Medieval Warm Period

The Medieval Warm Period (MWP), 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 950 CE to about 1250 CE. 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 (LIA). 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 950 CE to about 1250 CE, during the European Middle Ages. Some researchers divide the MWP into two phases: MWP-I, which began around 450 CE and ended around 900 CE, and MWP-II, which lasted from about 1000 CE to about 1300 CE; 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 In 1965, Hubert Lamb, one of the first paleoclimatologists, published research based on data from botany, historical document research, and meteorology, combined with records indicating prevailing temperature and rainfall in England around 1200 CE and around 1600 CE. He proposed,

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 1000–1200 CE, and was followed by a decline of temperature levels till between around 1500–1700 CE the coldest phase since the last ice age occurred. The era of warmer temperatures became known as the Medieval Warm Period and the subsequent cold period the Little Ice Age (LIA). However, the view that the MWP was a global event was challenged by other researchers. The IPCC First Assessment Report of 1990 discussed the:

Medieval Warm Period around 1000 CE (which may not have been global) and the Little Ice Age which ended only in the middle to late nineteenth century. It stated that temperatures in the:

late tenth to early thirteenth centuries (about 950–1250 CE) appear to have been exceptionally warm in western Europe, Iceland and Greenland. The IPCC Third Assessment Report from 2001 summarized newer research:

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' are chiefly documented in describing northern hemisphere 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 the Earth may have been slightly cooler globally (by 0.03 °C or 0.1 °F) 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 (the autumns only from 1500 onwards).

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. Likewise, peer-reviewed research, compensating for the bias of more studies in northern areas, documents global temperatures were rather stable for 7000 years until the start of the industrial revolution without a global Medieval Warm Period or Little Ice Age.

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 1 °C (1.8 °F) cooler approximately 400 years ago, during the LIA, and 1700 years ago, and was approximately 1 °C (1.8 °F) 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 (δ18 O) and carbon (δ13 C) 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

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