One of the aims of the ice2sea programme is to produce projections of how the volume of ice contained in glaciers and ice sheets will change as they adapt to a changing climate. It is important to understand the effect that this will have on global sea levels in coming decades to centuries. There is plenty still to be learned and discovered about the cryosphere, which comprises all ice on Earth, but what do we already know about glaciers and ice sheets?
A glacier is defined by glaciologists as a body of snow and/or ice in which the ice itself is moving from an area of accumulation to an area of loss (ablation). That is, glaciers constantly move, but their slow speed – usually measured in metres per year, although a few flow at up to several km per year – means that we cannot directly observe this movement. Areas where snow simply falls and rests, eventually melting where it has fallen, are described as having “snow-cover,” which is usually temporary.
Ice sheets are particularly large glaciers that cover vast areas – there are only two ice sheets in existence today, one covering Antarctica and the other covering most of Greenland. During the last ice age, ice sheets also covered most of North America and large parts of Scandinavia and northern Europe, reaching as far south as East Anglia in the UK. The vast quantities of water stored in these ice sheets meant that sea levels were up to 120m lower than they are today.
In the present day, the world’s valley glaciers and ice caps contain enough ice to raise global sea levels by 50cm if they were all to melt. The Greenland ice sheet contains far more ice; if all of that were to melt global sea levels would rise by around 7 metres! Antarctica is much bigger still; global sea levels would rise around 60 m if that was to melt completely – although it has not done so at any point in the last 25 million years.
Equilibrium and Climate Change
Glaciers are naturally self-regulating systems that have internal processes that control their size and the volume of ice that they contain. These processes are most easily understood by a simple analogy:
Imagine a bucket that stands under a tap and that has a hole near its bottom. When the tap is opened the bucket will begin to fill (see figure). If the hole is small enough, the water level will rise above the hole, and eventually adjust itself until the rate of flow out of the hole exactly matches the flow into the bucket from the tap. Once this happens, equilibrium is established and the water level remains constant. The key feature that allows this to happen is that flow out of the hole increases as the water level rises. If the rate of flow of the tap is increased (see figure, right), the water-level and therefore pressure rises, until once again the flow increases to match the in-flow. A new stable water level is therefore found.
In this analogy the bucket is a self-regulating system, which is actually very similar to a glacier. The input into a glacial system is supplied not by a tap but by annual snowfall on parts of the glacier. The hole in the bucket is replaced by the glacier terminus, where snow and ice are removed from the glacier by melting, and sometimes by iceberg production. The movement of ice between the two areas is driven by gravity, and crucially becomes faster as the glacier volume and mass increases.
Suppose then that an increase in snowfall occurs: the glacier will flow faster and have more mass, meaning it will flow further down a slope. Generally, temperatures are warmer at lower elevations so that this advance into lower areas causes higher melt rates, which will equal the higher snowfall rate if a new equilibrium is reached. So overall, the situation in a glacier is the same as in our analogy: once equilibrium is established the volume of ice contained in the glacier remains stable at a level that is dependent on the rate of snowfall. Any change, either on the inflow, or indeed the outflow, will produce a period of glacier growth or retreat until a new equilibrium is established.
Conceptually, the polar ice-sheets behave in a similar way as glaciers – they just contain a lot more ice!
There is sufficient ice held in glaciers in the mountainous regions of the world that if they were all to melt, they would cause global sea levels to rise by about 50 cm. The Greenland ice sheet contains more ice; if all of that was to melt global sea levels would rise around 7 metres! Antarctica is much bigger still; global sea levels would rise around 60 m if that was to melt completely – although it has not done this at any point in the last 25 Million years.
The ice2sea programme will produce projections of how the volume of ice contained in glaciers and ice sheets will change and what effect that will have on global sea levels in coming decades to centuries.
How do Glaciers move?
There are three different processes which cause a glacier to move flow towards the coast.
1. Ice Deformation: Like all materials, ice will change its shape and deform if it is put under enough pressure. Since pressure from the weight of the overlying ice is greatest near the bottom of a glacier, these bottom layers will deform more than layers near the top. However, since this movement adds up from the bottom layers upwards, the ice surface moves the furthest due to deformation while the bottom layers moves the least. This is easily visualised using a pack of cards: if the whole pack except for the bottom card is shifted, and the whole pack except for the second-from-bottom card is shifted again, then the top fifty cards will have moved more than the bottom two. This process can be carried on right the way to the top card, or the surface of the glacier.
2. Basal Sliding: This is best visualised as the glacier, as a whole, sliding over a slippery ground surface. In order for the ground to be slippery, water is required – this means that the interface between ice and ground must be at the melting point of ice – otherwise, any water would simply freeze on to the bottom of the glacier. Of course, the bed underneath a glacier is not usually smooth but rather bumpy, so one might think that this extra friction would slow the glacier down. However, these bumps do the opposite: they increase the pressure at the bed of the glacier so that the ice deforms and “flows” around these bumps. In many ways, then, glaciers actually behave like slow, frozen rivers!
3. Bed Deformation: Sometimes, glaciers rest not on solid bedrock but on loose sediment and soft, deformable material. In this case, water at the ice-bed interface will not simply lubricate the glacier and allow it to slide. Much rather, the water will soften the bed even further, eventually allowing it to deform and almost flow along at the bottom of the glacier, carrying the overlying ice along with it and adding to its speed.
Valley Glaciers and Ice Caps
The steady-state view of glaciers as static input-output systems is a little too simplistic – after all, a perfect steady state is practically never achieved in reality. Besides the climatic control on long time-scales, unusual weather patterns, or even one-off events such as rock-fall or avalanches onto the ice surface or volcanic eruptions can have an impact on the amount of mass it gains or loses over the course of a year (its mass balance). There are also a few glaciers that completely fail to self-regulate in this steady-state manner but instead exhibit cyclical behaviour called surging. These glaciers will flow unusually slowly for a long time and gradually gain mass before suddenly accelerating dramatically and losing the excess mass in a short period of time.
Generally, no two glaciers are the same – they can differ in a huge range of characteristics. They may end on land and have meltwater streams at their front, or in the sea (tidewater glaciers) where they can release icebergs. Their altitude, latitude and general location determines the amount of snow (or rainfall) they get every year, as well as winter and summer temperatures and the length of the summer melt season. All these factors can have a profound impact on a glacier’s response to climatic change. For example, a glacier in southern Alaska may not shrink at all despite strong warming in the region because higher snowfall during the winter helps offset growing summer mass loss. Other glaciers may be situated in regions that are so cold throughout the year that slightly higher temperatures will not cause any more melting. Yet other glaciers have already been affected by changing climate so drastically that they will not survive in the long term.
Since the ice lost from glaciers will eventually make its way to the ocean, it is crucial for us to gain an understanding of how exactly changing climate will affect glaciers around the world. After all, if glaciers around the world are losing mass on average, they will make a significant contribution to sea-level rise.
Ice sheets contain far more ice than all the other glaciers in the world combined, but their sheer size also means that changes in climate will take far longer to affect them than a smaller glacier. For example, there are theories that the Antarctic ice sheet is still responding to changes which occurred at the end of the last ice age some 10,000 years ago. Simply put, this is because a change occurring at the surface of an ice mass will propagate to the bottom of that ice mass more slowly if the ice is thicker.
For the most part, ice sheets consist of extremely slow-moving ice, with a few vast, fast-flowing glaciers draining these huge expanses of near-stationary ice. Nevertheless, ice sheets are not just static “blocks” of ice that sit upon the ground. They are governed by their own internal dynamics, the detailed workings of which are only just being discovered.
More importantly, recent research indicates that it is not warming air temperatures that affect ice-sheet mass balance most. Since most glaciers flowing from the Greenland and Antarctic ice sheets terminate in the sea, unusually warm sea temperatures have caused high rates of melting and iceberg production which have contributed to ice-sheet mass loss. This is because the floating parts of glaciers, called ice tongues or ice shelves, act as buttresses for up-glacier ice, which flows more slowly thanks to this effect. If these buttresses shrink or disappear, then the up-glacier ice will accelerate, flowing into the sea faster and melting – a vicious cycle can ensue. This faster flow also means that more ice is drawn down to lower elevations, where temperatures are generally warmer – this can further enhance melt.
It is unlikely that either of the two great ice sheets will melt completely in the foreseeable future, if ever. Nevertheless, they are among the least understood, and potentially biggest, contributors to future sea-level rise, and as such it is of the utmost importance to understand how these great ice masses will develop in the future.