ABSTRACT
AS THE NUMBER AND RESOLUTION of paleoclimatic and historical climate observations have improved over the past two decades, so too has our knowledge that climate is capable of changing in surprising ways. Examples include reorganizations in deep ocean circulation, decadal drought in North America, and apparent shifts in the frequency and magnitude of the El Niño Southern Oscillation. In each case, physical change in the climate system is abrupt, occurs over years to decades, is spatially widespread (from continental to global), and involves shifts from one stable state to another, each lasting from years to millennia.
