By Dr. Sebastian Lüning and Prof. Fritz Vahrenholt
(German text translated/edited by P Gosselin)
How do today’s temperatures fit into the climate-historical context?
This is one of the main tasks of today’s climate research. A group of researchers led by Milos Rydval have presented a reconstruction of summer temperatures in Scotland over the past 800 years. The results were produced from tree ring examinations.
Surprisingly, the scientists found that the current level of heat in Scotland had been reached and even exceeded several times in the past. These heat spells occurred in the 14th, 16th, and 18th centuries and each spanned over several decades (Figure 1). In between there were cold phases that fit well into the context of the Little Ice Age.
What follows is the abstract of the study published in November 2017 in the journal Climate Dynamics:
Reconstructing 800 years of summer temperatures in Scotland from tree rings
This study presents a summer temperature reconstruction using Scots pine tree-ring chronologies for Scotland allowing the placement of current regional temperature changes in a longer-term context. ‘Living-tree’ chronologies were extended using ‘subfossil’ samples extracted from nearshore lake sediments resulting in a composite chronology >800 years in length. The North Cairngorms (NCAIRN) reconstruction was developed from a set of composite blue intensity high-pass and ring-width low-pass filtered chronologies with a range of detrending and disturbance correction procedures. Calibration against July–August mean temperature explains 56.4% of the instrumental data variance over 1866–2009 and is well verified. Spatial correlations reveal strong coherence with temperatures over the British Isles, parts of western Europe, southern Scandinavia and northern parts of the Iberian Peninsula. NCAIRN suggests that the recent summer-time warming in Scotland is likely not unique when compared to multi-decadal warm periods observed in the 1300s, 1500s, and 1730s, although trends before the mid-sixteenth century should be interpreted with some caution due to greater uncertainty. Prominent cold periods were identified from the sixteenth century until the early 1800s—agreeing with the so-called Little Ice Age observed in other tree-ring reconstructions from Europe—with the 1690s identified as the coldest decade in the record. The reconstruction shows a significant cooling response 1 year following volcanic eruptions although this result is sensitive to the datasets used to identify such events. In fact, the extreme cold (and warm) years observed in NCAIRN appear more related to internal forcing of the summer North Atlantic Oscillation.”
Figure 1: Reconstruction of summertime July-August temperatures in Scotland over the past 800 years, based on tree rings. Source: Rydval et al. 2017.