ning roughly 50 million years ago, something happened to bring about a gradual deterioration of climate. This deterioration culminated, about 2 million years ago, in the arrival of a new mode of climate, characterized by a long sequence of perhaps as many as 20 major glacial-interglacial oscillations, which presumably continues to grip the world today.
The history of glacial-interglacial events is revealed in some detail for the past one million years by oxygen-18 analysis of deep-sea sediments and by several other kinds of paleoclimatic indicators. In that period of time, and most particularly during the last half-million years, the principal variations of ice volume and other conditions of climate appear to have been of a quasi-periodic nature, with a characteristic wavelength close to 100,000 years (see Figure 2.2). Lesser ice volume changes seem also to have occurred, with characteristic periods of about 20,000 and 40,000 years. The world has been in a relatively warm extreme-interglacial phase during the past 10,000 years, similar to other relatively brief warm phases that have occurred earlier at roughly 100,000-year intervals. In these interglacial phases, ice has been confined primarily to the Antarctic and Greenland ice sheets. In the intervening glacial phases, continental ice sheets have developed primarily in high northern latitudes, which covered a maximum of about 9 percent of the earth’s surface and reached a volume of about 75 × 106 km3—some three times present-day figures. The variations of ice conditions have been accompanied by midlatitude temperature changes of the order of 6 to 10°C and by sea-level changes of the order of 100 m.
Since recovery of the earth from the last major glacial stage about 10,000 years ago, global climate has been found from a variety of paleoclimatic indications to have varied within narrower limits (see Figure 2.3). An intercontinental survey of mountain glacier moraines and tree lines has revealed three periods of glacier expansion (each of about 1000 years’ duration), alternating with three periods of glacial contraction (each of 1000 to 2000 years’ duration) in the past 8000 years. There is evidence, derived primarily from pollen analyses of lake and bog sediments, that significant shifts of the earth’s vegetation zones accompanied these glacial variations, in what is sometimes referred to as the “neoglacial cycle.” General temperature levels are believed to have varied by about 1 or 2°C during the course of this cycle.
Narrowing our attention to the past millennium, we can begin to call into play various additional climatic indices such as human chronicles, tree-ring sections, and annually laminated ice cores from Greenland and Antarctica, to add further detail to our reconstruction of climatic history (see Figure 2.4). The period from about A.D. 1430 to A.D. 1850 was one of comparatively cold climate and expanded mountain glaciation. A member of the “neoglacial cycle,” noted above, this cold period is commonly referred to as the “Little Ice Age.” Earlier centuries were milder, although probably not everywhere as warm as the climate of today. From long instrumental records and human chronicles, typical fluctuations of 30-year averages of climatic variables over the past several centuries have been as follows: Major circulation features, such as the centers of subpolar lows and subtropical highs, and the position of the Intertropical Convergence Zone, have varied by 2 or more degrees of latitude. The positions of midlatitude troughs and ridges have shifted east or west