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Ice Cover on Lakes and Rivers
Climate Trends Inferred from Historical Records
The longer and colder a winter is, the earlier lakes freeze and the
later they thaw. This is obvious intuitively even if we ignore the
underlying meteorological complexities. EAWAG decided to take a
closer look at this phenomenon by analyzing long series of historical
observations of the timing of freeze-up and break-up of
lakes such as the Lej da San Murezzan in Switzerland and Lake
Baikal in Siberia. From these data, conclusions can be drawn
about past and future climate forcing.
Causally, the formation of an ice cover on
lakes and rivers in winter and its disappearance
in the following spring are the end
results of complex series of mixing, freezing
and thawing processes driven ultimately by
many meteorological forcing factors. By far
the most important of these factors is air
temperature, so that the formation or disappearance
of ice on a lake or river can often
be viewed in an empirical sense simply as
a temporally integrated response to the
seasonal variability in air temperature. The
reverse is also true, allowing conclusions
about the air temperatures that prevailed
in the vicinity of a water body at some time
in the past to be drawn from historical observations
of the timing of freeze-up and
break-up. Since air temperature is a very
spatially coherent meteorological variable –
Freeze-up date
January
December
November
19 EAWAG news 58
Lake Baikal
Mendota
Kallavesi
Red River
air temperatures are usually well correlated
over several hundred kilometers – historical
variations in the timing of break-up do not
merely reflect variations in local weather, but
– much more usefully – variations in regional
or even supraregional climate.
Shorter and Thinner Ice Cover –
A Response to Climate Change?
Historical observations of the calendar
dates of freeze-up (defined as the first day
of total ice cover) and break-up (defined as
the first completely ice-free day) of lakes
and rivers in Canada, the USA, Finland,
Switzerland, Russia and Japan provide
consistent evidence that lakes and rivers
around the Northern Hemisphere have been
freezing later and thawing earlier since at
least the middle of the 19th century (Fig. 1)
Fig. 1: Time-series of the calendar dates of freeze-up (left) and break-up (right) for a small but representative selection of lakes and rivers distributed around the northern hemisphere.
Data presented as a 10-year running mean, from [1].
Break-up date
May
Kallavesi
Lej da
S. Murezzan
Tornionjoki
Lake Baikal
Red River
Mendota
1800 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000 1800 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000
Year
Year
June
April
March
[1]. These long-term historical freezing and
thawing trends average around 6 days per
100 years, which corresponds to an air temperature
increase of 1.2 °C per 100 years.
Thus historical cryophenological data from
lakes and rivers lend support to the hypothesis
of past and current global warming, but
2
1 Mendota
2 Red River
1
6
3 Tornionjoki
4 Kallavesi
3 4 5
5 Lake Baikal
6 Lej da
S. Murezzan