evidence: When most of us think about Ice Ages, we imagine a slow transition into a colder climate on long time scales. Indeed, studies of the past million years indicate a repeatable cycle of Earth’s climate going from warm periods ("interglacial", as we are experiencing now) to glacial conditions. [...] Since the end of the present interglacial and the slow march to the next Ice Age may be several millennia away, why should we care? In fact, won’t the build-up of carbon dioxide (CO²) and other greenhouse gasses possibly ameliorate future changes? [...] These changes are twice as large as those experienced in the worst winters of the past century in the eastern US, and are likely to persist for decades to centuries after a climate transition occurs. They are of a magnitude comparable to the Little Ice Age, which had profound effects on human settlements in Europe and North America during the 16th through 18th centuries. Their geographic extent is in doubt; it might be limited to regions bounding the N. Atlantic Ocean. High latitude temperature changes in the ocean are much less capable of affecting the global atmosphere than low latitude ones, such as those produced by El Niño.