Tag Archives: glaciers

Important New Science on Melting Glaciers

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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.

ResearchBlogging.orgConsider 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.

ResearchBlogging.orgThis 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

The Science of Melting Ice Sheets: New review in Nature

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A paper came out in today’s Nature about glacial melting and its contribution to sea level rise. This paper does not present new research, but rather summarizes and evaluates the last several years of research on modeling and measuring contiental glaciers and their dynamics.

From the Abstract:

Since the 2007 Intergovernmental Panel on Climate Change Fourth Assessment Report, new observations of ice-sheet mass balance and improved computer simulations of ice-sheet response to continuing climate change have been published. Whereas Greenland is losing ice mass at an increasing pace, current Antarctic ice loss is likely to be less than some recently published estimates. It remains unclear whether East Antarctica has been gaining or losing ice mass over the past 20 years, and uncertainties in ice-mass change for West Antarctica and the Antarctic Peninsula remain large. We discuss the past six years of progress and examine the key problems that remain

ResearchBlogging.orgThere are many difficulties with measuring and understanding the dynamics of melting of large continental glaciers, the large ice sheets that cover Antarctica and Greenland. As ice melts from these glaciers, they grow lighter and this allows the underlying bedrock to rise up, and conversely, if snow is added to the surface this increases the amount of depression of the underlying bedrock. For this reason you can’t just measure the surface of the ice to estimate how much has been added or removed. When ice melts on the surface, some of it travels down into the glacier and some comes right off the surface. The ice that goes into the glacier may cause deeper ice to melt, or it may provide lubrication to the base of moving streams of ice. As a glacier loses mass at the edge through calving of ice bergs, and the margin retreats away from the sea, the degree of calving, which is an ice-ocean interaction effect probably decreases. Large masses of ice are “grounded” at the outer margin on a “grounding line” beyond which is floating glacier (not sea ice, but large masses of ice undercut by the sea). The grounding line can move towards the sea or away from it, and the dynamics of this movement are complex and difficult to model or measure. Many of the Antarctic grounding lines occur on surfaces that slope downwards in the inland direction, which makes the dynamic a bit more complicated to measure.

Major changes that have improved estimates include adding dimensions to some of the models, such as considering both vertical and horizontal forces along grounding lines. Also, newer models use a finer resolution. However, the increase in resolution is thought to be insufficient; current models are not calculated at fine enough resolution to include numerous smaller ice streams that are narrower than the sampling density of the models.

It appears that the range of uncertainty of ice-melting models has improved significantly over the years so greater confidence in their predictions may be warranted. The best estimates of future contribution to sea level rise of melting glaciers is still highly variable, however.

The current estimates of contributions to sea level rise in mm per year from various studies are between 0.59 and 0.82 from the major ice sheets, between 0.71 and 1.4 for ice caps and glaciers, about 1.1 for thermal expansion, and a negligible but positive amount from changes in terrestrial water storage. These modeled amounts sum to 1.66 mm per year or 3.11 mm per year depending on the set of sources that are used. The observed change in sea level rise over the period from 1993=2008 is 3.22, so there is good agreement though the models are a bit light.

These numbers are small, but they are larger than previous estimates and observations. Still, compared to the potential sea level rise when one considers that the ice in the continental glaciers equals several meters of ocean water, near future sea level rise may be expected to be relatively low if these models are correct and account for everything. Over a century of time, this amounts to about 300 mm, or one foot, of sea level rise. If, however, oceans are warming more than the air at present and a few more episodes of that occur over the next century, this may be considered a minimal estimate. One foot does not sound like a lot of sea level rise, but it is enough to remove extant barrier beaches. Also, flood tides would not be increased by one foot, but rather, more exponentially. This is how a sea level rise of about this order of magnitude over the last century managed to contribute to the flooding of the lower Manhattan subway tunnels when the region was struck by Hurricane Sandy last year.

But there is a problem. Several areas of uncertainty exist in the models that are currently in use, and my impression is that these areas of uncertainty could be associated with dramatic errors in sea level rise estimate. The dynamics of grounding line changes, the role of lubrication at the base of glaciers (which can cause ice streams to speed up on their way to the sea) and the effects of warm currents shifting their position in Antarctic to cause more melt at the boundaries are among those factors that are least known and that have the highest uncertainty. Also, the seaward edge of continental glaciers are not only held in place by their grounding line on the continent, but also by more distal parts of the floating segment of the glaciers being pinned on prominence. As far as I know the effects of pinning being disrupted or lost are not included in any of the models. Also, I’m pretty sure that the effects of sea level rise on grounding and pinning have not been adequately addressed.

That these issues may be a problem is empirically suggested. The paleo-record shows that continental ice melting and associated sea level rise may occur in fits and starts, with steady melting punctuated by brief periods of extreme melting. The current models don’t seem to predict this sort of event, though these events probably happen.

Hanna, E., Navarro, F., Pattyn, F., Domingues, C., Fettweis, X., Ivins, E., Nicholls, R., Ritz, C., Smith, B., Tulaczyk, S., Whitehouse, P., & Zwally, H. (2013). Ice-sheet mass balance and climate change Nature, 498 (7452), 51–59 DOI: 10.1038/nature12238

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