By Tim Radford

Scientists in the U.S. have identified an ominous trend in the Southern Ocean—the creation of enormous icebergs as rifts develop in the shelf ice many miles inland.

And although three vast icebergs have broken from the Pine Island glacier in West Antarctica and drifted north in this century alone, researchers have only just worked out what has been going on.

Their first clue came from a telltale shadow in the south polar ice, caught by a NASA satellite and visible only while the sun was low in the sky, casting a long shadow.

It was the first sign of a fracture 20 miles inland, in 2013. Two years later, the rift became complete and the 580 sq km iceberg drifted free of the shelf.

Significant Collapse

"It's generally accepted that it's no longer a question of whether the West Antarctic Ice Sheet will melt—it's a question of when," said study leader Ian Howat, a glaciologist in the School of Earth Sciences at Ohio State University in the U.S.

"This kind of rifting behavior provides another mechanism for rapid retreat of these glaciers, adding to the probability that we may see significant collapse of West Antarctica in our lifetimes."

The scientists report in Geophysical Research Letters journal says that they discovered that although shelf ice could be expected to wear at the ocean edge, something else was happening in West Antarctica.

The Pine Island glacier is grounded on continental bedrock below sea level, which means that warming ocean water could penetrate far inland beneath the shelf, without anyone being conscious of any change.

The first evidence of something unusual was a valley—the one highlighted by shadows visible only at a particular time and captured by NASA imagery—in the ice, where it had thinned. The valley was the first outward sign that ice was melting far below the surface.

The shelf ice plays an important role in slowing the progress of south polar glaciers: remove the shelf ice and the glacier flow accelerates.

Researchers have already identified evidence of glacier retreat in the West Antarctic and warned that bodies of ice massive enough together to raise global sea levels by three meters could—thanks to global warming as a consequence of fossil fuel combustion—be increasingly unstable.

Calving Icebergs

Dr. Howat said:

The really troubling thing is that there are many of these valleys further up-glacier. If they are actually sites of weakness that are prone to rifting, we could potentially see more accelerated ice loss in Antarctica.

Antarctica is home to more than half the world's fresh water. The Pine Island glacier and its neighbor and twin, the Thwaite glacier, are at the outer edge of an ice stream. In effect, they have "corked" the flow.

But West Antarctica is warming far more swiftly than the rest of the south polar region. And the calving of huge icebergs fuels researchers' fear that, within 100 years, the entire West Antarctic ice sheet could collapse, with disastrous consequences for many coastal cities worldwide.

Reposted with permission from our media associate Climate News Network.