Antarctic ice 'close to melting tipping point'

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 yesterday 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 7 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 km, 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 Centre in Wellington, said in a briefing on the drilling study.

The seabed drill samples also showed changes in the tilt of Earth's axis, placing polar regions towards 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 7 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 5 C, a temperature required to generate enough ice shelf melting to cause significant retreat of the West Antarctic ice sheet.

Reuters

(China Daily 03/19/2009 page11)