Revealed: What the Southern Ocean's ice belt means for sea level rise

File photo / NZ Herald

Changes to the Southern Ocean's sea ice belt could mean future ice sheet melt and global sea level rising several metres in coming centuries, according to a new study which has shed more light on a long-standing ice-age mystery.

The sea ice belt - comprised of frozen ocean water, and which grows as a protective fringe around Antarctica's ice sheets - is susceptible to ocean warming as greenhouse gases continue to rise.

Work by PhD student Molly Patterson, under the supervision of Dr Robert McKay and Professor Tim Naish from Victoria University's Antarctic Research Centre, shows that the stability of the world's largest ice sheet is influenced by the presence of a sea ice belt in the Southern Ocean.

Dr McKay said the research contributes to a long-standing ice-age mystery, resolving how exactly the Earth's orbit around the sun contributes to natural ice-age cycles.

"It sheds new light on how natural climate processes can dramatically amplify ice sheet responses to relatively small changes in energy that were provided by changes in Earth's orbit," he said.

"As we have seen repeatedly in geological records, these ice sheets are highly sensitive to changes in the energy they recieve, and are capable of driving global sea level changes by many tens of metres."

Current emissions of large quantities of greenhouse gases provide a far greater amount of energy than the orbits alone would - and at an unprecedented rate.

"Changes from global warming and any future reduction of the ice belt may have profound effects on the stability of the giant East Antarctic ice sheet, because its major parts sit below present day sea level and if melted would total more than 20 metres of increased sea level."

The researchers travelled to a remote region offshore from the East Antarctic ice sheet and examined iceberg remains from between 2.2 and 4.3 million years ago.

The research, published recently by the Nature Geoscience, showed that prior to 2.5 million years ago, when atmospheric carbon dioxide levels were at the same high level that they are today, the East Antarctic Ice Sheet melt was widespread.

In this higher carbon dioxide world, the Southern Ocean was too warm to support a large protective sea ice belt during the summer months, and this allowed wind-driven ocean currents to penetrate deep into the south and melt the marine margins of the East Antarctic ice sheets.

Due to these factors, Dr McKay said there was strong potential for widespread ice loss and sea level rise worldwide in the coming centuries, unless carbon dioxide levels are significantly reduced.

According to the most recent projections by the UN's Intergovernmental Panel on Climate Change (IPCC), the upper end prediction for sea level rise is about 1m by the end of the century.

But if the vast West Antarctic Ice Sheet collapsed, the impact could mean 3m of rise.

Under present sea level rise scenarios, many coastal areas in New Zealand were predicted to face a greater threat from higher tides and storm surges.