Trends in temperature extremes for Southern New Zealand

This research investigates the spatial and temporal trends of temperature extremes in Southern New Zealand, with particular emphasis on those elements that influence the resource economy. Many of the resources of Southern New Zealand are strongly influenced by climate variability, whether it be pastoral agriculture, horticulture, viticulture, forestry, winter and summer tourism or hydro-electricity generation. Past research has examined temperature trends for Southern New Zealand over time periods of a few decades, but detailed research on trends and changes in variability over the 150 year period of available record for the region is needed. There is also growing interest in the impacts of global warming on the thermal regime and consequent changes in risk factors such as frost.

This research utilises a newly expanded daily temperature database of 31 climate stations of past temperature data for the period 1852 – 2003. After rigorous homogenisation, the data used in this project provides an unbiased record of temperature changes over the instrumental period. Using a number of indices to measure the change in temperature extremes, such as the occurrence of frost days, growing degree days, heating and cooling degree days, and extreme maximum and minimum temperatures based on percentiles and thresholds. Results show a large amount of spatial variability in the changes in temperatures extremes over the 150 year period with some regions displaying large amounts of warming, while others showed no warming. Warming trends are strongest in the last 50 years. Fewer cold minimum temperatures and a corresponding increase in warm minimum temperatures. Changes in maximum temperatures show less of a response. The semi-continental inland regions show the strongest response in maximum temperatures with an increase in the number of days over 25°C. Little change is shown in absolute minimum and maximum temperatures.