 The drastic change in vegetation is typically interpreted to be an effect of a sudden decrease in (annual) temperature, unfavorable for the forest vegetation that had been spreading northward rapidly.The cooling not only favored the expansion of cold-tolerant, light-demanding plants and associated steppe fauna, but also led to regional glacial advances in Scandinavia and a lowering of the regional snow line.The change to glacial conditions at the onset of the Younger Dryas in the higher latitudes of the Northern Hemisphere, between 12,900 and 11,500 calendar years BP, has been argued to have been quite abrupt.