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.

References
Allison, I. and Kruss, P.  1977.  Estimation of recent change in Irian Jaya by numerical modeling of its tropical glaciers.  Arctic and Alpine Research 9: 49-60.

Begét, J.E.  1983.  Radiocarbon-dated evidence of worldwide early Holocene climate change.  Geology 11: 389-393.

Bergthorsson, P.  1969.  An estimate of drift ice and temperature in 1000 years.  Jökull 19: 94-101.

Bernabo, J.C.  1981.  Quantitative estimates of temperature changes over the last 2700 years in Michigan based on pollen data.  Quaternary Research 15: 143-159.

Blyth, J.R.  1982.  Storofsen i Ottadalen.  Unpublished Dissertation, Department of Geography, University of Cambridge, Cambridge, UK.

Campbell, I.D. and McAndrews, J.H.  1991.  Cluster analysis of late Holocene pollen trends in Ontario.  Canadian Journal of Botany 69: 1719-1730.

Campbell, I.D. and McAndrews, J.H.  1993.  Forest disequilibrium caused by rapid Little Ice Age cooling.  Nature 366: 336-338.

Crowley, T. J. and North, G.R.  1991.  Paleoclimatology, Oxford University Press, New York, NY.

Dansgaard, W., Johnsen, S.J., Reeh, N., Gundestrup, N., Clausen, H.B. and Hammer, C.U.  1975.  Climate changes, Norsemen, and modern man.  Nature 255: 24-28.

Davis, O.K.  1994.  The correlation of summer precipitation in the southwestern U.S.A. with isotopic records of solar activity during the medieval warm period.  Climatic Change 26: 271-287.

De'er, Z.  1994.  Evidence for the existence of the medieval warm period in China.  Climatic Change 26: 289-297.

Dean, J.S.  1994.  The medieval warm period on the southern Colorado Plateau.  Climatic Change 26: 225-241.

Druffel, E.M.  1982.  Banded corals: changes in ocean Carbon-14 during the Little Ice Age.  Science 218: 13-19.

Grimm, E.C.  1983.  Chronology and dynamics of vegetation change in the prairie-woodland region of southern Minnesota, USA.  New Phytologist 93: 311-350.

Grove, J.M.  1988.  The Little Ice Age.  Cambridge University Press, Cambridge, UK.

Grove, J.M. and Switsur, R.  1994.  Glacial geological evidence for the medieval warm period.  Climatic Change 26: 143-169.

Hastenrath, S.  1981.  The Glaciation of the Ecuadorian Andes, A.A. Balkema, Rotterdam, The Netherlands.

Henderson, E.  1819.  Iceland: or the Journal of a Residence in that Island, During the Years 1814 and 1815, Wayward Innes, Edinburgh, UK.

Houghton, J.T., Jenkins, G.J. and Ephraums, J.J.  (Eds.).  1990.  Climate Change: The IPCC Scientific Assessment.  Cambridge University Press, Cambridge, UK.

Imbrie, J. and Imbrie, K.P.  1979.  Ice Ages.  Enslow Publishers, Short Hills, NJ

Innes, J.L.  1985.  Lichenometric dating of debris flow deposits on alpine colluvial fans in southwest Norway.  Earth, Surface Processes and Landforms 10: 519-524.

Keigwin, L.D.  1996a.  Sedimentary record yields several centuries of data.  Oceanus 39 (2): 16-18.

Keigwin, L.D.  1996b.  The little ice age and the medieval warm period in the Sargasso Sea.  Science 274: 1504-1508.

Lamb, H.H.  1977a.  Climate History and the Future.  Methuen, London, UK.

Lamb, H.H.  1977b.  Climate: Present, Past and Future, v.2.  Barnes and Noble, New York, NY.

Lamb, H.H.  1984.  Climate in the Last Thousand Years: Natural Climatic Fluctuations and Change.  In: The Climate of Europe: Past, Present and Future.  H. Flohn and R. Fantechi (Eds.).  D. Reidel, Dordrecht, The Netherlands, pp. 25-64.

Lamb, H.H.  1988.  Weather, Climate and Human Affairs. Routledge, London, UK.

Le Roy Ladurie, E.  1971.  Times of Feast, Times of Famine: A History of Climate Since the Year 1000.  Doubleday, New York, NY.

Leavitt, S.W.  1994.  Major wet interval in White Mountains medieval warm period evidenced in d13C of bristlecone pine tree rings.  Climatic Change 26: 299-307.

Luckman, B.H.  1994.  Evidence for climatic conditions between ca. 900-1300 A.D. in the southern Canadian Rockies.  Climatic Change 26: 171-182.

MacCracken, M.C., Budyko, M.I., Hecht, A.D. and Izrael, Y.A.  (Eds.).  1990.  Prospects for Future Climate: A Special US/USSR Report on Climate and Climate Change.  Lewis Publishers, Chelsea, MI.

Manley, G.  1969.  Snowfall in Britain over the past 300 years.  Weather 24: 428-437.

Manley, G.  1971.  The mountain snows of Britain.  Weather 26: 192-200.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1998.  Global-scale temperature patterns and climate forcing over the past six centuries.  Nature 392: 779-787.

Naftz, D.L., Klusman, R.W., Michel, R.L., Schuster, P.F., Reddy, M.M., Taylor, H.E., Yanosky, E.A. and McConnaughey, E.A.  1996.  Little Ice Age evidence from a south-central North American ice core, U.S.A.  Arctic and Alpine Research 28 (1): 35-41.

Payette, S., Filion, L., Gautier, L. and Boutin, Y.  1985.  Secular climate change in old-growth treeline vegetation of northern Quebec.  Nature 315: 135-138.

Petersen, K.L.  1994.  A warm and wet little climatic optimum and a cold and dry little ice age in the southern Rocky Mountains, U.S.A.  Climatic Change 26: 243-269.

Porter, S.C. and Orombelli, G.  1980.  Catastrophic rockfall of September 12, 1717 on the Italian flank of the Mont Blanc massif.  Zeitschrift für Geomorphologie N.F. 24: 200-218.

Porter, S.C. and Orombelli, G.  1981.  Alpine rockfall hazards.  American Scientist 67: 69-75.

Pringle, H.  1997.  Death in Norse Greenland.  Science 275: 924-926.

Serre-Bachet, F.  1994.  Middle Ages temperature reconstructions in Europe, a focus on Northeastern Italy.  Climatic Change 26: 213-224.

Smith, D.J. and Laroque, C.P.  1995.  Dendroglaciological dating of a Little Ice Age glacier advance at Moving Glacier, Vancouver Island, British Columbia.  Géographie physique et Quaternaire 50 (1): 47-55.

Smith, D.J., McCarthy, D.P. and Colenutt, M.E.  1995.  Little Ice Age glacial activity in Peter Lougheed and Elk Lakes provincial parks, Canadian Rocky Mountains.  Canadian Journal of Earth Science 32: 579-589.

Smith, I.N. and Budd, W.F.  1981.  The derivation of past climatic changes from observed changes of glaciers.  In: Sea Level, Ice and Climatic Change. I.  Allison (Ed.).  Int. Assoc. Hydrol. Sci., Pub. 131: 31-52.

Stahle, D.W. and Cleaveland, M. K.  1994.  Tree-ring reconstructed rainfall over the southeastern U.S.A. during the Medieval Warm Period and the Little Ice Age.  Climatic Change 26: 199-212.

Thoroddsen, T.  1905-1906.  Island.  Grundriss der Geographie und Geologie, Petermanns Geographische Mitteilungen, Ergänzungsband 32, Heft 152/3.

Thórarinsson, S.  1959.  Um möguleika á thví ad segja fyrir næsta Kötlugos.  Jökull 9: 6-18.

Thórarinsson, S.  1957.  The jökulhlaup from the Katla area in 1955 compared with other jökulhlaups in Iceland.  Jökull 7: 21-25.

Villalba, R.  1994.  Tree-ring and glacial evidence for the medieval warm epoch and the little ice age in southern South America.  Climatic Change 26: 183-197.

Zhang, J. and Crowley, T.J.  1989.  Historical climate records in China and reconstruction of past climates.  Journal of Climate 2: 833-849.