Global Warming

The Medieval Warm Period (Medieval Warm Period) or Medieval Climate Optimum was a time of warm climate in the North Atlantic region, lasting from about the tenth century to about the fourteenth century. It was followed by the a cooler period in the North Atlantic termed as the Little Ice Age. The Medieval Warm Period is often invoked in discussions of global warming. Some refer to the event as the Medieval Climatic Anomaly as this term emphasizes that effects other than temperature were important. The Medieval Warm Period was a time of warm weather around AD 800-1300 during the European Medieval period. Initial research on the Medieval Warm Period and the following Little Ice Age (Little Ice Age) was largely done in Europe, where the phenomenon was most obvious and clearly documented. It was initially believed that the temperature changes were global. However, this view has been questioned; the 2001 IPCC report summarises this research, 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".

Palaeoclimatologists developing region-specific climate reconstructions of past centuries conventionally label their coldest interval as "Little Ice Age" and their warmest interval as the "Medieval Warm Period". Others follow the convention and when a significant climate event is found in the "Little Ice Age" or "Medieval Warm Period" time frames, associate their events to the period. Some "Medieval Warm Period" 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.

A radiocarbon-dated box core in the Sargasso Sea shows that the sea surface temperature was approximately 1 °C (1.8 °F) cooler than today approximately 400 years ago (the Little Ice Age) and 1700 years ago, and approximately 1 °C warmer than today 1000 years ago (the Medieval Warm Period).

The Vikings took advantage of ice-free seas to colonize Greenland and other outlying lands of the far north. Around 1000AD the climate was sufficiently warm for the north of Newfoundland to support a Viking colony and lead to the descriptor "Vinland". The Medieval Warm Period was followed by the Little Ice Age, a period of cooling that lasted until the 19th century, and the Viking settlements eventually died out. In the Chesapeake Bay, researchers found large temperature excursions during the Medieval Warm Period (about 800–1300) and the Little Ice Age (about 1400–1850), 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 western Great Basin.Alaska experienced three time intervals of comparable warmth: A.D. 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 MCA has broadened, some of these early patterns (e.g. violence and health problems) have been found to be more complicated and regionally varied than previously thought while others (e.g., settlement disruption, deterioration of long distance trade, and population movements) have been further corroborated.

The climate in equatorial east Africa has alternated between drier than today, and relatively wet. The drier climate took place during the Medieval Warm Period (~AD 1000–1270). An ice core from the eastern Bransfield Basin, Antarctic Peninsula, clearly identifies events of the Little Ice Age and Medieval Warm Period. The core shows a distinctly cold period about AD 1000–1100, neatly illustrating the fact that "Medieval Warm Period" 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. 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 Eyreduring the ninth and tenth 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.

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