In the April 3rd edition of Science a collaborative group of scientists from Switzerland, California and the UK report that medieval climate over Europe was heated by the North Atlantic Oscillation (NAO). This oscillation pattern, defined as the pressure difference between the Icelandic Low and the Azores High, also influences modern-day weather conditions and has contributed to the recent droughts in North Africa and floods in North-Central Europe.
Critique of PhysOrg article.
A comparison of tree rings from 1000-year old trees in Morocco and growth layers in a stalagmite from a cave in Scotland now reveal the mechanism behind the 'Medieval climate anomaly' - a period of global warmth between 1000 and 1400 AD. During this period, the pressure difference between the Azores High and the Icelandic Low was large and, by driving warm Atlantic winds over the cold European continent in winter time, was heating the European mainland.
Trees and stalagmites are "proxy archives", meaning that they are natural data sources from which past climatic conditions can be derived. Old cedar trees from the Atlas Mountains in Morocco grew extremely slowly during Medieval Times and thus reflect much drier conditions during this period compared to following centuries. These dry conditions, in turn, are an indicator for a strong Azores High. Opposite to the African tree rings, the Scottish stalagmite shows that during the same period it was much wetter than normal in northern Europe, reflecting a strong Icelandic Low.
Comment: Take notice of the use of the language here. Specifically this article's minimization of the Medieval Warm Period is now the 'Medieval climate anomaly'.
Comment: The attention focused on this article is because of the work and intent of the authors' of the paper discussed. The paper basically reduces the Medieval Warm Period to a regional European phenomenon using the old bag of tricks employed by Michael Mann of the infamous hockey-stick graphic (used to eliminate the Medieval warm period and show man-made global warming).
The discussion is rather in depth but it does show how low the man made global warming proponents will go to validate their beliefs.
Climate Audit's discussion has become quite expansive with several articles demonstrating the deceptiveness of the methods used in the paper.
Discussions can be reviewed here:
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Additionally the Medieval Warm Period was a world wide climatic event. As IceCap pointed out CO2 Science has setup a database to track research on the medieval Warm period. (Interactive map here java plugin)
The database preface reads: