Warmer temperatures, especially in the spring, cause many plants to flower earlier in the year. In 2003, scientists at Boston University and the Arnold Arboretum carefully observed the flowering times of 229 individual plants in the Arnold Arboretum (located in the Jamaica Plain area of Boston) and compared those times with herbarium records for the same individual plants going back over a hundred years.
The Arnold Arboretum/Boston University chart shows the historical trend that has resulted in plants flowering about 10 days earlier than in the past.
Ordering the results by average spring temperature--rather than by year--confirms that the earlier flowering is due to warmer weather. Projected climate change--including increasingly warmer temperatures, especially coupled with the urban heat-island effect--means that the earlier flowering trend is likely to continue, though there are likely to be biological limits, and results will vary by plant species.
D.Primack et al. 2004. Herbarium specimens demonstrate earlier flowering times in response to warming in Boston, American Journal of Botany 91(8): 1260-1264. http://people.bu.edu/primack/Primack_edu_etal_2004_herbarium.pdf