Sea rise from Antarctic ice melt overestimated

While a collapse of the West Antarctic ice sheet will have devastating impacts on global sea levels, a study published Thursday found the anticipated impact has been seriously overestimated.Using new measures of the ice sheet's geometry, British and Dutch researchers predict its collapse would cause sea levels to rise by 3.2 meters (11 feet) rather than previous estimates of five to seven meters.However, the study published in the journal Science found that even a one meter rise in sea levels would be significant enough to weaken the Earth's gravity field in the southern hemisphere and affect the Earth's rotation.That rotational shift would cause water to pile up in the northern oceans and could result in dramatic regional differences in sea levels, with the largest rise on the east and west coast of the United States."The pattern of sea level rise is independent of how fast or how much of the (Western Antarctic Ice Sheet) WAIS collapses," said lead author Jonathan Bamber of the University of Bristol in England."Even if the WAIS contributed only a meter of sea level rise over many years, sea levels along North America's shorelines would still increase 25 percent more than the global average."Antarctica holds about nine times the volume of ice as Greenland and is considered a sleeping giant when it comes to sea levels.The western ice sheet is of particular concern because enormous sections sit in inland basins on bedrock that is entirely below sea level.Vast floating ice shelves currently limit ice loss to the ocean but scientists fear the sheet could collapse if the floating ice shelves break free.The study authors based their predictions on the assumption that only ice on the downward sloping and inland-facing side of the basins would be lost while ice grounded on bedrock that is above sea level or slopes upward would survive.Researchers do not know how quickly the shelf would collapse. But if such a large amount of ice melted steadily over 500 years it would raise sea levels by about 6.5 millimeters per year.That's about twice the current rate due to all sources."Though smaller than past predictions, the scale of the fully manifested instability is enormous," cautioned Erik Ivins of the California Institute of Technology in an accompanying article."The total mass gained by the oceans ... would be roughly equal to the mass showered to Earth by the impact of about 2000 Halley-sized comets."Further complicating the situation is the fact that Greenland seems to be losing as much or more ice than Antarctica, even though it doesn't have the same unstable configuration."Greenland needs only half the mass loss rate of Antarctica to have an equivalent effect on polar motion due to its less polar position," he wrote.Even "more ominous" are the current accelerations of ice flow into the Amundsen Sea Embayment in Antarctica, he wrote."Should the ice sheet grounding line migrate farther inland, ice resting on bedrock well below sea level could become unstable."