Global warming has received much attention, but evidence from the past shows that sudden global cooling has occurred with severe failures of agriculture. Extrapolating from dendrochronological evidence, one can predict the following: Approximately once per century there will be a drop of about 0.5-1 degreesC in mean temperature worldwide. In some of these cases, perhaps once every 200 or 300 years this might endanger agricultural production globally. About once per millenium there will be periods of 5-20 years where the temperature is seriously below normal. The last major one year temperature drop was 1816, the year without a summer, probably caused by the cooling effect of the eruption of the volcano Tambora, Indonesia. The last decade-long cooling event was A.D. 536-545 where dust veil, cold, famine, and plague was recorded in Byzantium and China. Very large volcanic eruptions or a comet/asteroid impact have been suggested as cause. Nuclear winter after large-scale nuclear war is a well-known scenario, but climate instabilities may also be caused by changes in the sun, Milankovitch cycles, changes in ocean currents, volcanoes, asteroid impacts, dusting from comets passing close, methane released from its hydrate, and pollution, The risks associated with sudden global cooling are rather smaller than the risks of global warming, but they are real. A dangerous sudden cooling event will happen sooner or later. Ability to change to cold-resistant crops rapidly in large parts of the world may be necessary to avoid major famines. With some important exceptions, fundamental research in abrupt climate change is in place, but agricultural or economic research on volcanic/comet-dusting/nuclear winters and their mitigation is lacking. (C) 2003 Elsevier Science B.V. All rights reserved.