Marlowe Hood Yahoo News 18 Mar 09;
PARIS (AFP) – Manmade climate change is set to hasten the disintegration of a massive ice sheet in Antarctica by 100,000 years, boosting sea levels some five metres (16 feet), according to a pair of studies published Thursday.
The research, which matches new ice core data with a simulation of past and future changes in the West Antarctica Ice Sheet (WAIS), reveals for the first time regular cycles of "catastrophic collapse" and reformation reaching back five million years.
Cycles lasted 40,000 years during the first three-fifths of this period, but have since more than doubled in length, explained David Pollard, a scientist at Pennsylvania State University and lead author of one of the studies.
"But with global warming we are cutting short a natural cycle," he told AFP by phone.
"The two studies combined show it is really likely that the WAIS will collapse in the next few thousand years. In the absence of human influence, it would probably happen only 100,000 years from now," he said.
Rising sea levels is arguably the most serious long-term threat from climate change.
The global ocean water mark is likely to go up by at least a metre before the end of the century, recent research has shown.
That is enough to wipe out several small island nations, and to disrupt or displace tens of millions of people living in heavily-populated and low-lying delta areas in East Asia, African and the Indian subcontinent.
Part of that rise will come from thermal expansion as ocean temperatures rise, a process scientists understand well and are able to forecast.
But the world's two great ice sheets sitting atop Greenland and Antarctica remain climate change wild cards, with great uncertainty as to whether -- or how quickly -- they might shed their mass.
A team of more than 50 scientists led by Tim Naish of Victoria University in Wellington, New Zealand extracted sediment samples reaching 600 metres below the surface of the WAIS.
The findings showed a geological metronome of massive change across five million years, and provided the first direct evidence of total collapse.
"Before there were hints of it collapsing like that, but we really didn't know until now," said Pollard, a co-author of the study, published in Nature.
The new data also confirmed that the cycles of ice destruction and formation are closely linked to shifts in the tilt of Earth's axis as it rotates around the Sun, a process called obliquity.
The period covered by the sediment samples -- the early Pliocene -- is of special interest to climate scientists because it so closely resembles the conditions forecast for Earth over the next 100 years.
With global temperatures set to rise about 3.0 degrees Celsius (5.4 degrees Fahrenheit) by 2100, "more significance is being placed on the early Pliocene as an analogue for understanding the future behaviour of the WAIS and its contribution to global sea levels," the study says.
The concentration of carbon dioxide in the atmosphere -- some 400 parts per million (ppm) -- was also in line with projected 21st century levels, which have already hit 385 ppm and are still rising.
In the second study, Pollard and Robert DeConto of the University of Massachusetts in Amherst simulate Antarctic ice sheet variations in a mathematical model over the past five million years in order to track the "grounding line", the shifting border between land and sea ice.
"We found that the dominant mechanism attacking the West Antarctic ice has been variations in ocean melting under its large floating ice shelves," rather than changes in temperature or surface melt, Pollard said.
"One of the next steps is to determine if human activity will make it warm enough to start the collapse," said Pollard.
Antarctic Ice Close To Melting Tipping Point-Study
Michael Perry, PlanetArk 19 Mar 09;
SYDNEY - A large part of the ice covering West Antarctica could be lost if greenhouse gases in the atmosphere increase only slightly from today's levels and ocean temperatures continue to rise, a study released on Thursday says.
Another related study said if the West Antarctic ice sheet collapsed and the East Antarctic ice sheet continued to melt at its marine margins, global sea level would rise seven meters from today's level.
Antarctica stores about 90 percent of the world's freshwater.
Both studies, published in the journal Nature, are a result of extensive drilling into the seafloor under the Ross Ice Shelf by a team of New Zealand, Italian, American and Germany scientists.
The floating ice shelf won't elevate sea levels if melts because it is already displacing water. The real threat comes when the ice sheet behind, which is below sea level, is exposed to the ocean.
The 50-plus core samples, down to 1.2 kilometers (0.7 mile), allowed the scientists to study how previous periods of rising carbon dioxide affected ocean temperatures, ice movements and sea levels.
The cores showed the West Antarctic ice sheet in the Ross Embayment area collapsed and reformed every 40,000 years. It has collapsed 38 times in the past 5 million years.
"Most of it sits below sea level and is very vulnerable to rising ocean and atmospheric temperatures," Tim Naish, Director of Victoria University's Antarctic Research Center in Wellington, said in a briefing on the drilling study.
RISKS
The seabed drill samples also showed changes in the tilt of Earth's axis, placing polar regions toward and away from the Sun, had played a major role in ocean warming and cycles of growth and retreat of the West Antarctic ice sheet, between three to five million years ago, said the scientists.
Around four million years ago, rising carbon dioxide levels in the atmosphere, to around 400 parts per million, enhanced the warming effect of the tilt cycles, they said.
"Carbon dioxide concentration in the atmosphere is again approaching 400 parts per million," said Naish. It is currently around 387 ppm, up from about 280 ppm since the start of the Industrial Revolution.
"Geological archives, such as the ANDRILL (Antarctic drilling) core, highlight the risk that a significant body of permanent Antarctic ice could be lost...as earth's climate continues to warm," said Naish.
Computer modeling of ice sheet behavior and the drilling data found that the ice shelves protecting the West Antarctic ice sheet could disappear in centuries and the majority of the ice sheet could collapse within a thousand years, said Naish.
A study by Pennsylvania State University in the United States said computer modeling based on the core drilling found that oceanic warming was the primary driver of ice sheet melt and that a five degrees Celsius rise was enough to collapse the West Antarctic ice sheet.
The modeling showed that when the West Antarctic ice sheet collapsed, and the much larger East Antarctic ice sheet continued to melt at the edges, global sea level rose seven meters above present day levels, said Pennsylvania University's David Pollard, who led the study.
The two studies have given scientists an insight into Antarctica's glacial history in the early to middle Pliocene period, between 5 million and 3 million years ago, when the earth's temperatures were in the range of those projected for the coming centuries, said an accompanying Nature article.
It said it might take several centuries for the ocean to warm five degrees Celsius, a temperature required to generate enough ice shelf melting to cause significant retreat of the West Antarctic ice sheet.
"But such an outcome could result from the accumulation of total greenhouse-gas emissions projected for the 21st Century, if emissions are not greatly reduced," said the article.
(Editing by David Fogarty)
Temperature Rise May Trigger West Antarctic Thaw
Alister Doyle, PlanetArk 19 Mar 09;
OSLO - The West Antarctic ice sheet may start to collapse if sea temperatures rise by 5 degrees Celsius (9 degrees Fahrenheit), triggering a thaw that would raise world ocean levels by 5 meters (16 ft), U.S. scientists said.
Such a rise in sea levels -- taking thousands of years -- would swamp many coasts and cities and wipe some low-lying Pacific islands off the map.
West Antarctica, the part of the frozen continent most vulnerable to climate change, has thawed several times in the past few million years, most recently 400,000 years ago, according to Thursday's edition of the journal Nature.
The study "suggests the Western Antarctic ice sheet will begin to collapse when nearby ocean temperatures warm by roughly 5 C," David Pollard of Pennsylvania State University and Robert DeConto of the University of Massachusetts wrote.
The study helps plug big gaps in understanding Antarctica's likely reaction to modern global warming by improving knowledge of the history of the ice.
Pollard told Reuters the 5 C estimate for triggering a collapse was a rough guide, based on an computer model. The bigger East Antarctic ice sheet had not thawed in past warm periods studied.
The U.N. Climate Panel has projected a best estimate that world atmospheric temperature will rise by between 1.8 and 4.0 Celsius by 2100 because of emissions of greenhouse gases that could bring floods, droughts, heatwaves and more powerful storms.
Higher rises were possible unless the world reined in the growth of emissions, it said. Oceans temperatures lag far behind the rise in air temperatures.
"The required ocean warmings, of the order of 5 Celsius, may well take several centuries to develop," wrote Philippe Huybrechts of Vrije University in Brussels in a commentary.
"But such an outcome could result from the accumulation of total greenhouse-gas emissions projected for the twenty-first century, if emissions are not greatly reduced," he wrote.
A related paper in Nature suggested that past collapses of the West Antarctic ice were linked to the earth's rotation.
"The pattern of collapse suggests an influence of 40,000-year cycles in the tilt of Earth's rotational axis," Nature said of the study led by scientists in New Zealand.
(Editing by Matthew Jones)
