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All about Glaciers

Objective: Consider what glaciers are and why they are important. Have a look at where glaciers are located around the world today and where they have been located in the past.

The Cambridge English dictionary defines glaciers simply as "large masses of ice that move slowly". While this definition is, in essence, correct, it also misses the vast variety of and complexity of glaciers. A permanent snow patch transitions to being a glacier the moment is is thick enough to move and flow under its own weight- a threshold usually reached at thicknesses of a few tens of metres. A newly formed glacier, recently born out of a small permanent snow patch may be on the scale of a few hundreds of metres. On the other end of the scale, continental ice sheets may be many thousands of kilometres across and lock up orders of magnitude more fresh water than even the largest lakes. Some glaciers are "large", while others may be fairly small or absolutely enormous.

The definition also proposes that glaciers move slowly. For the smallest examples, freshly converted snowpatches or small valley glaciers, this is true. Glaciers deform slowly under their own weight and for thin glaciers will move only a few metres a year. They move, but barely on human timescales. This is not the case however for many of the larger glaciers, particularly in regions where large volumes of ice are channelled into a thin outlet valley, known as an "outlet glacier" or "ice stream" depending on the scale/ and geometry. In these scenarios ice can move at hundreds to thousands of metres a year, moving vast volumes of fresh water across the landscape.

One of the most dramatic examples, Jakobshavn Glacier in Greenland has been measured flowing at nearly 20 kilometres per year. To give a measure of how rapid this is, scientists could install a sensor on the ice to measure temperatures, come back a month later to recuperate it and find it more than 1500 m downstream of where they installed it! With a discharge of a couple of hundred cubic kilometres a year, Jakobshavn glacier is moving roughly the same volume of fresh water as large rivers such as the Indus in Asia, the Columbia in North America or the Danube in Europe (and more than double that of the longest river in the world, the Nile). The total flux of the Antarctic Ice Sheet to the ocean (around 2000 gigatons of ice a year), it is equal to more than four times that of the Mississippi river .

Satellite image of Jakobshavn Glacier, Greenland. This large outlet has experienced rapid and accelerating retreat over the past 150 years, contributing to sea-level rise. It is hard to describe this enormous glacier as "slow moving", given that it flows several kilometres a year and discharges twice as much freshwater to the ocean as the river Nile.

These small glaciers on Mont Blanc, France may be barely thick enough to flow with speeds of a few metres a year. Excluding meltwater, this makes for freshwater discharges of less than a litre a second, roughly equal to an average kitchen tap. Climate warming has caused many of these smallest glaciers to disappear entirely.

So some of the largest glaciers discharge as much water as continental rivers, while the smallest barely more than a dripping tap. Some of the processes driving the flow of both are the same, while others are very different. Let's start by having a look at the two main ways in which glaciers flow:

Internal deformation
Basal sliding

1) Internal deformation

Ice, just like any other fluid (simply over longer timescales due to its high velocity), flows downslope due to gravity. In this endmember, the glacier is frozen to the bed and flow occurs entirely due to internal deformation. Flow velocity is highest at the surface and drops to zero at the base.

2) Basal sliding

If ice is either thick enough to melt at the base, or located in a warm region where meltwater is present it may slide at the base. The more water is present at the base, the faster this slip may be. In this scenario, ice velocity will be identical throughout the ice column with the base sliding as fast as the surface.

In reality, many glaciers experience a combination of the two above behaviors with varying proportions in space and time (many glaciers will have a seasonal cycle with slip in the summer and only deformation in the winter). A few other types of motion may also be locally important, including brittle failure (faulting and crevassing in glaciers) and sliding on a deformable bed of weak or water-filled sediment.

The map above shows mean ice velocities of the entire Antarctic Ice Sheet as calculated from radar satellites. Ice velocity ranges from ~1 metre per year (red areas) to several kilometres per year (light purple). Purple areas in general are dominated by basal sliding, with internal deformation dominant over the rest of the continent. Image from NASA.

The map above shows that large ice sheets exhibit some very complex flow dynamics: flow is slow and steady on the crest of the ice sheet and is focussed into thin bands of high velocity (ice streams) which direct this ice to the coast. These ice streams often excavate deep subglacial valleys due to their prolonged rapid flow and are particularly vulnerable to rapid retreat. Interactions between these ice streams and the surrounding ocean and atmosphere can be extremely complex and will be discussed more in the specific article on Climate-Ice interactions. The diagram below showcases a number of these processes.

Example of some interactions between the atmosphere, oceans and ice sheets. Marine terminating glaciers, also known as 'tidewater' glaciers are particularly vulnerable to changes in conditions due to their close connection with the rest of the climate-ocean system.

The Earth is warmer today than it has been for much of the past million years, and with anthropogenic climate change is becoming warmer still. Nevertheless, there are two major ice sheets remaining and glaciers on 6 out of the 7 continents (Australia is entirely ice free... although there are glaciers on New Zealand). Recent glacier inventories identify on the order of 200,000 individual glaciers dotted around mountain ranges and high latitudes, with the vast majority of these thinning and retreating. Only 10-15,000 years ago ice coverage of Earth was considerably more extensive, with major ice sheets covering large parts of North America and Europe. Smaller ice sheets were also present in Asia and South America, and small glaciers were present in Australia and on Hawaii. The total mass of ice led to global sea levels being ~100 metres lower than today, exposing large parts of the present-day seabed at the surface. The two following maps show the extent of glaciers today and at the last glacial maximum.

Extent of ice at the present day (2015 Randolph Glacier Inventory). Note at least small glaciers on all continents except Australia.

Locations of glaciers at the Last Glacial Maximum around 15-20 thousand years ago (Ehlers et al., 2011) . The world has fluctuated in and out of glacial periods for more than a million years.