An ice sheet forms when snow falls on land, compacts into ice, and forms a system of interconnected glaciers which gradually flow downhill like play-dough. In Antarctica, it is so cold that the ice flows right into the ocean before it melts, sometimes hundreds of kilometres from the coast. These giant slabs of ice, floating on the ocean while still attached to the continent, are called ice shelves.
For an ice sheet to have constant size, the mass of ice added from snowfall must equal the mass lost due to melting and calving (when icebergs break off). Since this ice loss mainly occurs at the edges, the rate of ice loss will depend on how fast glaciers can flow towards the edges.
Ice shelves slow down this flow. They hold back the glaciers behind them in what is known as the “buttressing effect”. If the ice shelves were smaller, the glaciers would flow much faster towards the ocean, melting and calving more ice than snowfall inland could replace. This situation is called a “negative mass balance”, which leads directly to global sea level rise.
Respect the ice shelves. They are holding back disaster.
Ice shelves are perhaps the most important part of the Antarctic ice sheet for its overall stability. Unfortunately, they are also the part of the ice sheet most at risk. This is because they are the only bits touching the ocean. And the Antarctic ice sheet is not directly threatened by a warming atmosphere – it is threatened by a warming ocean.
The atmosphere would have to warm outrageously in order to melt the Antarctic ice sheet from the top down. Snowfall tends to be heaviest when temperatures are just below 0°C, but temperatures at the South Pole rarely go above -20°C, even in the summer. So atmospheric warming will likely lead to a slight increase in snowfall over Antarctica, adding to the mass of the ice sheet. Unfortunately, the ocean is warming at the same time. And a slightly warmer ocean will be very good at melting Antarctica from the bottom up.
This is partly because ice melts faster in water than it does in air, even if the air and the water are the same temperature. But the ocean-induced melting will be exacerbated by some unlucky topography: over 40% of the Antarctic ice sheet (by area) rests on bedrock that is below sea level.
Elevation of the bedrock underlying Antarctica. All of the blue regions are below sea level. (Figure 9 of Fretwell et al.)
This means that ocean water can melt its way in and get right under the ice, and gravity won’t stop it. The grounding lines, where the ice sheet detaches from the bedrock and floats on the ocean as an ice shelf, will retreat. Essentially, a warming ocean will turn more of the Antarctic ice sheet into ice shelves, which the ocean will then melt from the bottom up.
This situation is especially risky on a retrograde bed, where bedrock gets deeper below sea level as you go inland – like a giant, gently sloping bowl. Retrograde beds occur because of isostatic loading (the weight of an ice sheet pushes the crust down, making the tectonic plate sit lower in the mantle) as well as glacial erosion (the ice sheet scrapes away the surface bedrock over time). Ice sheets resting on retrograde beds are inherently unstable, because once the grounding lines reach the edge of the “bowl”, they will eventually retreat all the way to the bottom of the “bowl” even if the ocean water intruding beneath the ice doesn’t get any warmer. This instability occurs because the melting point temperature of water decreases as you go deeper in the ocean, where pressures are higher. In other words, the deeper the ice is in the ocean, the easier it is to melt it. Equivalently, the deeper a grounding line is in the ocean, the easier it is to make it retreat. In a retrograde bed, retreating grounding lines get deeper, so they retreat more easily, which makes them even deeper, and they retreat even more easily, and this goes on and on even if the ocean stops warming.
Diagram of an ice shelf on a retrograde bed (“Continental shelf”)
Which brings us to Terrifying Paper #1, by Rignot et al. A decent chunk of West Antarctica, called the Amundsen Sea Sector, is melting particularly quickly. The grounding lines of ice shelves in this region have been rapidly retreating (several kilometres per year), as this paper shows using satellite data. Unfortunately, the Amundsen Sea Sector sits on a retrograde bed, and the grounding lines have now gone past the edge of it. This retrograde bed is so huge that the amount of ice sheet it underpins would cause 1.2 metres of global sea level rise. We’re now committed to losing that ice eventually, even if the ocean stopped warming tomorrow. “Upstream of the 2011 grounding line positions,” Rignot et al., write, “we find no major bed obstacle that would prevent the glaciers from further retreat and draw down the entire basin.”
They look at each source glacier in turn, and it’s pretty bleak:
- Pine Island Glacier: “A region where the bed elevation is smoothly decreasing inland, with no major hill to prevent further retreat.”
- Smith/Kohler Glaciers: “Favorable to more vigorous ice shelf melt even if the ocean temperature does not change with time.”
- Thwaites Glacier: “Everywhere along the grounding line, the retreat proceeds along clear pathways of retrograde bed.”
Only one small glacier, Haynes Glacier, is not necessarily doomed, since there are mountains in the way that cut off the retrograde bed.
From satellite data, you can already see the ice sheet speeding up its flow towards the coast, due to the loss of buttressing as the ice shelves thin: “Ice flow changes are detected hundreds of kilometers inland, to the flanks of the topographic divides, demonstrating that coastal perturbations are felt far inland and propagate rapidly.”
It will probably take a few centuries for the Amundsen Sector to fully disintegrate. But that 1.2 metres of global sea level rise is coming eventually, on top of what we’ve already seen from other glaciers and thermal expansion, and there’s nothing we can do to stop it (short of geoengineering). We’re going to lose a lot of coastal cities because of this glacier system alone.
Terrifying Paper #2, by Mengel & Levermann, examines the Wilkes Basin Sector of East Antarctica. This region contains enough ice to raise global sea level by 3 to 4 metres. Unlike the Amundsen Sector, we aren’t yet committed to losing this ice, but it wouldn’t be too hard to reach that point. The Wilkes Basin glaciers rest on a system of deep troughs in the bedrock. The troughs are currently full of ice, but if seawater got in there, it would melt all the way along the troughs without needing any further ocean warming – like a very bad retrograde bed situation. The entire Wilkes Basin would change from ice sheet to ice shelf, bringing along that 3-4 metres of global sea level rise.
It turns out that the only thing stopping seawater getting in the troughs is a very small bit of ice, equivalent to only 8 centimetres of global sea level rise, which Mengel & Levermann nickname the “ice plug”. As long as the ice plug is there, this sector of the ice sheet is stable; but take the ice plug away, and the whole thing will eventually fall apart even if the ocean stops warming. Simulations from an ice sheet model suggest it would take at least 200 years of increased ocean temperature to melt this ice plug, depending on how much warmer the ocean got. 200 years sounds like a long time for us to find a solution to climate change, but it actually takes much longer than that for the ocean to cool back down after it’s been warmed up.
This might sound like all bad news. And you’re right, it is. But it can always get worse. That means we can always stop it from getting worse. That’s not necessarily good news, but at least it’s empowering. The sea level rise we’re already committed to, whether it’s 1 or 2 or 5 metres, will be awful. But it’s much better than 58 metres, which is what we would get if the entire Antarctic ice sheet melted. Climate change is not an all-or-nothing situation; it falls on a spectrum. We will have to deal with some amount of climate change no matter what. The question of “how much” is for us to decide.
ClimateSight 
Reblogged this on uknowispeaksense.
I especially enjoyed this posting, Kaitlin It was very well organised, concise, clearly explained and well balanced with fact and feeling. I am so happy to read articles and blogs where scientists are willing to express their opinions on what the facts mean to all of us. Best wishes, Sky
Hi Kaitlin. Great post — very clearly set out and explained. Those are terrifying papers indeed.
This is not true: The question of “how much” is for us to decide. It IS too late for us to decide. The amplifying feedbacks DWARF the initial forcing – which is our release of CO2 the past 150 years. The amplifying feedbacks have begun and are IRREVERSIBLE on any human timescale. Since we don’t have any way to pull CO2 out of the atmosphere anywhere near the scale at which we have released it, is is going to be an extinction level event, regardless of what we do going forward.
Yesterday I suddenly realized why the most brilliant book I’ve ever read about evolution and human nature isn’t famous. It’s because he doesn’t conclude with the sort of treacle that people like to hear. http://megafauna.com/table-of-contents/
Good point, well made, about the ‘treacle’.
Thanks for the link to ‘MEGAFAUNA’. I looked for the conclusion to which you refer and couldn’t find it… ‘The brink of full understanding‘ is missing. Apt.
Hello, Kaitlin. I’m a climate scientist from Fortaleza, Brazil and I also run a blog to popularize climate science (and of course fight denialism). I would like to ask to translate some of your material into Portuguese and publish it there.
Hi Alexandre, you are more than welcome to translate my work, just include a link back to the original. Best of luck with your blog! What area of climate science do you study?
Hi Kaitlin.. Somehow I missed your response, sorry. Yes, I will link the original articles always, for sure. =)
I have a PhD in Atmospheric Science and I work mostly with atmospheric modeling. My students currently are working on regional climate modeling over the tropical Americas.
Cheers,
Alex
Terrifying Paper #3. The price of gasoline has dropped below two dollars per gallon in the United States, and we’re buying gas guzzlers in record numbers. We’re too busy spewing CO2 to be concerned about Terrifying Papers #1 and #2.
An interesting read, thanks Kaitlin. A shame the deniers brigade can’t understand the science behind the destruction we are causing.
It would be interesting to still be alive in a few hundred years just to see how it all pans out (although, maybe it wouldn’t be such a good idea).
I’m not disputing the ice melting possibilities, but as a mere layman, I find it hard to visualise that there’s enough ice to melt which would cause sea level rises of many metres. It just doesn’t compute in my feeble mind.
However, any effective action is in the hands of the politicians and major corporations with influence; and whilst “greed” remains as a dominating human characteristic, I’m afraid we’re doomed.
They won’t heed the climate scientists messages because it doesn’t suit their agenda.
Oh well, I guess this cartoon kinda covers that aspect . . . .
Cheers
Mick
However bad is locked in we can make it worse. However soon is inevitable, we can make it happen quicker. An extinction event to rival the PETM sometime in the centuries to come, is not as bad as an End Permian type event later this century.
We are in an extinction event, but to date that has had little to do with climate change. This extinction event has been seventy thousand years in the making so far. The naked ape escaped from Africa, destroyed almost all the worlds mega fauna in short order. Then learned agriculture and the decimation of the natural systems stepped up orders of magnitude. Now we have industry and the destruction of nature has notched up several more orders of magnitude.
Maybe we are locked in to five meters of sea level rise by 2107, but that would be better than seven meters by 2070. There are so many known, unknowns about sea level rise. There are factors we know will adversely affect sea level rise, but by how much is most uncertain. However bad, we can make it worse.
Best of luck trying to get it all in the models. Such a fine line between realism and destroying all hope.
Reblogged this on educators climate alliance and commented:
“Climate change is not an all-or-nothing situation; it falls on a spectrum. We will have to deal with some amount of climate change no matter what. The question of “how much” is for us to decide.”
Well, we could all be fried. ‘”…Planets are being destroyed all the time. One misplaced comet or asteroid hurtling through the cosmos and whump! There goes another planet.”‘—”Termination: Project Earth”, Keith Trezise
Thanks for quoting my book :)
“Terrifying Paper #3. The price of gasoline has dropped below two dollars per gallon in the United States, and we’re buying gas guzzlers in record numbers”
Now there’s a great example of how short-sighted humans are (or downright stupid, take your pick); it’s not like gas is going to stay cheap.
Good read.
Nice post Kate. Reposted on SkS
http://skepticalscience.com/most-terrifying-papers-i-read-last-year.html
Kaitlin, this is a wonderfully clear explanation of the “irreversible Antarctic ice loss” that’s been described–much less clearly–in the main stream media. Thanks very much!
NOAA reported that on January 1 daily average CO2 levels at their Mauna Loa facility was over 400 parts per million (ppm).
Last time this happened, 3 millions years ago, temperatures were 3ºC higher and sea level 9 metres higher….
http://www.esrl.noaa.gov/gmd/ccgg/trends/weekly.html
Sea level rise map: Set 9 meters http://www.floodmap.net/
The rise of sea level and the flooding of major cities will put the governments between the devil and the deep blue sea, literally, either they cut emissions or they lose those cities.
With all the ice melt the sea level would rise 65 meters and many coastal cities would perish under the water. http://ngm.nationalgeographic.com/2013/09/rising-seas/if-ice-melted-map http://www.floodmap.net/ (set 65 meters)
‘Nothing can stop retreat’ of West Antarctic glaciers http://www.bbc.com/news/science-environment-27381010?ocid=socialflow_facebook
This layman is impressed by your ability to explain very clearly. A good price of writing.
I keep hearing “hundreds of years,” but we are dealing with multiple postive feedbacks in a chaotic system, and previous estimates have proven far too conservative. This could all happen much more quickly than thought as a bifurcation point is reached.
I suspect the ‘hundreds of years’ meme is simply the latest manifestation of denial.
To echo what others have said, this is the clearest explanation I’ve read of exactly why certain parts of Antarctica are vulnerable to irreversible melting.
I hope someone will publish a study on global ice mass loss incorporating the latest research and observations from various parts of the cryosphere. The limited data we have at the moment suggests that the total volume being added to the oceans from ice melt is doubling every 5 or 6 years, which implies 1 metre of sea level rise by about mid-century and several metres by 2100 – enough to matter a lot to anyone who owns property at sea level. We already know that meltwater pulses in the past produced sea level rise of around half a metre per decade – I wouldn’t want to bet my life savings on that not happening again.
Kaitlin –
Thank you for this clear and helpfully dispassionate account for the lay reader of the long term threat, which warrants much reposting.
The finding by Michael Mann (whose persecution gives him an exceptional motivation to avoid unsupported assertions) that we face an acceleration of warming to realize 2.0C of AGW by 2036, would presumably advance the timescale of these two threats as well as that of the loss of the Greenland ice sheet.
The latter case is more complex of course, being a sheet mostly on bedrock below sea level encircled by mountains with few gateways to the sea. Your discussion of the ice-filled troughs below the Wilkes Basin sheet brought to mind the ~300kms canyon and ‘riverine’ trenches recently observed under the GIS. As these are presumably transporting the rising volume of surface melt-lake water that is generated within the mountain ring, they must also be open to sea-water incursion when that flow stops in the autumn. If you know of any studies of the implications of this dynamic I’d be most grateful if you’d do a post on them.
Given that proximity is a large part of a threat’s emotional impact, I’m afraid the two papers you link are not the most terrifying I’ve seen in the last year, as both concern quite long term threats. Therefore I see your two and raise you two which, in my view, bring the issue of Geo-E into sharper focus.
The first of these, “Observational determination of albedo decrease caused by vanishing Arctic sea ice” (see: http://www.pnas.org/content/111/9/3322.abstract ) reports that Arctic sea ice loss during the satellite record since ’79 has imposed an albedo loss whose warming equates on average to 25% of that from anthro-CO2 stocks during the period. Given how sea ice loss has progressed over the period, this is plainly a rising component of AGW that we have yet to see realized, and one that already equates roughly to a new China’s-worth of annual CO2 output. In addition, Albedo Loss is only the most advanced of the eight Major Interactive Feedbacks that are reported in the literature to be accelerating, with at least several score of ‘direct coupling mechanisms’ identified as operating between them alongside their interactions via feedback GW.
The second paper, “Food Security: Near future projections of the impact of drought in Asia” (see: Food security effects of climate change will be felt in 10 years ) is a study from Oct 2012 led by Prof Piers Forster, an IPCC lead author, which shows that China, Pakistan and Turkey are the most seriously affected of the region’s major producers of wheat and maize.
From the press release:
– Research released today shows that within the next 10 years large parts of Asia can expect increased risk of more severe droughts, which will impact regional and possibly even global food security. On average, across Asia, droughts lasting longer than three months will be more than twice as severe in terms of their soil moisture deficit compared to the 1990-2005 period. This is cause for concern as China and India have the world’s largest populations and are Asia’s largest food producers.
Dr Lawrence Jackson, a co-author of the report, said: “Our work surprised us when we saw that the threat to food security was so imminent; the increased risk of severe droughts is only 10 years away for China and India. These are the world’s largest populations and food producers; and, as such, this poses a real threat to food security.”
Given that comparable projections are emerging for other major food-producing regions, it appears that during the 2020s we are liable to see crop failures in two or more regions coinciding to form the onset of serial global crop failures. Beyond the attendant suffering, mass-migration, strife and insurgencies, these also pose a potential for geopolitical destabilization on a scale to preclude the operation of the vital Climate Treaty.
The need for a crash research program of both modes of Geo-E as an addition to the rapid contraction of global GHG outputs is strongly affirmed by these papers, but it can be argued that that research program itself demands the prior mandating of a UN scientific agency for its stringent supervision and for the options’ assessment, with any deployment being legal only by the collective decision of the member nations of the UN General Assembly. There is also a need to legislate on reliable CDR (better named Carbon Recovery) having to precede the use of SRM (better named Albedo Restoration) both to address ocean acidification and to minimize the scale of risk of interruptions to the latter’s steady deployment before Carbon Recovery is completed. In addition, as a pragmatic measure for public acceptance, it would be well to agree that no such deployments can occur until a commensurate Emissions Control treaty is in operation.
Since 2012 Prof Forster has been focussing on the issue of Geo-E to the extent of acting as the principal investigator of the Integrated Assessment of Geoengineering Proposals project. His findings were not encouraging – neither the two CCS versions of Carbon Recovery (actually CO2CS) nor the six Albedo Restoration options he assessed were anywhere near credible as practical options, though some, such as ‘Cloud Brightening’, clearly had strong research potential, and one promising option, ‘Native Coppice Afforestation for Biochar’ was not considered. Such is the import of this field that the Bulletin of Atomic Scientists recently commissioned an article by him entitled ” Not enough time for geoengineering to work?
From this perspective I applaud your recognition that serving the research of Geo-E may well be a part of your career, but, given that we are now heading directly towards the emergency deployment of the only ready option for Albedo Restoration, namely the cheap, dirty and dangerous mass-deployment of sulphate aerosols, I hope you’ll consider making that transition just as soon as you are able.
With thanks for your highly accessible writing,
regards,
Lewis Cleverdon
(colon missing in “best posts” sidebar link)
Just found this. Scary papers, but in view of everything else…
I’m more worried about this at the moment: http://news.uaf.edu/scientists-predict-gradual-permafrost-greenhouse-gas-emissions-embargoed/
So if average permafrost temperatures have gone up by approximately 11 degrees Fahrenheit in 30 years, and assuming the rate of change doesn’t increase, that would imply average temperatures will still be above freezing by 2030. Since heating will continue after 2030, and if anything the rate of heating is likely to increase, we can probably assume no permafrost at all by the middle of this century.
As the effects of this have been an unknown (in quantitative terms) and have therefore apparently been ignored in climate modelling, it’s likely that current estimates of future temperatures are on the low side of what is actually going to happen, even with the same emissions scenarios.
There probably won’t be a sudden “bomb” but there will still be more CO2, and methane, and sooner, than present modelling has accounted for.