Evidence from deep ice cores shows that in Greenland, at least, the temperature changed rapidly toward the end of the last ice age. Around 15,000 years ago, temperatures were gradually rising as Earth rebounded from the ice ages. The period was named for an Arctic plant that first gave paleoclimatologists a signal of the cooling. When the warming resumed, temperatures rose 15 degrees C in about 40 to 50 years. That's fast. .
Nobody's sure, and ice core records are rare.
But there is some evidence that a rapid cooling like the Younger Dryas occurred in Antarctica at least. This kind of erratic behavior could cause havoc on a highly industrialized planet where greenhouse gases are rapidly accumulating in the atmosphere. Yet they also indicate that rapid warming took place long before the invention of the internal combustion engine. About 10 years ago, Wallace Broecker of Columbia University pinned the rapid cooling of the Younger Dryas on changes in ocean circulation. He suggested that the warming before the Younger Dryas had melted vast amounts of glacial ice, feeding fresh water to the seas around Labrador and Greenland and preventing the downturn of cold, salty ocean water to the sea bottom.