Recent warming in a small region with semi-oceanic climate, 1949-1998: what is the ground truth?

Trends of monthly air temperature extremes were investigated in five meteorological stations of the Grand-Duchy of Luxembourg during the period 1949-1998. The application of an innovative homogenization method based on the concept of relative homogeneity to climatic time series allows identifying multiple break points, as well as correcting data series in an objective and robust statistical way. The rise of maximum temperature (Tmax) has occurred at a rate of 1.5 times that of the minimum temperature (Tmin) in winter (+ 1.4 degrees C versus +0.9 degrees C) and summer (+ 1.4 degrees C versus +0.8 degrees C). No trend in temperature extremes was found in autumn, while spring was affected by a small warming (+0.3 degrees C) of Tmin and no change in Tmax resulting in a decrease of the diurnal temperature range (DTR) (-0.3 degrees C). In spring, a strong positive linear relationship between Tmin warming and local terrain slope could be found. Comparison to new-gridded large-scale climatologies indicates generally close agreement to temperature trends during the 1949-1998 period, while a lower local warming was observed in summer during the post- 1975 period following the changing-point year of atmospheric circulation over North-western Europe. This study shows that the question of data homogeneity is not trivial and should receive careful attention before quantifying historical temperature trends and identifying their spatial patterns at regional scale.