Monday, February 8, 2016

Watch The Ice Shelves - 3

Fig. 1 Ice Shelf loss
In previous posts of this series I focused on research that implicates Antarctica's ice shelves as harbingers of things to come (Watch The Ice Shelves, 2).

Since they hold back or resist the flow of the ice streams on the land mass, when those ice shelves go, the ice streams go faster.

I mean, more ice from the ice streams on land can flow faster and reach the ocean more easily.

Which will cause sea level change (SLC) in the form of both sea level rise (SLR) and sea level fall (SLF).

So, today let's do three things:
1) review some new research regarding ice shelf integrity,

2) further discuss ice sheet gravity as it relates to ice shelves,

3) engage in a hypothesis concerning ice shelf gravity.
I. New Research

Here is the abstract at the journal Nature concerning new research:
The floating ice shelves along the seaboard of the Antarctic ice sheet restrain the outflow of upstream grounded ice. Removal of these ice shelves, as shown by past ice-shelf recession and break-up, accelerates the outflow, which adds to sea-level rise. A key question in predicting future outflow is to quantify the extent of calving that might precondition other dynamic consequences and lead to loss of ice-shelf restraint. Here we delineate frontal areas that we label as ‘passive shelf ice’ and that can be removed without major dynamic implications, with contrasting results across the continent. The ice shelves in the Amundsen and Bellingshausen seas have limited or almost no ‘passive’ portion, which implies that further retreat of current ice-shelf fronts will yield important dynamic consequences. This region is particularly vulnerable as ice shelves have been thinning at high rates for two decades and as upstream grounded ice rests on a backward sloping bed, a precondition to marine ice-sheet instability. In contrast to these ice shelves, Larsen C Ice Shelf, in the Weddell Sea, exhibits a large ‘passive’ frontal area, suggesting that the imminent calving of a vast tabular iceberg will be unlikely to instantly produce much dynamic change.
(The Safety Band of Antarctic Ice Shelves). This paper attempts to describe some of the characteristics of the shelves and identify weak and strong parts.

A member of the scientific commentariat at ESA explains it this way:
It transpires that about 13% of the total ice-shelf area contains what is called ‘passive shelf ice’. This is the part of the floating ice body that provides no additional buttressing – so if lost there wouldn’t be an instant increase in glacial velocity.

However, behind this – there is an area of ice called the ‘safety band’, which is the most critical portion of the ice shelf restraining the ice flow.

Dr Johannes Fürst, from the University of Erlangen-Nuremberg’s Institute of Geography explained, “For some decades now satellite remote-sensing has allowed us to track changes and movement of Antarctic ice fronts. In some regions we have seen continuous ice-shelf recession.

“Once ice loss through the calving of icebergs goes beyond the passive shelf ice and cuts into the safety band, ice flow towards the ocean will accelerate, which might well entail an elevated contribution to sea-level rise for decades and centuries to come.”

However, there are some contrasting results across the continent as not all ice shelves have this passive ice.

Dr Fürst added, “The Amundsen and Bellingshausen seas have limited or almost no passive ice shelf, which implies that further retreat of current ice-shelf fronts will have serious dynamic consequences.
(Antarctic Safety Band At Risk). I guess this is at least a code yellow or perhaps even a code red for some locations there  (Proxymetry3 - 3).

II. Ice Sheet Gravity vs. Ice Shelf Gravity

The ice sheets have gravitational pull that pulls sea water toward the coastline and holds it there (The Ghost-Water Constant - 4).

Additionally, this ice sheet mass created gravitational pull must have an impact on the ice shelf.

It would be in the form of pressure put on the ice as the ocean water is pulled toward the landmass coastline.

Lunar and solar gravity caused tides do the same, except much more intensely I would think (Tidal Impact On Glaciers).

The ice shelves must also have that same effect on the sea water near them, a pull.

When the ice shelf breaks away, sea water is going to have an easier time getting to the shoreline.

III. Ice Shelf Gravity

Some of the ice shelves around Antarctica are massive, larger than some states or countries.

Fig. 2 Ice shelf impediments
They must add resistance to the ice sheet mass gravity pull on sea water (Fig. 2).

That is a pull that would bring sea water up against the coastline like a high tide or a storm surge.

But the ice shelf on top of the water would resist that.

So, when the ice shelf breaks away, a contradictory thing takes place.

Water will flow to the coast, the ice shelf now ice berg will float away and melt, then the resulting melt water will flow towards the bulge at the equator.

Back at the shore, the ice stream will speed up and dump ice into the sea, thereby losing mass and gravity.

Then, finally that will cut loose some ghost-water too.

In other words, ultimately near shore there will be SLF.

The bottom line, then, is an increase in both SLF and SLR, depending on location (latitude, longitude).

IV. Conclusion

While it may be true that SLC is not complicated, it is also true that it has a lot of moving parts.

And it is all happening at once .... I mean Greenland is losing ice at the same time Antarctica and Glacier Bay are.

And doing it on their own schedule.

So, predicting the intensity of catastrophes that SLC is bringing and will continue to bring for a long time, is not an exact science in terms of exactly predicting precise future time frames.

But it can determine what is coming, and give useful warnings.

Then it is up to those who should do something about it to do that something.

The previous post in this series is here.

5 comments:

  1. Dredd a question or two. Having been watching the AIS daily this melt season it would look like a large pulse of run off is coming from under the Shirase glacier. The open water forming off to right looks like upwelling and the obvious valley in the ice sheet out to open water is what under melt tends to look like. If a large pulse of run off were to happen that could cause noticeable SLR (several centimetres or so) how long would you expect the time lag between event and effect to take? Also the Amery shelf is looking worse all the time with ponding in particular. Do you think the hydro fracturing of the ponding will increase the rate of collapse should the seaward edge give way suddenly?

    Red

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    Replies
    1. Red,

      First Section (Only 4096 chars are allowed in a comment, so I must use two sections to reply).

      In today's post (Don't Believe In Abrupt Sea Level Change - Know About It - 4) the second video (Dr. Chambers, who wrote the IPCC report's section related to your questions) points out that E. Antarctica is stable, so, your reference to Shirase glacier and the Amory shelf, which are in E. Antarctica, must be an illusion eh?

      He states that E. Antarctica is stable, so no need to imagine it is melting eh?

      Just kidding.

      I pointed out that "As Dr. Chambers was writing down the myth in the IPCC report, East Antarctica was already melting but he didn't want to be aware of it ..." (ibid).

      Fear is the parent of denial, and there is both official fear and official denial in the most recent IPCC when it comes to E. Antarctica.

      Anyway, your focus on E. Antarctica is well placed (not that Greenland and W. Antarctica are not important too).

      It is difficult to pinpoint exactly when these or any other ice shelves will calve bergs or when melt water or pulses along the basal areas of their ice sheets will flow under into the ocean waters under the ice shelves.

      However, to your question about how long will it take for any melt water (not ghost water) or ice bergs to relocate, Dr. Mitrovica expressed the current consensus of "about two weeks" concerning melt water.

      (the second section follows)

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    2. As to ghost water, which is a substantial amount of water already in the ocean, it is released in proportion to the loss of ice mass of the ice sheet involved (see The Ghost-Water Constant, 2, 3, 4, 5, 6, 7). I might add that the IPCC is void of any mention of ghost water (Dr. Chambers does not mention it either, even though he does get the impact of ice sheet gravity on ocean water near the ice sheets).

      Ice bergs are a different story ... their fate depends on more forcing dynamics than melt water does, such as wind, temperatures, and currents, in terms of how long it takes for their complete relocation.

      The big picture in E. Antarctica is the Totten area, its counterpart in Greenland is the NEGIS (The NEGIS Revisited).

      One suggestion I would make is that you grasp, if you don't already, that only about 1% of the total ice of the cryosphere (Greenland ice sheet, Antactica ice sheet, and land glaciers) need to melt to begin to disrupt worldwide coastal areas (The 1.14% vs. The 100%, 2, 3).

      Finally, your question of how much "(several centimetres or so)" ... I don't do the bathtub model (The Bathtub Model Doesn't Hold Water, 2, 3, 4).

      The sea level near an ice sheet drops as it loses mass caused by global warming of air, land, and sea, which releases both melt water and ghost water to be relocated elsewhere.

      The quantity and destination of relocated water is in proportion to Earth's global gravity and its rotational forces which cause, among other things, "the bulge" (The Battle of the Bulge).

      In closing, let me say: "keep your lab coat on Red" (Put Your Lab Coats On).

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    3. Thanks for your time Dredd the query about the centimetres was more about the timing of it being noticed than the actual amount. I understand the bathtub thing or more accurately the lack of it. The speed any larger amounts of SLR or SLF would be noticed after an event was more what I was wondering. So two weeks +/-. That's fast yes? And yes I do grasp the 1% which is some of the drive behind my inquiry. If the whole of Antarctica is softening at the same time 1% is closer than anybody is willing to say.

      Red

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    4. Red,

      You wrote "The speed any larger amounts of SLR or SLF would be noticed after an event was more what I was wondering. So two weeks +/-. That's fast yes?"

      This is pointed out beginnin @ ~28:00 into this video

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