GLACIERS AND ICE-SHEETS
Cliff Ollier*
To understand the relationship between global warming and the breakdown of
ice-sheets it is really necessary to know how ice-sheets work. Ice-sheets do
not simply grow and melt in response to average global temperature. Anyone with
this naïve view would have difficulty in explaining why glaciation has been
present in the southern hemisphere for about 30 million years, and in the
northern hemisphere for only 3 million years.
In general glaciers grow, flow and melt continuously. There is a budget of
gains and losses.
A glacier budget
Snow falls on high ground.
It becomes more and more compact with time, air is extruded, and it turns into
solid ice. A few bubbles of air might be trapped, and may be used by scientists
later to examine the air composition at the time of deposition.
More precipitation of snow forms another layer on the top, which goes through
the same process, so the ice grows thicker by the addition of new layers at the
surface. The existence of such layers, youngest at the top, enables the glacial
ice to be studied through time, as in the Vostok cores of Antarctica, a basic
source of data on temperature and carbon dioxide over about 400,000 years.
When the ice is thick enough it starts to flow under the force of gravity. In a
mountain glacier it flows downhill, in an ice-sheet from the depositional high
centre towards the edges of the ice-sheet. The flow is generally slow, as
expressed in the common metaphor. "glacially slow". The Upernivek Glacier in
Greenland flows at about 40 metres per day, which is as much as a smaller Alpine
glacier covers in a year.
When the ice reaches a lower altitude or lower latitude where temperature is
warmer it starts to melt and evaporate. (Evaporation and melting together are
called ablation, but for simplicity I shall use 'melting' from now on).
If growth and melting balance the glacier appears to be 'stationary'. If
precipitation and growth exceeds melting the glacier grows. If melting exceeds
precipitation the glacier appears to recede.
How glaciers move
Flow is by a process called creep, essentially the movement of atoms from one
crystal to another, and the size of crystals grows by a thousand times from the
tiny crystals deposited as snow to the large crystals found at the glacier
snout.
There are three laws of creep:
1. Creep is proportional to temperature.
2. Creep is proportional to stress (essentially proportional to the weight of
overlying ice)
3. There is a minimum stress, called the threshold stress, below which creep
does not operate.
All these laws have significant effects on glacier movement, and on how glacial
behaviour might be interpreted.
Creep is proportional to temperature.
In valley glaciers the ice is almost everywhere at the prevailing melting point
of ice, so it is not an important feature.
In ice-sheets the temperature gets very much below freezing point, so flow is
very limited in most of the very cold ice. At the base of the glacier the ice
is warmed by the Earth's heat, and the flow is concentrated at and near the base
of the glacier. This is why the stratified layers of ice are preserved in the
upper ice, and can be recovered in cores like the Vostok cores.
Creep is proportional to stress (essentially proportional to the weight of
overlying ice)
This means that the thicker the ice, the greater the stress at depth, and the
faster the flow.
In a valley glacier there is frictional drag at the base, and no flow at the top
because it is below threshold stress (explained below), so the maximum flow is
somewhere in the middle.
In an ice-sheet the greatest stress will be at the base under the thickest ice.
Again we see that the upper ice will be preserved, which we already know from
the many cores.
There is a minimum stress, the threshold, below which creep does not operate.
At the surface there is no stress, so the ice does not flow: at a certain depth
the weight of ice is sufficient to cause flow. Between these two limits the ice
is a brittle solid being carried along on plastic ice beneath. Since the flow
is uneven (greatest in the middle in valley glaciers) the solid, brittle ice is
broken up by a series of cracks called crevasses.
Some results of the laws of glacier flow
These simple rules allow us to understand some observations on glaciers
Glacial surges
The speed of valley glaciers has been measured for a long time, and is rather
variable. Sometimes a valley will flow several times faster than it did
earlier. Suppose we had a period of a thousand years of heavy precipitation.
This would cause a thickening of the ice, and more rapid glacial flow. The
pulse of more rapid flow would eventually pass down the valley. It is important
to understand that the increase in flow rate is not related to present day air
temperature, but to increased precipitation long ago.
Melting and climate
In the case of ice-sheets it may take many thousands of years for ice to flow
from the accumulation area to the melting area. The balance between movement
and melting therefore does not relate simply to today's climate, but to the
climate thousands of years ago.
Glaciers and precipitation
We have seen that glaciers and ice-sheets are in a state of quasi-equilibrium,
governed by rates of melting and rates of accumulation.
For a glacier to maintain its present size it must have precipitation as
snowfall at its source. This leads to a slightly complex relationship with
temperature. If the regional climate becomes too dry, there will be no
precipitation, so the glacier will diminish. This could happen if the region
became cold enough to reduce evaporation from the ocean. If temperatures rise,
evaporation is enhanced and so therefore is snowfall. Paradoxically a rise of
temperature may lead to increased growth of glaciers and ice-sheets. Today, for
example, the ice-sheets of both Antarctica and Greenland are growing by
accumulation of snow.
Icebergs
Where ice-sheets or individual glaciers reach the sea, the ice floats and
eventually breaks off to form icebergs. This is inevitable so long as glaciers
reach the sea. In the southern hemisphere Captain Cook saw icebergs on his
search for the great south land. Ice-bergs have long been familiar to sailors in
the northern hemisphere, and the Titanic struck one that had drifted farther
south than usual in 1912. The actual break is a sudden, one-off event, but can
be built into a typical greenhouse-horror scenario. Some weeks ago, when a piece
of the Greenland ice shelf broke away, the scientists interviewed all said they
were surprised at how suddenly it happened. But how else but suddenly would a
piece of ice shelf break off! And this was an area that was ice free before the
Little Ice Age, and possibly after as well - Arctic explorers used to get their
ships a lot closer to northern Greenland than you could now.
Hansen's view of glacier collapse
In a television interview on March 13, 2007, Jim Hansen claimed that a rise in
temperature of a few degrees in the next few years would cause 'collapse' of the
ice-sheets and a rise of sea level of many metres.
Hansen's view of ice-sheet 'collapse' is untenable.
Ice-sheets do not melt from the surface down - only at the edges.
Once the edges are lost, further loss depends on the rate of flow of the ice.
The rate of flow of ice does not depend on the present climate, but on the
amount of ice already accumulated, and that will keep it flowing for a very long
time.
It is possible that any increase in temperature will cause increased snowfall
thereby nourishing the growth of the ice-sheet, not diminishing it.
While Hansen concentrates on ice-sheets, evidence of glacier recession is more
obvious in alpine glaciers. In many parts of the world glaciers have been
receding since 1895, and with increasing pace since 1930. This is the wrong time
scale to be associated with Hansen's hypothesis, and the dates have no
counterpart in carbon dioxide records..
*) Emeritus Professor Cliff Ollier., D.Sc. Research Fellow at the University of
Western Australia. Formerly at A.N.U., U.N.E., Canberra University, University
of Papua New Guinea, Melbourne University. Has worked all over the world as a
geologist, geomorphologist and soil scientist. Author of about ten books,
several translated into foreign editions, and over 300 publications.