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The Holocene 10,1 (2000) pp. 9–19 Recent warming in a 500-year palaeotemperature record from varved sediments, Upper Soper Lake, Baffin Island, Canada Konrad A. Hughen,1* Jonathan T. Overpeck1,2 and Robert F. Anderson3 (1Institute of Arctic and Alpine Research and Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA; 2NOAA Paleoclimatology Program, NGDC, Boulder, CO 80303, USA; 3Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA) Received 24 June 1998; revised manuscript accepted 26 April 1999 Abstract: Laminated sediments from Upper Soper Lake on southern Baffin Island provide a new 500-year record of temperature change in the Arctic. Radiometric dating, using 210Pb and Pu, shows that the light- and dark-coloured laminae couplets are annually deposited varves. Dark laminae thickness is strongly correlated to average June temperature from Kimmirut (r = 0.82), reflecting the influence of temperature on snowmelt and fluxes of runoff and suspended sediment. This relationship allowed the construction of a palaeotemperature record that documents large-amplitude interannual to decadal variability superimposed on distinct century-scale trends, including 2°C average warming and maximum temperatures during the 1900s. Similar patterns of change are seen in individual and regionally averaged palaeotemperature records from around the circum-Arctic. Upper Soper Lake records temperatures, rates of change and variance during the twentieth century that are all anomalously high within the context of the last 500 years, and outside the observed range of natural variability. Comparisons of Upper Soper Lake and Arctic average palaeotemperature to proxy-records of hypothesized forcing mechanisms suggest that the recent warming trend is mostly due to anthropogenic emissions of atmospheric greenhouse gases. The magnitude of the warming and decade-scale variability throughout the records, however, indicate that natural forcing mechanisms such as changing solar irradiance and volcanic activity, as well as positive feedbacks within the Arctic environment, also play an important role. Key words: Varved sediments, Canadian Arctic, palaeoclimate, global warming, climate change. Introduction Annually resolved records of palaeoclimatic change in the Arctic provide an important contribution to understanding the global climate system. The Arctic is an important region with respect to climate, and is highly sensitive to global climatic changes. General circulation model (GCM) experiments with doubled atmospheric CO2 show the largest temperature increases on the globe occurring in the high northern latitudes, due to numerous feedback mechanisms (Mitchell et al., 1990; Kattenberg et al., 1996; Nicholls et al., 1996). For example, the difference in albedo of snow and ice compared to land and ocean surfaces, as well as the effects of sea-ice cover preventing the release of heat from the relatively *Present address: EPS Department, Harvard University, Cambridge, MA 02138 Arnold 2000 warm ocean to the overlying atmosphere, can greatly amplify small shifts in temperature (Houghton et al., 1990; Chapman and Walsh, 1993). Recent research indicates that, in the Arctic, cloud cover can serve as a positive temperature feedback (Curry et al., 1996). In addition, the stability of low-level temperature inversions in the Arctic atmosphere influences the degree to which changes in air temperature are focused near the surface (Houghton et al., 1990; Chapman and Walsh, 1993). Climatological studies and glaciological observations from Baffin Island in the eastern Canadian Arctic show that glaciers and snowcover respond sensitively to relatively small changes in Northern Hemisphere climate (Bradley and Miller, 1972; Andrews et al., 1972). Thus, the effects of global climate change may be recognizable in the Arctic before they are discernible elsewhere. In addition to showing sensitivity to global climatic changes, the Arctic environment may 0959-6836(00)HL362RP 10 The Holocene 10 (2000) in fact be capable of forcing widespread changes in climate. For example, vegetation and soil conditions in the Arctic may have an important influence on latitudinal temperature gradients and levels of atmospheric trace-gases such as carbon dioxide and methane (Rind, 1987; Houghton et al., 1990; Oechel et al., 1993). Changes in sea-surface temperature (SST) and freshwater flux to the high-latitude North Atlantic Ocean (via changes in the Arctic hydrologic cycle or glacial meltwater/iceberg calving rates) could alter the pattern and rate of oceanic thermohaline circulation, and thus heat transport and global climate (Broecker and Denton, 1989; Stocker and Wright, 1991; Manabe and Stouffer, 1997). Determining spatial patterns of abrupt climate changes is important in order to identify responsible forcing mechanisms. Rind and Overpeck (1993) showed that different mechanisms capable of driving decade to century-scale climate variability – such as fluctuations in atmospheric gas concentrations, solar irradiance, volcanic eruptions, or ocean circulation – should have resulted in palaeoclimate changes with different spatial and temporal patterns. A spatial network of sites with accurate, high-resolution palaeoclimate records is necessary in order to help discriminate between these forcing ‘fingerprints’. Annually laminated (varved) lake sediments are an ideal repository for studying the temporal and spatial patterns of palaeoclimatic change. Varved sediments provide excellent chronological control and have been shown to record climatic variability (Leonard, 1985; Deslodges, 1994; Leeman and Neissen, 1994; Hardy et al., 1996; Lamoureux and Bradley, 1996; Gajewski et al., 1997). In the Arctic, varved lake sediments are more widespread than other sources of high-resolution palaeoclimate records, and occur in a greater diversity of environments. Other types of archives, such as tree-rings or ice cores, are either lacking (i.e., north of tree-line), or are too limited geographically to adequately resolve spatial patterns by themselves. Here we present a new palaeotemperature record from varved sediments of Upper Soper Lake, on southern Baffin Island. Two independent radiometric dating methods demonstrate the annual nature of light/dark laminae couplet deposition. Dark laminae thickness is calibrated to June (snowmelt season) temperature using meteorological records from the nearby town of Kimmirut (previously named Lake Harbour). The nearly 500-year palaeotemperature record shows dramatic warming during the twentieth century, in agreement with instrumental and other palaeotemperature proxy-records from around the Arctic and Northern Hemisphere (Overpeck et al., 1997; Mann et al., 1998). The Upper Soper Lake record was used in a recent study by Overpeck et al. (1997) that utilized a network of annual palaeotemperature records, combining varved sediments, tree-rings and ice cores, to construct an Arctic-wide average record of temperature change for the past 400 years. Analysis and comparisons of both the Upper Soper Lake and Arctic average palaeotemperature records to proxy-records of potential forcing mechanisms indicate that the observed recent warming is most likely due to anthropogenic greenhouse gases, with a contribution from natural causes (i.e., changing solar irradiance and volcanism). Lake study area A coring expedition to Baffin Island in spring of 1993 recovered up to 12 freeze-cores with intact sediment-water interfaces from each of Upper Soper, Ogac and Winton Bay Lakes (Hughen et al., 1996). These small tidewater lakes all contain finely laminated sediments composed of couplets of light-coloured, diatom-rich layers and dark-coloured, mineral grain-rich layers. The light laminae reflect the amount of primary productivity occurring as diatoms bloom within the lake, whereas the dark laminae represent deposition of grains washed into the lake with runoff from snowmelt. The appearance, composition and sediment fabric of the light and dark laminae in each lake were described in detail by Hughen et al. (1996). Excess 210Pb concentrations from Ogac and Winton Bay Lake sediments showed that the laminae couplets in those lakes are annually deposited varves (Hughen et al., 1996). Upper Soper Lake is located within the tidal limit on southern Baffin Island, on the northern shore of Hudson Strait (Figure 1). The southern Baffin Island coastline is heavily dissected and regional geology consists primarily of Precambrian gneisses. Upper Soper Lake has a surface area of 0.8 km2 and a drainage basin that is 213 km2 in area with a maximum elevation of 1500 m. The drainage basin is presently non-glacierized. The lake reaches a maximum depth of 30 m near the centre of the basin (Figure 2). There is an emergent bedrock sill at the outlet, creating reversing falls with each tidal cycle (the tidal range on southern Baffin Island can reach 14 m). Salt water enters Upper Soper Lake during high tides, whereas freshwater input is primarily from a single large stream, with minor contributions from four smaller streams. The dense salt water overlain by fresh water creates a strong halocline at about 14 m depth (Figure 3), causing meromictic conditions and preventing wind mixing of the lake’s bottom water. Oxidation of organic matter depletes oxygen in the isolated bottom water, rendering it anoxic (Figure 3). Anoxia prevents the establishment of a benthic fauna, resulting in a lack of bioturbation and allowing the preservation of sub-millimetre-scale laminated sediments. In general, the limnology of Upper Soper Lake is similar to other coastal Arctic meromictic lakes (McLaren, 1967; 1969; Ludlam, 1996; Hughen et al., 1996). Dark laminae in Upper Soper Lake sediments are composed of silt and clay particles deposited from suspended sediment entering the lake with runoff. Studies of hydrology and sediment transport Figure 1 Location map of Upper Soper Lake on southern Baffin Island in the eastern Canadian Arctic. Dark-grey filled circles show the locations of sites with palaeoclimate records that are compared to Upper Soper Lake. C2 = Lake C2 (Lamoureux and Bradley, 1996), TB = Tuborg Lake (Smith, 1997), DVO9 = Lake DVO9 (Gajewski, 1997) varve thickness records; DI = Devon Island (Koerner, 1977), SG = South Greenland (Kameda et al., 1995) ice cap % melt records; KUUJ = Kuujuaq, CP = Castle Peninsula, CH = Churchill (Jacoby and D’Arrigo, 1989), OKAK = Okak Lake (Overpeck et al., 1997), SWP = Salt Water Pond (D’Arrigo et al., 1996) tree-ring width records. Light-grey filled circles are sites with meteorological data used in this study. KI = Kimmerut (Lake Harbour); IQ = Iqaluit; NI = Nottingham Island; RI = Resolution Island; KU = Kuujuaq; IL = Ilulissat (Jakobshavn); NU = Nuuk (Godthaab); IV = Ivittuut. Konrad A. Hughen et al.: Recent climatic warming in laminated lake sediments, Soper Lake, Baffin Island 11 and deposition in similar coastal lakes in the high Arctic have shown that suspended sediment entering the lake systems results primarily from early snowmelt, with a smaller contribution from summer rainstorm events (Hardy, 1996; Retelle and Child, 1996). Additional studies of Arctic and Alpine lakes with clastic varved sediments have shown that runoff, suspended sediment concentration and varve thickness vary logarithmically as a function of average temperature during the snow or glacier melting season (Leonard, 1985; Leeman and Niessen, 1994; Hardy et al., 1996). We propose similar mechanisms for the primary controls on runoff and suspended sediment fluxes into Upper Soper Lake. Specifically, average temperatures during the southern Baffin Island early snowmelt season (June) should influence the flux of runoff and therefore the flux of suspended sediment entering the lake. Methods Figure 2 Bathymetry of Upper Soper Lake. Mudflats surrounding the lake are marked by the 1 m contour. The major freshwater inlet is via the large stream flowing in from the northwest. The wide shallow outlet sill to the south becomes reversing falls with each tidal cycle, allowing dense saltwater to enter the deep basin. The locations of cores used in this study, Soper 93-2, 93-3, 93-4, 93-6 and 93-7, are shown by solid squares in the deepest part of the basin. Contour interval (below 10 m) is 5 m. Seven freeze-cores containing intact sediment-water interfaces from Upper Soper Lake correlate well on the basis of identifiable, millimetre-scale marker laminae. Three cores, Soper 93-2, 93-3 and 93-4, were sampled continuously, embedded in epoxy resin and thin-sectioned for image analysis using procedures outlined by Hughen et al. (1996). Core Soper 93-6 was sampled at 0.5 cm intervals for plutonium (Pu) analysis and core Soper 93-7 was sampled at 0.5 cm intervals for lead-210 (210Pb) analysis. Pu analysis was carried out using techniques from Anderson and Fleer (1982), and 210Pb analysis followed the methods described by Hughen et al. (1996). Individual light and dark laminae were identified and crosscorrelated in cores Soper 93-2, 93-3 and 93-4. Each lamina was counted and measured three times and then averaged to produce laminae thickness time-series for each core (Figure 4). Although records for both light and dark laminae were produced, only dark laminae thickness is shown. The process of freeze-coring preserves sediments intact, exactly as they lie on the lake bottom. As a result, laminae thicken toward the top of a freeze-core, due to the increased porosity and the differential influence of sediment compaction with depth. Each of the individual records, therefore, had to be corrected for this differential compaction. The laminae thickness time-series were fitted with curves derived from equation (2) that model sedimentation rate as a function of increasing porosity (Figure 4, a, b and c). Porosity was measured in the sediments and equals 99% at the surface, decreasing logarithmically with depth. The modelled sedimentation rate curves were standardized and multiplied by the laminae thickness time-series to correct for increasing porosity. The three compaction-corrected time-series were then averaged to create a composite Upper Soper Lake dark laminae thickness record (Figure 4d). The averaged record thus avoids gaps due to local disturbances or breaks within individual cores and represents basin-wide sediment deposition back to ad 1515. Results Figure 3 Hydrographic data for Upper Soper Lake: water temperature, percent transmissivity, dissolved oxygen concentration, and salinity. A sharp halocline is visible around 14 m depth, separating salt water below from fresh water above. The density differences isolate the bottom water from contact with the atmosphere. Dissolved oxygen concentrations decline logarithmically below 14 m, reaching zero at the sediment surface. Minima in transmissivity near the surface and across the epilimnion are associated with blooming plankton and bacterial communities (McLaren, 1969). Measurements of water chemistry were made on a Sea-Bird SBE 19 Seacat Profiler, equipped with a SBE 23Y dissolved oxygen sensor. The measurements were made in July of 1994, during ice-free conditions. Radiometric dating Excess 210Pb activity plotted versus depth for core Soper 93-7 drops to a plateau of 0.1 dpm/g around 30 cm depth (Figure 5a). The data closely fit a logarithmic curve (r2 = 0.95). The mass accumulation rate was calculated from excess 210Pb using the equation: Pb = 210Pbo exp(–z/MAR) 210 (1) where 210Pb is the unsupported 210Pb activity as a function of depth; 210Pbo is the initial 210Pb activity; is the radioactive decay 12 The Holocene 10 (2000) S = MAR/(1–⌽)s (2) where S is linear sedimentation rate in cm yr ; ⌽ is the sediment porosity as a function of depth; and s is the dry mineral density (assumed to be 2.3 g cm−3, based on the average density of surrounding gneissic bedrock). The resulting 210Pb and varve ages for core Soper 93-7 are plotted versus depth and agree well over most of the period (Figure 5b). The small disagreement between the varve and 210Pb chronologies within the last 30 years (Figure 5b) led us to independently date the sediments within that time frame using bomb nuclide dating. Atmospheric concentrations of Pu, similar to cesium-137 (137Cs), peaked with atmospheric nuclear testing in 1963. Thus sediment Pu concentrations can be used as an independent measure of age. The activity of 339+240Pu versus depth, plotted in 0.5 cm increments, shows a prominent peak from 5.5 to 6 cm depth (Figure 6). These depths correspond to varve dates of 1962–1964, in excellent agreement with the known atmospheric Pu maximum in 1963. −1 Figure 4 (a, b, c) Raw dark laminae thickness data from cores Soper 932, 93-3 and 93-4. Wherever possible, each lamina was identified in all three cores before measurement. Gaps in individual cores result from core breaks or local disturbances. The thin lines are models of linear sedimentation rate as a function of decreasing porosity, fit to the individual thickness curves, and used to correct the thickness records for sediment compaction. (d) Composite Upper Soper Lake dark laminae thickness record created by standardizing and then averaging the three compaction-corrected time-series. Record shown plotted as standard deviation units () using mean and standard deviations calculated for the entire time-series. constant for 210Pb; z is the cumulative mass as a function of depth; and MAR is the mass accumulation rate in g cm−2 yr−1. This results in a MAR of 0.014 g cm−2 yr−1, a realistic value for an oligotrophic lake. To convert MAR to a linear sedimentation rate, we used the equation: Laminae thickness calibration June is the first month each year in southern Baffin Island with above-zero average temperatures, and represents the season of maximum snowmelt. In order to evaluate the hypothesis that average June temperature controls Upper Soper Lake dark laminae thickness, we used meteorological data from Kimmirut, 6 km to the south (Figure 1). Unfortunately, climate conditions were only recorded there continuously from 1923 to 1945, and the record contains temporal gaps (Figure 7a). The two records show similarity from 1933 to 1945, but disagree prior to that. The prominent peaks in 1924/1925 suggest that the records at that time are offset by one year. Thin sections were re-examined to investigate whether a single varve could have been skipped or misinterpreted. A layer that had been identified initially as a sublamina (i.e., a thin couplet not considered to be an annual varve) was found in all three cores. This layer was reinterpreted and added to the chronology, shifting all points older than 1931 by one year (Figure 7a). Incorrectly identifying one varve during the last 60 years suggests that the approximate overall error for the varve chronology is about 1.7%. The adjusted chronology shows better agreement with average June temperature over the entire period from 1923 to 1945. A scatter plot of adjusted dark laminae thickness versus Figure 5 (a) Excess 210Pb plotted versus depth for core Soper 93-7. The data decline rapidly, reaching a background ‘supported’ value of 1 decay per minute (dpm) per gram dry weight at 30 cm depth. The fit to a logarithmic curve is excellent (r2 = 0.95), indicating that deposition has been continuous and undisturbed. (b) Varve and 210Pb age versus depth profiles for core Soper 93-7. The two curves agree well along most of their length, indicating that the laminae couplets are annually deposited varves. Konrad A. Hughen et al.: Recent climatic warming in laminated lake sediments, Soper Lake, Baffin Island 13 Figure 6 Plutonium concentration and varve age for core Soper 93-6 plotted versus same depth scale. A large peak, corresponding to the peak in atmospheric nuclear testing in 1963, is visible from 5.5 to 6 cm. This depth yields a varve age of 1962–1964, in excellent agreement with Pu and further confirming that the laminae couplets are annual varves. Figure 7 (a) Average June temperature from Kimmirut compared to raw and adjusted chronologies for Upper Soper Lake dark laminae thickness over the period 1923–1945. The dark laminae thickness time-series are plotted on a logarithmic scale. The raw laminae thickness and temperature records agree well after 1932, but disagree prior to that. An additional laminae couplet was identified and measured in all three cores and added to the chronology (see text). The adjusted dark laminae record is in better agreement with average June temperature over the entire period. (b) Scatter plot of adjusted dark laminae thickness versus Kimmirut average June temperature. The data sets show a good correlation (r = 0.82), indicating that 67% of the variance in dark laminae thickness can be explained by average June temperature. June temperature shows a good correlation (r = 0.82; Figure 7b). May, July, August and summer (JJA) monthly average temperatures from Kimmirut showed weaker relationships to dark laminae thickness (r = 0.26, 0.41, 0.36 and 0.57, respectively). Scatter plots of dark laminae thickness versus average June temperatures from more distant meteorological stations at Iqaluit, Nottingham Island, Kuujuaq and Resolution Island (Figure 1) show lower degrees of correlation (r = 0.64, 0.61, 0.41 and 0.28, respectively), just as correlations among the meteorological stations themselves reveal significant spatial variability. Palaeotemperature record The quantitative relationship described by the regression of dark laminae thickness on average June temperature (Figure 7b) was applied in order to calculate Upper Soper Lake June temperatures back to the year 1515 (Figure 8). The record shows large interannual-to-decade-scale variability, superimposed on century-scale trends. The most striking feature of the record is dramatic warming and high temperatures occurring during the twentieth century. The oldest part of the record, prior to 1700, is characterized by an average temperature of 3.7°C, 1.3°C cooler than the present. 14 The Holocene 10 (2000) Figure 8 Average June palaeotemperature record constructed from Upper Soper Lake dark laminae thickness, plotted versus units of degrees C and standard deviations calculated for the entire record. Thin line shows annual values, heavy line is a five-year running average. Dashed lines are ±1 standard deviation. An abrupt drop in temperature around 1700 led to an extended period nearly 2°C cooler than present, averaging 3.1°C. This relatively cold period ended with large rapid warming events in the 1920s and 1940s, leading to a period of 5°C average June temperatures. In order to evaluate the magnitude of this recent warming in the context of naturally occurring variability, we standardized the record using a mean and standard deviation calculated for the entire 486-year period. Average temperatures (five-year running smooth) during the latter part of this century are consistently greater than 1 standard deviation higher than those seen during the rest of the record (Figure 8). Within the context of the past five centuries, average temperatures during the late 1900s are outside the range of naturally occurring behaviour. In addition to the general warming trend, Upper Soper Lake also records changes in high-frequency variability. From 1700 to 1920, there are large (up to 3°C) interannual-to-decade-scale fluctuations, but after 1920 the fluctuations become larger. June temperatures increased by about 5°C from 1919, the coldest year in the record, to 1924. This was followed by a warming of more than 5.5°C from 1942 to 1949, the warmest year in the record (Figure 8). The rates of warming during these periods equal about 1.0 and 0.8°C yr−1, and represent the highest rates of change seen in the record. In order to quantify this increase in variability, timedependent variance was calculated continuously for the record, using moving windows from 5 to 60 years in length (Figure 9). Variance for most of the record is low (⬍1) and fairly constant for all window lengths. However, variance increases sharply and remains high (up to 4) during the 1900s. The increase in variance is abrupt and coincides with the beginning of rapid warming in the 1920s (Figure 9). Discussion Upper Soper Lake palaeotemperature record Construction of an annually resolved palaeotemperature record requires several steps, including using independent dating methods to demonstrate that the laminae couplets are varves, and using instrumental data to quantitatively calibrate the laminae thickness records to climatic parameters. The good fit of excess 210 Pb concentration to a logarithmic profile reaching a plateau at depth (Figure 5a) indicates that background (supported) levels of 210 Pb have been reached and that sediment deposition has been continuous and undisturbed. The systematics of 210Pb deposition in these sediments are good, lending credibility to the 210Pb age model. The good agreement between 210Pb and varve ages with depth (Figure 5b) demonstrates that the laminae couplets are annually deposited varves. Additional independent dating by sediment Pu concentrations agrees with varve counts (Figure 6) and provides further confirmation that the laminations are varves. The regression of dark laminae thickness versus June temperature (Figure 7b) shows that 67% of the variability in the sediment Figure 9 Variance diagram for Upper Soper Lake temperature record. Variance calculated continuously using moving windows of length ranging from 5 to 60 years in increments of 5. High-frequency variance (5–10 year windows) in particular shows increased interannual variability during the twentieth century. Konrad A. Hughen et al.: Recent climatic warming in laminated lake sediments, Soper Lake, Baffin Island 15 record can be explained by average temperature during the June snowmelt season. The remaining unexplained variability could be the result of other factors besides temperature, such as the amount of snowfall the previous winter, or sediment transport from large rainstorm events. However, precipitation data from Kimmirut are quite limited, making it difficult to test these hypotheses. Additional factors such as the timing of arrival of the snowmelt season or rate of warming in a given year could also have a large impact on the correlation between average June temperature and sediment flux into the lake. Nonetheless, the strong correlation seen here is supported by previous studies of similar lake environments (Retelle and Child, 1996; Hardy et al., 1996) and leads us to interpret the Upper Soper Lake dark laminae thickness record primarily as a snowmelt-season temperature record. The Upper Soper Lake palaeotemperature record shows several centuries of cold conditions prior to the 1900s, reminiscent of the ‘Little Ice Age’ seen throughout much of the Northern Hemisphere (Grove, 1988; Bradley and Jones, 1993; Overpeck et al., 1997; Mann et al., 1998). Based on the Upper Soper Lake record, it appears that the coldest conditions of the ‘Little Ice Age’ on southern Baffin Island only lasted from 1700 until 1920, when it ended with large and abrupt warming. Our record does not extend back in time far enough to record conditions during the potential ‘true’ onset of the ‘Little Ice Age’ (Bradley and Jones, 1993; Fischer et al., 1998), and therefore may only record maximum cooling towards the end. The rapid warming and high temperatures during the twentieth century are beyond the range of natural patterns of variability seen in the previous 400 years. It is reasonable to suspect that these changes could be the result of errors in the construction of the record itself. For example, underestimating the influence of sediment compaction could yield increased dark layer thickness near the surface and artificially bias reconstructed temperatures toward recent warming. The observed recent warming in the Upper Soper Lake record, however, occurs in extremely abrupt events and is therefore not attributable to artifacts of compaction modelling that would produce thicker laminae. Errors in constructing the record would also result in systematic offsets with time between reconstructed and instrumental temperatures during the calibration period, and such offsets are not seen. Upper Soper Lake is in a pristine natural environment and there is no human disturbance to the landscape within the basin catchment. We conclude that the observed changes record primary depositional signals resulting from changes in climate. Eastern Canadian Arctic climate change To evaluate further the regional significance of the Upper Soper Lake palaeotemperature record, we compared it to instrumental and high-resolution temperature proxy-records from the eastern Canadian Arctic and circum-Arctic regions. Instrumental records on Baffin Island only extend back to the early 1920s. However, records were kept at some sites in Greenland from as early as the 1860s. Meteorological stations at Ilulissat (formerly Jakobshavn), Nuuk (formerly Godthaab) and Ivittuut record abrupt warming in average June temperature during the 1920s. In general, significant warming is seen during the period from the mid-1920s to the mid1940s in instrumental records from Greenland and Iceland (Rogers, 1985). Summer temperatures have also been inferred from the history of response of ice cap outlet and local cirque glaciers, and dates of burial of organic deposits, from Cumberland Peninsula on eastern Baffin Island (Davis, 1985; Miller, 1973; Boulton et al., 1976). These records show warming occurring in recent times, preceded by cold conditions for at least a few centuries. However, the temporal resolution of these records is inadequate for a more detailed comparison. Annually dated and resolved palaeoenvironmental records provide more precise climate histories to help place Upper Soper Lake trends in regional perspective. Overpeck et al. (1997) and Mann et al. (1998) compiled palaeotemperature records of the past several centuries from around the Arctic and Northern Hemisphere. Many of the individual records, as well as Arctic and Northern Hemisphere averages, show temperature trends similar to Upper Soper Lake (Overpeck et al., 1997). Throughout the eastern Canadian Arctic region, cool conditions relative to the present prevailed in the 1700s and 1800s, with warmer intervals occurring generally around 1800 and after 1920. Comparisons between individual records and Upper Soper Lake for the period from 1700 to 1990 show an average correlation of r = 0.42 (Table 1). However, the records show more regional variability prior to 1850 than after. The most striking feature in every record is a warming trend from low temperatures in the mid-1800s to anomalously high temperatures during the 1900s. To isolate the period of potential human influence on climate, we divided the timeseries into pre- and post-industrial eras, before and after 1850, respectively. This resulted in different degrees of correlation (Table 1). The average correlation for the period 1700–1845 is low (r = 0.13), whereas the average correlation for 1850–1990 is much higher (r = 0.58), reflecting the strength of the warming trend in each record. Similar changes are recorded throughout the entire Arctic. We re-scaled the Arctic average temperature record of Overpeck et al. (1997) using a mean and standard deviation calculated for the entire 400-year record. Average temperatures for the Arctic during much of the 1900s are consistently warmer than the rest of the 400-year record by more than 1 standard deviation (Figure 10), and cannot be explained as a consequence of natural variability alone. The average record was constructed using five-year binned data and could not be used to evaluate interannual variability. However, variance diagrams were constructed for each of the individual annually resolved time-series used in the Arctic average record. Although there is regional variability, a consistent pattern emerges in that many of the diagrams (⬎50%) show significantly increased variance during the present century. Anomalously high temperatures and variance in individual palaeoclimate records as well as regional averages for the circum-Arctic (Figure 10; Overpeck et al., 1997) and the Northern Hemisphere (Mann et al., 1998), suggest that climate system behaviour has shifted recently from its pre-industrial state, possibly due to the introduction of new and different external forcing. Climate forcing Several studies (Bradley and Jones, 1993; Rind and Overpeck, 1993; Crowley and Kim, 1996; Lean et al., 1995; Overpeck et al., 1997; Mann et al., 1998) have identified forcing mechanisms that are capable of causing decade-to-century-scale climatic variability, including variations in atmospheric greenhouse gas concentrations, solar irradiance, volcanic aerosol loading, and ocean circulation. We have expanded on the analysis described in detail by Overpeck et al. (1997) by comparing Upper Soper Lake and Arctic average temperature records to proxy-records for atmospheric CO2 concentration (Dlugokencky et al., 1994; Etheridge et al., 1996), solar irradiance (Lean et al., 1995), and stratospheric loading of volcanic aerosols (Zielinski, 1995), using a time-dependent correlation with a 200-year moving window. The correlation between Upper Soper Lake and CO2 is poor during the pre-industrial period but increases with time (Figure 11a), reflecting the increasing trends of both records. The correlation becomes strongly positive (r = 0.73) during the most recent interval. Similar results are found by correlating CO2 with the Arctic average record (Figure 11b) – the correlation is initially low and increases sharply toward the present to a maximum (r = 0.69). The agreement of CO2 concentration with temperature, together with evidence that recent climatic behaviour lies outside the range observed in the natural system, suggests that much of the observed 16 The Holocene 10 (2000) Table 1 Correlation coefficients (r) between Upper Soper Lake and palaeotemperature records from the eastern Canadian Arctic Site name Archive type Total period (1700–1990) Lake C2 Tuborg Lake Devon Island Cap Lake DVO9 Kuujuaq Castle Peninsula Churchill Okak Lake Salt Water Pond S. Greenland Ice Cap varve varve % melt varve tree-ring tree-ring tree-ring tree-ring tree-ring % melt r r r r r r r r r r = = = = = = = = = = 0.37 0.44 0.39 0.65 0.35 0.35 0.59 0.52 0.32 0.21 Pre-industrial (1700–1845) Post-industrial (1850–1990) r= r= r= – r= r= r= r= r= r= r r r r r r r r r r 0.36 0.03 0.21 0.07 0.05 0.16 0.04 0.13 0.14 = = = = = = = = = = 0.42 0.36 0.72 0.65 0.53 0.59 0.74 0.68 0.70 0.39 Lake C2 (Lamoureux and Bradley, 1996), Tuborg Lake (Smith, 1997), Devon Island Ice Cap (Koerner, 1977), Lake DVO9 (Gajewski et al., 1997), Kuujuaq, Castle Peninsula, Churchill (Jacoby and D’Arrigo, 1989), Okak Lake (Overpeck et al., 1997), Salt Water Pond (D’Arrigo et al., 1996), South Greenland Ice Cap (Kameda et al., 1995). Figure 10 Arctic average temperature record re-standardized from Overpeck et al. (1997), using mean and standard deviations calculated for the entire record. recent warming is due to increasing concentrations of anthropogenic trace gases, in agreement with previous studies (Lean et al., 1995; Santer et al., 1996; Overpeck et al., 1997; Mann et al., 1998). However, estimates of the radiative forcing capacity of trace gases (Shine et al., 1990; Houghton et al., 1994; Schimel et al., 1996), together with GCM experiments assessing the climatic response to increased greenhouse gases, including the effects of anthropogenic aerosols (Kattenberg et al., 1996; Santer et al., 1996), indicate that trace gases alone are insufficient to cause the full magnitude of observed warming. Positive feedback mechanisms, such as snow and ice albedo, sea ice and cloudiness, have probably served to amplify radiatively forced warming (Mitchell et al., 1995; Kattenberg et al., 1996). In addition to the amplitude of recent warming, the large observed decade-scale fluctuations can not be explained by gradually increasing trace-gas concentrations. This implies the operation of forcing mechanisms with variability at the decade scale, such as solar irradiance (Lean et al., 1995). Upper Soper Lake palaeotemperature and solar irradiance show generally similar patterns from the 1700s through the 1900s, including a rapid increase in this century. However, the large negative solar anomaly during the Maunder Minimum does not coincide with low temperatures at Upper Soper Lake. The correlation between the records is negative for most of the comparison (Figure 11a), largely due to the strong disagreement during the Maunder Minimum. The correlation starts to improve and becomes highly positive (r = 0.62) toward the present, though weaker than the correlation for CO2. Although this agreement indicates a possible solar contribution toward recent warming, estimates for radiative forcing from changes in solar irradiance since 1850 (about +1.3 W/m2) are only half those for greenhouse gases (about +2.6 W/m2; Houghton et al., 1994; Schimel et al., 1996). While solar forcing is potentially significant in causing recent warming (Lean et al., 1995), it probably does not play a dominant role. The negative correlation between solar forcing and Upper Soper Lake prior to 1850 is in contrast to previous studies finding a positive correlation using regionally averaged temperature records (Lean et al., 1995; Overpeck et al., 1997; Mann et al., 1998). Arctic average temperature and solar irradiance are positively correlated throughout the pre-industrial period (average r = 0.25; Figure 11b). The correlation reaches a maximum in the twentieth century (r = 0.50), but remains below CO2. The difference between Upper Soper Lake and Arctic average correlations may well be the result of climatic “noise” from local effects present in the Upper Soper Lake record. For example, changes in Northern Hemisphere atmospheric circulation could affect the strength of the upper westerlies, in turn shifting the location of the Baffin Trough of low pressure in the upper atmosphere (Keen, 1980). This could have a large influence on storm tracks and the advection of southerly versus northerly air masses over southern Baffin Island (Keen, 1980; Rogers, 1985). Together with other potential local effects, this could introduce noise to the Upper Soper Lake temperature record not seen in regional averages. A comparison of Upper Soper Lake palaeotemperature to volcanic activity reconstructed from sulfate concentrations in the GISP2 Greenland ice core (Zielinski, 1995) revealed periods of increased explosive volcanism coincident with decade-scale cold periods. The agreement includes several of the coldest intervals, such as 1720–1740, 1780–1790 and particularly 1900–1920, as well as individual cold years such as 1642, 1696 and 1809. The correlation between Upper Soper Lake temperature and sulfate concentration (Figure 11a) is negative for the entire period of record (average r = –0.23) and is comparable for pre- and post-industrial periods. Very similar results are seen in the correlation (Figure 11b) of sulfate with Arctic average temperature (average r = −0.18). In general, volcanic activity appears to play some role in causing decade-scale variability in the Upper Soper Lake and Arctic average records. A proxy-record of North Atlantic SST from off the coast of southeast Greenland (Jennings and Weiner, 1996) records steady warming and anomalously high temperatures during this century (Overpeck et al., 1997). However, it is difficult to determine whether this is a record of climate forcing (i.e., strength of thermohaline circulation) or a climatic response to radiative forcing. Additional marine records of variability in thermohaline circulation with high temporal resolution are needed in order to evaluate the ocean’s potential role in forcing recent climate changes. Konrad A. Hughen et al.: Recent climatic warming in laminated lake sediments, Soper Lake, Baffin Island 17 Figure 11 (a) Time-dependent correlations between Upper Soper Lake palaeotemperature and proxy-records for hypothesized forcing mechanisms. Solid lines are correlations for atmospheric CO2 (Etheridge et al., 1996), dashed lines for solar irradiance (Lean et al., 1995), and dotted lines for explosive volcanism (Zielinski, 1995). Correlation coefficient r was calculated using a moving 200-year window (thin grey lines) advanced in increments of five years. (b) Correlations for Arctic average temperature. Conclusions Sediments of Upper Soper Lake record abrupt 2°C average warming during the twentieth century. Mean temperatures, rates of change, and variance at that time are all anomalously high within the context of the last 500 years. Similar changes are seen in individual records from around the eastern Canadian Arctic and in a circum-Arctic average temperature record showing approximately 1.5°C warming from the nineteenth to the twentieth centuries (Overpeck et al., 1997). Our results suggest that the observed recent warming was caused primarily by increasing atmospheric trace-gas concentrations, with contributions from changes in solar irradiance and volcanic activity. In addition, positive feedback mechanisms that render high northern latitudes extremely sensitive to global climate changes have probably served to amplify the temperature increase in the Arctic. Although anthropogenic activity appears to play a dominant role in forcing recent climatic change, it probably is not the only factor. Additional warming brought on by the buildup of greenhouse gases will probably continue to be modified, and possibly amplified, by natural processes in the future. Acknowledgements The authors wish to thank R. Bradley, D. Hardy, P. Sauer, S. Lamoureux, G. Miller, J. Cole, M. Retelle, L. Barlow and S. Leh- man for helpful comments and discussion, and M. Duvall and J. Moore for their commitment to making thin sections. We also would like to thank the staff of the Iqaluit Research Center for years of invaluable support, the employees of Bradley Air and the DEW-line sites at Brevoort Island and Cape Dyer, and the inhabitants of Iqaluit and Kimmirut. This work was supported by a NASA Earth Systems Science Fellowship, by NSF grants ATM94-02657 and ATM-930072, and by the NOAA Office of Global Programs. This is PALE contribution number 101. 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