Comments 1 to 10:

1. mspelto at 09:40 AM on 18 December, 2012
   There are a few points worth adding to put the Winklemann et al (2012) paper into perspective. The paper is making the point that more interior snow does not lead to a lack of sea level rise and quantified this using several models that focused on the grounded portions of the ice sheets. This project has been a leader in pushing the limits of our modelling capabilities. The models used are not yet designed to physically reconstruct the pattern of basal melt rates under ice shelves. Basal melt rates are simply parameterized. It has become clear that changes in the ice shelves are the key and this is driven not by surface melt as is noted in the article, but by basal melt. It is not clear at this point that dynamic changes outlined above lead to greater changes than basal ice shelf melt. To put this in perspective a few recent papers are worth noting
   
   Prictchard et al (2012) observed-“We deduce that this increased melt is the primary control of Antarctic ice-sheet loss, through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow. This is illustrated in a NASA new release . The Amery Ice Shelf in East Antarctica has net basal melting accounting for about half of the total ice-shelf mass loss, with the rest being from iceberg discharge
   (Wen et al, 2010). Pine Island Glacier has surface features that suggest ice-shelf-wide changes to the ocean’s influence on the ice shelf as the grounding line retreated and reveal a spatially dependent pattern of basal melt with an annual basal melt flux of 40.5 Gt
   (Bindschadler et al, 2011). The influence of ocean temperature on melt rate is well illustrated in Holland et al (2008) Figure 1. It is also worth noting that GRACE does not examine ice shelves, that the models are poor at addressing dynamic changes in ice shelves or in basal melt rates. That is why Operation Icebridge has been so valuable in assessing the changes in ice shelves.
   Moderator Response: [DB] Fixed link urls. You used the ” (italic double-quote mark) instead of the " (vertical double-quote mark) throughout.
2. John Hartz at 10:54 AM on 18 December, 2012
   mspelto:
   
   Thank you for supplementing the OP with additional material. It would be extremely helpful to our readers if you would describe what "basal melting" is.
3. Philippe Chantreau at 11:26 AM on 18 December, 2012
   John, I believe that basal melt is melting that occurs at the base of the ice layer (i.e. between the ice and the underlying ground), where the liquid water layer acts as a lubricant and enhances glacial flow. If I remember Mauri's other explanations, that lubrication effect has the potential of increasing the flow of a glacier in rather spectacular fashion, as has been seen already in the Northern hemisphere.
   Moderator Response: [DB] Note that you describe the Zwally effect. This is explored in-depth in this SkS post, itself based heavily on input from Mauri himself.
4. mspelto at 22:30 PM on 18 December, 2012
   The basal melting I am referring to and is discussed in the resources is at the bottom of ice shelves. This is where the magnitude of basal melting is crucial in terms of ice thickness and hence stability of the ice shelves. Under the ice sheet it is not the amount of melt that matters it is really whether it is melting or not that influences flow.
5. John Hartz at 04:46 AM on 19 December, 2012
   mispelto @#4:
   
   Thank you for your prompt response to my initial request. A follow-up question:
   
   The first sentence under the header, “Sea-level is rising – that is a fact” of the OP is:
   
   A number of processes are relevant for ice-loss in Antarctica, most notably to sub-shelf melting caused by warming of the surrounding ocean water.
   
   I presume that the term "sub-shelf melting" is equaivalent to "basil melting."
   
   Am I correct?
6. mspelto at 08:45 AM on 19 December, 2012
   You are correct, the papers cited indicate that sub shelf is the most important not just one of the relevant processes. Thwaites Glacier is one location where the basal melt is leading to acceleration and rifting and calving losses.
7. Agnostic at 10:34 AM on 21 December, 2012
   “Between 30 and 65 percent of the ice gain due to enhanced snowfall in Antarctica is countervailed by enhanced ice loss along the coastline,” says lead-author Ricarda Winkelmann.
   
   I understand this to mean that between 35 and 70 percent of ice gain is NOT countervailed by enhanced ice loss – in other words Antartica is gaining ice. If so, Winkelmanns finding does not appear to be supported by GRACE gravity measurement.
   
   Given that EAIS and WAIS are expected to be affected by snowfall/ice gain and ice loss in quite different ways, failure to differentiate between the tow is not helpful.
8. michael sweet at 11:56 AM on 21 December, 2012
   Agnostic,
   
   This paper states that 30-65% of snow accumulation is countered by increased ice flow caused by the snow fall. In addition there is increased ice loss at the edges caused by warm ocean water melting the edge. Snow accumulation in the center of the Antarctic has long been known. The mechanism of ice loss caused by increased ice flow is new and increases the ice loss from the Antarctic.
   
   The data coming from the Antarctic is mixed and a clear pattern has not emerged. We will have to watch the measurements as they come to see the final result. The West Antarctic is more vulnerable to warm ocean water and the East has a larger snow accumulation area.
9. markx at 01:03 AM on 23 December, 2012
   Some degree of 'self regulation' occurs due to gravitational effects - new research by Gomez (student of Jerry Mitrovica)
   
   

   Evolution of a coupled marine ice sheet–sea level model
   
   Natalya Gomez,1 David Pollard,2 Jerry X. Mitrovica,1 Peter Huybers,1 and Peter U. Clark3
   
   Received 16 June 2011; revised 18 November 2011; accepted 6 December 2011; published 14 February 2012.
   
   [1] We investigate the stability of marine ice sheets by coupling a gravitationally self-consistent sea level model valid for a self-gravitating, viscoelastically deforming Earth to a 1-D marine ice sheet-shelf model. The evolution of the coupled model is explored for a suite of simulations in which we vary the bed slope and the forcing that initiates retreat.
   
   We find that the sea level fall at the grounding line associated with a retreating ice sheet acts to slow the retreat; in simulations with shallow reversed bed slopes and/or small external forcing, the drop in sea level can be sufficient to halt the retreat. The rate of sea level change at the grounding line has an elastic component due to ongoing changes in ice sheet geometry, and a viscous component due to past ice and ocean load changes. [......]

10. KR at 02:18 AM on 23 December, 2012
    markx - Very interesting. Of course, as has been said in many contexts, "size matters".
    
    Drop in relative sea level at the grounding edge of an ice sheet is an influence on ice sheet loss rates (-). So are the warming water at the grounding level (+), changes in precipitation due to atmospheric water vapor levels (-?), lubrication of the ice sheet from percolated melt water (+), and acceleration of sheet movement due to reduction of the grounding line dam effect (+), among others.
    
    Unfortunately, given the observations on ice sheet thicknesses, the sum of these influences is still (as far as I can see) leading to ice sheet loss in Greenland and parts of Antarctica.

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