 For example, excavations at one settlement site have shown the presence of birch trees during the early Viking period.In the case of the Norse, the Medieval warm period was associated with the Norse age of exploration and Arctic colonization, and the later colder periods led to the decline of those colonies.The same period records the discovery of an area called Vinland, probably in North America, which may also have been warmer than at present, judging by the alleged presence of grape vines.