Abstract
Despite the spatially homogenous summer temperature pattern in Fennoscandia, there are large spreads among the many existing reconstructions, resulting in an uncertainty in the timing and amplitude of past changes. Also, there has been a general bias towards northernmost Fennoscandia. In an attempt to provide a more spatially coherent view of summer (June–August, JJA) temperature variability within the last millennium, we utilized seven density and three blue intensity Scots pine (Pinus sylvestris L.) chronologies collected from the altitudinal (Scandinavian Mountains) and latitudinal (northernmost part) treeline. To attain a JJA temperature signal as strong as possible, as well as preserving multicentury-scale variability, we used a new tree-ring parameter, where the earlywood information is removed from the maximum density and blue intensity, and a modified signal-free standardization method. Two skilful reconstructions for the period 1100–2006 CE were made, one regional reconstruction based on an average of the chronologies, and one field (gridded) reconstruction. The new reconstructions were shown to have much improved spatial representations compared to those based on data from only northern sites, thus making it more valid for the whole region. An examination of some of the forcings of JJA mean temperatures in the region shows an association with sea-surface temperature over the eastern North Atlantic, but also the subpolar and subtropical gyres. Moreover, using Superposed Epoch Analysis, a significant cooling in the year following a volcanic eruption was noted, and for the largest explosive eruptions, the effect could remain for up to 4 years. This new improved reconstruction provides a mean to reinforce our understanding of forcings on summer temperatures in the North European sector.
Highlights
Understanding past climate change and variability allows us to quantify the impact of the recent anthropogenic influence on the climate system by quantifying its natural behaviour and variability (Frank et al 2010)
The results presented here and, perhaps more convincingly those of Björklund (2014), who showed that a set ΔDensity chronologies standardised with the RSFi method were far more coherent compared to their RCS standardised MXD counterparts which displayed more of a “spaghetti plot”, suggests that our method could solve at least some of the discrepancies observed among current data
Based on 10 ΔDensity and Δ blue intensity (ΔBI) chronologies from 8 sites from along the Scandinavian Mountains and the northern treeline, we provided a new skilful reconstruction of Fennoscandian summer (JJA) temperatures for the last 900 years
Summary
Understanding past climate change and variability allows us to quantify the impact of the recent anthropogenic influence on the climate system by quantifying its natural behaviour and variability (Frank et al 2010). Since the first high-resolution reconstruction of Northern Hemisphere temperatures (Mann et al 1999) was introduced, an increasing number of attempts to reconstruct global or hemispheric climate has been made (e.g. MassonDelmotte et al 2013). They all display their own characteristics, depending on methods used and proxies included, in general they provide a broadly coherent picture of the climate evolution over the last two millennia. The efforts of the PAGES 2K consortium (2013) is an excellent example of looking at regional climate change through time
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