Global climate change poses threats to ecosystems and the goods and services they provide not only by its magnitude but by its rapidity. Geohistorical records provide abundant evidence for abrupt changes in climate and other aspects of the earth system during the distant and even recent past. In fact, vulnerability of the earth system to abrupt change was first recognized from geohistorical records of such phenomena as the late-glacial Younger Dryas, glacial-age Heinrich Events, Holocene megadroughts, and Paleocene-Eocene Thermal Maximum. Much attention has been paid to what these and other abrupt changes reveal about the mechanisms underlying climatic regime shifts and the accompanying risks for the future. Paleoclimatology played a prominent role in the 2007 Fourth Assessment of the Intergovernmental Panel on Climate Change (IPCC), focusing particularly on abrupt change. In contrast, geohistorical records of ecological responses to abrupt changes have been underutilized in the global change dialogue, despite rich and detailed records. These records provide the only direct means – short of awaiting abrupt changes of the future – to learn what happens to ecosystems when climate undergoes rapid change. The proposed symposium brings together scientists studying abrupt changes of the past in a variety of ecosystems (tropical to arctic, terrestrial to marine) and in periods ranging from the past few centuries to tens of millions of years ago. The first speaker will discuss abrupt changes of the past from an IPCC perspective, outlining the risks of abrupt change in the future. Two talks will respectively cover terrestrial and marine impacts of the Paleocene-Eocene Thermal Maximum, an event that included rapid temperature increase, atmospheric CO2 increase, and ocean acidification. Two speakers will concentrate on terrestrial consequences of rapid glacial-to-interglacial transitions. One will discuss the Eemian and earlier transitions using a new record from Lake Urmia in Iran and integrating it with other records from Europe. The following speaker will review the European and North American records of rapid warming events (Bölling-Alleröd and Younger Dryas termination). Three speakers will discuss consequences of rapid mid-Holocene hydrological excursions in eastern North America, the role of vegetation-atmosphere feedbacks in the termination of the African mid-Holocene humid period, and recent and ongoing changes in the boreal forest. The Workshop will conclude with a 30-minute panel discussion on the ecological risks of abrupt changes of the future.