Understanding ice

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).  Areas where snow simply falls and rests, eventually melting where it has fallen, is described as having “snow-cover”.  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.

Glaciers are naturally self-regulating systems that have internal processes that control their size and the volume of ice they contain.  The basic concepts are most easily understood by a simple analogy.

Imagine a bucket that has a hole near its bottom that stands under a tap.  When the tap is opened the bucket will begin to fill.  If the hole is small enough, the level of the water level will rise above the hole, and eventually adjust itself under the rate of flow out of the hole exactly matches the flow into the bucket from the tap.  Once this happens, equilibrium is established so that the flow out of the bucket matches the flow in, 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, the water-level rises, until once again the flow increases to match the in-flow, and a new stable water level is found.

In this example the bucket is a self-regulating system, which is actually very similar to a glacier, except that in the glacier the inflow is supplied not by a tap, but by annual snowfall on part of the glacier, and the hole in the bucket is replaced by the glacier terminus, where the snow and ice are removed from the glacier by melting, and sometimes iceberg production.  The movement of ice between the two areas is driven by gravity, and crucially becomes faster as the glacier volume increases.  So overall, the situation in a glacier is the same as in our example, 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.

The only glaciers that don’t share this ability to self-regulate and establish an equilibrium are a small number that are prone to surging, and others where the climate has altered so drastically that in the long-term they will not survive.  For the rest, during periods when climate is stable, the volume of ice contained in the glacier remains roughly constant, at least when averaged over a period of years.  But as climate changes the volume of ice in a glacier will alter, and as it does, the lost ice with eventually find its way into the oceans.  If, around the world, glaciers are on average losing ice into the oceans, they will make a significant contribution to sea-level rise.

Conceptually, the polar ice-sheets behave in exactly the same way as glaciers – they just contain a lot more ice!

The ice2sea programme will produce projections of how the volume of ice contained in glaciers and ice sheets will change and the effect that they will have on global sea levels in coming decades to centuries.

Glaciers vs. ice sheets

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 m!  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.

to be continued…