Near the beginning of the current interglacial, global temperatures rose considerably about 10,000 years ago to usher in a period of time referred to as the Holocene. On the basis of temperature reconstructions derived from studies of latitudinal displacements of terrestrial vegetation (Bernabo and Webb, 1977; Wijmstra, 1978; Davis et al., 1980; Ritchie et al., 1983; Overpeck, 1985) and vertical displacements of alpine plants (Kearney and Luckman, 1983) and mountain glaciers (Hope et al., 1976; Porter and Orombelli, 1985), it has been concluded (Webb et al., 1987; COHMAP, 1988) that mean annual temperatures in the Midwestern United States were about 2 °C warmer than those of the past few decades (Bartlein et al., 1984; Webb, 1985), that summer temperatures in Europe were 2 °C warmer (Huntley and Prentice, 1988), as they also were in New Guinea (Hope et al., 1976), and that temperatures in the Alps were as much as 4 °C warmer (Porter and Orombelli, 1985; Huntley and Prentice, 1988). In the Russian Far East, temperatures are also reported to have been from 2 °C (Velitchko and Klimanov, 1990) to as much as 4-6 °C (Korotky et al., 1988) higher than they are today; while the mean annual temperature of the Kuroshio Current between 22 and 35 °N was 6 °C warmer (Taira, 1975), and the southern boundary of the Pacific boreal region was positioned 700 to 800 km north of its present location (Lutaenko, 1993).
A graphical representation of the mean global air temperature that results from the amalgamation of these several records, as prepared by the Intergovernmental Panel on Climate Change (Houghton et al., 1990) and presented in the accompanying figure, indicates that temperatures during the Holocene maximum were warmer than those of the past few decades for a period of time on the order of several thousand years.
In addition to exhibiting temperatures that were significantly warmer than those of today, the first half of the Holocene also produced several regions of significantly enhanced precipitation. The hyper-arid core of the Sahara, for example, was actually moist at this time and contained many lakes (Fabre and Petit-Marie, 1988; Petit-Marie, 1991), as the summer monsoon migrated northward by some 600 kilometers (Ritchie and Haynes, 1987), helping to create the extensive groundwater deposits of that region (van Zinderen Bakker and Coetzee, 1980) and giving life to ecosystems that supported crocodiles, giraffes, elephants and gazelles (Crowley and North, 1991). Increased monsoon moisture also extended across Saudi Arabia, Mesopotamia, and the Rajastan Desert of India (Bryson and Swain, 1981), drastically altering and enhancing the productivity of those lands as well. Indeed, because of the perception that these many biospheric changes were of a positive nature, this much warmer period than the present is often referred to as the Holocene Climatic Optimum (MacCracken et al., 1990; Ciaia et al., 1992; Lutaenko, 1993; Lambin et al., 1996). And it was during this particular warm interval that the world experienced perhaps the greatest of all anthropogenic advancements - "the rise of human civilization, based on the development of agriculture (Whyte, 1995)."
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