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 the rise of a money economy began to weaken the bonds of serfdom that tied peasants to the land. Land was at first plentiful while labour to clear and work the land was scarce; lords who owned the land found new ways to attract and keep labour. Urban centres were able to attract serfs with the promise of freedom. As new regions were settled, both internally and externally, population naturally increased.