Summer temperature variations in Lapland during the Medieval Warm Period and the Little Ice Age relative to natural instability of thermohaline circulation on multi-decadal and multi-centennial scales

New tree ring-based analysis for climate variability at a regional scale is presented for high latitudes of Europe. Our absolutely dated temperature reconstruction seeks to characterise the summer temperatures since AD 750. The warmest and coolest reconstructed 250-year periods Occurred at AD 931-1180 and AD 1601-1850, respectively. These periods share significant temporal overlap with the general hemispheric climate variability due to the Medieval Warm Period (MWP) and the Little Ice Age (LIA). Further, we detect a multi-decadal (ca. 50- to 60-year) rhythm, attributable to instability of the North Atlantic Deep Water, in the regional climate during the MWP but not during the LIA. Intensified formation of the North Atlantic Deep Water further appeared coincident to the initiation and continuation of MWP, the mid-LIA transient warmth occurring during the period AD 1391-1440, and to recent warming. Our results support the view that the internal climate variability (i.e. thermohaline circulation) could have played a role in the earlier start of the MWP in several proxy reconstructions compared to the externally forced model simulations. Copyright (C) 2009 John Wiley & Sons, Ltd.