The Medieval climatic optimum (AD 700-1200) was a time of extremely favorable climate in northern Europe. Harvests were good, fishing was abundant, sea ice remained far to the north, vineyards flourished 300 miles north of their present limits, and famine was rare. This was the period of great Viking expansion from Scandinavia. Viking settlements were based on cereal grains (wheat and barley) and dairy herds (goats, sheep, and cattle).
Iceland began settling in AD 874 and soon became an independent republic. Greenland was colonized in AD 985 by Erik the Red. By the 12th century, two sizeable communities existed in southwestern Greenland.
During the Medieval climatic optimum, sea level stood at least a half meter higher in southern Florida than today from the first through tenth centuries.
Comment: In other words, sea levels in the Atlantic have fallen at least 19 inches in the last 1,000 years.
Climatic deterioration began in the 1200s; glaciers expanded in Iceland and in the Alps. Vineyards began declining in Germany and by the 1300s had completely disappeared in England. Fishing replaced cereal grains as the main source of food in Iceland, and sea ice expanded southward between Greenland and Iceland.
Around 1340-50 the more northerly of the two Greenland communities was abandoned to the Inuits. By 1510, only Inuits remained. Cold climate reduced dairy production, and extensive sea ice hampered essential trade with Europe.
Across the Pacific Islands, during the period AD 1270-1475, sea level fell
by more than a meter and temperatures declined an average 1½EC. El Niño increased in frequency, and precipitation increased.
From Climatic History of the Holocene, by James S. Aber