Abrupt climate change refers to an event where large and widespread shift in climate occurs within a short period, perhaps a decade. The phrase was coined because of worldwide, centuries-long events seen in ice cores of past climate. The archetypical such event was the Younger Dryas at the ending of the last ice age, which saw a rapid brief return to glacial conditions. There are also abrupt climate changes with sudden onset and gradual recovery, such as the 8.2 kiloyear event associated with a meltwater pulse into the Labrador Sea. About 25 "short-term" (by geological standards) climate shifts, called Dansgaard-Oeschger cycles, have been identified in the ice core record during the glacial period over the past 100,000 years (the Wisconsin glaciation). The last one was the Younger Dryas which began 12,900 years ago and moved back into a warm-and-wet climate regime about 11,600 years ago.
The best current theory for the cause of abrupt climate change is the slowing of the ocean's thermohaline circulation (THC). The onset of the Younger Dryas and the 8.2 kiloyear event were caused by freshwater input from the Laurentide ice sheet; the Antarctic Cold Reversal, c. 14,500 years before the present (BP), was caused by a meltwater pulse from the Antarctic ice sheet.
Other, comparatively small events, such as the Little Ice Age, might also be placed in a category of rapid climate change.