2.2.5.5 Lake and river ice
Numerous studies suggest the importance of lake and river ice break-up as an
index of climate variability and change, especially as related to temperature
and snow cover (Palecki and Barry, 1986; Schindler et al., 1990; Robertson et
al., 1992; Assel and Robertson, 1995; Anderson et al., 1996; Wynne et al., 1998;
Magnuson et al., 2000). Records of lake and river ice can be used to independently
evaluate changes of temperature and, to some extent, snow cover. Like other
proxy measurements they have limitations, and are subject to their own time-dependent
biases such as changes in observers and protocols related to the identification
of �ice on� and �ice off� conditions. Larger lakes often
have the best records, but are often located near human settlements which can
affect the homogeneity of the record, e.g., associated cooling water discharges
and urban heat islands, so care is needed to select suitable lakes.
A recent analysis has been made of trends in 39 extensive Northern Hemisphere
lake and river ice records over the 150-year period from 1846 to 1995. Ice break-up
dates now occur on average about nine days earlier in the spring than at the
beginning of the record, and autumn freeze-up occurs on average about ten days
later (Magnuson et al., 2000). Only one of the 39 records, in Japan, showed
changes that indicate a slight cooling.
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