Studies of forests from Finland, Siberia, and Wyoming show that temperature-limited treelines were, at times, higher in elevation and extended farther north than at present. These studies provide additional observational evidence that current warming is neither unprecedented nor unusual and that warmer periods occurred in the past due to natural variation.
A paleoclimate study by a Finnish researcher of northeastern European Russia and Finnish Lapland shows maximum distribution of pine in Finland between 8300 and 4000 cal. yr BP ( calendar years before present). “During this period, pine was absent only from the highest peaks in Finnish Lapland and the distribution area was 13 000 km2 more extensive than at present. In northern Finland the gradual withdrawal of pine resulted in a 2500 km2 decrease in the pine distribution area by 3000 cal. yr BP and a further decrease of 3200 km2 by ca. 1000 cal. yr BP.”
In the Russian forests, “The more extensive distribution of conifer forest and presence of Typha at the present arctic treeline suggest that temperatures were up to 2-4 ° C higher during summer thermal maximum than at present. A gradual cooling started at ca. 6300 cal. yr BP. 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. Between 2500 and 1700 cal. yr BP, megafossil evidence of a wider distribution of pine in Finnish Lapland is lacking. During the Medieval Warm Period the reconstructed minimum shift in mean July temperature was ca. + 0.5 °C.”
In a study of the treeline in the Rocky Mountains of Wyoming, researchers also record a warmer period in the past. “Both pollen and stomata record a sequence of vegetation and climate change similar in most respects to other regional studies, with sagebrush steppe and lowered treeline during the Late Pleistocene, rapid upward movement of treeline beginning about 11,500 cal. yr BP, treeline above modern between ~9000 and 6000 cal. yr BP, and then moving downslope ~5000 cal. yr BP, reaching modern limits by ~3000 cal. yr BP. Between 6000 and 5000 cal. yr BP sediments become increasingly organic and sedimentation rates increase. We interpret this as evidence for lower lake levels during an extended dry period with warmer summer temperatures and treeline advance.”
These past periods of warming occurred when atmospheric carbon dioxide was at least 100 parts per million lower than at present (if you believe ice-core reconstructions).