On time scales and causes of abrupt paleoclimatic events

During the last 7 × 10⁵ years the occurrence of abrupt climatic variations, of an intensity probably reaching 5°C/50 yr and with a duration of the order of several centuries can be demonstrated; their frequency is of the order 10^?4 (sometimes even 10^?3) per year. Most impressive examples are sudden coolings in earlier interglacials; in some periods the variability of past climates was obviously much greater than now. Due to the effective spatial coherence of the atmospheric and oceanic circulation their extension, not necessarily of similar intensity, is probably hemispheric or even global. They are modified by feedback mechanisms within the geophysical climatic system; orbital changes play a selective role leading either to suppression or to growth. Any physical interpretation of such abrupt paleoclimatic events remain as yet speculative. One of the most attractive models is the occurrence of clusters of major volcanic eruptions which is more frequent than expected in random series. This is similar to the clustering of severe earthquakes in recent years; both events are probably interrelated responses to the (apparently discontinuous) motions of tectonic plates.