Ice sheets and sea-level rise
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?
The findings of the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4), in 2007, was that the Antarctic Ice Sheet, as a whole, was contributing to sea level rise at a rate 0.2 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 warmed over the last 50 years.
Since late 2005 (the cut-off date for work assessed by IPCC AR4), further studies of ice accumulation and 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.
These new estimates suggest that the total annual loss from Antarctica since 1993 is around 100 Gt/yr (100 billion tonnes of ice per year; equivalent to ~0.25 mm/yr of global sea level rise). While the range of estimates from the different studies is large (from near zero to 0.5 mm/yr of sea level rise) they all suggest a net loss. Ice loss has been greatest along coastal sectors of the Antarctic Peninsula and West Antarctica. However, ice thickening (gain) further inland and over most 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.
This ‘balance flux’, as depicted within the image gallery above, is the volume of ice that must be discharged to balance the annual snow fall onto the ice sheet. This is derived by a computer model for a given snow fall distribution: the blue areas are low ice discharge rates and the red are high rates, on a logarithmic scale. The plot compares the modelled mass flux across the 200 m surface elevation contour. The discharge is derived from ice velocity and the ice thicknesses, measured during over-snow traverses between 40 and 130 degrees East (black dots). Where the balance flux is greater than the measured flux, the interior ice sheet is growing, and vice versa.
Much of this part of Antarctica is nearly in balance, although gains in the Lambert Glacier Basin (LGB, top) and Wilkes Land (bottom) lead to an overall gain for this part of the ice sheet that is equivalent to a drop in sea level of 0.1 mm per year. Different balance conditions in other parts of Antarctica, and between the 2000 m elevation contour and the coastline, also impact sea level, and overall Antarctica is contributing to a net sea level rise. From the report: Australia’s contribution to Antarctic Climate Science (2008).
What about the Greenland Ice Sheet?
In Greenland the average ice mass loss since 1993 has been about 120 GT/yr (contributing ~0.35 mm/yr to sea level rise). There is evidence that the rate of mass loss may be increasing, with recent values as high as 0.5 mm/yr of sea level rise. However there can be large variability from year to year in the surface melt in Greenland and the short term changes, from satellite gravity data in particular (which are only available since 2003), may reflect this, rather than a long-term trend. There has been thickening of the high central ice sheet in Greenland, but this has been more than offset by increased melting near the coast. Flow speed has also increased for some Greenland outlet glaciers.
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 warming air temperature. The rate of temperature rise in this region (2.5ºC over the last 50 years) is among the greatest 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.
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 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?
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. This is comparable to loading too many ice cubes in your gin and tonic – the bottom one touches the bottom of the glass even though it's well below the water level.
Where the bedrock under a marine ice sheet slopes down towards the interior, 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 the entire West Antarctica ice sheet, leading to rapid disintegration.
What will be the contribution of the ice sheets to future sea level rise?
The IPCC AR4 projected that sea level rise from thermal expansion of the ocean, melt of small glaciers and ice caps, and from Greenland and Antarctica (for a wide range of emission scenarios) would be in the range 0.18 to 0.59 m by 2090–2100. This estimate does not include further accelerated discharge from ice sheet outlet glaciers.
The ice sheet contribution to this estimate comes mostly from melt in Greenland and from the Antarctic Peninsula. Surface temperatures over most of East Antarctica are well below the freezing point and direct melt of the East Antarctic ice sheet is not expected to contribute significantly to sea level rise over the next century.
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 lubricating water at the bed of the glacier allows it to move more rapidly) are not well understood. The 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. But the AR4 report emphasized that even larger values might be possible.
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.
A more generally accepted upper bound of sea level rise over the next century is 2 m. The probable rise will be less than this, although possibly toward the upper end of the IPCC AR4 estimate of around 0.8 m.
What is happening to ice shelves – and do they 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 the base, which thins and makes them more vulnerable, and warmer summer temperatures which cause 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.
The most recent example of this is the Wilkins Ice Shelf. The Wilkins Ice Shelf has undergone significant changes since 2008 after two significant break-up events in February and May 2008 and further losses in June and July 2008. These changes have been attributed to strong regional warming, and melting of the ice shelves 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 flow 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.
What are the gaps in our knowledge that restrict better estimates of future sea level rise?
The main gaps are in our understanding are of some aspects of ice sheet dynamics. There is a need to improve our mathematical models of ice streams, 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 sliding processes, 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.
- More information on Australian scientists’ contributions to ice sheet research (2008 report)
- Latest climate information from the Department of Industry Intergovernmental Panel on Climate Change (IPCC) website