In order to reconstruct the air temperature variations in Northern Poland for the last millennium observational and proxy (tree-ring widths) data were used. For the first time the ground surface temperature (GST) for Northern Poland was reconstructed based on geothermal data (well temperature profiles). A general warming trend is observed for both the GSTH (GST Histories) derived from geothermal data and instrumental data, in particular, for the last 200 years
