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Pmt111500

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Melting of the Greenland ice sheet now unstoppable
« on: August 18, 2020, 06:17:38 AM »
Says Mottram & al. (2020)
Adding their estimates are conservative, so bye bye GIS.
https://rmets.onlinelibrary.wiley.com/doi/abs/10.1002/joc.6771

gerontocrat

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #1 on: August 18, 2020, 07:03:55 PM »
Who am I to have some reservations about their paper? But what the hell.

Their paper only seems to consider air temperatures, although presumably the Greennland blocking high (included in their analysis) is likely to keep Greenland dry and sunny (but not this year).

This year surface melt has been well above average for most days since early June.
But SMB losses this melting season are average. Precipitation has been high.
So far the SMB gain for the year 1 Sep 2019 to 31 Aug 2020 looks like being pretty much average. But that average conceals that it is caused by above average high energy weather.

Other papers talk about a warmer wetter Greenland - so SMB losses or gains may well depend on how much precipitation falls as rain and how much falls as snow.

Meanwhile the GIS loses mass from glacial calving and ocean melting of marine terminating glaciers. What happens to the surrounding ocean must also be a big factor.

Like everything else we look at I get the feeling that we are in a permanent state of ever accelerating transition, and the science is going to have a tough job keeping up.

« Last Edit: August 18, 2020, 07:55:29 PM by gerontocrat »
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gerontocrat

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #2 on: August 19, 2020, 12:37:09 PM »
Some more evidence on the fate of the GIS...

https://sustainabilitycommunity.springernature.com/posts/the-role-of-greenland-glaciers

BEHIND THE PAPER
Quantifying the impact of retreating glaciers on Greenland ice loss
Large outlet glaciers in Greenland are changing rapidly. New analyses of over three decades of satellite data reveal that glaciers are draining more ice due almost entirely to observed retreat, with important implications for net ice sheet mass loss.

Michalea King
Scientist, Byrd Polar and Climate Research Center
Quote

Fortunately, many large-scale glacier processes can be studied from space. Our study integrated three and a half decades of satellite data to measure changes in ice thickness and flow speeds, and derived how the volume of ice drained from glaciers, called ice "discharge", varied through time. We found that outlet glacier discharge increased by about 14% since the 1980's. Rather than a steady rise, however, this increase was largely concentrated in a short period between 2000-2005. This resulted in an apparent step-increase from stable discharge rates to relatively consistent, but elevated, rates of ice loss after 2006. We also resolved a distinct seasonal signal, with discharge spiking in the summer months, throughout the full study period. This summertime spike is likely related to how meltwater (that forms on the surface in warm months) drains beneath glaciers and impacts their flow speeds. This was an exciting result because it suggested outlet glacier dynamics have been sensitive to seasonal melt for several decades.

click this link for a super little gif ; Gerontocrat

https://images.zapnito.com/uploads/zxbKZ9dfTPaS1RxDDPMZ_netanomalies3.gif

Therefore, a return to mass balance would now require both (1) a return to surface mass gains seen prior to 2000 (through a combination of increased snowfall and reduced runoff) and (2) a significant reduction in ice discharge. Meeting both conditions is challenging because processes that effectively decrease discharge, including glacier thinning and/or retreat onto higher topography, likely inherently require reductions in surface mass balance. Further, many large glaciers are vulnerable to continued retreat, and modeling work suggests glacier discharge will remain a significant contributor to mass loss in the coming centuries. How rapidly these changes occur, however, is dependent on rates of atmospheric and oceanic warming.

Link to the paper....

https://www.nature.com/articles/s43247-020-0001-2?utm_campaign=MultipleJournals_USG_SUSTAIN&utm_source=Nature_community&utm_medium=Community_sites&utm_content=BenJoh-Nature-MultipleJournals-Human_Ecology-Global

Underlying data for the main manuscript figures is included as an excel file in source data. Individual glacier time series, including monthly ice discharge, front position change, velocity, and ice thickness, are available and can be accessed at https://doi.org/10.5061/dryad.qrfj6q5cb.


I have downloaded the data - one HUGE file & one (by comparison) small file, and am having a go at analysing the data in the smalishl excel file. Wait small...
« Last Edit: August 19, 2020, 12:46:36 PM by gerontocrat »
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BornFromTheVoid

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #3 on: August 19, 2020, 12:41:21 PM »
Where does the initial paper suggest the melting is unstoppable?
I recently joined the twitter thing, where I post more analysis, pics and animations: @Icy_Samuel

gerontocrat

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #4 on: August 19, 2020, 12:50:39 PM »
Where does the initial paper suggest the melting is unstoppable?

In the discussion section...

Quote
As a result of the new, semi-static rate of GrIS-wide D, that is >60 Gt yr−1 higher than in 1985, annual SMB greater than two standard deviations above the 19-year (2000–2018) mean is required for the ice sheet to gain mass and is thus likely in a long-term state of persistent loss.
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BornFromTheVoid

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #5 on: August 19, 2020, 01:16:47 PM »

In the discussion section...

Quote
As a result of the new, semi-static rate of GrIS-wide D, that is >60 Gt yr−1 higher than in 1985, annual SMB greater than two standard deviations above the 19-year (2000–2018) mean is required for the ice sheet to gain mass and is thus likely in a long-term state of persistent loss.

Thanks, Gero, but I meant in the opening paper posted by PMT.

With yours, it appears that the extra loss is associated with increased glacier flow and calving, which is also related to the glacier/ocean interface. Not quite a tipping point, but certainly suggests continued mass loss for decades to come, or longer, regardless of our actions.

I recently joined the twitter thing, where I post more analysis, pics and animations: @Icy_Samuel

gerontocrat

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #6 on: August 19, 2020, 01:52:37 PM »

In the discussion section...

Quote
As a result of the new, semi-static rate of GrIS-wide D, that is >60 Gt yr−1 higher than in 1985, annual SMB greater than two standard deviations above the 19-year (2000–2018) mean is required for the ice sheet to gain mass and is thus likely in a long-term state of persistent loss.

Thanks, Gero, but I meant in the opening paper posted by PMT.

With yours, it appears that the extra loss is associated with increased glacier flow and calving, which is also related to the glacier/ocean interface. Not quite a tipping point, but certainly suggests continued mass loss for decades to come, or longer, regardless of our actions.
Add, or perhaps multiply, the following
- glacial retreat,
- global heating + polar amplification,
- increased precipitation, increasingly as rain. https://tc.copernicus.org/articles/13/815/2019/
Increased Greenland melt triggered by large-scale, year-round cyclonic moisture intrusions
- sub-glacial discharge, https://www.nature.com/articles/s41467-018-05002-0
Hydrology and the future of the Greenland Ice Sheet

We need a science paper that puts all this together.
But my guess is that  Humpty Dumpty has fallen off the wall.

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Hefaistos

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #7 on: August 19, 2020, 02:12:55 PM »
...
But my guess is that  Humpty Dumpty has fallen off the wall.

He seems to have fallen off one wall, question is, is it a structurally bearing wall?

Your chart of the gross and net SMB is very valuable!
Do you have the data to make it for a longer time series? As there is an apparent step change taking place summer 2019 in the cumulative NET MASS LOSS, it'd be interesting if there were similar step changes during previous years?
And are we going to get a similar wall for Humpty to fall off from this year, as more data come in?

Edit: I now see that the answer about longer time series for this chart is negative, in your post here:
https://forum.arctic-sea-ice.net/index.php/topic,2899.msg273863.html#msg273863

gerontocrat

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #8 on: August 19, 2020, 11:36:43 PM »

In the discussion section...

Quote
As a result of the new, semi-static rate of GrIS-wide D, that is >60 Gt yr−1 higher than in 1985, annual SMB greater than two standard deviations above the 19-year (2000–2018) mean is required for the ice sheet to gain mass and is thus likely in a long-term state of persistent loss.
Thanks, Gero, but I meant in the opening paper posted by PMT.

With yours, it appears that the extra loss is associated with increased glacier flow and calving, which is also related to the glacier/ocean interface. Not quite a tipping point, but certainly suggests continued mass loss for decades to come, or longer, regardless of our actions.
& here is some of the data they used that I dragged out from their excel file.
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Ken Feldman

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #9 on: September 01, 2020, 05:54:07 PM »
The original story was an exaggeration, as explained below:

https://climatefeedback.org/evaluation/article-by-cnn-exaggerates-studys-implications-for-future-greenland-ice-loss/

Quote
Article by CNN exaggerates study’s implications for future Greenland ice loss with “point of no return” claim

Analysis of "Greenland's ice sheet has melted to a point of no return, according to new study"
Published in CNN, by Brandon Miller, Max Claypool on 14 August 2020

Three scientists analyzed the article and estimate its overall scientific credibility to be 'low'.
A majority of reviewers tagged the article as: Exaggerating.

Quote
As described by the reviewers below, the CNN article also overlooks the role of human-caused greenhouse gas emissions in altering the future rate of ice loss from the Greenland Ice Sheet as well the consequences for global sea level rise. For example, one study found that under a low-emissions scenario (RCP 2.6) the Greenland Ice Sheet will lose 8-25% of its present-day mass over the long-term, compared to a loss of 72-100% under a high-emissions scenario(RCP 8.5)[2].

Quote
These comments are the overall assessment of scientists on the article, they are substantiated by their knowledge in the field and by the content of the analysis in the annotations on the article.

Stefan Rahmstorf, Professor, Potsdam University:
While most of this article is correct, I have to give it a low credibility rating because the attention-grabbing headline conclusion is not supported by the study the article is about: “Greenland’s ice sheet has melted to a point of no return, and efforts to slow global warming will not stop it from disintegrating.”

What the paper actually finds is well described in its press release: “Even if humans were somehow miraculously able to stop climate change in its tracks […] the ice sheet would continue to shrink for some time.”[1]

The key phrase here is “for some time”. Based on the study, I conclude that the time scale meant here is decades, maybe a century. However, complete loss of the ice sheet would take about a millennium even with unmitigated warming, and the process which decides over the complete ice loss is surface melt—not the ice discharge by glaciers flowing into the ocean at the margins of the ice sheet, which the study is about. When the ice sheet shrinks, it will withdraw further and further from the coast and ice discharge into the ocean will become less important. This is shown by model simulations that continue all the way until complete ice loss.

Quote
Luke Trusel, Assistant Professor, Pennsylvania State University:
Greenland ice sheet melt and its discharge of ice into the ocean has increased over the last few decades, making it one of the largest contributors to global sea level rise. This is important and concerning, and there’s a clear human fingerprint on Greenland ice sheet mass trends. The referenced study by King and coauthors represents important new observations of Greenland’s mass loss and the large increases that have recently occurred[1].

However, the CNN article’s suggestion that Greenland has passed a tipping point is not well established. For example, a paper published in Nature Climate Change in 2018 by Pattyn and coauthors found that the tipping point (that is, the point where potentially irreversible change is set in motion) would be in the neighborhood of 1.5 to 2°C warming above pre-industrial[3]. We’re close, but not quite there yet.

Ken Feldman

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #10 on: September 01, 2020, 06:00:41 PM »
This explanation is clearer.

https://climatecrocks.com/2020/08/17/greenlands-demise-some-clarification/

Quote
Greenland’s Demise – Some Clarification
August 17, 2020

A new paper about Greenland has been released with headlines about “Greenland’s point of no return”.
We are definitely in trouble, but…don’t wave the white flag just yet.

Paleo Climate expert Stefan Rahmstorf and NASA’s Gavin Schmidt have a number of useful qualifiers – a lot to digest here, so I’m going to bookmark this one and keep as a reference:

Stefan Rahmstorf on Twitter:

    News about the death of the Greenland Ice Sheet is greatly exaggerated. What the new paper shows is an observed increase in solid ice discharge into the ocean, which has been at a higher rate than before during the past 15 years. The authors basically say: if this continues, ……then the Greenland Ice Sheet will continue to lose mass.

    But due to that, the ice front retreats from the ocean, as nicely shown e.g. in Andy Aschwanden’s detailed simulations (Above).

    And when the ice loses contact with the ocean, the ice discharge stops.

    The tipping point for losing the Greenland Ice Sheet altogether therefore depends on surface melt permanently exceeding snowfall and does not involve solid ice discharge. Whether this real tipping point of Greenland has been crossed, we do not know.

Quote
Gavin Schmidt on Twitter:

A lot of interest in the Greenland ice loss study from King et al.

Quote
This is an analysis of 35 years of data, not a modeling study, and so while it can do a good job at attributing the current rates of loss to dynamic responses of the ice sheet, it says nothing about where the process would end up in the future under any plausible scenario.
Nonetheless, it is a very valid question (and subject of much research) to ask at what point the Greenland ice sheet is unviable.

From the Pliocene records, we know that a global mean of ~3ºC above the pre-industrial does not seem to be compatible with a substantial GIS.From the history of past interglacials, with differing temperature changes, the ice sheet responded to different extents:

Quote
From the last interglacial, we suspect that an Arctic warming of > 6ºC is compatible with a loss of ~1/3 of the GIS (ie. ~2m of global sea level rise). (fig from royalsocietypublishing.org/doi/pdf/10.109…)

gerontocrat

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #11 on: September 01, 2020, 07:59:49 PM »
Every year, on average, the Surface Mass Balance (SMB) of Greenland increases by about 365GT.
In no year on record, (including 2012) has melting / run-off exceeded precipitation (snow and rain).

So at the moment, it is calving and melting of marine-terminating glaciers that, by being greater than SMB increase , is causing the Mass Balance of the GIS to fall.

This year Greenland melt was very high over most of the melting season, but during this time precipitation was higher than usual. This high energy weather produced an end result of about a 350 GT SMB increase - close to average. Perhaps looking at the past may not be a good way of looking at what comes next. AGW + Polar amplification, warming of the seas and reduction of winter sea ice, especially in the Baffin. More warmth, more snow, more rain? What will be the result both for SMB and melting / calving of the glaciers ?

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Tor Bejnar

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #12 on: September 01, 2020, 11:46:18 PM »
One line in the quoted material caught my attention:
Quote
When the ice sheet shrinks, it will withdraw further and further from the coast and ice discharge into the ocean will become less important.
This phenomenon would apply to East Antarctica, but not to Greenland or West Antarctica.  The later two regions are largely iced over archipelagos, so the ice sheet won't 'withdraw from the coast', in fact, the coast will become more and more icy (less rocky) as the ice sheet retreats (until only a handful of mountain glaciers remain and there is no ice sheet).
Arctic ice is healthy for children and other living things.

Ken Feldman

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #13 on: September 02, 2020, 12:56:04 AM »
One line in the quoted material caught my attention:
Quote
When the ice sheet shrinks, it will withdraw further and further from the coast and ice discharge into the ocean will become less important.
This phenomenon would apply to East Antarctica, but not to Greenland or West Antarctica.  The later two regions are largely iced over archipelagos, so the ice sheet won't 'withdraw from the coast', in fact, the coast will become more and more icy (less rocky) as the ice sheet retreats (until only a handful of mountain glaciers remain and there is no ice sheet).

There's a huge difference between West Antarctica, which has a great deal of exposure to the ocean and retrograde slopes beneath the ice sheet, to Greenland, which has fewer outlet glaciers to the central portion of its ice sheet.





Greenland appears to be less at risk than east Antarctica, based on the topographic maps with the ice sheets removed.

sidd

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #14 on: September 02, 2020, 08:11:51 AM »
East antarctica has large regions below sea level ...

sidd

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #15 on: September 02, 2020, 10:12:45 AM »
East antarctica has large regions below sea level ...

sidd
And parts are now losing ice mass instead of gaining as was the case in the last century.
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kassy

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #16 on: September 02, 2020, 03:05:10 PM »
The original story was an exaggeration, as explained below:

Quote
Article by CNN exaggerates study’s implications for future Greenland ice loss with “point of no return” claim

Quote
As described by the reviewers below, the CNN article also overlooks the role of human-caused greenhouse gas emissions in altering the future rate of ice loss from the Greenland Ice Sheet as well the consequences for global sea level rise. For example, one study found that under a low-emissions scenario (RCP 2.6) the Greenland Ice Sheet will lose 8-25% of its present-day mass over the long-term, compared to a loss of 72-100% under a high-emissions scenario(RCP 8.5)[2].

Quote

However, the CNN article’s suggestion that Greenland has passed a tipping point is not well established. For example, a paper published in Nature Climate Change in 2018 by Pattyn and coauthors found that the tipping point (that is, the point where potentially irreversible change is set in motion) would be in the neighborhood of 1.5 to 2°C warming above pre-industrial[3]. We’re close, but not quite there yet.



OK, last point first. We will easily hit that because we are much too slow in our actions to curb CO2.

Then the scenarios. They range from 8% loss to 100% loss.
So we are going to lose at least a big chunk.
If we rule out both 2.6 and 8.5 then we will lose between 25-70%. So lets say 40%. That is already a calamity.

If we want to stop it melting we have to go zero carbon and then negative.

So in a very practical way it is unstoppable for the near future.



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Ken Feldman

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #17 on: September 02, 2020, 06:11:02 PM »
The original story was an exaggeration, as explained below:

Quote
Article by CNN exaggerates study’s implications for future Greenland ice loss with “point of no return” claim

Quote
As described by the reviewers below, the CNN article also overlooks the role of human-caused greenhouse gas emissions in altering the future rate of ice loss from the Greenland Ice Sheet as well the consequences for global sea level rise. For example, one study found that under a low-emissions scenario (RCP 2.6) the Greenland Ice Sheet will lose 8-25% of its present-day mass over the long-term, compared to a loss of 72-100% under a high-emissions scenario(RCP 8.5)[2].

Quote

However, the CNN article’s suggestion that Greenland has passed a tipping point is not well established. For example, a paper published in Nature Climate Change in 2018 by Pattyn and coauthors found that the tipping point (that is, the point where potentially irreversible change is set in motion) would be in the neighborhood of 1.5 to 2°C warming above pre-industrial[3]. We’re close, but not quite there yet.



OK, last point first. We will easily hit that because we are much too slow in our actions to curb CO2.

Then the scenarios. They range from 8% loss to 100% loss.
So we are going to lose at least a big chunk.
If we rule out both 2.6 and 8.5 then we will lose between 25-70%. So lets say 40%. That is already a calamity.

If we want to stop it melting we have to go zero carbon and then negative.

So in a very practical way it is unstoppable for the near future.

If you look closely at the graph, we're on the SSP1-2.6 track now and the stated policies are under the SSP2-4.5 path.  And the stated policies haven't really caught up with the economics of the energy transition, in part because the fossil fuel industry had a lot of cash with which to influence politicians in the democracies.

A lot has changed since that graph was published. The energy transition is well underway.  We've already seen peak coal and peak oil is probable in the 2020s.  Renewable electricity generation, which was thought to be too expensive to provide a significant amount of capacity when the RCPs were developed, is now cheaper than fossil fuels and last year renewables were 2/3s of all new electric generation built globally.  Fossil fuel companies are going bankrupt at a record rate as the Covid shut-ins have destroyed demand, particularly for aviation fuel and gasoline.

Regenerative agriculture (both in ranching and in staple crops) which will sequester billions of tons of CO2 is becoming more widely used.  Reforestation and afforestation is already being done (and has helped reverse desertification in Africa) which will also sequester a significant amount of carbon.  And there is research being done on accelerated weathering (olivine on beaches and other minerals elsewhere), to help decrease CO2 concentrations in the atmosphere.

Don't give up.  That's what a certain group of deniers (often those closely tied to the fossil fuel industry) want you to do.

kassy

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #18 on: September 02, 2020, 10:21:37 PM »
The current policies outcome is above the the SSP2-4.5 path until 2040 and pledged is well off 2.6.
The only scenario hitting negative emissions before 2100 is 2.6 and since i am too lazy too extrapolate the blue line that just means there is 80+ years of additional damage locked in.

It´s not about giving up but maybe realizing we should do more now. This is not a thing we can leave to the market and bright new ideas to solve but we seem to have a hard time understanding that.

The IPCC never defined dangerous climate change. If you include logical things like not triggering Greenland melt we are already well within that territory.
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RoxTheGeologist

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Re: Melting of the Greenland ice sheet now unstoppable
« Reply #19 on: September 03, 2020, 03:19:42 PM »
One line in the quoted material caught my attention:
Quote
When the ice sheet shrinks, it will withdraw further and further from the coast and ice discharge into the ocean will become less important.
This phenomenon would apply to East Antarctica, but not to Greenland or West Antarctica.  The later two regions are largely iced over archipelagos, so the ice sheet won't 'withdraw from the coast', in fact, the coast will become more and more icy (less rocky) as the ice sheet retreats (until only a handful of mountain glaciers remain and there is no ice sheet).

There's a huge difference between West Antarctica, which has a great deal of exposure to the ocean and retrograde slopes beneath the ice sheet, to Greenland, which has fewer outlet glaciers to the central portion of its ice sheet.





Greenland appears to be less at risk than east Antarctica, based on the topographic maps with the ice sheets removed.

Are there maps adjusted for PGR?