Map showing parts of Antarctica’s land surface that are actually below sea level, making the glacial ice there (which is much too thick to float) more vulnerable to retreat.
It’s been a bit of a head scratch. Sea level data over the last few million years tells us that there have been some warm periods when sea levels could have been this high. 20 meters higher than today. However, with the conditions prevailing at the time, our computer models of ice sheets have not been able to reproduce such ocean swelling.
The models can simulate that much sea level rise, but it requires temperatures much higher than during those warm spells. Realistic loss of ice from Greenland and the fragile, western part of Antarctica (the West Antarctic Ice Sheet) could yield only something close to 3 to 10 meters of sea level rise. That leaves 10 to 17 meters for the East Antarctic Ice Sheet – the largest and most stable ice sheet – to break off. It’s not easy to convince the miserly East Antarctic Ice Sheet to be so generous with its content, which is why the models required such high temperatures.
Updating the models
So what are the models missing? David Pollard and Richard Alley of Penn State, and Robert DeConto of the University of Massachusetts, Amherst had an idea to try something. Two things to try, actually. They added a few physical processes to an ice sheet model that hadn’t been simulated before. The first was hydraulic fracturing. When water reaches the ice sheet through rain or ice melt at the surface, it fills crevasses in the ice.
When they’re filled deep enough, the water pressure forces the crevasse to open even deeper — that’s called hydraulic fracturing. The other process is due to the simple fact that a sheer ice cliff can only be so high before collapsing under its own weight – a condition not seen in too many places today.
One place where it does occur is where drifting glaciers calve large icebergs. These occur on the coastal outlet glaciers at the edges of ice sheets most vulnerable to warming. The glacier thins out towards the outer edge and at some point it becomes so thin that it begins to float. The point at which it floats off the bottom is called the “ground line” – from there to the end of the ice is called an ice shelf. Ice shelves that rub against the shore (think floating in a bay or fjord) work to hold back the ice flow behind them. These planks gradually melt from below as they float in their seawater bath. But they can also melt from above, shedding large icebergs on their outer edges.
Both hydraulic fracturing and cliff failure can increase the shedding of icebergs from shelves, hastening their demise and uncorking the glacier behind them. However, once these things happen near the ground line, they can really speed up the retreat if it’s in an unstable configuration where the ground surface dips as you go inland. (Significant parts of Antarctica fit that description.) Once you begin to retreat into that situation, the glacier may have to retreat quite a distance to regain a stable position.
A hasty retreat
After adding representations of these two processes to the model, the researchers simulated a sudden change from modern conditions to warmer conditions as seen in past periods of very high sea levels. Then they watched how the virtual Antarctic glaciers reacted.
The results were dramatic. Together, the new processes have had a huge impact. Instead of about 2 meters of sea level rise, Antarctica lost enough ice to raise global sea levels by 17 meters over several thousand years. The fragile West Antarctic ice sheet is collapsing in no time decades, rather than centuries or millennia. There is a sea level rise of 5 meters in the first two centuries, after which the retreat in parts of the East Antarctic ice sheet really starts.
Much more work will be needed to ensure these new processes are accurately simulated, but the early results show that researchers can get in on the ballpark to solve the puzzle of past high sea levels.
The relevance to our current situation is less direct, as the warming in the simulation was not realistic, but the possibility that West Antarctica could be losing ice faster than we thought is serious. Richard Alley, whose work on this possibility we discussed earlier, explained to Ars via email: “I believe (and I suspect many people do) that it’s important for us as scientists not only to know the most likely future outcome but also the range of possibilities, including some kind of assessment of best-case and worst-case outcomes. At best it’s fairly straightforward, I believe, but at worst it’s not; however, offering both will likely to be useful to many people.
“The physical knowledge that too high cliffs give way is very old and known to every miner or quarry. The physical knowledge that ice is not the strongest rock on Earth is also quite old. And the suggestion that a cliff failure could affect the stability of West Antarctica dates back to 1962,” Alley wrote. “We now have stronger evidence that a sufficient retreat to West Antarctica could lead to a calving front higher than any other in the world, and higher than a stability limit suggested in recent papers. Turning that understanding into projections of the future, as in our new paper, has implications for the worst-case scenario, and testing against the paleoclimatic record supports that understanding.
He continued: “It is too early to say that this is an accurate worst-case scenario. Step-by-step application of the [warming] is clearly too extreme… but it is within the realm of the possibility that for the timescale of collapse, the true worst-case scenario could be even a little faster than modeled here; the renewed interest in this topic is recent and the number of scientific papers examining physics remains low.
Earth and planetary science letters2014. DOI: 10.1016/j.epsl.2014.12.035 (About DOIs).