In contrast, for most of northern Eurasia there has been creases, due to changes in the albedo of the land surface
a long-term increase in snow depth and the duration of (see Chapter 3). In Alaska, 95 per cent of recent summer
snow cover
17
. At Abisko in subarctic Sweden, increases warming trends have been attributed to the decrease in
in snow depth have been recorded since 1913
18
. Dur- snow-cover duration
20
.
ing 1935–1995, snow-cover duration increased by about
four days per decade in northern European Russia and Shallow snow cover at low elevations in temperate re-
small areas of west central Siberia and decreased by gions is the most sensitive to temperature fluctuations
about two days per decade over southern and southeast- and hence most likely to decline with increasing tem-
ern Siberia
19
. perature
4
. In locations where snow accumulates at tem-
peratures close to its melting point, small increases in
Outlook for snow cover temperature will have large effects on snow cover. For
example, in the Pacific Northwest region of the United
Decreases in snow-cover extent and duration will con- States, the temperate snow cover of the Cascade Range
tribute to continued and accelerated temperature in- of mountains could be reduced by over 20 per cent with
an increase in mid-winter temperatures of only 2° C
21
.
Mountain regions are particularly sensitive to climate
change
22
, and increases in mean minimum tempera-
tures are more pronounced at higher elevations than
in valleys
23
. Temperatures are projected to continue
rising in the mountains of the western United States,
with accompanying reductions in snow cover
24
. Similar
changes are expected in other mountainous regions of
the world. In central Chile, air temperature data from
1975 to 2001 show an increase in elevation of the 0°
C isotherm (the line on a map linking points at which
the mean temperature is 0° C) by 122 m in winter and
200 m in summer
25
. It is estimated that the snow line
of the European Alps will rise about 150 m for every
Observed change in spring snow cover
1.0
o
C increase in winter temperatures
26
. Climate mod-
duration 1970-2004 (days/yr) el projections indicate that the Alps and Pyrenees will
experience warmer winters with possible increases in
-4 - -2 -2 - -1 -1 - -0.25 -0.25 - +0.25 +0.25 - +3
precipitation
27
, which, as in the western United States,
will raise snow lines, reduce overall snow cover, and de-
Figure 4.4: Trend (days/year) in spring (February–July) snow-cover
duration from 1970–2004 from the NOAA weekly snow-cover da-
crease summer runoff.
taset. Changes exceeding ~ ±1 represent significant local changes
at the 95% level. Greenland was excluded from the analysis.
Snow water equivalent and snow-covered area are mod-
Source: R. Brown, Environment Canada; data from D. Robinson, Rutgers
elled in General Circulation Model experiments to predict
University global changes in snow cover. A comparison of results
44 GLOBAL OUTLOOK FOR ICE AND SNOW
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