The benign conditions were not confined to Greenland and Iceland, of course. During the early Middle Ages the climate was warmer over most
of the Northern Hemisphere. A prolonged period of weather that is warmer than the weather before or after it is known as a climatic optimum. This period is the medieval optimum.
For about 200 years, beginning around 880, farmers were growing wheat around Trondheim, Norway, and they grew barley as far north as about 69.5°N. This is north of Narvik and well inside the Arctic Circle, far beyond the northern limit for present-day arable farming. Farms also expanded up the sides of the valleys during this period, moving onto land 330-660 feet (100-200 m) higher than any that had been cultivated earlier.
Farmers were plowing higher ground all over Europe. Remains of these old fields and long abandoned farming settlements can still be seen in parts of England, on what are now upland moors, more than 1,000 feet (300 m) above sea level. There are examples on Dartmoor and Bodmin Moor, in the southwest of England, for example, in Northumberland in the northeast, and elsewhere. Indeed, this expansion was causing problems. Sheep farmers relied on the upland grazing and in the 1280s there are records of complaints from them that the lowland farmers were plowing too much of the high ground and clearing away the pasture.
In recent years vineyards have become popular in southern England and they are now fairly common. They produce enough grapes to be viable, but modern Britain is not renowned for its fine wines. The late summer sunshine is barely strong enough for the grapes to produce a high enough sugar content to make good wine. Perhaps new grape varieties, cultural techniques, and somewhat warmer weather mean this will change in years to come, but if it does it will be a reversal to the situation that occurred in the Middle Ages. Then there were many vineyards, some as far north as 53°N, in Yorkshire. Some of those vineyards are sited in what are now frost hollows—depressions, such as valleys, where cold air collects on clear nights to produce late spring and early autumn frosts. Vines would be severely damaged or killed by frosts in such places nowadays. Clearly, the medieval growers were not troubled by early frosts that would harm the blossom or prevent the fruit from setting, or by late frosts that would damage the grapes before they had ripened. This suggests that average summer temperatures must have been 1.26-1.8°F (0.7-1.0°C) higher than the English average in the 20th century. Central Europe was even warmer. Temperatures there were 1.8-2.52°F (1.0-1.4°C) higher than the 20th century summer average. The industry was so successful, and the quality of the wine so good, that French wine producers sought a treaty to have the English trade stopped.
Thousands of miles away, in Kampuchea, the climate was also changing. It was already warm, of course, but all climatic changes are associated with alterations in the distribution of pressure and winds. The warm, dry weather of Europe was probably linked to a northward movement of the subtropical high-pressure belt, producing a summer anticyclone with a center extending from the Azores to northern Germany or Scandinavia. In Asia the effect was to produce a fairly permanent anticyclone over Thailand, Laos, and Kampuchea. This brought much drier weather. The tropical forest receded, allowing the Khmer empire, centered on Angkor, to flourish.
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