Medieval Warm Period
The Medieval Warm Period (MWP), Medieval Climate Optimum, or Medieval Climatic Anomaly 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 China and other areas, lasting from about 950 to 1250. It was followed by a cooler period in the North Atlantic termed the Little Ice Age. Some refer to the event as the Medieval Climatic Anomaly as this term emphasizes that effects other than temperature were important.
Despite uncertainties, especially for the period prior to 1600 for which data are scarce, the warmest period of the last 2,000 years prior to the 20th century very likely occurred between 950 and 1100. Proxy records show peak warmth occurred at different times for different regions, indicating that the Medieval Warm Period was not a time of globally uniform change. Temperatures in some regions matched or exceeded recent temperatures in these regions, but globally the Medieval Warm Period was cooler than recent global temperatures. Temperatures were probably between 0.1 °C and 0.2 °C below the 1961 to 1990 mean and significantly below the level shown by instrumental data after 1980.
Possible causes of the Medieval Warm Period include increased solar activity, decreased volcanic activity, and changes to ocean circulation.
The Medieval Warm Period (MWP) is generally thought to have occurred from about AD 950–1250, during the European Middle Ages. In 1965 Hubert Lamb, one of the first paleoclimatologists, published research based on data from botany, historical document research and meteorology combined with records indicating prevailing temperature and rainfall in England around 1200 and around 1600. He proposed that "Evidence has been accumulating in many fields of investigation pointing to a notably warm climate in many parts of the world, that lasted a few centuries around A.D. 1000–1200, and was followed by a decline of temperature levels till between 1500 and 1700 the coldest phase since the last ice age occurred."
The warm period became known as the MWP, and the cold period was called the Little Ice Age (LIA). However, this view was questioned by other researchers; the IPCC First Assessment Report of 1990 discussed the "Medieval Warm Period around 1000 AD (which may not have been global) and the Little Ice Age which ended only in the middle to late nineteenth century." The IPCC Third Assessment Report from 2001 summarised research at that time, 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". Global temperature records taken from ice cores, tree rings, and lake deposits, have shown that, taken globally, the Earth may have been slightly cooler (by 0.03 degrees Celsius) during the 'Medieval Warm Period' than in the early and mid-20th 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 are actually "dry periods" or "wet periods."
A 2009 study by Michael Mann et al. examining spatial patterns of surface temperatures shown in multi-proxy reconstructions 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 parts of North America which appears to substantially exceed that of the late 20th 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 the South Atlantic, exhibit anomalous coolness.
Lloyd D. Keigwin's 1996 study of radiocarbon-dated box core data from marine sediments in the Sargasso Sea found that the sea surface temperature there was approximately 1 °C (1.8 °F) cooler approximately 400 years ago (the Little Ice Age) and 1700 years ago, and approximately 1 °C warmer 1000 years ago (the Medieval Warm Period).
Using sediment samples from Puerto Rico, the Gulf Coast and the Atlantic Coast from Florida to New England, Mann et al. (2009) found consistent evidence of a peak in North Atlantic tropical cyclone activity during the Medieval Warm Period followed by a subsequent lull in activity.
Through retrieval and isotope analysis of marine cores and examination of mollusc growth patterns from Iceland, Patterson et al were able to reconstruct a mollusc growth record at a decadal resolution from the Roman Warm Period through the Medieval Warm Period and into the Little Ice Age.
The 2009 Mann et al. study found warmth exceeding 1961–1990 levels in Southern Greenland and parts of North America during the Medieval climate anomaly (defined for this purpose as 950 to 1250) with warmth in some regions exceeding temperatures of the 1990–2010 period. Much of the Northern hemisphere showed significant cooling during the Little Ice Age (defined for the purpose as 1400 to 1700) but Labrador and isolated parts of the United States appeared to be approximately as warm as during the 1961–1990 period.
Norse colonization of the Americas has been associated with warmer periods. Popular books say that Vikings took advantage of ice-free seas to colonize areas in Greenland and other outlying lands of the far north  but this has been challenged. Around 1000 AD the climate was sufficiently warm for the Vikings to journey to Newfoundland and establish a short-lived European outpost.
From around 985 AD Vikings founded the Eastern Settlement and Western Settlement, both near the southern tip of Greenland. In the colony's early stages they kept cattle, sheep, and goats, with around a quarter of their diet from seafood. After the climate became colder and stormier around 1250 their diet steadily shifted towards ocean sources; by around 1300 seal hunting provided over three quarters of their food. By mid century there was reduced demand for their exports and trade with Europe fell away. The last document from the settlements dates from 1412, and over the following decades the remaining Europeans left in what seems to have been a gradual withdrawal mostly due to economic factors such as increased availability of farms in Scandinavian countries.
In Chesapeake Bay (modern day Maryland and Virginia in the United States), researchers found large temperature excursions (changes from the mean temperature of that time) during the Medieval Warm Period (about 950–1250) and the Little Ice Age (about 1400–1700, with cold periods persisting into the early 20th century), 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 west of 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. Review of more recent archaeological research shows that as the search for signs of unusual cultural changes during the MWP has broadened, some of these early patterns (for example, violence and health problems) have been found to be more complicated and regionally varied than previously thought while others (for example, settlement disruption, deterioration of long distance trade, and population movements) have been further corroborated.
A sediment core from the eastern Bransfield Basin, Antarctic Peninsula, preserves climatic events in the Little Ice Age and Medieval Warm Period. The core shows a distinctly cold period about AD 1000–1100, illustrating 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. 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 9th and 10th centuries is consistent with this La Niña-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), whilst investigating sediments in Lake Nakatsuna in central Japan, finding a warm period from AD 900 to 1200 that corresponded to the Medieval Warm Period and three cool phases, of which two could be related to the Little Ice Age. Another research in northeastern Japan shows that there is one warm/humid interval from AD 750 to 1200, and two cold/dry intervals from AD 1 to 750 and 1200 to present. Ge et al. studied temperatures in China during the past 2000 years; they found high uncertainty prior to the 16th century but good consistency over the last 500 years, highlighted by the two cold periods 1620s–1710s and 1800s–1860s, and the warming during the 20th century. They also found that the warming during the 10–14th centuries in some regions might be comparable in magnitude to the warming of the last few decades of the 20th century which was unprecedented within the past 500 years.
A 1979 study from the University of Waikato found that "Temperatures derived from an 18O/16O profile through a stalagmite found in a New Zealand cave (40.67°S, 172.43°E) suggested the Medieval Warm Period to have occurred between AD 1050 and 1400 and to have been 0.75 °C warmer than the Current Warm Period." The MWP has also been evidenced in New Zealand by an 1100-year tree-ring record.
A reconstruction based on ice cores found the Medieval Warm Period could be distinguished in tropical South America from about 1050 to 1300, followed in the 15th century by the Little Ice Age. Peak temperatures did not rise as high as those from the late 20th century, which were unprecedented in the area during the study period going back around 1600 years.
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