Most of the current models of glacial ice melting (and contribution to sea level rise) focus on ice melting and less than they need to on the process of glaciers falling apart in larger chunks such as ice bergs. Also, current understanding of glacial ice melting due to global warming indicates that the Western Antarctic Ice Sheet (WAIS) is more vulnerable to melting over coming decades or centuries than is the Eastern Antarctic Ice Sheet (EAIS). New research from two different teams seems to provide a major corrective to these assumptions.
First, about how glaciers turn into ocean water.
Consider this experiment. Take a large open-top drum of water and poke a hole near the bottom. Measure the rate at which water comes out of the hole. As the amount of water in the drum goes down, the rate of flow out of the hole will normally decrease because the amount of water pressure behind the hole decreases. Now, have a look at a traditional hourglass, where sand runs from an upper chamber which slowly empties into a lower chamber which slowly fills. If you measure the rate of sand flow through the connecting hole, does it decrease in flow rate because there is, over time, less sand in the upper chamber? I’ll save you the trouble of carrying out the experiment. No, it does not. This is because the movement of sand from the upper to lower parts of an hourglass is an entirely different kind of phenomenon than the flow of water out of the drum. The former is a matter of granular material dynamics, the latter of fluid dynamics.
Jeremy Bassis and Suzanne Jacobs have recently published a study that looks at glacial ice as a granular material, modeling the ice as clumped together ice boulders that interact with each other either by sticking together or, over time, coming apart at fracture lines. This is important because, according to Bassis, about half of the water that continental glaciers provide to the ocean comes in the form of ice melting (with the water running off) but the other half consists of large chunks (icebergs) that come off in a manner that has been very hard to model. By treating the ice as a granular substance, Bassis and Jacobs have been able to look at the relationship between the large scale geometry of glacial ice and the smaller scale process of ice berg calving.
From the abstract of their paper:
…calving is a complex process and previous models of the phenomenon have not reproduced the diverse patterns of iceberg calving observed in nature… Our model treats glacier ice as a granular material made of interacting boulders of ice that are bonded together. Simulations suggest that different calving regimes are controlled by glacier geometry, which controls the stress state within the glacier. We also find that calving is a two- stage process that requires both ice fracture and transport of detached icebergs away from the calving front. … as a result, rapid iceberg discharge is possible in regions where highly crevassed glaciers are grounded deep beneath sea level, indicating portions of Greenland and Antarctica that may be vulnerable to rapid ice loss through catastrophic disintegration.
This is interesting in light of a second recent paper, by Carys Cook and a cast of dozens, which looks at Antarctica during the Pliocene. Green house gas levels were about the same during much of the Pliocene as the current elevated levels, and sea levels may have been many meters higher at various points in time as well. From the abstract of that paper:
Warm intervals within the Pliocene epoch (5.33–2.58 million years ago) were characterized by global temperatures comparable to those predicted for the end of this century and atmospheric CO2 concentrations similar to today. Estimates for global sea level highstands during these times imply possible retreat of the East Antarctic ice sheet, but ice-proximal evidence from the Antarctic margin is scarce. Here we present new data from Pliocene marine sediments recovered offshore of Adélie Land, East Antarctica… Sedimentary sequences deposited between 5.3 and 3.3 million years ago indicate increases in Southern Ocean surface water productivity, associated with elevated circum-Antarctic temperatures. The [geochemistry]… suggests active erosion of continental bedrock from within the Wilkes Subglacial Basin, an area today buried beneath the East Antarctic ice sheet. We interpret this erosion to be associated with retreat of the ice sheet margin several hundreds of kilometres inland and conclude that the East Antarctic ice sheet was sensitive to climatic warmth during the Pliocene.
This is, to me, one of the most disturbing facts about climate change that we learn from the paleo record. It may be reasonable to say that our near doubling of greenhouse gasses have brought us to a situation in which it is normal to have perhaps something like 20 meters more sea level than we have today, and that the only thing keeping that from happening is … well, nothing, really, other than time. Glaciers tend to behave glacially, after all. Cook et al. look at sediments offshore from Antarctica deposited during the Pliocene periods. Using fingerprinting with specific stable isotopes they were able to determine that at certain times during the Pliocene sediments were being deposited in the ocean from an eroding landscape that is currently deeply and firmly buried under the EAIS. This seems to suggest that under conditions not necessarily very different from today, large areas of Eastern Antarctic, thought to be iced over long term, can be ice-free. If those vast areas were ice free, than the ocean would have been much higher, and it seems that the ocean was, in fact, higher at that time.
I asked Jeremy Bassis, lead author of the ice-as-granular-material paper, if he could translate the modeling work done by him and Jacobs into an estimate of how fast glaciers could disintegrate. He told me that it was hard to say. Their models help them “… understand the different patterns of calving that occur and based on that, it looks like some regions of Antarctica and Greenland might be vulnerable to disintegration. However, the simulations we did took place over a few hours so to translate that into an actual sea level rise estimate we would need to run the models for much longer. The best I can say for sure is that based on our model, important processes are not included in current estimates of sea level rise.” He also noted that most models that don’t use paleo data assume iceberg calving at present rates from their current position at the sea. Their paper, however, suggests that these may not be good assumptions.
Sadly, none of this work will be included in the upcoming IPCC reports. The time cycle for IPCC is rather ponderous, which may be good in some ways, but also has disadvantages. These two papers exemplify an effort to address one of the biggest unknowns in climate change, the nature and character of meltdown of the polar ice caps. We need to put more resources into this sort of study.
Meanwhile, don’t throw away your knickers.
Bassis, J. N., & Jacobs, S. (2013). Diverse calving patterns linked to glacier geometry Nature Geoscience DOI: 10.1038/ngeo1887
Cook, Carys, Flierdt, Tina van de, Williams, Trevor, & et al (2013). Dynamic behaviour of the East Antarctic ice sheet during Pliocene warmth Nature Geoscience DOI: 10.1038/ngeo1889