Between the 10th and 14th centuries A.D., Earth's average global temperature may have been warmer than it is today (Lamb, 1977a; Lamb, 1984; Grove, 1988; Lamb, 1988). The existence of this Medieval Warm Period was initially deduced from historical weather records and proxy climate data from England and Northern Europe. Interestingly, the warmer conditions associated with this interval of time are known to have had a largely beneficial impact on Earth's plant and animal life. In fact, the environmental conditions of this time period have been determined to have been so favorable that it is often referred to as the Little Climatic Optimum (Imbrie and Imbrie, 1979; Dean, 1994; Petersen, 1994; Serre-Bachet, 1994; Villalba, 1994).
The degree of warming associated with the Medieval Warm Period varied from region to region; and, hence, its consequences were manifested in a number of different ways (Dean, 1994). In Europe, temperatures reached some of the warmest levels of the last 4,000 years, allowing enough grapes to be successfully grown in England to sustain an indigenous wine industry (Le Roy Ladurie, 1971). Contemporaneously, horticulturists in China extended their cultivation of citrus trees and perennial herbs further and further northward, resulting in an expansion of their ranges that reached its maximum extent in the 13th century (De'er, 1994). Considering the climatic conditions required to successfully grow these species, it has been estimated that annual mean temperatures in the region must have been about 1.0 °C higher than at present, with extreme January minimum temperatures fully 3.5 °C warmer than they are today (De'er, 1994).
In North America, tree-ring chronologies from the southern Canadian Rockies have provided evidence for higher treelines and wider ring-widths between 950 and 1100 A.D., suggesting warmer temperatures and more favorable growing conditions (Luckman, 1994). Similar results have been derived from tree-ring analyses of bristlecone pines in the White Mountains of California, where much greater growth was recorded in the 11th and 12th centuries (Leavitt, 1994). By analyzing 13C/12C ratios in the rings of these trees, it was also found that soil moisture conditions were more favorable in this region during the Medieval Warm Period (Leavitt, 1994). Simultaneous increases in precipitation were additionally found to have occurred in monsoonal locations of the United States desert southwest, where there are indications of increased lake levels from A.D. 700-1350 (Davis, 1994). Other data document vast glacial retreats during the Medieval Warm Period in parts of South America, Scandinavia, New Zealand and Alaska (Grove and Switsur, 1994; Villalba, 1994); and ocean-bed cores suggest global sea surface temperatures were warmer then as well (Keigwin, 1996a, 1996b).
In the area of human enterprise, the climatic conditions of the Medieval Warm Period proved providential. The Arctic ice pack, for example, substantially retreated, allowing the settlement of both Iceland and Greenland; while alpine passes normally blocked with snow and ice became traversable, opening trade routes between Italy and Germany (Crowley and North, 1991). Contemporaneously, on the northern Colorado Plateau in America, the Anasazi Indian civilization reached its climax, as warmer temperatures and better soil moisture conditions allowed them to farm a region twice as large as is presently possible (MacCracken et al., 1990).
Between the 16th and 19th centuries global temperatures were about 1.0°C cooler than present (Allison and Kruss, 1977; Lamb, 1977b; Smith and Budd, 1981; Druffel, 1982; Beget, 1983; Grove, 1988; Zhang and Crowley, 1989; Mann et al., 1998; Mann et al., 1999), gripping the Earth in the jaws of a climatic regime that has universally been acknowledged to have been a Little Ice Age.
As a result of the lower temperatures of this cool climatic excursion, snowfall occurred at lower latitudes and elevations throughout most of the world (Manley, 1969; Manley, 1971; Hastenrath, 1981; Grove, 1988). In some places, such as the Ben Nevis area of Scotland, snowlines were 300-400 meters lower in the 17th and 18th centuries then they are presently (Grove, 1988). The combination of lower snowlines and cooler temperatures provided excellent conditions for glacial growth; and a vast array of studies indicate that alpine glaciers advanced in virtually all mountainous regions of the globe during this period (Luckman, 1994; Villalba, 1994; Smith et al., 1995; Naftz et al., 1996).
Glacial advances during the Little Ice Age typically eroded large areas of land and produced masses of debris. Like an army of tractors and bulldozers, streams of ice flowed down mountain slopes, carving paths through the landscape, moving rocks, and destroying all vegetation in their paths (Smith and Laroque, 1995). These advances often were relatively swift, with one Norwegian account recording a glacial advance of 200 meters in just 10 years (Grove, 1988).
Continental glaciers and sea ice expanded their ranges as well (Grove, 1988; Crowley and North, 1991). Near Iceland and Greenland, in fact, the expansion of sea ice during the Little Ice Age was so great that it essentially isolated the Viking colony established in Greenland during the Medieval Warm Period, leading to its ultimate demise (Bergthorsson, 1969; Dansgaard et al., 1975; Pringle, 1997).
Two closely associated phenomena that often occurred during the Little Ice Age were glacial landslides and avalanches (Porter and Orombelli, 1981; Innes, 1985). In Norway, an unprecedented number of petitions for tax and land rent relief were granted in the 17th and 18th centuries on account of the considerable damage that was caused by landslides, rockfalls, avalanches, floods and ice movement (Grove, 1988). In one example of catastrophic force and destruction, the Italian settlements of Ameiron and Triolet were destroyed by a rockfall of boulders, water, and ice in 1717. The evidence suggests that the rockfall had a volume of 16-20 million cubic meters and descended 1860 meters over a distance of 7 kilometers in but a few minutes, destroying homes, livestock, and vegetation (Porter and Orombelli, 1980). Other data suggest rockslides and avalanches were also frequent hazards in mountainous regions during this period (Porter and Orombelli, 1981; Innes, 1985).
Flooding was another catastrophic hazard of the Little Ice Age, with meltwater streams from glaciers eroding farmland throughout Norway (Blyth, 1982; Grove, 1988). In Iceland, flooding also wreaked havoc on the landscape when, on occasion, subglacial volcanic activity melted large portions of continental glaciers (Thoroddsen, 1905-06; Thorarinsson, 1959). Peak discharge rates during these episodes have been estimated to have been as high as 100,000 cubic meters per second - a value comparable in magnitude to the mean discharge rate of the Amazon River (Thorarinsson, 1957). During one such eruption-flood in 1660, glacial meltwater streams carried enough rock and debris from the land to the sea to create a dry beach where fishing boats had previously operated in 120 feet (36.6 m) of water (Grove, 1988); while flooding from a later eruption carried enough sediment seaward to fill waters 240 feet (73.2 m) deep (Henderson, 1819).
There is also evidence to suggest that some regions of the globe experienced severe drought during the Little Ice Age as a result of large-scale changes in atmospheric circulation patterns (Crowley and North, 1991; Stahle and Cleaveland, 1994). In Chile, for example, dendrochronology studies have revealed that the most intense droughts of the past 1,000 years occurred during this period of time (Villalba, 1994). Similar findings have been obtained from tree-ring analyses in the southeastern United States, where the most prolonged dry episode of spring drought in the last 1,000 years occurred during the mid-18th century (Stahle and Cleaveland, 1994). Elsewhere in the southwestern United States, dendrochronology data indicate that the warm and moist conditions experienced during the Medieval Warm Period gave way to progressively cooler and drier conditions during the Little Ice Age; and it is suspected that this transformation of the climate led to the demise of the Anasazi Indian civilization by reducing the area of land on the Colorado Plateau that was suitable for agriculture (Petersen, 1994). Indeed, cold temperatures and glacial advances resulted in problematic farming in many areas of the world during the Little Ice Age; and failed crops and disrupted ecosystems produced much human misery (Bernabo, 1981; Grimm, 1983; Payette et al., 1985; Campbell and McAndrews, 1991; Cambpell and McAndrews, 1993).
On the basis of many of the reports cited above, the Intergovernmental Panel on Climate Change (Houghton et al., 1990) has determined that the mean air temperature of the globe over the last thousand years most likely varied as shown in the figure to the left.
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