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Threats to Water Availability in Canada

14. Climate Variability and Change - Crysophere

Ross D. Brown,1 Michael N. Demuth,2 Barry E. Goodison,1 Philip Marsh,3 Terry D. Prowse,4 Sharon L. Smith2 and Ming-ko (Hok) Woo5

1 Environment Canada, Meteorological Service of Canada, Dorval, QC
2 Natural Resources Canada, Geological Survey of Canada, Ottawa, ON
3 Environment Canada, National Water Research Institute, Saskatoon, SK
4 Environment Canada, National Water Research Institute, Victoria, BC
5 McMaster University, Department of Geography, Hamilton, ON

 


Current Status

The cryosphere (snow, ice, glaciers and frozen ground/permafrost) is one of the most important components of the Canadian environment. Most of the country experiences several months of snow cover, virtually the entire country experiences seasonally frozen ground, almost half of the terrestrial land surface is underlain by permafrost, most water bodies have a winter ice cover, and glaciers and ice caps constitute the largest volume of land ice in the Northern Hemisphere outside Greenland. The hydrology of most Canadian river basins is dominated by phase changes and seasonal long-term storages involving the cryosphere and cryospheric processes (Woo, 1996).

Cryospheric responses to warming (e.g., an increase in the fraction of precipitation received as rainfall, reduced snow and ice cover duration, disappearance of mountain glaciers, increase in active layer thickness, melting of ground ice) will have important consequences for regional hydrological cycles, particularly in permafrost environments (Woo et al., 1992; Woo, 1996; Rouse et al., 1997). However, quantifying this response and its impact on the hydrological cycle and freshwater supply is a challenge as the cryosphere is characterized by complex linkages and feedbacks, in addition to variable time lags and storages. Current fully coupled global climate models do not incorporate many of the important cryospheric processes for terrestrial areas, or they display important limitations in their ability to simulate key characteristics of cold region climates (Allison et al., 2001). The purpose of this chapter is to assess the current state of understanding of the cryosphere and its role in hydrological systems in Canada, document the cryospheric response to recent warming and implications for freshwater resources, and present a summary of key knowledge gaps and program needs.

Precipitation: Hydrological models require accurate information on precipitation amount (adjusted for systematic errors, e.g., Goodison et al., 1998), timing, and type. Winter precipitation, especially snow, is notoriously difficult to measure accurately, and changes in methods of observations, including automation and network reductions, compromise our capacity to define adequately the spatial and temporal variability of winter precipitation across Canada. While some effort has been made to develop gridded precipitation datasets for hydrological studies (e.g., Louie et al., 2002), these are currently only available at monthly time steps. Operational weather forecast models and reanalysis projects have precipitation products at the required spatial and temporal scales, but the accuracy of these datasets is a concern. In addition, large uncertainties in climate model precipitation projections pose similar concerns for investigating cryospheric and hydrological responses to global warming.

Snow Cover: Accurate information on the spatial and temporal evolution of snow cover (extent, depth, snow water equivalent) is required to monitor changes in climate and hydrological systems, as well as for input to a range of operational needs such as drought monitoring, forest fire potential, flood and flow forecasting, and initialization of weather and hydrological models. Canada has extensive in situ snow depth and snow course networks, but these are concentrated in southern latitudes and lower elevations, and experienced major contractions during the 1990s (Brown et al., 2000). In spite of these limitations, in situ data represent a valuable database for monitoring regional variability and change, and for validating satellite products and climate-weather-hydrology model output.

A variety of satellite datasets is available for mapping snow cover extent and snow water equivalent (SWE). Weekly SWE maps derived from SSM/I passive microwave satellite data are provided in near-real time for the Canadian Prairie Provinces (Goodison and Walker, 1995) at a 25-km resolution. The resolution will increase to 10 km when AMSR-E satellite data become available in 2003. Research to extend this product to other regions of Canada has shown some success (Goïta et al., 2003), but densely forested, mountainous and tundra regions remain challenges.

A range of energy balance models of varying sophistication is available for modelling seasonal snowpack accumulation and ablation, varying from a single layer representation of snow cover (e.g., CLASS - Verseghy, 1991) to multi-layer models that take account of snow grain size evolution (e.g., CROCUS - Brun et al., 1992). Validation studies (e.g., Slater et al., 2001) show that land surface process and snow models are able to capture the broad features of snow cover (duration, depth and SWE) and snowmelt regimes on both an intra- and inter-annual basis for open grass-covered sites. However, modelling the spatial variability of snow cover poses more of a challenge because this requires consideration of blowing snow transport and sublimation, canopy interception and loss, and parameterizations to handle patchy snow conditions. Significant recent advances have been made in understanding these processes at the site scale (Marsh, 1999; Woo et al., 2000). However, further work is needed to incorporate results into land surface process and climate models where surface properties and processes need to be treated at larger spatial scales, e.g., 10-100 km.

Interaction of snow and vegetation is important for water resources (Greene et al., 1999). For example, prairie farmers routinely exploit the snow-trapping properties of grain stubble to increase the amount of snow retained on the surface to enhance soil moisture recharge and runoff (Steppuhn, 1981). Forest management practices also influence runoff as outlined in Chapter 8. Recent climate-induced changes to vegetation are modifying water budgets and soil thermal regimes, and it is expected these changes will continue in the future. For example, a transition in vegetation over northern Canada from tundra to shrub-tundra is estimated to generate a deeper, more evenly distributed snow cover, with greater meltwater production (Sturm et al., 2001; Liston et al., 2002). However, the annual change in water supply due to such a vegetation change is more difficult to predict.

Frozen Ground/Permafrost: Frozen ground plays an important role in hydrology through its influence on infiltration, runoff, and groundwater storage and flow. Frozen ground also has indirect influences on hydrology through rooting zone depth (important for vegetation succession and growth) and length of the thaw season (important for evapotranspiration and carbon uptake). In areas with seasonally frozen ground, potential reductions in frost penetration and the duration of soil freezing in response to warming will affect water supply through increased potential for evapotranspiration and changes to the runoff regime. In northern environments, permafrost plays an important role in the water/moisture balance (e.g., control of water table height) as well as the surface energy balance (e.g., delaying the melt runoff process) (Marsh and Woo, 1984).

Large quantities of moisture in permafrost are locked in as ground ice, and thawing will provide additional water. Arctic warming is likely to be associated with an increase in thaw duration and thickness of the surface thaw layer ("active layer") and disappearance of permafrost in some locations (Kane et al., 1991; Anisimov et al., 1997). However, local factors (snow, vegetation, organic layer, proximity to water bodies) add complexity to the ground thermal response (Smith and Riseborough, 1983). Reductions in winter snow depth (observed over much of Canada in the post-1976 period), for example, result in less insulation of the ground layer, which could offset the effect of warmer winter air temperatures.

Extensive ground temperature and active layer monitoring networks in Canada have permitted mapping of ground temperature, permafrost distribution and ground ice conditions (Smith et al., 2001a), and are important data sources for GCM model validations, permafrost sensitivity modelling (Smith and Burgess, 1998) and mapping and impact assessments. Remote sensing also offers capabilities for monitoring the freeze/thaw status of surface soil and changes in surface water area that may be associated with thermokarst development.

The hydrological implications of changes to areas with permafrost are summarized in Woo (1996). These include greater storage capacity for groundwater from a thickened active layer, lowering of the water table, enhanced vertical drainage, and greater potential for evaporation. Differential thaw settlement related to variation in ground ice content can lead to thermokarst topography and exert an influence on distribution of surface water and on drainage patterns. Such changes would modify the runoff regime (e.g., lower spring runoff peak, increased and longer baseflow) and the hydrological behaviour of northern wetlands (e.g., some wetland areas may drain, while some lakes may expand). A study that employed satellite monitoring of thermokarst lakes in the Old Crow Flats area of the Yukon revealed evidence of a drying trend since the early 1970s (Labrecque and Duguay, 2001). In other areas of northwest Canada, lakes in regions with high ground ice content may catastrophically drain (Mackay, 1992; Marsh and Neumann, 2001) due to changes in lake water balance, active layer thickness or slope instability. Potential increases in regional groundwater flow from thawing ground ice may offset the effect of increased evaporation. However, there may be issues of water quality tied to an increased exchange between surface and ground water, as groundwater can have a high dissolved load (Michel and van Everdingen, 1994).

Taliks may develop beneath the active layer if the active layer does not completely freeze back during winter and groundwater may be sustained throughout the winter (Hinzman and Kane, 1992). This could lead to higher winter stream levels, more ice formation, and greater possibility of flooding during breakup (Woo et al., 1992). Slope failures along rivers and streams associated with thaw of ice-rich permafrost would result in increased siltation and may result in damming of rivers and possible upstream flooding (Aylsworth et al., 2000).

Glaciers: The mountain glaciers and icefields in the Cordillera are an important freshwater source (drinking water, irrigation, industrial use, fisheries and hydropower), and there are significant hydrological and ecosystem impacts involved in a reduction or loss of this supply. Runoff from arctic glaciers and ice caps is less important in terms of water supply, but does contribute to sea level rise, and plays a role in near-shore nutrient stratification in the water column and the local sea-ice regime. Hopkinson and Young (1998) determined that glacier wastage in the Bow River at Banff contributed 13% of summer flow in low-flow years, with the contribution exceeding 50% in extreme low-flow years. The glaciers feeding the Bow River have undergone extensive retreat and there is evidence (Moore and Demuth, 2001; Demuth et al., 2002b) that glacier melt contribution is declining. This raises concerns about the sustainability of summer flows in this region of Canada, particularly in light of uncertainties in the volume of ice currently stored in glaciers.

Currently, annual mass balance measurements are carried out at five sites in the Cordillera as part of the National Glaciology Program (Demuth, 1996). Monitoring sites are selected to represent major glacier-climate response regions, but there are several important gaps in the network (Cogley and Adams, 1998; Demuth and Koerner, 2001). These historical in situ measurements are indispensable (e.g., for validation of models and satellite algorithms), but need to be augmented with remotely sensed information and numerical modelling to assess glacier response to climate variability and change. Recent advances have been made in applying remote sensing to glacier monitoring (Cogley et al., 2001; Copland et al., 2002; Demuth et al., 2002a, submitted for publication), and work has started on generating baseline information on the areal extent of selected glaciers in the Canadian Rockies, Selkirk range and coastal Cordillera of northern B.C. (Sidjak and Wheate, 1999).

Recent advances in glacier-runoff modelling have been made by combining remotely sensed and in situ data (Brugman and Pietroniro, 1995; Rott et al., 2000), but the processes take place at scales not yet resolved by most regional or global climate models. Investigating the future response of cordilleran glaciers to climate change will therefore require development of methods for downscaling climate model output to individual ice masses. Moreover, representation of glacier cover within distributed hydrological models requires further development. An improved understanding of glacier hydrology is also needed with respect to glacier hazards. Outburst floods ("jokulhlaup events") from supraglacial, ice marginal and englacial/subglacial sources are a concern in high mountain areas, particularly during periods of rapid glacier wasting. The dynamic component of glacier response to climate change must also be considered, since changing glacier geometry feeds back to the flow regime, which in turn affects the evolution of glacier morphology. Changes in water flux through glaciers may also affect the nature of subglacial drainage systems and runoff response.

Evaporation/Sublimation: The loss of snow mass from sublimation of blowing snow is important in exposed landcover regions such as prairie and tundra. Pomeroy and Gray (1995) estimated sublimation loss over prairie environments to be 15-41% of annual snowfall. They also estimated that approximately one-third of total snowfall falling on spruce and pine was lost through canopy sublimation. More recent modelling work (Dery and Yau, 2001) showed that computed sublimation rates from blowing snow were highly sensitive to assimilation of humidity measurements and evolving thermodynamic fields in the atmospheric boundary layer during blowing snow events. Accurate estimates of sublimation loss at the basin scale are needed to close the water budget. However, there still appear to be major uncertainties in the science of blowing snow sublimation such as the threshold wind speed and particle number density and size distribution near the surface of a layer of blowing snow (Xiao and Taylor, 2002). Detailed observational data are required to address these uncertainties.

As noted in other sections, the cryosphere is an important control of surface evaporation, and reductions in ice and snow cover accompanying warming are likely to be associated with a corresponding increased potential for evaporative losses. This has important implications for lakes (Chapter 12), wetlands (Chapter 13) and agricultural regions.

Freshwater Ice: The duration and thickness of ice cover exert important influences on evaporation, river and lake discharge, ecology, and flooding from ice jams. Potential changes in ice-cover climate (and discharge) will also affect river and on-ice transportation. Recent reviews of the biological and hydrological aspects of river ice were provided by Prowse (2001b,c) and Prowse and Beltaos (2002). The ice-jam process is especially significant in Canada, not only because of the potential for flooding and property damage (Chapter 4), but also because it is an important natural process in cold region river basins that is intimately linked with river ecology (Prowse and Culp, 2003) and the freshwater pulse into the Arctic basin (e.g., Lewis et al., 2000).

Projected warming over Canada is likely to be associated with a reduction in ice cover and an increase in the evaporation season. Ice cover is also implicated in local climate (e.g., lake effect snowfall), which includes the marine cryosphere in the case of Hudson Bay, where sea ice has an important influence on snowfall accumulation over one of the main hydroelectricity producing regions of Quebec. Ice formation and decay are sensitive to changes in climatic conditions, and Beltaos and Prowse (2001) found that increased incidence of mid-winter melt and associated breakup events in some temperate regions of Canada could actually enhance the frequency and severity of ice jams.

A variety of in situ observations for lake and river ice has been collected in Canada (see summary in Brown et al., 2002). In general, the in situ databases are characterized by relatively short periods of record (little data prior to 1950) with major contractions in networks during the 1990s. Canada's in situ networks are no longer adequate to provide the primary information desired for lake ice monitoring. Remote sensing offers a viable alternative observing strategy, but the satellite data record is still too short for documenting variability and change. This situation requires the development of approaches to merge available in situ and satellite observations to create a consistent, long-term time series of lake ice freeze-up/breakup and ice cover processes. River ice is more difficult to monitor routinely, but high-resolution SAR data have been used successfully in experimental trials (Pietroniro and Leconte, 2000).

Considerable progress has been made in our ability to model formation and decay of lake ice, and one-dimensional thermodynamic lake ice models (e.g., Ménard et al., 2002) have been able to reproduce observed variability in lake ice cover using local meteorological forcing data. Similar progress has been made in modelling the hydraulic effects of ice cover on river discharge (Hicks and Healy, 2003) and the ice-jamming process (Beltaos, 1995). Modification of the ice-jam flood process through flow regulation has also been suggested and field tested as an adaptation strategy for dealing with the drying effects of climate change (Prowse 2001a; Prowse et al., 2002b).

Cryospheric Response to Recent Climate Warming

Glaciers: There is widespread evidence of a major retreat of small alpine and continental glaciers in response to twentieth century warming (Dyurgerov and Meier, 1997; Cogley and Adams, 1998). Demuth and Keller (2002) documented recent acceleration of glacier contraction in Canada's southern Cordillera consistent with global trends, and Demuth et al. (2002b) documented a dramatic contraction of outlet glaciers and glacier cover over the eastern slopes of the Rocky Mountains since the Neo-glacial maximum stage (ca. 1850). These decreases in the areal extent of glacier ice cover during the twentieth century have been accompanied by corresponding decreases in the contribution to downstream summer flow volumes to the western Prairies, which exacerbate the impact of drought conditions in this region of Canada because less water is available from irrigation reservoirs on major rivers.

It has been suggested that a temporary period of enhanced flow from glacier sources may be established when large areas of upland ice (e.g., Columbia Icefield) are subject to persistent melting conditions (Demuth et al., 2002c). However, forecasting the timing, duration and magnitude of this enhanced glacier contribution to runoff is complicated by uncertainties in how temperature and precipitation regimes may change at higher elevations, as well as by the dynamic component of glacier response. There is also the possibility of major shifts in drainage divides and even in the basins to which glacierized areas contribute runoff when major icefields are melting.

Glacier mass balance over the Cordillera is also strongly influenced by variations in atmospheric circulation, notably the Pacific Decadal Oscillation (PDO) and Pacific-North America (PNA) patterns (Moore and Demuth, 2001; Demuth and Keller, 2002). Documenting the sensitivity of glaciers to these modes of atmospheric variability is important since this response will be superimposed on greenhouse gas-induced climate changes, and because certain modes of atmospheric variability may be more persistent under a warmer climate (IPCC, 2001).

Snow Cover (SWE, depth, extent): Analysis of in situ snow depth data showed the post-1970s period was characterized by significant reductions in winter snow depth and spring snow cover (earlier melt) over much of the country and in western Canada in particular (Brown and Braaten, 1998). This finding was confirmed by satellite data from the early 1970s that showed extensive reductions in spring season snow cover over western Canada and the Arctic. The satellite data show little change in snow cover at the start of the snow season, although in situ data from the early 2000s are starting to show evidence of a delay to the start of the snow season. Brown (2000) reported somewhat similar contrasting trends in estimated SWE over mid-latitudes of North America, with winter showing significant increases in SWE over the 1915-1992 period in response to increasing precipitation, and spring showing a significant decrease over the same period (Fig. 1). The contrasting seasonal response is consistent with the observations of Groisman et al. (1994) that snow cover exerts the strongest feedback to the earth radiation balance in the spring period.

Figure 1: Historical variation in estimated snow water equivalent for December and April over the mid-latitude region of North America

Fig. 1 Historical variation in estimated snow water equivalent for December and April over the mid-latitude region of North America. Source: Brown(2000).

The response of mountain snowpacks to a changing climate is a major concern over western Canada where snowmelt runoff is a key component in reservoir recharge. Moore and McKendry (1996) showed that snowpack conditions in southern British Columbia were dominated by atmospheric circulation patterns linked to decadal-scale shifts in sea surface temperatures. They also found evidence of an abrupt shift to less winter snow accumulation after 1976, which coincided with a well-documented shift in the Pacific-North America (PNA) teleconnection pattern to more positive values (Leathers and Palecki, 1992). This shift has been associated with reduced snow cover, earlier runoff, and more negative glacier mass balances over much of western North America. Brown (1998) showed that ENSO was responsible for significant anomalies in regional snow cover over western Canada with El Niño associated with below-average winter snow cover extent, and La Niña with above-average SWE. Abrupt shifts in atmospheric circulation such as the 1976 change in the PNA pattern and a recent tendency toward more frequent El Niño events add an additional level of uncertainty onto the regional snow cover response to global climate warming.

Freshwater Ice: Magnuson et al. (2000) documented a hemispheric-wide trend toward earlier breakup of river ice consistent with a widespread cryospheric response to twentieth century warming (IPCC, 2001). Within Canada, the available river and lake freeze-up/breakup data suggest there is more regional complexity to this trend. Zhang et al. (2001) analyzed a 249-station subset of data from the Reference Hydrometric Basin Network to infer information on river freeze-up/breakup trends across Canada over the 1947-1996 period. Results showed an interesting regional difference with rivers over western Canada generally showing trends toward earlier breakup, and rivers over the Maritimes showing later breakup. The patterns have been linked to changes in seasonal warming (0°C-isotherm) and specific atmospheric circulation patterns (Bonsal and Prowse, 2003; Bonsal et al., 2001; Prowse et al., 2002a). Marsh et al. (2002) documented a similar trend towards earlier breakup of the Mackenzie River near its mouth, but did not find a trend in the magnitude of breakup. Further evidence of important regional differences in lake ice freeze-up and breakup trends were provided by Duguay et al. (2002).

Frozen Ground/Permafrost: Young and Woo (2002) argued that it is very difficult to separate the freeze-thaw response of frozen ground to interdecadal climate variability and climatic trend. Recent analyses (IPCC, 2001) revealed that permafrost in many regions of the earth is warming; however, monitoring of shallow permafrost only began in earnest over the last few decades. In Canada, the onset, magnitude, and rate of warming was shown to vary regionally due to different regional climates and effects of snow cover and surface layer properties. For example, the Mackenzie Delta and High Arctic regions showed evidence of recent permafrost warming (Romanovsky et al., 2002), while parts of northeastern and northwestern Canada showed evidence of recent permafrost cooling (Allard et al., 1995; Burn, 1998). There is also evidence of increased thaw depths in the Mackenzie Delta. Large increases were associated with the extreme warming encountered during the summer of 1998. At sites with ice-rich soil, the increase in thaw penetration was accompanied by significant ground subsidence and, hence, very little change in active layer thickness (Smith et al., 2001b; Wolfe et al., 2000). Recent evidence of permafrost melting in the northern Prairie Provinces was reported by Beilman et al. (2001).

Bow Glacier, Canadian Rockies, Alberta | Photo: Chris Hopkinson

Knowledge and Program Needs

Data/Monitoring Needs

  • Gridded, high-resolution (~10 km) spatially representative precipitation data at a 6-hourly interval (or better) are needed for running climate and hydrological models. Observed precipitation datasets should be corrected for systematic errors and a concerted effort made to understand the impact of automation (e.g., through intercomparisons of manual and autogauge measurements under operational conditions).
  • Reliable cryospheric datasets of variables such as SWE and ice cover are needed for documenting variability and change, and for model input and validation. This requires the combination of in situ, satellite and model-derived information, as well as application of data assimilation methods. Accurate datasets covering a range of spatial scales are urgently needed for development and validation of scaling approaches.
  • A systematic approach should be implemented to determine the amount of water stored in cordilleran glaciers and the rate at which it is melting. This will require satellite monitoring of areal extent for decadal reassessments of glacier area, development of upscaling/downscaling methods to perform regional mass balance simulations (validated with field or remote sensing data), and systematic (approximately 5-year interval) airborne laser altimetry surveying to detect elevation changes. Digitizing of historical data at the Canadian Glacier Inventory is needed to document glacier response to climate over the past 40-50 years (Munro, 2000).
  • Basic research is required to understand the interrelationships and feedbacks between cryospheric and hydrological systems and provide reliable estimates of changes in water yields in response to a changing climate. The WCRP Climate and Cryosphere (CliC) project implementation plan proposed development of field campaigns at a number of “Super Sites” to improve knowledge of cold climate processes and interactions and to develop parameterizations of important cryospheric processes: for example, sublimation, snow-vegetation interactions, and the role of frozen ground in infiltration and drainage for coupled climate-hydrological models. The “Super Site” concept can also be used to validate remotely sensed products, evaluate measurement systems, and investigate scaling issues.

Modelling Needs

  • Controlled intercomparisons of coupled climate-hydrological models with high quality Canadian basin-scale datasets are needed. This entails a major effort to develop the required surface property and model forcing fields.
  • We need a better understanding of the role increased regional groundwater flow from ground ice melt may have in the water balance, as well as in determining the future permafrost distribution under climate warming. In addition, an improved understanding of the development of more coherent drainage systems in peatlands/wetlands in response to thawing is required. This is important in identifying water routing pathways and for future carbon sources and sinks (Chapter 13).
  • New knowledge of cryospheric processes such as frozen soil infiltration should be incorporated into Canadian coupled climate-hydrological models (Woo et al., 2000).
  • Regional climate and hydrological model runs for current and future climate conditions are required for testing our ability to model the northern cryosphere and for considering future changes (e.g., Mackenzie GEWEX Study - MAGS).
  • Determining the future response of glaciers to climate change requires new approaches and methods for generating realistic future climates in mountainous regions (e.g., statistical downscaling and nested regional climate models). It also requires a new type of glacier model that includes the higher order stress terms important for flow over complex topography, but at the same time can be run at a regional scale.

Other Issues

  • Canada is losing its field expertise in cryospheric and hydrological science. Improved field training and incentives for young scientists to work in snow and ice are urgently needed.
  • Dwindling logistical support for northern research activities is having a debilitating impact on Canada’s capacity to carry out fundamental scientific research on northern climate and hydrological systems. This situation contributes to the loss of field expertise described above.

Acknowledgements

The authors are very grateful for the valuable review comments provided by external reviewers Wayne Rouse (McMaster University), Martin Sharp (University of Alberta), and Fred Wrona (NWRI).

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