Glaciers and Ice-Sheets

Cliff Ollier

Introduction

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. 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 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.