The medieval period of hot climate or Medieval Warm Period has always aroused controversies and various interpretations among historians and scientists. Yet it did exist just as the colder periods that the earth has known and historical accounts confirm its worldwide occurrence.
Don J. Easterbrook is Professor Emeritus of Geology at Western Washington University in Evidence-Based Climate Science gives a clear description of this period that lasted for almost half a century.
The Medieval Warm Period (MWP) was a time of warm climate from about 900–1300 AD, when global temperatures were somewhat warmer than at present. The effects of the warm period were particularly evident in Europe, where grain crops flourished, alpine tree lines rose, many new cities arose, and the population more than doubled.
The Vikings took advantage of the climatic amelioration to colonize southern Greenland in 985 AD, when milder climates allowed favorable open-ocean conditions for navigation and fishing. Greenland settlements lasted about 500 years before cooling during the Little Ice Age ended the settlements.
During the Medieval Warm Period, wine grapes were grown as far north as England, where growing grapes is now not feasible and about 300 miles (500 km) north of present vineyards in France and Germany. Grapes are presently grown in Germany up to elevations of about 1800 ft (560 m), but from about 1100 to 1300 AD, vineyards extended up to about 2500 ft (780 m), implying that temperatures were warmer by about 2–2.5°F (1–1.4°C). Wheat and oats were grown around Trondheim, Norway, suggesting that the climate was about 2°F (1°C) warmer than present (Fagan, 2007).
Elsewhere in the world, prolonged droughts affected the southwestern United States and Alaska warmed. Sediments in central Japan record warmer temperatures. Sea surface temperatures in the Sargasso Sea were approximately 1°C warmer than today and the climate in equatorial east Africa was drier from 1000 AD to 1270 AD. An ice core from the eastern Antarctic Peninsula shows warmer temperatures during this period.
At the end of the Medieval Warm Period, around 1300 AD, temperatures dropped dramatically and the cold period that followed is known as the Little Ice Age. This period occurred from the 14th century through the mid-19th century.
(Source: Evidence-Based Climate Science/ Don J. Easterbrook)