Summer temperatures in the Canadian Rockies during the last millennium: a revised record
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- Luckman, B.H. & Wilson, R.J.S. Clim Dyn (2005) 24: 131. doi:10.1007/s00382-004-0511-0
We present a significant update to a millennial summer temperature reconstruction (1073–1983) that was originally published in 1997. Utilising new tree-ring data (predominantly Picea engelmannii), the reconstruction is not only better replicated, but has been extended (950–1994) and is now more regionally representative. Calibration and verification statistics were improved, with the new model explaining 53% of May–August maximum temperature variation compared to the original (39% of April–August mean temperatures). The maximum latewood density data, which are weighted more strongly in the regression model than ringwidth, were processed using regional curve standardisation to capture potential centennial to millennial scale variability. The reconstruction shows warm intervals, comparable to twentieth century values, for the first half of the eleventh century, the late 1300s and early 1400s. The bulk of the record, however, is below the 1901–1980 normals, with prolonged cool periods from 1200 to 1350 and from 1450 to the late 19th century. The most extreme cool period is observed to be in the 1690s. These reconstructed cool periods compare well with known regional records of glacier advances between 1150 and the 1300s, possibly in the early 1500s, early 1700s and 1800s. Evidence is also presented of the influence of solar activity and volcanic events on summer temperature in the Canadian Rockies over the last 1,000 years. Although this reconstruction is regional in scope, it compares well at multi-decadal to centennial scales with Northern Hemisphere temperature proxies and at millennial scales with reconstructions that were also processed to capture longer timescale variability. This coherence suggests that this series is globally important for the assessment of natural temperature variability over the last 1,000 years.
Although there are many dendroclimatic reconstructions of temperatures that span the last 300–500 years, there are relatively few that extend back prior to AD 1000. It is therefore critical to obtain data from the early part of the last millennium to assess regional and global records of a possible Medieval warm period (Lamb 1965; Hughes and Diaz 1994) and evidence of an early Medieval cooler (glacial) interval in some alpine areas (Luckman 2004). Luckman et al. 1997 published a mean summer (April–August) temperature reconstruction based on ringwidth (RW) and maximum latewood density data (MXD) from tree-ring sites in the Columbia Icefield area of the Canadian Rockies. This reconstruction was the longest then available for boreal North America (1073–1983). Although developed essentially from a single site, this record was considered to be regionally representative owing to its correspondence with available proxy climate records (Luckman et al. 1997, 2000) and has, subsequently, been verified by comparison with an independently developed regional summer temperature record from several treeline sites in interior British Columbia (Wilson and Luckman 2003). The original Icefield data and chronology have been utilised in several compilations of millennial records for Northern Hemisphere temperatures (Briffa 2000; Esper et al. 2002; Mann et al. 1999). In this paper we present a revised reconstruction (869–1994) that extends and increases replication of the earlier Icefield record, uses additional sites from the Rockies and employs different standardisation techniques to capture more low frequency information. We also provide signal strength and related confidence information not reported in the previous reconstruction.
2 Materials, methods and results
2.1 Tree-ring data
Tree-ring data sources
MXD and RWc
MXD and RWd
Additional data for this study
MXD and RWc
2.2 Climate data
The L1997 reconstruction was calibrated against a regional temperature record developed by Luckman and Seed (1995), primarily using data from Jasper, Valemount, Banff and Golden records from 1880 to 1984. Recently, the Meteorological Service of Canada has developed a gridded 50×50 km data-set using homogenised data that extends back to 1895 (Zhang et al. 2000; Milewska and Hogg 2001). For this study, calibration was based on the mean of the four grid squares in this data-set immediately adjacent to the Columbia Icefield area (approximately 51°45′–52°45′N by 116°23′–117°52′W). The correlation between the gridded series and Luckman and Seed’s series over their common interval (1895–1984) is 0.94 for May–August mean temperatures. Data for minimum, mean and maximum temperatures were extracted from the gridded series. Previous work (Wilson and Luckman 2002, 2003) suggested that, as the relationship between these temperature variables has changed in recent decades, maximum temperatures (Tmax) might provide a more stable calibration than the more conventionally used mean temperatures (Tmean).
2.3 Chronology development
Luckman et al. (1997) originally detrended the tree-ring data using traditional individual series standardisation methods. For the RW data, age-related trends were removed by subtracting either negative exponential or straight line functions, while the MXD data were detrended using straight-line functions. This approach captures multi-decadal to century-scale information, but may remove potential millennial scale low frequency trends (Cook et al. 1995). In the present study, we aim to address the potential loss in centennial-to-millennial scale variability that may bias, in the frequency domain, the original Luckman et al. (1997) study.
For this new tree-ring data-set, the mean sample length (MSL) for the RW and MXD series are 242 years and 232 years, respectively, and, when calculated as a running time series, never fall below 200 years over the length of both composite chronologies. Cook et al. (1995) state that the lowest frequency of climate information that can be realistically recovered from traditional detrending methods is 3/n cycles per year (where n = the MSL). Therefore, using traditional individual series detrending methods for either the RW or MXD data, frequencies at timescales greater than a century will not be captured effectively.
In this study therefore, we explored the utilisation of the regional curve standardisation method (RCS, Mitchell 1967; Briffa et al. 1992, 1996; Cook et al. 1995; Esper et al. 2003) that aims to capture secular scale variability at frequencies greater than the MSL. Significantly more low-frequency information was captured using the MXD data (see Appendix) but no significant gain was observed by using the RCS method on the RW data (analysis not shown). Therefore, a RCS chronology was not developed for the RW data. The signal strength and statistical confidence of the chronologies were assessed using both the expressed population signal (EPS, Wigley et al. 1984) and bootstrapped error bars (Efron 1987). EPS values were calculated for each chronology using a 30-year moving window. This ‘moving window’ EPS approach provides an absolute measure of signal quality through time (Briffa 1995). The final RCS MXD chronology meets the 0.85 EPS criterion for signal strength acceptance (Wigley et al. 1984) after AD 1000, except for minor short periods (ca. 1090, 1140, 1280–1320 and 1500), and remains above 0.70 back to AD 900 (Fig. 2a).
2.4 Calibration and verification
As May–August Tmax shows no significant long term trends (first-order AC=0.01) over the calibration period, it is statistically impossible to quantify which reconstruction (RCS2004 or STD2004; Fig. 3a, c respectively) most robustly portrays summer temperature variability over the last 1,000 years. However, as it is known that ‘traditional’ individual series standardisation procedures remove long term trends (i.e. centennial to longer scales) from TR series (Cook et al. 1995), we hypothesise that the RCS2004 reconstruction is a more representative model of past temperature variation. This hypothesis is partially supported by the fact that the RCS2004 reconstruction verifies marginally better than STD2004 and the linear trend of its model residuals is almost zero (Fig. 3a).
Most extreme non-overlapping 20-year periods in the RCS2004 reconstruction
The pre-1300 reconstruction replaces and extends the poorly replicated part of L1997, with some confidence (Fig. 2) to ca. AD 950. The poorly replicated, apparently warm, interval in the early 1100s in L1997 is now discounted. RCS2004 indicates cooler conditions through the 1100s (a time of glacier advance in the region, Luckman 2000; Fig. 5). However, the early eleventh century was as warm as the twentieth century and the late tenth century was probably cooler. Development of the new long composite RW chronology also allowed cross-dating of a critical Athabasca snag that was previously undated. Sample A78-S2 was identified as Larix lyallii (Y. Bégin, 2004, personal communication) and lived between AD 960 and 1107 (the previous chronology only had one sample between AD 1073 and 1107). This is the only known sample of Larix from Jasper National Park, and this is approximately 30 km north of the present range limit of the species, supporting the possibility that conditions were as warm at that time as at present. It is also interesting to note that most of the trees at the Wilcox site that died during the late 1600s began growth in the late 1300s and early 1400s (Luckman 1994) which are reconstructed as relatively warm in RCS2004.
Overall, all three Icefields reconstructions (Fig. 4) are entirely consistent with the known regional glacier history (Luckman 2000). Specifically, the cool periods inferred from the RCS2004 reconstruction coincide very well with periods of glacial advance and moraine formation (Fig. 5b). Although the glacial story becomes less clear further back in time, there is evidence of glacial advance at the Stutfield (Osborn et al. 2001), Robson and Peyto Glaciers during the late twelfth to early fourteenth centuries. This period of glacial activity coincides both with extreme reconstructed 20-year cool intervals (Table 2) and the Wolfe solar minimum. Cool conditions and extreme 20-year cool periods are reconstructed for the mid-to-late 1400s during the Spörer minimum. There is limited evidence of a glacier advance towards the end of this period from minimum lichenometric ages obtained from small moraine fragments in Jasper National Park (Luckman 2000). The reconstructed cold spell in the late 1600s immediately precedes the formation of the outer LIA moraines at about 20% of the glaciers in the Canadian Rockies suggesting that the glaciers responded directly to cool conditions at the end of the seventeenth century. The last major cool period in the RCS2004 reconstruction, at the beginning of the nineteenth century, is again associated with a known period of low solar activity (the Dalton minimum) and immediately precedes the maximum LIA extent of most glaciers in the Rockies ca. 1840–1850 (Luckman 2000) and a series of readvance moraines at many glaciers in the late nineteenth century.
The 20 coldest reconstructed years in RCS2004 after processing the series with a 150-year high pass filter (Fig. 5c)
Mayon, Luzon, Philippines (1897)
Hyaynaputina, Peru (1600)
Gamkonora, Halmahere, Indonesia (1673)
Askja, Northeastern Iceland (1875)
Fuego, Guatemala (1737)
Babuyan Claro, Philippines (1831)
22 (1832), 28 (1831)
48 (1832), 8 (1831)
Soufriere St. Vincent, West Indies (1812)
Awu, Sangihe Islands, Indonesia (1812)
Suwanose- Jima, Ryukyu Islands, Japan (1813)
Llaima, Central Chile (1640)
32 (1641), 8 (1640)
Komaga-Take, Hokkaido, Japan (1640)
Kelut, Java, Indonesia (1641)
Paker, Mindanao, Philippines (1641)
Raung, Java, Indonesia (1817)
31 (1817), 4 (1816)
Colima, Mexico (1818)
Katla, Southern Iceland (1490?)
Although the RCS2004 reconstruction is regionally focused, it does show evidence of large-scale external forcing (both solar and volcanic) that suggests that this series is potentially important for studies of large-scale (i.e. global) climate variability. One of the primary objectives in the development of millennial length reconstructions is to add to the meagre database of available millennial records. Figure 6 compares the RCS2004 reconstruction with available composite Northern Hemisphere temperature reconstructions. Overall, the RCS2004 record shows stronger similarities with the BRIFFA2000 and ESPER2002 records until replication becomes a problem prior to ca. 1000. In general, the three records show cool conditions during the thirteenth century to the first half of the fourteenth century, around the late fifteenth century, and from the late sixteenth to late nineteenth centuries. Although all the records show cool conditions around 1700, the relative magnitude (though not timing) of the 1690s crash in the RCS2004 series is unusual. The RCS2004, BRIFFA2000 and ESPER2002 records portray consistent warm conditions in the eleventh and twentieth centuries, implying that temperatures at the beginning of the millennium were at least similar to those of the twentieth century. RCS2004 also shows conditions in the early 1400s that are relatively warmer than the other reconstructions though all four reconstructions show similar multi-decadal trends from ca. 1300 to 1500. Conditions in the sixteenth century are most similar between RCS2004 and MANN1999 reconstruction.
Ignoring the obvious coherent multi-decadal variability, the RCS2004, BRIFFA2000 and ESPER2002 series show a centennial–millennial scale trend of warm conditions in the eleventh century, followed by cooler conditions until the twentieth century warming, punctuated by warmer intervals between 1400–1600. The MANN1999 record shows a general linear decrease from 1000 until the beginning of the twentieth century when warming starts. Esper et al. (2004) suggest that this difference in long term trend possibly reflects differences in the processing of the tree-ring data between these reconstructions.
The similarity between RCS2004 and large scale reconstructions of Northern Hemisphere temperatures (Fig. 6) highlights the importance of these data and the region for the assessment of large scale temperature variability. Diaz (1996) noted that western North America was one of the key locations where palaeoclimate research should be targeted because temperature variability within this region could explain a high fraction of Northern Hemispheric temperatures on decadal and longer time scales. Therefore continued work in this area will aim to extend the present data-set further back in time.
The revised Icefield reconstruction (RCS2004) remains the longest summer temperature reconstruction from boreal North America and calibrations explain over 50% of the variance of the instrumental temperature record. It is a more regionally based record than the original reconstruction but confirms the general pattern of L1997 in that the 1200s and early 1300s, late 1400s through to the mid-1800s are generally colder (ca. 0.5°C–1°C below 1900–1980 average temperatures). Warm intervals, comparable to twentieth century values, are reconstructed for the first half of the eleventh century, the late 1300s and early 1400s. The 1690s are exceptionally cold (>0.4°C cooler than other intervals) in RCS2004 (and all component chronologies), probably reflecting a combined response to both volcanic and solar forcing. There is also some evidence of colder conditions in the 900s but this early record needs stronger replication. Compared with large-scale NH temperature reconstructions, the Icefield record is not as cold around the early seventeenth century. As with L1997, the general pattern of reconstructed summer temperatures conforms to the known regional record of glacier advances in the 1150–1300s, possibly the early 1500s, early 1700s and 1800s (Luckman 2000). The RCS reconstruction produces lower average temperatures than more traditional standardisation techniques (−0.53°C:−0.34°C over the 1000–1900 period) and more extended colder intervals. The record from 950–1300 is new and/or better replicated and shows a short warmer interval in the early eleventh century but generally cooler conditions in the later eleventh through fourteenth centuries. The periods of greatest transition and change in the last millennium are cooling in the mid-1400s and late 1600s and the warming trend from the mid-1800s onwards.
The RCS2004 record appears to indicate a reasonable response of local trees to large-scale forcing of climates, with reconstructed cool conditions comparing well with periods of known low solar activity, and also extreme cold reconstructed years coinciding with known volcanic events over the last 500 years. Comparison with Northern Hemisphere reconstructions suggests this record is more similar to the Briffa (2000) and Esper et al. (2002) reconstructions than the Mann et al. (1999) reconstruction, though all these records show periods of strong multi-decadal-centennial common variance (Esper et al. 2004). In fact, the strong similarity of the Icefields reconstruction with these Northern Hemisphere reconstructions of temperature suggest that this data-set is important for the assessment of natural temperature variability over the last 1000 years.
The reconstruction presented in this paper is a significant update to Luckman et al. (1997). Not only have the calibration/verification statistics been markedly improved by the inclusion of new data and utilising different temperature parameters, but more low-frequency information has been captured by using non-traditional detrending methods. The continued recovery of buried wood from several sites in the region may provide the potential to link older ‘floating chronologies’ to develop a continuous regional chronology extending back several thousand years.
Funding support from the Natural Sciences and Engineering Research Council of Canada, the Canadian Foundation for Climate and Atmospheric Sciences, Parks Canada and the Inter-American Institute for Global Change Research is gratefully acknowledged. We thank Fritz Schweingruber, Ernest Schär and Theodor Forster at WSL for processing the densitometric data; Trudy Kavanagh for permission to use her Hilda data-set; Yves Bégin (Laval University) for wood identification; Carla Aruani for calculating pith offset data and many individuals who, over the years, have assisted in the collection of samples in the Canadian Rockies; and Dave Frank for proof-reading the final manuscript.