What we can learn from the Larsen Ice Shelf

The Larsen Ice Shelf is a huge piece of ice that sits on the other side of the Antarctic Peninsula from Rothera Station. It is divided into sections, which are named, from north to south (left to right, in the map), the Larsen A, B, and C.

Back in 2002, a large piece of the Larsen B broke off into the ocean. The piece that broke off was 1,250 square miles, about the size of Rhode Island! It broke apart and fell into the ocean over a course of about a month. (You can see the satellite images of its breakup on NASA’s website.) The loss of the Larsen B was a huge event, because it was the loss of a LOT of ice over a relatively short period of time.

Satellite image of the Larsen B breakup from NASA's website

The Larsen B section of ice was mostly covering the ocean (but connected to ice on the land). Because it was already floating, the amount of water in the ice did not add new water to the ocean when it broke off. That means it did not cause any rise in overall sea level. Think about it… When the ice melts in your glass of water, the glass doesn’t get more full of water. That’s because the ice was already taking up space while it was a frozen cube. Whether the water in the glass is frozen or liquid, it’s still part of the overall level of water in the glass, so when it melts, the water level doesn’t change. But, if you add new ice cubes to your glass, the water level would rise because you added more water (frozen water) that wasn’t already there. The remaining ice on the Larsen Ice Shelf is “grounded ice”, meaning it covers land, not water. If it breaks off, it would add new water to the ocean and cause sea level rise.

Many of the scientists working from Rothera Station study the Larsen Ice Shelf. Some of them study why the Larsen B section broke off. (Scientists think it was caused by higher temperatures creating many pools of meltwater on the surface. The meltwater leaks into cracks and crevasses in the ice, to then act like wedges that deepen the cracks and break the ice into pieces.) The scientists I met study what that break-off means for the ice that remains. Is the remaining ice less stable now that it’s lost a huge chunk of itself? They have put GPS stations around the ice so that they can track the speed of its movement. It’s important to know if the rest of the Larsen Ice Shelf is stable, because its breaking would contribute to sea level rise.

I also met scientists who study what that loss of ice means for the rock and earth beneath the section that broke off. Ice is very heavy and can actually squash the rock and earth beneath it. We don’t usually think of rock and being squishy enough to be mashed down by ice, but the ice is that heavy! It can squash rock! When large pieces of ice disappear, the rock beneath it can re-expand now that the weight is no longer pushing it down. (Think about pushing down on a sponge with your hand. When you remove your hand, the sponge re-expands.) That happens pretty quickly after the ice is gone. Even after that re-expansion, the ground will continue expanding because the magma in the mantle is able to flow back in to the crust to keep pushing up on the earth. (Think again about pushing down on a sponge with your hand, but think about pushing it down in a bowl of water. When you move your hand, it would not only re-expand because the weight of your hand is gone, but it would also start to soak up water to expand even bigger.) Scientists are using radar and GPS to measure that ground expansion after the Larsen B fell off. Most of the expansion they’ve measured so far is actually from the mantle flowing back in, which surprised them, because they expected that part would happens much more slowly.

The Larsen B receives a lot of attention, but it was not the first or last of the ice shelves to break apart. There have been many other examples of major ice shelves breaking up over recent years, including the Wilkins Ice Shelf. Loss of ice is expected to continue of warming continues in this area.