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Alaska's Glacier Bay has fascinated
naturalists and glaciologists for
centuries named the National Monument in
1925 and a national park in 1980 Glacier
Bay today attracts more than 250,000
visitors a year just 200 years ago the
entire Bay was covered by glacier ice
with the help of ground and satellite
measurements study of the recession of
the glaciers of Glacier Bay as well as
in other areas is providing us with
important clues about changes in global
climate
a glacier is a dynamic system consisting
of snow ice and often rock debris that
transports material from higher
elevations where snow accumulates to
lower elevations where snow and ice melt
snow becomes glacier ice over time when
the pressure of increasing layers are
snow accumulating year after year
transforms the snow first into fern
which is very dense snow and eventually
into ice today there are over 200
separate smaller glaciers in Glacier Bay
National Park to the delight of park
visitors who view them from cruise ships
private boats and kayaks glaciers at the
head of tar inlet of the most
spectacular these include the Grand
Pacific and the margaery together they
comprise three kilometers of ice front
which is cowling into the sea the Johns
Hopkins and Marjorie glaciers are each
about one-and-a-half kilometres wide and
22 kilometers long they are both dwarfed
by the Grand Pacific glacier a tall
towers 60 to 90 meters above the water
at their term and I with another 120
meters below the water surface these
glaciers calve icebergs into the ocean
on a regular basis grilling visitors
with the splash and thunder
written records of the glaciers in
Glacier Bay began with George
Vancouver's visit in 1794 ground
observations beginning in the late 1800s
and ground-based and satellite
measurements in recent years have shown
that many glaciers in and near Glacier
Bay have been receding some rapidly
when a glacier recedes water from the
melting ice is released into the ocean
and sea level rises though the greatest
potential for sea level rise is from
melting of the Antarctic and Greenland
ice sheets the small glaciers of the
world such as those in Glacier Bay would
contribute to a sea-level rise of about
six-tenths of a meter if they were to
melt completely precise measurements of
glacier changes may be made by
ground-based surveying the light
glaciologist William O field began
photographing and measuring glaciers in
Glacier Bay in 1926 many of the glacier
terminus positions map by field and his
colleagues are shown on the map of
Glacier Bay prepared by the National
Park Service glacier work in the early
part of this century was even more
difficult and hazardous than it is today
scientists had to roll through iceberg
laden water to reach many of the glacier
fronts detailed measurements in Glacier
Bay made by field and others have
provided an excellent description of the
dates and magnitude of changes in the
glaciers there
the once enormous Muir glacier located
in what is now mere Inlet in the East
arm of Glacier Bay was named for John
Muir the famous naturalist and explorer
who visited Glacier Bay in the late 19th
century in 1905 just 26 years after
mirrors first visit Fremont Morris wrote
formally the mirror presented a
perpendicular front at least 200 feet in
height from which huge Berg's were
detached at frequent intervals the sight
and sound of one of these vast masses
falling from the cliff or suddenly
appearing from the submarine I slit was
something which once witness was not to
be forgotten it was grand and impressive
beyond description fortunately the
recent changes in the mirror have not
increased its impressiveness from a
scenic standpoint instead of the
imposing cliff of ice the front is
sloping and seems to be far less active
than formerly the Muir glacier is now
but a small remnant of its former glory
and has nearly retreated up out of the
ocean in fact many of the large
Tidewater glaciers that John Muir first
observed in 1879 have been reduced to
small glaciers that terminate on land
land that is uncovered as glaciers
recede from its plant and animal life to
appear and flourish beginning with the
launch of the first Landsat satellite in
1972 we have with a spatial resolution
of up to 30 meters been able to acquire
detailed satellite images of the
glaciers of Glacier Bay and measure
changes in those glaciers to satellite
sensors the multispectral scanner and
the more advanced thematic mapper allow
us to measure glaciers without having to
visit the area
maps drawn by earlier explorers of
Glacier Bay can be registered to or
overlaying on satellite images using
computer techniques common stable points
between the map and the images are
located and digitally overlaying using
image processing techniques one image is
stretched in relationship to the other
once the images are registered we can
measure changes in glacier terminus
positions from the time of the earliest
maps to the present
using computer techniques we can
register elevation data to satellite
images this allows us to obtain a
three-dimensional perspective and
actually appear to fly by an area
enabling more realistic visualization
measurements of the Muir glacier have
shown that the terminus retreated up the
mirror inlet at a rate of about four
tenths of a kilometer per year between
1794 and 1892 for a total retreat to
more than 40 kilometers satellite
measurements derived from images
acquired in 1973 1980 1983 and 1986 show
that the Muir glacier terminus went back
more than seven kilometres between 1973
and 1986 with most of that retreat
occurring between 1973 and 1983
mcBride glaciers the most active glacier
and mirror inlet it is still receding
and over the last two decades has formed
an impressive Inlet as recorded by
satellite datum the terminus of the
McBride glacier retreated almost three
kilometres between 1973 in 1986 and the
nearby Burroughs glacier shrank
considerably filling lakes with its
meltwater note the increase in the
amount of vegetation the vicinity of
burrows glacier in 1986 the burrows is
now a dying ice field having been cut
off from a source of nourishment
still other glaciers have been in
equilibrium or advancing some like the
Johns Hopkins have stopped their retreat
and have advanced during the period
covered by the satellite record also the
lamp Lou Reed Marjorie and Grand Pacific
glaciers are all advancing at this time
the nearby Brady glacier has been
advancing since 1794 however the major
feature of Glacier Bay is the
large-scale retreat of its Tidewater
glaciers and both mirror and tar inlets
Tidewater glaciers like the mirror and
the Marjorie terminate in the sea
Tidewater glaciers follow their own
cycles that are independent of
short-term climate changes there are
also non Tidewater glaciers near Glacier
Bay many of which are receding recession
of non Tidewater glaciers in the area is
probably due to amelioration of the
regional climate meteorological data
from the nearby stations at juneau Sitka
and Yakutat show a tendency toward an
increase in average summer air
temperature since about nineteen forty
when the meteorological record began
combining the extensive records of
explorers beginning the late 18th
century with more recent efforts of
glaciologists and computer scientists a
wealth of information has been obtained
about the d glaciation of Glacier Bay
Alaska as these glaciers and others
continue to change or if they remain
stable over time the satellite record
will permit assessment of the impact of
changing climate on the glaciers and
many of the non Tidewater glaciers are
excellent indicators of regional climate
and climate change
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