Medieval Climate Anomaly

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 and elsewhere 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 in the Northern Hemisphere 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.

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 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 In 1965 Hubert Lamb, one of the first paleoclimatologists, published research based on data from botany, historical document research and meteorology, combined with records indicating prevailing temperature and rainfall in England around 1200 and around 1600.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, "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 AD, 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 Medieval Warm Period, and the cold period was called the Little Ice Age (LIA). However, that 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 then summarized 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' 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 the Earth may have been slightly cooler globally (by 0.03 °C) then than in the early and mid-20th century.

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