Over recent years, a large amount of attention has been paid to ice sheets in the Arctic and Antarctic, by both scientists and the media.  But why are ice sheets so important?

Satellite images of the Greenland and Antarctic ice sheets. The divide between the two main Antarctic ice sheets is shown by the orange dashed line, and a further Antarctic Peninsula Ice Sheet exists on the finger of the continent.


What is an ice sheet? How does one form?

An ice sheet is defined as mass of glacial ice which is over 50,000 km².  Ice sheets form when it is cold enough for snow to remain in the same place for a number of years.  Over time this snow is buried by fresh snow, compressed, and turned into ice.  To form glacier ice in this way can take several hundreds of years, and thousands of years to create large ice sheets.  There are currently three main ice sheets, the Greenland Ice Sheet, the East Antarctic Ice Sheet, and the West Antarctic Ice Sheet (the latter two are often collectively called the Antarctic Ice Sheet).  However, during the height of the last glacial period (about 24,000 years ago) ice sheets covered much of North America, northern Europe, Scandinavia, and Patagonia.

 

Ice extent in the Northern Hemisphere during the last glaciation (in black). Map centred over the North Pole.

Why is it important to understand how they work and what’s happening to them?

Together, the ice sheets from Greenland and Antarctica hold more than 99% of the world’s freshwater ice.  These are enormous stores of frozen water, held on land.  If they fully melted, Greenland and Antarctic would respectively cause 7.2 m and 61.1 m of global sea level rise.  Though highly unlikely that the near future will see the complete melting of these ice sheets, they remain an important water store.

As the climate changes, becoming either warmer or colder, important changes happen to these ice sheets at the pole.  Increases in air temperature have two effects upon these large ice masses.  Warmer temperatures cause more evaporation from the oceans, and therefore more snow will fall on the ice sheet.  However, higher temperature also cause heating of the ice sheet surface, leading to increased melting, at a rate greater than the increase in snowfall.  As the edges of both the Greenland and Antarctic ice sheets are in the ocean, the temperature of the ocean also has a large effect on ice sheets.  Changes in ocean temperature can increase or decrease the amount of melting the ice sheet experiences, adding more or less water to the ocean.

What other parts of the Earth’s system do they affect?

Ice sheets also have other important impacts upon the Earth’s climate system.  As they are large, land masses at high altitudes (over 2000 m above sea level), they have impacts upon atmospheric circulation, often affecting storm tracks, and creating their own wind systems.  Ice sheets also affect the Earth’s climate through a phenomenon called “albedo”.  The albedo of a surface is the amount of sunlight it reflects back to the atmosphere.  So if a surface’s albedo is low (as in water or land), most of the energy from the sun is absorbed.  However, if it is high (as in snow and ice), the majority of energy is reflected back into space.  Therefore, if the ice sheets in Greenland and Antarctica get smaller, more ocean and land will be revealed, meaning the Earth will absorb more energy, causing further warming of the Earth.

Part of an ice core from the West Antarctic Ice Sheet

Finally, the ice sheets can provide scientists with very important details about past changes in the Earth’s climate.  Ice cores can be extracted from the centre of ice sheets and analysed.  As shown above, ice sheets are formed by layers of compressed snow, collected over thousands of years.  Ice cores through these layers can be used as an important palaeo-record.  Each layer (representing one year of snow fall) contains trapped gases, dust, and frozen water.  Scientists can study these to reconstruct the Earth’s climate and atmosphere in the past.