It's important to note that there is melting involved in Antarctica's contribution to sea level rise. Especially in West Antarctica. But it's not the grounded ice (land ice, glaciers) melt that is responsible for West Antarctic ice loss.
Mass balance is an important term for glaciology, but certain types of mass balance don't account for everything that happens to the ins and outs of glaciers and ice sheets. Surface mass balance is the mass balance (net ice gained or lost) at the surface of glaciers and ice sheets. Snow can fall on the glacier adding to the surface mass balance, ice can melt or sublimate off the glacier reducing surface mass balance. It's like being an observer at the front door of a hotel: you can get a rough estimate of how many people are in the hotel at a time by tracking how many people leave and enter through the front door.
But some folks at that hotel might be parked behind the hotel, and they have to leave through the back door. But you can't see the back door! And if you're only paying attention to the people coming and going through the front door, you might not notice the hotel becoming vacant.
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| Surface mass balance of Antarctica and Greenland from 1989-2009. Notice that Antarctica mostly has a positive surface mass balance. Figure from Van den Broeke et al., 2011. |
Observing Antarctica through surface mass balance is like watching people enter and leave through the front door of a hotel. We might not be capturing what is actually happening to the ice. Because of that back door, it is entirely possible for a glacier or an ice sheet to have a positive surface mass balance and still undergo thinning and retreat.
How is this possible? Antarctica, particularly the West Antarctic Ice Sheet, is vulnerable to changes that do not depend on the surface mass balance -- like a back door. And these changes occur underneath the ice, and on the margins of the ice sheet itself. At the margins of ice sheets like the West Antarctic ice sheet is the very important system between glaciers (grounded ice) and ice shelves (floating ice, attached to glaciers). The physical mechanism of total mass loss is the dynamics that occur within this system.
When I say total mass loss, I am talking about total mass balance. Total mass balance involves processes at the surface and processes below the ice that may contribute to ice thinning or thickening. Front doors and backdoors.
Antarctic scientists are concerned with the melting at the base of ice shelves. Ice shelves alone cannot contribute to sea level rise, however, because they are already floating in water. But ice shelves do play an important role in glacier dynamics at ice sheet margins, dynamics that can cause total mass loss.
Think of glaciers as flowing rivers. Glaciers flow toward the ocean. Attached to many important glaciers in Antarctica are ice shelves. Think of them as dams. They push back against the flow of the glacier in a way that they regulate how quickly ice flows towards the sea. This relationship exists due to Newton's third law of motion. Every force is negated by an equal and opposite force. The glacier pushes on the ice shelf, and the ice shelf pushes back (put very simply).
The boundary between the glacier and the ice shelf is called the grounding line. This line separates the grounded ice (ice than can contribute to sea level rise) and floating ice (ice that can't). Ice that passes the grounding line adds to sea level rise. By melting ice shelves, we weaken their ability to push back on the flow of the glacier. This allows more ice to cross the grounding line.
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| Antarctic ice mass change tracked through GRACE, a NASA mission that observes gravitational differences across the Earth's surface and time. This shows that the West Antarctic ice sheet is losing mass, despite a positive mass balance. From NASA |
And when more ice crosses the grounding line, we get sea level rise. Going forward, the grounding lines in Antarctica will be very important. But scientists are currently working on finding out what the future will hold for the ice sheet. And sometimes to answer questions about the future, we have to look to the past. But that's a different post...
Sources and Whatnot:
- Antarctic Ice Sheet surface mass balance (antarcticglaciers.org)
- An introduction to Glacier Mass Balance (antarcticglaciers.org)
- Antarctic Ice Loss 2002-2020 : GRACE Tellus (nasa.gov)
- Van den Broeke, Michiel R., et al. “Ice Sheets and Sea Level: Thinking Outside the Box.” Surveys in Geophysics, vol. 32, no. 4-5, 22 June 2011, pp. 495–505, https://doi.org/10.1007/s10712-011-9137-z.
