The Medieval Warm Period (MWP) or Medieval Climate Optimum was an unusually warm period in history lasting from about the 10th century to about the 14th century.
During this time winegrapes were grown in Europe up to 300 miles north of their present northerly growing limit. The Vikings took advantage of ice-free seas to colonize Greenland and other outlying lands of the far north. The period was followed by the Little Ice Age (LIA), a period of cooling that lasted until the 19th century when the current period of global warming began.
Initial research on the MWP and LIA 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, recently this view has been questioned by the IPCC. The 2001 IPCC report says: "...current evidence does not support globally synchronous periods of anomalous cold or warmth over this timeframe, 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." [1] (http://www.grida.no/climate/ipcc_tar/wg1/070.htm).
See also the 1000 year temperature reconstruction of Mann et al [2] (http://www.grida.no/climate/ipcc_tar/wg1/figspm-1.htm). The IPCC TAR says of the MWP that the posited Medieval Warm Period appears to have been less distinct, more moderate in amplitude, and somewhat different in timing at the hemispheric scale than is typically inferred for the conventionally-defined European epoch. The Northern Hemisphere mean temperature estimates of Jones et al. (1998), Mann et al. (1999), and Crowley and Lowery (2000) show temperatures from the 11th to 14th centuries to be about 0.2°C warmer than those from the 15th to 19th centuries, but rather below mid-20th century temperatures. A paper by Soon and Baliunas (http://w3g.gkss.de/G/Mitarbeiter/storch/pdf/soon+baliunas.cr.2003.pdf) disagrees with the IPCC and Mann and states that those warm and cold periods were indeed widespread climatic anomalies (Soon & Baliunas don't disagree altogether with Mann because Soon & Baliunas state in their table 1 that Mann shows a widespread Little Ice Age). However, the Soon and Baliunas report itself has been extensively criticised [3] (http://w3g.gkss.de/G/Mitarbeiter/storch/CR-problem/cr.2003.htm). As a result, there is no definitive answer to this question.
American Heritage Dictionary (http://bartleby.com/61/34/M0193425.html)- "The period from about 1000 to 1400 in which global temperatures are thought to have been a few degrees above those of the preceding and following periods. The climatic effects of this period were confined primarily to Europe and North America. Also called Medieval Warm Epoch." (American Heritage Dictionary)
The "Medieval Warm Period" (http://greenpeace.org/~climate/database/records/zgpz0231.html)- Greenpeace article citing journal article: (M.K. Hughes and H.F. Diaz, "Was there a 'Medieval Warm Period?", Climatic Change, v.26, p.109-142, March 1994).
John L. Daly's graphs (http://www.john-daly.com/hockey/hockey.htm) - claims that the Medieval Warm Period may have been global in character
Climate in a narrow sense is usually defined as the “average weather”, or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years.
When the original conception of climate as a long-term average came to be considered, perhaps towards the end of the 19th century, the idea of climate change was not current, and a 30 year average seemed reasonable (but see note 1).
Climate indices are generally identified or devised with the twin objectives of simplicity and completeness, and each typically represents the status and timing of the climate factor they represent.
Climate variations, both in the mean state and in other statistics such as, for example, the occurrence of extreme events, may result from radiative forcing, but also from internal interactions between components of the climate system.
The response of the climate to the internal variability of the climate system and to external forcings is further complicated by feedbacks and non-linear responses of the components.
Similarly, although the climate system is highly non-linear, the quasi-linear response of many models to present and predicted levels of external radiative forcing suggests that the large-scale aspects of human-induced climate change may be predictable, although as discussed in Section 1.3.2 below, unpredictable behaviour of non-linear systems can never be ruled out.
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