date: Wed, 14 Aug 1996 14:59:37 -0500 (EST) from: LUCKMAN@SSCL.UWO.CA subject: Icefields paper to: K.BRIFFA@UEA.AC.UK Dear Keith/ Phil, I have been working on the paper but am now in need of some help and an outside perspective. THis is particualrly true for the introductory and concluding sections where some broader vision is needed to put this study in perspective. I have attached a revised text with a list of questions and figures. I have faxed new figures to you that you have not seen: several of the figures you already have in the earlier report. Comments, suggestions and corrections are needed and welcome. I look forward to hearing from you in the near future. Cheers Brian ******************************************************* general questions/comments TABLE 1: check Table 1 reconstructions; Is this set up correctly? I do not see how you can get negative anomalies if you feed in MXD chronology data which are positive. Am I missing something here? INTRODUCTION I have severely curtailed in arm-waving contextual stuff e.g. importance of millennial high resolution records etc. DO we need some of this in the introduction? Do we need ( wish) a further elaboration of which densitometry is etc or do we assume that the readers either know or can find out? CALIBRATION/ nature of the climate signal. This is unchanged from the earlier report. Do we wish to report calibration tests or should this section be cut. I think this calibration section needs some strengthening. ABIES PROBLEM Until I plotted up Figure 2 I did not realise the proportion of the chronology that is Abies in the 14-1600 period. Although the picea density chronology does (as stated) correlate very well with the combined chronology throughout this interval, the correlation between the abies and picea chronologies themselves is much lower (ca. 0.5-0.7 see draft diagram). Examination of the statistics for the Abies MXD series indicates that their mean density is about 0.1 greater than picea (which should not be a problem because the series were indexed before averaging in the chronology-right?) and their mean sensitivity value is about half that of picea. i.e. there appears to be less interannual variability. It seems to me that this could be a serious problem and/or be picked up as such by a referee. The obvious question would be if we substituted the picea chronology and did the reconstruction over the results be any different over this interval? The justification for including the abies was to increase replication in the snag record but I assumed the climate signal was similar from the two species. We have no data from the Icefields to test this- Colenutt and Luckman 1991 did develop abies and picea chronologies from the same site at Larch Valley which show a similar pattern of response (see Figure 4, xerox sent) but they are clearly not identical. The question is basically do we address this issue head on, citing these data and indicate that reconstruction from picea alone is very similar to that produced in the paper- and present (or have available) data to back it up? Although I could probably find picea snags (and Fritz has cores) to address this issue it would take a year or more to process this material and is not a solution at this stage. so- can you try a reconstruction using picea alone and see if they are different. I have assumed that because the two chronologies are very highly correlated then reconstructions from these two chronologies (using the same transfer function) would be equally similar. Is that a valid assumption? COMPARISON TREE_RING AND RECONSTRUCTED CHRONOLOGIES Do we need this or should I just omit the tree-ring record from the diagram? COMPARISON WITH OTHER RECORDS Do you wish to maplify this with some other records and/or diagrams etc.? CONCLUSION weak! needs a broader vision? any ideas? ******************************************************************* SUMMER TEMPERATURES AT THE COLUMBIA ICEFIELD, ALBERTA, 1072-1987 Luckman, Briffa, Jones and Schweingruber INTRODUCTION Recent studies have established the potential for reconstructing summer temperature conditions from tree-ring densitometry of conifers growing in boreal and subalpine sites (Schweingruber 1988a, Briffa et al etc). In particular the maximum latewood density of each annual ring has proven to be quite sensitive to summer conditions and has been extensively used in climate reconstruction work (Schweingruber 1988a). In North America the principal application of this technique has been in the development of sample site networks that provide regional overviews for the last 2-300 years (FHS 1988b, Briffa etc). Ongoing dendrochronological studies have also demonstrated the potential for millennial length chronologies based on old living trees and snag material (Luckman etc Jacoby, Briffa et al.,). This paper presents a quantitative reconstruction of past summer temperatures based on densitometric and ring-width measurements from trees that grew at sites near the Columbia Icefield in the Canadian Rockies. The reconstruction spans most of the last millennium and is evaluated against other paleoenvironmental data from the same site plus similar, shorter reconstructions from a variety of North America sources. DEVELOPMENT OF THE TREE-RING CHRONOLOGIES The history of tree-ring studies in the Canadian Rockies has been reviewed previously (Luckman and Innes 1991, Luckman 1993). Recent studies have built up a data base of almost 50 ringwidth chronologies focused primarily on high elevation (treeline) sites using Picea engelmannii (Engelmann spruce), Larix lyallii (alpine larch) or Pinus albicaulis (whitebark pine). Almost all chronologies exceed 300 years and millennial-length chronologies have been developed for all three species utilising living trees and ring series from dead (snag) material lying on the surface close to treeline. Although the strong relationships between maximum latewood density and summer temperatures were first demonstrated using Picea engelmannii from a treeline site within the Rockies (Parker and Henoch 1970) there have only been limited subsequent studies using densitometry. Luckman et al. (1985) used several density and ringwidth variables to reconstruct June-July temperatures and December-March precipitation from Picea engelmannii at Lake Louise. In the only study utilising Psuedotsuga menziesii Robertson and Jozsa (1988) used 15 ringwidth and density chronologies from 12 cores to reconstruct June-July precipitation and temperature data from 1800 from a site near Banff. Both of these studies were exploratory, have limited sample depth and only reconstruct the high frequency climate signal. Tree-ring investigations at the Columbia Icefield The Columbia Icefield is the largest icemass in the Canadian Rockies and feeds several major outlet glaciers (e.g. Athabascsa, Dome, Saskatchewan, Columbia and Stutfield). The eastern edge of the Icefield is easily accessible by road and this area has been the focus of considerable paleoenvironmental work in and around Sunwapta Pass and the terminii of Athabasca and Dome Glaciers (Luckman 1988, 1994a, Beaudoin and King 199?, etc). Several tree-ring investigations have been carried out in this area over the last 15 years associated with studies of glacier fluctuations and paleoclimate. The subalpine forest at the Icefield consists principally of Picea engelmannii and Abies lasiocarpa with minor components of Pinus albicaulis and Pinus contorta (Holland and Coen, 1982?). Dendrochronological studies have focused almost exclusively on picea because abies rarely attains ages of >250 years. For descriptive convenience four principal sampling areas can be identified- the Athadome, Ancient Forest, Icefields and Sunwapta sites (Figure 1). Initially tree-ring investigations were used to document the Little Ice Age history of Dome and Athabasca Glaciers using dendrogeomorphic evidence to date glacier fluctuations (Luckman, 1986, 1988). The first ringwidth chronology was developed for the ice-proximal Athadome site (1660-1980) that lies between the lateral moraines of Athabasca and Dome Glaciers. A second longer (1346-1981) ringwidth and densitometric chronology was developed from a site north of the Athabasca Glacier using living picea up to 680 years old (Jozsa et al., 1983, Luckman et al 1984). This "ancient forest" site is bordered downslope by the Little Ice Age terminal moraines and outwash of Athabasca and Dome Glaciers and grades upslope into the treeline ecotone. The "Icefields site" (Figures 2 and 3) is a more open area on the lower slopes of Mount Wilcox adjacent to and upslope of the Little Ice Age moraines of the Athabasca Glacier. Studies of seedling establishment and tree-line dynamics on this slope (Kearney, 1982; Kavanagh and Luckman 1996) indicate a complex history. Treeline is presently advancing upslope but abundant snag material lying on the slope surface indicates that treeline was formerly higher (Figure 3). Radiocarbon dates of 980-1160 14C yr B.P. from these snags initially suggested that this phase of higher treelines pre-dated the Little Ice Age (Luckman 1986) and promised the potential to extend the living tree chronology by several hundred years. Ring-width and densitometric data were obtained from Forintek Canada Corporation for a limited number of these snags but crossdating trials were only partially successful leading to the development of a "floating" (undated) chronology from some of these materials (see Hamilton 1987, Luckman and Colenutt 1992). Subsequent development of an extensive ring-width data base from snags at this site led to the crossdating of the floating chronology within the existing living tree chronology, demonstrating that some of the previously-obtained 14C dates were up to 500 years too old (Luckman 1994a). Ring-width series from these snags and living picea at the Athadome, Icefields and Sunwapta sites were used to develop a composite site chronology (the Athabasca chronology) extending from 1072-1987 (Luckman 1992, 1993). During the summer of 1984, Schweingruber sampled a network of 63 tree-ring sites in western North America (Schweingruber 1988b). Ring width and densitometric chronologies were developed for all sites in this network and have been used to reconstruct summer temperature patterns in western North America over the last 3-400 years (Schweingruber et al., 1991; Briffa et al, 1992). This network includes a Picea engelmannii chronology from Sunwapta Pass that extends from 1608-1983 A.D. This Sunwapta site is about 5km upvalley of the Icefields site and at a similar elevation (Figure 1). Following the development of the Athabasca Chronology additional sampling of snag material at the Icefield site was carried out in 1990. Selected samples of cross-dated snag material from this site were submitted to the Swiss Federal Forestry Institute Laboratory in Birmensdorf for densitometric analysis in 1992. These samples were selected on the basis of the record length, dating and quality of the ringwidth series. They were not identified by species prior to selection and actually consisted of 2 Pinus albicaulis, 14 Abies lasiocarpa and 22 Picea engelmannii discs (identifications by F. Scharr, Birmensdorf). Ring-width and maximum density chronologies were initially developed separately for both the abies and picea samples but, as the majority of samples were picea and the combined and picea chronologies are very highly correlated over the interval from 1400-1690 A.D. (Figure 4), both species were used to develop the chronology presented in this paper. The two pinus samples provide important replicates in the earliest part of the record (1138- 1315; 1179-1383) and are also included. The densitometric chronology developed from this snag material covers the interval from 1073-1897 but is poorly replicated after ca 1700. Few of the snag records extend into the 19th century and only one living tree (1778-1991) was sampled for densitometry at the Icefields site. Therefore the densitometric series from the snag material at the Icefields site were combined with the living picea record developed from the Sunwapta site to produce the chronology used in this paper (Figure 4). Note that, although the chronology ends in 1991, there are only two radii (from a single tree) that postdate 1983. The ring-width chronology used in this study is the Athabasca chronology and was developed using a larger composite sample of snags from the Icefield site and living trees from the Sunwapta, Icefields, Ancient Forest and Athadome Sites. Tree-ring width measurements were either obtained using the TRIM measurement system (McIver et al, 19xx) at The University of Western Ontario or by densitometry at Birmensdorf. All the densitometric data used were obtained at the Birmensdorf facility. Technical details of sample preparation techniques are given by Schweingruber (refs). Crossdating was verified using standard software (Holmes, etc,). Standardization techniques used during chronology development were very conservative. Individual ringwidth series were standardized using either a simple negative exponential or linear fit. Maximum density values were standardized using a linear transformation. ASSEMBLING A REGIONAL CLIMATE DATA BASE In the Canadian Rockies the instrumental climate records begin with the initial European settlement of the region that followed the development of the transcontinental railways. The oldest meteorological stations are associated with the Canadian Pacific Railway, namely Calgary (1880), Banff (1887, first complete year 1890), plus Donald (1891-1901) and Golden (1902-present) in the Rocky Mountain Trench. Further north the earliest records are from Edmonton (1880) and along the Grand Trunk Railway beginning in 1914 at Jasper and Cranberry Lake (the present Valemount). The highest elevation meteorological station with a long record in the Rockies is Lake Louise (on the CPR) at ca 1530m, commencing observations in 1915. Higher elevation sites have only short or seasonal records (Janz and Storr 1977) that are inadequate for calibration purposes. These climate data were recently reviewed and summarised by Luckman and Seed (1995). Precipitation and temperature records were checked for homogeneity and missing values estimated by regression techniques from adjacent stations. Complete monthly climate data records were developed for precipitation and temperatures at Jasper (1916-1994), Banff (1888-1994) Lake Louise (1915-1990), Valemount (1916-1989) and Donald-Golden (1891-1990). The Donald and Golden records do not overlap but, as the sites are at similar elevations and close together within the Rocky Mountain Trench, these records were combined without adjustment and the linking 1902 data estimated from Banff. These meteorological stations are all located in valley floor sites 500-1200m below and 100-150km from the Icefields site (Figure 5). A regional composite record was developed to maximise the length and representativeness of these records for calibration purposes. Monthly temperature series for Banff, Jasper, Golden/Donald and Valemount were converted to anomalies (øC) from the 1961-90 means for each station and the four series were averaged to provide monthly anomaly values for the Canadian Rockies from 1890-1994. These data were preferred to the Bradley and Jones (1991) gridded temperature records because of their more restricted geographical coverage within the Rocky Mountains (see Luckman, in press). CLIMATE CALIBRATION AND TEMPERATURE RECONSTRUCTION Based on previous work, climatic calibrations were attempted using a variety of temperature combinations with both ring-width and maximum density data. After several trials, reconstructions were developed for April-August temperatures which provided the best verification when compared to the regional climate record. The April-August interval combines elements of both spring and summer conditions and is often referred to as the "summer half- year". Two reconstructions were carried out. The first used maximum latewood density of the growth year and prior year. The second reconstruction also included ring-width for the same two years. The reconstructed chronologies are very similar but the reconstruction using both ring-width and density data explains slightly more of the variance during the calibration interval. Details of the chronologies and calibration statistics are given in Table 1. THE RECONSTRUCTED APRIL-AUGUST TEMPERATURE RECORD EVALUATION OF THE RECONSTRUCTION The quality of a reconstructed climate record can be verified in several ways. Conventionally the reconstruction is initially calibrated using only half of the available instrumental climate record. These relations are then tested by using them to reconstruct the climate for the period covered by the climate data held back from the calibration and then comparing the reconstruction with the observed climate record for this interval. This process is then repeated reversing the two climate data sets. If the reconstructions are of similar quality the data set is then combined in a final run using the entire climate data set to develop the reconstruction. DO WE WISH TO PRESENT THE DATA IN THIS FORM? (i) Comparison with the instrumental climate record. The calibration results are highly statistically significant (Table 1). Over the 1890-1982 period the correlation between the regional temperature record and the reconstructed record is 0.651 for the annual data and 0.686 using decadal values (1890-1980). Figure 4 shows a plot of the actual and reconstructed records. The reconstruction captures the general rise in temperatures between 1890-1930 but performs less well during the latter half of the calibration period (see Table 2). Examination of the individual station records that comprise the network used to generate the regional series also shows greater differences between the component sites during this interval (Luckman and Seed, 1995). The greatest discrepancy in the decadal series occurs during the 1960s and this period, with the 1950s, has the most extreme interannual variability in the instrumental record (Figure 6). This may in part explain some of these differences. Examination of the residual values (measured temperature minus the reconstructed values) shows a strong negative correlation with the actual values (Table 3) which indicates that the reconstruction provides a conservative estimate of the more extreme years in the calibration period. Examination of both the decadal (Table 4) and annual data (Figure 6) indicates that both the warmest and coldest years are underestimated. Possibly this situation could be improved with a larger sample of "warmer" years in the calibration period: the lack of densitometric data after 1983 severely restricts the available sample for calibration purposes. General comment Figure 7 shows the reconstructed temperature record for the Icefield site and the reconstructed decadal averages are given in Table 4. All reconstructed and instrumental temperature data are reported as anomalies from the April-August average for the 1961- 1990 period. During the 1961-90 interval the decadal temperature anomalies were, respectively, -0.08øC, -0.06øC and +0.15øC. The 1961-1983 mean anomaly is -0.083øC. and the 1981-90 decade contains half of the 14 positive years during the entire 30-year interval. As the reconstructed record essentially terminates in 1983, it should be anticipated that the most recent part of the reconstructed record would have a dominance of negative anomalies. The last 30 years also contain some of the warmest decades in the instrumental climate record (Tables 4,5) and the mean temperature anomaly over the 1891-1990 period is -0.375øC. The fact that almost all of the reconstructed values are negative (Figure 7) indicates that summer temperatures at the Icefield for most of the last 900 years were considerably lower than those experienced during recent decades. The reconstructed mean for the period 1073-1990 is -0.656øC and is almost 0.3øC lower than the mean of the instrumental record from 1891-1990, indicating that conditions during the period of the instrumental record are, on the average, warmer than the preceding 800 years. This is not unexpected because the greatest extent of the Athabasca Glacier during the last 10,000 years occurred in the 1840s (Luckman 1988). The only part of the reconstructed record that has decades with positive temperature anomalies is the late 11th century at the beginning of the record. As this reconstruction is based on fewer than 5 samples prior to 1140 and these are all young trees, it is possible that these results may be unduly influenced by juvenile effects and should be interpreted more cautiously than the better replicated sections of the reconstruction. Description of the reconstruction Figure 7 shows the annual reconstruction of summer temperature anomalies fitted with a 10-year Gaussian filter to highlight the overall trends. Sample depth, species and composition of the chronology are shown in Figure 4. The reconstructed decadal averages are given in Table 4 and the extreme years and decades in the reconstruction are listed in Table 5. The most obvious comment about the reconstruction is that almost all of the record is well below the 1961-1990 mean i.e. most of the last 900 years were significantly cooler than the last 30 years of the instrumental record. The other striking feature of the reconstruction is the severity of conditions during the 19th century which contains 8 of the 10 coldest years and 3 of the 10 coldest decades (Table 5). The average reconstructed temperature anomaly over the 120 year interval from 1781-1900 is -1.04øC and three separate six year intervals within this period have mean anomalies <-1.6øC. (Table 5). The four coldest decades were the 1780s, 1810s, 1830s and 1870s. In addition to the 1780-1900 period, several other cold intervals are reconstructed particularly ca. 1190-1250, 1280-1340, 1440- 1500 and the 1690s. Temperature anomalies for the 1197-1240 interval average -1.09øC and six of the decades in the 13th century rank in the coldest 20% of decades in the reconstruction (Table 5). The 1445- 1500 average anomaly is -0.96øC with three decades during this interval falling below -1.0øC. The 1690s are unique in having six consecutive years (1696-1701) with temperature anomalies colder than -1.5øC. Nine of the 10 warmest decades in the reconstruction are in the periods 1073-1110 and 1921-1980 (Tables 4 and 5). Apart these two periods the major warmer intervals were ca. 1160-1180, 1260-1280, 1350-1440 and 1710-1770. The 16th and 17th centuries are, on the whole, rather unremarkable. This result was unexpected given the variability of ringwidth values during this period (see below and Luckman 1996a) and the fact that the early 1600s are considered to be the coolest period in the northern hemisphere during the last 500 years (Bradley and Jones, 1993). It is possible that some of this complacency may be attributed to the admixture of abies samples in the snag chronology but, as the mixed chronology is very highly correlated with the less well replicated picea chronology throughout this interval (Figure 4), this effect is thought to be of minor significance. This record indicates that summer temperatures during the period from ca 1120-1960 at the Columbia Icefield have been almost universally cooler than those recorded during the 1961-1990 reference period. However, although summer conditions were particularily severe in the 19th century, the presence of several episodes of relatively warmer and cooler conditions throughout this record does not permit the clear demarcation of a classical "Little Ice Age" period of cooler temperatures. The reconstruction does suggest that warmer summers did occur in the 1073-1120 interval that may be interpreted as a period when conditons were similar or warmer than present. However, low sample depth and possible juvenile density effects suggest these data be interpreted with caution until complementary evidence and or longer chronologies are available. Comparison with other proxy climate data from the Canadian Rockies Previous work at the Athabasca Glacier and in adjacent areas has provided a rich source of proxy environmental data that may be used to evaluate the temperature reconstruction shown here (Figure 8). The local glacial record indicates the maximum extents of Dome and Athabasca Glaciers were in 1843/4 and 1846 respectively (Luckman 1988). There is also evidence from a tilted tree at the Athadome site that the Athabasca Glacier had advanced very close to its maximum position earlier in 1714 (Heusser 1956, Luckman 1988). Examination of the regional glacial record also shows two major periods of glacier advance in the last 300 years (Luckman 1996a, see also Figure 9). The first of these is a tightly dated set of moraines between 1700 and 1725 found at about 25% of the glaciers studied in this region. The most extensive glacier event was usually in the nineteenth century: dates derived from trees tilted or killed by this event at 7 different glaciers suggest this maximum extent occurred in the 1840's (Luckman 1996a). At many glaciers in the Rockies there are several moraines between the LIA maximum moraine and the first known position of the glacier margin based on human records (usually photographs). Where dated, most of these moraines were formed between 1850 and ca 1920. At the Athabasca Glacier there are two such moraines between the outermost limit and the first photographed position of the glacier in 1906 (Luckman 1986b). Almost all of the evidence presented above strongly supports the reconstructed temperature history from the Athabasca site (see Figure 8). The severest conditions during the last millennium are clearly during the first half of the 19th century and the 1840s marks the beginning of a period of slightly milder summer temperatures that would have led to glacier recession after the period of coldest summers. The renewal of colder summers during several decades of the late 19th century could clearly account for the glacial events that produced the well defined moraine ridges that lie between the LIA maximum limit and the known 1906 ice front position of Athabasca Glacier. As the dating of moraines at the Athabasca parallels that found at many other glaciers within this region this would seem to be a good indication that; (i) the reconstructed temperature record is very useful in understanding the local glacier history of Athabasca and Dome Glaciers; and, (ii) the local glacier history is reasonably representative of the regional glacier history of the Canadian Rockies and is therefore a useful summary of regional climate trends over this interval. Most of the 1700s are reconstructed as being warmer than the 1800s and this corresponds with the evidence of wider ringwidths from many tree-ring chronologies in this area (see Figure 9 and Luckman, submitted). Several climate reconstructions from northern North America also show that the late 18th century was generally milder than the early 18th and 19th centuries (e.g. D'Arrigo and Jacoby, 1992). However, in many of these records the onset of severe cooling occurs after ca. 1810 (similar to the ringwidth plot shown in Figure 9) rather than in the 1780s. This difference may reflect the fact that the tree-ring maximum