Using An Outdated Reference Period, Germany’s DWD Weather Service Turns Cooling Into Warming

Why does the DWD use the internationally no longer valid 1961-1990 reference period? It writes itself elsewhere: “To record the climate and its changes, mean values are formed over a period of 30 years…With the end of the year 2020, the reference period for current climatological assessments was replaced by the 1991 to 2020 period.”