The Climatic History of the European North Atlantic Seaboard
Kullman, L. 1998. Tree-limits and montane forests in the Swedish Scandes: Sensitive biomonitors of climate change and variability. Ambio 27: 312-321.
Among other things, Kullman discovered that "some exceptionally warm and stable centuries, with high tree-limits and dense montane forests, occurred during the Medieval period." He also finds "an episode of warmer climate during the first half of the [20th] century," but notes that tree-limits and high-elevation forests at that time "were far from restored to their medieval levels," which by AD 900-1100 "were situated 80-100 meters higher" than they were about a century ago, i.e., ~1900. In addition, he reports that "during the past few decades," i.e., during the latter part of the 20th century, there was widespread "rapid cold-induced dieback."
In discussing his findings, the Swedish scientist states that "the slight cooling and associated tree-limit and forest responses since the climate optimum in the late 1930s [italics added] fit a more general pattern, common to the entire North Atlantic seaboard and adjacent continental areas." And what is that pattern? He says "there appear to have been no detectable regional or global tree-limit progression trends over the past 2-3 decades matching the GCM climate projections based on increasing CO2 levels." Hence, he concludes that "since tree-limits in Scandinavia or elsewhere in the world have not reestablished at their Medieval levels, it is still possible that today's climate, despite centennial net warming, is within its natural limits."