5.6: Permafrost
Permafrost
temperature has increased over the past 3-4 decades (very high confidence).
Regional observations identify warming rates of about 0.1ºC per decade in the
central Mackenzie Valley and 0.3ºC to 0.5ºC per decade in the high Arctic.
Active layer thickness has increased by approximately 10% since 2000 in the
Mackenzie Valley. Widespread formation of thermokarst landforms have been
observed across northern Canada.
Increases in
mean air temperature over land underlain with permafrost are projected under
all emissions scenarios, resulting in continued permafrost warming and thawing
over large areas by mid-century (high confidence) with impacts on northern
infrastructure and the carbon cycle.
Permafrost
is an important component of the Canadian landscape, underlying about 40% of
the landmass and extending under the ocean in parts of the Canadian Arctic.
Soil properties (including both the deep mineral soil and any overlying layers
of organic matter), ground cover, and the thickness of overlying snow cover
(because of snow’s insulating properties) have important influences on ground
temperatures and, therefore, permafrost characteristics. The soil layer above
the permafrost that thaws and freezes annually is referred to as the “active
layer.”
Understanding
current permafrost conditions and how they may evolve in response to a changing
climate is essential for the assessment of climate change impacts and the
development of adaptation strategies in northern Canada. Permafrost conditions
are linked to hydrological (e.g., drainage) and land surface processes (e.g.,
erosion and slope movements); ground warming and thawing can therefore affect
ecosystems. Thawing of ice-rich permafrost results in ground instability; if
not considered in the design phase, this can affect the integrity of
infrastructure such as buildings and airstrips. Coastal communities face unique
challenges because of processes related to thawing of the shore face (Ford et
al., 2016). The northern circumpolar permafrost region holds reserves of carbon
(approximately 1000 petagrams [Pg] in the upper 3 m) as large as the total
amount of carbon in the atmosphere (Hugelius et al., 2014; Olefeldt et al.,
2016). If permafrost thaws, it could therefore release massive amounts of
greenhouse gases (carbon dioxide and methane) into the atmosphere (Romanovsky
et al., 2017a). Northern soils efficiently store mercury, which is vulnerable
to release as a consequence of permafrost thaw (Schuster et al., 2018).
Permafrost thawing can also release other compounds and dissolved material
(e.g., Kokelj and Jorgenson, 2013; Kokelj et al., 2013), including contaminants
associated with waste facilities that may depend on permafrost for containment
(e.g., Prowse et al., 2009; Thienpont et al., 2013).
5.6.1: Observed changes in permafrost
Permafrost
conditions are challenging to monitor because they cannot be directly
determined using satellite measurements. They are therefore determined largely
from in situ monitoring, which results in gaps in the spatial distribution of
measurement sites because of the relative inaccessibility of large portions of
northern Canada and historical emphasis in monitoring regions with
infrastructure development potential (such as the Mackenzie Valley; Smith et
al., 2010). Changes in permafrost conditions over the last few decades can be assessed
by tracking changes in two key indicators: permafrost temperature and thickness
of the active layer. Ground temperature, measured below the depth where it
varies from one season to the next, is a good indicator of decadal to century
changes in climate, while the active layer responds to shorter-term climate
fluctuations (Romanovsky et al., 2010).
Ground
temperature is measured in boreholes, generally up to 20 m deep, across
northern Canada. Some of these monitoring sites have been operating for more
than two decades, while many others were installed during the International
Polar Year (IPY, 2007–2009) to establish baseline measurements of the
temperature of permafrost (Smith et al., 2010; Derksen et al., 2012). A
comparison of data collected for about five years after the establishment of
the IPY baseline indicates that permafrost has warmed at many sites from the
boreal forest to the tundra (Smith et al., 2015a), with greater changes in the
colder permafrost of the eastern and high Arctic, where temperatures increased
by more than 0.5ºC at some sites over this short time period. Continued data
collection has extended the time series beyond 30 years for some sites,
allowing researchers to place the changes since IPY in the context of a longer
record.
The
temperature of warm permafrost (above −2ºC) in the central and southern
Mackenzie Valley (i.e., Norman Wells, Wrigley) has increased since the
mid-1980s, but the rate of temperature increase has generally been lower since
2000 — less than about 0.2ºC per decade. The low rate of increase is observed
because permafrost temperatures are already close to 0ºC in this region, so
energy is directed toward the latent heat required to melt ground ice rather
than raising the temperature further. In the Yukon, comparison of recent ground
temperature measurements with those made in the late 1970s and early 1980s
suggests similar warming of approximately 0.2ºC per decade (Duguay, 2013; Smith
et al., 2015b). In contrast, in the northern Mackenzie Valley (sites designated
Norris Ck and KC-07, recent increases in permafrost temperature have been up to
0.9ºC per decade, likely associated with the greater increases in surface air
temperature in this region over the last decade when compared with the southern
Mackenzie Valley (Wrigley, Norman Wells in Figure 5.17; Smith et al., 2017).
Since 2000,
high Arctic permafrost temperatures have increased at higher rates than those
observed in the sub-Arctic, ranging between 0.7ºC and 0.9ºC at 24 m depth
and more than 1.0ºC per decade at 15 m depth, consistent with greater increases
in air temperature since 2000 (Smith et al., 2015a). Short records from sites
in the Baffin region indicate warming at 10–15 m depth since 2000 (see Figure
5.17 and Table 5.1), but there has been a decline in permafrost temperatures
since 2012 (Ednie and Smith, 2015) that likely reflects lower air temperatures
in this region since 2010. In northern Quebec, where measurements at some sites
began in the early 1990s, permafrost continues to warm at rates between 0.5ºC
to 1.0ºC per decade (Smith et al., 2010; Allard et al., 2016). Permafrost can
exist at high elevations in more southerly locations. Canada’s most southerly
occurrence of permafrost, at Mont Jacques-Cartier on the Gaspé Peninsula, shows
an overall warming trend at 14 m depth of 0.2ºC per decade since 1977 (Gray et
al., 2017).
A network of
thaw tubes throughout the Mackenzie Valley has provided information on trends
in the active layer thickness (ALT) between 1991 and 2016 (see Figure 5.18;
Smith et al., 2009). ALT exhibits greater variability among years than does
deeper ground temperature, with higher values of ALT in extremely warm years
such as 1998 (Duchesne et al., 2015). ALT generally increased between 1991 and
1998 but decreased over the following decade in response to lower annual air
temperatures in the region. Since 2008, there has been a general increase in
ALT in Mackenzie Valley, with peak values in 2012 (Duchesne et al., 2015; Smith
et al., 2017). At sites where the permafrost is ice-rich, increases in summer
thawing have been accompanied by significant settlement (subsidence) of the
ground surface (Duchesne et al., 2015).
A number of
recent studies provide other evidence of changing permafrost conditions.
Observations of landscape change over time, often based on air photo or
satellite imagery interpretation, have identified areas undergoing thermokarst
processes, such as lake formation and collapse of peat plateaus and palsas
(e.g., Olefeldt et al., 2016; Kokelj and Jorgenson, 2013). Over the last 50
years in northern Quebec, there has been a loss of permafrost mounds, collapse
of lithalsas, and increases in the size of thermokarst ponds (Bouchard et al.,
2014; Beck et al., 2015; Jolivel and Allard, 2017), while palsa decay has been
observed in the Mackenzie mountains of the Northwest Territories (Mamet et al.,
2017). A recent repeat of a 1964 survey of permafrost conditions along the
Alaska Highway corridor between Whitehorse and Fort St. John indicated that permafrost
continues to persist in organic-rich soils, but is no longer found at other
sites (James et al., 2013). Changes in lake area in Old Crow Flats since 1951
have also been linked to thermokarst processes (Lantz and Turner, 2015). A
recent intensification of thaw slumping may also be tied to changes in climate,
including increases in precipitation (Kokelj et al., 2015, 2017a; Segal et al.,
2016; Rudy et al., 2017). In the southern Northwest Territories, forest die-off
has been attributed to permafrost thawing and ground subsidence (Sniderhan and
Baltzer, 2016). Erosion of Arctic coasts in the form of retrogressive thaw
slumps can result from a combination of mechanical (wave action) and thermal
(warming permafrost) processes, potentially exacerbated by sea-level rise (see
Chapter 7, Section 7.5; Ford et al., 2016; Lamoureux et al., 2015; Lantuit and
Pollard, 2008).
