An iceberg in Pine Island Bay. Photo: Björn Eriksson

The Earth contains hints about its past everywhere. And as we hurtle toward a much warmer future, those clues can tell us a lot about what we face.

In Antarctica, that’s never been more important. The glaciers tumbling down from West Antarctica’s ice sheet hold back a massive amount of ice that could raise sea levels up to 13 feet if it all ends up in the ocean. They’re also in a precarious position and some scientists have suggested they’re at or near the point of unstoppable retreat.

To understand just how close we are, new research published in Nature this week looks at the seafloor near Pine Island and Thwaites glaciers—two of the most vulnerable in West Antarctica—and eons of scars, scratches and dings caused by icebergs. The findings show that during a 1,100-year period starting 12,300 years ago, massive clusters of small icebergs broke off from the glaciers. That’s bad news for us. The climate today is similar to that period and will only get warmer due to rising carbon pollution. Atmospheric carbon dioxide levels are at 400 parts per million now, well above the roughly 265 ppm they were at at the end of all that activity in the Antarctic thousands of years ago.

The scientists used high resolution underwater imaging to look at a history of iceberg violence on the seafloor. They found 10,831 features, and the results show that during a warm period 12,300 years ago, most were caused by small icebergs.

Scars left on the seafloor by icebergs. Image: Martin Jakobsson and Matthew Wise

Advertisement

Those swarms of icebergs happened because the ice shelves ended in towering frozen cliffs that stood up to a kilometer high from the top to the cold water below. Those cliffs also happened to be incredibly unstable when faced with warmer conditions and continually collapsed, leading to rapid retreat.

“A major challenge in the Earth sciences is interpreting evidence from the ancient record,” Tom Wagner, a cryosphere expert at NASA, told Earther. “In this case they’ve taken the shape of the sea floor and used it to interpret how ice sheets fall apart and raise sea level.”

While the glaciers eventually stabilized thousands of years ago, the research suggests we could be doomed to a repeat now that human carbon emissions are once again cranking up the heat.

Advertisement

Since the 1980s, the glaciers have been losing mass at an accelerating rate largely due to warm water clawing away at them from below. The Pine Island and Thwaites glaciers break off huge icebergs measured in Manhattan-sized units, but an unfortunate circumstance of geology could lead to increasing instability and more smaller icebergs associated with rapid retreat.

Here’s the deal. The grounding line—the point where the ice shelf begins to float—until recently has been sitting on a little ridge. That means the cliff at the end of the iceberg has been relatively stable because it’s only a few hundred feet tall. Big to be sure, but not unstable big.

But as warm water cuts further inland, the grounding line will recede further to an area where the bedrock the glacier sits on slopes downward. That will give rise to ice cliffs that could precede a rapid collapse (at least in geological terms).

Advertisement

“Inland of the present-day grounding-line, there are no ‘pinning points’ such as bedrock rises for the grounding-line to stabilize on once ice retreat has started,” Matthew Wise, a PhD candidate at Scott Polar Research Institute who led the research, told Earther. “There is therefore nothing to stop future retreat from extending deep into the West Antarctic hinterland.”

The melt of the West Antarctic ice sheet could unleash up to 13 feet of sea level rise. The Pine Island Thwaites glaciers alone are responsible for keeping 4 feet of that rise in check. But this and other research increasingly shows they’re not up to the challenge.

That means its up to coastal communities around the world to start planning for a watery future. At least some of them appear to be up for the challenge.

Advertisement

This post has been updated to include comments from Matthew Wise.

[Nature]