The effects induced on the climate by human activity have become a major issue for the new millennium. In order to arrive at sustainable conclusions it is necessary, first of all, to assess and quantify natural climatic changes. In general this is done by analysing available time series. In the case of historical hydrometeorological data sets, a comparative analysis with solar cycles is not usually conducted. This work, however, demonstrates that the effect of solar cycles observed at the Equator is also visible at middle and high latitudes with multiple periodicity of the basic solar frequency (roughly 11 years). This could well be due to the interaction between solar forcing and circulation mechanisms within the atmosphere, i.e. water-air-soil interactions coupled with anthropogenic forcing. This theory has been tested by comparing different types of historical data series with the River Po discharges and cyclic appearance of slime bloom in the Adriatic Sea.