Read here. Treeline reconstructions are a superior means to determine overall long-term temperature changes versus tree-ring widths. Using treelines, researchers can determine past growth areas for tree species.
Finnish researchers using pollen samples and fossils were able to reconstruct the Scots Pine tree growth areas as far back as 6,000 years before the present. These trees prefer warmer temperatures than today's High North offers, but in the past their range extended much farther north indicating temperatures were at times some 2.0+ Celsius degrees higher. (click on image to enlarge)
"The inferred minimum shift in mean July temperatures between 8300 and 4000 cal. yr BP in Finnish Lapland suggests a ca. + 2.5 °C warmer climate. Until 3000 cal. yr BP, the results indicate a shift of ca. +1 °C in mean July temperatures....During the Medieval Warm Period the reconstructed minimum shift in mean July temperature was ca. + 0.5 °C."