Glaciers and Ice-Sheets
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
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
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
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.
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. Icebergs 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
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.