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to a much earlier date due to the earlier onset of melting, which re
ects the change in the
radiation balance and the fact that there is less ice to melt. The resulting ice break-up is
5 days earlier for the 1
fl
°
C warming, 25 days earlier for the 4
°
C warming, and 75 days
earlier for the 6
C warming. This illustrates the high instability of the ice season to
warming. In a warm climate, ice may form in mid-winter but since it is relatively thin it
melts, if the weather becomes warmer, or breaks at signi
°
cant wind load.
The results show that the 1
°
C change brings the thickness curve down by 5
10 cm but
-
has no effect on the form of the thickness cycle. With a 5
C warming there are growth and
decay periods through the winter and the maximum ice thickness is about 20 cm. In
southern Finland the thickness of 20 cm is enough for a stable ice cover in small lakes, but
in large or medium-size lakes stormy winds could break this cover and give rise to areas of
open water at any time during the winter. This means a remarkable qualitative change in
the lake ice seasons. The length of the ice season is only three months and the ice cover is
also mechanically unstable throughout the winter.
°
8.4.4 Future Ice Seasons
Climate warming has taken place during the last 30 years as shown in the rise of Northern
Hemispheric temperatures (IPCC 2001, 2007). Climate impact on lakes is thoroughly
discussed in the topic of George et al. (2009). The recent climate warming has affected the
ice regime of lakes as shown by numerous publications. A review on the time series work
was given by Adrian et al. (2009). Magnuson et al. (2000) reported trends of 5.8 days later
freeze-up dates and 6.5 days earlier break-up dates per 100 years in lake and river ice in
the northern hemisphere (1846
1995). Trends of the same order of magnitude were found
in Berlin and Brandenburg lakes by Bernhardt et al. (2011). For the irregularly freezing
M
-
ü
ggelsee, Berlin, trends were reported for later freezing (5.7 days) and earlier thawing
(
2007. Concurrently, the lake revealed increasing number
of ice-free years, shortening in the total ice duration of
6.8 days) for the period 1947
-
-
15.6 days (that is, longer duration
of intermediate ice-free periods), and thinning of the ice cover. In Lake Kilpisj
-
rvi, Arctic
tundra in northern Finland, since 1964 the trend has been later freezing by 11.5 days and
earlier ice breakup by 5.0 days per 50 years (Lei et al. 2012).
Notably, trends in the earlier breakup are sometimes reported to be stronger than those
in the freezing date (Korhonen 2006; Jensen et al. 2007), despite ice melting in spring is
driven, as we have shown above, by absorption of solar radiation, which is affected by the
global warming only via the albedo. Prokacheva and Borodulin (1985) examined the
variability of lake ice seasons in Northwest Russia, in particular Lake Ladoga. No clear
connection was found with sunspots (Wolff index) in Lake Ladoga. Spatial coherence in
was seen with large lakes in the region, with Lake Onega, Lake Il
ä
men, Lake Peipsi and
Rybinski Reservoir. Thus, climatic factors other than higher local air temperature in spring
affect the ice-off. They may include decreased ice thickness due to milder autumn/winter
conditions, trends in winter precipitation affecting the snow amount, or more cloudy
'
 
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