Ice-core records and other paleoclimatic
indicators show that large (up to 10 degrees C), abrupt (in about 10 years),
widespread (hemispheric to global) climate changes have been common for much of
the last 100,000 years and beyond, but rare during the most recent few
millennia. Changes in the coupled
ocean-atmosphere system with a center of activity in the north Atlantic
probably have been important, but several hypotheses remain possible including
solar influence and a stochastically resonant interaction with changing
freshwater fluxes. Our current
understanding does not allow us to exclude the possibility that human or
natural processes could "flip the switch" of another abrupt change in
the future.