HYDROLOGICAL PROCESSES
Hydrol. Process. 20, 781– 801 (2006)
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.6131
Recent trends in Canadian lake ice cover
Claude R. Duguay,1 * Terry D. Prowse,2 Barrie R. Bonsal,3 Ross D. Brown,4
Martin P. Lacroix2 and Patrick Ménard5
1 Geophysical Institute, University of Alaska Fairbanks, PO Box 757320, Fairbanks, AK 99775-7320, USA
National Water Research Institute, Water & Climate Impacts Research Centre, University of Victoria, PO BOX 1700 STN CSC, Victoria,
British Columbia V8W 2Y2, Canada
3 National Water Research Institute, Saskatoon, Saskatchewan S7N 3H5, Canada
4 Service météorologique du Canada—Région du Québec, Place Bonaventure, 800, rue de la Gauchetière Ouest, bureau 7810, Montreal,
Quebec H5A 1L9, Canada
5 145 Dew Street, King City, Ontario L7B 1L1, Canada
2
Abstract:
Recent studies have shown that ice duration in lakes and rivers over the Northern Hemisphere has decreased over
the 19th and 20th centuries in response to global warming. However, lake ice trends have not been well documented
in Canada. Because of its size, considerable variability may exist in both freeze-up and break-up dates across the
country. In this paper, results of the analysis of recent trends (1951–2000) in freeze-up and break-up dates across
Canada are presented. Trends toward earlier break-up dates are observed for most lakes during the time periods of
analysis which encompass the 1990s. Freeze-up dates, on the other hand, show few significant trends and a low
degree of temporal coherence when compared with break-up dates. These results are compared with trends in autumn
and spring 0 ° C isotherm dates over the time period 1966–95. Similar spatial and temporal patterns are observed,
with generally significant trends toward earlier springs/break-up dates over most of western Canada and little change
in isotherm and freeze-up dates over the majority of the country in autumn. Strong correlations (r > 0Ð5) between
0 ° C isotherm dates and freeze-up/break-up dates at many locations across the country reveal the high synchrony of
these variables. These results are also consistent with more recent observations of other cryospheric and atmospheric
variables that indicate, in particular, a general trend toward earlier springs in the latter part of the 20th century. The
results of this study provide further evidence of the robustness of lake ice as a proxy indicator of climate variability
and change. Copyright  2006 John Wiley & Sons, Ltd.
KEY WORDS
lake ice; freeze-up and break-up dates; 0 ° C isotherm dates; climate change; Canada
INTRODUCTION
Lake ice is an important component of the Canadian terrestrial cryosphere. Surface–atmosphere interactions
are altered by the presence of ice on a lake and the formation of lake ice has important ecological and
economic implications (Brown and O’Neill, 2002). The dates of ice freeze-up and break-up have been
shown to be good indicators of climate variability and global climate change (e.g. Palecki and Barry,
1986; Robertson et al., 1992; Barry and Maslanik, 1993; Assel and Robertson, 1995; Livingstone, 1997;
Magnuson et al., 2000; Assel et al., 2003). For example, using long historical ice records from a limited
set of lakes (and rivers), Magnuson et al. (2000) have shown that freeze-up and break-up dates provide
consistent evidence of later freezing and earlier break-up around the Northern Hemisphere from 1846 to
1995. Over that period, changes in freeze-up dates averaged 5Ð8 days per 100 years later, and changes in
break-up dates averaged 6Ð5 days per 100 years earlier; these translate to increasing air temperatures of about
1Ð2 ° C per 100 years. These trends are in general agreement with various studies suggesting that each 1 ° C
* Correspondence to: Claude R. Duguay, Geophysical Institute, University of Alaska, Fairbanks, Fairbanks, AK 99775-7320, USA.
E-mail: claude.duguay@gi.alaska.edu
Copyright  2006 John Wiley & Sons, Ltd.
Received 11 June 2005
Accepted 23 September 2005
782
C. R. DUGUAY ET AL.
change in autumn or spring air temperature causes a 4 to 6 day change in the mean freeze-up or break-up
date for lakes in the middle latitudes of the Northern Hemisphere, all other things being equal (Walsh, 1995).
However, recently, the linear temperature response of lake ice break-up has been questioned: Weyhenmeyer
et al. (2004) showed that the relationship between the timing of break-up and air temperature from four
decades of lake ice break-up observations from 196 Swedish lakes was best described by an arc cosine
function.
Over the last 10 years or so, many studies have used long time-series of historical observations of lake
ice break-up dates for clarifying the relationship between break-up and air temperature, and establishing the
significance of break-up as an indicator of climate variability and change (Anderson et al., 1996; Livingstone,
1999; Magnuson et al., 2000; Hodgkins et al., 2002). Some of the most recent studies describe trends and
variability in ice cover in relation to changes in air temperature and large-scale (atmospheric circulation)
oscillations (e.g. North Atlantic oscillation, Arctic oscillation, and the North Pacific index) in Europe,
Scandinavia and the USA (Benson et al., 2000; Yoo and D’Odorico, 2002; Todd and Mackay, 2003; Blenckner
et al., 2004; George et al., 2004).
In Canada, few investigations have examined patterns and trends in lake ice cover time series, and
most of the results have appeared in the grey literature (Da Silva, 1984, 1985; Anderson, 1987; Skinner,
1992). Schindler et al. (1990) examined climatic and hydrologic records from the 1960s to the 1980s for
the Experimental Lakes Area (ELA) of northwestern Ontario (boreal region) and found that the ice-free
season duration had increased by about 20 days over the 20-year period due mainly to earlier breakup dates in spring. Autumn freeze-up dates were not observed to change significantly. The earlier spring
break-up of the lakes in the ELA was attributed to the increased April–May air temperatures and reduced
snow cover and warmer temperatures in March causing earlier snowmelt and increased solar radiation
absorption by the lake in early spring. Skinner (1992) used freeze-up and break-up records from 30 lakes
across various regions of Canada (most regions with data over the period 1956–57 to 1988–89, other
regions with shorter records 1971–72 to 1988–89) and showed that trends toward earlier and warmer
spring seasons in western and central Canada were reflected in break-up dates for most lakes. As in the
Schindler et al. (1990) study, no specific trends were seen in either air temperatures or regional freezeup dates during the autumn season in any part of the country. The trends toward shorter ice seasons in
most parts of Canada reflected the earlier spring break-up in response to warmer temperatures during the
spring season. More recently, Futter (2003) presented results from the analysis of 46 break-up and 15 icefree season phenology time series from lakes of southern Ontario (1853–2001) obtained largely through
volunteer monitoring efforts. He observed monotonic trends toward earlier break-up dates and longer icefree seasons across this region of Canada over the full length of the time series and in the last 30 years
of data. The trends are believed to be indicative of the warming in spring temperature across the study
area.
The primary objective of this study was to analyse spatial and temporal trends in lake ice freeze-up and
break-up dates across Canada for the period 1951–2000. This study extends the scope of previous lake ice
investigations by providing a Canada-wide picture of the response of lake ice covers to climate in Canada
at different time intervals during the last five decades. More specifically, this paper addresses the following
questions:
1. What has been the magnitude of changes in freeze-up and break-up dates in different regions of Canada
for the 30-year climatological periods (1951–80, 1961–90, and 1971–2000)?
2. How do the lake ice trends compare with trends in other cryospheric and atmospheric variables observed
by other workers (e.g. Serreze et al., 2000) over a comparable time period (1966–95)?
3. How do the trends and variability in freeze-up and break-up dates compare with those of the autumn and
spring 0 ° C isotherm dates?
Copyright  2006 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 781– 801 (2006)
783
TRENDS IN CANADIAN LAKE ICE COVER
DATA AND METHODOLOGY
Data sources
Lake ice observations. The lake ice observations were extracted from the Canadian Ice Database (CID;
Lenormand et al., 2002). The CID is a national historical ice database that contains records relating to the state
of Canadian ice covers (lake ice, river ice, landfast sea ice) since 1822. The CID contains in situ observations
from 757 sites across Canada that were originally kept on digital or paper records at the Meteorological
Service of Canada Headquarters (MSC-HQ) and the Canadian Ice Service (CIS). As shown in Figure 1, the
network of lake ice observation sites reached a peak between the early 1950s and the mid 1980s, but has since
then experienced a dramatic decline. A complete description of the ice database and its historical evolution
are given in Lenormand et al. (2002).
Data provided by the CIS relate to (near-)weekly ice thickness and on-ice snow depth measurements made
during an ice season, and the records from the MSC-HQ contain detailed information pertaining to several
parameters describing the freeze-up and break-up processes. These include, to name a few, the first ice date,
the date of complete freeze over (CFO), the maximum ice thickness, the date of beginning of ice melt, and
the date when the water body becomes clear of ice (WCI). The terms freeze-up and break-up dates as used
throughout this paper correspond to CFO dates and WCI dates respectively. Freeze-up and break-up dates, the
terms most often used in the literature, are also referred to at times as ice-in and ice-out dates (e.g. Hodgkins
et al., 2002). In this paper, the term ‘lake site’ is favoured over ‘lake’, since shore-based ice observations
may not represent ice conditions over an entire lake.
Since the main objective of this study is to analyse spatial patterns in freeze-up and break-up dates across
Canada, a decision was taken to examine trends over 30-year time periods for which there were relatively
good spatial distributions of lakes across the country with at least 20 years of observations during those
periods. The use of a longer 50-year period would have been preferred, but there were only 13 lake sites in
the CIS with data covering a 50-year period. Of these 13 sites, a number had periods of missing data and the
periods of observations did not overlap sufficiently to compare time series over the same time period.
Three 30-year periods (1951–80, 1961–90, and 1971–2000) corresponding to the conventional periods used
in the calculation of climate normals and anomalies in Canada were selected for trend analysis. An additional
120
100
Number of Sites
80
60
40
1999
1989
1979
1969
1959
1949
1939
1929
1919
1909
1899
1889
1879
1869
1859
1849
1839
0
1829
20
Years
Figure 1. Evolution of lake ice observation sites from the CID
Copyright  2006 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 781– 801 (2006)
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C. R. DUGUAY ET AL.
period, 1966–95, was included so that trends could also be compared with trends in other cryospheric and
atmospheric variables from recent studies, as well as trends in autumn and spring 0 ° C isotherm dates calculated
over the same time period. Lake sites with at least 20 years of records over the four periods of analysis are
shown in Figure 2. Noteworthy is the larger number of sites for the periods 1961–90 and 1966–95 compared
with the previous (1951–80) and later (1971–2000) periods. The establishment of the DEW line (short for
Distant Early Warning Line) in 1957 contributed to the growth of the lake ice network in the north. The
DEW line was an integrated chain of 63 radar and communications systems stretching 3000 miles from the
northwest coast of Alaska to the eastern shore of Baffin Island, Canada, roughly along the 69th parallel (The
DEW Line Sites in Canada, Alaska & Greenland, 2005). The operations ceased for many locations in the late
1980s to early 1990s, which is reflected in the smaller number of lake sites in northern Canada with at least
20 years of observations during the period 1971–2000.
Air temperature data. Air temperature data employed in this study consisted of daily mean values for
187 high-quality, relatively evenly distributed MSC stations across Canada for the period 1966–95. The 0 ° C
autumn and spring isotherm dates were determined at each meteorological station using the approach described
in Bonsal and Prowse (2003). Briefly, the autumn (spring) 0 ° C isotherm dates are defined as the date when
mean daily temperature falls below (rises above) 0 ° C. Because of the large degree of variability inherent in
daily temperatures, mean daily temperature values are filtered using a 31-day running mean and autumn and
spring isotherms are defined as the date when the running mean daily temperature crosses 0 ° C. The MSC
station network does not adequately represent mountainous regions of Canada, but this is not considered
problematic for this study as few of the lakes were located in mountain regions.
The strength and the significance of relations between freeze-up/break-up dates and autumn/spring isotherm
dates were determined using the Pearson product-moment correlation r. Freeze-up and break-up dates at lake
sites were correlated with 0 ° C isotherm dates calculated from their closest meteorological station. A high
(low) correlation means that there is a high (low) degree of synchrony between the 0 ° C isotherm dates and
ice freeze-up/break-up dates. In total, 66 correlation coefficients were derived relating freeze-up dates and
0 ° C autumn isotherm dates and 74 relating break-up dates to 0 ° C spring isotherm dates. Maps displaying
isotherm trends across Canada were also produced through spatial interpolation (ordinary kriging method
available in ArcGIS 9Ð0 software package) using spring and autumn 0 ° C isotherm dates derived from the 187
meteorological stations.
Trend analysis
The presence of first-order trends in freeze-up/break-up and 0 ° C isotherm time series was tested using
the non-parametric Mann–Kendall test and the magnitude (slope) of the trends was estimated with Sen’s
method (Sen, 1968). The Mann–Kendall test is widely used in environmental science because it can cope
with missing values and values below a detection limit. Recently, the test has been utilized to detect linear
trends in long-term time series of river ice and lake ice observations (e.g. Smith, 2000; Hodgkins et al., 2002;
Futter, 2003).
With the Mann–Kendall test, the presence of a statistically significant trend is evaluated using the
standardized Z-statistic, which has a normal distribution. A positive (negative) value of Z indicates an upward
(downward) trend. To test for either a downward or an upward monotone trend (a two-tailed test) at ˛ level
of significance, the null hypothesis (H0 : no trend) is rejected if the absolute value of Z is greater than
Z1˛/2 , where Z1˛/2 is obtained from the standard normal cumulative distribution table. The estimation of
the slope of the trends was determined using Sen’s non-parametric method (Sen, 1968). The Mann–Kendall
test and Sen’s slope estimates were both performed using the Excel template implementation (MAKESENS:
Mann–Kendall test for trend and Sen’s slope estimates) developed by Salmi et al. (2002).
Copyright  2006 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 781– 801 (2006)
785
TRENDS IN CANADIAN LAKE ICE COVER
(a) 1951-1980
Climatic Region
1 Pacific
5 Arctic
2 Cordillera
Great Lakes /
3 Prairies
6 Saint Lawrence
4 Boreal
7 Atlantic
5
2
1
7
3
4
6
(b) 1961-1990
Climatic Region
1 Pacific
5 Arctic
2 Cordillera
Great Lakes /
6 Saint Lawrence
3 Prairies
4 Boreal
7 Atlantic
5
2
1
7
3
4
6
Figure 2. Distribution of lake ice observation sites for periods (a) 1951– 80, (b) 1961– 90, (c) 1966– 95, and (d) 1971– 2000. The thick lines
represent the limits of the climatic regions of Canada (Hare and Thomas, 1974)
Copyright  2006 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 781– 801 (2006)
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C. R. DUGUAY ET AL.
(c) 1966-1995
Climatic Region
1 Pacific
5 Arctic
2 Cordillera
Great Lakes /
3 Prairies
6 Saint Lawrence
4 Boreal
7 Atlantic
5
2
1
7
3
4
6
(d) 1971-2000
Climatic Region
1 Pacific
5 Arctic
2 Cordillera
3 Prairies
Great Lakes /
6 Saint
Lawrence
4 Boreal
7 Atlantic
5
2
1
7
3
4
6
Figure 2. (Continued )
Copyright  2006 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 781– 801 (2006)
TRENDS IN CANADIAN LAKE ICE COVER
787
RESULTS AND DISCUSSION
Trends for periods 1951–80, 1961–90, and 1971–2000
Freeze-up dates. The spatial distribution of freeze-up trends across Canada for periods 1951–80, 1961–90,
and 1971–2000 is shown in Figure 3. With a few exceptions, clear spatial patterns are difficult to discern
owing to the large regional variability. One exception can be found in the map of 1961–90, a period during
which there appears to be group responses of lakes (spatial coherence) toward later freeze-up dates along
the DEW line (69th parallel) and in the Quebec–southeastern Ontario region, and earlier freeze-up dates in
the Cape-Breton–Newfoundland region. During the period 1951–80, there is a greater concentration of sites
showing later freeze-up dates in western Canada (including sections of British Columbia and the Northwest
Territories) and earlier dates in the central and eastern parts of the country (Great Lakes–St Lawrence and
Atlantic regions). For the period 1971–2000, lakes in the Great Lakes–St Lawrence region show trends toward
later freeze-up dates (three out of four that are significant at the 10% level), and earlier dates for a group of
lakes in the Boreal region of northern Ontario and Manitoba, and those of the Atlantic region. It is worth
noting that trends toward earlier freeze-up dates (colder climate) persist for the latter region throughout the
three 30-year periods. Overall, the spatial coherence in freeze-up trends appears to be more local or regional
in nature than for break-up trends, which will be discussed in the next section. Also, of all the trends, few
are statistically significant at the 10% level over the three periods.
The large spatial variability in freeze-up trends contrasts with Canadian river freeze-up results reported by
Zhang et al. (2001) that showed widespread trends toward earlier freeze-up over the period 1967–96. This
difference may be related to the different spatial distribution of observing stations in the two studies. Trends in
snow cover onset date over the Northern Hemisphere for the period 1972–98 from the NOAA dataset (Hall
et al., 2004) confirm the results presented here, i.e. of strong regional contrasts in fall-season cryospheric
response (e.g. earlier snow cover onset over the Maritimes, southern Manitoba and the Mackenzie Valley, and
later snow cover onset dates over British Columbia, northern Quebec and the high Arctic).
Lake sites that have experienced some of the strongest freeze-up trends are shown in Figure 4. Freezeup anomalies are computed as departures from the 30-year mean. Some lakes have experienced very
significant trends, especially toward later freeze-up dates for their respective periods. For example, Lake
Utopia (1971–2000), which has a complete 30-year record, shows the most significant trend (significant at
the 0Ð1% level), with its freeze-up date occurring 37 days later (1Ð23 days per year). Two of the lakes in
the Atlantic region, Grand Lake and Deadman’s Pond, both show significant trends toward earlier freezeup dates (about 2 weeks earlier over the 30-year periods). In addition to the trends, some lake sites show
larger interannual variability than others, as well as some indications of regime shifts. Without discarding the
role of local factors on the interannual variability of freeze-up dates, some of the fluctuations and shifts are
likely associated with the influence of large-scale atmospheric/oceanic circulation variability (oscillations).
This subject merits further investigation, but is beyond the scope of this paper.
Break-up dates. In contrast to freeze-up dates, break-up dates show a greater spatial coherence (Figure 5).
There is a dipole-like distribution of break-up dates across the country for the period 1951–80, with most
of the lake sites west of Hudson Bay experiencing earlier springs, which is particularly strong along the
Manitoba–Saskatchewan border and in the Northwest Territories. Later break-up dates, on the other hand,
are more prominent in eastern Canada, but with few sites displaying statistically significant trends at the
10% level. The period 1961–90 shows a strong, generalized trend toward earlier break-up dates over all of
Canada: 71 (88%) of the 81 lake sites exhibit this trend (21 sites significant at the 10% level). Noteworthy
is the fact that none of the sites experiencing later break-up dates shows statistically significant trends at the
10% level. Trends during the period 1971–2000 reveal the persistence of the warmer spring conditions for
much of Canada: 31 (72%) of the 43 lake sites show a trend toward earlier break-up dates. During that period,
a ‘boomerang shape’ distribution of lakes displaying earlier break-up dates (negative trends) is noticeable in
Figure 5, beginning in Labrador and ending in northern British Columbia. A similar pattern is observed in
Copyright  2006 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 781– 801 (2006)
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C. R. DUGUAY ET AL.
(a) 1951-1980
Freeze up: 1951-1980
+ 13
Significant postive trend
Non-Significant postive trend
Non-Significant negative trend
− 23
Significant negative trend
No trend
5
2
1
7
4
3
Climatic Region
6
5 Arctic
1 Pacific
2 Cordillera
Great Lakes
3 Prairies
6 Saint Lawrence
4 Boreal
7 Atlantic
(b) 1961-1990
Freeze up: 1961-1990
+ 29
Significant postive trend
Non-Significant postive trend
Non-Significant negative trend
− 29
Significant negative trend
No trend
5
2
1
7
3
4
Climatic Region
1 Pacific
2 Cordillera
5 Arctic
6
Great Lakes
3 Prairies
6 Saint Lawrence
4 Boreal
7 Atlantic
Figure 3. Trends in freeze-up dates across Canada for the periods (a) 1951– 80, (b) 1961– 90, and (c) 1971– 2000. Triangles pointing up
indicate later freeze-up dates (warming trend) and those pointing down indicate earlier freeze-up dates (cooling trend). Lake sites with
significant trends at the 10% level are denoted by filled triangles. The upper and lower limits of the number of days of change determined
for each period are given on the left-hand side of the freeze-up legend
Copyright  2006 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 781– 801 (2006)
789
TRENDS IN CANADIAN LAKE ICE COVER
(c) 1971-2000
Freeze up: 1971-2000
+37
Significant postive trend
Non-Significant postive trend
Non-Significant negative trend
−17
Significant negative trend
No trend
5
2
1
7
3
4
Climatic Region
1 Pacific
5 Arctic
2 Cordillera
3 Prairies
4 Boreal
6
6
Great Lakes /
Saint Lawrence
7 Atlantic
Figure 3. (Continued )
trends of last date of spring snow cover over the period 1972–98 (Hall et al., 2004). A few lake sites in
northern Manitoba and Saskatchewan, as well as the Northwest Territories, though, show trends toward later
break-up dates (colder conditions). Some significant late break-up trends appear in the Great Slave Lake area,
which also shows up as an area with a trend toward later spring snow cover over the period 1972–98 (Hall
et al., 2004).
Break-up anomalies for the lakes showing some of the strongest positive (later dates, colder climate) and
negative (earlier dates, warmer climate) trends in the country over the three 30-year periods are illustrated
in Figure 6. From this set of graphics it can be seen that break-up dates became earlier (or later) by about
10–15 days, with earlier dates being the norm, as shown in the maps of Figure 5. With the exception of the
Colpoys Bay site (Georgian Bay, Lake Huron), break-up dates appear to show smaller departures from their
means than freeze-up dates. From the sample of lake sites of Figure 6, break-up anomalies rarely exceed
20 days, whereas for freeze-up dates (see Figure 4) the anomalies more frequently surpass 20 days. The
examination of a larger number of sites would be necessary, however, before making any strong generalization
on this particular point.
Trends for the period 1966–95
Albeit with a few exceptions, trends in freeze-up and break-up dates for the period 1966–95 (Figure 7)
mirror those of the period 1961–90 (Figures 3b and 5b), with break-up dates becoming increasingly earlier for
most of the country and no strong regional signals in freeze-up dates, except perhaps for the Great Lakes–St
Lawrence region (later freeze-up) and the Atlantic region (earlier freeze-up). Also shown in Figure 7, as a
backdrop, are the trends in 0 ° C isotherm dates presented in the form of classes. As explained in an earlier
section, these were obtained through interpolation of autumn and spring 0 ° C isotherm dates derived from 187
meteorological stations across Canada. Their trends in relation to freeze-up and break-up trends will be the
focus of a discussion in a later section.
Copyright  2006 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 781– 801 (2006)
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C. R. DUGUAY ET AL.
(a) Frame Lake - Boreal region
Grand Lake – Atlantic region
(b) Lake Athabasca – Boreal region
Deadman's Pond – Atlantic region
(c) Lake Utopia – Great Lakes/St. Lawrence region
Island Lake – Boreal region
Figure 4. Freeze-up anomalies for lake sites showing some of the strongest positive (left: later dates, warmer) and negative (right: earlier
dates, colder) trends for the periods (a) 1951– 80, (b) 1961– 90, and (c) 1971– 2000. The number of years n for each record, the number of
days per year of change (in parentheses) and the significance level of the trends (C: 10%; Ł : 5%; ŁŁ : 1%; ŁŁŁ : 0Ð1%) are also indicated on
the graphics
Comparison with trends in other cryospheric and atmospheric variables. In recent years, several papers
have been published on the trends in atmospheric and cryospheric variables (e.g. Brown and Goodison, 1996;
Brown and Braaten, 1998; Serreze et al., 2000; Zhang et al., 2001; Huntington et al., 2003; Overland et al.,
2004), as well other parameters such as the greenness of vegetation (e.g. Myneni et al., 1997; Slayback et al.,
Copyright  2006 John Wiley & Sons, Ltd.
Hydrol. Process. 20, 781– 801 (2006)
791
TRENDS IN CANADIAN LAKE ICE COVER
(a) 1951-1980
Break-up : 1951-1980
−18
Significant postive trend
Non-Significant postive trend
Non-Significant negative trend
+17
Significant negative trend
No trend
5
2
1
7
4
3
Climatic Region
5 Arctic
1 Pacific
2 Cordillera
6
Great Lakes
3 Prairies
6 Saint Lawrence
4 Boreal
7 Atlantic
(b) 1961-1990
Break-up : 1961-1990
−24
Significant postive trend
Non-Significant postive trend
+13
Non-Significant negative trend
Significant negative trend
No trend
5
2
1
7
3
4
Climatic Region
1 Pacific
2 Cordillera
5 Arctic
6
Great Lakes
3 Prairies
6 Saint Lawrence
4 Boreal
7 Atlantic
Figure 5. Trends in break-up dates across Canada for the periods (a) 1951– 80,
indicate later break-up dates (cooling trend) and those pointing down indicate
significant trends at the 10% level are denoted by filled triangles. The upper and
for the periods are given on the left-hand side
Copyright  2006 John Wiley & Sons, Ltd.
(b) 1961– 90, and (c) 1971– 2000. Triangles pointing up
earlier break-up dates (warming trend). Lake sites with
lower limits of the number of days of change determined
of the break-up legend
Hydrol. Process. 20, 781– 801 (2006)
792
C. R. DUGUAY ET AL.
(c) 1971-2000
Break-up : 1971-2000
−16
Significant postive trend
Non-Significant postive trend
Non-Significant negative trend
+10
Significant negative trend
No trend
5
2
1
7
4
3
Climatic Region
1 Pacific
2 Cordillera
5 Arctic
6
Great Lakes
3 Prairies
6 Saint Lawrence
4 Boreal
7 Atlantic
Figure 5. (Continued )
2003) and plant phenology (e.g. Beaubien and Freeland, 2000; Cayan et al., 2001; Menzel, 2003) in response
to 20th century climate warming in the Northern Hemisphere. Trends in freeze-up/break-up dates for the
period 1966–95 were examined, as they can be compared with trends in other cryospheric and atmospheric
variables previously documented over the same time period. The major trends identified in these references
are summarized in Table I. Of all the variables listed in the table, air temperature is the one that has the
largest influence on freeze-up and break-up dates. As stated by (Magnuson et al., 1997):
Ice formation and break-up are dependent on many climatic forcing variables, such as air temperature,
solar radiation, wind, and snow depth. Yet, air temperature alone often provides a reasonable prediction
of ice phenologies.
The air temperature trends (winter and spring) are generally consistent with the break-up trends from this
study, in that most of Canada, with the exception of northeastern Canada and Newfoundland, has experienced
a warming trend over the period 1966–95. As discussed earlier, freeze-up trends are highly variable across
the country, so that it is difficult to establish a clear relation with the air temperature trends summarized in
Table I.
Relation to 0 ° C isotherm dates. It has been shown in several lake ice studies that freeze-up and breakup dates correlate most strongly with air temperatures in the 1 to 3 months before the event (e.g. Palecki
and Barry, 1986; Robertson et al., 1992; Livingstone, 1997, 1999). Magnuson et al. (2000) reported that, in
more northern areas of the Northern Hemisphere (such as Lake Kallavesi, Finland), freeze-up dates reflect the
climate prevailing around October to November, whereas in more southern areas, such as Grand Traverse Bay
(connected to Lake Michigan), the dates reflect the climate from January to February. Break-up dates, on the
other hand, reflect February to March climates in more southern areas, such as Lake Mendota in Wisconsin
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TRENDS IN CANADIAN LAKE ICE COVER
(a) Colpoys Bay (Lake Huron) − Great
Lakes/St. Lawrence region
Brochet Bay (Reinder Lake) − Boreal region
(b) Gull Lake − Great Lakes/St. Lawrence
region
Lake Utopia − Great Lakes/St. Lawrence
region
(c) Back Bay (Great Slave Lake) − Boreal
region
Diefenbaker Lake − Prairies region
Figure 6. Break-up anomalies for lake sites showing some of the strongest positive (left: later dates, colder) and negative (right: earlier
dates, warmer) trends for the periods (a) 1951– 80, (b) 1961– 90, and (c) 1971– 2000. The number of years n for each record, the number
of days per year of change (in parentheses) and the significance level of the trends (C: 10%; Ł : 5%; ŁŁ : 1%; ŁŁŁ : 0Ð1%) are also indicated
on the graphics
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Hydrol. Process. 20, 781– 801 (2006)
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C. R. DUGUAY ET AL.
(a)
Freeze-up : 1966-1995
+20
Significant postive trend
Non-Significant postive trend
Non-Significant negative trend
−28
Significant negative trend
No trend
5
2
1
7
4
3
Climatic Region
1 Pacific
5 Arctic
2 Cordillera
6
Great Lakes
3 Prairies
6 Saint Lawrence
4 Boreal
7 Atlantic
(b)
Break-up : 1966-1995
−20
Significant postive trend
Non-Significant postive trend
+11
Non-Significant negative trend
Significant negative trend
No trend
5
2
1
7
3
4
Climatic Region
1 Pacific
2 Cordillera
5 Arctic
6
Great Lakes
3 Prairies
6 Saint Lawrence
4 Boreal
7 Atlantic
Figure 7. Trends in (a) freeze-up and (b) break-up dates across Canada for period 1966– 95. Triangles pointing up indicate later
freeze-up/break-up dates and those pointing down indicate earlier freeze-up/break-up dates. Lake sites with significant trends at the 10%
level are denoted by filled triangles. Also shown in the background are the trends in autumn and spring 0 ° C isotherm dates estimated across
Canada (number of days change over the period 1966– 95)
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TRENDS IN CANADIAN LAKE ICE COVER
Table I. Summary of trends in other cryospheric and atmospheric variables in Canada
Documented trendb
Variable
Air temperature
Precipitation
Snow-covered area
Snow depth
River ice freeze-up/break-up
River ice duration
Glaciers
Arctic sea ice extent
Sea ice duration
Permafrost
Autumn. Colder over part of central Canada, Hudson Bay, and into western Nunavut,
Northwest Territories and Yukon. Close to no change over the rest of the country
Winter. Warmer west of Hudson Bay; particularly strong over western provinces,
Northwest Territories, and Yukon. Colder from Hudson Bay eastward. Close to no
change over Ontario
Spring. Warmer over most of the country; particularly strong over western Canada. Colder
on southern portion of Baffin Island. Close to no change over Hudson Bay, northern
Quebec, and Atlantic provinces
Summer. Warmer over part of western Canada and the western Arctic in particular. Close
to no change over the rest of the country
Positive trends in annual precipitation, as well as snowfall (up to 20% increase), during the
past 40 years over Canada north of 55° N
Statistically significant decreases in spring snow cover (1946–95) over most of western
Canada and the high Arctic
Decrease in January–March snow depths for most of Canada (1946–95), with the largest
decreases in March. Decreases are most prominent in Mackenzie basin, Prairies, and
lower St Lawrence Valley. Increases are noted only on the east coast. Snow cover
duration declined over most of western Canada and in the Artic in summer. Sharp
transition to lower depths in the mid 1970s
Statistically significant trends toward earlier freeze-up, particularly in eastern Canada, and
earlier break-up, especially in British Columbia (1967–96)
Increased ice cover duration over the Maritimes; variable response in other regions of
Canada
Canadian mountain glaciers characterized by generally negative mass balance and retreat
during the second half of 20th century. Glacier mass balance over the western Cordillera
closely linked to North Pacific climate variability, e.g. Pacific decadal oscillation
Small negative trend since 1979, with more pronounced reduction since the late 1980s
Statistically significant increase in length of ice-free season in southwestern Hudson Bay
region (1971–2003). Much of the increase is attributed to earlier break-up (3 days per
decade)
Temperature increases for western Canada, but not consistent
Decrease in temperatures for northern Quebec from mid-1980s, tentatively attributed to
lower air temperatures in this region, to about mid-1990s
a Period 1966– 95 unless specified otherwise.
b Sources: Brown and Goodison (1996); Brown
and Braaten (1998); Serreze et al. (2000); Zhang et al. (2001); Brown et al. (2004); Gough
et al. (2004); Overland et al. (2004).
(Magnuson et al., 2000). How well freeze-up and break-up dates correlate with a simpler air temperature
index, such as the 0 ° C isotherm date, had not yet been investigated.
The arrival of the 0 ° C isotherm date in autumn initiates, for example, snow accumulation and animal
hibernation. In spring it brings about, for example, snowmelt, early plant growth, and flood hazards. In a
recent paper, Bonsal and Prowse (2003) summarized some relations between trends in autumn and spring
0 ° C isotherm dates and trends in hydro-cryospheric variables (e.g. snowmelt, river and lake ice) in general
terms. Here, the strength of the relation between autumn/spring 0 ° C isotherm dates and freeze-up/break-up
dates is quantified. Trends in both sets of dates, as well as their degree of synchrony, are described.
Figure 7 allows one to get a sense of the spatial distribution of trends in both freeze-up/break-up dates and
0 ° C isotherm dates, and their possible relation from a qualitative standpoint. In Figure 7a, autumn isotherm
trends are represented in three classes. The first class shows areas of the country with a trend toward later
0 ° C isotherm dates (warming trend) and the two other classes a trend toward an earlier arrival of the 0 ° C
isotherm. Few lake sites exhibit any statistically significant trends in freeze-up dates. However, for those that
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C. R. DUGUAY ET AL.
do, they tend to fall in regions of the country where trends in autumn isotherms generally follow the same
direction (i.e. positive or negative trends). The Great Lakes–St Lawrence and Atlantic regions, as well as
northern Ellesmere Island (Upper Dumbell Lake–Eureka weather station), are good examples of such trends.
As shown in Figure 7b, the break-up dates follow much the same trends as the spring 0 ° C isotherm dates,
with a strong signal toward earlier break-up and 0 ° C isotherm arrival dates. Only one small region of the
country shows a trend toward later 0 ° C isotherm arrival dates (eastern Newfoundland), and the lake site
(Deadman’s Pond) closest to this region exhibits a trend toward later break-up dates.
The maps of Figure 8 provide a clear picture of the spatial and temporal coherence between freezeup/break-up dates and 0 ° C autumn/spring isotherm dates: 52% (34/66) of the paired sites (lake and closest
meteorological station) have r > 0Ð5 in the relation freeze-up/autumn isotherm dates. For the spring break-up
period, r > 0Ð5 for 78% of the paired sites (58/74). So, although 0 ° C isotherm dates mark the beginning of
the freeze-up and break-up periods, they show a high degree of synchrony at many locations throughout the
country, and generally more so during the spring break-up period.
However, it should be noted that, although the 0 ° C isotherm dates and freeze-up/break-up dates are often
in tempo, there is a lag (delay) between the onset of melting and freezing conditions and the complete
freeze-over and disappearance of ice from lakes. This point is clearly illustrated in Figures 9 and 10. From
the sample of sites shown in these figures (one per climatic region), it appears that the lag (i.e. the number
of days of difference) is shorter during the freeze-up period (10–15 days on average) than the break-up
period (25–30 days on average). There are no complete explanations that can be provided at this point,
other than the likelihood that the lake sites selected to show the synchrony between freeze-up dates and
autumn 0 ° C isotherm dates are generally shallow. Shallow lakes have thermal turnover rates in the order of
a week (they store less heat); hence, they form ice more quickly than larger lakes. However, no lake depth
measurements are available for these lakes to support this affirmation. The greater lag during the spring period
(see Deadman’s Pond in Figures 9 and 10 for comparison) can be attributed to some degree to the amount
of time needed to melt the snow on the ice covers and to melt the slab of ice formed on the lakes during
the winter period. Overall, the correlations obtained between the 0 ° C isotherm dates and freeze-up/break-up
dates are as strong as the correlations involving the use of air temperatures from the 1 to 3 months preceding
the events (break-up/freeze-up).
SUMMARY AND CONCLUSION
The primary intent of this paper was to analyse spatial and temporal trends in lake ice freeze-up and break-up
dates across Canada for various time periods between 1951 and 2000. Three questions were being addressed:
1. What has been the magnitude of changes in freeze-up and break-up dates in different regions of Canada
for the 30-year climatological periods (1951–80, 1961–90, and 1971–2000)?
2. How do the lake ice trends compare with trends in other cryospheric and atmospheric variables observed
by other workers over a comparable time period (1966–95)?
3. How do the trends and variability in freeze-up and break-up dates compare with those of the autumn and
spring 0 ° C isotherm dates?
The main findings, which provide answers to the above questions, can be summarized as follows:
1. Trends in freeze-up dates. Trends toward later and earlier freeze-up dates were found at various locations
across Canada. However, many of these trends were not significant at the 10% level and their spatial
coherence was generally weak. The latter may be due to the effect of lake morphometry (depth and area)
and local meteorological conditions such as wind, which can play a significant role during the freeze-up
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TRENDS IN CANADIAN LAKE ICE COVER
(a)
Correlation
< 0.2
0.2 - 0.4
0.4 - 0.6
0.6 - 0.8
0.8 - 1.0
5
2
4
1
7
3
6
Climatic Region
1 Pacific
5 Arctic
2 Cordillera
6 Great Lakes
Saint Lawrence
3 Prairies
7 Atlantic
4 Boreal
(b)
Correlation
< 0.2
0.2 - 0.4
0.4 - 0.6
0.6 - 0.8
0.8 - 1.0
5
2
4
1
7
3
Climatic Region
1 Pacific
6
5 Arctic
2 Cordillera 6 Great Lakes
Saint Lawrence
3 Prairies
4 Boreal
7 Atlantic
Figure 8. Correlation coefficients between (a) freeze-up and autumn 0 ° C isotherm dates and (b) break-up and spring 0 ° C isotherm dates
across Canada for the period 1966– 95
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C. R. DUGUAY ET AL.
(a) Watson Lake, Cordillera region
(d) Unnamed Lake 684009748 − Arctic region
(b) Ekapo Lake − Prairies region
(e) Base Lake − Great Lakes/St. Lawrence region
(c) Attawapiskat Lake − Boreal region
(f) Deadman’s Pond − Atlantic region
Figure 9. Comparisons between freeze-up and autumn 0 ° C isotherm dates for: (a) Watson Lake, Yukon; (b) Ekapo Lake, Saskatchewan;
(c) Attawapiskat Lake, Ontario; (d) Unnamed Lake 684 009 748, Nunavut; (e) Bass Lake, Ontario; (f) Deadman’s Pond, Newfoundland.
Each lake is representative of a different climatic region. Correlation coefficients r are based on annual time series for the period 1966– 95
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TRENDS IN CANADIAN LAKE ICE COVER
(a) Watson Lake, Cordillera region
(d) Unnamed Lake 684009748 – Arctic region
(b) Ekapo Lake – Prairies region
(e) Bass Lake – Great Lakes/St. Lawrence region
(c)Attawapiskat Lake – Boreal region
(f) Deadman’s Pond – Atlantic region
Figure 10. Comparisons between break-up and spring 0 ° C isotherm dates: (a) Watson Lake, Yukon; (b) Ekapo Lake, Saskatchewan;
(c) Attawapiskat Lake, Ontario; (d) Unnamed Lake 684 009 748, Nunavut; (e) Bass Lake, Ontario; (f) Deadman’s Pond, Newfoundland.
Each lake is representative of a different climatic region. Correlation coefficients r are based on annual time series for the period 1966– 95
period. Where trends toward later freeze-up were observed they corresponded to areas with increasing fall
snow cover from the NOAA dataset.
2. Trends in break-up dates. Trends toward earlier and later break-up dates were found at various locations
across Canada, though earlier break-up dates dominated over the periods of analysis. Several trends were
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significant at the 10% level and their spatial coherence was strong. Western Canada showed the most
consistent trends toward earlier break-up dates over all 30-year periods. The spatial patterns were consistent
with changes in spring snow cover duration derived from the NOAA dataset.
3. Comparison with trends in other cryospheric and atmospheric variables. The trends observed in lake ice
cover are consistent with other cryospheric variables (e.g. snow cover, river ice) and provide further evidence
of the enhanced spring warming that began over North America during the second half of the 20th century.
Warming has played a role, but changes in the main modes of atmospheric circulation over North America
have also played a role (Walsh et al., 2005).
4. Relation between freeze-up/break-up dates and 0 ° C isotherm dates. A strong relation was found between
0 ° C isotherm dates. Although autumn and spring 0 ° C isotherm dates mark the beginning of the freezing
and thaw periods respectively, there was a high degree of synchrony between freeze-up and break-up dates
and the 0 ° C isotherm dates. This was well reflected in both the trends and interannual variability depicted
in the 1966–95 time series. Ice freeze-up and break-up dates lagged behind the 0 ° C isotherm dates by a
few days to about 1 month.
Finally, the results of this study leave the door open for future, regionally focused investigations on
the response of lake ice to climate over longer time periods and in relation to large-scale atmospheric
oscillations/teleconnections. The recent availability of lake ice databases such as CID (Lenormand et al.,
2002) and IceWatch on the Canadian Cryospheric Information Network (CCIN) will facilitate these types of
investigations.
ACKNOWLEDGEMENTS
This research was made possible with funding obtained from the Natural Sciences and Engineering Research
Council of Canada, Environment Canada and a start-up grant from the Geophysical Institute, University of
Alaska Fairbanks, to C. Duguay. The CID was developed at Laval University with financial support from the
MSC to C. Duguay. The CID is available through the CCIN at http://www.ccin.ca. We are grateful for the
helpful comments of Andrew Klein and two anonymous reviewers.
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