Antonine Plague (165–180), the Plague of Cyprian (250 to 260), and the Crisis of the Third Century. European population probably reached a minimum during the extreme weather events of 535–536 and the ensuing Plague of Justinian (541–542). Some have connected this demographic transition to the Migration Period Pessimum, when a decrease in global temperatures impaired agricultural yields. Early Middle Ages A major plague epidemic struck the Mediterranean, and much of Europe, in the 6th century. The Early Middle Ages saw relatively little population growth with urbanization well below its Roman peak, reflecting a low technological level, limited trade and political, social and economic dislocation exacerbated by the impact of Viking expansion in the north, Arab expansion in the south and the movement of Slavs and Bulgarians, and later the Magyars in the east. This rural, uncertain life spurred the development of feudalism and the Christianization of Europe. Estimates of the total population of Europe are speculative, but at the time of Charlemagne it is thought to have been between 25 and 30 million, of which perhaps half were in the Carolingian Empire that covered modern France, the Low Countries, western Germany, Austria, Slovenia, northern Italy and part of northern Spain. Most medieval settlements remained small, with agricultural land and large zones of unpopulated and lawless wilderness in between. High Middle Ages In the 10th–13th centuries, agriculture expanded into the wilderness, in what has been termed the "great clearances". During the High Middle Ages, many forests and marshes were cleared and cultivated. At the same time, during the Ostsiedlung, Germans resettled east of the Elbe and Saale rivers, in regions previously only sparsely populated by Polabian Slavs. Crusaders expanded to the Crusader states, parts of the Iberian Peninsula were reconquered from the Moors, and the Normans colonized England and southern Italy. These movements and conquests are part of a larger pattern of population expansion and resettlement that occurred in Europe at this time. Reasons for this expansion and colonization include an improving climate known as the Medieval warm period, which resulted in longer and more productive growing seasons; the end of the raids by Vikings, Arabs, and Magyars, resulting in greater political stability; advancements in medieval technology allowing more land to be farmed; 11th-century reforms of the Church that further increased social stability; and the rise of Feudalism, which also brought a measure of social stability. Towns and trade revived, and