Hanging by an icy thread

Some 3000 kilometres due south of New Zealand, a giant Antarctic ice sheet is crumbling like a cheap supermarket pavlova.

The collapse of the Wilkins Ice Shelf has sparked fears of rising sea levels, but is it time to update the house insurance, or buy some water wings?

Scientists think the loss of Antarctic ice shelves such as Wilkins will let inland glaciers slide to the ocean faster, pumping vast quantities of ice into the sea and contributing to sea-level rises.

If it goes, the Wilkins Ice Shelf the size of Jamaica would become the 10th Antarctic ice shelf to recede or vanish into the sea since 1950.

Sea-level rises are also caused by thermal expansion of the oceans as they become warmer.

The Intergovernmental Panel on Climate Change has predicted a sea-level rise of up to 59 centimetres by the end of the century.

But it did not include the possible effects of melting ice sheets and said it could not yet predict the full extent of a future sea-level rise.

Greater Wellington regional council hazards analyst Iain Dawe says councils are already preparing for rising sea levels and resulting coastal erosion.

Options include hazard setback zones, a retreat from the coast, and even building homes on piles so they can be easily moved back from the sea.

In parts of Wairarapa and the Kapiti Coast, developments less than 50 metres from the sea are already banned.

Mr Dawe says planners are plagued by uncertainty over how much seas may rise. The Kapiti Coast faces real challenges, while Wellington's south coast is less at risk.

"On big gravel beaches the gravel will just move further up the shore and absorb the impact."

Insurance Council insurance manager John Lucas says rising sea levels will increase the chance of flooding from storm surge and could lead to increased premiums for coastal dwellers.

"Insurers look at information from local councils on flood risk areas."

Rising sea levels also threaten low-lying islands in the Pacific and Indian oceans.

Victoria University climate change professor Martin Manning, who was on the climate panel, says new studies talk of sea-level rises of between 1.6 metres and 2 metres by 2100.

"The bottom line is that things are happening quite quickly for example, the fact that we have only seen rapid change in the ice sheets since 2003."

On the other hand, Waikato University oceanographer Willem de Lange says the threat of rising sea levels is exaggerated.

He puts the upper limit of sea-level rise at 20cm in the next 100 years and says the effects of a sea-level rise depend on a host of factors, including the rate of sediment supply and where the waves come from.

The Wilkins Ice Shelf collapse is interesting, but not unusual, he says. "It happens. There has long been a pattern of ice shelves building up and collapsing. All it shows is that the world has got warmer since the last ice age."

- © Fairfax NZ News