Wayne R. Rouse

EFFECTS OF CLIMATE CHANGE ON THE FRESHWATERS OF ARCTIC AND SUBARCTIC NORTH AMERICA

Region 2 comprises arctic and subarctic North America and is underlain by continuous or discontinuous permafrost. Its freshwater systems are dominated by a low energy environment and cold region processes. Central northern areas are almost totally influenced by arctic air masses while Pacific air becomes more prominent in the west, Atlantic air in the east and southern air masses at the lower latitudes. Air mass changes will play an important role in precipitation changes associated with climate warming. The snow season in the region is prolonged resulting in long-term storage of water so that the spring flood is often the major hydrological event of the year, even though, annual rainfall usually exceeds annual snowfall. The unique character of ponds and lakes is a result of the long frozen period, which affects nutrient status and gas exchange during the cold season and during thaw. GCM models are in close agreement for this region and predict temperature increases as large as 4°C in summer and 9°C in winter for a 2 × CO2 scenario. Palaeoclimate indicators support the probability that substantial temperature increases have occurred previously during the Holocene. The historical record indicates a temperature increase of > 1°C in parts of the region during the last century. GCM predictions of precipitation change indicate an increase, but there is little agreement amongst the various models on regional disposition or magnitude. Precipitation change is as important as temperature change in determining the water balance. The water balance is critical to every aspect of hydrology and limnology in the far north. Permafrost close to the surface plays a major role in freshwater systems because it often maintains lakes and wetlands above an impermeable frost table, which limits the water storage capabilities of the subsurface. Thawing associated with climate change would, particularly in areas of massive ice, stimulate landscape changes, which can affect every aspect of the environment. The normal spring flooding of ice-jammed north-flowing rivers, such as the Mackenzie, is a major event, which renews the water supply of lakes in delta regions and which determines the availability of habitat for aquatic organisms. Climate warming or river damming and diversion would probably lead to the complete drying of many delta lakes. Climate warming would also change the characteristics of ponds that presently freeze to the bottom and result in fundamental changes in their limnological characteristics. At present, the food chain is rather simple usually culminating in lake trout or arctic char. A lengthening of the growing season and warmer water temperature would affect the chemical, mineral and nutrient status of lakes and most likely have deleterious effects on the food chain. Peatlands are extensive in region 2. They would move northwards at their southern boundaries, and, with sustained drying, many would change form or become inactive. Extensive wetlands and peatlands are an important component of the global carbon budget, and warmer and drier conditions would most likely change them from a sink to a source for atmospheric carbon. There is some evidence that this may be occurring already. Region 2 is very vulnerable to global warming. Its freshwater systems are probably the least studied and most poorly understood in North America. There are clear needs to improve our current knowledge of temperature and precipitation patterns; to model the thermal behaviour of wetlands, lakes and rivers; to understand better the interrelationships of cold region rivers with their basins; to begin studies on the very large lakes in the region; to obtain a firm grasp of the role of northern peatlands in the global carbon cycle; and to link the terrestrial water balance to the thermal and hydrological regime of the polar sea. Overall, there is a strong need for basic research and long-term monitoring.

Role of the Hudson Bay lowland as a source of atmospheric methane

Based on point measurements of methane flux from wetlands in the boreal and subarctic regions, northern wetlands are a major source of atmospheric methane. However, measurements have not been carried out in large continuous peatlands such as the the Hudson Bay Lowland (HBL) (320,000 km2) and the Western Siberian lowland (540,000 km2), which together account for over 30% of-the wetlands north of 40°N. To determine the role the Hudson Bay Lowland as a source of atmospheric methane, fluxes were measured by enclosures throughout the 1990 snow-free period in all the major wetland types and also by an aircraft in July. Two detailed survey areas were investigated: one (≈900km2) was in the high subarctic region of the northern lowland and the second area (≈4,800 km2) straddled the Low Subarctic and High Boreal regions of the southern lowland. The fluxes were integrated over the study period to produce annual methane emissions for each wetland type. The fluxes were then weighted by the area of 16 different habitats for the southern area and 5 habitats for the northern area, as determined from Landsat thematic mapper to yield an annual habitat-weighted emission. On a per unit area basis, 1.31±0.11 and 2.79±0.39 g CH4 m−2 yr−1 were emitted from the southern and northern survey areas, respectively. The extrapolated enclosure estimates for a 3-week period in July were compared to within 10% of the flux derived by airborne eddy correlation measurements made during the same period. The aircraft mean flux of 10±9 mg CH4 m−2 d−1 was not statistically different from the extrapolated mean flux of 20±16 mg CH4 m−2 d−1. The annual habitat-weighted emission for the entire HBL using six wetland classes is estimated as 0.538±0.187 Tg CH4 yr−1 (range of extreme cases is 0.057 to 2.112 Tg CH4 yr−1). This value is much lower than expected, based on previous emission estimates from northern wetlands.

Eddy covariance measurements of evaporation from Great Slave Lake, Northwest Territories, Canada

The first direct measurements of evaporation from a large high-latitude lake, Great Slave Lake, Northwest Territories, Canada, were made using eddy covariance between July 24 and September 10, 1997, and June 22 and September 26, 1998. The main body of the lake was ice-free between June 20 and December 13, 1997, and June 1, 1998, and January 8, 1999, with the extended ice-free season in 1997-1998 coinciding with 48C above normal air temperatures and an abnormally strong El Nino. Measurements extending roughly 5.0 to 8.5 km across the lake were made from a small rock outcrop located near the main body of the lake. The lake was thermally stratified between mid- July and September, with the thermocline extending down to approximately 15 m. High winds were effective in mixing warm surface waters downward and, when accompanied by cold fronts, resulted in large, episodic evaporation events typically lasting 45 hours. The daily total evaporation was best described as a function of the product of the horizontal wind speed and vapor pressure difference between the water surface and atmosphere. Seasonally, the latent heat flux was initially negative (directed toward the surface) followed by a steady increase to positive values (directed away from the surface) shortly after ice breakup. The latent heat flux then remained positive for the remainder of the ice-free period, decreasing midsummer and then steadily increasing until freeze-up. The sensible heat flux was small and often negative most of the spring and summer yet switched to positive and began to increase in the early fall. Extrapolation of evaporation measurements for the entire ice-free periods gave totals of 386 and 485 mm in 1997 and 1998 -1999, respectively.

Interannual variability of net ecosystem CO2 exchange at a subarctic fen

Landscape-scale net ecosystem CO 2 exchange (NEE) and the energy balance of a subarctic fen were studied during five growing seasons near Churchill, Manitoba. Interannual variability in NEE was large and ranged from a net sink of -235 g CO 2 m -2 in 1996 to a net source of +76 g CO 2 m -2 in 1994. Annual estimates of CO 2 exchange indicate that during the present period the fen is losing carbon nearly 3 times faster than its long-term historical gain of about 11 g CO 2 m -2 yr -1 . Our estimates suggest that gross ecosystem photosynthesis may be more variable than ecosystem respiration on diurnal, seasonal, and interannual timescales. Our data strongly indicate that an early snowmelt combined with wet and warm conditions during the spring period lead to large carbon acquisition even when drier conditions were experienced over the majority of the growing season. The phenological stage of the vegetation relative to the climatic conditions experienced is an important cause of the interannual variability in NEE. An accurate representation of phenology in climate models is, therefore, critical to the success of forecasting the carbon budgets of northern wetlands.

The Role of Northern Lakes in a Regional Energy Balance

There are many lakes of widely varying morphometry in northern latitudes. For this study region, in the central Mackenzie River valley of western Canada, lakes make up 37% of the landscape. The nonlake components of the landscape are divided into uplands (55%) and wetlands (8%). With such abundance, lakes are important features that can influence the regional climate. This paper examines the role of lakes in the regional surface energy and water balance and evaluates the links to the frequency–size distribution of lakes. The primary purpose is to examine how the surface energy balance may influence regional climate and weather. Lakes are characterized by both the magnitude and temporal behavior of their surface energy balances during the ice-free period. The impacts of combinations of various-size lakes and land–lake distributions on regional energy balances and evaporation cycles are presented. Net radiation is substantially greater over all water-dominated surfaces compared with uplands. The seasonal heat storage increases with lake size. Medium and large lakes are slow to warm in summer. Their large cumulative heat storage, near summer’s end, fuels large convective heat fluxes in fall and early winter. The evaporation season for upland, wetland, and small, medium, and large lakes lasts for 19, 21, 22, 24, and 30 weeks, respectively. The regional effects of combinations of surface types are derived. The region is initially treated as comprising uplands only. The influences of wetland, small, medium, and large lakes are added sequentially, to build up to the energy budget of the actual landscape. The addition of lakes increases the regional net radiation, the maximum regional subsurface heat storage, and evaporation substantially. Evaporation decreases slightly in the first half of the season but experiences a large enhancement in the second half. The sensible heat flux is reduced substantially in the first half of the season, but changes little in the second half. For energy budget modeling the representation of lake size is important. Net radiation is fairly independent of size. An equal area of medium and large lakes, compared with small lakes, yields substantially larger latent heat fluxes and lesser sensible heat fluxes. Lake size also creates large differences in regional flux magnitudes, especially in the spring and fall periods.

Interannual and Seasonal Variability of the Surface Energy Balance and Temperature of Central Great Slave Lake

This paper addresses interannual and seasonal variability in the thermal regime and surface energy fluxes in central Great Slave Lake during three contiguous open-water periods, two of which overlap the Canadian Global Energy and Water Cycle Experiment (GEWEX) Enhanced Study (CAGES) water year. The specific objectives are to compare the air temperature regime in the midlake to coastal zones, detail patterns of air and water temperatures and atmospheric stability in the central lake, assess the role of the radiation balance in driving the sensible and latent heat fluxes on a daily and seasonal basis, quantify magnitudes and rates of the sensible and latent heat fluxes and evaporation, and present a comprehensive picture of the seasonal and interannual thermal and energy regimes, their variability, and their most important controls. Atmospheric and lake thermal regimes are closely linked. Temperature differences between midlake and the northern shore follow a seasonal linear change from 68C colder midlake in June, to 68C warmer in November‐December. These differences are a response to the surface energy budget of the lake. The surface radiation balance, and sensible and latent heat fluxes are not related on a day-to-day basis. Rather, from final lake ice melt in mid-June through to mid- to late August, the surface waters strongly absorb solar radiation. A stable atmosphere dominates this period, the latent heat flux is small and directed upward, and the sensible heat flux is small and directed downward into the lake. During this period, the net solar radiation is largely used in heating the lake. From mid- to late August to freeze up in December to early January, the absorbed solar radiation is small, the atmosphere over the lake becomes increasingly unstable, and the sensible and latent heat fluxes are directed into the atmosphere and grow in magnitude into the winter season. Comparing the period of stable atmospheric conditions with the period of unstable conditions, net radiation is 6 times larger during the period of stable atmosphere and the combined latent and sensible heat fluxes are 9 times larger during the unstable period. From 85% to 90% of total evaporation occurs after mid-August, and evaporation rates increase continuously as the season progresses. This rate of increase varies from year to year. The time of final ice melt exerts the largest single control on the seasonal thermal and energy regimes of this large northern lake.

The influence of surface cover and climate on energy partitioning and evaporation in a subarctic wetland

Energy partitioning and evaporation were measured over three wetland surfaces in a subarctic coastal marsh during pre-growing and growing periods. These surfaces included an alder/willow woodland, a sedge marsh and a raised backshore sedge meadow. A combination model analysis was used to assess the relative importance of surface resistance and meteorological conditions on the magnitude of the Bowen ratio, Β, during the growing period.Overall, the three surfaces experienced important site-to-site and seasonal differences in Β and evaporation, QE. During the non-foliated period, QE was largest and Β was smallest for the open water marsh, while the dry backshore site experienced the smallest QE and largest Β. The non-foliated woodland assumed intermediate values of Β and QE. After the vegetation covers were established, the woodland assumed the smallest Β and largest QE flux. It was also found that Β at the marsh site increased with the presence of a vegetation cover.Wind direction was always an important factor in determining QE and Β at all sites. Β was substantially larger and QE was smaller for onshore winds (i.e., originating from James Bay) than for offshore winds. The combination model analysis showed that canopy resistance at all sites was largest during warm offshore winds, which were associated with large saturation deficits. However, the effect of increased canopy resistance on Β during offshore winds was offset by a large climatological resistance, resulting in small Β values and large QE. When winds originated from James Bay, canopy resistance was smaller than for offshore winds, but the climatological resistance also was much smaller, resulting in larger Β and small QE. The results have important implications for changes in land cover and climate on the regional water balance.

IMPACTS OF HUDSON BAY ON THE TERRESTRIAL CLIMATE OF THE HUDSON BAY LOWLANDS

Hudson Bay remains frozen or is dominated by ice over the summer solstice and throughout much of the high-sun season. This contributes directly to the winterization of summer. The juxtaposition of the mean summer position of the Arctic front and of treeline to the west of Hudson Bay has been clearly documented, and the coincidence of treeline and the southern boundary of continuous permafrost is well known. The strong southward thrust of the Arctic front in summer is, in major degree, a response to cold air masses spawned over Hudson Bay. On a mesoscale, Hudson Bay generates onshore winds across a strong temperature and pressure gradient. This mesoscale regime dominates the temperature and surface energy balance for a large distance inland across the Hudson Bay Lowlands. Superimposed on the mesoscale wind field is a land-sea breeze, which is sometimes well developed in coastal areas and which can be traced up to 65 km inland. It is, however, of low frequency occurrence and is usually overridden by the regional wind. Progressing inland from the coast, in the central Hudson Bay Lowlands, the landscape becomes dominated by deep peat soils and a surface vegetation which is resistant to evapotranspiration. As a result, it is much drier than the wetlands near the coast and evapotranspiration is reduced.

Application of an energy combination model for evaporation from sparse canopies.

This study tests the evaporation energy combination model developed by Shuttleworth and Wallace (1985) with data collected in a subarctic wetland. The modelled evaporation was compared with evaporation calculated from the Bowen ratio energy balance technique over a range of leaf area indices (LAI) from non-vegetated to fully vegetated conditions. The Shuttleworth-Wallace (SW) model was in excellent agreement with the measured evaporation for hourly and day-time totals for all values of LAI. This gives a particular advantage to the SW model compared to the simple Penman-Monteith combination equation. A comparison of measured and modelled total evaporation for all days yielded a root mean square error and mean bias error of 0.98 and −0.13 MJ m−2 day−1, respectively. The model also shows good agreement with the measured evaporation on an hourly basis. Although the results of this study are encouraging, we are cautious because this test is not truly independent. The need for additional investigation and testing of certain model parameters is recognized. In this study, we assume that eddy diffusivity within the canopy decreased exponentially and was controlled by a decay constant which varied with LAI. However, there is little information available to validate this treatment. Net radiation at the soil surface was computed from net radiation over the canopy and an exponential function of LAI, which was held constant over the course of the growing season.

Northern Canadian wetlands : Net ecosystem CO2 exchange and climatic change

Northern Canadian peatlands represent a long term sink for atmospheric carbon dioxide (CO2), however there is concern they may become a net source of CO2 due to climatic change. Climatic change is expected to result in significant changes in regional hydrology in boreal and subarctic regions of Canada. A hydrologic model predicted a summer water table drop of 0.14 m in northern Canadian fens given an increase in summer temperature and rainfall of 3°C and 1 mm d-1, respectively. Moreover, surface peat temperature increased by 2.3°C. Net ecosystem exchange of CO2 was modelled using these modelled hydrologic and thermal changes with respiration:peat temperature and water table:net ecosystem production relationships developed from measurements at wetlands in northern Sweden and near Churchill, Manitoba. Model results indicate that the net atmospheric CO2 sink function of fens may be enhanced under future 2 × CO2 scenarios, while bogs may become a net source of atmospheric CO2. If the net ecosystem productivity response to the new hydrologic conditions was ignored then the model predicts a decrease in summer carbon storage for all peatland types.