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            MEDIEVAL WARM PERIOD

The Medieval Warm Period occurred from about AD 950- 1250, during the European Middle Ages.  It was a time of warm climate in the North Atlantic region that may also have been related to other climate events around the world during that time, including in China, New Zealand and other countries.  It was followed by a cooler period in the North Atlantic termed the Little Ice Age.

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.  Global temperature records taken from ice cores, tree rings, and lake deposits, have shown that, taken globally, the Earth may have been slightly cooler (0.03 degrees Celsius)  during the "MWP” than in the early and mid 20^th century. 

Palaeoclimatologists developing region-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. 

By world region

Evidence exists across the world, often very sparsely, for changes in climatic conditions over time.  Some of the "warm period” events documented below is actually "dry periods” or "wet periods”.

Globally

A study by Michael Mann et al. finds that the MWP shows "warmth that matches or exceeds that of the past decade in some regions, but which falls well below recent levels globally”.  Their reconstruction of MWP pattern is characterised by warmth over large part of North Atlantic, southern Greenland, the Eurasian Arctic, and pars of North America which appears to substantially exceed that of modern late 20^th century (1961-1990) baseline and is comparable or exceeds that of the past one-to-two decades in some regions.  Certain regions such as central Eurasia, northwestern North America, and (with less confidence) parts of South Atlantic exhibit anomalous coolness

North Atlantic

A radiocarbon-dated box core in the Sargasso Sea shows that the sea surface temperature was approximately 1 ºC warmer than today 1000 years ago (MWP).

North America

The Vikings took seas to colonize Greenland and other outlying lands of the far north.  Around 1000 AD the climate was sufficiently warm for the north of Newfoundland to support a Viking colony and led to the descriptor "Vinland”.  Due to the cooling in the Little Ice Age, which lasted up until the 19^th century, the Viking settlements eventually died out.  Sediments in Piermont Marsh of the lower Hudson Valley show a dry Medieval Warm period from AD 800-1300. 

Prolonged droughts affected many pars of the western United States and especially eastern California and the western Great Basin.  Alaska experienced three time intervals of comparable warmth: AD 1-300, 850-1200, and post-1800.  Knowledge of the North American Medieval Warm Period has been useful in dating occupancy periods of certain Native American habitation sites, especially in arid parts of the western U.S. 

Other regions

The climate in equatorial east Africa has alternated between drier than today, and relatively wet. 

An ice core from the eastern Bransfield Basin, Antarctic Peninsula, identifies events of the Little Ice Age and Medieval Warm Period.  The core shows a distinctly cold period about AD 1000-1100, illustrating 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 Niña-like configuration of the El

Niño-Southern Oscillation patterns. 

Adhikari and Kumon (2001), whilst investigation sediments in Lake Nakatsuna in central Japan, verified the existence there of both the MWP and the LIA.   

Temperatures derived from an ¹⁸O/¹⁶O profile through a stalagmite found in a New Zealand cave suggested the Medieval Warm Period to have occurred between AD 1050 and 1400 and to have been 0.75ºC warmer than the Current Warm Period.  The MWP has also been evidenced in New Zealand by an 1100-year old tree-ring record.

Stalactites and stalagmites are what are known as speleothems, deposits of minerals that form into cave structures and line the insides of a cave. Stalactites are the formations that hang from the ceilings of caves like icicles, while stalagmites look like they're emerging from the ground and stand up like a traffic cone. Some may take thousands of years to form, while others can grow quite rapidly. The two formations are also sometimes referred to collectively as dripstone.

Note:   (¹⁸O/¹⁶O)  Oxygen isotope ratio cycles are cyclical variations in the ratio of the abundance of oxygen with an atomic mass of 18 to the abundance of oxygen with an atomic mass of 16 present in some substance, such as polar ice or calcite in ocean core samples. The ratio is linked to water temperature of ancient oceans, which in turn reflects ancient climates.