Read here. The past treeline of high latitude northern areas are excellent indicators of historical temperatures. The higher a treeline it indicates that the growing season was warmer and longer. As the red curve in the below representation indicates, the modern treeline is well below the past treeline of the Siberian Larch (Larix sibirica). Also note the estimated past Arctic Ocean shoreline some 10,000 years ago. That much farther north shoreline was prior to the interglacial global warming that melted the ice sheets and glaciation over thousands of years. (click image to enlarge)
"The treeline was at its highest elevation [Polar Ural Mtns.] during the MWP [Medieval Warming Period] between ca AD 900 and 1300 when it reached 340 m [meters]. Following that time the treeline descended to approximately 270 m during the LIA and then ascended to its present elevation of approximately 310 m during the recent warming of the late nineteenth and twentieth centuries....Thus, some areas that were forested or supported sparse trees during
the MWP remain treeless at present. In effect, at the Russian sites
studied, the impact of twentieth century warming has not yet
compensated fully for the mortality and range constriction caused by
the cold temperatures of the LIA [Little Ice Age]. These results are similar to
observations in some other northern treeline regions such as uplands in
eastern Quebec and interior Labrador where Picea mariana (P. Mill.) B. S. P. and Picea glauca (Moench) Voss trees remain below their pre-LIA limits despite recent warming."
