a short (1–2-year) overlap period with comparison stations and consider only inhomogeneities in monthly mean (rather than monthly maximum and minimum) temperatures. This leads to underestimates of the statistical significance of individual temperature discontinuities and hence rejection of many valid adjustments. Since there was a systematic tendency for the seven-station sites to be relocated to colder locations as the early half of the twentieth century progressed, this rejection of valid adjustments produces an artificially low rate of warming. We therefore disagree with the trend calculations in the dFDB paper and consider there is no reason to reject the previous estimates of around 0.9 °C warming per century.
Highlights
IntroductionA paper by de Freitas et al ([1], hereafter referred to as dFDB) claims to find a very small warming rate for New Zealand
A paper by de Freitas et al ([1], hereafter referred to as dFDB) claims to find a very small warming rate for New ZealandA number of site moves and instrument changes have occurred over the century at the 7SS stations, necessitating the calculation of adjustments before joining sections of the record together. dFDB depend on an adjustment methodology they refer to as RS93, from a paper published in the early 1990s [4]
Trends from an Independent Analysis of New Zealand Temperature Records BBerkeley Earth^ was a project conceived in 2010 to reanalyze the Earth’s surface air Retreat of NZ Glaciers Chinn [27] reported on a study of 127 Southern Alps glaciers, which indicated an average shortening by 38% and loss of 25% in area in the century up to 1990
Summary
A paper by de Freitas et al ([1], hereafter referred to as dFDB) claims to find a very small warming rate for New Zealand. A number of site moves and instrument changes have occurred over the century at the 7SS stations, necessitating the calculation of adjustments before joining sections of the record together. DFDB depend on an adjustment methodology they refer to as RS93, from a paper published in the early 1990s [4]. RS93 was developed for New Zealand data, and many publications followed from it (see Section 2).
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