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The Medieval Warm Period and Little Ice Age in Ireland
Reference
McDermott, F., Mattey, D.P. and Hawkesworth, C. 2001. Centennial-scale Holocene climate variability revealed by a high-resolution speleothem 18O record from SW Ireland. Science 294: 1328-1331.

What was done
The authors derived a 18O record - with a time resolution they say is "approximately an order of magnitude better than in the North Atlantic cores that record evidence for quasi-periodic (1475 500 year) ice rafting during the Holocene" - from a stalagmite discovered in Crag Cave in southwestern Ireland, after which they compared this record with the 18O records from the GRIP and GISP2 ice cores from Greenland.

What was learned
In the words of the authors, the study provided evidence for "centennial-scale 18O variations that correlate with subtle 18O changes in the Greenland ice cores, indicating regionally coherent variability in the early Holocene." They also note that the Crag Cave data "exhibit variations that are broadly consistent with a Medieval Warm Period at ~1000 200 years ago and a two-stage Little Ice Age, as reconstructed by inverse modeling of temperature profiles in the Greenland Ice Sheet." Also evident in the Crag Cave data were the 18O signatures of the earlier Roman Warm Period and Dark Ages Cold Period that comprised the prior such cycle of climate in that region.

What it means
The authors state that the coherent 18O variations in the records from both sides of the North Atlantic "indicate that many of the subtle multicentury 18O variations in the Greenland ice cores reflect regional North Atlantic margin climate signals rather than local effects." And, of course, their data confirm the reality of the Medieval Warm Period / Little Ice Age cycle (which climate alarmists refuse to acknowledge), as well as the even-more-strongly-expressed preceding Roman Warm Period / Dark Ages Cold Period cycle, once again demonstrating there is nothing unusual - or unprecedented, as climate alarmists are fond of saying - about the global warming of the past century or so.