Summary. State of knowledge on abrupt impacts of climate change ; Anticipating surprises ; This way forward -- 1. Introduction. Previous definitions of abrupt climate change ; Definition of abrupt climate change for this report ; Historical perspective: previous reports on abrupt change ; This report -- 2. Abrupt changes of primary concern. Abrupt changes in the ocean ; Abrupt changes in the atmosphere ; Abrupt changes at high latitudes ; Abrupt changes in ecosystems -- 3. Areas of concern for humans from abrupt changes. Ecosystem services ; Infrastructure ; Other areas of importance for humans from abrupt changes -- 4. The way forward. What has been learned? ; Anticipating surprises ; ACEWS: need for action. "There is still uncertainty about how we will arrive at that future climate state. Although many projections of future climatic conditions have predicted steadily changing conditions giving the impression that communities have time to gradually adapt, for example, by adopting new agricultural practices to maintain productivity in hotter and drier conditions, or by organizing the relocation of coastal communities as sea level rises, the scientific community has been paying increasing attention to the possibility that at least some changes will be abrupt, perhaps crossing a threshold or "tipping point" to change so quickly that there will be little time to react. This concern is reasonable because such abrupt changes--which can occur over periods as short as decades, or even years--have been a natural part of the climate system throughout Earth's history. The paleoclimate record--information on past climate gathered from sources such as fossils, sediment cores, and ice cores--contains ample evidence of abrupt changes in Earth's ancient past, including sudden changes in ocean and air circulation, or abrupt extreme extinction events. One such abrupt change was at the end of the Younger Dryas, a period of cold climatic conditions and drought in the north that occurred about 12,000 years ago. Following a millennium-long cold period, the Younger Dryas abruptly terminated in a few decades or less and is associated with the extinction of 72 percent of the large-bodied mammals in North America."--p. 1.