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Natural Climate Variability on Decade-to-Century Time Scales
Time series of the variations of the annual mean temperature and the annual diurnal temperature range from two long-term stations: the Prague Klementinum-Observatory, Czechoslovakia, and Sodankylä, Finland, which has been designated as a climate reference station. Smooth curve was obtained with a nine-point binomial filter.
is the fact that so many areas share an overall decrease of the DTR.
Recently, other investigators have also compiled information on the change of the DTR over other portions of the globe. Frich (1992) provides evidence to indicate that there has been a general decrease over Denmark since about 1950, based on an analysis of several long-term stations, most located in rural areas. Bücher and Dessens (1991) analyzed a long record of maximum and minimum temperatures from the Pic du Midi de Bigorre Observatory in the Pyrenees at a height of over 2,800 m. Their analysis revealed a significant decrease of the DTR since the late nineteenth century, but an inhomogeneity in the record prevented a continuation of the analysis beyond 1970. Kruss et al. (1992) reported on changes of maximum and minimum temperature between the two 30-year periods 1931 to 1960 and 1961 to 1990 over Pakistan. Despite considerable missing data, they managed to obtain at least 20 years of data from each of the two periods for 35 stations across Pakistan. Their analysis revealed a mix of decreasing and increasing changes of the DTR. In our analysis of Pakistani data we could only manage to identify five stations with adequate data to analyze year-to-year changes, and these were all located in the northern half of the country. These stations also depicted a decrease of the DTR.
Relation to Variations of the Seasonal and Annual Extremes
For a variety of practical considerations, it is important to know whether the decrease in the mean DTR translates to a decrease in the extreme temperature range. Karl et al. (1991a) provide evidence to suggest that, indeed, over the United States and the former Soviet Union (the only areas for which they had access to daily data) there was often a significant and substantial decrease in the seasonal and annual temperature extremes similar to the decrease in the seasonal and annual mean DTR. This similarity is also reflected in the time series of monthly extreme maximum and minimum temperature over Sudan (Jones, 1992).
The reliability of the data used to calculate the changes of the DTR is a critical question. The data presented and discussed here have been subjected to various degrees of