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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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