Homogenization of air temperature and its long-term trends in Poznań (Poland) for the period 1848–2016

A long-term series of meteorological measurements will allow for a better understanding of the rate and nature of climate change. Such analysis presumes a historical knowledge of the particular place of measurements and changing measurement techniques, as well as a further evaluation of the quality of the available data. This research focuses on the city of Poznań and temperature measurements conducted from 1848 to 2016. By 1919, the location of meteorological stations had changed five times, causing time-series inhomogeneities. In 1921, meteorological measurements began to be carried out at the Poznań-Ławica airport, which have there been maintained to this day. The monthly means of air temperature for the period 1848–1919 clearly indicate a break in their homogeneity. For this reason, the main objective of this study is to reconstruct the 169-year long air temperature records in a homogeneous way so as to make it coherent with the surrounding series of long-term measurements. Data gaps are supplemented by the constant difference method based on the homogenized data series from the nearest stations. The monthly means are homogenized according to the Alexanderson’s Standard Normal Homogeneity Test. In addition, evaluation of the annual, seasonal, and monthly temperature trends is also provided. An increasing long-term trend can be observed for the entire period of analysis. For the years 1848–2016, the air temperature grew at a rate of 1.1 °C per 100 years. As far as seasons go, the highest increase was observed in Winter (+ 1.5 °C/100 years) with the lowest in Summer (+ 0.6 °C/100 years). In the last 30 years, the pace of trend has increased to a rate of + 4.6 °C per 100 years, with the highest values recorded for Summer (+ 7.5 °C/100 years).