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6 RECORDED RESPONSE TO CLIMATE VARIABILITY
As a result of the complexities in climatic factors controlling river-ice processes, river ice has rarely been used as an environmental indicator of climate change. Instead most attention has been focussed on lake ice which has a higher sensitivity to a single meteorological variable and a much lower signal/noise ratio (e.g., Barry, 1985). The myriad of potential impacts that could result from climate-induced changes to river-ice regimes, however, has spurred some
recent assessments. These focus primarily on easily retrievable measures, such as ice-cover dates, and not on physical-process variables such as variations in ice thickness, flow hydrograph, or breakup severity.
Historical records pertaining to the river-ice season have been routinely collected in many Eurasian countries for up to several centuries but long-term records are available for only a handful of stations in North America and even these extend to little more than one century. The most comprehensive analyses of long-term trends in river-ice freeze-up and breakup dates have been conducted in the Former Soviet Union (FSU) by Ginzburg et al.
(1992) and Soldatova (1993). Ice data have been summarized for homogenous hydrologic regions of the European and Asian FSU over the period 1893-1985, and modified to account for any effects of water-resource development which could influence climatic signals (e.g., Soldatova, 1992). Although appreciable inter-decadal variability was found, significant long- term spatial patterns and temporal trends of ice-cover dates could also be identified for the almost one-hundred year period. The most significant regional trend was for later- river-ice freeze-up dates in the European FSU and Western Siberia. Freeze-up on such as the Danube, breakup dates and, hence, an overall expansion of the ice season.
Relative to climatic trends, both studies (G&burg et al., 1992; Soldatova, 1993) note that trends in the ice-formation and breakup dates agreed with those for air temperature.
Specifically, it was found that freeze-up and breakup dates correlated (r’ = 0.6-0.7) with the mean air temperature in the preceding autumn and spring months respectively. In attempting to accurately forecast future changes from climatic change, however, Ginzburg et al. (1992) found a major constraint to be that regional predictions of air temperature existed only for the summer and winter months, and not for the critical shoulder seasons of spring and autumn.
Similar to the results for the western portions of the FSU, an advance in breakup dates has been recorded on one of the major rivers in northern Scandinavia, the River Tomea,“lven (Zachrisson, 1989). Historical records as far back as 1693 indicate that breakup has occurred much earlier in the 20th century than in earlier times. As in the FSU, the strongest correlation to breakup dates was made with spring (April), not winter, air temperatures. This suggests that it is the pre-breakup melt and runoff period that is more important to the timing of breakup than the overall winter severity and peak ice thickness. Although forest clearing may have complicated the spring runoff regime of the Tomea,“lven, a 3°C rise in April air temperatures between 1870 and 1950 was associated with about a 15-day advance in breakup.
Interestingly, Williams (1970) noted that breakup of the Saint John River in Canada and the
America. In general, the average duration of the Red River ice season has been shortened by approximately three weeks between the 19th and 20th centuries. Specifically, median dates of freeze-up and breakup were 12 and 10 days earlier and later in the 19th than in the 20th century. The earlier-breakup trend is corroborated by the findings of Burn (1994), who examined records of 84 hydrometric gauging stations in west-central Canada: the snowmelt peak flow tends to arrive earlier than it did in the past at an average rate of 25 days per century, though a few stations showed the opposite trend.
Except for the case of some rivers in Central and Western Siberia, the general trend of available results for other circumpolar areas is towards a shorter river-ice season. Although the current data set is meagre, a crude first approximation of river-ice response to climatic changes in these areas would be that a long-term mean rise of 2 to 3°C in autumn and spring air temperature has produced an approximate 10 to 15 day delay in freeze-up and advance in breakup respectively. It is important to remember again, however, that other climate- dependent variables, such as winter snow accumulation and river discharge, also control river-ice processes, and these are not reflected in a simple heat index such as air temperature.
Brimley and Freeman (1997) considered the duration of the ice season in Atlantic Canada, as revealed by hydrometric gauge records in the past 45 years or so. The results highlight the spatial variability of Maritime climate, and are consistent with local winter temperature trends though not entirely explainable by temperature. The largest rates of change were found in Cape Breton and Newfoundland (up to +3 days/year!) in response to colder winter temperatures. The magnitude of change is strikingly greater than that of previously quoted data. A possible explanation may be in the brief and often intermittent ice cover formation in Atlantic Canada streams due to the occurrence of one or more mid-winter thaws.
Such events may not happen as often under colder winter temperatures.
For south-central British Columbia, Doyle (1997, personal communication) examined the ice-in, ice-out dates at seven Water Survey of Canada hydrometric stations, where the ice compensating changes in flow discharge may neutralize temperature effects.
In general, therefore, historical records suggest that climatic warming is able to alter the timing of break, but its effect on breakup flooding is less clear because of the complicating effects of, for example: changes in the winter snow accumulation available for runoff, seasonal variations in the radiation-induced decay of ice strength, and the increased potential for sudden mid-winter breakups. It is thus difficult to draw any general conclusions from the limited evidence at hand as to what is happening to the river ice cover, and to use them to predict what may occur in the future due to greenhouse-gas impacts. Conspicuously lacking is information on the frequency and severity of ice jam events, perhaps because it is not as readily obtainable as “ice-in” and “ice-out” dates. A study of past and present breakup and ice- jam conditions is required, perhaps through a comprehensive analysis of hydrometric station
data, taking into account both hydrologic and morphologic aspects.
As a first step in this direction, Beltaos (1999) examined hydro-climatic records over the past 80 years for the Saint John River near Edmundston & Clair, New Brunswick.. The study was prompted by the unexpected occurrence of three mid-winter breakup events, one in 1995
and two in 1996. It was found that peak winter flows have increased dramatically (Figure 5) as a result of increased rainfall.
800 -
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1920 1930 1940 1950 1960 1970 1980 1990 2000
Year
Figure 5. Variation of peak ‘winter’ flow in past seventy years (Saint John River at Fort Kent (Beltaos, 1999).
In turn, this was produced by a marked increase in the incidence of mild (air temperature
> OOC) winter days, despite a modest concomitant rise of the average monthly temperatures during the winter (order of 1°C per century). Considering that a mild winter day in Canada is a rare event, the latter finding is consistent with the well-known sensitivity of probability distribution functions near their extremes to changes in mean values (Figure 6).
Increased flows were also noted in April, the time of the spring breakup event. This is consistent with the findings of Hare et al. (1997a, b) who examined annual Saint John River flows further downstream (Grand Falls, Mactaquac). This effect could explain the relatively high frequency of ice-jam induced spring flood events experienced in recent years (e.g. 1976,
1987,1991, and 1993).
Furthermore, Beltaos (1999) reported that the slight warming and, more importantly, the increasing winter/spring flows appear to have resulted in later freeze-up and earlier breakup by about 11 days per century, thus truncating the ice season by about three weeks.