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Medieval warm period was a time of global warming.

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Medieval Warm Period was a time of unusually warm climate in the North Atlantic region. The medieval warm period lasted from about the tenth century to about the fourteenth century.

The medieval warm period is often invoked in contentious discussions of global warming and the greenhouse effect. Some refer to the event as the Medieval Climatic Anomaly as this term emphasizes that effects other than temperature were important. Medieval warm period was also known as the Medieval Climate Optimum.

Medieval warm period initial research.

Medieval Warm Period.
The Medieval Warm Period varies little between different studies.

The Medieval Warm Period was a time of unusually warm weather around 800-1300 AD, during the European medieval period. Initial research on the MWP and the following Little Ice Age (LIA) was largely done in Europe, where the phenomenon was most obvious and clearly documented.

It was initially believed that the temperature changes were global. However, this view has been questioned; the 2001 IPCC report summarises this research, saying "…current evidence does not support globally synchronous periods of anomalous cold or warmth over this time frame, and the conventional terms of 'Little Ice Age' and 'Medieval Warm Period' appear to have limited utility in describing trends in hemispheric or global mean temperature changes in past centuries".

Palaeoclimatologists developing regionally specific climate reconstructions of past centuries conventionally label their coldest interval as "LIA" and their warmest interval as the "MWP". Others follow the convention and when a significant climate event is found in the "LIA" or "MWP" time frames, associate their events to the period. Some "MWP" events are thus wet events or cold events rather than strictly warm events, particularly in central Antarctica where climate patterns opposite to the North Atlantic area have been noticed.

The Medieval Warm Period partially coincides with the peak in solar activity named the Medieval Maximum (1100-1250).

Medieval warm period climate events in the North Atlantic and North American regions.

During the MWP wine grapes were grown in Europe as far north as southern Britain although less extensively than they are today (however, factors other than climate strongly influence the commercial success of vineyards and the time of greatest extent of medieval vineyards falls outside the MWP). The Vikings took advantage of ice-free seas to colonize Greenland and other outlying lands of the far north. The MWP was followed by the Little Ice Age, a period of cooling that lasted until the 19th century. In Maryland's Chesapeake Bay, researchers found large temperature excursions during the Medieval Warm Period (about 800-1300) and the Little Ice Age (about 1400-1850), possibly related to changes in the strength of North Atlantic thermohaline circulation. Sediments in Piermont Marsh of the lower Hudson Valley show a dry Medieval Warm period from AD 800-1300.

Prolonged droughts affected many parts of the western United States and especially eastern California and the western Great Basin. Alaska experienced three time intervals of comparable warmth: A.D. 1-300, 850-1200, and post-1800.

A radiocarbon-dated box core in the Sargasso Sea shows that the sea surface temperature was approximately 1ºC cooler than today approximately 400 years ago (the Little Ice Age) and 1700 years ago, and approximately 1ºC warmer than today 1000 years ago (the Medieval Warm Period).

Other regions of the medieval warm period.

The climate in equatorial east Africa has alternated between drier than today, and relatively wet. The drier climate took place during the Medieval Warm Period (~AD 1000-1270).

An ice core from the eastern Bransfield Basin, Antarctic Peninsula, clearly identifies events of the Little Ice Age and Medieval Warm Period. The core clearly shows a distinctly cold period about AD 1000-1100, neatly illustrating the fact that "MWP" is a moveable term, and that during the "warm" period there were, regionally, periods of both warmth and cold.

Corals in the tropical Pacific ocean suggest that relatively cool, dry conditions may have persisted early in the millennium, consistent with a La Niņa-like configuration of the El Niņo-Southern Oscillation patterns. Although there is an extreme scarcity of data from Australia (for both the Medieval Warm Period and Little Ice Age) evidence from wave built shingle terraces for a permanently full Lake Eyre during the ninth and tenth centuries is consistent with this La Niņa-like configuration, though of itself inadequate to show how lake levels varied from year to year or what climatic conditions elsewhere in Australia were like.

Adhikari and Kumon (2001) in investigating sediments in Lake Nakatsuna in central Japan have verified there the existence of both the Medieval Warm Period and the Little Ice Age.

For further discussion of regional and global temperature variations.

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