5.6.2: Projected changes in
permafrost
Climate
models project large increases in mean surface temperature (approximately 8ºC)
across present-day permafrost areas by the end of the 21st century under a high
emission scenario (RCP8.5) (Koven et al., 2013) (see Chapter 3, Section 3.3.3).
While this dramatic warming will no doubt affect permafrost temperatures and
conditions (e.g., Slater and Lawrence, 2013; Guo and Wang, 2016; Chadburn et
al., 2017), it is challenging to project associated reductions in permafrost
extent from climate model simulations because of inadequate representation of
soil properties (including ice content) and uncertainties in understanding the
response of deep permafrost (which can exceed hundreds of metres below the
surface). Simulations from a model considering deeper permafrost and driven by
low and medium emission scenarios project that the area underlain by permafrost
in Canada will decline by approximately 16%–20% by 2090, relative to a 1990
baseline (Zhang et al., 2008a). These declines are smaller than projections
from other modelling studies that only examined near-surface ground temperature
(Koven et al., 2013; Slater and Lawrence, 2013). These simulations also show
that permafrost thaw would continue through the late 21st century, even if air
temperatures stabilize by mid-century (Zhang et al., 2008b).
Other
climate-related effects also influence the future response of permafrost to
warming and complicate modelling of future conditions (e.g., Kokelj et al.,
2017b; Romanovsky et al., 2017a). For example, intensification of rainfall
appears to be strongly linked to thaw slumping (Kokelj et al., 2015). New shrub
growth in the tundra can promote snow accumulation and lead to warmer winter
ground conditions (Lantz et al., 2013). Thaw and collapse of peat plateaus and
palsas into adjacent ponds increase overall permafrost degradation, and gullies
that form because of degrading ice wedges can result in thermal erosion and
further permafrost degradation (Mamet et al., 2017; Beck et al., 2015; Quinton
and Baltzer, 2013; Godin et al., 2016; Perreault et al., 2017). Damage to
vegetation and the organic layer due to wildfires (which are projected to occur
more frequently under a warming climate) can lead to warming of the ground,
increases in ALT, and degradation of permafrost (Smith et al., 2015c; Zhang et
al., 2015; Fisher et al., 2016). Similarly, vegetation clearing and surface
disturbance due to human activity and infrastructure construction can also lead
to ground warming and thawing and enhance the effects of changing air
temperatures on permafrost environments (Smith and Riseborough, 2010; Wolfe et
al., 2015).
6.2: Surface runoff: streamflow
The seasonal
timing of peak streamflow has shifted, driven by warming temperatures. Over the
last several decades in Canada, spring peak streamflow following snowmelt has
occurred earlier, with higher winter and early spring flows (high confidence).
In some areas, reduced summer flows have been observed (medium confidence).
These seasonal changes are projected to continue, with corresponding shifts
from more snowmelt-dominated regimes toward rainfall-dominated regimes (high
confidence).
There have
been no consistent trends in annual streamflow amounts across Canada as a
whole. In the future, annual flows are projected to increase in most northern
basins but decrease in southern interior continental regions (medium
confidence).
treamflow-related
floods result from many factors, and in Canada these mainly include excess
precipitation, snowmelt, ice jams, rain-on-snow, or a combination of these
factors. There have been no spatially consistent trends in these flood-causing
factors or in flooding events across the country as a whole. Projected
increases in extreme precipitation are expected to increase the potential for
future urban flooding (high confidence). Projected higher temperatures will
result in a shift toward earlier floods associated with spring snowmelt, ice jams,
and rain-on-snow events (medium confidence). It is uncertain how projected
higher temperatures and reductions in snow cover will combine to affect the
frequency and magnitude of future snowmelt-related flooding.
Canada has
more than 8500 rivers and streams of various lengths (Monk and Baird, 2011).
Many are affected by human alterations, such as flow regulation (dams, weirs,
and locks), water withdrawals, and diversions, often associated with
hydroelectric facilities (CDA, 2016). Studies on climate-related past changes
in streamflow rely heavily on data from streams that are not subject to these
forms of human regulation (i.e., unregulated) or those with limited regulation.
In a few cases, studies have attempted to account for regulation by determining
naturalized flow using various hydrological models (Peters and Buttle, 2010).
Future streamflow changes are assessed using climate output (e.g.,
precipitation and temperature) from various GCMs and/or RCMs that provide input
to a hydrological model. The multitude of climate and hydrological models used
in these studies adds uncertainty to future streamflow changes (e.g.,
Seneviratne et al., 2012).
6.2.1: Streamflow magnitude
Streamflow
magnitude (runoff) is a key indicator for evaluating changes in surface water.
It is assessed at monthly, seasonal, and annual timescales to determine changes
in overall flow volumes, and on daily to weekly scales to assess high and low
streamflow extremes. In all cases, pan-Canadian analyses are infrequent and, in
most cases, older than regional studies. For Canada as a whole, annual
streamflow trends were mixed. Significant declines occurred at 11% of stations
and significant increases at 4% of stations for the 1967–1996 period. Most
decreases were in southern Canada (Zhang et al., 2001; similar results in Burn
and Hag Elnur, 2002). Seasonal runoff over the 1970–2005 period, in most of the
172 stations evaluated, was dominated by natural variability. Twelve per cent
of stations showed significant increases in winter runoff (December to
February), while only 5% had significant winter decreases. Spring and summer
trends were mixed, with no spatial pattern (Monk et al., 2011). From 1960 to
1997, significant increases in April flow occurred at almost 20% of stations,
and significant decreases in summer flow (May to September) were observed at
14% of the sites (Burn and Hag Elnur, 2002). The increases in April flow were
also found (25% of stations) for the longer 1950–2012 period (Vincent et al.,
2015).
Regional
studies of trends in annual and seasonal streamflow magnitudes are summarized.
Although these individual studies use different time periods, hydrometric
stations, and analysis techniques, the findings are mostly consistent with the
Canada-wide analyses. Annual flows over western Canada have varied from one
region to another, with both increasing and decreasing trends since
approximately the 1960s and 1970s (e.g., DeBeer et al., 2016). Most declines
were observed in rivers draining the eastern slopes of the central/southern Rocky
Mountains, including the Athabasca, Peace, Red Deer, Elbow, and Oldman rivers
(Burn et al., 2004a; Rood et al., 2005; Schindler and Donahue, 2006; St.
Jacques et al., 2010; Yip et al., 2012; Peters et al., 2013; Bawden et al.,
2014). Long-term streamflow records (over more than 30 years) from the
Northwest Territories, including the Mackenzie River, indicated increasing
annual flows (St. Jacques and Sauchyn, 2009; Rood et al., 2017). However,
annual runoff from rivers draining into northern Canada as a whole (western
Arctic Ocean, western Hudson and James Bay, and Labrador Sea) showed no
significant trends for the period 1964–2013 (Déry et al., 2016). Rivers in
Yukon, British Columbia, Ontario, and Quebec reported mixed annual trends
(Fleming and Clarke, 2003; Brabets and Walvoord, 2009; Fleming, 2010; Fleming
and Weber, 2012; Déry et al., 2012; Nalley et al., 2012; Hernández-Henríquez et
al., 2017).
Seasonally,
there has been a consistent pattern of increasing winter flows in many regions
(see Table 6.1), particularly for more northern basins, such as the Mackenzie
and Yukon rivers and those draining into Hudson Bay. Summer flows have been
generally declining over most regions of Canada, although the declines are not
as widespread as for winter. Note that these studies are mostly consistent on
the direction of change, but there are large differences in the rate of these
changes. Many of these regional trends in flow were linked to precipitation
trends or variability affecting the entire basin, although winter warming and
associated snowmelt explained several of the increases in winter/early spring
flow (e.g., DeBeer et al., 2016). In addition, several flows were associated
with naturally occurring internal climate variability (mainly El Niño–Southern
Oscillation, Pacific Decadal Oscillation [PDO], and Arctic Oscillation [AO];
see Chapter 2, Box 2.5), particularly for western Canada during winter (e.g.,
Bonsal and Shabbar, 2008; Whitfield et al., 2010), the Mackenzie Basin (St.
Jacques and Sauchyn, 2009), and rivers draining into Hudson Bay (Déry and Wood,
2004).
Changes in
extreme short-term streamflow are important indicators of flood risk. One-day
maximum flow magnitudes (the highest one-day flow recorded during the year)
from 1970 to 2005 revealed that 11% of hydrometric sites across Canada have
significantly decreasing trends (lower maximum flow levels), while only less
than 4% have increasing trends (higher maximum flow levels) (Monk et al.,
2011). A more recent study using an expanded set of RHBN stations (280) for the
1961–2010 period yielded very similar results, with 10% of the sites showing
significant decreases and less than 4% significant increases (Burn and
Whitfield, 2016) (see Figure 6.3).
Equally
important to aquatic ecosystems and society are low flows, since they represent
periods of decreased water availability. More stations show significantly lower
one-day minimum flow trends (18%) than show significantly higher ones (8%) (see
Figure 6.4) (Monk et al., 2011). Results from a smaller subset of RHBN stations
(Ehsanzadeh and Adamowski, 2007, for 1961–2000 and Burn et al., 2010, for
1967–2006) revealed similar tendencies in seven-day low flows, with more sites
having significantly lower values (36% and 18% for each study, respectively)
than higher values (7% and 5%, respectively).
Another
indicator of freshwater availability is baseflow, the portion of streamflow
resulting from seepage of water from the ground (related to groundwater; see
Section 6.5). Baseflow often sustains river water supply during low-flow
periods. For the vast majority of sites in Canada, annual baseflow trends did
not significantly change from 1966 to 2005 (Rivard et al., 2009). However, an
analysis of a baseflow index across Canada found some locations with
significantly decreasing trends (11% of stations) and others with increasing
trends (9%) (Monk et al., 2011). Additionally, in northwestern Canada, winter
baseflow has increased significantly in 39% of the 23 rivers analyzed. The
likely explanation is enhanced water infiltration from permafrost thawing due
to climate warming (St. Jacques and Sauchyn, 2009).
Only one
published study directly attributed changes in streamflow magnitude within
Canada to anthropogenic climate change. This included recent observed declines
in summer (June–August) streamflow in four British Columbia rivers (Najafi et
al., 2017b). The decreases were due to smaller late-spring snowpacks (and
consequently, lower summer runoff), which were attributed to the human
influence on warming of cold-season temperatures (Najafi et al., 2017a) (see
Chapter 4, Section 4.3.1.2).
Projected
future changes in Canadian streamflow magnitudes have not been extensively
examined on a national scale, although several regional assessments have been
conducted (see Table 6.2 and Figure 6.5). The majority of these studies are
based on the third phase of the Coupled Model Intercomparison Project (CMIP3)
climate models and SRES emission scenarios (see Chapter 3, Section 3.3) unless
otherwise specified. The findings are mostly consistent on the direction of
change, although there are large uncertainties in magnitude. In general, for
the mid-21st century, watersheds in British Columbia and northern Alberta are
projected to have increases in annual and winter runoff, whereas some watersheds
in Alberta, southwest British Columbia, and southern Ontario are projected to
have declines in summer flow (Kerkhoven and Gan, 2011; Poitras et al., 2011;
Bennett et al., 2012; Bohrn, 2012; Harma et al., 2012; Schnorbus et al., 2011;
2014; Shrestha et al., 2012a; Eum et al., 2017; Islam et al., 2017). In the
Prairie region, most rivers in southern Alberta and Saskatchewan are projected
to have decreases in both annual and summer runoff (Lapp et al., 2009; Shepherd
et al., 2010; Forbes et al., 2011; Kerkhoven and Gan, 2011; Kienzle et al.,
2012; Tanzeeba and Gan, 2012; St. Jacques et al., 2013, 2017). However, rivers
in southern and northern Manitoba are projected to have increasing flow
(Poitras et al., 2011; Shrestha et al., 2012b; Stantec, 2012). Projected
changes in future annual runoff are mixed in Ontario (EBNFLO Environmental and
AquaResource Inc., 2010; Grillakis et al., 2011), while in Quebec the majority
of studies project increasing annual flows (Quilbe et al., 2008; Minville et
al., 2008, 2010; Boyer et al., 2010; Chen et al., 2011; Guay et al., 2015). A
Quebec study using several models from the fifth phase of the Coupled Model
Intercomparison Project (CMIP5) found that mid-century (2041–2070) flows for
southern rivers under both medium and high emission scenarios (RCP4.5 and
RCP8.5) will be characterized by earlier and smaller spring peak flow and lower
summer runoff. Annual mean flow is anticipated to increase in northern regions
and decrease in the south (CEHQ, 2015). Annual streamflow is projected to
increase for New Brunswick (El-Jabi et al., 2013) and Labrador (Roberts et al.,
2012). In northwestern Canada, there is evidence that watersheds such as the
Mackenzie and Yukon river basins will see an increase in annual flow, mainly
due to the higher precipitation amounts projected at higher latitudes (e.g.,
Poitras et al., 2011; Thorne, 2011; Vetter et al., 2017).
