Medieval warm period was natural, according to an article in New Scientist, 2 Apr 2009 and Science, vol. 24, p78, 3 Apr 2009. The "Medieval Warm Period" extended from the ninth to the fourteenth century AD. Following that the climate rapidly cooled to the "Little Ice Age" that lasted to the nineteenth century. A group of climate scientists have studied tree rings Morocco and stalactites in Scotland to reconstruct the climate over the last thousand years in Britain, Europe and North Africa. They concluded that during the warm period there was an intensification of the North Atlantic Oscillation (NAO) associated with increased circulation of warm ocean currents in the North Atlantic resulting in more warm westerly winds blowing over these regions. The researchers collated climate data from other parts of the world and used computer models to come up with a hypothesis of why the ocean currents and winds changed. They suggest there was a La Nina, or cool region, in the tropical western pacific at the same time, and this combined with the increased NAO set up a positive feedback loop that kept the warm period going for five centuries.

The New Scientist article claims the study "scuppers one of the favourite arguments of climate-change deniers" because it "means the medieval warm period was mainly a regional phenomenon caused by altered heat distribution rather than a global phenomenon." The original report in SCIENCE makes no such claim, but concludes: "The persistent positive phase reconstructed for the MCA (Medieval Climate Anomaly) appears to be associated with prevailing La Nina-like conditions possibly initiated by enhanced solar irradiance and/or reduced volcanic activity and amplified and prolonged by enhanced AMOC (Atlantic meridional overturning circulation). The relaxation from this particular ocean atmosphere state into the LIA (Little Ice Age) appears to be globally contemporaneous and suggests a notable and persistent reorganization of large-scale oceanic and atmospheric circulation patterns."

New Scientist

Editorial Comment: The twist given by the New Scientist article to the above data is a classic example of the fallacy of peer review publishing. The changes in climate at the beginning and end of the Medieval Warm Period was as great as recent changes in climate, but the original authors in Science admit they were caused by changes in solar irradiance, volcanic activity, ocean currents and winds. These are all natural phenomena and they are all still happening, so it is inconsistent to blame the recent climate changes on human activity. We tend to agree with one New Scientist letter to the editor which suggested "readers will start to think of the magazine as Green Scientist" because it pushes the green political agenda. (New Scientist Letters, 1 Nov 2008, p20) (Ref. history, weather, politics)

Evidence News 8 April 2009


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