similar rate, resulting in an almost two-week per centu- on spring temperatures and ice break up) largely paral-
ry reduction in the ice-covered season. Numerous other leling trends in major atmospheric patterns that have
regional and continental studies have been conducted produced regional climatic warming or cooling
14,15
.
using the more spatially-detailed sets of observations
available for the latter half of the 20th-century. Results Overall, the data for river ice indicate that long-term
reveal strong contrasts in freeze-up and break-up tim- increases of 2–3°C in autumn and spring air tempera-
ing between decades and between regions
9–13
(see box tures have produced an approximate 10 to 15 day delay
Spring temperatures and ice break up
Although ice-cover duration on rivers and lakes
of the Northern Hemisphere has significantly
decreased in response to increasingly warmer
climate conditions during the 20th century, the
response has been shown to vary regionally and
to be strongly related to the variability and regime
shifts in large-scale atmospheric and oceanic os-
cillations.
In Canada, recent evidence indicates a shorten-
ing of the freshwater-ice season over much of the
country with the reduction being mainly attributa-
ble to earlier break ups. These trends match those
in surface air temperature during the last 50 years
(Figure 8.2). For example, similar spatial and tem-
poral patterns have been found between trends
(1966 to 1995) in autumn and spring 0°C iso-
therms
14
(lines on a map showing location of 0°C
air temperatures) and lake freeze-up and break-up
dates, with generally significant trends toward ear-
lier springs and earlier break-up dates over most
Lake ice break-up Spring 0°C Isotherm
of western Canada and little change in the onset
trends (1966-1995) trends (1966-1995)
of cooler temperatures and in freeze-up dates over
Earlier trend,
the majority of the country in autumn
9
.
-22.5 – -15
significant
Earlier trend,
-15 – -7.5
non-significant
Later trend,
-7.5 – 0
non-significant
Figure 8.2: Trends in spring temperatures and in
ice break-up dates in Canada.
No trend
0 – 7.5
Source: Based on Duguay and others 2006
9
Number of days
204 GLOBAL OUTLOOK FOR ICE AND SNOW