Bolling-allerod

A period of warm climate beginning abruptly approximately 14 700 years ago, following the end of the Pleistocene, and extending to approximately 12 700 years ago.
This warm period ended with a return to cold conditions during the Younger Dryas. Traditionally, this period is divided into the Bolling (warm), Older Dryas (cold), and Allerod (warm) intervals, but recent, more detailed climatic records indicate that the entire Bolling–Allerod period was generally warm with several abrupt coolings.
Broeker, W. S. 1992. Defining the boundaries of the late-glacial isotope episodes. Quat. Res.. 38, No.1. 135– 138.

Any function of time f(t) is periodic with period t if f(t) = f(t + τ) for all times t, where τ is the smallest number for which this equality holds.