The breakup of just two Antarctic ice shelves could push global sea levels up by 26mm, scientists have warned.

The Larsen C and George VI ice shelves are considered to have the highest risk of collapse of those the Antarctic Peninsula, where average temperatures have climbed by 0.5C each decade in the last five, and where billions of tonnes of ice mass loss have been recorded.

Because they hold back inland glaciers, the ice carried by these glaciers can flow faster into the sea when the ice shelves collapse, which contributes to sea-level rise.

While Larsen C has received much attention due to the break-away of a trillion-tonne iceberg from it last summer, its collapse would contribute only a few millimetres to sea-level rise.

The break-up of the smaller George VI shelf, a new study has found, would have a much larger impact.

A collapse of Larsen C would result in inland ice discharging about 4mm to sea level, while the response of glaciers to George VI collapse could contribute over five times more to global sea levels, around 22mm.

A collapse of the George VI Ice Shelf could mean 22mm of global sea level rise. Photo / British Antarctic Survey

"These numbers, while not enormous in themselves, are only one part of a larger sea-level budget including loss from other glaciers around the world and from the Greenland, East and West Antarctic ice sheets," explained study lead author and University of Birmingham glaciologist Nicholas Barrand.

"The Antarctic Peninsula may be seen as a bellwether for changes in the much larger East and West Antarctic ice sheets as climate warming extends south."

Warming in the Antarctic Peninsula led, in 2002, to the dramatic collapse of Larsen B, an ice shelf just north of Larsen C.

Unprecedented in its size, almost the entire ice shelf broke up in just over two weeks after being stable for the last 10,000 years.

"Larsen C is the most northerly remaining large ice shelf, therefore subject to the warmest temperatures, and the likeliest candidate for future collapse," study lead author Clemens Schannwell added.

"George VI is further west and south, in a slightly cooler climate, but is still vulnerable to a warming atmosphere and ocean."

Last summer, an iceberg twice the size of Luxembourg broke away from Larsen C.
But despite the recent attention on this ice shelf, the team found its future collapse would have a modest effect on global sea level.

By using computer models to simulate the interactions between the Antarctic Peninsula ice sheet and the ice shelves, the team found that the glacier response to collapse of Larsen C would add up to 2.5 mm to sea level by 2100 and 4.2 mm by 2300.

"The vulnerability to change at George VI Ice Shelf and the possible sea level implications from these changes, are far greater," Schannwell said.

Sandwiched between the Antarctic Peninsula and Alexander Island, George VI Ice Shelf is, at 24,000 square kilometres, around half the size of Larsen C.

But it would contribute far more to sea-level rise because it was fed by larger glaciers and was very effective at holding back the ice that drains into it from these glaciers.

According to the simulations presented in the new study, adjustment of the glaciers flowing into it following a collapse could contribute up to 8mm to global sea levels by 2100 and 22mm by 2300.

"Prior to our work, we didn't know what would happen to the upstream ice in the Antarctic Peninsula if these shelves were to be lost," Schannwell said.

"This could have important implications for the local environment and for global sea levels, information that is essential for climate change mitigation planning and policy."

The wider continent, which bore in its icy expanses an equivalent 58m of potential sea-level rise, has lost three trillion tonnes of ice in less than three decades.

The study comes a month after New Zealand scientists helped sketch two 2070 scenarios for Antarctica in a special edition of major scientific journal Nature.

In the first scenario, the world's greenhouse gas emissions kept rising, with little policy action taken to slow climate change's impact on Antarctica.

That meant a dramatic loss of major ice shelves, sea warming, sea-ice retreat and ocean acidification and metres of global sea-level rise over coming centuries.

But under the low emissions scenario - which could be achieved if future temperature rise was limited to the 2C threshold sought by the Paris Agreement - Antarctica would look much as it did in the earlier decades of the century, and global sea-level rise would remain under the one-metre mark.