Ice sheets and sea-level rise

'Fish scale' or 'dragon scale' ice, formed when wave action breaks apart newly forming ice and sudden pressure causes the scales to pile up.
'Fish scale' or 'dragon scale' ice, formed when wave action breaks apart newly forming ice and sudden pressure causes the scales to pile up. (Photo: Tony Worby)
MODIS satellite image of Wilkins Ice Shelf break up Aerial photo of the ice of the Lambert Glacier moving between two landforms.Schematic showing the relationship between ice sheets, attached to the continent, ice shelves, attached to the ice sheet but floating in the ocean, and sea ice, formed when the ocean surface freezes. A graphic showing how the West Antarctic ice sheet is attached to bedrock below sea level.Satellite image showing the break-up of the Wilkins Ice Shelf in 2008. This 60 km section of radar signals over the Aurora Basin shows the lower half of the East Antarctic ice sheet. The strong bedrock reflection is seen through about 4 km of ice, and internal layers in the ice can be seen sweeping over an 800 m change in bedrock height.

Australia has collected data on ice flow, ice thickness and other ice sheet characteristics in East Antarctica for over 50 years. These data allow researchers to calibrate and validate new satellite measurements of the ice sheet and to develop models used to project the ice sheet's response to climate change.

The intense cold of the Antarctic ice sheet affects the global climate system through changes in surface energy and moisture, clouds, precipitation, and atmospheric and ocean circulation. If the Antarctic ice sheet melted, it would raise global sea level by nearly 60 metres. However, the response of the ice sheet to global warming is the largest unknown in projecting future sea level over the next 100–1000 years.

A major focus of Australia's ice sheet research has been in the Lambert Glacier basin. At the Amery Ice Shelf, where the Lambert Glacier flows out to sea, research aims to understand the thermal and salinity interactions between the ice and the underlying ocean. Scientists have found that ice near the base of the ice shelf is porous and infiltrated with sea water, making it highly vulnerable to rapid melting.

Is the Antarctic ice sheet growing or shrinking?

In 2013, the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5) concluded that the Antarctic ice sheet, as a whole, was contributing to sea level rise at a rate of 0.41 mm/yr. Ice loss occurred mostly from increased discharge of icebergs by large outlet glacier systems in the Amundsen Sea and Bellingshausen Sea regions of West Antarctica. Loss also occurred by melt along the Antarctic Peninsula, where air temperatures have risen over the last 50 years. 

Studies of snowfall and ice loss (‘mass budget’) in Greenland and Antarctica have been made using satellite altimetry, satellite gravity measurements and estimates of the difference between net snowfall and discharge of ice. These confirm that both the Greenland and Antarctic ice sheets are losing ice mass and contributing to sea level rise. 

The average rate of ice loss from the Antarctic ice sheet has likely increased from about 30 Gt (giga tonnes; 1 Gt = 1 billion tonnes) per year between 1992 and 2001 to about 147 Gt/year between 2002 and 2011. (100 Gt of ice per year is equivalent to approximately 0.28 mm/yr of global sea level rise). While the range of estimates from the different studies is large, they all suggest a net loss.

While ice loss has been greatest along coastal sectors of the Antarctic Peninsula and West Antarctica, ice thickening (gain) further inland and over much of East Antarctica may have partially offset this loss. All of the available estimates, however, show that the loss of mass in West Antarctica is greater than any added mass in East Antarctica.

What about the Greenland Ice Sheet?

In Greenland the average ice mass loss from 2002 to 2011 has been about 215 Gt/yr – almost 1.5 times that of Antarctica and contributing approximately 0.60 mm/yr to sea level rise. This loss has increased from about 34 Gt/year between 1992 and 2001. This negative mass balance is the result of an increase in runoff and enhanced ice discharge, due to the increased speed of some outlet glaciers.

For a few days in July 2012 there was an extreme surface melt event covering more than 90% of the Greenland ice sheet. Such extreme melt events are rare and have been observed in ice core records only twice; once in 1889, and once more, seven centuries earlier in the Medieval Warm Period.

Why is Antarctic ice melting faster over the Peninsula and in West Antarctica than in East Antarctica?

The Antarctic Ice Sheet is complex, and different regions respond differently.

Ice loss by melting along the Antarctic Peninsula is a direct result of increased ocean and air temperatures. The rate of temperature rise in this region (2.5°C over the last 50 years) is one of the highest on our planet.

Increased ice discharge from glaciers is, in some cases, a result of the collapse of floating ice shelves. In the more northerly parts of the Antarctic Peninsula, large ice shelves are eroded from beneath by warming ocean waters, and a number of these ice shelves have catastrophically disintegrated. Although the collapse of a floating ice shelf does not add to sea level, the removal of buttressing by the ice shelves may ‘unplug’ land-based glaciers behind the former ice shelves, and these can then flow more rapidly into the sea. As sea ice is also believed to have a buttressing effect, the loss of sea ice in polar regions could also contribute to this process.

The cause of acceleration of other large outlet glaciers in West Antarctica is not fully understood, but may be related to marine ice shelf instability (discussed under the next question).

Over most of East Antarctica surface temperatures are well below the freezing point, and a small increase in temperature cannot initiate surface melt. Warmer temperatures, however, allow the atmosphere to hold more water vapour, and thus lead to increased snowfall. An increased input of snow may be causing East Antarctica to grow slightly, but any gain here is more than offset by loss from West Antarctica and the Antarctic Peninsula.

Could the West Antarctic ice sheet continue to add to sea level rise?

Yes. The West Antarctic ice sheet forms what is called a marine ice sheet – the ice is resting on bedrock, but that bedrock is below sea level (image five in panel above). Where the bedrock under a marine ice sheet slopes down towards the interior of the continent, such as under parts of West Antarctica, the ice sheet may be unstable. If the coastal part of the ice sheet thins, it will start to float and is then able to flow more rapidly. This drains more ice from further inland, which may also start to float and, with bedrock that slopes backwards and becomes deeper further in, continued retreat of the grounded ice sheet may proceed very rapidly. A small retreat could in theory destabilize large sections of West Antarctica ice sheet, leading to rapid disintegration.

What will be the contribution of the ice sheets to future sea level rise?

The IPCC AR5 states that sea level rise from thermal expansion of the ocean, melt of small glaciers and ice caps, and from Greenland and Antarctica (based on a wide range of emission scenarios) would be in the range 0.26 to 0.98 m by 2081–2100. The ice sheet contribution to this estimate comes mostly from melt in Greenland and from the Antarctic Peninsula. This estimate does not include further accelerated discharge from ice sheet outlet glaciers.

Estimating any extra sea level rise from further acceleration of outlet glaciers is not straight forward. Processes such as those controlling basal sliding of glaciers (where water at the bed of the glacier lubricates it and allows it to move more rapidly) are not well understood. The previous Assessment Report of the IPCC (IPCC AR4) estimated that dynamic ice sheet accelerations from processes such as marine ice sheet instability and accelerated basal sliding might add another 0.1 to 0.2 m of sea level rise over the next century. IPCC AR5 reported that this additional contribution would not exceed several tenths of a metre of sea level rise during the 21st century.

Are reports of sea level rise of 6 m correct?

With recent observations of the speed-up of some glaciers in both Greenland and Antarctica, it has been argued that the IPCC estimate of the ice dynamic effect may be too low. Total sea level rise of as much as 6 m over the next century has been proposed based on a comparison with sea level rise rates at the end of the last ice age.

However, at the end of last ice age there was three times as much ice to melt as there is presently on the Earth. A rise of sea level by 6 m over the next century is improbable within constraints of the area of present day ice sheets, and the rate at which glaciers can accelerate.

IPCC AR5 concludes that there is currently insufficient evidence to evaluate the probability of specific sea levels above the assessed likely range of 0.26–0.98 m by 2081–2100 depending on carbon emission scenarios.

Do ice shelves contribute to sea level rise?

A number of floating ice shelves along the Antarctic Peninsula have disintegrated dramatically over the last decade. The cause of their catastrophic collapse is a combination of melting at their base, which thins and makes them more vulnerable, and warmer summer temperatures causing increased surface melt that can lead to rapid disintegration. Large areas of ice shelves (thousands of square kilometres of ice that is 100 to 200 m thick) have broken into small pieces and disintegrated within a few weeks.

An example of this is the Wilkins Ice Shelf in the western Antarctic Peninsula. The Wilkins Ice Shelf underwent significant changes in 2008 after two significant break-up events in February and May 2008 and further losses in June and July 2008. These changes were attributed to strong regional warming, and melting of the ice shelf from below.

Loss of ice shelves does not contribute to sea level rise as they are already floating. But where ice shelves buttress glaciers flowing into the sea, accelerated glacier outflow can add to sea level rise. This is not the case for Wilkins Ice Shelf, but did occur when the Larsen B Ice Shelf dramatically collapsed in 2002.

What are the gaps in our knowledge that restrict better estimates of future sea level rise?

The main gaps in our understanding are of some aspects of ice sheet dynamics.

There is a need to improve our mathematical models of glaciers, ice sheets and ice shelves to be able to better project future changes. We also need more detailed measurements of how deep the bedrock is under the ice sheets, to use in the models.

Another major gap concerns what is happening at the bed of the ice sheets – how they react with liquid water at the base, what role water may have in the ice sliding over the bedrock, and the role of gravels and slurry at the base. We now know there is a lot of liquid water under the ice sheets, but we don't really know how changes in this may affect the ice flow.

What ice sheet research is Australia involved in?

To help address gaps in the scientific knowledge about ice sheets, an international team from Australia, the US and UK, are involved in the ‘ICECAP’ project - 'Investigating Cryospheric Evolution through Collaborative Aerogeophysical Profiling'. This project is studying the underlying geology and the structure of the East Antarctic ice sheet and processes beneath the ice sheet.

East Antarctica was previously regarded as less responsive to climatic changes than the smaller marine-based West Antarctic ice sheet. However, numerous sub-glacial lakes have recently been discovered beneath East Antarctica, indicating that the ice sheet is potentially more mobile than if it were frozen solid to the bedrock.

Using Casey and McMurdo stations as bases, the ICECAP team is exploring the vast Aurora and Wilkes sub-glacial basins, using instruments fitted to a Basler aircraft, to measure the glaciological and geological properties of the basins. These instruments include a high resolution ice-penetrating radar, to image the underside of the ice sheet and layers within the ice, providing insight into bedrock conditions and past ice flow (image 7 in panel above). A gravity sensor and magnetometer are also measuring the density and composition of the rock lying beneath the ice, providing information on the geological character of this region. The aircraft also carries a laser altimeter to map the ice surface, and GPS receivers to accurately locate the aircraft.

Data collected about the ice structure and conditions at the ice-bedrock interface, will be used to improve computer models of ice flow for Antarctica. This will improve estimates of ice sheet stability, and forecasts of its reaction to climate change and impacts on global sea level.

Related links

This page was last modified on 2 February 2014.