Environment Counts | Warm ocean currents dominant cause of ice loss from Antarctica
Author: Geoff Zeiss – Published At: 2012-04-30 14:27 – (1879 Reads)
Scientists used a combination of measurements from NASA’s Ice, Cloud, and land Elevation Satellite (ICESat) and models to differentiate between warm ocean currents thawing ice sheets from below and warm air melting them from above. It was found that 20 of 54 ice shelves, mostly in West Antarctica, are being melted by warm ocean currents. Ocean-driven thinning is responsible for the majority of Antarctic ice sheet loss during the period studied. Nature, 484, 502â€“505 (26 April 2012)
To map the changing thickness of almost all the floating ice shelves around Antarctica, researchers from the British Antarctic Survey, Utrecht University in the Netherlands, the University of California at San Diego and Earth and Space Research in Oregon used a time series of 4.5 million surface height measurements taken by a laser instrument mounted on ICESat from October 2003 to October 2008. They measured how the ice shelf height changed over time and ran computer models to discard changes in ice thickness because of natural snow accumulation and compaction. The researchers also used a tide model that eliminated height changes caused by tides raising and lowering the ice shelves. Space-based, laser altimetry coupled with NASA’s other ice sheet research using data from the GRACE mission, satellite radars and aircraft provides a comprehensive view of ice sheet change and improves estimates of sea level rise.
ICESat was the first satellite specifically designed to use laser altimetry to study the Earth’s polar regions. It operated from 2003 to 2009. Its successor, ICESat-2, is scheduled for launch in 2016.
Glacier acceleration has intensified along the coastal margins of the Antarctic ice-sheet. Atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves, potentially limiting their ability to buttress the flow of grounded tributary glaciers. This study uses satellite laser altimetry and modelling of the surface firn (compressed snow) layer to reveal the pattern of ice-shelf thinning through increased basal melt. It is concluded that this increased melt is the primary control of Antarctic ice-sheet loss, through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow. The highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf.
It is speculated that wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of Antarctic ice shelves, through ocean upwelling in the Amundsen and Bellingshausen seas, and atmospheric warming on the Antarctic Peninsula. This implies that climate forcing through changing winds influences Antarctic ice-sheet mass balance, and hence global sea level, on annual to decadal timescales.
The researchers conclude that thinning can be attributed to ocean-driven basal melt on 20 of 54 ice shelves, with the most widespread and rapid losses (up to ~7â€‰mâ€‰yrâˆ’1) on the coast of West Antarctica, where warm waters at depth have access to thick ice shelves via deep bathymetric troughs.
There is evidence that suggests that changes in wind forcing explain both the increased oceanic supply of warm water to thinning West Antarctic ice shelves, and the atmospheric warming on the Antarctic Peninsula that caused the loss of Larsen A and B and now dominates the thinning of Larsen C. Both mechanisms imply that Antarctic ice shelves can respond rapidly to Southern Hemisphere wind patterns that vary on timescales of years to decades.
It is also concluded that ocean-driven ice-shelf thinning is in all cases coupled with dynamic thinning of grounded tributary glaciers that together account for about 40% of Antarctic discharge and the majority of Antarctic ice-sheet mass loss. It is concluded that reduced buttressing from the thinning ice shelves is driving glacier acceleration and dynamic thinning.
The researchers suggest that that the most profound contemporary changes to the ice sheets and their contribution to sea level rise can be attributed to ocean thermal forcing that is sustained over decades.
They also warn that this may already have triggered a period of unstable glacier retreat.
Nature, 484, 502â€“505 (26 April 2012)