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Observed soil temperature trends associated with climate
change in Canada
Budong Qian,
1
Edward G. Gregorich,
1
Sam Gameda,
1
David W. Hopkins,
2,3
and Xiaolan L. Wang
4
Received 3 September 2010; revised 1 November 2010; accepted 8 November 2010; published 21 January 2011.
[1] Trends in soil temperature are important, but rarely reported, indicators of climate
change. On the basis of the soil temperature data from 30 climate stations across
Canada during 1958–2008, trends in soil temperatures at 5, 10, 20, 50, 100, and 150 cm
depths were analyzed, together with atmospheric variables, such as air temperature,
precipitation, and depth of snow on the ground, observed at the same locations. There was
a significant positive trend with soil temperatures in spring and summer means, but not
for the winter and annual means. A positive trend with time in soil temperature was
detected at about two‐thirds of the stations at all depths below 5 cm. A warming trend of
0.26–0.30°C/decade was consistently detected in spring (March–April–May) at all depths
between 1958 and 2008. The warming trend in soil temperatures was associated with
trends in air temperatures and snow cover depth over the same period. A significant
decreasing trend in snow cover depth in winter and spring was associated with increasing
air temperatures. The combined effects of the higher air temperature and reduced snow
depth probably resulted in an enhanced increasing trend in spring soil temperatures, but no
significant trends in winter soil temperatures. The thermal insulation by snow cover
appeared to play an important role in the response of soil temperatures to climate change
and must be accounted for in projecting future soil‐related impacts of climate change.
Citation: Qian, B., E. G. Gregorich, S. Gameda, D. W. Hopkins, and X. L. Wang (2011), Observed soil temperature trends
associated with climate change in Canada, J. Geophys. Res. , 116, D02106, doi:10.1029/2010JD015012.
1. Introduction
[2] The Intergovernmental Panel on Climate Change
(IPCC) has reported that the 100 year linear warming trend
(1906–2005) of 0.74°C [0.56–0.92°C] in global surface
temperature is larger than the corresponding trend of 0.6°C
[0.4–0.8°C] for 1901–2000 given in the IPCC TAR (Third
Assessment Report) [Intergovernmental Panel on Climate
Change (IPCC ), 2007]. The linear warming trend over the
50 years from 1956 to 2005 (0.13°C [0.10–0.16°C] per
decade) is nearly twice that for the 100 years from 1906 to
2005. This temperature increase is widespread over the globe
and is greater at higher latitudes. Furthermore, land has
warmed faster than the oceans. Zhang et al. [2000] found a
distinct pattern of temperature trends across Canada for the
period 1950–1998, showing warming in the south and west,
and cooling in the northeast; these trends were most evident
in winter and spring. They also found that precipitation had
increased by 5–35% across Canada, although significant
negative trends occurred in southern regions during winter.
The ratio of snowfall to total precipitation also increased
overall, but significant negative trends in the ratio occurred
mostly in southern Canada during spring. Decreases in snow
cover were observed over a large, well‐defined area extending
from the west coast of Canada, across the southern Prairies
and northern Great Plains, to the Great Lakes [Brown and
Goodison, 1996]. Decreased snow cover occurred in the
second half of the snow cover season, while the spring tem-
peratures were significantly higher in the Prairies [Brown and
Goodison, 1993]. The strong positive feedback between
spring snow cover and temperature through the radiative
balance [Groisman et al., 1994] suggests that decreased snow
cover has an important role in the process of climate warm-
ing. Moreover, projected warming in the 21st century shows
scenario‐independent geographical patterns similar to those
observed over the past several decades. Warming is expected
to be greatest over land and at higher latitudes, continuing
recent observed trends [IPCC,2007].
[
3] Soil temperatures have not been analyzed as much as
other climate variables, such as air temperature and precipi-
tation, because data are not widely available for either spatial
or temporal coverage. Zhang et al. [2001] analyzed long‐
term changes over the 1890s to 1990s in soil temperatures at
1
Eastern Cereal and Oilseed Research Centre, Research Branch,
Agriculture and Agri‐Food Canada, Ottawa, Ontario, Canada.
2
Scottish Crop Research Institute, Dundee, UK.
3
School of Biologic al and Environmental Sciences, University of
Stirling, Stirling, UK.
4
Climate Research Division, Science and Technology Branch,
Environment Canada, Toronto, Ontario, Canada.
Published in 2011 by the American Geophysical Union.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D02106, doi:10.1029/2010JD015012, 2011
D02106 1of16
Irkutsk, Russia, finding that soil temperature at 40 cm depth
increased by up to 9°C during the winter, while air temper-
ature increased about 4–6°C. They suggested that an increase
in snowfall during early winter (October and November)
and early snowmelt in spring might play a major role in the
increase of soil temperatures through the effects of insula-
tion and albedo changes. García‐Suárez and Butler [2006]
showed an increasing trend of 0.04–0.25°C/decade in soil
temperatures over the past century for three sites in Ireland.
Du et al. [2007] found an even greater trend (0.43–0.66°C/
decade) in seasonal mean soil temperatures at near‐surface
layers (top 40 cm) observed at Lhasa in Tibet, China, during
1961–2005, especially in spring. They also indicated that the
increasing trend in soil temperatures was stronger than that
for air temperature.
[
4] Ground surface temperature (GST), based on borehole
temperature, has been used as a proxy to reconstruct surface
air temperature (SAT) [Mann and Schmidt, 2003]. In light
of the importance of soil temperature at shallow depths as
the source of deep soil temperature variations, Hu and Feng
[2005] examined how soil temperature has been affected by
the surface air temperature and precipitation in the Eurasian
continent, based on historical soil temperature data at 153
stations in Russia, Mongolia, and China. At some locations
it was found that the relationship between air and ground
temperatures was not as straightforward as previously thought
[Beltrami and Kellman,2003;Beltrami et al., 2003, 2006].
Woodbury et al. [2009] examined long‐term SAT and GST
changes since the 1960s at eight sites west of the Canadian
cordillera, concluding that GST observations showed no
obvious climate‐induced perturbations, even though all sites
showed significant increasing trends in SAT. Their compari-
son of GST and SAT temperatures suggested that any trend
in increased SAT temperatures was masked by freeze–thaw
and latent energy effects in the winter and spring.
[
5] As an alternative to soil temperature observations,
simulated soil temperatures derived from process‐based
models have been used to study the change in soil tempera-
tures in response to climate change as characterized by
warmer air temperatures and precipitation variability. Isard
et al. [2007] studied soil temperature trends in the Great
Lakes region by examining the simulated vertical profiles
of soil water content and temperature, calculated using a
modified form of a soil water and temperature algorithm
Figure 1. Map showing the 30 stations with soil tempera-
ture data across Canada (Ottawa is at 45°23′N, 75°43′W).
Table 1. Number of Stations Hav ing Statistically Significant Trends in Annual and Seasonal Soil Temperature at Different Depths
During 1958–2008
Time Period
a
Soil Depth
5 cm 10 cm 20 cm 50 cm 100 cm 150 cm
Annual
N 8 18 17 18 16 14
NPT 4 12 9 8 10 9
NST 4 3 0 2 4 0
NSPT 3 2 0 1 4 0
Dec, Jan, Feb
N222726272826
NPT 13 14 11 14 13 15
NST 0 0 0 3 2 4
NSPT 0 0 0 1 2 1
Mar, Apr, May
N212727262826
NPT 15 20 20 19 20 17
NST 4 7 3 3 3 5
NSPT 3 6 2 1 3 3
Jun, Jul, Aug
N222727252725
NPT 15 20 18 15 18 18
NST 0 4 4 5 8 8
NSPT 0 4 3 2 8 7
Sep, Oct, Nov
N212928262926
NPT 14 19 15 11 17 16
NST 0 4 2 2 3 2
NSPT 0 2 1 1 3 2
a
N, number of stations (out of 30) available for trend analysis; NPT, number of stations with a positive (either statisti cally significant or not); NST,
number of stations with a trend (positive or negative) statistically significant at the 0.05 level; NSPT, number of stations with a significant positive trend.
QIAN ET AL.: SOIL TEMPERATURE TRENDS ACROSS CANADA D02106D02106
2of16
[Schaetzl and Isard, 1991, 1996; Isard and Schaetzl, 1993,
1995]. They modeled soil temperatures at 50 cm in the
soil profile, using 1951–2000 air temperature and precipita-
tion data from 194 National Weather Service stations in
Wisconsin and Michigan. Their modeling results suggested
that, even though winter air temperatures increased, winter
soil temperatures decreased, especially at sites downwind
from the Great Lakes, many of which are in snowbelt loca-
tions. Rising winter air temperatures over the past 50 years
coincide with (and probably have led to) more variable and
thinner snowpacks, lessening their insulating effect and
contributing to decreasing winter time soil temperatures.
[
6] Zhang et al. [2003] developed a process‐based model
of northern ecosystem soil temperature (NEST) to simulate
the transient response of soil thermal regime to climate
change in Canada. Their results [Zhang et al., 2005] show
that, depending on the location, changes in annual mean soil
temperature during the 20th century differed from those in air
temperature by −3°C to +3°C, and that the difference was
more significant in winter and spring than in summer. They
found that on average, for the whole of Canada, the annual
mean soil temperature at 20 cm depth increased by 0.6°C
while the annual mean air temperature increased by 1.0°C.
[
7] Changes in soil temperature as a result of a warmer
climate will have profound effects on terrestrial ecosystems.
Increases in soil temperatures will likely thaw permafrost in
high latitudes, change the distribution and growth of plants,
including agricultural crops, and enhance decomposition of
soil organic matter. Permafrost melting, enhanced decom-
position and some changes in plant growth generate greater
CO
2
emissions from the soil to the atmosphere, thereby
setting up a positive feedback to climate change [Trumbore
et al., 1996; Goulden et al., 1998; Nelson, 2003; Davidson
and Janssens, 2006].
[
8] Given the evidence from several studies for increasing
soil temperatures during the second half of the 20th century,
it is now important to understand the controls and feedbacks on
soil temperature, how it interacts with other climate variables,
and quantify the magnitude of the increases. Here we present
trends observed from soil temperatures at six soil depths: 5,
10, 20, 50, 100 and 150 cm, measured at 30 stations across
Canada for the period 1958–2008. Trends in other climate
variables, such as air temperature, precipitation, and depth of
snow cover at the same locations, were also observed for the
same period. We also examined the associations between
trends in soil temperature and other climate variables in Canada
to better understand the effects of future climate change on
soils and the associated biophysical and biochemical processes.
2. Data and Methods
2.1. Soil Temperature and Other Climate Data
[
9] Soil temperature is not observed as widely as other
climate variables across Canada. Measurement of soil tem-
perature started as early as 1958, with 62 stations recording
these data in 1990. In contrast, air temperature and precip-
Figure 2. Maps showing the trends across Canada of spring (March–April–May) mean soil temperature
at the depths of (a) 10 cm and (b) 100 cm and summer (June–July–August) at the depths of (c) 100 cm
and (d) 150 cm for the period 1958–2008. Upward and downward triangles show positive and negative
trends, respectively. Solid triangles indicate trends significant at the 5% level.
QIAN ET AL.: SOIL TEMPERATURE TRENDS ACROSS CANADA D02106D02106
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Table 2. Number of Stations Having Statistically Significant Trends in Monthl y Soil Temperature Across Canada During
1958–2008
a
Time Period
b
Soil Depth
5 cm 10 cm 20 cm 50 cm 100 cm 150 cm
Jan
N 24282828 29 28
NPT 13 14 16 14 12 15
NST 3 0 0 2 2 5
NSPT 2 0 0 1 2 2
Feb
N 24293030 30 29
NPT 13 16 13 15 16 15
NST 5 3 3 4 7 5
NSPT 2 2 2 2 6 2
Mar
N 24293029 30 28
NPT 16 18 21 16 17 15
NST 7 5 4 5 8 6
NSPT 4 3 2 3 6 3
Apr
N 24282927 29 27
NPT 17 23 23 18 19 17
NST 4 5 6 8 10 7
NSPT 4 5 5 6 8 4
May
N 24282827 29 27
NPT 17 20 20 20 21 20
NST 3 5 2 7 10 11
NSPT 1 5 2 5 10 9
Jun
N 25292928 29 27
NPT 15 19 19 19 25 23
NST 0 2 6 4 8 9
NSPT 0 2 4 2 8 8
Jul
N 24272727 27 25
NPT 15 17 17 16 19 18
NST 0 0 4 5 10 5
NSPT 0 0 1 2 9 3
Aug
N 23292728 28 26
NPT 16 19 17 15 21 18
NST 0 2 2 5 11 2
NSPT 0 2 1 3 10 2
Sep
N 23292826 30 27
NPT 15 23 17 14 20 18
NST 2 0 2 2 6 3
NSPT 1 0 1 1 6 2
Oct
N 24293028 30 28
NPT 14 16 17 15 17 18
NST 5 0 2 4 3 4
NSPT 4 0 1 2 3 3
Nov
N 23293028 30 28
NPT 13 18 15 14 17 18
NST 2 3 4 2 2 4
NSPT 2 2 2 1 2 3
Dec
N 23282828 29 27
NPT 15 16 12 14 18 13
NST 0 2 4 5 4 4
NSPT 0 2 2 4 4 2
a
Out of 30 stations.
b
N, number of stations (out of 30) available for trend analysis; NPT, number of stations with a positive (either statistically significant or not); NST,
number of stations with a trend (positive or negative) statistically significant at the 0.05 level; NSPT, number of stations with a significant positive trend.
QIAN ET AL.: SOIL TEMPERATURE TRENDS ACROSS CANADA D02106D02106
4of16
itation were recorded at more than 2000 stations that year,
and records go back as far as 1840 [Phillips, 1990].
[
10] For this study, Environment Canada provided daily
soil temperature data at depths of 5, 10, 20, 50, 100, and
150 cm, recorded at about 80 stations across Canada, begin-
ning between 1958 and 1991 and ending between 1969 and
2008. Almost all stations had some missing data so we
selected 30 stations from this data set (Figure 1) which the
criterion of having soil temperature data for at least 25 years
between 1958 and 2008. The stations are not evenly distrib-
uted across Canada, but they represent Canada’sregional
climate regimes well in the west coast, Prairies, central
Canada, and Atlantic Canada, with one station in the Arctic
(Resolute). Although soil temperatures were recorded twice
daily, morning (0800 h) and afternoon (1500 h), we used only
morning observations to calculate monthly mean soil tem-
peratures because there were more missing data in the after-
noon data. A monthly value was retained if soil temperature
readings were available for at least 25 d in the month; other-
wise, a missing value was assigned for the month. Seasonal
and annual means were calculated from monthly values if all
monthly values were available and, as above, missing values
were assigned when there were monthly values missing. For
interpreting the results it is important to note that most of
the stations are located in southern Canada and that only
morning observations were used in the analyses.
[
11] Monthly means of daily maximum and minimum air
temperatures, monthly rainfall, snowfall, total precipitation,
and snow cover depth at the 30 stations were derived from
daily climate data archived at Agriculture and Agri‐Food
Canada (AAFC) and provided by Environment Canada for
1958–2008. Seasonal and annual series were then created
from monthly data.
2.2. Homogenization of the Data Series
[
12] Homogeneity of climate data is often a concern,
especially in detecting and analyzing long‐term trends. For
example, it has been noticed that linear‐trend estimates are
reliable only when the data series are homogeneous in time
[Easterling and Peterson, 1995; Lu et al., 2005; Hanesiak
and Wang , 2005; Wang, 2006]. A lack of data homogene-
ity is often related to discontinuities, or change points, in
Figure 3. Trendsofmonthlymeansoiltemperaturesin(a)Marchat5cm,(b)Aprilat10cmand
(c) April at 20 cm, (d) May at 10 cm, (e) May at 100 cm, (f) May at 150 cm, (g) June at 100 cm, and
(h) June at 150 cm for the period 1958–2008. Upward and downward triangles show positive and neg-
ative trends, respectively. Solid triangles indicate trends significant at the 5% level.
QIAN ET AL.: SOIL TEMPERATURE TRENDS ACROSS CANADA D02106D02106
5of16
climate data series, which can be introduced by changes in
measurement instruments, observation locations, and mea-
surement practices and procedures. Some change points can
be identified from climate metadata, and are referred to as
documented shifts; however, metadata are often incomplete
and inaccurate. Change points may still exist in climate data
that cannot be identified from metadata; these are referred to
as undocumented shifts [Wang et al., 2007]. Since no me-
tadata were available for this study, we first had to detect
undocumented shifts in the climate data series and then
Table 3. Median of the Magnitudes of the Trends Estimated From Seasonal and Annual Mean Soil Temperature at Different Depths and
Daily Maximum and Minimum Air Temperatures (Tmax and Tmin) Across Canada During 1958–2008
Time Period
a
Soil Depth Air Temperatures
5 cm 10 cm 20 cm 50 cm 100 cm 150 cm Tmax Tmin
Annual
N 8 18 17 16 18 14 30 30
M 0.10 0.08 0.01 0.16 −0.07 0.11 0.16 0.28
Dec, Jan, Feb
N 22272628 27 26 3030
M 0.17 0.07 −0.15 −0.07 0.01 0.04 0.29 0.48
Mar, Apr, May
N 21272728 26 26 3030
M 0.30 0.27 0.28 0.29 0.26 0.29 0.35 0.39
Jun, Jul, Aug
N 22272727 25 25 3030
M 0.30 0.28 0.22 0.34 0.07 0.30 0.02 0.23
Sep, Oct, Nov
N 21292829 26 26 3030
M 0.12 0.15 0.03 0.09 −0.04 0.19 −0.02 0.00
a
N, number of stations (out of 30) with sufficient data to estimate a trend; M, the median magnitude (°C/decade) of the trends from all available stations.
Table 4. Median of the Magnitudes of the Trends Estimated From Monthly Mean Soil Temperature at Different Depths and Daily
Maximum and Minimum Air Temperatures (Tmax and Tmin) Across Canada During 1958–2008
Month
a
Soil Depth Air Temperatures
5 cm 10 cm 20 cm 50 cm 100 cm 150 cm Tmax Tmin
Jan
N2428 28 28 29 28 3030
M 0.25 0.03 0.06 0.02 −0.05 0.07 0.34 0.69
Feb
N2429 30 30 30 29 3030
M 0.04 0.10 −0.05 0.00 0.06 0.07 0.34 0.60
Mar
N2429 30 29 30 28 3030
M 0.18 0.25 0.10 0.09 0.20 0.08 0.40 0.52
Apr
N2428 29 27 29 27 3030
M 0.29 0.34 0.26 0.26 0.22 0.19 0.44 0.32
May
N2428 28 27 29 27 3030
M 0.28 0.37 0.32 0.36 0.47 0.34 0.17 0.33
Jun
N2529 29 28 29 27 3030
M 0.28 0.40 0.28 0.24 0.30 0.37 0.01 0.23
Jul
N2427 27 27 27 25 3030
M 0.14 0.19 0.19 0.17 0.32 0.27 −0.06 0.16
Aug
N2329 27 28 28 26 3030
M 0.25 0.18 0.07 0.17 0.30 0.24 0.11 0.17
Sep
N2329 28 26 30 27 3030
M 0.18 0.20 0.16 0.08 0.15 0.21 0.08 0.10
Oct
N2429 30 28 30 28 3030
M 0.10 0.07 0.04 0.03 0.13 0.17 −0.16 −0.13
Nov
N2329 30 28 30 28 3030
M 0.27 0.09 −0.01 −0.01 0.09 0.10 −0.09 0.10
Dec
N2328 28 28 29 27 3030
M 0.17 0.04 −0.02 0.01 0.10 0.00 0.18 0.31
a
N, number of stations (out of 30) with sufficient data to estimate a trend; M, the median magnitude (°C/decade) of the trends.
QIAN ET AL.: SOIL TEMPERATURE TRENDS ACROSS CANADA D02106D02106
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adjust the data series to eliminate the detected shifts by using
statistical methods. Several statistical methods have been
proposed for detecting undocumented shifts [Reeves et al.,
2007]. We used a statistical algorithm of the penalized
maximal F test accounting for autocorrelation (PMFred)
[Wang, 2008a, 2008b; Wang and Feng, 2010].
2.3. Statistical Analysis
[
13] A nonparametric Kendall’s tau‐based slope estimator
[Sen, 1968] was used to estimate trends. An iterative pro-
cedure, originally proposed by Zhang et al. [2000] and
refined by Wang and Swail [2001], was adopted to take the
effect of lag‐1 autocorrelation into account when testing the
significance level of a trend. A p value was obtained in each
test, and a trend was considered to be statistically significant
if the p value was smaller than the significance level a (e.g.,
0.05). We maximized the power conditional on the Type I
error rate being at or below some level a when testing a
single hypothesis. However, a compound error rate should
be controlle d when mult iple t esting is cond ucted [Storey,
2002]. In our study, the same test was conducted at differ-
ent locations. The null hypothesis that there is no trend in a
series can be rejected at a rate higher than the specified
significance level a even if the hypothesis is true in this
case. Therefore, we applied a false discovery rate (FDR)
controlling procedure [Benjamini and Hochberg, 1995]
when a statistical test was performed at multiple sites. In this
procedure, considering testing H
1
, H
2
, …, H
m
based on the
corresponding p values P
1
, P
2
, …, P
m
, let P
(1)
≤ P
(2)
≤ … ≤
P
(m)
be the ordered p values, and denoted by H
(i)
the null
hypothesis corresponding to P
(i)
. To define the following
Bonferroni‐type multiple‐testing procedure, let k be the
largest i for which P
(i)
≤ (i/m)q, then reject all H
(i)
i = 1, 2,
…, k for the FDR at q. We chose the FDR at q = 0.05.
[
14] Correlation analysis was performed to identify the
relationships between soil temperature variability and other
climate variables at the same locations. The FDR controlling
procedure was also taken in the tests of correlation significance.
3. Results and Discussion
[15] Change points were not very common in the data;
they were detected in less than one‐third of the monthly
data series tested, for soil temperature, air temperature and
precipitation. To account for the potential effects of these
change points, trend analysis and correlation analysis were
applied to the original data series and those that were
adjusted. Significant trends were detected at slightly more of
the stations in some cases when the adjusted series were
used. However, our conclusions are not different whether the
original or adjusted data series were used. Here we present
only results from the adjusted data, but it is important to note
that most data series were not adjusted, because change
points were not detected in the two‐thirds of the data series.
3.1. Trends in Soil Temperatures
[
16] Only a few stations had sufficient data to estimate
trends over the period 1958–2008 in annual mean soil
Figure 4. Trends in annual mean (a) daily minimum air temperature (Tmin) and (b) daily maximum air
temperature (Tmax) and (c) spring (March–April–May) mean Tmin and (d) summer (June–July–August)
mean Tmin for the period 1958–2008. Upward and downward triangles show positive and negative
trends, respectively. Solid triangles indicate trends significant at the 5% level.
QIAN ET AL.: SOIL TEMPERATURE TRENDS ACROSS CANADA D02106D02106
7of16
temperature and there were only 8 stations with data at the
5 cm depth (Table 1). It appears however that seasonal mean
soil temperatures increased in spring (March–April–May),
especially in surface layers (e.g., 10 cm) (Table 1). A positive
trend in soil temperature over time was estimated at about
two‐thirds of the stations at all depths from 5 to 150 cm. A
map of the trends across Canada of spring mean soil tem-
perature at 10 cm depth is shown in Figure 2a. Significant
soil warming was found at six stations across the country, but
a significant soil cooling was seen in Nova Scotia, in the
Maritimes. Soil cooling was observed at more locations in
eastern Canada than anywhere else; this observation is
related to several factors, one of the most important of which
is reduced snow cover in this region (discussed below). A
similar distribution of trends is shown in Figure 2b for spring
soil temperature at 100 cm, but with fewer significant trends.
Soil warming during summer (June–July–August) was more
often found in deeper layers (at 100 and 150 cm depths,
Figures 2c and 2d) than near the surface, compared to the
spring (Figures 2a and 2b). This implies that the warming
occurring in surface soil layers could take some time to reach
the deeper soil layers, but once warmed, the deeper layers
remain warmer later in the year. Significant trends were
found at fewer stations in autumn (September–October–
November) (Table 1).
[
17] Examining monthly mean soil temperatures, a sig-
nificant trend was detected at more stations in March, April,
and May than other months at the near surface layers, as
well as in May, June, July, and August in the deeper layers
(Table 2). This is consistent with the results from seasonal
means. A positive trend in soil temperature at 5 cm was
observed at most locations across the country in March
(Figure 3a). However, a significant negative trend was also
seen in eastern Canada. Several factors may contribute to
this observation. The depth of snow cover affects the near‐
surface soil temperature because of high albedo, insulation
effects, and soil‐water content related to snowmelting. The
cooling trends could also be associated with declining air
temperature in eastern Canada observed by Zhang et al.
[2000]. Significant soil warming in April soil temperatures
at 10 and 20 cm depths were found mostly in western
Canada (Figures 3b and 3c). A significant soil warming
trend in May at 10 cm was also found at stations in eastern
Canada (Figure 3d). Nevertheless, a significant warming
was detected at more stations across the country at deeper
layers, for example, in May and June at 100 and 150 cm
depths (Figures 3e–3h). It is possible that a significant trend
in soil temperatures may be more easily detected at deeper
soil layers than at near‐surface layers because soil tem-
peratures at deeper layers are less variable.
[
18] The median values of the trends across the country in
seasonal and annual mean soil temperatures are shown in
Table 3. The median values may be more representative
than the means with respect to the magnitude of the trends,
as they are less influenced by an occasional extreme values.
A warming rate of 0.26–0.30°C/decade was consistently
seen in spring (March–April–May) at all depths (5, 10, 20,
50, 100, and 150 cm). The warming rate was even higher
Table 5. Number of Stations Having Statistically Significant Trends in Annual and Seasonal Series of Climate Variables Across Canada
During 1958–2008
Time Period
a
Climate Variables
Tmax Tmin Rainfall Snowfall Precipitation Snow on Ground
Annual
N3030303030 19
NPT 24 24 26 5 23 0
NST 4 17 0 0 0 3
NSPT 2 15 0 0 0 0
Dec, Jan, Feb
N3030293030 24
NPT 25 24 17 7 7 4
NST 0 4 0 0 0 5
NSPT 0 3 0 0 0 0
Mar, Apr, May
N3030303030 24
NPT 28 28 25 8 23 1
NST 0 10 0 0 0 4
NSPT 0 9 0 0 0 0
Jun, Jul, Aug
N3030301530 7
NPT 16 25 22 8 22 4
NST 0 9 0 0 0 0
NSPT 0 7 0 0 0 0
Sep, Oct, Nov
N3030303030 23
NPT 13 15 21 11 18 6
NST 0 0 0 0 0 0
NSPT 0 0 0 0 0 0
a
N, number of stations (out of 30) available for trend analysis; NPT, number of stations with a positive (either statisti cally significant or not); NST,
number of stations with a trend (positive or negative) statistically significant at the 0.05 level; NSPT, number of stations with a significant positive trend.
QIAN ET AL.: SOIL TEMPERATURE TRENDS ACROSS CANADA D02106D02106
8of16
when evaluated on a monthly scale, e.g., the median value
of monthly mean soil temperatures ranged from 0.27 to
0.47°C/decade at different depths in May (Table 4).
3.2. Trends in Other Climate Variables
[
19] Significant increases were detected more often in
annual mean daily minimum air temperatures (Tmin) than in
daily maximum air temperatures (Tmax) (Table 5). The
significant increase in Tmin was often found in spring and
summer (Figure 4). These findings are consistent with those
of Zhang et al. [2000], who observed that air temperatures
were higher everywhere in the country except for northern
Quebec and Labrador, where it was significantly cooler.
Although no significant trends have been detected in
snowfall, a negative trend was seen in most parts of the
country in the annual and the seasonal snowfall totals in
winter and spring, accompanied by a decrease in the average
snow depth (Table 5), which occurred almost everywhere
across the country (Figure 5). A trend could not be estimated
in regions where snow did not accumulate on the ground in
most years, such as the west coast and the southernmost
part of Ontario.
[
20] On a monthly time scale, a significant positive trend
was not detected as widely as on annual and seasonal scales,
especially for mean Tmin (Table 6). Examples of the trends
in monthly mean Tmin are shown in Figure 6 for two spring
months (April, May), one midwinter month (January), and
one midsummer month (July). These trends in monthly
means are consistent with those for the annual and the sea-
sonal means.
[
21] It is useful to compare the magnitudes of the trends in
soil temperature that we found with trends in air tempera-
ture, and also to compare our results with those from sim-
ulation studies such as that by Zhang et al. [2005]. They
showed that for the whole of Canada the annual mean soil
temperature at 20 cm depth increased by 0.6°C while the
annual mean air temperature increased by 1.0°C during
the 20th century. The median magnitudes of the trends in the
annual mean soil temperatures at different depths are all
smaller than the median magnitudes of the trends in the
annual mean Tmin (Table 3). The median magnitude of the
trends in winter mean Tmin is much larger, compared with
that in soil temperatures; however, it is only slightly larger
in spring and even smaller in autumn. On a monthly scale,
the largest discrepancy was found in January, February, and
March, with a large increase in air temperatures but a much
smaller change in soil temperatures (Table 4). This is most
likely associated with the effects of snow cover because the
monthly mean depths of snow on ground in February and
March declined significantly over the period. From April
to summer, the magnitudes of the trends in monthly
mean soil temperatures were comparable or even larger
than those for the air temperatures. It is difficult to make
a direct comparison of the differences we observed
between changes in soil temperature and changes in air
temperature with those observed by Zhang et al. [2005],
because we used the median magnitude of the trends,
estimated from only a limited number of stations with
observed soil temperature data. Nevertheless, even though
Figure 5. Trends in seasonal mean snow depth on ground in (a) winter (December–January–February)
and (b) spring (March–April–May) and monthly mean in (c) February and (d ) Ma r ch fo r th e pe r io d
1958–2008. Upward and downward triangles show positive and negative trends, respectively. Solid
triangles indicate trends significant at the 5% level.
QIAN ET AL.: SOIL TEMPERATURE TRENDS ACROSS CANADA D02106D02106
9of16
the actual values are different between the two studies,
the general pattern that annual mean air temperature
exhibits a greater change than soil temperature holds true
for both studies.
3.3. Linking Soil Temperature to Other Climate
Variables
[
22] Correlations between soil temperatures at different
depths and simultaneous air temperatures (Tmax and Tmin),
Table 6. N umber of Stations Having Statistically Significant Trends in Monthly Series of Climate Variables Across Canada During
1958–2008
a
Month
b
Climate Variables
Tmax Tmin Rainfall Snowfall Precipitation Snow on Ground
Jan
N30302930 30 26
NPT 26 23 18 9 11 5
NST 0 4 0 0 0 0
NSPT 0 3 0 0 0 0
Feb
N30302930 30 26
NPT 22 25 13 9 9 5
NST 0 2 0 0 0 8
NSPT 0 1 0 0 0 0
Mar
N30302930 30 25
NPT 28 26 24 10 17 2
NST 0 0 0 0 0 5
NSPT 0 0 0 0 0 0
Apr
N30303029 30 24
NPT 27 26 25 13 21 3
NST 0 7 0 0 0 2
NSPT 0 6 0 0 0 0
May
N30303026 30 25
NPT 23 25 26 9 25 9
NST 0 4 0 0 0 0
NSPT 0 3 0 0 0 0
Jun
N30303014 30 5
NPT 17 22 24 9 23 3
NST 0 3 0 0 0 0
NSPT 0 2 0 0 0 0
Jul
N 30 30 30 5 30 2
NPT 11 22 23 3 23 2
NST 0 5 0 0 0 0
NSPT 0 4 0 0 0 0
Aug
N 30 30 30 5 30 2
NPT 20 22 13 3 14 1
NST 0 3 0 0 0 0
NSPT 0 1 0 0 0 0
Sep
N30303020 30 16
NPT 19 20 17 8 16 4
NST 0 0 0 0 0 0
NSPT 0 0 0 0 0 0
Oct
N30303030 30 26
NPT 4 10 20 16 19 12
NST 0 3 0 0 0 0
NSPT 0 0 0 0 0 0
Nov
N30302930 30 26
NPT 11 19 22 11 13 8
NST 0 0 0 0 0 0
NSPT 0 0 0 0 0 0
Dec
N30302930 30 24
NPT 20 21 13 9 9 6
NST 0 0 0 0 0 0
NSPT 0 0 0 0 0 0
a
Out of 30 stations.
b
N, number of stations (out of 30) available for trend analysis; NPT, number of stations with a positive (either statistically significant or not); NST,
number of stations with a trend (positive or negative) statistically significant at the 0.05 level; NSPT, number of stations with a significant positive trend.
QIAN ET AL.: SOIL TEMPERATURE TRENDS ACROSS CANADA D02106D02106
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rainfall, snowfall, precipitation totals, and mean snow depth
were computed on monthly, seasonal, and annual time scales.
As expected, air temperatures had a consistent influence on
soil temperatures, showing a significant positive correlation,
although this relationship was weaker in winter compared to
other seasons (Table 7). The correlations between soil tem-
peratures and both Tmax and Tmin are very similar, thus only
the results with Tmin are shown in Table 7. Because of the
influence of snow cover (i.e., soil temperatures are higher
when there is more snow on ground in winter) a significant
correlation between soil temperatures and Tmin was found
at fewer stations during the winter than any other season. It is
also apparent that the influence of the air temperature on soil
temperatures in deeper layers is weaker than in the near‐
surface layers, probably in part because there were fewer
stations with a significant correlation from the near‐surface to
the deeper layers (Table 7).
[
23] Except in northern Canada, snow cover disappears by
summer. More snow on the ground normally results in lower
soil temperatures in spring and autumn, but higher soil
temperatures in winter because of the thermal insulation
effect of snow cover. Goodrich [1982] interpreted the
influence of snow cover on the ground thermal regime in
detail. This insulation effect could explain why the warming
trend for air during the winter was not accompanied by soil
warming, because there was a significant decline in snow
depth. In contrast, spring warming trend of the air was
accompanied by both soil warming and less snow.
[
24] Such relationships between soil temperature and other
climate variables may not always occur at a particular loca-
tion, especially those associated with snow cover. For
example, there was often a significant positive correlation
between near‐surface soil temperature and air temperature
in spring, but not winter because of snow cover (Figure 7).
However, there was a significant positive correlation in
winter at the stations near the west coast, where there is rarely
any snow cover. Furthermore, near the west coast soil tem-
perature and mean snow depth were negatively correlated for
stations where there are sufficient snow data (Figure 8). Soil
temperature and snow depth were significantly negatively
correlated at one station near the west coast in midwinter
(January) when the correlations were positive at all other
stations. This may be because snow did not remain on the
ground for an extended period near the west coast (particu-
larly in the southern part) thus the thermal insulation effect
would not have been significant. In fact, the negative corre-
lation is more likely because of the effect of low air tem-
perature accompanying snow.
[
25] Significant correlations between soil temperatures
and monthly rainfall, snowfall, and total precipitation were
also observed but trends in these variables over time were
usually not significant (Table 6). We expect some relation-
ship between soil temperature and climatic variables, but
there are many factors that influence soil temperatures at a
local or site‐specific scale. In a broad‐scale study such as
this, it is difficult to tease apart and determine which specific
factors, if any, influence whether or not a relationship exists.
For example, soil profile features, such as surface organic
layers (forest floor or peat), perched water tables and ice or
sand lenses, can insulate the soil from warming in summer,
Figure 6. Trends in monthly mean daily minimum air temperature in (a) January, (b) April, (c) May, and
(d) July for the period 1958–2008. Upward and downward triangles show positive and negative trends,
respectively. Solid triangles indicate trends significant at the 5% level.
QIAN ET AL.: SOIL TEMPERATURE TRENDS ACROSS CANADA D02106D02106
11 of 16
resulting in a lower soil temperature. For example, Smith
[1975] reported cooler soils beneath an organic layer com-
pared to soil without an organic layer, and organic layers
can shield the ground thermal regime from the effects of
atmospheric climate change [Riseborough, 1985].
4. Conclusions
[26] Significant soil warming has been detected in Canada
since the late 1950s, driven mainly by warming during the
spring and summer. This warming is consistent with con-
comitant atmospheric warming. Soil warming during the
winter was often not detectable, probably because of
declining snow cover in the winter. Reduced winter snow
cover probably reduced the thermal insulation of snow on
the underlying soil during winter months, but it enhanced
soil temperature during the spring, producing soil tempera-
ture trends comparable to or larger than the trends in air
temperature. The insulation effect of snow cover should be
Table 7. Nu mber of Stations Ha ving Statistically Si gnificant Simultaneous Correlations Between Seasonal and Annual Mean Soil
Temperature and Daily Minimum Air Temperature (Tmin) and Depth of Snow on Ground Across Canada During 1958–2008
Time Period
a
Soil Depth
5 cm 10 cm 20 cm 50 cm 100 cm 150 cm
Tmin
Annual
N303030303029
NPT 29 29 30 27 30 29
NST 24 21 19 19 20 16
NSPT 24 21 19 18 20 16
Dec, Jan, Feb
N303030303029
NPT 26 27 25 27 23 21
NST 9 9 7 6 5 2
NSPT 9 9 7 6 5 2
Mar, Apr, May
N303030303029
NPT 29 30 30 29 30 26
NST 24 28 26 21 17 9
NSPT 24 28 26 21 17 9
Jun, July, Aug
N303030303029
NPT 30 29 30 28 30 28
NST 27 29 24 18 13 11
NSPT 27 29 24 18 13 11
Sep, Oct, Nov
N303030303029
NPT 30 30 30 29 30 27
NST 24 27 26 21 21 13
NSPT 24 27 26 21 21 13
Snow on Ground
Annual
N303030303029
NPT 14 15 16 17 16 19
NST 0 2 2 2 2 0
NSPT 0 1 1 1 1 0
Dec, Jan, Feb
N303030303029
NPT 25 25 23 26 25 24
NST 11 14 11 4 8 7
NSPT 11 14 11 4 8 7
Mar, Apr, May
N303030303029
NPT 10 8 10 12 13 14
NST 3 4 4 4 0 0
NSPT 0 0 0 0 0 0
Jun, Jul, Aug
N777777
NPT 2 4 4 3 4 4
NST 0 0 0 0 0 0
NSPT 0 0 0 0 0 0
Sep, Oct, Nov
N303030303029
NPT 15 12 10 8 8 7
NST 0 0 0 0 0 0
NSPT 0 0 0 0 0 0
a
N, number of stations (out of 30) available for correlation analysis; NPT, number of stations with a positive correlation (either statistically significant or
not); NST, number of stations with a correlation (positive or negative) statistically significant at the 0.05 level; NSPT, number of stations with a significant
positive correlation.
QIAN ET AL.: SOIL TEMPERATURE TRENDS ACROSS CANADA D02106D02106
12 of 16
Figure 7. Correlation between simultaneous seasonal and monthly mean soil temperature at 10 cm depth
and daily minimum air temperature (Tmin) in (a) winter (December–January–February), (b) spring
(March–April–May), (c) January, and (d) April for the period 1958–2008. Upward and downward trian-
gles show positive and negative correlations, respectively. Solid triangles indicate correlations significant
at the 5% level.
QIAN ET AL.: SOIL TEMPERATURE TRENDS ACROSS CANADA D02106D02106
13 of 16
Figure 8. Correlation between simultaneous seasonal and monthly mean soil temperature at 10 cm depth
and average depth of snow on ground in (a) winter (December–January–February), (b) spring (March–April–
May), (c) January, and (d) April for the period 1958–2008. Upward and downward triangles show positive
and negative correlations, respectively. Solid triangles indicate correlations significant at the 5% level.
QIAN ET AL.: SOIL TEMPERATURE TRENDS ACROSS CANADA D02106D02106
14 of 16
taken into account in projecting the response of soils to
climate change, because it is possible that the current sea-
sonal snow cover may disappear in some regions in the
future. Our results suggest that the thermal insulation effect
of snow was the major factor masking the increasing trend
in annual mean soil temperatures that could have resulted
from the widely observed increasing trend in annual mean
air temperatures.
[
27] In association with precipitation regimes and perhaps
other local factors, the trends in soil temperatures were dif-
ferent from one location to another across Canada. However,
the spatial distribution of trends in soil temperatures across
Canada could often be explained by trends in air temperatures
and snow cover. In response to atmospheric changes in
temperature, it was observed that trends in soil temperatures
were delayed at deeper layers (e.g., 100 and 150 cm depths);
this is a result of the temperature changes being transmitted
with depth through the soil profile. A significant trend was
detected in soil temperatures at more locations in deeper
layers than at the near surface, perhaps because of lower
variability in soil temperatures in deeper layers.
[
28] Acknowledgm ent s. The au thors thank Dirk Anderson for his
assistance in obtaining and extracting daily soil temperature data from
Environment Ca nada. We are also gratefu l to Yang Feng for his help in
using the statistical package RHtest V3.
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