About glacier retreat
Glaciers are masses of ice moving downhill or
downslope by either flowing as a solid and/or sliding along their
base. Glaciers grow and shrink in size over time, much like your checking
account; glaciers have budgets. Glaciers grow (lengthen, widen and
thicken) under favorable conditions, either by increasing deposits
(more snowfall, more snow avalanches, etc.), by decreasing withdrawals
(less melting, less calving of icebergs, etc.), or both until a positive
mass balance is achieved. Glaciers decrease in size (shrink or retreat)
by decreasing deposits (less snowfall, etc.), increasing withdrawals
(less melting, etc.), or both, until a negative mass balance is achieved.
As an alpine or valley glacier shrinks in size, the end of the glacier
is found farther and farther uphill; the glacier is said to retreat.
Shrinking glaciers can have profound effects on
man and the environment. In the Pacific Northwest of the US, glaciers
are found at the heads of many major river systems in both the Cascade
and Olympic Mountains. Melting of these glaciers provides water for
streams and rivers, especially in the months of August, September
and October, when streamflows are low due to lack of precipitation
(Washington gets very little rain in the summer). The smaller the
glacier, the smaller the outflow of meltwater. Low streamflows lead
to reduced electricity generation at hydropower plants downstream
from shrinking glaciers. And low streamflows can also have profound
impacts on salmon viability, especially for the early fall spawners.
Data on the recent changes in length of the Rainbow
Glacier on Volcano Baker in the North Cascades of Washington State
are shown in the table and figure (Pelto, 2001, personal communication).
The data show the position of the snout (terminus) of the Rainbow
Glacier as a function of time, compared with the 1985 position. The
changes in position of the snout are essentially equivalent to changes
in the total length of the glacier, as the head of the glacier does
not change position significantly. As can be seen, the Rainbow Glacier
has shrunk almost monotonically over the last 15 years. The students
can fit a linear regression to these data to determine the typical
rate of change, though they should first change calendar years to
more practical values. There's a missing year; can linear interpolation
be used to estimate a reasonable value? The Rainbow Glacier was 3750
m long in 1950; assuming a constant rate, how much did the glacier
retreat in the previous 35 years? When will the glacier be half its
1950 length?
Why is the Rainbow Glacier retreating? Students
will be quick to answer "melting", but should remember that
decreasing deposits can have just as big an impact on their bank account
as increasing withdrawals.
Reference: Mauri Pelto, Nichols College,
North Cascades Glacier Climate Project
http://www.nichols.edu/departments/Glacier/
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