Medieval Warm Period The Medieval Warm Period (MWP) is the name given to the warm period observed in many parts of the world between 900 and 1200 ad approximately. The peak of this period is sometimes referred to as the ‘Little Optimum’, distinguishing it from the longer ‘Post-glacial Climatic Optimum’ that lasted from the fifth to the second millennium bc.

Climate reconstruction is possible from two main sources: historical archives and dating techniques. Knowledge of the climatic history and its effects upon people owes much to the independent archival research of the British climatologist Hubert Lamb and the French historian Le Roy Ladurie. The most useful sources include agricultural records and accounts—proxy indicators of climate as a potentially destabilizing influence on society.

Archival research in and around Europe

Scrutiny of archive sources by Hubert Lamb in the 1960s revealed how climate affected human prosperity in medieval Europe. The Norse colonization of Greenland and Iceland—itself probably made possible only by the moderation in climate—provided a legacy of records in a climatically sensitive zone. Indeed, high latitudes provide the most emphatic evidence for increasing temperature. This is usually attributable to variations of sea ice; for most of the eleventh and twelfth centuries the waters around Iceland and southern Greenland were largely clear of ice.

The discovery early in the twentieth century of plant roots and Norse burial grounds in subsequently frozen soil suggests that temperatures were 2–4 °C higher than in the twentieth century. A chronology of annual temperature for Iceland, derived from sea-ice records, indicates a peak of warmth around 1100 ad, a finding confirmed by evidence of agricultural prosperity: the largest surviving foundations of farmhouses are of Norse origin.

Central European vineyards have been found at elevations more than 200 m higher than today, indicating that summers were up to 1.5 °C warmer. Vineyards spread northwards as well as upwards; they were quite widely distributed over southern England. This implies a freedom from late spring frosts, and summers that were relatively dry and sunny. This is confirmed by estimates of seasonal temperature and rainfall compiled by Lamb, who estimated the mean temperature for July and August in central England from 1150 to 1300 at 16.3 °C compared with 15.8 °C for 1900–50.

Apart from obvious problems of exaggeration, Lamb had to make adjustment for changes in the perception of ‘normal’ weather. Reconstructions can be verified by using the atmospheric circulation as a frame of reference for individual reports. Depression tracks are temperature-sensitive. This is illustrated by the more northerly track in summer, initiated by the poleward retreat of polar ice. It follows that depression tracks will be further north in warm periods, promoting drier summers. Lamb has drawn attention to the care with which medieval water supply channels were constructed in Alpine valleys. Conversely, bridges dating from medieval times over some rivers in the Mediterranean have longer spans than the modern river width would require.

Evidence from dating techniques: assessing the global distribution of the MWP

Dating techniques have provided confirmation of archival evidence. In Europe, radiocarbon dates of wood found in moraines indicate a glacial minimum before 1200 ad, and it is known that tracks made over several Alpine passes were later overrun by glaciers. Ladurie concluded that a glacial advance in central Europe had begun by the early 1200s.

Elsewhere, evidence for warmth has been less conspicuous than increased supply of moisture. This could reflect the fact that agriculture in lower latitudes might be expected to be more sensitive to scarcity of moisture. In the drier western areas of North America, archaeological evidence indicates productive agriculture on land that is now arid. The Indian subculture of New Mexico and Arizona, for example, developed field terracing and irrigation.

A recent advance is the analysis of dendrochronologies that extend over a thousand years. The Laboratory of Tree Ring Research at the University of Arizona has analysed carbon isotope ratios in wood from bristlecone pines (Pinus longaeva) growing in the White Mountains region, which indicate a moist anomaly from 1080 to 1129. This is thought to represent the wettest period of the past thousand years. Rings from trees in the Sierra Nevada region indicate that temperatures exceeded late twentieth-century values between 1100 and 1375. A tree-ring chronology in the southern Canadian Rockies shows a relatively early temperature peak from 950 to 1100 ad, followed by a deterioration in which glaciers are known to have overrun mature forest. This early onset of the MWP at high latitudes is consistent with the results of Greenland ice cores that show rapid cooling as early as 1130 to 1190.

Evidence for warmth can be found in many parts of the world, especially at high latitudes. There is a bias to the northern hemisphere, but as dating techniques become more widely applied—and as interest in the event develops—the global distribution of climate anomalies is becoming clearer. A warming in the twelfth century was detected in a tree-ring chronology for Tasmania. Documentary evidence of citrus cultivation in China suggests that a peak in warmth occurred in the thirteenth century. Hughes and Diaz, at the University of Arizona, do not support the notion that the MWP was a global event. This is contradicted by radiocarbon dating of glacial deposition by Grove and Switsur. There is, however, little doubt that the period represents the last occasion when substantial parts of the world were warmer than in the late twentieth century.

Julian Mayes

Bibliography

Hughes, M. K. and and Diaz, H. F. (1994) Was there a ‘Medieval Warm Period’, and if so, where and when? Climatic Change, 26 (2–3), 109–42. (See also other papers in the same issue.)
Lamb, H. H. (1965) The early Medieval warm epoch and its sequel. Palaeogeography, Palaeoclimatology, Palaeoecology, 1, 13–37.
Le Roy Ladurie, E. (1972) Times of feast, times of famine: a history of climate since the year 1000. Allen and Unwin, London.
Grove, J. M. and and Switsur, R. (1994) Glacial geological evidence for the Medieval Warm Period. Climatic Change, 26 (2–3), 143–69.