My attention was drawn to a Working Paper by Alan Carlin (1) which was basically about how emissions reductions may be a dangerous strategy to avoid climate change. Much of his perceived threat is based on papers by Hansen (2007) and others who propose rapid melting of the Greenland and West Antarctica (henceforth Antarctica) Ice Sheets that causes a sea level rise of 5 m or more.
RAPID MELTING OF THE GREENLAND AND ANTARCTIC ICE SHEETS IS IMPOSSIBLE!
Hansen is a modeller, and his scenario for the collapse of the ice sheets is based on a false model.
Hansen has a model of an ice sheet sliding along an inclined plane, lubricated by meltwater, which is itself increasing because of global warming. The same model is adopted in many copy-cat papers. Christoffersen and Hambrey (2006) and Bamber et al. (2007). are typical papers, a popular article based on the same flawed model appeared in the June 2007 issue of National Geographic, and the idea is present in textbooks such as The Great Ice Age (2000) by R.C.L. Wilson et al.
Hansen’s model, unfortunately, includes neither the main form of the Greenland and Antarctic Ice Sheets, nor an understanding of how glaciers flow. The predicted behaviour of the ice sheets is based on melting and accumulation rates at the present day, and on the concept of an ice sheet sliding down an inclined plane on a base lubricated by meltwater, which is itself increasing because of global warming. The idea of a glacier sliding downhill on a base lubricated by meltwater seemed a good idea when first presented by de Saussure in 1779, but a lot has been learned since then.
It is not enough to think that present climate over a few decades can affect the flow of ice sheets. 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.
To understand what is possible it is necessary to know something about the physics of glacier flow, which explains a few things not accounted for in the Hansen model, including:
Why are ice crystals at the foot of a glacier a thousand times bigger than those in the snow that feeds them?
Why does lake ice deform at lower stress than other ice?
Why do crevasses only reach a limiting depth?
In reality the Greenland and Antarctic ice sheets occupy deep basins, and cannot slide down a plane. Furthermore glacial flow depends on stress (including the important yield stress) as well as temperature, and much of the ice sheets is well below melting point. The accumulation of kilometres of undisturbed ice in cores in Greenland and Antarctica (the same ones that are sometimes used to fuel ideas of global warming) show hundreds of thousands of years of accumulation with no melting or flow. Except around the edges, ice sheets flow at the base, and depend on geothermal heat, not the climate at the surface. It is impossible for the Greenland and Antarctic ice sheets to ‘collapse’.
A glacier budget
In general glaciers grow, flow and melt continuously, with a budget of gains and losses. 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. A mountain glacier flows mainly downhill, but can flow uphill in places. In an ice sheet the flow is from the depositional high centre towards the edges of the ice sheet. When the ice reaches a lower altitude or lower latitude where temperature is higher 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 exceeds melting the glacier grows. If melting exceeds precipitation the glacier recedes.
How glaciers move
Flow is mainly by a process called creep, essentially the movement of atoms from one crystal to another. The first clues to this came from the study of lake ice, which can flow at a stress much below the shear strength of ‘regular’ ice if the stress is applied parallel to the lake surface. This results from the crystal properties of ice. Ice is a hexagonal mineral with glide planes parallel to the base. Lake ice is a sheet of crystals with the c-axes vertical and the glide planes all parallel to the lake surface, so a push parallel to the glide planes deforms the ice readily. Much greater stress is needed to deform ice perpendicular to the glide planes.
Another method of flow is important in ‘regular’ ice. There is constant gain-and-loss of atoms between different crystals in a mass of ice, and in the absence of any stress an individual grain of ice will lose about the same number of atoms that it gains, and so remain unchanged. But if a crystal is stressed it will lose more atoms than it gains and so shrink, while a nearby unstressed grain will gain more than it loses and so grow. In this way there will be preferential growth of those ice crystals which are oriented in such a way that their glide planes are parallel to the stress, and grains in other orientations will tend to disappear. This is observed in glaciers, where it is found that a preferred crystal orientation appears with distance down-valley, and the ice crystals at a glacier snout have a volume about a thousand times greater than that of the first-formed ice crystals at the source of the glacier. These observations cannot be explained by mechanisms that ignore the crystal structure of ice.
The flow of material in a solid crystalline state is known as creep, and there are three laws of creep relevant to the flow of ice:
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 yield stress, below which creep does not operate.
All these laws have significant effects on glacier movement. Alpine glaciers differ significantly from the ice caps of Greenland and Antarctica, and care is needed to transfer knowledge of one kind of glacier to the other. Incidentally, the physics of ice as described here was worked out over 60 years ago, by people such as Perutz (1940)
Creep is proportional to temperature.
The closer the temperature comes to the melting point the greater the creep rate. In experiments at a fixed stress it was found that the creep rate at -1oC is 1000 times greater than at -20oC. In valley glaciers the ice is almost everywhere at the prevailing melting point of ice, because the latent heat of ice is very much greater than its specific heat. Very little heat is required to raise the temperature of an ice block from -1oC to 0oC; it takes about 80 times as much heat to turn the same ice block at 0oC into water at 0oC. Since the temperature does not vary in valley glaciers they are not affected by this first law of creep.
But ice caps are very different. They are cooled to temperatures well below freezing point, which reduces their capacity to flow very greatly. Ice caps can be kilometres thick, and their warmest part is actually the base, where the ice is warmed by the Earth’s heat, and where flow is concentrated. The drilling of the Northern Greenland Ice Core Project (NGRIP) was stopped by relatively high temperatures near the base and new equipment had to be designed to drill the core from 3001 m to 3085 m. It is because ice only flows at the base that great thicknesses of stratified ice can accumulate, as revealed in the ice cores.
Some Greenland cores show no flow at all. This is cold-based ice. A large geomorphology literature describes delicate landforms such as tors and patterned ground in areas that were formerly covered by an ice sheet. The general view is that cold-based ice essentially preserves any pre-existing landforms, and the erosion potential of cold-based ice is zero or minimal. Importantly for ideas of ‘collapse’, the ice is not sliding: it is not moving at all.
Greenland differs from Antarctica in that the ice sheet spills out through gaps in the mountain rim, and the glaciers overlie deep narrow valleys. According to van der Veen et al. (2007) such valleys have higher than usual geothermal gradients, so it might be geothermal heat, rather than global warming, that causes some Greenland glaciers to have higher than usual flow rates. The overspills have some of the characteristics of alpine glaciers, where evidence of glacier recession is more obvious.
Creep is proportional to stress (essentially proportional to the weight of overlying ice)
This means that the thicker the ice the faster the flow, but a great stress is required if the ice is very cold. This is shown by the huge thicknesses of undisturbed ice revealed by the ice cores that are used to work out palaeoclimates. In Antarctica, in the Vostok cores the undisturbed ice that provides the desired information continued to a depth of 3310 m or 414,000 years, but below this the ice starts to be deformed.
There is a minimum stress, the yield stress, 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, and all the ice below this limit must flow. The threshold boundary between non-flowing ice and flowing ice marks the yield stress level. The upper, brittle ice is a solid being carried along on plastic ice beneath. Since the flow is uneven the solid, brittle ice is broken up by a series of cracks called crevasses. The base of crevasses marks the position of the yield stress and the transition from brittle to plastic ice.
In Antarctic and Greenland ice sheets crevasses occur towards the edges, where the ice is flowing, but not in the areas of accumulation. In the middle of the ice sheets there are no crevasses to transmit meltwater to the base of the ice sheet, even if it were present (which is impossible).
Some results of the laws of glacier flow
These simple rules of creep allow us to understand some observations on glaciers
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
On July 21, 1983, the lowest reliably measured temperature ever recorded on Earth was at Vostok with −89.2 °C. The highest recorded temperature at Vostok is -19o C, which occurred in January 1992, and during the month of July 1987 the temperature never rose above -72.2o C. At these temperatures ice cannot flow under the pressures that prevail near the surface. Warming has no effect at such low temperatures: ice will not flow faster at -60oC than at -70o C.
Ice sheets may take many thousands of years 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
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 regional 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.
The age of ice sheets
In the Greenland ice sheet several cores have over 3 km of undisturbed ice which go back in time for over 105,000 years, much less than the Antarctic equivalent. The Vostok cores in Antarctica provide data for the past 414,000 years before the ice starts to be deformed. Dome F core reached 3035 m and Dome C core 3309 m, and both date back to 720,000 years. The Epica core in Antarctica goes back to 760,000 years, as does the Guliya core in Tibet. But what is more important than the age is that vast thicknesses of ice are preserved, and they retain complete records of deposition, in spite of the fact that temperatures at times during that period have been warmer than now. They do not fit the model of surface melting, even infrequently. After three quarters of a million years of documented continuous accumulation, how can we believe that right now the world’s ice sheets are collapsing!
The collapse of ice sheets
Some of the present-day claims that ice sheets ‘collapse’ are based on false concepts. 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 an ice sheet 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.
The very ice cores that are used to determine climates over the past 400,000 years also show that the ice sheet has grown over that period by accumulation of stratigraphic layers of snow, and has not been deformed or remelted. The mechanism portrayed by Christoffersen and Hambrey (2006), of meltwater lakes on the surface finding their way down through cracks in the ice and lubricating the bottom of the glacier is not compatible with accumulation of undisturbed snow layers. It might conceivably work on valley glaciers, but it tells us nothing of the ‘collapse’ of ice sheets.
The global warming doomsday writers claim the Greenland and Antarctic ice sheets are melting catastrophically, and will cause a sudden rise in sea level of 5 or more metres. This ignores the mechanism of glacier flow which is by creep. Glaciers are not melting from the surface down, nor are they sliding down an inclined plane lubricated by meltwater. The existence of ice over 3 km thick preserving details of past snowfall and atmospheres, used to decipher past temperature and CO2 levels, shows that the ice sheets have accumulated for hundreds of thousands of years without melting. Variations in melting around the edges of ice sheets are no indication that they are collapsing. Indeed ‘collapse’ is impossible.
Appenzeller, T. 2006. The Big Thaw. National Geographic, June 2007. 56-71.
Bamber, J.L., Alley, R.B. and Joughin, I. 2007. Rapid response of modern day ice sheets to external forcing. Earth and Planetary Science Letters, 257, 1-13.
Carlin, A. 2007. NCEE Working Paper #07-07.
Christoffersen, P. & Hambrey, M.J. 2006. Is the Greenland Ice Sheet in a state of collapse? Geology Today, v.22, pp. 98-103.
De Saussure, H-B. 1779-1796. Voyages dans les Alpes.(4 volumes) Manget, Geneva.
Hansen, J. 2007. Scientific reticence and sea level rise. Environmental Research Letters, May 24.
M.F. Perutz. Mechanism of glacier flow. Proc.Phys.Soc., 52, 132-135, 1940.
van der Veen, C.J., Leftwich, T., von Frese, R., Csatho, B.M. & Li, J. 2007. Subglacial topography and geothermal heat flux: Potential interactions with drainage of the Greenland ice sheet, Geophysical Research Letters, v.34, LI2501, doi:10.1029/2007 GL030046.
Copyright 2007, Cliff Ollier – reproduced with permission
Cliff Ollier, School of Earth and Geographical Sciences, The University of Western Australia, Crawley, WA 6009, Australia [email@example.com]