What is happening to sea levels? That was perhaps the most controversial issue in the 4th IPCC report of 2007. The new report of the Intergovernmental Panel on Climate Change is out now, and here I will discuss what IPCC has to say about sea-level rise (as I did here after the 4th report).
Let us jump straight in with the following graph which nicely sums up the key findings about past and future sea-level rise: (1) global sea level is rising, (2) this rise has accelerated since pre-industrial times and (3) it will accelerate further in this century. The projections for the future are much higher and more credible than those in the 4th report but possibly still a bit conservative, as we will discuss in more detail below. For high emissions IPCC now predicts a global rise by 52-98 cm by the year 2100, which would threaten the survival of coastal cities and entire island nations. But even with aggressive emissions reductions, a rise by 28-61 cm is predicted. Even under this highly optimistic scenario we might see over half a meter of sea-level rise, with serious impacts on many coastal areas, including coastal erosion and a greatly increased risk of flooding.
Fig. 1. Past and future sea-level rise. For the past, proxy data are shown in light purple and tide gauge data in blue. For the future, the IPCC projections for very high emissions (red, RCP8.5 scenario) and very low emissions (blue, RCP2.6 scenario) are shown. Source: IPCC AR5 Fig. 13.27.
In addition to the global rise IPCC extensively discusses regional differences, as shown for one scenario below. For reasons of brevity I will not discuss these further in this post.
Fig. 2. Map of sea-level changes up to the period 2081-2100 for the RCP4.5 scenario (which one could call late mitigation, with emissions starting to fall globally after 2040 AD). Top panel shows the model mean with 50 cm global rise, the following panels show the low and high end of the uncertainty range for this scenario. Note that even under this moderate climate scenario, the northern US east coast is risking a rise close to a meter, drastically increasing the storm surge hazard to cities like New York. Source: IPCC AR5 Fig. 13.19.
I recommend to everyone with a deeper interest in sea level to read the sea level chapter of the new IPCC report (Chapter 13) – it is the result of a great effort by a group of leading experts and an excellent starting point to understanding the key issues involved. It will be a standard reference for years to come.
Past sea-level rise
Understanding of past sea-level changes has greatly improved since the 4th IPCC report. The IPCC writes:
Proxy and instrumental sea level data indicate a transition in the late 19th to the early 20th century from relatively low mean rates of rise over the previous two millennia to higher rates of rise (high confidence). It is likely that the rate of global mean sea level rise has continued to increase since the early 20th century.
Adding together the observed individual components of sea level rise (thermal expansion of the ocean water, loss of continental ice from ice sheets and mountain glaciers, terrestrial water storage) now is in reasonable agreement with the observed total sea-level rise.
Models are also now able to reproduce global sea-level rise from 1900 AD better than in the 4th report, but still with a tendency to underestimation. The following IPCC graph shows a comparison of observed sea level rise (coloured lines) to modelled rise (black).
Fig. 3. Modelled versus observed global sea-level rise. (a) Sea level relative to 1900 AD and (b) its rate of rise. Source: IPCC AR5 Fig. 13.7.
Taken at face value the models (solid black) still underestimate past rise. To get to the dashed black line, which shows only a small underestimation, several adjustments are needed.
(1) The mountain glacier model is driven by observed rather than modelled climate, so that two different climate histories go into producing the dashed black line: observed climate for glacier melt and modelled climate for ocean thermal expansion.
(2) A steady ongoing ice loss from ice sheets is added in – this has nothing to do with modern warming but is a slow response to earlier climate changes. It is a plausible but highly uncertain contribution – the IPCC calls the value chosen “illustrative” because the true contribution is not known.
(3) The model results are adjusted for having been spun up without volcanic forcing (hard to believe that this is still an issue – six years earlier we already supplied our model results spun up with volcanic forcing to the AR4). Again this is a plausible upward correction but of uncertain magnitude, since the climate response to volcanic eruptions is model-dependent.
The dotted black line after 1990 makes a further adjustment, namely adding in the observed ice sheet loss which as such is not predicted by models. The ice sheet response remains a not yet well-understood part of the sea-level problem, and the IPCC has only “medium confidence” in the current ice sheet models.
One statement that I do not find convincing is the IPCC’s claim that “it is likely that similarly high rates [as during the past two decades] occurred between 1920 and 1950.” I think this claim is not well supported by the evidence. In fact, a statement like “it is likely that recent high rates of SLR are unprecedented since instrumental measurements began” would be more justified.
The lower panel of Fig. 3 (which shows the rates of SLR) shows that based on the Church & White sea-level record, the modern rate measured by satellite altimeter is unprecedented – even the uncertainty ranges of the satellite data and those of the Church & White rate between 1920 and 1950 do not overlap. The modern rate is also unprecedented for the Ray and Douglas data although there is some overlap of the uncertainty ranges (if you consider both ranges). There is a third data set (not shown in the above graph) by Wenzel and Schröter (2010) for which this is also true. The only outlier set which shows high early rates of SLR is the Jevrejeva et al. (2008) data – and this uses a bizarre weighting scheme, as we have discussed here at Realclimate. For example, the Northern Hemisphere ocean is weighted more strongly than the Southern Hemisphere ocean, although the latter has a much greater surface area. With such a weighting movements of water within the ocean, which cannot change global-mean sea level, erroneously look like global sea level changes. As we have shown in Rahmstorf et al. (2012), much or most of the decadal variations in the rate of sea-level rise in tide gauge data are probably not real changes at all, but simply an artefact of inadequate spatial sampling of the tide gauges. (This sampling problem has now been overcome with the advent of satellite data from 1993 onwards.) But even if we had no good reason to distrust decadal variations in the Jevrejeva data and treated all data sets the same, three out of four global tide gauge compilations show recent rates of rise that are unprecedented – enough for a “likely” statement in IPCC terms.
Future sea-level rise
For an unmitigated future rise in emissions (RCP8.5), IPCC now expects between a half metre and a metre of sea-level rise by the end of this century. The best estimate here is 74 cm.
On the low end, the range for the RCP2.6 scenario is 28-61 cm rise by 2100, with a best estimate of 44 cm. Now that is very remarkable, given that this is a scenario with drastic emissions reductions starting in a few years from now, with the world reaching zero emissions by 2070 and after that succeeding in active carbon dioxide removal from the atmosphere. Even so, the expected sea-level rise will be almost three times as large as that experienced over the 20th Century (17 cm). This reflects the large inertia in the sea-level response – it is very difficult to make sea-level rise slow down again once it has been initiated. This inertia is also the reason for the relatively small difference in sea-level rise by 2100 between the highest and lowest emissions scenario (the ranges even overlap) – the major difference will only be seen in the 22nd century.
There has been some confusion about those numbers: some media incorrectly reported a range of only 26-82 cm by 2100, instead of the correct 28-98 cm across all scenarios. I have to say that half of the blame here lies with the IPCC communication strategy. The SPM contains a table with those numbers – but they are not the rise up to 2100, but the rise up to the mean over 2081-2100, from a baseline of the mean over 1985-2005. It is self-evident that this is too clumsy to put in a newspaper or TV report so journalists will say “up to 2100”. So in my view, IPCC would have done better to present the numbers up to 2100 in the table (as we do below), so that after all its efforts to get the numbers right, 16 cm are not suddenly lost in the reporting.
| Scenario |
Mean
|
Range
|
| RCP2.6 |
44
|
28-61
|
| RCP4.5 |
53
|
36-71
|
| RCP6.0 |
55
|
38-73
|
| RCP8.5 |
74
|
52-98
|
Table 1: Global sea-level rise in cm by the year 2100 as projected by the IPCC AR5. The values are relative to the mean over 1986-2005, so subtract about a centimeter to get numbers relative to the year 2000.
And then of course there are folks like the professional climate change down-player Björn Lomborg, who in an international newspaper commentary wrote that IPCC gives “a total estimate of 40-62 cm by century’s end” – and also fails to mention that the lower part of this range requires the kind of strong emissions reductions that Lomborg is so viciously fighting.
The breakdown into individual components for an intermediate scenario of about half a meter of rise is shown in the following graph.
Fig. 4. Global sea-level projection of IPCC for the RCP6.0 scenario, for the total rise and the individual contributions.
Higher projections than in the past
To those who remember the much-discussed sea-level range of 18-59 cm from the 4th IPCC report, it is clear that the new numbers are far higher, both at the low and the high end. But how much higher they are is not straightforward to compare, given that IPCC now uses different time intervals and different emissions scenarios. But a direct comparison is made possible by table 13.6 of the report, which allows a comparison of old and new projections for the same emissions scenario (the moderate A1B scenario) over the time interval 1990-2100(*). Here the numbers:
AR4: 37 cm (this is the standard case that belongs to the 18-59 cm range).
AR4+suisd: 43 cm (this is the case with “scaled-up ice sheet discharge” – a questionable calculation that was never validated, emphasised or widely reported).
AR5: 60 cm.
We see that the new estimate is about 60% higher than the old standard estimate, and also a lot higher than the AR4 attempt at including rapid ice sheet discharge.
The low estimates of the 4th report were already at the time considered too low by many experts – there were many indications of that (which we discussed back then), including the fact that the process models used by IPCC greatly underestimated the past observed sea-level rise. It was clear that those process models were not mature, and that was the reason for the development of an alternative, semi-empirical approach to estimating future sea-level rise. The semi-empirical models invariably gave much higher future projections, since they were calibrated with the observed past rise.
However, the higher projections of the new IPCC report do not result from including semi-empirical models. Remarkably, they have been obtained by the process models preferred by IPCC. Thus IPCC now confirms with its own methods that the projections of the 4th report were too low, which was my main concern at the time and the motivation for publishing my paper in Science in 2007. With this new generation of process models, the discrepancy to the semi-empirical models has narrowed considerably, but a difference still remains.
Should the semi-empirical models have been included in the uncertainty range of the IPCC projections? A number of colleagues that I have spoken to think so, and at least one has said so in public. The IPCC argues that there is “no consensus” on the semi-empirical models – true, but is this a reason to exclude or include them in the overall uncertainty that we have in the scientific community? I think there is likewise no consensus on the studies that have recently argued for a lower climate sensitivity, yet the IPCC has widened the uncertainty range to encompass them. The New York Times concludes from this that the IPCC is “bending over backward to be scientifically conservative”. And indeed one wonders whether the semi-empirical models would have been also excluded had they resulted in lower estimates of sea-level rise, or whether we see “erring on the side of the least drama” at work here.
What about the upper limit?
Coastal protection professionals require a plausible upper limit for planning purposes, since coastal infrastructure needs to survive also in the worst case situation. A dike that is only “likely” to be good enough is not the kind of safety level that coastal engineers want to provide; they want to be pretty damn certain that a dike will not break. Rightly so.
The range up to 98 cm is the IPCC’s “likely” range, i.e. the risk of exceeding 98 cm is considered to be 17%, and IPCC adds in the SPM that “several tenths of a meter of sea level rise during the 21st century” could be added to this if a collapse of marine-based sectors of the Antarctic ice sheet is initiated. It is thus clear that a meter is not the upper limit.
It is one of the fundamental philosophical problems with IPCC (causing much debate already in conjunction with the 4th report) that it refuses to provide an upper limit for sea-level rise, unlike other assessments (e.g. the sea-level rise scenarios of NOAA (which we discussed here) or the guidelines of the US Army Corps of Engineers). This would be an important part of assessing the risk of climate change, which is the IPCC’s role (**). Anders Levermann (one of the lead authors of the IPCC sea level chapter) describes it thus:
In the latest assessment report of the IPCC we did not provide such an upper limit, but we allow the creative reader to construct it. The likely range of sea level rise in 2100 for the highest climate change scenario is 52 to 98 centimeters (20 to 38 inches.). However, the report notes that should sectors of the marine-based ice sheets of Antarctic collapse, sea level could rise by an additional several tenths of a meter during the 21st century. Thus, looking at the upper value of the likely range, you end up with an estimate for the upper limit between 1.2 meters and, say, 1.5 meters. That is the upper limit of global mean sea-level that coastal protection might need for the coming century.
Outlook
For the past six years since publication of the AR4, the UN global climate negotiations were conducted on the basis that even without serious mitigation policies global sea-level would rise only between 18 and 59 cm, with perhaps 10 or 20 cm more due to ice dynamics. Now they are being told that the best estimate for unmitigated emissions is 74 cm, and even with the most stringent mitigation efforts, sea level rise could exceed 60 cm by the end of century. It is basically too late to implement measures that would very likely prevent half a meter rise in sea level. Early mitigation is the key to avoiding higher sea level rise, given the slow response time of sea level (Schaeffer et al. 2012). This is where the “conservative” estimates of IPCC, seen by some as a virtue, have lulled policy makers into a false sense of security, with the price having to be paid later by those living in vulnerable coastal areas.
Is the IPCC AR5 now the final word on process-based sea-level modelling? I don’t think so. I see several reasons that suggest that process models are still not fully mature, and that in future they might continue to evolve towards higher sea-level projections.
1. Although with some good will one can say the process models are now consistent with the past observed sea-level rise (the error margins overlap), the process models remain somewhat at the low end in comparison to observational data.
2. Efforts to model sea-level changes in Earth history tend to show an underestimation of past sea-level changes. E.g., the sea-level high stand in the Pliocene is not captured by current ice sheet models. Evidence shows that even the East Antarctic Ice Sheet – which is very stable in models – lost significant amounts of ice in the Pliocene.
3. Some of the most recent ice sheet modelling efforts that I have seen discussed at conferences – the kind of results that came too late for inclusion in the IPCC report – point to the possibility of larger sea-level rise in future. We should keep an eye out for the upcoming scientific papers on this.
4. Greenland might melt faster than current models capture, due to the “dark snow” effect. Jason Box, a glaciologist who studies this issue, has said:
There was controversy after AR4 that sea level rise estimates were too low. Now, we have the same problem for AR5 [that they are still too low].
Thus, I would not be surprised if the process-based models will have closed in further on the semi-empirical models by the time the next IPCC report gets published. But whether this is true or not: in any case sea-level rise is going to be a very serious problem for the future, made worse by every ton of CO2 that we emit. And it is not going to stop in the year 2100 either. By 2300, for unmitigated emissions IPCC projects between 1 and more than 3 meters of rise.
Weblinks
I’m usually suspicious of articles that promise to look “behind the scenes”, but this one by Paul Voosen is not sensationalist but gives a realistic and matter-of-fact insight into the inner workings of the IPCC, for the sea-level chapter. Recommended reading!
And the IPCC sea-level authors have a good letter to Science about their findings.
—
(*) Note: For the AR5 models table 13.6 gives 58 cm from 1996; we made that 60 cm from 1990.
(**) The Principles Governing IPCC Work explicitly state that its role is to “assess…risk”, albeit phrased in a rather convoluted sentence:
The role of the IPCC is to assess on a comprehensive, objective, open and transparent basis the scientific, technical and socio-economic information relevant to understanding the scientific basis of risk of human-induced climate change, its potential impacts and options for adaptation and mitigation.
References
- J.A. Church, and N.J. White, "Sea-Level Rise from the Late 19th to the Early 21st Century", Surveys in Geophysics, vol. 32, pp. 585-602, 2011. http://dx.doi.org/10.1007/s10712-011-9119-1
- R.D. Ray, and B.C. Douglas, "Experiments in reconstructing twentieth-century sea levels", Progress in Oceanography, vol. 91, pp. 496-515, 2011. http://dx.doi.org/10.1016/j.pocean.2011.07.021
- M. Wenzel, and J. Schröter, "Reconstruction of regional mean sea level anomalies from tide gauges using neural networks", Journal of Geophysical Research: Oceans, vol. 115, 2010. http://dx.doi.org/10.1029/2009JC005630
- S. Jevrejeva, J.C. Moore, A. Grinsted, and P.L. Woodworth, "Recent global sea level acceleration started over 200 years ago?", Geophysical Research Letters, vol. 35, 2008. http://dx.doi.org/10.1029/2008gl033611
- S. Rahmstorf, M. Perrette, and M. Vermeer, "Testing the robustness of semi-empirical sea level projections", Climate Dynamics, vol. 39, pp. 861-875, 2011. http://dx.doi.org/10.1007/s00382-011-1226-7
- S. Rahmstorf, "A Semi-Empirical Approach to Projecting Future Sea-Level Rise", Science, vol. 315, pp. 368-370, 2007. http://dx.doi.org/10.1126/science.1135456
- M. Schaeffer, W. Hare, S. Rahmstorf, and M. Vermeer, "Long-term sea-level rise implied by 1.5 °C and 2 °C warming levels", Nature Climate Change, vol. 2, pp. 867-870, 2012. http://dx.doi.org/10.1038/NCLIMATE1584
AbruptSLR (146: 31 Oct 2013 at 9:33 AM -8:00 GMT), you write:
Looking at the links that you provided to a board forum, all I see are posts referencing Antarctica, the West Antarctic Ice Sheet and sea level rise. While I find links to official websites, I am not immediately seeing any references to any peer reviewed literature, e.g. the Proceedings of the National Academy of Sciences, which incidentally becomes open access after six months. Moreover, judging at least from the titles of the posts themselves, I am not seeing anything referencing El Nino or its increased intensity.
Perhaps you could provide a link or two to a peer reviewed paper on that topic here? Preferably something open access, if possible. Requiring Hank and others (including myself) to sift through posts at a message board to find your references to on topic peer reviewed papers when nearly all the posts are about something else seems a bit unfair. Thank you!
> on the forum-pages he linked to….
> you find many papers supporting his statement
It’s good to cite primary sources (with DOI) for claims.
Science isn’t any one single paper — it’s a process over time.
The DOI takes you to where you can see that. Blog posts don’t.
From ASLR’s links, I find this paper on El Nino:
http://www.nature.com/nature/journal/v502/n7472/full/nature12580.html
And for example these papers on many of the possible ice loss mechanisms, forcings and feedbacks:
http://forum.arctic-sea-ice.net/index.php/topic,70.msg934.html#msg934
And:
http://forum.arctic-sea-ice.net/index.php/topic,70.msg936.html#msg936
It’s a lot of work to cite all or some of these separately linked, but feel free to look them up. I’m sure ASLR would be more specific if he has time.
Hasis,
You are correct. If anything, accumulation will continue there. As Lennart mentioned earlier, we simply do not understand the ice sheets well enough to propose an accurate picture. The Rosenthal paper
http://www.sciencemag.org/content/342/6158/617
showed much warmer waters around 10,000 years ago without substantial melting, although that was when Conway (1999) determined that melting first started occurring on the WAIS.
All,
It is true that I do not have time to customize my posts to each new topic, but I will try posting some of my references about the risks of ASLR:
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ASLR, if you want to do science, or even science writing, dumping your entire list is no more helpful than handwaving at your prior blog posts.
Most of that list is familiar to those who’ve read here a while. No surprises, no news. Quite a few of the authors are participants in the conversation here. Do you know which of them are here?
Either you do the work of citing sources for your claims, or you don’t.
Either way, it’s your task. You don’t acquire a reputation by using a pseudonym, having no publications, and assuming others will do the work.
Your heart is no doubt in the right place. That’s not the problem.
Ok, so that’s all reposted from
Critique of Most Common SLR Guidance Criteria w.r.t Abrupt SLR
http://forum.arctic-sea-ice.net/index.php?topic=70.0
“… On the off chance that some of the authors of the IPCC AR5 WG1 on SLR (and/or other Forum readers) might have missed (or missed the significance of) some of the following references …”
You can use HTML to point to the blog rather than copypaste it.
Thanks aSLR. That’s enough I think, for the time being: a valuable ready reference.
Question: how abrupt? In geological time, I think it’s arguable that we have it now. Certainly the overflow in big tides where I live (near Boston harbor) is noticeably more regular and encroaching more since the 1980s, but I’m expecting an acceleration.
A lot of people tend to think you might mean overnight. Or months. Or just 2 or 3 years. But in geological time a hundred years would be a wink.
On Holocene sea level:
http://www.skepticalscience.com/Sea-Level-Isnt-Level-Ocean-Siphoning-Levered-Continents-and-the-Holocene-Sea-Level-Highstand.html
On Holocene surface temperature:
https://www.realclimate.org/index.php/archives/2013/09/paleoclimate-the-end-of-the-holocene/
On the new paper by Rosenthal et al, see Mike Mann’s take here:
http://www.huffingtonpost.com/michael-e-mann/pacific-ocean-warming-at-_b_4179583.html
This new paper says ocean heat content is rising 15 times faster than at any other time during the Holocene, so pretty soon SLR may be many times faster than earlier in the Holocene as well, would be my first conclusion.
Sea level rise could be as much as “6 feet in 100 years” — NPR’s Science Friday today, that’s the worst case from a Corps of Engineers guy interviewed about protecting New York/New Jersey (a retrospective on Hurricane Sandy)
1)AbruptSLR was requested to post citations for his claims. He did so, pointing to the original web site where he had posted them, together with discussion. Some readers were not satisfied, so he posted a citation list here. He was again attacked, and advised to post a reference to the original site (which he had already done …) If there are some shortcomings in the format of the reference list posted here, that matter might be best discussed on the “Unforced Variations” thread.
2)More fruitfully, let us discuss if the citations do indeed support his claims. Apparently the citations posted were not novel to some readers. Therefore those readers might be best situated to respond as to whether the papers do support the claims put forward by AbruptSLR ?
3)I take my own advice in 2). Work cited by AbruptSLR by Ding and Steig (among others), who point out that it is not just a stronger Southern Annular Mode forced by ozone depletion over Antarctica that enhances the influx of Circumpolar Deep Water (CDW) under ice shelves. They argue that a large part of the enhancement comes from tropical influences, particularly in austral fall, and that modelling indicates that these influences are likely to increase, so that ozone recovery may not stem hot CDW influx melting southern ice shelves. This is unfortunate, I had hoped for reduced influx as ozone recovered. Perhaps Prof. Steig might comment further as to how robust the projections are ?
4)The paper by Pfeffer et al. on kinematic constraints on glaciers reports upper bound of 2m SLR from icesheets by 2100. But the more I think on this, the weaker the analysis seems, especially in West Antarctica. Glaciers are not melting by rushing to the sea and melting at calving front in West Antarctica. Rather, the sea is tunnelling under them and subtracting volume above flotation from below.
5)In a larger sense: every new feedback we find seems to act for the worse and not for the better. Our hopes grow frailer with every discovery. At this point, I see no way to rule out events on the scale of Melt Water Pulse 1A, but perhaps those among us who are already so familiar with the literature might ?
sidd
Hank, come on,
First you ask references for the almost 20 feedbacks that ASLR listed. He points to a list and to posts where these have been discussed before, but then you say that’s not good enough, and that the list doesn’t contain any news. If you’re asking someone to put more effort into this dialogue, it’s only fair to put some more effort in it yourself, in my opinion. It almost sounds like you think you’re the schoolmaster who’s testing his students. Or am I missing something?
And the good news is [Not]: http://www.sciencedaily.com/releases/2013/10/131031142738.htm … Parts of Oceans warming 15 times faster now than in the previous 10,000 years.
LOL. Having seen ASLR’s posting on other sites, I knew we were in for a treat when hank asked him for references.
Perhaps it would be more productive to the conversation if ASLR would pick out one or two papers that he thinks are most convincing wrt his point of view, then we could discuss those in more details, and other articles specifically relevant to whatever issues come up could then be introduced at the right time?
Good catches, LvdL.
> pick out one or two papers that
> he thinks are most convincing
> wrt his point of view
Since
(1) Mr. ASLR’s list is the papers he in March wrote that the forthcoming IPCC report, when available, “might have missed (or missed the significance of)” and
(2) that 5th IPCC Report on sea level is available now, and
(3) this topic is about that 5th IPCC report on sea level,
Mr. ASLR can find out if the IPCC 5th “missed (or missed the significance of)” any paper on his list by looking at their sources.
Chapter 13 IPCC WGI Fifth Assessment on sea level are at (PDF) cites the sources used at pp. 13-71 through 13-88 of the Final Draft (7 June 2013) as of today. Go to ipcc.ch for the current links, as they do change.
The list (today) starts with
Ablain, M., A. Cazenave, G. Valladeau, and S. Guinehut, 2009: A new assessment of the error budget of global mean sea level rate estimated by satellite altimetry over 1993–2008. Ocean Science, 5, 193-201, cited by 59 later papers
and ends with
Zwally, H. J., and M. B. Giovinetto, 2011: Overview and assessment of Antarctic Ice-Sheet mass balance estimates: 1992-2009. Surveys in Geophysics, 32, 351-376, cited by 28 later papers.
You go to the IPCC chapter; download the PDF (get the actual current work, not someone’s old copy on some blog somewhere).
Copy the cite; paste it into Scholar.
Lo — current information, as best we amateur readers can do.
Science works, not by picking papers that support what you believe, but by reading _all_ the work. Ideas that can’t survive being tested aren’t useful.
It’s not an “attack” to expect references for claims.
Anyone who’s ever defended a science thesis can tell you.
Science isn’t a popularity contest. This is how it works.
AbruptSLR had stated in 146:
In this context, I had specifically asked for peer reviewed references regarding the increasing intensity of El Nino under global warming.
In response to my request, Lennart van der Linde stated in 153:
From the abstract, I find:
As such there would seem to have been very little support for the view that El Niño will become more intense as the result of global warming.
However, they continue:
As such, this would appear to be a new result, first published 2013-10-24, a little over a week ago. I wouldn’t be surprised if this result stands, but there may very well be direct responses to this study, and it will likely be tested at least in part by other studies in the same area over the next few years. Personally, while I suspect there will be an intensification of El Niño, I do not believe that one can claim that there is as of yet much justification for this in the peer reviewed literature, that is, unless there are papers that the authors of the above study missed.
Zwally(2011) doi:10.1007/s10712-011-9123-5
available freely at
http://link.springer.com/article/10.1007/s10712-011-9123-5/fulltext.html
seems to have been overtaken. That paper argues for a very small Antarctic contribution to SLR, but this causes problems with overall SLR budget as shown in Chen(2013) doi: 10.1038/NGEO1829 and conflicts with newer estimates as in Shepherd(2012) doi: 10.1126/science.1228102 and Velicogna(2013) doi:10.1002/grl.50527
The latter papers use improved ice models and GIA estimates and find an accelerating component to mass waste from both Greenland and Antarctica.
I see that Chen(2013) alludes to the new ICE6G model
” The improvement in the upcoming ICE6G model is expected to significantly reduce the discrepancy among PGR models, … ”
PGR is post glacial rebound, which leads to a large part of the uncertainty for Antarctica. Has anyone seen any work based on ICE6G lately relating to current mass waste loss rates ? I see some paleo work, but nothing on GRACE corrections. I will look harder.
In this regard, i recall that at this time last year Fettweis or Tedesco or somebody posted the then latest GRACE data for Greenland. Seems like this years results should be available about now, has anyone spotted it ?
sidd
Good points, hank. Except that we should not fool ourselves into thinking we are doing science here, we are having a conversation. Science may require a fool appraisal of all available evidence, but it is very hard to have a conversation about hundreds of articles at once, imho.
Unfortunately, my last post appears to have been caught in the spam filter, but essentially it said that:
In order to focus discussion (per Wili’s suggestion), I propose that the Pine Island Glacier (PIG) – Thwaites Glacier system is the best case to consider to begin the discussion of the risk of abrupt sea level rise, ASLR, (including being influenced by all of the multiple feedback factors that I previously posted), as by themselves this drainage basins could contribute over 2-feet of SLR.
In this regard I cite the following two references focused on the Thwaites Glacier
First: “Dynamic (in)stability of Thwaites Glacier, West Antarctica”, B. R. Parizek, K. Christianson, S. Anandakrishnan, R. B. Alley, R. T. Walker, R. A. Edwards, D. S. Wolfe, G. T. Bertini, S. K. Rinehart, R. A. Bindschadler, S. M. J. Nowicki, Article first published online: 16 MAY 2013, DOI: 10.1002/jgrf.20044; Journal of Geophysical Research
Second: “Weak bed control of the eastern shear margin of Thwaites Glacier, West Antarctica”; Joseph A. MacGREGOR, Ginny A. CATANIA, Howard CONWAY, Dustin M. SCHROEDER, Ian JOUGHIN, Duncan A. YOUNG, Scott D. KEMPF, & Donald D. BLANKENSHIP; Journal of Glaciology, Vol. 59, No. 217, 2013 doi: 10.3189/2013JoG13J050
Per the following abstract, the first reference includes the following quote citing the “ephemeral” nature of the stability of the Thwaites Glacier if circulating waters substantially reduce the basal resistance in the gateway area:
Parizek et al 2013 Abstract:
“In addition to the SeaRISE data sets, we use detailed aerogeophysical and satellite data from Thwaites Glacier as input to a coupled ice stream/ice-shelf/ocean-plume model that includes oceanic influences across a several kilometers wide grounding zone suggested by new, high-resolution data. Our results indicate that the ice tongue provides limited stability, and that while future atmospheric warming will likely add mass to the surface of the glacier, strong ice stream stabilization on bedrock highs narrower than the length of the grounding zone may be ephemeral if circulating waters substantially reduce basal resistance and enhance melting beneath grounded ice within this zone.”
MacGregor et al 2013 clearly cite: (a) the possibility that the Thwaites Glacier may have retreated back at least to the eastern shear margin during the Eemian, as the radar signal might indicate the occurrence of marine sediment beneath the glacier; and (b) the SW tributary glacier could be activated by one more major calving event for the Pine Island Ice Shelf (PIIS); which in turn could active the eastern shear margin for the Thwaites Glacier, that should accelerate ice velocities out of the Thwaites Gateway, with associated ice thinning and grounding line retreat.
Furthermore:
– The continued retreat of PIG combined with the recurring major El Nino events (though 2060) could synergistically increase what I call “horizontal advection” of warm CDW from the trough leading to the PIG to the trough in the Thwaites Gateway leading to the Byrd Subglacial Basin (BSB); where the ice is current thinner and has more crevasses since the local ice tongue surge event during the late austral winter and spring of 2012; and thus the ice is this trough area is much more susceptible to calving acceleration from the warm Circumpolar Deep Water (CDW).
– The possibility that Glacial Isotatic Adjustment (GIA) corrections will increase estimate ice mass loss estimate from PIG/Thwaites by up to 40%, raises the possibility that the basal meltwater subglacial hydrological system is more active under both the PIG and especially under the Thwaites Glacier than previously expected; and if so this active subglacial drainage system would promote ice mass loss.
– The austral winter of 2013 was the warmest on record, thus raising the probability that in the near future there will be more days of surface melt during the austral summer, which would likely flow into the increasing number of surface crevasses in the ice in the Thwaites Gateway (especially as it thins); which should promote accelerated calving of the ice in this area (which is not constrained laterally as is the PIIS).
– The observed trend of increasing concentration of methane in the atmosphere over Antarctica will likely lead to increased coastal wind velocities which will likely increase the flow of warm CDW into the Amundsen Sea Embayment (ASE); which will promote ice mass loss for both the PIG and the Thwaites Glacier.
– Based on the observed snowfall trend it is unlikely that snowfall will increase before the grounding line for the Thwaites Glacier retreats to upstream of the gateway; at which point an increase in snowfall will actually accelerate the local calving by providing more driving force to promote rapid calving and groundling line retreat after the 2040 to 2060 timeframe.
– It should be remembered that any significant acceleration of ice mass loss from the Greenland Ice Sheet (GIS) in the 2013 to 2060 timeframe will help to de-stabilize the PIG/Thwaites system by raising sea level in the ASE due to the fingerprint effect.
While there are many other feedback factors, it is impossible at this time to predict the rate and amount of their synergistic interaction; and thus we will need to keep a close watch on this critical area in the coming years in order better assess the timing of any possible tipping point in the PIG/Thwaites system.
First a quibble about the Parizek paper: they seem to be using the words “lee” and “leeward” as a synonym for “landward.” This is not my understanding, “leeward” or “alee” is an antonym for windward” as far as I know. Am i correct in my interpretation of their usage or have i got it wrong ?
Now to more substantial matters. Three points struck me as important.
1) the use of a grounding zone instead of a grounding line, whose importance is illustrated by the comment:
“Removing the ice shelf, equivalent to greatly increased melting beneath floating ice but no change in melting beneath grounded ice across a discrete grounding line, has little influence on TG. However, the entire domain deglaciates in only 40 years if a smaller but still substantial increase in melting is applied not only to the floating ice but also to the node at the grounding line. Numerically, this is equivalent to a grounding zone of the same length as the last grounded finite element (typically 1 km), with the melt rate increasing from near zero at the upglacier end to the ice-shelf value (200 m/yr) at the downglacier end.”
2)The importance of coupling the ocean to ice as is done in run M4:
A weakness in some of the runs is the imposed basal melt rate in M1,M2, and M3 runs. The coupled ocean ice model in M4 i think is more reliable, and shows features absent in M1, M2 and M3, and breaks the linearity of response:
“Short of the M3-magnitude forcing, VAF changes increase linearly with spatially constant prescribed melt rates (Figure 7c). However, with the coupled ice-ocean simulation (M4), this linearity breaks down … As peak melt rates ( 40 m/yr, lower right vertices) are concentrated near the grounding line, spatially variable melt leads to efficient removal of the ice shelf (cf. Figure 6d and 6e) [Little et al., 2009; Walker and Holland, 2007; Parizek and Walker, 2010] and a nonlinear response to the average melt magnitude (roughly 50–70% more volume loss than a linear trend would predict for a spatially constant 15 m/yr melt rate; see arrows in Figure 7c).”
3)the presence and influence of subglacial water several kilometers upstream of the grounding “line.” This relates to 1) above. They see evolution of subglacial hydrology in their models, and this evolution coupling with the width of the grounding zone greatly influences retreat (or growth.) In this context, another weakness of their model is the reduced dimensionality, as they admit:
“As discussed above, the strong variations in basal topography transverse to flow suggest that seawater may penetrate inland in some regions and then spread laterally. Melting of ice above such a layer would be less than in the ice-shelf cavity, where ocean circulation is vigorous. However, under some conditions, it is likely to be notably larger than beneath fully grounded ice where the enhanced melting from transport of warmed ocean waters exceeds the reduced melting from lubrication suppressing basal friction.”
They go on to find:
“Thus, if the grounding zone is narrower than the distance between the peak of the bedrock ridge and the downstream edge of the subglacial lake, then grounding line stabilization occurs; otherwise, catastrophic retreat begins (136 km in 55 years), with no other points of stabilization in our model domain.”
They repeat the caveat about reduced dimensionality:
“Because of our reduced dimensional modeling, we note that these cavities are potentially interconnected with the ocean in the transverse dimension.”
In the Appendix they point out the importance of tidal pumping (which is not explicitly included in the model):
” Model estimates for likely subglacial materials yield a low-tide pressure drop beneath this flexural uplift that is larger in magnitude than the oceanic pressure drop from the tide. This may cause tidal pumping of seawater upglacier. This ocean water then may be spread farther inland by strong water pressure variations associated with ice flexure [Murray and Clarke, 1995; Walker et al., 2013]. Such pumping may explain why radar data collected across grounding lines typically show a bright reflection typical of ice over seawater extending inland and only gradually fading to a weaker reflector more consistent with a thin layer of freshwater [Walker et al., 2013].”
They warn:
” … our results do not yet provide reliable projections of best estimate or upper limit sea level rise from TG. While our assumptions of a linear decrease in basal melt and weakening of the basal drag coefficient in the grounding zone are aggressive, they are not worst case scenarios.”
I think this is an important advance in modelling, highlighting the importance of accurate basal topo measurement. I eagerly await the next iteration of such models, especially to remove the limitation of reduced dimensionality, and utilizing detailed topo.
I shall try to put some of the grafs on the web with comments in my copious(not!) spare time …
sidd
I should add one important point to my comment above
4)the importance of bed rheology: an effectively plastic bed can _stabilize_ against retreat. This might be a ray of hope. I do no clearly see if such a “plastic” bed is realistic, but the possibility that the glacier can sculpt its own bed to stabilize itself is a tantalizing, if far-fetched glimmer of good news.
“However, with a nearly plastic basal rheology and xgz = 7 km, the GZ3P simulation forms two subglacial lakes in the grounding zone but ultimately stabilizes (Figures 10e, 12c, and 12d) as basal stresses are spread across a larger length scale. Furthermore, thinning waves propagate rapidly across the entire glacier (cf., m = 1 bed), and only minor geometric adjustments are required to deliver the ice flux necessary to maintain grounding on the bedrock ridge. Because little change in driving stress is required to compensate large marginal forcings arising from the 7 km grounding zone, only the last ~45 km of grounded ice has a significantly different profile
(|s| > 20 m) when compared to the end of the T1 standard simulation. Therefore, given the high-resolution basal topography in the grounding zone, higher bed exponents tend to stabilize the TG system. This novel result further highlights the need for additional data and analyses to determine bed type [Anandakrishnan et al., 2003; Joughin et al.,2004, 2009; Walker et al., 2012].”
They go on to warn however:
” This significant stability can be overcome, however, if we reduce the upstream flux by 11%. Even though episodic advances persist late into the deglaciation, this additional forcing leads to rapid retreat (Figures 10f, 12e, and 12f)”
sidd
As a follow-on to my previous post, the following abstract come from the linked sources and are relevant to the question of potential abrupt SLR contribution from the Thwaites/PIG drainage basins:
http://www.igsoc.org/symposia/2013/kansas/proceedings/procsfiles/procabstracts_63.htm
Contact: Secretary General, International Glaciological Society
67A028
Ice sheets and sea level – data, models and ways forward
Richard B. ALLEY, Sridhar ANANDAKRISHNAN, Byron R. PARIZEK, David POLLARD, Kiya L. RIVERMAN, Nicholas D. HOLSCHUH, Atsu MUTO, John M. FEGYVERESI, Nathan T. STEVENS, T. LUTHRA, D.E. VOIGT, P.G. BURKETT, K. CHRISTIANSON, J.P. WINBERRY
Corresponding author: Richard B. Alley
Corresponding author e-mail: rba6@psu.edu
Abstract:
“The ‘unknown unknowns’ of ice-sheet behavior have been shrinking rapidly under the coordinated efforts of surface observations, airborne and satellite remote sensing, and modeling, together with atmospheric, oceanic and geologic investigations around the ice sheets, including paleoclimatic studies. For most ice-sheet regions, it is now possible to place useful limits on likely rates of change, quantify uncertainties and define research plans for reducing those uncertainties. Unfortunately, this optimistic outlook does not apply universally. Sufficient retreat of the Thwaites Glacier grounding zone, for example, could shift a calving front into a region of combined width and water depth larger than any outlet on Earth today, raising physical questions that are not as yet close to being answered and that may prove very difficult to constrain tightly. The community faces the challenge of continuing the highly successful work of reducing uncertainties in well-characterized flow regimes, while identifying and characterizing those physical processes that are not yet well represented in key places. Furthermore, policy-makers would like guidance from plausible scenarios until those physical processes are better represented. The need for coordinated observations and modeling is thus growing, not shrinking.”
I strongly agree with Alley et al 2013’s statements cited above about the Thwaites Glacier, and I note that our current lack of certainty on the topic of the (in)stability of the Thwaites/PIG system poses a hazard that merits considerable focus on coordinated observations and advanced modeling (beyond what is practicable today).
wili wrote in 171:
Well, if someone wishes to suggest that their position is well-supported by the literature then I would recommend citing a review or two. Absent that, it might help to do a little of their own objective survey. A cherry pick need not apply. If they wish instead to discuss the latest finding, I certainly don’t have a problem with that, either, so long as they make it clear that this is what they are discussing rather than something that has wider support.
We may not be doing science, but we should try to insure that, when we appeal to science we are actually appealing to the science, representing it accurately, not our own personal hunches. This does not however mean that we must always hold with the most conservative estimate, either. If we represent the uncertainties accurately, we can point out that uncertainty oftentimes implies risk.
That should have been ‘full appraisal’ instead of ‘fool appraisal,’ though perhaps I should just let the Freudian slip stand?
Sidd,
Thanks for the great input about the Parizek et al 2013 paper.
For those interested in the details of the MacGregor et al 2013 paper, I provide the following link to a pdf and I post the abstract to this paper:
http://www.igsoc.org/journal/59/217/j13J050.pdf
“ABSTRACT. Recent acceleration and thinning of Thwaites Glacier, West Antarctica, motivates investigation of the controls upon, and stability of, its present ice-flow pattern. Its eastern shear margin separates Thwaites Glacier from slower-flowing ice and the southern tributaries of Pine Island Glacier. Troughs in Thwaites Glacier’s bed topography bound nearly all of its tributaries, except along this eastern shear margin, which has no clear relationship with regional bed topography along most of its length. Here we use airborne ice-penetrating radar data from the Airborne Geophysical Survey of the Amundsen Sea Embayment, Antarctica (AGASEA) to investigate the nature of the bed across this margin. Radar data reveal slightly higher and rougher bed topography on the slower-flowing side of the margin, along with lower bed reflectivity. However, the change in bed reflectivity across the margin is partially explained by a change in bed roughness. From these observations, we infer that the position of the eastern shear margin is not strongly controlled by local bed topography or other bed properties. Given the potential for future increases in ice flux farther downstream, the eastern shear margin may be vulnerable to migration. However, there is no evidence that this margin is migrating presently, despite ongoing changes farther downstream.”
The following linked reference and associated abstract presents relatively recent numerical findings that certain GIS and WAIS glaciers (including the Thwaites Glacier) may be at risk of ” catastrophic disintegration”:
http://www.nature.com/ngeo/journal/v6/n10/full/ngeo1887.html
Bassis, J.N., and Jacobs,S., (2013), “Diverse calving patterns linked to glacier geometry”, Nature Geoscience, 6, 833–836, doi:10.1038/ngeo1887
Abstract:
“Iceberg calving has been implicated in the retreat and acceleration of glaciers and ice shelves along the margins of the Greenland and Antarctic ice sheets. Accurate projections of sea-level rise therefore require an understanding of how and why calving occurs. Unfortunately, calving is a complex process and previous models of the phenomenon have not reproduced the diverse patterns of iceberg calving observed in nature. Here we present a numerical model that simulates the disparate calving regimes observed, including the detachment of large tabular bergs from floating ice tongues, the disintegration of ice shelves and the capsizing of smaller bergs from grounded glaciers that terminate in deep water. Our model treats glacier ice as a granular material made of interacting boulders of ice that are bonded together. Simulations suggest that different calving regimes are controlled by glacier geometry, which controls the stress state within the glacier. We also find that calving is a two-stage process that requires both ice fracture and transport of detached icebergs away from the calving front. We suggest that, as a result, rapid iceberg discharge is possible in regions where highly crevassed glaciers are grounded deep beneath sea level, indicating portions of Greenland and Antarctica that may be vulnerable to rapid ice loss through catastrophic disintegration.”
While the following linked, referenced (with abstract), paper is not specifically directed towards SLR contribution from the West Antarctic Ice Sheet, WAIS; nevertheless, if correct, it does indicate the potential for ice sheet collapse under conditions comparable to modern conditions:
http://www.nature.com/ngeo/journal/v6/n9/full/ngeo1890.html
http://www.nature.com/ngeo/journal/v6/n9/fig_tab/ngeo1890_F1.html
O’Leary, M.J., Hearty, P.J., Thompson, W.G., Raymo, M.E., Mitrovica, J.X., and Webster, J.M., (2013), “Ice sheet collapse following a prolonged period of stable sea level during the last interglacial”, Nature Geoscience; doi:10.1038/ngeo1890.
Abstract
“During the last interglacial period, 127–116 kyr ago, global mean sea level reached a peak of 5–9 m above present-day sea level. However, the exact timing and magnitude of ice sheet collapse that contributed to the sea-level highstand is unclear. Here we explore this timing using stratigraphic and geomorphic mapping and uranium-series geochronology of fossil coral reefs and geophysical modelling of sea-level records from Western Australia. We show that between 127 and 119 kyr ago, eustatic sea level remained relatively stable at about 3–4 m above present sea level. However, stratigraphically younger fossil corals with U-series ages of 118.1±1.4 kyr are observed at elevations of up to 9.5 m above present mean sea level. Accounting for glacial isostatic adjustment and localized tectonics, we conclude that eustatic sea level rose to about 9 m above present at the end of the last interglacial. We suggest that in the last few thousand years of the interglacial, a critical ice sheet stability threshold was crossed, resulting in the catastrophic collapse of polar ice sheets and substantial sea-level rise.”
op. cit.
ASLR, you can search the site, e.g. for that paper:
https://www.google.com/search?q=site%3Arealclimate.org+“doi%3A10.1038%2Fngeo1887”
Stefan, I keep re-reading your opening post on this and getting more out of it each time. I’d encourage others to do the same, treat this as studying the science, not as a chatroom conversation. Most of us are the audience here.
I particularly appreciated Stefan’s pointer to http://www.glaciology.net/Home/Miscellaneous-Debris/ar5sealevelriseuncertaintycommunicationfailure
Every link in the opening post leads to more reading worth serious attention on this subject. The scientists have, I think, made very clear how we’ve changed the future of the planet already.
The following linked reference (and abstract) indicates that due to modeling issues associated with the Glacial Isostatic Adjustment, GIA, it is possible that the ice mass loss from the Amundsen Sea sector glaciers (including PIG and Thwaites) may be under estimated by about 40%. Currently, British researchers are installing more GPS sites in this area in order to try to resolve this issue (which may take many years-worth of data). However, in the meantime this uncertainty implies greater public risk not greater public safety.
http://www.sciencedirect.com/science/article/pii/S0921818112001567
Groh, A., Ewert, H., Scheinert, M., Fritsche, M., Rulke, A., Richter, A., Rosenau, R., and Dietrich, R., (2012), “An investigation of Glacial Isostatic Adjustment over the Amundsen Sea sector, West Antarctica”, Global and Planetary Change, December, Vols. 98-99, pp 45-53 http://dx.doi.org/10.1016/j.gloplacha.2012.08.001.
“Abstract
The present study focuses on the Amundsen Sea sector which is the most dynamical region of the Antarctic Ice Sheet (AIS). Based on basin estimates of mass changes observed by the Gravity Recovery and Climate Experiment (GRACE) and volume changes observed by the Ice, Cloud and Land Elevation Satellite (ICESat), the mean mass change induced by Glacial Isostatic Adjustment (GIA) is derived. This mean GIA-induced mass change is found to be 34.1 ± 11.9 Gt/yr, which is significantly larger than the predictions of current GIA models. We show that the corresponding mean elevation change of 23.3 ± 7.7 mm/yr in the Amundsen Sea sector is in good agreement with the uplift rates obtained from observations at three GPS sites. Utilising ICESat observations, the observed uplift rates were corrected for elastic deformations due to present-day ice-mass changes. Based on the GRACE-derived mass change estimate and the inferred GIA correction, we inferred a present-day ice-mass loss of − 98.9 ± 13.7 Gt/yr for the Amundsen Sea sector. This is equivalent to a global eustatic sea-level rise of 0.27 ± 0.04 mm/yr. Compared to the results relying on GIA model predictions, this corresponds to an increase of the ice-mass loss or sea-level rise, respectively, of about 40%.”
Researcher such as Stefan Rahmstorf and Aslak Grinsted, certainly do not deny that the risk of the collapse of portions of the West Antarctic Ice Sheet, WAIS, may not be adequately included within the AR5 SLR projections, as indicated in the information in the following website links (maintained by Aslak Grinsted):
http://www.glaciology.net/Home/Miscellaneous-Debris/optimisticicesheetprojectionsinar5
http://www.glaciology.net/Home/Miscellaneous-Debris/optimisticicesheetprojectionsinar5
Therefore, readers should not think that I am criticizing any such researchers. However, I will state that picking a date for non-linear SLR projections like 2100 is deceptive because a few years thereafter (say one hundred years from now 2014) the SLR could be very much higher; and also non-linear interactions not fully captured by current “ice experts” [such as those assessed by Bamber & Aspinall (2013), see reference below)] projections, may cause the “ice experts” to change (possibly increase) their projections of ice mass loss from the WAIS in the future, when their models become more powerful/calibrated (possibly to incorporate data from some of the recent scientific findings that I am referencing).
Bamber, J.L. and Aspinall, W.P. (2013), “An expert judgment assessment of future sea level rise from the ice sheets”, Nature Climate Change; Volume:3, Pages: 424–427; doi:10.1038/nclimate1778.
ASLR, so far you’re mostly restating points made by Stefan in the original post that opens this thread.
Yes, Aslak Grinsted’s pages are very helpful, as are the other links Stefan provides. You should read them.
You use the word “deceptive” to describe the use of 2100 as a marker. I’d say you’re deceiving yourself if you think that’s the end of what’s discussed. Look at the very last lines of the AGU chapter, in the PDF.
Your claim the scientists are being “deceptive” would mislead anyone who hasn’t read the AGU chapter.
Note more than 200 people have recommended reading this (bottom of the main post, click the button).
Please be aware you’re writing for an audience.
Check the assumptions you’re making before posting what you believe.
If you’re just posting without reading, then, well, bless your heart.
Whoever you are.
The following linked reference, and associated abstract, indicates that the CDW entering the troughs leading to the PIG (which contributes to the CDW leading to the Thwaites Glacier) have increased in volume between 2000 and 2010. I note that the year from 2000 to 2010 were all El Nino hiatus years, and that it is likely that when the current El Nino hiatus period ends, that the volume of CDW passing through the troughs leading to the PIG and the Thwaites Glacier, may accelerate during El Nino periods:
http://www.journals.elsevier.com/deep-sea-research-part-i-oceanographic-research-papers/recent-articles/
From circumpolar deep water to the glacial meltwater plume on the eastern Amundsen Shelf
Y. Nakayama | M. Schröder | H.H. Hellmer
Deep Sea Research Part I: Oceanographic Research Papers; Volume 77, July 2013, Pages 50–62
“Abstract: The melting of Pine Island Ice Shelf (PIIS) has increased since the 1990s, which may have a large impact on ice sheet dynamics, sea-level rise, and changes in water mass properties of surrounding oceans. The reason for the PIIS melting is the relatively warm (∼1.2°C) Circumpolar Deep Water (CDW) that penetrates into the PIIS cavity through two submarine glacial troughs located on the Amundsen Sea continental shelf. In this study, we mainly analyze the hydrographic data obtained during ANTXXVI/3 in 2010 with the focus on pathways of the intruding CDW, PIIS melt rates, and the fate of glacial meltwater. We analyze the data by dividing CTD profiles into 6 groups according to intruding CDW properties and meltwater content. From this analysis, it is seen that CDW warmer than 1.23°C (colder than 1.23°C) intrudes via the eastern (central) trough. The temperature is controlled by the thickness of the intruding CDW layer. The eastern trough supports a denser CDW layer than the water mass in Pine Island Trough (PIT). The eastern intrusion is modified on the way into PIT through mixing with the lighter and colder CDW from the central trough. Using ocean transport and tracer transport calculations from the ice shelf front CTD section, the estimated melt rate in 2010 is ∼30myr−1, which is comparable to published values. From spatial distributions of meltwater content, meltwater flows along the bathymetry towards the west. When compared with earlier (2000) observations, a warmer and thicker CDW layer is observed in Pine Island Trough for the period 2007–2010, indicating a recent thickening of the CDW intrusion.”
For those not familiar with the Antarctic, Bertler et al (2006, see reference below) explains the reason that more warm CDW flows into the ASE during El Nino periods [due to shifts in the Amundsen Sea Low, ASL (or the Amundsen-Bellingshausen Seas Low, ABSL)], see my associated statement to this affect in my preceding post.
Bertler, N.A., Naish, T.T., Mayewski, P.A. and Barrett, P.J., (2006), “Opposing oceanic and atmospheric ENSO influences on the Ross Sea Region, Antarctica”, Advances in Geosciences, 6, pp 83-88, SRef-ID: 1680-7359/adgeo/2006-6-83.
Furthermore, the following reference (and abstract) not only indicates how important/unique the Bellingshausen – Amundsen Seas Sector is but also just how poorly the current GCMs, and RCMs, model this critical area (and as cited above the ABSL together with El Nino events can accelerate ice mass loss from the Amundsen-Bellingshausen Seas Sector Ice Sheets, as the warm CDW promotes grounding line retreat for the affected glaciers):
The influence of the Amundsen-Bellingshausen Seas Low on the climate of West Antarctica and its representation in coupled climate model simulations. J. Scott Hosking, Andrew Orr, Gareth J. Marshall, John Turner, and Tony Phillips, Journal of Climate 2013; doi: http://dx.doi.org/10.1175/JCLI-D-12-00813.1
Abstract:
“In contrast to earlier studies, we describe the climatological deep low-pressure system that exists over the South Pacific sector of the Southern Ocean, referred to as the Amundsen-Bellingshausen Seas Low (ABSL), in terms of its relative (rather than actual) central pressure by removing the background area-averaged mean sea level pressure (MSLP). In doing so, we remove much of the influence of large-scale variability across the ABSL sector region (e.g., due to the Southern Annular Mode), allowing a clearer understanding of ABSL variability and its effect on the regional climate of West Antarctica. Using ERA-Interim reanalysis fields the annual cycle of the relative central pressure of the ABSL for the period 1979 to 2011 shows a minimum (maximum) during winter (summer), differing considerably from the earlier studies based on actual central pressure which suggests a semi-annual oscillation. The annual cycle of the longitudinal position of the ABSL is insensitive to the background pressure, and shows it shifting westwards from ~250° E to ~220° E between summer and winter, in agreement with earlier studies. We demonstrate that ABSL variability, and in particular its longitudinal position, plays an important role in controlling the surface climate of West Antarctica and the surrounding ocean by quantifying its influence on key meteorological parameters. Examination of the ABSL annual cycle in seventeen CMIP5 climate models run with historical forcing showed that the majority of them have definite biases, especially in terms of longitudinal position, and a correspondingly poor representation of West Antarctic climate.”
Again the poor CMIP5 model projections of the ABSL annual cycle raises more uncertainties about the AR5 SLR projections; which does not reflect well on the use of the AR5 projections for protecting the public.
Also, the following reference (as abstract) explains the fingerprint effect of rapid ice melt on SLR, which indicates that ice mass loss from the WAIS has a much larger effect on Northern Hemisphere, NH, areas that such an ice mas loss has on eustatic sea level. As most of the world’s affected populated centers occur in the NH the consequence of AR5 underestimating SLR contribution from the WAIS would be more serious that from non-Antarctic sources:
Hay, C.C., Morrow, E., Kopp, R.E., and Mitrovica, J.X., 2012, “Estimating the sources of global sea level rise with data assimilation techniques”, Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.1117683109.
Abstract
“A rapidly melting ice sheet produces a distinctive geometry, or fingerprint, of sea level (SL) change. Thus, a network of SL observations may, in principle, be used to infer sources of meltwater flux. We outline aformalism, based on a modified Kalman smoother, for using tide gauge observations to estimate the individual sources of global SL change. We also report on a series of detection experiments based on synthetic SL data that explore the feasibility of extracting source information from SL records. The Kalman smoother technique iteratively calculates the maximum-likelihood estimate of Greenland ice sheet (GIS) and West Antarctic ice sheet (WAIS) melt at each time step, and it accommodates data gaps while also permitting the estimation of nonlinear trends. Our synthetic tests indicate that when all tide gauge records are used in the analysis, it should be possible to estimate GIS and WAIS melt rates greater than ∼0.3 and ∼0.4 mm of equivalent eustatic sea level rise per year, respectively. We have also implemented a multimodel Kalman filter that allows us to account rigorously for additional contributions to SL changes and their associated uncertainty. The multimodel filter uses 72 glacial isostatic adjustment models and 3 ocean dynamic models to estimate the most likely models for these processes given the synthetic observations. We conclude that our modified Kalman smoother procedure provides a powerful method for inferring melt rates in a warming world.”
I return to Parizek, and my misgivings increase.
Consider: ” … spatially variable melt leads to efficient removal of the iceshelf …” in relation to Fig. 7 where they show that spatially variable melt patters remove much more ice than imposing an average melt rate would suggest.
I think the reason is the following. Melt depends linearly on heat delivered, so we can say
dM/dt=dQ/Ldt, where dM/dt is mass melted per unit time, L is latent heat, dQ/dt is heat delivered per unit time. But dQ/dt is driven by CDW influx, which increases as meltwater efflux sucks warm CDW in below the freshwater going seaward. dQ/dt can be written as
dQ/dt=C(Tcdw-Tmelt)v,
Tcdw and tmelt are self explanatory, C is of course specific heat, and v is velocity of CDW influx (equal to melt efflux modulo density differences) and v increases as melt rate increases, v=kdM/dt. So the faster the melt, the more heat is delivered.
dM/dt = C(Tcdw-Tmelt)k(dM/dt)/L, implying exponential increase in melt rate in local hotspots, the efficiency of spatially variable melt is much larger than a linear average indicates.
Now, this leads to another remark in Parizek, and the Bertler and Nakayama papers referred to by AbruptSLR. Parizek states
“We note that recent observations by Jacobs et al. [2011] indicate that in some regions near Pine Island Glacier, the CDW layer has since warmed … ”
So the heat flux is temporally variable as well as spatially variable. Precisely the same argument I have made above leads to the conclusion that temporally variable melt will be much more efficient at removing ice than once would imagine from a linear temporal average.
sidd
More support evidence for the point the Sidd makes in post 190 can be found in the following two articles about the PIG:
T. P. Stanton, W. J. Shaw, M. Truffer, H. F. J. Corr, L. E. Peters, K. L. Riverman, R. Bindschadler, D. M. Holland, S. Anandakrishnan (2013), “Channelized Ice Melting in the Ocean Boundary Layer Beneath Pine Island Glacier, Antarctica”, Science,13 September 2013: Vol. 341 no. 6151 pp. 1236-1239 , DOI: 10.1126/science.1239373.
Anne M. Le Brocq, Neil Ross, Jennifer A. Griggs, Robert G. Bingham, Hugh F. J. Corr, Fausto Ferraccioli, Adrian Jenkins, Tom A. Jordan, Antony J. Payne, David M. Rippin & Martin J. Siegert, (2013), “Evidence from ice shelves for channelized meltwater flow beneath the Antarctic Ice Sheet”, Nature Geoscience; doi:10.1038/ngeo1977.
and more generalized information on this matter (with regard to marine terminating glaciers) can be found in the following two references:
Enderlin, E. M., Howat, I. M., and Vieli, A.: The sensitivity of flowline models of tidewater glaciers to parameter uncertainty, The Cryosphere, 7, 1579-1590, doi:10.5194/tc-7-1579-2013, 2013.
Callens, D., Matsuoka, K., Steinhage, D., Smith, B., and Pattyn, F.: Transition of flow regime along a marine-terminating outlet glacier in East Antarctica, The Cryosphere Discuss., 7, 4913-4936, doi:10.5194/tcd-7-4913-2013, 2013.
Some readers may not be aware that the West Antarctic Ice Sheet, WAIS, is the last remaining marine ice sheet in the world and thus is particularly sensitive to both volume increases and temperature increases of warm CDW advecting to both the grounding lines and beneath associated ice shelves in this area (particularly for the Thwaites Glacier and associated Amundsen Sea Embayment, ASE, marine terminating glaciers). Indeed, the following reference makes it clear that the flow of warm CDW through the troughs leading into the ASE, continues year round, whether, or not, sea ice, or El Nino events, are extant:
L. Arneborg, A. K. Wåhlin, G. Björk, B. Liljebladh & A. H. Orsi, (2012), “Persistent inflow of warm water onto the central Amundsen shelf”, Nature Geoscience, Volume: 5, pp 876–880, doi:10.1038/ngeo1644.
Furthermore, my posts 187 and 188 show this flow of warm CDW increases in El Nino periods, yet most CMIP5 hind-castes of the ABSL (ASL) do not adequately correlate to the ENSO historical record, and thus are likely to under predict ice mass loss from the ASE glaciers during El Nino periods (which may not have been noticed given the present El Nino hiatus, but which by definition also implies that when the current El Nino hiatus period end that the frequency of El Nino events will be more likely on average to the end of the century). Furthermore, my first post referenced the Power et al 2013 (see reference repeated below) that indicates that El Nino events are likely to become more intense with increasing global warming, which was not previously understood in the AR5 process. Thus the AR5 projections of SLR contributions from the marine glaciers in the ASE in particular (and marine terminating glaciers in general) are likely to be too low with regard to the influence of El Nino events.
Power, S., Delage, F., Chung, C., Kociuba, G. and Keay, K., (2013), “Robust twenty-first-century projections of El Nino and related precipitation variability”, Nature, 502, 541-545, doi:10.1038/nature12580.
Sidd mentioned that CDW advection can increase due to increased saline pump interaction with the ice shelf/glacier, and I have mentioned that: (a) the volume of CDW has been measured to be increasing in the troughs leading to the ASE (possibly due to the measured increase of volume in the Southern Ocean); (b) the CDW flow will increase due to the increase frequency of El Nino events (due to the end of the hiatus period) and in intensity as global warming increases. Furthermore, global warming will (among numerous other things) : (a) increase the temperature of the CDW; (b) increase local storm action (which increases ice mass loss, see Fogt et al 2011); and (c) will increase surface ice melting (and associated water drainage to the basal water). However: (a) the Central West Antarctica is among the most rapidly warming regions on Earth [see: Bromwich, D.H., et al., (2013), “Central West Antarctica among the most rapidly warming regions on Earth”. Nature Geoscience; Vol. 6, p. 139; doi:10.1038/ngeo1671]; and (b) I believe that there is considerable evidence that the RCP radiative forcing scenarios used in the AR5 SLR projections are insufficient to fully characterize the risk of higher global temperatures (than that characterized by the RCP scenarios), for reasons such as the following:
Schuur and Abbott (see Schuur, E.A.G. and Abbott, B., (2011), “High risk of permafrost thaw”, Nature, 480, 32-33, Dec. 2011.) were the first to identify that the RCP radiative forcing scenarios do not include methane emissions from the expected permafrost degradation (this has been confirmed by the IPCC), which is particularly disturbing for RCP 8.5 scenario (which is projected to induce considerable permafrost degradation this century), and even more so if the Arctic Sea Ice grades sooner than AR5 projects, because methane has a Global Warming Potential (GWP) that is currently at least 25, and more likely 35 (see Shindell et al 2009 and Shindell et al 2013, cited below), times greater than that for carbon dioxide (which is a significant error as for RCP 8.5 methane emissions from permafrost degradation is estimated by Schuur and Abbott (2011) to be approximately 2% of all GHG emitted by the degraded permafrost. Thus both due to ignoring methane emissions from permafrost degradation and due to being calibrated to match lower a GWP for methane than is likely due to methane chemistry in the atmosphere, the IPCC global warming projections and associated SLR projections are mostly likely lower than what will be experienced in the future.
Shindell, D.T., Faluvegi, G., Koch, D.M., Schmidt, G.A., Unger, N., and Bauer S.E. (2009), “Improved Attribution of Climate Forcing to Emissions”, Science, Vol. 326 no. 5953 pp. 716-718, DOI: 10.1126/science.1174760.
Shindell, D.T., O. Pechony, A. Voulgarakis, G. Faluvegi, L. Nazarenko, J.-F. Lamarque, K. Bowman, G. Milly, B. Kovari, R. Ruedy, and G. Schmidt, (2013), “Interactive ozone and methane chemistry in GISS-E2 historical and future climate simulations”, Atmos. Chem. Phys., 13, 2653-2689, doi:10.5194/acp-13-2653-2013.
ASLR, there’s a difference between an ore and a metal.
The difference is called refinement.
Stefan started this thread with a careful discussion of what’s in the IPCC report, what’s not in it, and what he considers the facts to be and where he and others differ from what’s in the IPCC report.
You’re just duplicating at great and wordy length some of what’s in other people’s writing, without sorting it out at all.
Refinement is helpful.
You could be.
Please. Make the effort.
If you’re reading anything anyone here says at all.
If you aren’t reading the comments, others won’t either.
I’m done with ya. Carry on.
For those not familar with Fogt’s work (related to ASE weather and the ENSO teleconnection), please review the following references:
2012 Fogt, R. L., A. J. Wovrosh, R. A. Langen, and I. Simmonds. The Characteristic Variability and Connection to the Underlying Synoptic Activity of the Amundsen-Bellingshausen Seas Low. J. Geophys. Res., 117, doi:10.1029/2011JD017337.
2011Fogt, R. L., D. H. Bromwich, and K. M. Hines. Erratum to: Understanding the SAM influence on the South Pacific ENSO teleconnection. Climate Dynamics, 37, 2127-2128.
2011Bromwich, D. H., D. F. Steinhoff, I. Simmonds, K. Keay, and R. L. Fogt. Climatological aspects of cyclogenesis near Adèlie Land Antarctica. Tellus A, 63, 921-938.
2011Fogt, R.L., (associate editor and author). Antarctica [In “State of the Climate in 2010”]. Bulletin of the American Meteorological Society, 92, S161-S171.
2011Fogt, R. L., D. H. Bromwich, and K. M. Hines. Understanding the SAM influence on the South Pacific ENSO teleconnection. Climate Dynamics, 36, 1555-1576.
ASLR
I for one am enjoying your posts, very interesting, take Hanks advice and ‘carry on’
ASLR
I for one, am enjoying your posts, very interesting. As Hank advises ‘carry on’.
Just to be a little bit clearer about my meaning, I believe that the references that I have posted provide support to Hansen’s position that a SLR of 2-3 meters by 2100 may be possible under our current BAU scenario.
Furthermore, I would like to point out that I do not believe that RCP 2.6 is a plausible scenario anymore; yet the IPCC includes this scenario in all of its risk probability distributions; while if it were to be eliminated, the probabilities of greater forcing scenarios occurring would increase proportionally. Also, I believe that the GCMs used in the AR5 findings do not do a good job of projecting the extent of Arctic Sea ice in the future, and I believe that the Arctic Sea extent retreat sufficiently to have a significantly greater positive feedback effect (than considered by the IPCC) due to changes in both albedo and atmospheric humidity (due to reduced sea coverage and increased water temperatures).
I believe that Lempert et al 2012 (see linked reference below) do a relatively good job of quantifying the risk of abrupt sea level rise, ASLR, by statistically expanding previously existing Probability Distribution Functions, PDF, from both extended scenarios of Pfeffer et al 2008 and from CO-CAT 2010 (see figures 7 and 8 of the linked pdf)to include ASLR considerations ignored by the IPCC WG1. The probabilities of SLR this century shown in Figure 8, fully support my (and Hansen’s) position that from 2 to 3 m of SLR is feasible by the end of this century. If nothing else, Pfeffer et al 2008 is a widely cited reference, and statistically extending this work to include the risk of ASLR is a practical approach/methodology that the IPCC could consider adopting in order to provide policy makers with a better idea of how much risk that the public actually faces from SLR:
http://www.energy.ca.gov/2012publications/CEC-500-2012-056/CEC-500-2012-056.pdf
Lempert, Robert, Ryan L. Sriver, and Klaus Keller (RAND). 2012. Characterizing Uncertain Sea Level Rise Projections to Support Investment Decisions. California Energy Commission. Publication Number: CEC-500-2012-056.
ASLR: …I do not believe that RCP 2.6 is a plausible scenario anymore…
You’re not alone. For all the criticism of the IPCC, RCP 2.6 is the actual nugget of science fiction with which critics could take legitimate issue.
RCP 2.6 is how the emissions path would appear in the years just after the final scenes of “12 Monkeys,” or in the Koch brother’s most fevered nightmares. Not going to happen, the horse is out of the barn, etc.
I’ll hazard a guess that RCP 2.6 was included only for purposes of illustration of some kind, what might have been if only Al Gore had not been fat.