Download Glaciology INTRODUCTION A Topics B What are Glaciers?

Glaciology
INTRODUCTION
Glaciology brings together knowledge from:
• Physics: Snow and ice crystals, phase transformations, ice flow dynamics, thermodynamics,
...
• Geology: Landscape, erosion, sedimentation, . . .
• Chemistry: Isotopes, composition, . . .
• Meteorology: Climate, precipitation, . . .
• Oceanography: Ocean circulation, . . .
and many other branches of science.
The best-known examples of glaciological research today are undoubtedly the detailed records
of past climate that the deep ice cores in Greenland and Antarctica have revealed.
A
Topics
Glaciologist are concerned with a wide range of topics regarding the past, present and future
behavior of glaciers. The basic questions of where do glaciers form (distribution of glaciers), and
are they growing or shrinking (mass balance) give us ideas about where and when to expect glaciers
to form and/or dissappear. Related to that is research into how snow is transformed into ice (snow
to ice transformation). Sea ice and avalanche studies are areas of active research and practical
concern. The glaciers themselves are studied in relation to their heat budget and effects on local
(micro-) climate. The size, thickness and changes of glaciers are observed using, for instance, GPS,
satellite data, photographs, radio-echo soundings, and ice core drilling. Glaciologist also look for
answers relating to the temperature distribution within the glacier, the flow and deformation of
the glacier, sliding at the base, and water within and discharging from the glacier (hydrology and
jökulhlaup’s). The glaciers preserve one of the best records of past climate.
B
What are Glaciers?
A glacier is a large mass of ice (also, firn and snow as we will see later) flowing, and possibly sliding,
as a result of the pull of gravity; individual glaciers or ice masses may link up into one large ice
cap or ice sheet (Greenland or Antarctic ice sheet).
Glaciers can in some ways be thought of as rivers of ice, with the ice flowing down an inclined
surface, ice streams being an example. There are still considerable differences between the way
water flows down a river and ice flows down a glacier! The deformation behavior of a glacier is in
some ways similar to deformation in the earth ’s crust (there are even ice-quakes) and the mantle,
but the timescales are very different with glacier movement occurring at speeds of several meters
to several hundreds of meters per year (and sometimes even more, ice streams).
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C
Why study Glaciers?
From the earliest of times, people have been in close interaction with glaciers and ice-sheets. A
significant part of our ancestors lived during times of vast ice sheets, covering a large part of the
northern hemisphere. Changes in sea-level associated with the buildup of the great Laurentide
Ice Sheet in North America and the Fennoscandian Ice Sheet in northern Europe were crucial in
creating the Bering land bridge and making Alaska the stepping stone into the Americas. At times
much earlier Earth’s history (Neoproterozoic) snow and ice may have covered almost the entire
surface; happened as many as four times between 750 million and 580 million years ago according
to the snow-ball Earth theory (Hoffman and others, 1998).
Today, as mankind’s impact on global climate becomes increasingly apparent, it is the impact
of glacier and ice sheet melt on global sea-level that causes concerns. During the past 100 years
sea level has risen by 1-2 mm per year due to glacier melt and thermal expansion of the ocean
(Oerlemans, 1994). Snow and ice also play an important role in the global climate system because
of their ability to reflect a significant fraction of the incoming sunlight, a quantity called albedo
(snow has an albedo of around 0.7 to 0.95, such that 70 to 95 % of the short-wave radiation coming
from the sun is reflected back from the surface). Changes in the extent of glaciers and ice sheets
may not only impact the development of climate but are also something of an early indicator of
impending climate change.
Glaciers are of immediate importance in areas where water supply and hydroelectric power
depend on glacier-fed streams; in fact, it has even been discussed to tow icebergs from the Antarctic
to arid regions to alleviate the problems of water shortage. Given the large area of industrialized
regions settled on glacial or peri-glacial (i.e. in the vicinity of glaciers or ice sheets), glaciers also
have an indirect impact on questions of water supply, foundation of structures and so on. Glaciers
also have a more direct impact when it comes to estimating the risk of jökulhlaup (outburst floods).
Apart from these and other practical considerations, glaciers, and ice in general, are anesthetically and culturally important. Our planet would be considerably less appealing in the absence of
ice (Ball, 2000).
D
Distribution of snow and ice on Earth
Snow and ice distribution (coverage) changes considerably with season. Satellite data can show the
distribution at various times and seasonal averages. Figure 1 shows the global average snow depth
in January and June over a 10 year period.
1
Where do glaciers form?
Throughout the solar system there are different types of large ice bodies, not only water ice but
also ice made up of ammonia, carbon dioxide and other substances that are gases or liquids at
conditions typical for the Earth’s surface, but may freeze solid at much lower temperatures. The
planets between the Sun and Earth (Venus and Mercury) are too warm for ice to build up. Mars
has a big polar icecap composed of water and carbon dioxide ice; this ice cap grows and shrinks
with the seasons, similar to the seasons on earth with different amounts of solar radiation received
at the earth’s surface in winter and summer. The surface layers of Jupiter and Saturn are composed
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Figure 1: Average snow depth in January (top) and June (bottom) for the years 1993 - 2003. Maximum
(black) in January is 1841 mm, and in June it is 1641 mm (data from from NASA ISLSCP GDSLAM
Snow-Ice-Oceans: Global data sets).
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of different types of ices and one of Jupiter’s moons, Europa, actually has an ice crust of several
tens to possibly hundreds of kilometers in thickness which is in many ways similar to ice features
visible on earth; even our moon may have some ice present within the upper soil layers.
On earth, we find glaciers and ice-sheets on every continent (if New Zealand taken as a part
of the Australian continent). Roughly 10 % of the total land surface is covered by glaciers or ice
sheets, and sea ice covers roughly 10 % of the world’s ocean surface.
Polar sea ice is one of the most variable features of the Earth’s climate, changing considerably
from summer to winter and from one year to another. At any given time, global sea ice covers an
area approximately the size of the North American continent. The presence of the ice restricts the
transfer of heat between the ocean and the atmosphere. The ice also restricts evaporation into the
atmosphere and affects the circulation of the ocean.
If we take into account the distribution of ice sheets during previous glaciations, glaciers have
at some point in time covered an even larger fraction of the earth’s surface; as a result, a significant
fraction of the landscapes on earth are influenced by glacial action. Glaciers are also an important
part of the hydrological cycle, see Table 1 which lists the distribution of water on earth.
Table 1. The distribution of Water on Earth
Ocean
Polar ice caps, icebergs, glaciers
Ground water, soil moisture
Lakes and rivers
Atmosphere
SUM
Volume (km3)
1 370 000 000
29 000 000
9 500 000
225 000
13 000
1 408 738 000
%
97.250
2.060
0.674
0.016
0.001
100.000
36 020 00
2.600
Fresh water
Fresh water as a % of its total
Polar ice caps, glaciers
Ground water to 800 m depth
Ground water from 800 m - 4 km
Soil moisture
Lakes (fresh water)
Rivers
Hydrated earth minerals
Plants, animals, humans
Atmosphere
SUM
77.230
9.860
12.350
0.170
0.350
0.003
0.001
0.003
0.040
100.000
Cryosphere. Cryosphere is the region where ice can form.
On Earth it ranges from about 600-800 m depth (where ice formation is limited by heat), to
6-18 km height where moisture is too low for formation of ice.
Snowline. The lower limit where snow can survive for years.
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The snowline is at sea level at the poles, 5500 m at 30◦, and 5000 m at the equator.
The snowline also generally rises from West to East on the continents. Moist air is carried
with lows that come from East. For example, on the W-coast of Norway the snow line is at 1000
m, but in Sweden it is at 2000 m. In Iceland the precipitation is greatest on the south coast.
Glaciers on the S-coast receive precipitation of 5000 mm/a and there the snow-line is 1100 m.
The snowline is above Herdubreid (1688 m) north of Vatnajökull, and the lowest snowline is at 700
m on Drangjökull in the NW-fjords.
Distribution Problems
1 Why is the snow line lower at the equator than at 30◦?
2 The surface area of Iceland is about 103 000 km2 , and the total volume of ice is about 4 000
km3 (glaciers cover about 11% of the surface of Iceland).
a) What is the mean thickness of Icelandic glaciers?
b) How thick would the ice cover be if the total volume were spread evenly out over Iceland?
3 Explain why the seasonal variation in the area covered by sea-ice is much greater in the
Southern hemisphere than the Northern hemisphere. (Name at least two reasons.)
Ball, P. 2000. Life’s Matrix: A biography of water . Farrar, Strauss and Giroux, New York, first
edition.
Hoffman, P. F., A. J. Kaufman, G. P. Halverson and D. P. Schrag. 1998. A neoproterozoic snowball
earth. Science, 281, 1342–1346.
Oerlemans, J. 1994. Quintifying global warming from the retreat of glaciers. Science, 264, 243–245.
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