Comments 1 to 16:

1. Riduna at 10:12 AM on 13 September, 2014

   The article concludes that … By the end of the century, at that rate, [decadal doubling of ice loss] Greenland alone would be accounting for about one meter per year [of sea level rise].  

   This is akin to the conclusion reached by Dr Hansen in 2007 – one which has been studiously ignored by glaciologists and rejected by those contributing to 2013 IPCC report on sea level rise this century. Why? Apparently because of doubt that doubling of ice sheet mass loss per decade will be sustained over the rest of this century.

   Hansen was referring to mass loss from both the Arctic and Antarctic, concluding that consequential average global sea level rise of 5m by 2100 should be an expected outcome. IPCC rejection of this does smack of common agreement by contributing scientists to ignore Hansen. In the absence of cogent scientific refutation of Hansen, does this amount to cherrypicking on their part – or is it just a healthy “concensus”?

   Given the observations referred to in the article, the expectation that greenhouse gas emissions associated with permafrost thawing, on-going aerosol soot deposits on the Greenland ice sheet and loss of albedo due to sea-ice depletion – should we really cling to the notion that average global sea level will rise by no more than one metre by 2100?

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2. Tom Curtis at 14:49 PM on 13 September, 2014

   Riduna @1, the IPCC discusses these issues in FAQ 13.2, saying of Greenland:

   "In Greenland, mass loss through more surface ablation and outflow dominates a possible recent trend towards increased accumulation in the interior. Estimated mass loss due to surface ablation has doubled since the early 1990s. This trend is expected to continue over the next century as more of the ice sheet experiences surface ablation for longer periods. Indeed, projections for the 21st century suggest that increasing mass loss will dominate over weakly increasing accumulation. The refreezing of melt water within the snow pack high up on the ice sheet offers an important (though perhaps temporary) dampening effect on the relation between atmospheric warming and mass loss.

   Although the observed response of outlet glaciers is both complex and highly variable, iceberg calving from many of Greenland’s major outlet glaciers has increased substantially over the last decade, and constitutes an appreciable additional mass loss. This seems to be related to the intrusion of warm water into the coastal seas around Greenland, but it is not clear whether this phenomenon is related to inter-decadal variability, such as the North Atlantic Oscillation, or a longer term trend associated with greenhouse gas–induced warming. Projecting its effect on 21st century outflow is therefore difficult, but it does highlight the apparent sensitivity of outflow to ocean warming. The effects of more surface melt water on the lubrication of the ice sheet’s bed, and the ability of warmer ice to deform more easily, may lead to greater rates of flow, but the link to recent increases in outflow is unclear. Change in the net difference between surface ablation and accumulation is projected to contribute between 10 and 160 mm to sea level rise in 2081-2100 (relative to 1986-2005), while increased outflow is projected to contribute a further 10 to 70 mm (Table 13.5).

   
   The Greenland ice sheet has contributed to a rise in global mean sea level over the last few decades, and this trend is expected to increase during this century. Unlike Antarctica, Greenland has no known large-scale instabilities that might generate an abrupt increase in sea level rise over the 21st century. A threshold may exist, however, so that continued shrinkage might become irreversible over multi-centennial time scales, even if the climate were to return to a pre-industrial state over centennial time scales. Although mass loss through the calving of icebergs may increase in future decades, this process will eventually end when the ice margin retreats onto bedrock above sea level where the bulk of the ice sheet resides."

   The most important points are the uncertainty as to whether the large increase in ice loss is due to the NAO or to global warming, and the fact that any runaway process will ground out (last sentence in the quote).

   As it happens, Jason Box was a contributing author to that chapter.  That does not mean the chapter reflects his views, but it does mean his views were given due consideration.  Therefore to the extent that his views differ, they do not reflect the consensus of relevant experts.

   Having said that, there is a divide among the relevant experts between those favouring "process based" and those favouring "semi empirical" based projections.  The later give consistently higher projections than those obtained by the process based methods (on which the IPCC headline results are based, as shown in the IPCC Fig 13.12:

   

   Figures a, b, c and d are for RCP scenarios 2.6, 4.5, 6.0, and 8.5 respectively.  The black line is the median process based projection with the likely (66%) range shown in grey.  Blue estimates are based on temperatures, red on forcings, with the bars showing the "extremely likely" (95%) range.  

   Based on that, sea level rise one or even two meters greater than IPCC projections cannot be excluded, but there is no basis in the literature for Hansen's projection of 5 meters.  It should be noted that the IPCC preffers the process based projections because they are assessed as having medium confidence, while the semi emperical projections are assessed as having low confidence.

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   Moderator Response:

   [Rob P] - Tom, recent ice sheet modelling, (yet to be published in the peer-reviewed literature AFAIK) that incorporates new physics, simulates a collapse of parts of the Antarctic ice sheet over the next half-millennia.  

   "The new model simulates an Antarctic contribution to sea-level rise of ~15m during peak mid-Pliocene warmth and ~4.25m during the LIG, in approximate agreement with (albeit uncertain) geological sea-level indicators. When applied to long-term future simulations assuming extended RCP greenhouse gas emission scenarios and using high resolution atmosphere and ocean components, the same model physics show a dramatic retreat of Antarctic marine-based ice over the next 500 years, beginning within a few decades in the Pine Island Bay sector of West Antarctica. In the most extreme RCP scenarios, subsequent retreat of the Siple Coast margin results in the near-total collapse of the West Antarctic Ice Sheet (WAIS) within a few centuries, followed by retreat into the deep subglacial basins underlying the East Antarctic Ice Sheet (EAIS). Antarctica is shown to contribute up to 9m of sea level rise within the next five centuries."

   This seems more consistent with marine sediment core samples taken from around Antarctica which imply swift dynamic responses to warming.  

3. Tom Curtis at 18:41 PM on 13 September, 2014

   Rob P inline @2:

   First, google search shows the paper to not yet have been published, but only to have been presented at a conference in 2014.  Therefore it cannot have been considered by the IPCC for AR5.  If asked (and sometimes if not asked), I consistently advise non-climate scientists to accept IPCC findings as the best statement of the consensus on the science at time of writing.  As non-experts, they are certainly entitled to their own opinion but should be aware that their own opinion is likely to have been formed by making a number of subtle (and not so subtle) gaffes from looking at limited and often non-representative data.  As in any area where we consult experts, they are far wiser to accept the expert opinion even when they, with their little knowledge, disagree.

   Given that, I would be hypocritical to not accept the IPCC consensus in areas where I or my friends think there is a reasonable chance the IPCC has been too cautious.  Even if I am told they have been too cautious by some climate scientists, I know I can find others who do not think so (and who are well within the 97% consensus on the attribution issue).  Nor am I, and nor can I be in a position to judge which of the individual experts has it right unless I first get up to speed on the issue by an intense reading of relevant textbooks, and keep up to speed by reading something in the order of 10 relevant scientific articles a week, while maintaining regular communication with a network of peers to keep up with what is current.

   Jason Box has done, and continues to do the equivalent of that.  He is a genuine expert and is entitle to disagree with the IPCC.  I have not, and have not the time to do that (unless I want to restrict my commentary solely to icesheet dynamics), and so must stick with the IPCC - even, indeed especially, when giving greater weight to a subsection of the experts would appear to warrant more urgent action.

   (I understand that you are far more up on the sea level literature than I am, and are thus far more entitled to a different opinion to the IPCC - but at best that just makes you one more expert on the more alarmed side of the IPCC opinion to me.  Further, I do not think even you would claim to be expert on the topic, as opposed to well informed.)

   2)  More directly, the IPCC explained in detail the prospect of rapid collapse of the WAIS ice sheet, so is not in disagreement with the new study per se.  In particular, they showed the following image:

   

   The key factor show is the increased depth increased distance shoreward.  That ensures the process does not ground out, and indeed can become self sustaining even in the event that we control surface temperatures to current levels.  That is the key difference between the WAIS and the Greenland Ice Sheet where the water becomes shallower as the shore is approached, thus limiting the extent of any rapid collapse induced by other means.

   Therefore the only thing new added by the as yet unpublished study is the quantification of "up to" 9 meters sea level rise "within the next five centuries".  That equates to "up to" 1.55 meters of sea level rise by the end of this century assuming a constant rate, and much less assuming (as it must) the rate accelerates from a slow start.  That would raise sea levels by more than the process based estimates, but by about the same amount as the semi-impirical based estimates.  It does not justify belief in a 5 meter sea level rise by 2100, or of 1 meter sea level rise from Greenland alone by 2100.

   3)  I would love for SkS to publish an article by Jason Box in which he explains not only the processes that are accelerating the ice loss from Greenland, but also the processes that might limit the acceleration, and gives a median estimate of sea level rise from the GIS by 2100 taking all factors into account.  Given such a fuller account, he might persuade me to change my personal opinion - although I would still recommend accepting the current IPCC position for formulating policy even if my opinion was changed.

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4. Rob Painting at 19:26 PM on 13 September, 2014

   Tom - a few things:

   1. I was simply drawing attention to recent research on the topic. There is no suggestion that this was even available for consideration for inclusion into the latest IPCC assessment. As is normally the case, this was presented at a scientific conference earlier this year and will likely find it's way into the scientific literature in due course. Regardless, cutting edge research is unlikely to find it's way into the IPCC reports given the panel's conservative nature. That seems a reasonable stance.

   2. Previous ice sheet modelling work by Pollard and DeConto is often cited in the scientific literature as a reason for caution over the semi-empirical methods. By incorporating better understood physics into the model, that may no longer stand.

   3. Modelling now is a much better fit to paleo sea-level records and the physical evidence (ice-rafted debris in sediment cores for instance) around the Antarctic continent. Rates of sea level rise near 1.2 metres per century are not uncommon in the last few glacial/interglacial cycles.  

   3. Riduna brought up the 5 metre sea level rise this century claim, not me. There is no evidence AFAIK to support such high rates of sea level rise, and such a scenario seems implausible according to all the literature on the topic that I have read.

   4. If you use the IPCC as a guide then you will have noticed that your projections of future sea level rise have often been at the upper bound.        

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5. ubrew12 at 21:32 PM on 13 September, 2014

   World's largest Slip-n-Slide...

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6. MA Rodger at 01:50 AM on 14 September, 2014

   The statement of 1 metre per year SLR "from Greenland alone" by the end of the century isn't entirely correct (when it should be squeeky clean give it is so controversial). Also the Hansen argument mentioned @1 is a long long way from the statement by Jason Box in the post/video.

   The simple arithmatical progression described in the post would lead from a 2010's rise of 1mm pa today to a 1m pa in the 2110's which is actually a significant period after "the end of this century."

   The Hansen thesis was 1 metre per decade SLR from both Greenland and Antarctica, and certainly not 1 metre per year from Greenland alone.

   There is however a little ambiguity in Hansen's position. And I am not convinced by his 5m SLR by 2100, although his warning that humanity shouldn't be complacent about SLR is surely correct. The potential delivery of ~80m SLR with just a few degrees of global temperature rise is convincing and that would inundate 90% of human endeavour. That is not trivial, even if it were a 2,000 year process.

   I take issue with Hansen when he argues that a 5m SLR by 2100 is not unrealistic. He makes plain that there is a negative feedback that will prevent a continued doubling of SLR. "Our simulations (Hansen and Sato, 2012) suggest that a strong negative feedback kicks in when sea level rise reaches meter-scale, as the ice-melt has a large cooling and freshening effect on the regional ocean." (From Hansen & Sato (2012a) Update of Greenland Ice Sheet Mass Loss : Exponential ? here)

   Yet, his Figuree 8 of 21st century SLR showing 5m SLR by 2100 in Hansen & Sato (2012) Paleoclimate Implications for Human-Made Climate Change (here) shows rates of rise that are well above 'reaching meter-scale' SLR. In Fig 8, the last metre SLR before 2100 taking 3.25 years, a rate of 3.1m per decade. And this is not me being picky in my literal interpretation of 'reaching meter-scale'. Hansen & Sato (2012) talks of "...a negative feedback that comes into play as ice discharge approaches a level of the order of a meter per decade."

   The only basis I can see for Hansen's argument for 5m SLR by 2100 is the accelerating mass loss of Greenland and Antarctica as shown by GRACE. If the data is updated to include 2013, the accelerations are a lot less worrying than those curves graphed by Hansen & Sato, with doubling rates faster than per 10 years looking unlikely. Indeed, the Greenland mass data from NOAA Report Card 2013 yields a graph of rate-of-loss (two clicks down here) that could easily show a linear acceleration, although that 30 Gty^-2 (from data centred on 2008) is steeper than the 21Gty^-2 in Hansen & Sato (2012a) Figure 1 (from data centred on 1999) suggesting linearity isn't entirely on a solid footing.

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7. Hyperactive Hydrologist at 09:16 AM on 14 September, 2014

   Where did the 1m per year figure come from? Jason Box stated that the yield of SLR from Greenland alone would be 1m in total by 2100. Jason is just extrapolating current melt acceleration, which is perhaps an over exaggeration, as it would mean the entire Greenland Ice sheet would be gone by 2130.

   The question I would like answering is how much ice mass would Greenland need to lose before all the major glacier snouts are grounded? I think once that occurs melt rates will begin to slow quite significantly due to a lack of carving by the sea and a decrease in basal lubrication. By then however, it will probably already be too late. 

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8. sidd at 14:19 PM on 14 September, 2014

   Hansen is more often right than wrong. Here is a tidbit to chew on:pulling one plug on WAIS drains more than one basin http://www.the-cryosphere-discuss.net/8/4885/2014/tcd-8-4885-2014.pdf

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   Moderator Response:

   [PS] Fixed link per later message (and again, checked this time)

9. noa at 17:06 PM on 14 September, 2014

   @6 MA Rodger

   As far as I know there seem to be some issues with the GRACE data for 2013. See repley 156 here on the ASIF. Answer from the DMI Polar Portal (see here there GRACE chart):

   Your question is really good, and the answer is actually quite simple: The GRACE mission is already way past the originally intended duration, but the satellites are still flying. But systems do fall out once in a while and, as an example, the 2013 summer data are unavailable due to power system problems. The 2013 summer data are thus missing from the Polar Portal GRACE figure. The linear interpolation across the summer negative peak suggests an extremely low (even no) loss summer and therefore is very misleading. We will work on a different way of representing this.

   Recent GIS MIB estimates derived from CryoSat-2 published in TC (Helm 2014) do not look that promissing (within doubling rates faster than  10 years). From p. 13 table 4:

   IMBIE 2003-2008: −189 ± 20 Gt

   ICESat 2003-2009: −146 ± 13 Gt

   CryoSat 2011-2014: −375 ± 24 Gt

     See also  McMillan (2014)

   Reagarding the SLR projections of AR5. Here is the latest paper from one of the lead authors of chapter 13 SLR, Anders Levermann, albeit just for the AIS. His new estimates seem to be 3 times as high as the ones in the AR5 (but still with very big error margins).

   re stability of GIS, an interesting papers is (I am sure most here are aware of it):

   Morlighem (2014):

   Abstract: "... Here, we combine sparse ice-thickness data derived from airborne radar soundings with satellite-derived high-resolution ice motion data through a mass conservation optimization scheme5. We infer ice thickness and bed topography along the entire periphery of the Greenland ice sheet at an unprecedented level of spatial detail and precision. We detect widespread ice-covered valleys that extend significantly deeper below sea level and farther inland than previously thought. Our findings imply that the outlet glaciers of Greenland, and the ice sheet as a whole, are probably more vulnerable to ocean thermal forcing and peripheral thinning than inferred previously from existing numerical ice-sheet models.

   @sidd thanks for the link.

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10. noa at 17:54 PM on 14 September, 2014

    I just remembered that the Morlighem paper was covered on SkS here, of course. ;-)

    An other paper re SLR projections is this one from Spence et al. 2014 (free PDF). They seeked to improve the modeling of the consequences from the poleward shifting westerlies in the SH, which is - according to them - projected to persist under continued anthr. forcing. They find that the westerlies weaken the Antarctic Slope Front (a counter-current on the outer antarctic shelfs, if I am not mistaken) and this opens the doors for the relative warm circumpolar deep waters. From that they project a warming of the waters near the grounding lines of some outlet-glaciers up to 2 degrees till 2100. This feedback is, as they say, not included in the AR5 SLR projections, because the resolution of the CMIP5-models is not high enough to account for the changes of the Antarctic coastal waters.

    (Please bear with me if I made some basic errors in my summary - I am just an interested layman).

    Well, I'll better quote the abstract of the paper:

    The southern hemisphere westerly winds have been strengthening and shifting poleward since the 1950s. This wind trend is projected to persist under continued anthropogenic forcing, but the impact of the changing winds on Antarctic coastal heat distribution remains poorly understood. Here we show that a poleward wind shift at the latitudes of the Antarctic Peninsula can produce an intense warming of subsurface coastal waters that exceeds 2°C at 200 – 700 m depth. The model simulated warming results from a rapid advective heat fl ux induced by weakened near-shore Ekman pumping and is associated with weakened coastal currents. This analysis shows that anthropogenically induced wind changes can dramatically increase the temperature of ocean water at ice sheet grounding lines and at the base of fl oating ice shelves around Antarctica, with potentially signi fi cant rami fi cations for global sea level rise.

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11. MA Rodger at 19:20 PM on 14 September, 2014

    noa @10.

    Thanks for the info. Sad to hear that Tom & Jerry the two GRACE satellites are running out of umph.

    The GRACE data at the NOAA's 2013 Greenland Report runs up to July 2013 and it looks from the graph in the link you provide to the Polar Portal that the last 4 moths after March 2013 have been pulled.

    The IMBIE/ICESat/CryoSat data does show a doubling, but so indeed does the GRACE data. The numbers scaled from the NOAA graph show  2011-13 averaging ~415Gt/yr loss, while the 2003-2009 data gives a ~210Gt/yr loss. But I do not see this, a doubling in 6 years, as being inconsistent with even a constant acceleration of ~30Gt/yr/yr as illustrated in the graph I link to @6.

    Not that such a linear trend is small. If sustained to 2100 we will still be receiving 420mm SLR from Greenland alone.

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12. noa at 22:54 PM on 14 September, 2014

    MA Rodger @  12.

    Absolutely agreed. I wanted to edit my post @10 and add the caveat that there is to little data  to decide wether the trend is linear or exponential  (but I needed a break from my heroic struggles with the foreign language monster).

    I think Hansen himself says that in the first paper you linked to. But as you show with your projection, even the SLR from a constant acceleration  would be quite substantial. Thanks for that.

    I guess it is time to buy a boat, a rubber boat will have to do it for now ... "Way hay and up she rises, ..."   :-)

    
    Have a nice day

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13. sauerj at 12:07 PM on 16 September, 2014

    Request: Could someone post two sets of curves here? This would be a good reference visual aid for sharing purposes.

    Set 1) Rate of SLR (Greenland contribution) vs year (2000 thru ~2120) in mm/yr; A family of 3 curves with doubling periods of 5, 10 and 12 years.

    Set 2) Accumulated SLR (integration of above) (GL contrib) vs year (same). With same 3 doubling period curves. ... thanks in advance, sauerj

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    Moderator Response:

    [ps] sentence fixed as requested.

14. sidd at 08:26 AM on 17 September, 2014
    Mr sauerj: see fig 8 in Hansen(2012) "Paleoclimate ..." linked to by Mr. M. A. Rodger at 1:50 AM 14 sep 2014. Then divide y axis by 360.
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15. sidd at 08:31 AM on 17 September, 2014
    result of calculation in last sentence of my last post will be in mm and mm/yr
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16. sauerj at 12:53 PM on 18 September, 2014

    sidd, Thanks! Very helpful!

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