Medieval Warm Period
The Medieval Warm Period, also known as the Medieval Climate Optimum or the Medieval Climatic Anomaly, was a time of warm climate in the North Atlantic region that lasted from about to about. Climate proxy records show peak warmth occurred at different times for different regions, which indicate that the MWP was not a globally uniform event. Some refer to the MWP as the Medieval Climatic Anomaly to emphasize that climatic effects other than temperature were also important.
The MWP was followed by a regionally cooler period in the North Atlantic and elsewhere, which is sometimes called the Little Ice Age.
Possible causes of the MWP include increased solar activity, decreased volcanic activity, and changes in ocean circulation. Modelling evidence has shown that natural variability is insufficient on its own to explain the MWP and that an external forcing had to be one of the causes.
Research
The MWP is generally thought to have occurred from about to about, during the European Middle Ages. Some researchers divide the MWP into two phases: MWP-I, which began around and ended around, and MWP-II, which lasted from about to about ; MWP-I is called the early Medieval Warm Period while MWP-II is called the conventional Medieval Warm Period. In 1965, Hubert Lamb, one of the first paleoclimatologists, published research based on data from botany, historical document research, and meteorology, combined with records indicating prevailing temperature and rainfall in England around and around. He proposed,The era of warmer temperatures became known as the Medieval Warm Period and the subsequent cold period the Little Ice Age. However, the view that the MWP was a global event was challenged by other researchers. The IPCC First Assessment Report of 1990 discussed the:
It stated that temperatures in the:
The IPCC Third Assessment Report from 2001 summarized newer research:
Global temperature records taken from ice cores, tree rings, and lake deposits have shown that the Earth may have been slightly cooler globally than in the early and the 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" timeframes, they 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 that are opposite to those of the North Atlantic have been noticed.
Using methods of historical climatology, Christian Pfister and Heinz Wanner reconstructed the seasonal temperature conditions for Western and Central Europe in 2021 on the basis of indices from CE 1000 to 1999.
Global climate during the Medieval Warm Period
The nature and extent of the MWP has been marked by long-standing controversy over whether it was a global or regional event. In 2019, by using an extended proxy data set, the Pages-2k consortium confirmed that the Medieval Climate Anomaly was not a globally synchronous event. The warmest 51-year period within the MWP did not occur at the same time in different regions. They argue for a regional instead of global framing of climate variability in the preindustrial Common Era to aid in understanding.North Atlantic
Lloyd D. Keigwin's 1996 study of radiocarbon-dated box core data from marine sediments in the Sargasso Sea found that its sea surface temperature was approximately cooler approximately 400 years ago, during the LIA, and 1700 years ago, and was approximately warmer 1000 years ago, during the MWP.Using sediment samples from Puerto Rico, the Gulf Coast, and the Atlantic Coast from Florida to New England, Mann et al. found consistent evidence of a peak in North Atlantic tropical cyclone activity during the MWP, which was followed by a subsequent lull in activity.
Iceland
Iceland was first settled between about 865 and 930, during a time believed to be warm enough for sailing and farming. By retrieval and isotope analysis of marine cores and from examination of mollusc growth patterns from Iceland, Patterson et al. reconstructed a stable oxygen and carbon isotope record at a decadal resolution from the Roman Warm Period to the MWP and the LIA. Patterson et al. conclude that the summer temperature stayed high but winter temperature decreased after the initial settlement of Iceland.Greenland
The Mann et al. study found warmth exceeding 1961–1990 levels in southern Greenland and parts of North America during the MWP, which the study defines as from 950 to 1250, with warmth in some regions exceeding temperatures of the 1990–2010 period. Much of the Northern Hemisphere showed a significant cooling during the LIA, which the study defines as from 1400 to 1700, but Labrador and isolated parts of the United States appeared to be approximately as warm as during the 1961–1990 period. Greenlandic winter oxygen isotope data from the MWP display a strong correlation with the North Atlantic Oscillation.The Norse colonization of the Americas has been associated with warmer periods. The common theory is that Norsemen took advantage of ice-free seas to colonize areas in Greenland and other outlying lands of the far north. However, a study from Columbia University suggests that Greenland was not colonized in warmer weather, but the warming effect in fact lasted for only very briefly. Around the climate was sufficiently warm for the Vikings to journey to Newfoundland and to establish a short-lived outpost there.
File:Authentic Viking recreation.jpg|thumb|right| L'Anse aux Meadows, Newfoundland, today, with a reconstruction of a Viking settlement.Around 985, Vikings founded the Eastern and Western Settlements, 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 1350, 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, which was caused mainly by economic factors such as increased availability of farms in Scandinavian countries.
Europe
Winters in Central Europe were almost consistently cold between 1000 AD and the late 19th century- The curve of summer temperatures is characterised by an almost continuous 140-year-long sequence of warm seasons between 1170 and 1310 including, however, a drastic cold setback between 1195 and 1197 Substantial glacial retreat in southern Europe was experienced during the MWP. While several smaller glaciers experienced complete deglaciation, larger glaciers in the region survived and now provide insight into the region's climate history. On the other hand, the Gorner glacier which is the second largest of the Alps advanced during the cool and wet summers between 1140 und 1151 culminating in 1168 In addition to warming induced glacial melt, sedimentary records reveal a period of increased flooding, coinciding with the MWP, in eastern Europe that is attributed to enhanced precipitation from a positive phase NAO. Other impacts of climate change can be less apparent such as a changing landscape. Preceding the MWP, a coastal region in western Sardinia was abandoned by the Romans. The coastal area was able to substantially expand into the lagoon without the influence of human populations and a high stand during the MWP. When human populations returned to the region, they encountered a land altered by climate change and had to reestablish ports. In the Iberian Central Range, there was elevated lake productivity and soil erosion, along with frequent intense runoff events.Other regions
North America
In Chesapeake Bay, researchers found large temperature excursions during the MWP and the Little Ice Age, which are possibly related to changes in the strength of North Atlantic thermohaline circulation. Sediments in Piermont Marsh of the lower Hudson Valley show a dry MWP from 800 to 1300. In the Hammock River marsh in Connecticut, salt marshes extended farther westward than they do in the present due to higher sea levels.Prolonged droughts affected many parts of what is now the Western United States, especially eastern California and the west of Great Basin. Alaska experienced three intervals of comparable warmth: 1–300, 850–1200, and since 1800. Knowledge of the MWP in North America has been useful in dating occupancy periods of certain Native American habitation sites, especially in arid parts of the Western United States. Aridity was more prevalent in the southeastern United States during the MWP than the following LIA, but only slightly; this difference may be statistically insignificant. Droughts in the MWP may have impacted Native American settlements also in the Eastern United States, such as at Cahokia. Review of more recent archaeological research shows that as the search for signs of unusual cultural changes has broadened, some of the early patterns have been found to be more complicated and regionally varied than had been previously thought. Other patterns, such as settlement disruption, deterioration of long-distance trade, and population movements, have been further corroborated.