North Cairngorms reconstruction

EPS values >0.85, coinciding with high replication in the RW and BI data, are evident back to the mid-1500s (Fig. 2). With generally lower replication before the mid-sixteenth century, EPS for the RW data (Fig. 2a) nevertheless generally remains reasonably high with a few weaker periods around 1400 and 1500 and in particular before ~1280 when replication decreases and EPS drops more noticeably below the commonly used threshold. The BI chronology replication and signal strength (Fig. 2b) generally mirror the RW results with the exception of an additional weak period around 1425–1550 (a particularly weakly replicated period comprising relatively short samples). Although reconstructions are generically deemed reliable when EPS is >0.85, here the pre ~ 1550 period, where replication is >10 series, is used to allow extension of the reconstruction further back in time—but with caveats of decreased confidence for these earlier periods. The full period of the presented reconstruction is 1200–2010.

Examining the four individual BI-high-pass/RW-low-pass composite chronologies presented in Fig. 3, the two RCS-SF chronologies, as expected, express more low frequency variance than their SF-only counterparts. Increased spread among the chronologies is apparent before ~1740 and is most evident in the period around 1300 and from the late 1500s until ~1700. The period 1280–1300 also appears as a distinct episode of peak tree establishment which might suggest a slight juvenile growth rate bias at this time.

Fig. 3 Four versions of composite (BI + RW) chronologies a untransformed and b smoothed with a 20-year low-pass Gaussian filter. Chronologies are expressed as Z-scores relative to the 1901–2009 period. Tree establishment represents the number of established dates (using pith estimates) in 20 year blocks divided by the mean replication in each respective period. (Note that because the pairs of CID v1.0 and CID v2.0 chronologies are virtually identical after ~1850, the figure appears to show only two chronologies after that time) Full size image

Table 1 details the calibration and verification statistics for each of the four time-series variants. Each series portrays a similar level of reconstruction skill as expressed by the calibration (r2 (1901−2009) = 0.54–0.56) and verification (r2 (1866−1900) = 0.54–0.57) results. However, the similarity of the results is partly a reflection of the limited length of instrumental data which restricts reconstruction assessment to the period covered predominantly by living-tree data (and hence why detrending uncertainty was included in estimation of the reconstruction error range). Nevertheless, a visual assessment of the chronologies in Fig. 3b suggests a high degree of similarity among the chronology variants back to ~1750. Figure 4 presents the instrumental record together with the final calibrated North Cairngorms reconstruction, which was derived by averaging the four variants and weighting them using the validation period RMSE (Table 1). Good agreement is observed between observed and reconstructed temperatures during the 1901–2009 full calibration period (r2 = 0.57) as well as over the 1866–1900 independent verification period (r2 = 0.56).

Table 1 Calibration and verification statistics of the individual reconstructions Full size table

Fig. 4 Untransformed and 10-year low-pass filtered NCAIRN reconstruction (1901–2009 calibration) and observed instrumental July–August temperature including split 1901–1954 and 1955–2009 calibration and verification periods and an 1866–1900 independent verification period Full size image

The final North Cairngorms (NCAIRN) reconstruction is presented in Fig. 5 and the warmest and coldest reconstructed years and decades summarised in Table 2. Importantly, although the range of uncertainty is small over the instrumental period and generally also back to the mid-1700s, before that time there is a wider spread in the confidence limits with variations in the error range reflecting periods of greater and lesser agreement among the individual chronology variants (Fig. 3). In general, however, the reconstruction captures well the late twentieth and early twenty-first century warming (Fig. 4). Considering the range of uncertainty, the recent warming is not unique with 2001–2010 representing the third warmest decade in the record (Table 2). Other notably warm reconstructed periods include two shorter periods (1280–1290 and 1300–1320) in the early part of the record, suggesting the possibility of previous warmer conditions during the late Medieval. Other warm decadal periods, similar to the present, are 1490–1510, 1370–1380 and 1730–1740. Despite containing two of the warmest decades (Table 2), the interval around 1300 is associated with considerable uncertainty and reduced replication. Furthermore, representation of this period in the RCS reconstruction versions may potentially be biased by a concentrated period of recruitment around 1300. However, historical records indicate that the 1280s were marked by climatically favourable conditions with hot, dry summers, though the early 1300s were characterised by deteriorating climate with poor harvests, famine and wet conditions (Dawson 2009; Lamb 1964). It is therefore not clear to what extent this reconstructed early fourteenth century warm period reflects actual climate and this period must therefore be interpreted cautiously at this time.

Fig. 5 NCAIRN a untransformed and b 20-year low-pass filtered reconstruction of July–August temperatures Full size image

Table 2 Ten coldest and warmest reconstructed years and five warmest and coldest reconstructed decades (anomalies relative to 1961–1990) Full size table

When examining extreme individual summers (Table 2), the top five warmest years occur in 1284, 1285, 1307, 1310 and 1282. However, as discussed above, if the 1300s period values are biased due to low replication and age structure, this representation of the warmest years may be misleading and caution is advised when assessing individual years as the expression of extreme years in such reconstructions may be limited (McCarroll et al. 2015). An examination of historical accounts for unusually warm years may also offer little help as documentary archives tend to focus on societally stressful extreme events which may translate into an under-representation of anomalously warm conditions. This is because (unless linked to severe drought) warm extremes were less likely to lead to societal disruption and hardship in a country such as Scotland and were therefore less likely to be recorded than for example cold or wet extremes (Dawson 2009; Dobrovolný et al. 2010).

The most evident extended cold period is centred on the seventeenth century and extends from the late sixteenth until the early eighteenth century (although this is also one of the periods of greatest uncertainty in the reconstruction). This cold period coincides with the so-called Little Ice Age (LIA—Matthews and Briffa 2005) reported in historical and various proxy records from both the Northern and Southern Hemispheres (Büntgen and Hellmann 2014; Neukom et al. 2014) and described as a period of deteriorating climate in Scotland after ~1550 (Lamb 1964). Three of the five coldest reconstructed decades (1631–1640, 1661–1670 and 1691–1700) occurred in the seventeenth century with the 1690s representing the coldest decade in the record (Table 2). The period (~1693–1700) was marked by exceptionally cold and wet summers with widespread famine in Scotland, failed or delayed harvests and southward expansion of sea ice in the northern North Atlantic, coinciding with the effects of volcanic eruptions including the Mt Hekla eruption in 1693 and an unidentified event in 1695 (Dawson 2009; Lamb 1964; Plummer et al. 2012). Other noteworthy cold periods in the reconstruction include the 1440s, the second half of the 1700s and the pre-1270 period, although, again, the latter should be interpreted with caution due to weak replication during that time (Fig. 2). However, historical accounts coinciding with cold periods in the NCAIRN reconstruction do suggest that severe cold winters and famines were frequent before the late thirteenth century and also occurred up to and after the middle of the fifteenth century with widespread crop failure in some years (Dawson 2009).

Exceptionally cold years appear to be well expressed. Of the most extreme single reconstructed years summarised in Table 2 the five coldest are 1232, 1782, 1698, 1799 and 1227. While early accounts are scarce and focus predominantly on cold winters, historical evidence of more recent summers, characterised by low temperature extremes, is insightful. Historical accounts, for example, suggest that the year 1799 (fourth coldest reconstructed year) was characterised by a “…remarkably cold summer…” with temperatures in Scotland well below average (Dawson 2009, p 151). Similarly, 1782 and 1698 are described as years of famine with very late and poor harvests and with very cold conditions overall (Dawson 2009; Walton 1952).

Comparison of NCAIRN with European reconstructions

Spatial correlations of the NCAIRN reconstruction with gridded 0.5° CRU TS 3.22 (Harris et al. 2014) July–August mean temperature (Fig. 6a, b) highlight strong agreement of reconstructed and instrumental temperatures over the British Isles, particularly over Scotland and much of east and northeast England with correlations >0.72. Although the strength of this relationship decreases with increasing distance, it nevertheless still remains high (r > 0.64) over western France, Belgium and the Netherlands. Correlations >0.4 are observed as far away as central Spain and Portugal, parts of central Europe, western parts of the Baltic states, southwest Finland and central Sweden. Though understandably weaker, the character of this spatial pattern is similar when compared to the correlation of the ESMT instrumental temperature record with gridded temperature series around Europe (Fig. 6c).

Fig. 6 Spatial plot of correlations between gridded CRU TS 3.22 (Harris et al. 2014) July–August mean temperatures and a the NCAIRN reconstruction for the United Kingdom and Ireland b the NCAIRN reconstruction for Europe and the c ESMT temperature series over the 1901–2009 period Full size image

Figure 7 compares the NCAIRN reconstruction against other temperature records for the UK including an independent “rest of the Cairngorms” (hereafter referred to as ROC) reconstruction from central Scotland (Rydval et al. 2016b), the Central England Temperature (CET) instrumental record (Parker et al. 1992) and other temperature reconstructions for the UK (Hughes et al. 1984; Lamb 1965; Luterbacher et al. 2004) (a comparison using 20 year low-pass versions is included as Supplementary Figure S4). The ROC reconstruction, derived using a principal component regression approach using all living TR data except for the four sites used in NCAIRN (Rydval et al. 2016b), compares very well back to ~1740 (Fig. 7a). Minor trend differences and deviations in the mean level can be explained as a consequence of both applying RCS to the subfossil series in NCAIRN, whereas no RCS detrending was applied to ROC, and also to weak replication in the early sections of the living chronologies used for ROC.

Fig. 7 Comparison of the NCAIRN reconstruction with a a reconstruction produced using all other living chronologies from the Cairngorms, b CET July–August mean temperature, c Luterbacher et al. (2004) 0.5° summer season (June–August) reconstruction for central Scotland, d Hughes et al. (1984) July–August reconstruction for Edinburgh and e the Lamb (1965) reconstruction of July–August temperature (digitised from Lamb 1965)—note x-axis scale change Full size image

Considering the distance (~500 km) of the North Cairngorms from central England, good agreement between the records is observed back to ~1800 (Fig. 7b), with some weakening in the late twentieth century. The 1750–1800 period appears warmer in CET and a weaker correlation is also observed around that time. The records depart before ~1720 with CET showing generally warmer conditions and the correlation is also weaker in this earliest part. While uncertainties in the CET record have been extensively examined and discussed by Parker and Horton (2005) for the period after 1878, no such exercise has been undertaken for the ~200 years prior to that time. Uncertainty in the record invariably increases back in time with the incorporation of a diverse range of sources and increasing reliance on non-instrumental data such as diaries in the early parts of the record or the placement of some thermometers indoors before 1760 (Manley 1974). These circumstances present a considerable challenge to producing a single homogenised and unbiased temperature record. Such considerations have been investigated and discussed in relation to other long European instrumental temperature records as for example in Sweden (Moberg et al. 2003), the European Alps and for the Northern Hemisphere more generally (Frank et al. 2007). Manley (1974) states that the earliest part (first ~60 years) of the record in particular should be considered less reliable.

The generally flatter Luterbacher et al. (2004) June–August reconstruction (Fig. 7c) shows a centennial departure around the mid-1600s and may possibly also be an expression of limited low frequency information contained in that record which also includes historical documentary evidence. Limitations in the use of historical indices to capture low frequency trends is a known issue (Dobrovolný et al. 2010). Additionally, it is uncertain whether truncation of TR records was performed in Luterbacher et al. (2004) for weakly replicated early sections of the records as this would also affect reconstruction quality in the earlier periods. In general, the correlation between the two series decreases back in time with particularly weak agreement around 1600 and the late seventeenth century, although this improves again in the sixteenth century. The variable degree of agreement can partly be explained by the changing representation of various predictor records from different locations through time in the Luterbacher reconstruction. The excellent agreement with NCAIRN after ~1800 is undoubtedly due to the inclusion of Scottish instrumental temperature records (including Edinburgh from 1764 onwards and Aberdeen from ~1870). As CET is also included as a predictor in the Luterbacher reconstruction, it undoubtedly strongly influences the late seventeenth to late eighteenth century estimates. Luterbacher et al. (2004) acknowledged that inhomogeneities in early (mid-eighteeth to mid-nineteenth century) instrumental records are a potential source of uncertainty, possibly causing a bias towards warmer estimates. The sixteenth century agreement is surprising as (1) this part of the Luterbacher record appears to be based on TR data from northern Norway and documentary evidence from the Low Countries and (2) because the sixteenth century is a period of weaker signal strength in NCAIRN based on EPS results (Fig. 2b). Therefore, this period of coherence with Luterbacher et al. (2004) provides some re-assurance about the reliability of this particular period in NCAIRN.

Given that NCAIRN and the Hughes et al. (1984) reconstructions are entirely independent, agreement with Hughes is good over most periods (Fig. 7d), though weaker in the earliest common period. The most apparent departure after 1800 occurs during a known period of increased disturbance related to more intensive tree harvesting associated with the Napoleonic Wars (Oosthoek 2013; Rydval et al. 2016a; Smout et al. 2005), which may be partly reflected in the Hughes record. The utilisation of polynomials to detrend TR series in the Hughes reconstruction presumably severely restricted the retention of any lower frequency trends. However, there is no indication of divergence when compared to NCAIRN as there appears to be little long-term trend in the period of overlap, although poor verification results in the Hughes record before 1810 are reported and the study cautions that the earliest section of the reconstruction may be less reliable (Hughes et al. 1984). Interestingly, although more qualitative in nature, the Lamb (1965) historical observation-based reconstruction suggests that the seventeenth century was a colder period (Fig. 7e) and also indicates the existence of a warmer period centred on 1300. While this qualitative agreement with NCAIRN is only indicative and does not in any way substantiate the existence of a warmer period around that time, it also highlights that such a possibility cannot be ruled out without careful consideration and evaluation.

Correlations between the UK instrumental and proxy temperature records discussed above (excluding the Lamb reconstruction) are presented in Table 3. The high correlation between CET and the Luterbacher et al. (2004) reconstruction, and the relatively weaker agreement with the three Scottish reconstructions indicates a high degree of dependence of the Luterbacher record on CET. The fact that the ROC reconstruction correlates more strongly with other records than NCAIRN is not surprising as the former includes data from 11 site chronologies—also including MXD data. Overall, the Hughes record shows the lowest correlation of the Scottish reconstructions with other records—indicating that the new data express a substantial update on this original work. An additional comparison of NCAIRN and ROC with an instrumental temperature record from Gordon Castle in northeast Scotland for 1781–1827 and 1879–1974 (Fig. 1; Table 3) shows very strong relationships between the instrumental series and the reconstructions over both periods (r (1879−1974) = 0.72 and r (1781−1827) = 0.70 for NCAIRN; r (1879−1974) = 0.69 and r (1781−1827) = 0.64 for ROC). These consistently high correlations with Gordon Castle provide additional validation of the temporal stability of NCAIRN and ROC outside the calibration period.

Table 3 Correlation matrix of NCAIRN reconstruction and other UK temperature records Full size table

We compare NCAIRN with European temperature reconstructions (Fig. 8), including central Europe (CEU—Dobrovolný et al. 2010), the Pyrenees (PYR—Liñán et al. 2012), the European Alps (ALPS—Büntgen et al. 2006), Jämtland in central Sweden (JÄM—Zhang et al. 2015) and northern Fennoscandia (N-EUR—Esper et al. 2014; Matskovsky and Helama 2014). It is possible to distinguish multidecadal scale periods of reconstruction agreement and disagreement within the spatial context of the location of those records (Fig. 6; see also Supplementary Figure S5 for a comparison of instrumental temperature targets for NCAIRN and other European records). Large differences exist in the magnitude and timing of warmer and colder episodes between most of the examined reconstructions, which can be expected considering the decreasing spatial correlation of Scottish TR and instrumental data over Europe with increasing distance (Fig. 6b, c). However, it is also important to examine agreement and disagreement between records in the context of the reconstructed target season (which is broader than July–August in the case of the Jämtland, Pyrenees, Alps and N-EUR), standardisation method applied and reconstruction uncertainty as such factors can also affect coherence between the series.

Fig. 8 Comparison of temperature reconstructions including a NCAIRN, b Northern Europe, c Central Scandinavia, d Central Europe, e the European Alps and the f Pyrenees. (Periods of solar minima and maxima are highlighted; Jirikowic and Damon 1994; Usoskin et al. 2007; Wagner and Zorita 2005) Full size image

Based on the correlation between NCAIRN target season instrumental data with instrumental target series of the other reconstructions, best agreement would be expected with CEU (r = 0.55) followed by PYR (r = 0.52), ALPS (r = 0.47), JÄM (r = 0.44) and N-EUR (r = 0.28). Correlations between the European reconstructions (Table 4) are largely consistent with geographical distance between the locations as reflected for example by very good agreement between N-EUR and JÄM (r (1500−2002) = 0.63) or the high frequency coherence between CEU and ALPS (r (1500−2002) = 0.56). Interestingly, although some of the European records are not significantly correlated (or only correlate very weakly) with each other, the new Scotland reconstruction correlates significantly with all examined records and most strongly with the Alps (r (1500−2002) = 0.46) and central Scandinavia (r (1500−2002) = 0.38) followed by central Europe (r (1500−2002) = 0.31), N-EUR (r (1500−2002) = 0.21) and the Pyrenees (r (1500−2002) = 0.21).

Table 4 Correlation matrix of the NCAIRN reconstruction and European temperature records using untransformed, 10-year Gaussian-filtered high-pass (HP) and low-pass (LP) series (results significant at the 95% confidence level are highlighted in bold font) Full size table

The reason for the weaker than expected correlation with PYR is unclear, though it may be related to the broader seasonal window (May–September) of that reconstruction and its weaker lower frequency match with instrumental data (Liñán et al. 2012). Poor late eighteenth century summer season verification statistics and a weak common signal between the historical records used for August around the 1650–1700 period in the CEU reconstruction may also account for the weaker correlation with NCAIRN at this time. While some degree of agreement with the other European records is expected considering Scotland’s location (Fig. 6), the correlations suggest that the Scottish record can be seen as intermediary as it shares temporally changing common variance with records from northern to southern Europe. NCAIRN arguably must also express unique variability related to North Atlantic climate dynamics.

The greater variance of N-EUR and CEU in Fig. 8 relative to the other reconstructions can be explained by the application of scaling in the case of N-EUR (instead of regression used for calibration of the other records with the exception of ALPS which also used scaling and PYR which used various approaches) and because CEU was based on historical documentary evidence. With the exception of CEU (and PYR which employed several methods), standardisation of all other records was performed using various forms of RCS standardisation and so would be expected to express low frequency trends well. Although Büntgen et al. (2006) argue that an offset between warmer instrumental and cooler reconstructed temperatures before ~1820 is likely a consequence of unreliable early instrumental temperature data, it is also possible that the reconstruction may over-estimate the extent of cooling prior to that time.

All six records show a warmer interval in the period leading up to the 1950s (see Supplementary Figure S5), although it is less distinct in the CEU reconstruction. While largely absent from other records, the ~1500s warming in the Scotland reconstruction is also present in the central Sweden and CEU records. Although the two Scandinavian records indicate warmer conditions before ~1200, only the Jämtland reconstruction suggests a warmer period around the mid-thirteenth century that is comparable to the twentieth/twenty-first century warming in that record, though it is present ca. 50 years earlier than in NCAIRN—a period which also coincides with greater uncertainty (lower EPS) in the Jämtland record and so should also be interpreted with caution. The absence of a distinct warm episode around 1300 in any of the other records other than NCAIRN supports the notion that the warm estimates for this period may be an artefact of juvenile detrending bias in a period of low replication. Nonetheless, its existence cannot be entirely ruled out based on this evidence alone as there is also some disagreement between the two reconstructions from northern Europe regarding the timing and magnitude of warm and cold events especially in the early periods.

There is reasonable agreement in general between the records regarding protracted cold periods which occur during the LIA and specifically around the Maunder solar minimum centred on the second half of the seventeenth century and to some extent also around the latter part of the fifteenth century coinciding with part of the Spörer minimum (Usoskin et al. 2007). The second half of the 1400s appears as a notably cold period in all of the records with the exception of the Pyrenees where it is less pronounced. However, although the exceptional cold period around 1700 in NCAIRN also stands out in the European Alps and Pyrenees records, the period is less apparent in the CEU, Jämtland and N-EUR reconstructions. There are also greater regional differences in the Dalton minimum period (Wagner and Zorita 2005). Specifically, the period of greatest cooling around that time is before 1800 in the NCAIRN and Jämtland reconstructions but after 1800 in the Central Europe, European Alps and Pyrenees records while only minimal cooling is noted at this time in the N-EUR record.

Exploring forcing of extreme years

As discussed above, and detailed in Table 2, there are a number of significant extreme warm and cold years expressed in the NCAIRN reconstruction. Understanding the forcing mechanisms of such annual extremes is fundamental towards understanding the climate dynamics controlling Scottish summer temperatures. The results of the SEA are presented in Fig. 9 (see Table 5 for a list of events). The GAO results (Fig. 9a) indicate a significant but moderate mean cooling response of 0.25 °C relative to pre-eruption conditions in the first post-eruption year in the NCAIRN record. Using the Sigl et al. (2015) data, the mean post volcanic cooling is slightly greater at 0.37 °C. While the SEA results are noisy, the response to some individual events may be clearer. For example, the single largest temperature reduction in NCAIRN (on the order of ~2 °C) compared to pre-eruption conditions occurred in 1816 following the eruption of Tambora in 1815, although this year is only the 15th coldest reconstructed year in NCAIRN.

Fig. 9 Superposed Epoch Analysis of the response to volcanic eruptions in a, c the NCAIRN reconstruction and b, d other European temperature reconstructions for the 1300–2000 period. a, b represent all events based on the Gao et al. (2008) and c, d on the Sigl et al. (2015) records. See Table 5 for details. Coloured dots in b, d indicate significantly (95% bootstrap—10,000 iterations) cooler post-eruption years in each record. Year ‘zero’ on the x-axis represents the year of the recorded event Full size image

Table 5 Overview of volcanic event years for SEA in Fig. 9 including NH sulphate aerosol injection events >15 Tg from Gao et al. (2008) Full size table

As well as NCAIRN, all of the examined European records consistently show significant cooling 1 year following volcanic events regardless of whether the GAO or SIGL lists are used with the exception of NEUR which shows a response in the year of the event using the SIGL list. There are, however, some additional differences when using the GAO and SIGL data-sets. For example, the Pyrenees reconstruction additionally shows a significant cooling response in year zero using the SIGL events whereas using the GAO list the fourth post-event year is significant instead. In comparison to the other records, the NCAIRN response to volcanic events using the GAO list is muted, which may perhaps be an expression of the oceanic influence on climate in Scotland. However, from this analysis it is also quite clear that the SEA results are sensitive to the specific list of events selected (see also discussion in Esper et al. 2013).

The results may additionally be affected by factors such as the temporal uncertainty of the sulphate deposition records and (when examining individual regional temperature reconstructions) differences in the spatial distribution (and therefore also the light scattering influence) of stratospheric sulphate aerosols which will also differ from event to event (Gao et al. 2008). It can therefore be expected that the expression of post-volcanic cooling from individual regional records may be less clear compared to one based on a larger scale (e.g. hemispheric) analysis (Schneider et al. 2015; Stoffel et al. 2015; Wilson et al. 2016). Furthermore, although limited to a small number of instances and unlikely to produce a significant bias, some additional limitation of the SEA analysis may result from overlapping windows of certain events (e.g. 1809 and 1815) which means that the response to some eruptions may not be entirely independent of others.

Ultimately, although major volcanic events are expressed in the NCAIRN record, there are clearly significant cold reconstructed summers which are not coincident with these externally forced volcanic perturbations of the atmosphere (Table 2). Some other factor must be influencing these reconstructed cold summers which we hypothesise must be related to internal dynamics of the climate system of the North Atlantic sector. To test this, we perform a spatial composite analysis for extreme (> ± 1 standard deviation away from a running 21-year local median high pass filter) warm and cold years against the 500 hPa geopotential height field using both observed (Compo et al. 2011) and reconstructed (Luterbacher et al. 2002) datasets. Clear consistent patterns emerge for both observed (Fig. 10a, 1851–1999; C20Cv2 (Compo et al. 2011), n warm = 6, n cold = 11) and reconstructed (Fig. 10b, 1659–1999; Luterbacher et al. 2002, n warm = 8, n cold = 21) extreme values, although the spatial expression of the height anomalies is larger using the reconstruction. Highly similar patterns are also obtained for SLP (not shown). These obtained patterns are very similar to those of the negative and positive phases of the summer North Atlantic Oscillation (SNAO, Folland et al. 2009). The correlation between NCAIRN and the SNAO (1851–2010) is 0.30 (p < 0.01). The composite results suggest that extreme high (low) temperature years are associated with the positive (negative) phase of the SNAO, where the storm track is shifted northwards (southwards) yielding anticyclone (cyclonic) conditions over Scotland causing warm and dry (mild and wet) summers. The frequency and distribution of warm and cold temperature extremes in the new Scottish reconstruction indicates that the negative phase of the SNAO dominated during the LIA, and that the high frequency of positive SNAO years during the twentieth century was anomalous, at least since the mid-seventeenth century, possibly related to a northward shift in the jet stream.

Fig. 10 a Observed twentieth century reanalysis data (C20C v2c, Compo et al. 2011) 500 hPa geopotential height (Z500) composite anomalies (in meters) for June–August during years with (left) extreme warm (1859, 1878, 1933, 1949, 1955, 1959) and (right) cold (1866, 1867, 1879, 1881, 1883, 1885, 1902, 1909, 1922, 1956, 1962) reconstructed summer temperatures in Scotland (1851–1999). b Reconstructed 500 hPa geopotential height composite anomalies (Luterbacher et al. 2002) for June–August during years with (left) extreme warm (1688, 1779, 1859, 1878, 1933, 1949, 1955, 1959) and (right) cold (1667, 1698, 1722, 1755, 1772, 1782, 1799, 1816, 1823, 1841, 1866, 1867, 1879, 1881, 1883, 1885, 1902, 1909, 1922, 1956, 1962) reconstructed summer temperatures in Scotland (1659–1999) Full size image

Further discussion

Other researchers have highlighted potential limitations of using subfossil material from lakes to make inferences about past climatic conditions. For example, Linderholm et al. (2014) cautioned that the sensitivity of lakeshore pine trees to temperature can be reduced in periods with wetter conditions as they may respond differently when compared to trees growing at tree-line. Although in Scandinavia some bias potential of lakeshore and non-lakeshore material has been noted (Esper et al. 2012), this is less relevant in the Scottish case since the original provenance of the subfossil samples is likely more spatially heterogeneous. Many of the lake-preserved subfossil samples had clear evidence of felling such as axe and saw marks and were also likely felled from a wider region (i.e. including non-lakeshore areas) and transported to the lakes overland or via rivers as a result of logging activities. The living trees were therefore sampled both close to and away from lakeshore environments.

It is worth mentioning that the availability of subfossil material in Scotland is limited in the sense that sampling sites cannot be strategically selected for proximity to upper tree-line as would be the ideal for finding temperature limited trees. Rather, the availability of subfossil material is restricted to specific lakes with suitable conditions for preservation. Therefore, the sites included in the NCAIRN reconstruction are located at an elevational range of 260–420 m a.s.l. and are therefore at least 200 m below the current theoretical tree-line (Miller and Cummins 1982). Nevertheless, strong calibration and verification results and the generally good agreement with the ROC reconstruction (Fig. 7a), which utilised sites closer to the upper tree-line, implies good overall reconstruction performance and does not indicate the existence of any systematic weakening of reconstruction fidelity because of the lower elevation situation of the sampled sites.

Although limitations in the ability of regression-based approaches to accurately represent the full amplitude in reconstructions have previously been highlighted (Esper et al. 2005; von Storch et al. 2004), the strong calibration results should counteract such methodological limitations to some extent. However, it is worth considering that the NCAIRN reconstruction may under-represent the absolute magnitude of past temperature changes.