An analysis of the evidence for climatic change in New Zealand in the last thousand years: Evidence from diverse natural phenomena and from instrumental records

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Abstract
An analysis is made of the diverse evidence for climatic variation in New Zealand in the last 1000 years. After a brief account of the pressure systems and circulation pattern near New Zealand, indirect evidence from a range of phenomena which have been influenced by climate is discussed. The climatological evidence derived from instrumental records and observations is then considered, with emphasis on precipitation and temperature. Little reliance may be placed on some evidence which is weak, equivocal, difficult to interpret or difficult to place in time. The most reliable evidence is summarized in two overlapping time periods, 1000 A.D. to 1900 A.D. and 1850 A.D. to the present. Dated glacial episodes, speleothem palaeotemperatures and tree palaeotemperatures demonstrate that there have been many fluctuations of temperature in the last 1000 years, over a range of not much more than ± 0.5°C. The high temperatures since about 1950 A.D. have probably not been exceeded. Expansion of glaciers in the Southern Alps requires consistent cold airflow from southerly directions onto the western side of the South Island. Iceberg irruptions into unusually low latitudes in the Southern Hemisphere occurred at the same times as glacial advances in New Zealand last century and early this century. Exceptional snowstorms in the South Island were generally most frequent at the same times. Closed basin lakes in the Central North Island were low in the 17th century, 18th century and la te 19th cen tury when glaciers were expanded in the Southern Alps. The lakes were high in the 20th century when glaciers were shrunken and temperatures and local precipitation were also high. Consistent warm northerly airflow onto the North Island provides the conditions promoting the high lake levels and other correlated phenomena. Periods of erosion and deposition in streams in Hawkes Bay since about 1650 A.D. are well dated and probably indicate increased storminess, but there may be some interaction with fire and tectonic events. Anomalies in distribution or population structure of tree species probably indicate climate variation (especially alternation of drought periods with wet periods) but most of the published data are not reliable enough for very definite conclusions to be made about the timing or amplitude of climate fluctuations. Smoothed curves for instrumental records (most reliahle since about 1880 A.D.) show fluctuating mean temperature, with similar trends in all localities and an amplitude of only about ± 0.5°C. Temperature rose in the 1950s and 60s to the highest levels since records began. Smoothed precipitation curves also show fluctuations over a small range in the same period, but trends vary from locality to locality. There are indications of a cyclic recurrence of drought since about 1850 A.D. Droughts and floods often occurred in the same years so that there must be rapid variation in the causal factors. It is suggested that most of the apparent variations in climate can be explained by latitudinal alterations in positions of pressure systems and changes in the Fate of circulation. Such changes cause fluctuations in amounts of zonal, as compared with meridional, flow and corresponding fluctuations in the climatic regimes experienced by different parts of the country.

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