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Tuesday 21 April 2009 (Updated on Tue 28 April)

RAPID ROUNDUP: Increasing ice in the East Antarctic – experts respond

With confusion in the media this week over whether ice is decreasing or increasing in the Antarctic, here experts clarify the apparent anomaly.

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Professor Barry Brook holds the Foundation Sir Hubert Wilkins Chair of Climate Change and is Director of the Research Institute for Climate Change and Sustainability at the University of Adelaide

"This is a common source of confusion among climate change sceptics. As the world warms, the atmosphere's ability to hold water vapour increases. Think of how humid it is in the tropics, and how dry the Arctic air is. The largest desert on Earth is the continent of Antarctica -- it receives very little annual precipitation. In a warming world, more water vapour allows for more snowfall in Antarctica, which accumulates particularly in East Antarctica where the temperature never rises above freezing point. So, ice accumulates on that side of the continent. In the Antarctic Peninsula and West Antarctica, this extra accumulation of snow is more than offset by summer surface melt.

Also, as the sea warms around the continent, especially in the most northerly parts of the continent (Antarctic Peninsula) large ice shelves are eroded from beneath, and the frequency with which they break up starts to accelerate. This melting of buttressing ice shelves unplugs the land-based glaciers, and they begin to flow into the sea more rapidly. As such, there is a large net loss of ice from the western half of the continent, and a slight gain in the eastern half. More sea ice builds up around the continent because as the surface waters warm, the ocean becomes more stratified (it 'turns over' less readily). Less ocean heat is brought up from below. So it's a battle between the negative effect of increased surface melt of sea ice, and the positive effect of more snowfall and decreased in melting from below, both of which reinforce sea ice formation. The result -- a steady state or slight increase in the amount of floating ice around the great southern continent."

Dr Ian Allison is Leader of the Ice, Ocean, Atmosphere and Climate Program at the Australian Government Antarctic Division and Antarctic Climate and Ecosystems CRC. He was also Lead Author, IPCC Chapter 4 – Observations – changes in snow, ice and frozen ground.

On the current status of the Greenland and Antarctic ice sheets:

“Modern satellite techniques are providing increasing evidence that mass loss from ice sheets is currently contributing to sea level rise. The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4) was able, for the first time, to conclude that, taken together, the ice sheets in Greenland and Antarctica have very likely (>90% probability) been contributing to sea level rise over the period 1993 to 2003 - at an average rate estimated at 0.4 mm of sea level rise per year (or 4 cm/century).

Since late 2005, the cut-off date for work assessed by the IPCC AR4, a number of further studies of the mass budget of Greenland and Antarctica have been made using satellite altimetry, satellite gravity measurements and estimates of mass influx and discharge from a variety of techniques. 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 global SLR), but the error range is large. In Antarctica, mass loss has been greatest along coastal sectors of the Antarctic Peninsula and West Antarctica, but with thickening further inland and over most of East Antarctica partially offsetting this. Recent changes in ice flow rates observed in satellite data explain much of the estimated Antarctic mass imbalance.

In Greenland the average mass loss since 1993 has been about 120 GT/yr (~0.35 mm/yr of SLR). As for Antarctica, the error range of the estimates is large, but for Greenland there is evidence that the rate of mass loss may be increasing, with recent estimates as high as 0.5 mm/yr of SLR. 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.”

Contribution of ice sheets to future sea level:

“The IPCC AR4 projected 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) to be in the range 0.18 to 0.59 m by 2090-2100. This estimate does not include further accelerated discharge from outlet glaciers.

The ice sheet contribution to this estimate comes mostly from 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 ice discharge (called the ice dynamic effect) is not straight forward, as processes such as those controlling basal sliding of outlet glaciers are not well understood.

The IPCC AR4 used an empirical argument to estimate that such acceleration might add another 0.1 to 0.2 m of slr over the next century. But they added that even larger values could not be excluded.

It has been argued that, with recent observations of speed-up of some glaciers in both Greenland and Antarctica, 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 have 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 is less than this, although possibly toward the upper end of the IPCC AR4 estimate.”

Professor Nathan Bindoff is a physical oceanographer and Director of the Tasmanian Partnership for Advanced Computing (TPAC). Partners include the University of Tasmania, CSIRO Marine & Atmospheric Research and the Antarctic Climate and Ecosystems CRC. He was a Coordinating Lead Author of the IPCC AR4 Working Group 1 chapter on oceanic climate change and sea level observations (Chapter 5).

"The Antarctic Ice Sheet is more complex than most readers appreciate. One of the surprising results from the last IPCC Fourth Assessment report in 2007 was that the Antarctic Ice sheet on average was contributing to sea-level rise at a rate 0.21mm/yr through the melt of the Antarctic Ice Sheet by warmed oceans, primarily in the Amundsen Bellinghausen Sea and around the Antarctic Peninsula. Prior to this report the general expectation had been that the Antarctic Ice sheet would actually grow from increased snow over the continent. More recent estimates of Antarctic Ice sheet mass loss have been significantly higher, but the error bars have remained large and these estimates are probably not significantly different from zero. In estimating the net mass loss of the Antarctic Ice Sheet, some regions will grow while other regions will lose mass. This is precisely what has occurred on the Antarctic Continent, but all of the available estimates show that the loss of mass in the west part of Antarctica is greater than the added mass in the eastern part of Antarctica."

Professor John Buckeridge is Head of the School of Civil, Environmental & Chemical Engineering at RMIT University, Melbourne.

"I see no issues with the current changes in Antarctica. Importantly, we are not dealing with a straight line graph. There are quite significant oscillations, and even though there may be some build-up of ice in some parts, the overall trend is one of warming. The report does in fact say this."

Dr Andrew Glikson is an Earth and paleoclimate scientist at the Australian National University's Research School of Earth Science in Canberra.

"The complex transient and regional variations in temperature, ice extent and ice thickness in West and East Antarctica do not mask the overall decade-scale warming trend of the continent, which in west Antarctica reach surface tmperature anomalies three and four times higher than at lower latitudes.

Formation of the Antarctic ice sheet was associated with abrupt cooling ~34 million years ago in connection with:
(1) decline of CO2 levels to below 500 ppm and temperature level by ~4- 5° C [1];

(2) Opening of the Drake passage, isolating Antarctica from South America and retarding introduction of warm currents to high southern latitudes.

Two major asteroid impacts (Popigai [D=90 km] and Chesapeake [D=100 km]) occurred at that time, possibly triggering transient cooling. With this perspective, the current rise of CO2-e (including radiative forcing by methane and N2O) to levels near 440 ppm is approaching conditions under which the Antarctic ice sheet formed.

Antarctic ice melt is affected by:
(1) polar-ward migration of climate zones by about 400 km [2];
(2) contraction and thereby acceleration of the Antarctic wind vortex, causing reduced temperature in parts of the continent;
(3) atmospheric ozone (greenhouse gas) depletion results in somewhat reduced greenhouse effect;
(4) stratospheric ozone depletion results in increased UV radiation;
(5) the increase in south oceans water temperature of ~0.2 to 0.3°C erodes ice shelves laterally and from below. Increased evaporation results in increased snow fall and thickened ice in some regions.

Melting is greatly enhanced by the ice melt/water interaction feedback process, the factor driving temperature rises to levels three or four times the mean global average [3]. NASA/GISS surveys show that entire Antarctic surface temperatures increased by a mean of +0.12C per decade and West Antarctica by 0.17°C per decade between 1957 and 2006 [4]. Manifestations of warming include reduced concentration of sea ice north of the West Antarctic peninsula and in the direction of Australia, and increase in sea ice in other areas [5]. West Antarctica ice shelves which overlies sub-sea level basement are vulnerable to sea water-induced melting, as exemplified by the Wilkins ice shelf collapse."

1. Zachos et al., 2008; http://www.nature.com/nature/journal/v451/n7176/full/nature 06588.html
2. Hansen et al., 2007, 2008; http://arxiv.org/abs/0804.1126
3.http://data.giss.nasa.gov/gistemp/
4. http://www.giss.nasa.gov/ research/news/20090121/
5. http://nsidc.org/data/seaice_ index/