Via Wired Science, a graph of long-term climate data:
In Japan, cherry blossom festivals are an ancient and wildly popular tradition, featuring days-long celebrations carefully timed to coincide with peak flowering. The festivals are so prominent in Japanese culture that their collective descriptions in diaries, literature and administrative records have been turned into a six-century-long record of blossoming dates and locations across the islands.The top row shows dates of full flowering, from the 11th century until now (lower number = closer to Jan 1). Bottom graph is mean March temps derived from flowering dates.
Because the trees blossom at certain temperatures, scientists can infer historical weather information, and ultimately climate trends, from these dates. Few other historical climate records contain such fine-grained detail.