East Antarctic Ice Sheet getting thicker from underneath

Image: Montana.edu

From AAAS online:

Widespread Persistent Thickening of the East Antarctic Ice Sheet by Freezing from the Base

Abstract

An International Polar Year aerogeophysical investigation of the high interior of East Antarctica reveals widespread freeze-on that drives significant mass redistribution at the bottom of the ice sheet. While surface accumulation of snow remains the primary mechanism for ice sheet growth, beneath Dome A 24% of the base by area is frozen-on ice. In some places, up to half the ice thickness has been added from below.

These ice packages result from conductive cooling of water ponded near the Gamburtsev Subglacial Mountain ridges and supercooling of water forced up steep valley walls. Persistent freeze-on thickens the ice column, alters basal ice rheology and fabric and upwarps the overlying ice sheet, including the oldest atmospheric climate archive, and drives flow behavior not captured in present models.

  • Received for publication 8 November 2010.
  • Accepted for publication 18 February 2011.
  1. Robin E. Bell1,
  2. Fausto Ferraccioli2,
  3. Timothy T. Creyts1,
  4. David Braaten3,
  5. Hugh Corr2,
  6. Indrani Das1,
  7. Detlef Damaske4,
  8. Nicholas Frearson1,
  9. Thomas Jordan2,
  10. Kathryn Rose2,
  11. Michael Studinger5, and
  12. Michael Wolovick1

+ Author Affiliations


  1. 1Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA.

  2. 2British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK.

  3. 3Center for the Remote Sensing of Ice Sheets, Kansas University, Lawrence, KS, USA.

  4. 4Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, Germany.

  5. 5Goddard Earth Science and Technology Center, University of Maryland Baltimore County, MD, and NASA Goddard Space Flight Center, MD, USA.
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53 Responses to East Antarctic Ice Sheet getting thicker from underneath

  1. Otter says:

    I know nothing of glacial mechanics, but I have to wonder if super-cooled water does not play a significant role in glacial flow, rather than melting.

  2. martin webster says:

    ….so more atmospheric warming results in more melt..results in more ice formation beneath..hhmmm Surely the header should read “East Antarctic Ice Sheet getting thicker from underneath, thinner from the top”…or is there some agenda on here…surely not

  3. etudiant says:

    Wow, that is impressive.
    Despite several thousand feet of ice insulating the rock surface, the cold has been severe enough for long enough to freeze out the water that is found down there by conduction.
    Also means the ice at the bottom is the youngest, not the oldest.
    Interpreting those cores will be interesting.

  4. Phenomenal! There are more things in heaven and earth…

    Right now, it”s about 2:30 in the morning in California, Anthony. I know all of us want you to keep up the good work, but if you are in your home state, you are going the extra distance! We should allow you to rest.

  5. Otter says:

    Additional to my earlier comment: I meant to say ‘melting, as called for by the Antarctic-ice-sheet-is-gonna-collapse crowd.’

  6. Alexander K says:

    Fascinating stuff. Funny how the universal quest for knowledge pushes all of us in different directions; I was diverted by the term ‘rheology’, utterly new to me, and found a whole world of non-Newtonion liquids that behave in ways I did not expect. When I am preparing meals, I will look inside the neck of my honey and mayo jars each time I open them with a new understanding. Education late in one’s life is truly exciting,

  7. Gary Mount says:

    super-cooled water is believed to be what brought down Air France Flight 447 in 2009.
    “Accident investigators believe that super-cooled water in the clouds – well below freezing, but too pure to turn into ice – could have disabled the pitot probes.”

    Read more: http://www.dailymail.co.uk/news/article-1282367/Air-France-crash-The-truth-disaster-killed-228-people.html#ixzz1G0Twk1Q6

  8. Baa Humbug says:

    In between the frivilous but humourous comments which I enjoy, I look forward to a comment from a knowledgeable person explaining what this really means.

  9. Geoff Sherrington says:

    Allied to this, it is often said that a deeper ice core hole at Vostok would meet bedrock. What does this mean? (a) Was there a period estimated at some 700,000 years or more ago when the globe was so hot that there was no ice where Vostok sits now? (b) has the basal ice from earlier times been squeezed sideways out of the picture? (c) would it not be interesting to discover where, on earth, there sits the oldest known ice in contact with rock below? (d) some people hypothesise that below Vostok there is a lake. Can it be that this lake is also freezing from the bottom up? (e) Is the time axis derived from Vostok horribly wrong?

    I can offer no suggestions. I do not have to, because the science is “settled”.

  10. E.M.Smith says:

    @Baa Humbug:

    Don’t know if I meet your qualifications, or if I’ve got it right, but what it said to me was:

    1) Ice can form at the bottom. Some ice cores may ‘have issues’ due to this, or not, if you are careful.

    2) Ice forms around the edges where there are puddles from the mountains. Ice growth is NOT just from snowfall. Measuring snowfall does not tell you total ice growth.

    3) We don’t really know where all the ice is coming from.

    4) We don’t really know the exact history of all the ice (as we now know some is from the bottom). So things like “it’s about this thick so about that old” are now broken.

    5) How does adding ice from the bottom change the implied CO2 in the ice (if at all)? Does it in any way mess up the nice snow layers? (I’d guess not, but…)

    6) Doing a “mass balance” that says “snow on / calving and melt off” will be saying “it’s MELTING” when it isn’t as you missed the “added from below” ice. It may well be growing a lot more than that technique would indicate. So you need something more like sats and radar measuring height…

    7) It’s just kind of way cool and a new idea!

    I’m sure there is more, but that’s the big bits I got at 4 am and sleepy ;-)

  11. Don Penim says:

    Someone should tell Tim Naish about this because he seems to be coming to very different and alarming conclusions:

    –Victoria University Antarctic and climate change researcher Professor Tim Naish has been watching the weather this year and has found nothing to reassure him.

    In Antarctica, where Tim’s research is focused, the trends are the same. Both the Greenland and West Antarctic ice sheets are beginning to melt and new research suggests the same is happening to the world’s biggest ice sheet in East Antarctica.

    “Rising sea levels present one of the biggest threats to civilisation as we know it,” says Tim.

    Tim is Director of Victoria University’s Antarctic Research Centre and one of the lead authors of the next international climate change assessment from the Intergovernmental Panel on Climate Change (IPCC) , due for release in 2014. –

    http://wiredcampus.chronicle.com/campusViewpointArticle/Climate-change-The-clock-is/388/

  12. This is evidence that at this region on our earth, it is getting colder rather than warmer. Now they need to quantify the rate of cooling and determine what natural cycle of climate change it is following. It isn’t following the rise in atmospheric CO2.

  13. For those of you who are questioning how water can exist below kilometers of ice, google “triple point of water”. Water flows and collects in the valleys and is not found in any significant quantities under ice cores taken over domes. But ice flows as well (much more slowly).

  14. tty says:

    “Someone should tell Tim Naish about this because he seems to be coming to very different and alarming conclusions”

    The most interesting thing about that interview is the things that are not mentioned. Note that now it’s the Pliocene, 3 or 4 million years back that is held up as the model for what will happen in 2100. Until quite recently it was the last interglacial 120,000 years ago, or MIS 11, the fourth interglacial back that were all the rage. The West Antarctic Ice sheet was supposed to have melted and the sea level to have been 6-9 or even 15-20 meters higher than now. Unfortunately the ANDRIL cores (that Tim Naish is involved with) showed that it simply didn’t happen. There has not been appreciably less ice than now in Antarctica for more than a million years, despite interglacials that were considerably warmer than the present one.
    So now the AD 2100 horror story has perforce had to move back into Pliocene, when the climate and geography was vastly different from the present (there was no Panama Isthmus, just for starters).

  15. John Marshall says:

    Normal glaciers melt at the contact surface with the valley floor because of geothermal heat. This is why glacier streams flow. This water also helps glacier movement. So I do not understand this as there will be a significant amount of geothermal heat there and the thick ice insulates this surface from the cold atmosphere above.

  16. John Marshall says:

    Normal glaciers melt at the contact surface with the valley floor because of geothermal heat. This is why glacier streams flow. This water also helps glacier movement. So I do not understand this report as there will be a significant amount of geothermal heat there and the thick ice insulates this surface from the cold atmosphere above.

  17. icicling says:

    This seems to be largely a mass redistribution process. The supercooling process has been recognized in modern glaciers. It occurs when liquid water, pressurized by the weight of overlying ice reaches temperatures below freezing but does not freeze due to this pressure. Freezing may occur as the water moves to areas of lower pressure. The critical element for this to occur seems to be the bed slope vs. ice surface slope ratio. If the bed slope is 1.2-1.7 times the ice surface slope, then freeze-on of the supercooled water may take place. This is essentially the process described in the abstract. The freeze-on process occurs along overdeepenings (or in this case valley sides where the appropriate slope ratios are liekly met).

    As some of you have suggested, and the abstract mentions, this will have implications for interpreting ice cores. However, there are likely very diagnostic ‘signatures’ of ice accreted by supercooling (physical characteristics and isotopic signatures) which should allow for its recognition in ice cores. The supercooling process also entrains sediments and freezes them in the ice layers (if sediments are present along the flow path prior to freezing), which maks these layers distinguishable form clean ice and layers of ice-rich sediment derived directly from erosion.

    This is highly interesting and yet another glimpse into the ever evolving understanding of the subglacial hydrology of Antarctica.

    BTW, regarding a previous comment, there is ice accreting at the base of Lake Vostok.

  18. icicling says:

    Fred H. Haynie,

    I don’t think the temperature linkages are that simple. The refreezing water is either 1) very old and it has been ‘trapped’ in topographic saddles where the pressure keeps it liquid, and/or 2) it is young and produced by the pressure melting and the very limited sliding that occurs (if at all). The geothermal heat flux also likely melts some of the basal ice to produce some water (ice thickness is like an insulating blanket).

    The main point is that there will be an enormously long lag time for a warming or freezing front to penetrate to that depth and tying it to an atmospheric cycle may be difficult due to this lag time. As well, by the time a warming/freezing front, reaches those depths, it may be difficult to differentiate the cause of the warming/cooling from the ‘background’ temperatures. There will also be some degree of feedbacks that will muddle the ‘signal’. For example, increased temperatures at the base (whatever the cause) could lead to greater melting, which could increase sliding and friction, leading to more heat generation. It might be possible to differentiate the sources of warming/cooling but it would depend on the magnitude of the changes relative to the ‘background’ and initial conditions, as well as the time scale(s) at which these changes can take place.

  19. icicling ,

    I agree. I have simplified the system for those who do not understand the basics. Still it is more complex than the simple model that increasing atmospheric levels of CO2 are increasing the rate of melting of Antarctic ice sheets.

  20. Billy Liar says:

    Gary Mount says:
    March 8, 2011 at 3:36 am

    super-cooled water is believed to be what brought down Air France Flight 447 in 2009.

    Supercooled water is pretty common in the atmosphere. Any time ice forms on a plane flying in cloud it is probably because of the prescence of supercooled water.

  21. Peter Plail says:

    Yet another real world phenomenon that the models fail to allow for.

  22. tty says:

    “Normal glaciers melt at the contact surface with the valley floor because of geothermal heat. This is why glacier streams flow. This water also helps glacier movement. So I do not understand this as there will be a significant amount of geothermal heat there and the thick ice insulates this surface from the cold atmosphere above.”

    It’s not that simple. Glaciers can be both warm-based with a water film at the bottom and cold-based, i. e. frozen to the bed. Cold-based glaciers also flow, but slower than warm-based since they can only flow by internal deformation. Large ice-sheets (like Antarctica) are usually polythermic, i e partly warm-based and partly cold-based, depending on the thickness of the ice and the geothermic heat-flow.

  23. Caleb says:

    “In some places, up to half the ice thickness has been added from below.”

    Whew! That is a concept I never dreamed of. It simply has to seriously alter, bend, or even interupt the top-side flow of the ice. It will cause problems for any model that does not include the concept of ice-added-from-below.

  24. greg holmes says:

    This is an absolute mind adjuster as far as I am concerned. I would not have guessed this was happening, thanks be to the scientists who have discovered this process, I look forward to more enlightenment on the subject. Thanks again.

  25. DesertYote says:

    I read “getting thicker from underneath”, leaned back to laugh and my chair creaked. Time to lay off the doughnuts :/

  26. wayne says:

    I would think this might be driven solely by capillary action, where a liquid (super-cooled water) is siphoned upward against graivty between two closely spaced solids until it can freeze. Then this new ice is merely one of the solids and this process continues, never ending. Can’t you see something like that occurring?

  27. BillyBob says:

    ““Rising sea levels present one of the biggest threats to civilisation as we know it,” says Tim”

    Lucky for us 2010 is on target for a DROP in sea levels.

  28. phlogiston says:

    Hmmm .. we should look out for any suspiciously palindromic sequences in Antarctic ice core temperature reconstructions. For instance those arguing for a long continuation of the current interglacial point to similarities between it and the 4th interglacial back, just over 400 kyrs ago.

    However I believe that frozen sea water does retain some salt. Snow by contrast is generally not salty. So salinity, plus entrapped plankton and particles, should identify ice frozen on from beneath.

    So what does an ice core analyst do, toiling diligently in the lab on a long ice core, to check that the core section under analysis is really from snowfall, thus getting older with depth, and not sea ice from below and younger with depth?

    Well there is a simple ad hoc solution, he/she should periodically lick the ice to taste if it is fresh or salty.

    Hang on though – maybe not such a good idea. A certain scene from one of the Dumb and Dumber movies springs to mind…

  29. edgar says:

    I guess everyone has seen the latest spew from the liars at nasa:

    http://www.nasa.gov/topics/earth/features/earth20110308.html

  30. DesertYote says:

    edgar
    March 8, 2011 at 2:45 pm

    Yup,
    OMG! ts worse then we thought :0

  31. mike g says:

    @martin webster

    Why would it be getting thinner from the top when it’s not any warmer at the top? Of course, thinning glaciers are proxy for precipitation, not temperature. But, the precip was already near zero, there.

  32. mike g says:

    @Gary Mount

    Leave it the French to design a passenger airplane that crashes if the pitot tubes freeze up.

  33. George E. Smith says:

    So Ok I’ll be the guinea pig. Now all this ice that is growing underneath the Antarctic ice; this is ordinary regulation H2O type ice is it not. Normal ice is made from normal H2O water which is a liquid substance, but it can also be made from gaseous H2O which is a gaseous substance; but in any case to make ice you generally need some source of H2O molecules: oodles of them; megatons of them in fact to grow much thickness of underbelly ice on Antarctica..

    So ‘splain me the origins of this H2O, underneath the ice, could it be leaking out of the oceans, and up rivers somehow underneath the ice by some sort of caterpillary action ? Izzat possible ?

  34. George E. Smith says:

    I should hasten to add, that I do believe the story; I’m just curious as to how it rains or snows under so much ice.

  35. George E. Smith,

    The water is ice that has melted at the earth/ice interface from geothermal heating. That water collects in low spots. Thus Lake Vostok. This refreezing of this melted ice is apparently the result of more heat being conducted through the insulating ice sheet. Either the upper surface is getting colder or the thermal conductivity is increasing.

  36. Clifford says:

    Here is some additional information about the phenomenon.

    http://www.sciencedaily.com/releases/2011/03/110303141551.htm

    It is somewhat misleading as it doesn’t mention a time period for the refreezing, much of which likely occurred during the last glacial period.

    It says nothing about the current ice balance.

    Keep in mind, for these very thick ice sheets… it likely takes centuries for an temperature change to travel from top to bottom. I’d love to see an actual direct measured temperature profile, top to bottom of the Vostok ice core, or other ice cores, but haven’t been able to find one.

    It will be interesting to see if they determine whether the lakes were existing at the time of initial glaciation of Antarctica, frozen with a layer of ice, then slowly thickened over time, or whether they were from glacier melt as likely regularly occurs during interglacial periods with less cooling from the top to maintain the integrity of the glaciers, and to counteract the thermal radiation of earth’s core heat.

    Perhaps if they get ice cores, salinity and trapped organisms will give some clues as to the origins of the lake ice, and hopefully something will indicate a time profile.

  37. tty says:

    “mike g. says:

    Leave it the French to design a passenger airplane that crashes if the pitot tubes freeze up.”

    You don’t know what you are talking about. Any aircraft will. Want some amurrican examples?

  38. tty says:

    The question about where the water is coming from is interesting. It’s certainly not from the ocean. This is very far from the sea and well above sea level. Neither can it be meltwater from the top of the ice – inland Antarctica is never above freezing. It´can hardly be meltwater from the Transantarctic mountains either, there is very little melting there. However large parts of the Antarctic icecap is warm-based, i e the temperature at the base of the ice is above freezing. Other areas are cold-based, i e frozen to bedrock. Presumably there is a hydrologic system beneath the ice where water melts, flows and refreezes, and probably this changes over time as the thickness of the ice and perhaps also the geothermic flow changes. There are cases in Antarctica where entire volcanoes grew in a waterfilled cavity in the ice (Gaussberg for example).
    What this illustrates is that there is a lot we don’t know about what happens beneath a permanent icecap like Antarctica. Most of Glaciology is based on studies of small northern hemisphere glaciers and the the landforms left by the intermittent Eurasian and Laurentide ice-sheets.
    Incidentally I don’t think this will be much of a problem for ice-core studies. Such re-frozen ice would be easy to separate from “virgin” ice by several criteria. However it does decrease the chances of finding really old ice in Antarctica.

  39. Steve Keohane says:

    This doesn’t invoke a warm fuzzy feeling about the stability of CO2 within this ever-morphing ice.

  40. Don K says:

    “Rising sea levels present one of the biggest threats to civilisation as we know it,” says Tim”

    I know that “climate scientists” often seem to have more in common with creation scientists than they do with practicioners of hard science, but are they aware that there are sea level rise data sets covering many decades? Have they ever looked at them? Wikipedia will do fine for this one. http://en.wikipedia.org/wiki/Current_sea_level_rise. Does that graph look like it is tracking either the CO2 concentration or the IPCC et al notion of temperature?

    I would agree that dramatically rising sea levels would be more of a threat to civilization than dubious science, but fortunately, it looks like we only have to deal one of the two and the threat doesn’t look to be sea level rise.

  41. ev tekstili says:

    It is somewhat misleading as it doesn’t mention a time period for the refreezing, much of which likely occurred during the last glacial period.

  42. Richard G says:

    George E. Smith says:
    March 8, 2011 at 5:10 pm

    “So Ok I’ll be the guinea pig. Now all this ice that is growing underneath the Antarctic ice; this is ordinary regulation H2O type ice is it not. Normal ice is made from normal H2O water which is a liquid substance, but it can also be made from gaseous H2O which is a gaseous substance; but in any case to make ice you generally need some source of H2O molecules: oodles of them; megatons of them in fact to grow much thickness of underbelly ice on Antarctica..

    So ‘splain me the origins of this H2O, underneath the ice, could it be leaking out of the oceans, and up rivers somehow underneath the ice by some sort of caterpillary action ? Izzat possible ?”
    _____________________________-

    It is my understanding that if we were to remove the ice cap, Antarctica would be an archipelago not a continent.

    What this is describing is basically an ice volcano slowly erupting ice from the center of Antarctica. The northern ice cap also freezes/adds ice from below does it not?

    The more I learn, the more I learn how little I know.

  43. Richard G says:

    George E. Smith says:
    March 8, 2011 at 5:10 pm

    George, Have you never seen a large spring issuing from the ground? Examples abound where rivers pour forth from underground aquifers in mind boggling volumes. Examples: Thunder River where Tapeats creek in the Grand Canyon pours out of a cave. Thousand Springs where millions of gallons pour from the Snake River Aquifer. Nature is astounding.

  44. Feet2theFire says:

    This is the way of learning new things: At first (IMHO the first century or five) simplistic ideas pop into mind, and we think we have a basic understanding of a phenomenon, and later on we learn of complications that tell us we have to add complexity to our understanding.

    It isn’t that we are stupid, but it IS because we think that the model we have conjured up in our heads matches the reality. And once the brain model takes form, it is that which we are seeing when we see the phenomenon’s evidence being unveiled over time – we color our perceptions by the expectations that we have about our brain model, that it will prove out to be correct. So we keep crowbar-ing the evidence to fit the model.

    Some of us will see the Emperor’s New Clothes and push for modification of the model/paradigm/construct, against some level of resistance by the rest. Progress does get made, but those brain models keep fighting for their very lives.

    ALL brain models are wrong, in that they oversimplify. The real reality is much more complex than we will admit. So far, every step forward in information manipulation (i.e., computer CPU speeds and capacities) we believe that now we can get the complexity figured out. But IMHO we are centuries away from having the CPU/brain power to handle the complexities. Of course, the programmers and the scientists don’t want anyone else to know any of this; they make a living off people who think the programmers and thinkers are equal to the task. But good programming and good thinking need enough evidence and very solid methods in order to know the right approach. We are making a good effort, but we simply are too early in the game to do more than find new questions to ask.

    And that is a GOOD thing, to get to asking those new questions. But we are only a very short way along the continuum. No, the programmers and the scientists don’t want people to know that we are a long way from really knowing what is going on. Our human hubris screams, like a 2-year-old, “I can do it myself!”

    But in reality, we are only stroking ourselves and our vanity. Science is best approached with humility. Someone please tell Michael Mann that.

    The AGWers are doing their bests, but their brain model is wrong. The skeptics’ brain model is wrong, too – but at least we admit it. And we can see to a good degree how wrong theirs is, too.

    This study flat out craps on the simplistic idea that they understand what the ice cores mean. Everyone – inside their circles and outside – knows that the “understanding” of ice cores is based on an assumption. Now we find out about ONE of the complexities that muddies the water. Others will come…

  45. Tenuc says:

    Ah good, another unknown unknown has been identified and become a known unknown about how much this process effects icecap mass, growth and decline.

    Lets throw away the now useless models and do some research so we know more…

  46. Leo Geiger says:

    There are many comments wondering where the water is coming from. It is simply coming from the ice sheet. What is being discussed is ice at the bottom of the ice sheet that becomes liquid (due to pressure or geothermal heat) and then refreezes.

  47. kadaka (KD Knoebel) says:

    Leo Geiger said on March 10, 2011 at 7:08 am:

    There are many comments wondering where the water is coming from. It is simply coming from the ice sheet. What is being discussed is ice at the bottom of the ice sheet that becomes liquid (due to pressure or geothermal heat) and then refreezes.

    1. Ice at bottom of sheet melts into liquid water.
    2. This liquid water at bottom of sheet then refreezes into ice.
    3. This makes bottom of sheet grow thicker, without any new water added?

    Some clarification of your comment seems warranted.

  48. kadaka (KD Knoebel) says:

    General musing:
    1. At the bottom of the ice sheet, there is liquid water.
    2. There is great pressure, as one finds deep in the ocean.
    3. There is a source of energy and nutrients, namely volcanic-type activity, as can be found deep in the ocean at hydrothermal vents.
    Question: Is there life at the bottom of the ice sheet, similar to life found deep in the ocean at hydrothermal vents?

  49. Richard G says:

    Leo,
    Are you really ready to presume that groundwater movement that is present in all other geologic provinces is simply not possible or present in Antarctica? Me thinks thou doth presume to much.
    Ground water venting at 56 degrees F. in Antarctica, considering the temperature regimes there, is still geothermally heated.

  50. Leo Geiger says:

    I was simply pointing out that the existence of liquid water at the base of ice sheets does not require mysterious flows from oceans (see George E. Smith’s comment on March 8, 2011 at 5:10 pm) or even springs. It can be there simply because of dynamical processes involving pressure and geothermal heat acting at the base of the ice sheet. No doubt springs exist, but the source of water from those would ultimately be from ice sheet melt too.

  51. Leo Geiger says:

    kadaka (KD Knoebel) says: “3. This makes bottom of sheet grow thicker, without any new water added? Some clarification of your comment seems warranted.”

    Maybe there is some confusion about *localized* growth at the bottom of the ice sheet by the refreezing of water transported from elsewhere (either at the rock-ice interface or beneath it via aquifers). This is not the same as thinking there might be an *overall* increase in the amount of Antarctic ice because it is growing from beneath. The “new” water would still have come from ice sheet melt somewhere else.

  52. Richard G says:

    Tapeats creek (http://web.mst.edu/~rogersda/cp_megalandslides/tapeats_cave_canyon.htm) is entirely spring fed, is a relatively small creek yet it continuously discharges an estimated 70 million gallons a day 24/7-365 days a year. Who can say what is happening under the icecap. I dare say it is not a simple answer. Volcanoes discharge “new” water that has never been part of the surface water cycle before. There are volcanoes under the icecap. We cannot simply assume the ice cap only gains water from precipitation. The polar world is a cold desert. We know little of the geology under the ice. Estimates of ice mass are WAGs.

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