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Sea level in the 5th IPCC report

Filed under: — stefan @ 15 October 2013

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.


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.


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.

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.


  1. 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.
  2. R.D. Ray, and B.C. Douglas, "Experiments in reconstructing twentieth-century sea levels", Progress in Oceanography, vol. 91, pp. 496-515, 2011.
  3. M. Wenzel, and J. Schröter, "Reconstruction of regional mean sea level anomalies from tide gauges using neural networks", Journal of Geophysical Research, vol. 115, 2010.
  4. 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.
  5. S. Rahmstorf, M. Perrette, and M. Vermeer, "Testing the robustness of semi-empirical sea level projections", Climate Dynamics, vol. 39, pp. 861-875, 2011.
  6. S. Rahmstorf, "A Semi-Empirical Approach to Projecting Future Sea-Level Rise", Science, vol. 315, pp. 368-370, 2007.
  7. 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.

234 Responses to “Sea level in the 5th IPCC report”

  1. 201
    Lennart van der Linde says:

    ASLR #199,

    I’m trying to understand what Lempert et al 2012 do and do not say. On pp.18-19 I read:
    “Pfeffer et al. (2008) analyze kinematic constraints on the sea level rise contributions from landbased ice and derive lower and upper bounds of 785 and 2008 mm for sea level rise in the year 2100 and a “more plausible” estimate of about 800 mm. We introduce two adjustments to the Pfeffer et al. (2008) results because these previous results neglect uncertainties due to thermosteric sea level rise and the divergence between global mean and local sea level change. The lack of uncertainty assessment of about the thermosteric sea level rise component is addressed by adding an additional rise of -230 to + 200 mm. This uncertainty range is derived from a comparison of observed sea levels and an ensemble of runs from an Earth System Model of Intermediate Complexity, that includes a three-dimensional dynamic ocean general circulation model and samples key parametric uncertainties (Sriver et al. 2012). The local circulation effects are approximated with an additional rise of +/- 300 mm. This range is approximately the range of projected local sea level rise anomalies with respect to the global mean at the end of this century (Meehl et al. 2007). This range is also roughly consistent with the divergence of the simple parabolic fit to the local (PoLA) and global (Jevrejeva et al. 2006) observations extrapolated to the year 2100 (results not shown). These two adjustments yield a modified lower and upper bounds for the annual mean local sea level in 2100 of 255 mm to 2508 mm with a more plausible value of 950 mm (Figure 7).”

    So for global mean SLR by 2100 they take into account a small chance of a maximum of 2208 mm and for local mean SLR they think this maximum is 2508 mm, if I understand their figure 7a correctly. It seems they don’t think 3000 mm of global (or local) mean SLR by 2100 is possible. Or do I miss something?

  2. 202
    AbruptSLR says:

    Understandably, some individuals would like to have clear well defined probability density functions, PDFs, for SLR. However, I have previously stated that the current generation of models (Global/Regional/Local Circulation Models, GCMs, RCMs and LCMs) cannot yet adequately characterize the risk of the collapse of marine ice sheets and marine glacier (such as the Thwaites, other ASE glaciers, and indeed all WAIS marine glaciers), sufficiently to discount the risk of the partial collapse of significant portions of the West Antarctic Ice Sheet, with in one hundred years.

    If this were to happen, sea level could rise 2 to 3 meters by 2100; however, I cannot provide definitive proof, only increasing amounts of evidence that points in this direction. In this regard I will continue to provide evidence such as the following links related to the recent identification of an elaborate subglacial hydrologic system beneath the Thwaites Glacier:

  3. 203
    AbruptSLR says:


    Your are missing that in the first part of Figure 7, Lempert et al 2012 first extend Pfeffer et al 2008’s work (which does not include abrupt ice sheet collapse) to the situation; then in Figure 8 Lempert et al 2012 use statistics to include the risk of abrupt ice sheet collapse.


  4. 204
    AbruptSLR says:

    First, In my response to Lennart, after the word “situation” I had meant to insert the words “for the Port of Los Angeles”.

    Second, I provide the following references of climate sensitivity factors related to: (a) the possible future emissions of Soil Organic Carbon (SOC) into the atmosphere; and (b)reduced sulpher emissions into the atmosphere due to the acidification of the ocean. If both of these feedback factors occur with greater activity than assumed in AR5, then global warming should occur faster than projected:

    Nishina, K., Ito, A., Beerling, D. J., Cadule, P., Ciais, P., Clark, D. B., Falloon, P., Friend, A. D., Kahana, R., Kato, E., Keribin, R., Lucht, W., Lomas, M., Rademacher, T. T., Pavlick, R., Schaphoff, S., Vuichard, N., Warszawaski, L., and Yokohata, T.: Global soil organic carbon stock projection uncertainties relevant to sensitivity of global mean temperature and precipitation changes, Earth Syst. Dynam. Discuss., 4, 1035-1064, doi:10.5194/esdd-4-1035-2013, 2013.

    First signs of carbon sink saturation in European forest biomass; Gert-Jan Nabuurs, Marcus Lindner, Pieter J. Verkerk, Katja Gunia, Paola Deda, Roman Michalak & Giacomo Grassi; Nature Climate Change; Volume: 3, Pp:792–796; (2013); doi:10.1038/nclimate1853.

    Global warming amplified by reduced sulphur fluxes as a result of ocean acidification; Katharina D. Six, Silvia Kloster, Tatiana Ilyina, Stephen D. Archer, Kai Zhang & Ernst Maier-Reimer; Nature Climate Change; (2013); doi:10.1038/nclimate1981.

  5. 205
    Lennart van der Linde says:


    Could you clarify for me how to read figure 8 in Lempert et ak 2012? It’s about the probability of a certain rate of ASLR starting in a certain year, but how can we deduce a total global mean SLR by 2100 from that figure, if at all?

    I do see in their table 1 that they seem to assume a (deeply uncertain) possibility of ASLR of 30 mm/yr starting somewhere between 2010 and 2100. So can we infer from this that they assume a possibility of 2700 mm of SLR between 2010 and 2100?

    Or put differently: how do you conclude from their figure 8, or otherwise, that they assume a possibility of 3 m of SLR by 2100?

  6. 206
    AbruptSLR says:

    I imagine that only a few readers will be interested in the details of the mechanics of glaciers degrading; however, for those who are, the following references provide new insights into the stability of marine glaciers (as opposed to glaciers on land) and why they may be less stable than previously expected (which increases the risk of ASLR):

    Ice-shelf buttressing and the stability of marine ice sheets G. H. Gudmundsson; The Cryosphere, 7, 647–655, 2013;; doi:10.5194/tc-7-647-2013.

    Pattyn, F., and G. Durand (2013), Why marine ice sheet model predictions may diverge in estimating future sea level rise, Geophys. Res. Lett., 40, doi:10.1002/grl.50824.

    Lampkin, D. J., N. Amador, B. R. Parizek, K. Farness, and K. Jezek (2013), Drainage from water-filled crevasses along the margins of Jakobshavn Isbræ: A potential catalyst for catchment expansion, J. Geophys. Res. Earth Surf., 118, 795–813, doi:10.1002/jgrf.20039.

  7. 207
    AbruptSLR says:


    The text says the annual mean sea level for time index “t” is zt = a +bt + ct2 +c*I (t-t*) (eq. 4)

    Where the first three terms are the well understood SLR (let’s say here AR5 RCP 8.5 SLR projection at the 83% Confidence Level, CL, ie from ocean water thermal expansion and ice mass loss from small glaciers = 980mm), and the fourth term is the uncertain abrupt SLR (from ice mass loss from ice sheets); where the terms are defined in Table 1 (e.g. the rate of abrupt sea level rise is c*, the year the abrupt rise begin is t*).

    Per Section 3.3: “Note that the condition c* > 14mm/yr + 0.3mm/yr(t*-2010) implies a sea level rise contribution from poorly understood processes of about 1400mm in 2100.” Furthermore, the text says that Figure 8 implies that there is a 14% to 16% probability of this being the case (ie say about a 84% to a 86% Confidence Level, CL).

    Thus combing the AR5 “Well-Characterized Uncertainty” upper end of the “likely” range (ie 83% CL) of 980mm plus the Lempert et al 2012 “Deep Uncertainties” “likely” value of 1400mm gives a “likely mean ASLR value” of 2380mm (2.38m) by 2100. However, many SLR experts believe that the AR5 SLR projects are too scientifically conservative (including NOAA), therefore, I believe that a mean ASLR range of 2 to 3m is reasonable (83% CL) for a BAU case.


  8. 208
    Lennart van der Linde says:

    Thanks for the clarification, although I still find Lempert et al pretty hard to follow (probably due to my lack of scientific training).

    I think the AR5 SLR-value of 98 cm does contain some contribution (also dynamical?) from GIS and AIS, judging from figures 13.10 and 13.11 on pp.104-105 of chapter 13:

    Other than that, i agree with your risk assessment approach and think we cannot exclude a risk of 2-3m of SLR by 2100, based on what several experts have said on this.

  9. 209
    AbruptSLR says:


    Your comment about the fact that the AR5 SLR projections already include some SLR contributions from both the GIS and the AIS is very true, and raises the question as to what does “Well-Characterized Uncertainty” mean with regard to SLR contributions. Certainly the base PDFs for “Well-Characterized Uncertainty” SLR that Lampert et all 2012 used (by Pfeffer et al 2008 extended and by CO-CAT 2010) to determine the “Deep Uncertainty” SLR contribution, also included larger contributions from the GIS and the AIS than AR5 assumes. Thus by logical extension you are pointing out that my estimate of ASLR of between 2 to 3m by 2100 might be too low. Furthermore if you believe that the RCP 8.5 scenario is too low (because it does not include all of the methane emission sources, albedo flip, and other positive feedback factors that I mentioned), then it is not difficult to believe that this radiative forcing scenario could be increased by about 33%, which could increase the 95% CL mean global temperature increase from 6oC to 8oC by 2100.
    Thus if you both believe that the RCP forcing scenarios are 33% too low and that it is better to use Rahmstorf, Perrett and Vermeer (2011, see reference below)’s semi-empirical method for determining “Well-Characterized Uncertainty” SLR contributions (which also include contributions from the GIS and the AIS), then by 2100 one gets a 95% CL “Well-Characterized Uncertainty” SLR contribution of about 2.11m, to which the “Deep Uncertainty” contribution would need to be added.

    Rahmstorf, S., Perrett, M., and Vermeer, M. (2011), “Testing the robustness of semi-empirical sea level projections”, Clim Dyn, Springer-Verlag, doi: 10.1007/s00382-011-1226-7.

    Furthermore, if all of the “Deep Uncertainty” SLR contribution comes from the AIS then for example for POLA that contribution would need to by multiplied by about 1.4 to account for the regional fingerprint effect; which would give an abrupt mean RSLR by 2100 for POLA of about 4.2m for a roughly 85% to 95% CL as indicated in my reply #1 at the following link, where I provide a full PDF for RSLR for California for RCP 8.5 (with a modified temperature history), which I developed using a methodology roughly similar to that used for Lempert et al 2012, assuming that most of the “Deep Uncertainty” contribution to ASLR comes from the WAIS with a fingerprint factor of approximately 1.4 (for that portion of the contribution), and that the “Well-Characterized Uncertainty” contributions to SLR by 2100 come from Rahmstorf, Perrett and Vermeer, RRV (with a 33% modified RCP 8.5 scenario):,60.0.html

  10. 210
    AbruptSLR says:

    Obviously, when one discusses SLR values with “Deep Uncertainty” at the 95% Confidence Levels, combined together with an aggressive interpretation of “Well Characterized Uncertainty” SLR contributions (note that the CO-CAT PDF used by Lempert et al 2012, is based on Vermeer & Rahmstorf 2009’s semi-empirical SLR values), then the values become uncomfortably high. Furthermore, such high ASLR (which include “Deep Uncertainty” considerations for all ice sheets, not just the WAIS) values can also significantly increase short-term water levels such as those due to such design factors (see Tebaldi et al 2012, cited below) as: (a) tides; (b) storm surge, (c) storm tide, and (d) regional SLR considerations (such as prevailing winds and other regional affects). Also, in areas of high subsidence (such as many river delta areas), these values also need to be included in infrastructure design values. Thus if such values actually occur (were in my last post were based on statistical, as well as semi-empirical SLR, methodologies, which may not be correct) under a BAU scenario, greater than RCP 8.5; then it is clear that society will need to be exposed to risks at higher levels (lower confidence levels), than society is currently use to operating under. Alternately, ice sheet researchers should be given more research funds in order to dispel concerns that such high ASLR values might actually occur:

    Tebaldi, C., Strauss, B.H., and Zervas, C.E. (2012), “Modelling sea level rise impacts on storm surges along US coasts”, Environ. Res. Lett. 7 (2012) 014032 (11pp), doi:10.1088/1748-9326/7/1/014032.

  11. 211
    Dan H. says:

    Now I see where are getting your numbers. Assuming a temperature rise of 6-8°C would generate higher values. Since these numbers are well above the highest estimates for the next century, it follows that 2-3 m would not be seen for centuries also.

  12. 212
    AbruptSLR says:

    The following linked reference discusses the nature of Dragon-King events comparable to the high-end projections for ASLR:
    and pdf version with supplemental material:

    Predictability and Suppression of Extreme Events in a Chaotic System
    Hugo L. D. de S. Cavalcante, Marcos Oriá, Didier Sornette, Edward Ott, and Daniel J. Gauthier, Phys. Rev. Lett. 111, 198701 (2013), DOI: 10.1103/Physics.6.120

  13. 213
    AbruptSLR says:

    It is true that unless you look at the high end cases, then the risk of ASLR is negligible. That said society is currently exceeding the RCP 8.5 90% CL scenario, and whether we stay on that path or not is anybody’s guess. Nevertheless, according to the following linked reference following the RCP 8.5 66% CL and 90% CL pathways will result in mean global temperatures of about 6oC and 7.5oC by 2100, respectively. So my assumptions are not terribly unreasonable if society stays on its current BAU pathway:

    Rogelj, J., Meinshausen, M. and Knutti, R., (2012), “Global warming under old and new scenarios using IPCC climate sensitivity range estimates”, Nature Climate Change – Letters, doi: 10.1038/NCLIMATE1385

  14. 214
    perwis says:

    ASLR #203 (and Lennart #201)

    ASLR, you say that Lempert et al (2012) “first extend Pfeffer et al 2008′s work (which does not include abrupt ice sheet collapse) to the situation; then in Figure 8 Lempert et al 2012 use statistics to include the risk of abrupt ice sheet collapse.”

    However, Lempert et al (2012) do not add any additional component for the risk of abrupt ice sheet collapse, they only add more thermal expansion, based on results from Sriver et al (2012), thus adding ca 20 cm to the upper limit in Pfeffer et al (2008). In addition they also add local effects (ca 30 cm), resulting in the graph in Figure 7.

    Regarding, Pfeffer et al (2008) it can be argued that they include some form of abrupt ice sheet collapse in Antarctica, as they assume accelerated discharges from WAIS.

    It is interesting to note that Pfeffer et al (2008) is often used as some kind of “upper bound” for SLR planning, but strangely enough they only include 30 cm for the thermometric component, while there are much higher figures abound. For example, IPCC (2007) go up to ca 45 cm and Katsman et al (2008) has 48 cm and Sriver et al (2012) 55 cm.

    I have yet to see a real “worst case” scenario for SLR…

    Katsman, C. a., Hazeleger, W., Drijfhout, S. S., Oldenborgh, G. J., & Burgers, G. (2008). Climate scenarios of sea level rise for the northeast Atlantic Ocean: a study including the effects of ocean dynamics and gravity changes induced by ice melt. Climatic Change, 91(3-4), 351–374. doi:10.1007/s10584-008-9442-9
    Sriver, R. L., Urban, N. M., Olson, R., & Keller, K. (2012). Toward a physically plausible upper bound of sea-level rise projections. Climatic Change, 115(3-4), 893–902. doi:10.1007/s10584-012-0610-6

  15. 215
    sidd says:

    “I have yet to see a real “worst case” scenario for SLR”

    1)Eemian (Last InterGlacial) 6-20 m above present
    2)MWP1A, need I say more ?5m/century for a few centuries

  16. 216
    AbruptSLR says:

    In my post #213, I mistakenly state that Rogelj et al (2012) give values of 6oC and 7.5oC for the RCP 8.5 66%CL and 90%CL mean global temperature projections by 2100, respectively; when I should have said that Rogelj et al (2012) give values of 6oC and 7.5oC for the RCP 8.5 83%CL and 95%CL mean global temperature projections by 2100, respectively. However, if one were to assume that RCP 2.6 is no longer a valid scenario, then my original post is probably much closer to the truth than Rogelj et al (2012)’s values.

    Perwis (post #214),
    I agree that in their Figure 7, Lempert et al (2012)do not add any additional component for the risk of abrupt ice sheet collapse to Pfeffer et al (2008) values. What I was trying to say is that in Figure 8, Lempert et al (2012) present probabilities for the risk of 1.4m of sea level rise contribution by 2100 due to abrupt ice sheet collapse. Also, while many people would agree with you that Pfeffer et al (2008) “.. include some form of abrupt ice sheet collapse in Antarctica, as they assume accelerated discharge from WAIS”, I would call that accelerated discharge from the WAIS “rapid ice mass loss” and not abrupt ice mass loss associated with “Deep Uncertainty”.

  17. 217
    Lennart van der Linde says:

    Perwis #14,

    Good points. As I said in reply to ASLR earlier, I don’t really understand figure 8 of Lempert et al 2012, but based on the opinion of several experts I think we cannot exclude a risk of 2-3 meters of SLR by 2100.

    Can we exclude more than 3m by 2100? I don’t know, but even 1.5m by 2100 would imply the risk of about 3-5m of SLR from 2100-2200, and maybe even faster SLR after 2200. So even then 3m of SLR could by passed (well) before 2150.

    Could the rate of SLR become as fast, or faster, as during Meltwater Pulse 1A, when it was probably as high as 4-5 meter/century? And how long could such a speed be sustained?

    John Englander has posed this question to Jim Hansen in his book ‘High Tide on Main Street: Rising Sea Level and the Coming Coastal Crisis’:

    His question to Hansen was (p.100, second edition): ‘Could all the ice melt in a thousand years, or even hundreds, causing 212 feet [65m] or more of sea level rise?’

    Hansen replied: ‘The rapidity of the business as usual human-made climate forcing, burning all the fossil fuels in the next century or two, has no paleoclimate analog. With such a forcing, I would expect the time scale for demise of the great ice sheets would be measured in centuries, not millennia.’

    So in principle, according to Hansen, an average rate of SLR of (at least) 6-8 meter/century could be sustained for centuries to a millennium. For shorter periods up to 1 meter/decade would then seem a possibility.

    Can we exclude such a risk? If not, it seems we should take such a risk into account in discussing options for mitigation and adaptation.

  18. 218
    AbruptSLR says:


    I appreciate that the statistical methodology used by Lempert et al (2012) to estimate the “Deep Uncertainty” risk of equaling, or exceeding, 1.4m of SLR contribution to ASLR by 2100 can be difficult to understand, even though the authors are indeed experts in their fields. Furthermore, I appreciate both that: (a) extending the projection period to 2150 (from 2100) can result in higher confidence levels that the 1.4m of SLR contribution from the ice sheets will be equaled or exceeded; and (b) that looking to paleo-evidence (such as the Meltwater Pulse 1A SLR rise event) is a good alternate to using statistical methodology in order to try to bound the risk of ASLR. Following your lead, I would like to make the following selected points regarding use of the paleo-record to try to bound the risk of ASLR:

    (1) With regard to Hansen’s response that: ‘The rapidity of the business as usual human-made climate forcing, burning all the fossil fuels in the next century or two, has no paleoclimate analog. With such a forcing, I would expect the time scale for demise of the great ice sheets would be measured in centuries, not millennia.’: I would like to point out that:
    (a) our current BAU rate of increase in radiative forcing is about 100 times faster than any time in the past several hundred million years, including the rate during the PETM; and
    (b) during the Eemain peak, and/or the Holsteinian peak, the WAIS likely collapsed abruptly, and that current collapse forcing conditions, equal or exceed those extant during the Holsteinian peak, and/or the Eemain peak.

    (2) The NEEM community members (2013) confirmed that the WAIS contributed close to 3.8m of SLR during the Eemian; which is much more than the 1.4m of ASLR contribution that Lempert et al (2012) address, see: NEEM community members, (2013), Eemian interglacial reconstructed from a Greenland folded ice core, Nature, Volume: 493, Pages: 489–494, doi:10.1038/nature11789.

    (3) In regards to paleo-evidence of stronger positive feedback factors than those currently considered by “Well-Characterized Uncertainty” SLR projections:
    (a)Regarding evidence of strong polar amplification, Brigham-Grette et al 2013 state: “Consequently, the distinctly higher observed [temperature and precipitation] at MIS 11c cannot readily be explained by the local summer orbital forcing or GHG concentrations alone, and suggest that other processes and feedbacks contributed to the extraordinary warmth at this interglacial, and the relatively muted response to the strongest forcing at MIS 5e.” Note that MIS 5e roughly matches the Eemian period. See: Brigham-Grette, J., Melles, M., Minyuk, P., Andreev, A., Tarasov, P., DeConto, R., Koenig, S, Nowaczyk, N., Wennrich, V., Rosen, P., Haltia-Hovi, E., Cook, T., Gebhardt, T., Meyer-Jacob, C., Snyder, J., Herzschuh, U. Pliocene Warmth, Polar Amplification, and Stepped Pleistocene Cooling Recorded in NE Arctic Russia. Science. Online. DOI: 10.1126/science.1233137; and
    (b) I note that today on of the best examples of a strange attractor phenomenon (per Chaos Theory) that is not fully represented in current GCMs is the ENSO; while the following reference by White et al (2002) indicates that there is a positive feedback between the Antarctic Circumpolar Wave and the global El Nino-Southern Oscillation Wave; which provide historical evidence that non-linear atmospheric/oceanic interactions can further amplify the rate of future SLR: White, W. B., S.-C. Chen, R. J. Allan, and R. C. Stone, “Positive feedbacks between the Antarctic Circumpolar Wave and the global El Niño–Southern Oscillation Wave”, J. Geophys. Res., 107(C10), 3165, doi:10.1029/2000JC000581, 2002.

    (4) The WAIS Project Members (2013, see following reference) find that data from the “WAIS Divide Core” hole showed abrupt climate change that occurred in the past on timescales of decades (circa 20,000 years ago), and that these abrupt climate changes accelerated the de-glaciation of the WAIS likely due to interactions between the Southern Ocean and the ice sheet: WAIS Project Members, (2013), Onset of deglacial warming in West Antarctica driven by local orbital forcing, Nature; doi:10.1038/nature12376.

  19. 219
    perwis says:

    sidd #215

    Sure, but I was thinking of published scenarios that can be taken seriously by planners. Published scenarios are very important, as the examples of Pfeffer et al (2008) or Vermeer and Rahmstorf (2009) shows, being used all over the world. But neither of these really try to assess worst-case scenarios.

    Your examples are also out there, but are less used as pure paleo-analogs are not really true analogs, because of the current forcing is different from previous forcings (Eeemian) or because the ice sheets are different (MWP1A). (A striking difference is the UK:s H++ scenario of 2.5 m SLR by 2100 (see Lowe et al 2009), which is based on Rohling et al (2008) assessment of rapid SLR during the Eeemian high stand from proxies from the Red sea )

  20. 220
    perwis says:

    Lennart #217,

    It is harder to exclude impossible scenarios than to say what is possible.

    The philosopher Gregor Betz has a nice discussion of this, where he says that:

    “Another illustrative case are the sea level rise scenarios as reported in the well known figure of the TAR. These scenarios did not contain the possible contribution from the melting Greenland and Antarctic ice sheets (though the IPCC text mentioned these separately). Is this reconcilable with the scenario approach? Admittedly, the dynamics of the ice caps were – and still are – little understood. Yet taking the above implementation serious, lack of knowledge is not a reason to consider some scenario as impossible. In contrast, independent scientific arguments are needed to do so. Therefore, worst case scenarios of Greenland and Antarctic ice melt should be integrated into the sea level rise scenarios because of our lack of understanding.” (Betz 2007, p 7)

    There is a strong bias in the literature against worst-case scenarios, and this is a very unfortunate situation, which I think will lead to many, many decisions all over the world, based on this biased information.

    Betz, G. (2007). Probabilities in climate policy advice: a critical comment. Climatic Change, 85(1-2), 1–9. doi:10.1007/s10584-007-9313-9

  21. 221
    AbruptSLR says:

    Perwis #220:
    Further to your statement: ” There is a strong bias in the literature against worst-case scenarios, and this is a very unfortunate situation, which I think will lead to many, many decisions all over the world, based on this biased information.”, I provide the following additional supporting considerations:
    First, I reference Stein & Geller (2012, see link, citation and abstract below) about the challenges of communicating uncertainties in natural hazard forecasts (including ASLR):

    Stein, S. and R. J.Geller, (2012), “Communicating uncertainties in natural hazard forecasts”, Eos Trans. AGU, 93(38), 361.

    “Natural hazards research seeks to help society develop strategies that appropriately balance risks and mitigation costs in addressing potential imminent threats and possible longer-term hazards. However, because scientists have only limited knowledge of the future, they must also communicate the uncertainties in what they know about the hazards. How to do so has been the subject of extensive recent discussion [Sarewitz et al., 2000; Oreskes, 2000; Pilkey and Pilkey-Jarvis, 2006]. One approach is General Colin Powell’s charge to intelligence officers [Powell, 2012]: “Tell me what you know. Tell me what you don’t know. Then tell me what you think. Always distinguish which is which.” In dealing with natural hazards, the last point can be modified to “which is which and why.” To illustrate this approach, it is helpful to consider some successful and unsuccessful examples [Stein, 2010; Stein et al., 2012].”

    From Stein & Geller (2012), I extract the following key passages:

    “One major challenge is that real uncertainties often turn out to have been underestimated. In many applications, 20%-45% of results are surprises, falling outside the previously assumed 98% confidence limits [Hammitt and Shyakhter, 1999]. …. This effect arise in predicting river floods [Merz, 2012] and earthquake ground motions and may arise for the IPCC uncertainty estimates [Curry, 2011].”
    “The Intergovernmental Panel on Climate Change (IPCC) [2007] report compares the predictions of 18 models for the expected rise in global temperature. … The report further notes that the models “cannot sample the full range of possible warming, in particular, because they do not include uncertainties in the carbon cycle.”

    Finally, I present one simple example of how poor communications can mask some of the actual risk of ASLR:

    The drainage basins determined by researchers around Antarctica are based on the areas that would drain out of a given gateway given the current ice surface gradients. One misconception worth discussing is that most researchers report the potential maximum SLR contribution from each of these basins as if the current ice surface gradients will be maintained into the future; which is not the case. For example it is frequently reported that the maximum SLR contribution from PIG and Thwaites Glacier are approximately: 9″ and 18″, respectively, and also many researchers project that ice mass loss from PIG may slow sufficiently in the next 5 to 10 years to limit the SLR contribution from PIG this century to an inch or two; however, as the Thwaites basin adjoins the PIG basin, should the Thwaites Glacier collapse as I have indicated may be possible; then it is possible that several inches of SLR of ice in the PIG basin could drain through the Thwaites Gateway. Such interactive logistics increase the likelihood that higher levels of SLR will occur by the end of this century; above that commonly thought likely.

  22. 222
    AbruptSLR says:

    perwis #219

    You state:
    “sidd #215
    Sure, but I was thinking of published scenarios that can be taken seriously by planners.”

    First, I would like to say that it appears to me that you (and probably a lot of other readers) are confused by Lempert et al (2012)’s analysis, as first Lempert works for the RAND Corp. and RAND is a very serious consultant to planners such as the Port of Los Angeles, POLA. Therefore, let me try to clarify, how this report does represent an example of what you are asking for, and I will do so using only the Extended Scenarion Pfeffer et al (2008) Beta PDF (which means the “Well-Characterized” SLR contribution corrected for POLA’s conditions) shown in Figure 7 and the probability of equaling or exceeding 1.4m (this value is selected as lower values do not cause POLA to make an investment) of abrupt sea level rise contribution shown in Figure 8 (of that reference for POLA), for a 86% confidence level, CL, case:

    (1) Graphically, the Beta PDF from Figure 7 gives a 86% CL “Well-Characterized” SLR contribution of about 1.8m by 2100.
    (2) Figure 8 gives a 14% probability (or 86% CL) that the “Deep Uncertainty” SLR contribution by 2100 will equal or exceed 1.4m.
    (3) Combining these two 86% CL SLR contribution values (using only extended Pfeffer et al 2008 SLR assumptions, which you note is widely cited by planners), one gets a value of about 3.2m of SLR by 2100, which planners could take seriously if they so decided, if they chose to take responsibility for accepting the methodology used by Lempert et al 2012 to extend Pfeffer et al 2008’s SLR projections.

    Typically, planners only choose to take seriously GCM SLR projections (such as by CMIP5), without making any additional corrections for “Deep Uncertainty” SLR contributions. Also, typically planners choose to ignore high-end SLR estimates using Bayesian approaches such as that given by Hoffman et al (1983, see reference below), which give an estimate of the High-End SLR estimate of 3.68m by 2100:

    Hoffman, J. S., D. Keyes, and J. G. Titus. 1983. Projecting Future Sea Level Rise; Methodology, Estimates to the Year 2100, and Research Needs. Washington D.C.: U.S. Environmental Protection Agency. 121 pp.

    Furthermore, Hoffman et al (1983)’s estimates are included and compared with other early estimates (before planners choose to become limited by GCM SLR projections) in NRC 1987 (see following reference):

    Committee on Engineering Implications of Changes in Relative Mean Sea Level, Marine Board, National Research Council, (1987), Responding to Changes in Sea Level: Engineering Implications, ISBN: 0-309-59575-4, 160 pages, 6 x 9

    The NRC 1987 PDF is available from the National Academies Press at:

    Finally, to put this in prospective I provide the following table of potential maximum SLR contributions from ice melting (to which you would need to add steric SLR contributions):

    Location Potential SLR (m)
    EAIS: 64.80
    WAIS: 8.06
    Ant. Pen.: 0.46
    Greenland: 6.55
    All other ice: 0.45
    Total: 80.32m (263.5ft)

  23. 223
    Lennart van der Linde says:

    Perwis, ASLR,

    During the last deglaciation increasing insolation in the Northern Hemisphere from about 20.000 years ago caused ice to melt, which lowered albedo and increased temperatures, which released GHG’s and melted more ice, which caused even higher temperatures, and so on, until insolation decreased again at the beginning of the Holocene about 10.000 years ago, which caused warming to stop and cooling to start.

    Roughly we can say that in about 10.000 years the planet warmed about 5 degrees C, CO2-levels rose about 100 ppm and sea level rose about 100 meters. On average temperature rose about 0.05 degrees/century, CO2-levels rose about 1 ppm/century, and sea level rose about 1 meter/century. Over this period lower albedo probably caused about the same radiative forcing as higher CO2/GHG levels.

    Today CO2-levels are almost 100 ppm higher than a century ago, rising at roughly 2 ppm/year, temperature is about 0.7 degrees higher than a century ago, rising at about than 0.15 degrees/decade, and sea level is about 20 cm higher than a century ago, rising at about 3-4 mm/yr. Albedo is decreasing.

    At this moment CO2 seems to be rising about 200 times faster than 10.000-20.000 years ago, temperature seems to be rising about 30 times faster, and sea level seems to be rising three times slower, so far. Albedo is decreasing, but it’s not clear to me how fast compared to this earlier period.

    CO2 rise, temperature rise, sea level rise and albedo decrease are probably all accelerating. The question is: will they accelerate in even proportion to each other, and what are the limits of this acceleration?

    Let’s assume that the current or near future climate forcing is indeed about 100 times as strong as during the last deglaciation. Will temperature and sea level rise then be about 100 times as fast as back then?

    If temperature rises about 5 degrees this century that would indeed be a 100 times faster rise than the 5 degrees rise from roughly 20.000-10.000 years ago. Could sea level then also rise 100 times faster than during this earlier period?

    I’ve not heard anyone suggest 100 meter of SLR would be possible in one century (apart from the fact there’s not enough ice for such a rise). But as quoted in my earlier comment: Jim Hansen thinks that 60-70 meters of SLR is possible, or even likely, in less than a millennium, under BAU forcing.

    That implies he thinks the average rate of SLR over the coming centuries could be up to about 10 times faster than during the last deglaciation, and about double the speed of Meltwater Pulse 1A.

    Does anyone here have reason to believe that Hansen’s estimate is too high, or too low, and if so, why?

  24. 224
    AbruptSLR says:

    To be honest, I have never thought about SLR beyond 2200 before; however, the following linked reference gives an Extended RCP 8.5 95% CL SLR value of about 11m by 2500 using relatively “Well-Characterized Uncertainty” (ie semi-empirical) methodology:

    Sea level projections to AD2500 with a new generation of climate change scenarios
    S. Jevrejeva, J.C. Moore, and A. Grinsted, Global and Planetary Change 80–81 (2012) 14–20.

    Regarding the “Deep Uncertainty” contribution to SLR, I imagine (based on such information as provided in the following links) that if we stay on a BAU pathway that sometime between 2100 and 2300 the Earth’s atmospheric circulation patterns would change to that of an equable climate, where heat from the equatorial regions are transported directly through the atmosphere to the polar regions (which then need not freeze even in winter), and if so then Hansen’s estimated SLR by/before the year 3000 seems reasonable to me:

    Department of Mathematics and Statistics; University of Guelph, Canada; Presented to the BioM&S Symposium on Climate Change and Ecology; University of Guelph; April 28, 2011

    Can Planetary Wave Dynamics Explain Equable Climates?By: Sukyoung Lee, Steven Feldstein, David Pollard, and Tim White; May 3, 2010.

    Persistent near-tropical warmth on the Antarctic continent during the early Eocene epoch, by: Jörg Pross, Lineth Contreras, Peter K. Bijl, David R. Greenwood, Steven M. Bohaty, Stefan Schouten, James A. Bendle, Ursula Röhl, Lisa Tauxe, J. Ian Raine, Claire E. Huck, Tina van de Flierdt, Stewart S. R. Jamieson, Catherine E. Stickley, Bas van de Schootbrugge, Carlota Escutia, Henk Brinkhuis; Nature; 488,73–77 (02 August 2012); doi:10.1038/nature11300.

    The Early Eocene equable climate problem: Can perturbations of climate model parameters identify possible solutions?, by: by Navjit Sagoo, Paul Valdes, Rachel Flecker, and Lauren Gregoire; Royal Society Philosophical Transactions A; 2013.

  25. 225
    sidd says:

    As far as I understand: Hansen argues that previous transgressions have been paced by the timescale of Milankovitch forcing and not by internal timescale of icesheet response, which could be smaller. As we see today under, geologically speaking, instantaneous, compared to Milankovitch, forcing, the internal icesheet timescale is indeed smaller.

    What it the upper bound on the internal icesheet timescale, how long does it stay around? ANDRILL indicates a millenium or so. That in itself, as Mr. van der Linde points out, is bad enuf.

    What is the lower bound, how fast can it go away ? That answer is worth a great deal of money …
    Hansen says a few hundred years. I tend to believe him, since we see already how fast the icesheets are responding.

    The recent results on sediment cores putatively dated to the PETM, indicating eyeblink fast geological change, is the only paleo example i am aware of that compares to the speed of human forcings. No large icesheet then, of course, too warm already. But there was a great dying then, too.


  26. 226
    AbruptSLR says:

    In addition to our “Deep Uncertainties” about ice sheet instability and climate sensitivity, most current SLR projections with “Well-Characterized Uncertainties” rely on the radiative forcing scenarios prescribed by the Recommended Concentration Pathway, RCP, scenarios. Unfortunately,there is considerable “Deep Uncertainty” about the probable future radiative forcing pathway, which are thus not captured by these “Well-Characterized Uncertainty” SLR projections, as partially indicated by the findings of the following two recent reports:

    A bi-annual report by the French Institute of Demographic Studies (INED, released in October 2013) projected that the world’s population will rise to 9.7 billion in 2050 from the current level of 7.1 billion and that India will overtake China as the world’s most populous nation. According to the report, the projected population of other countries in the world by 2050 (in millions) will be: Nigeria (444), US (400), Indonesia (366), Pakistan (363), Brazil (227), Bangladesh (202), Congo (182), Ethiopia (178), Philippines (152), Mexico (150), Russia (132), Tanzania (129), Egypt (126), Uganda (114), Vietnam (109), Iran (99), Japan (97), Kenya (97), Turkey (93), Iraq (83), UK (79), Germany (76), France (72), Sudan(69), Niger(66), South Africa (64), Mozambique (63) and Colombia (63). Due to the lag-time in the trend line for SLR, if all of these 9.7 billion people are not on their personal best behavior in 2050, then it is very likely that the 2100 SLR will exceed that projected for the RCP 8.5 95% CL radiative forcing scenario.

    Unfortunately, the RCP 8.5 scenario does not correctly account for GHG emissions (and thus does not exhibit sufficient radiative forcing) from the likely degradation of the polar permafrost, while the rate and mix of carbon dioxide and methane will be controlled by the moisture content of the thawed soil, according to the following reference (see following link, citation and abstract):

    Long-term CO2 production following permafrost thaw, Bo Elberling, Anders Michelsen, Christina Schädel, Edward A. G. Schuur, Hanne H. Christiansen, Louise Berg, Mikkel P. Tamstorf & Charlotte Sigsgaard, (2013) Nature Climate Change, 3,890–894doi:10.1038/nclimate1955.

    “Thawing permafrost represents a poorly understood feedback mechanism of climate change in the Arctic, but with a potential impact owing to stored carbon being mobilized. We have quantified the long-term loss of carbon ( C ) from thawing permafrost in Northeast Greenland from 1996 to 2008 by combining repeated sediment sampling to assess changes in C stock and >12 years of CO2 production in incubated permafrost samples. Field observations show that the active-layer thickness has increased by >1 cm yr−1 but thawing has not resulted in a detectable decline in C stocks. Laboratory mineralization rates at 5 °C resulted in a C loss between 9 and 75%, depending on drainage, highlighting the potential of fast mobilization of permafrost C under aerobic conditions, but also that C at near-saturated conditions may remain largely immobilized over decades. This is confirmed by a three-pool C dynamics model that projects a potential C loss between 13 and 77% for 50 years of incubation at 5 °C.”

  27. 227
    AbruptSLR says:

    To follow-up on Sidd’s point that we now know (and we continue to learn) a lot more about how fast marine glacier can loss ice mass that contributes to SLR, I provide a few specific examples of recent research that illustrates just how fast that ice mass loss can occur:

    The first reference by Fudge et al 2013 (see abstract below) indicates that the measured basal ice melt rate beneath the Thwaites drainage basin is at least three times faster than researchers had previously imagined (which both results in direct ice mass loss by drainage and also faster ice calving due to a reduction in basal friction due to the lubricating effect of basal water):

    “High Basal Melt at the WAIS-Divide ice-core site, by T.J. Fudge, Gary Clow, Howard Conway, Kurt Cuffey, Michelle Koutnik, Tom Neumann, Kendrick Taylor, and Ed Waddington, 2013:

    We use the depth-age relationship and borehole temperature profile from the WAIS-Divide ice core site to determine the basal melt rate and corresponding geothermal flux. The drilling of the WAIS-Divide ice core has been completed to 3400 m depth, about 60 m above the bed. The age of the deepest ice is 62 ka, younger than anticipated, with relatively thick annual layers of ~1 cm. The borehole temperature profile shows a large temperature gradient in the deep ice. We infer a basal melt rate of 1.5 (±0.5) cm yr-1 using a 1-D ice flow model constrained by these data sets.
    The melt rate implies a geothermal flux of ~230 mW m-2, three times the measured value of 70mW m-2 at Siple Dome. We compile radio-echo sounding data sets to assess the spatial extent of high melt. Deep internal layers are the most useful for inferring spatial patterns of basal melt. Unfortunately, the IceBridge WAIS-core flight and two site-selection surveys did not image consistent reflectors deeper than Old Faithful (2420 m and 17.8 ka). A ground-based survey by CReSIS (Laird et al., 2010) was able to image consistent layers as deep as 3000 m, but the survey is not oriented along the ice-flow direction making interpretation more difficult. There is no obvious draw down of deep internal layers that would indicate an area of localized melt. While this suggests a uniform melt rate within the survey, it might also indicate that other factors (e.g. accumulation gradients, rough bed topography) obscure the influence of basal melt on the internal layer depths.”

    For other publications related to the WAIS-Divide ice core project, see the following links:

    The following article indicates how surface water drainage from water-filled crevasses along the margin of the Jakobshaven glacier could result in the dynamic acceleration of ice mass loss from this marine glacier, and which indicates that the Thwaites glacier (and all other marine glaciers) will likely be subject to the same ice mass loss mechanism in the future:

    Lampkin, D. J., N. Amador, B. R. Parizek, K. Farness, and K. Jezek (2013), Drainage from water-filled crevasses along the margins of Jakobshavn Isbræ: A potential catalyst for catchment expansion, J. Geophys. Res. Earth Surf., 118, 795–813, doi:10.1002/jgrf.20039.

    “Saturated crevasses occur in local depressions within the shear margins of Jakobshavn Isbræ at inflections in the ice stream’s flow direction. Spatio-temporal variability of seven distinctive saturated crevasse groups was examined during the 2007 melt season. The area of saturated crevasses reached its maximum extent, ~1.8 km2, in early July, and remained largely constant until early August. Filling rates are correlated with regional melt production, while drainage rates are highly correlated with areal extent. Estimates on potential drainage volume from the largest crevasse system are ~9.23 × 10−3 km3 ± 2.15 × 10−8 km3 and ~ 4.92 × 10−2 km3 ± 3.58 × 10−8 km3, respectively, over a 16 day interval and are more than required for a distributed basal hydrologic system across this area to temporarily flood bedrock obstacles believed to control basal sliding. Future drainage events, likely extending farther inland with warming, could result in enhanced lateral mass discharge into the ice stream, with implications for the dynamic evolution of the entire basin.”

    Lastly for this post, the linked reference has a free pdf, and I agree with Drouet et al that:

    “Despite the recent important improvements of marine ice-sheet models in their ability to compute steady state configurations, our results question the capacity of these models to compute short-term reliable sea-level rise projections.”

    Drouet, A. S., Docquier, D., Durand, G., Hindmarsh, R., Pattyn, F., Gagliardini, O., and Zwinger, T.: Grounding line transient response in marine ice sheet models, The Cryosphere, 7, 395-406, doi:10.5194/tc-7-395-2013, 2013.

    “Abstract. Marine ice-sheet stability is mostly controlled by the dynamics of the grounding line, i.e. the junction between the grounded ice sheet and the floating ice shelf. Grounding line migration has been investigated within the framework of MISMIP (Marine Ice Sheet Model Intercomparison Project), which mainly aimed at investigating steady state solutions. Here we focus on transient behaviour, executing short-term simulations (200 yr) of a steady ice sheet perturbed by the release of the buttressing restraint exerted by the ice shelf on the grounded ice upstream. The transient grounding line behaviour of four different flowline ice-sheet models has been compared. The models differ in the physics implemented (full Stokes and shallow shelf approximation), the numerical approach, as well as the grounding line treatment. Their overall response to the loss of buttressing is found to be broadly consistent in terms of grounding line position, rate of surface elevation change and surface velocity. However, still small differences appear for these latter variables, and they can lead to large discrepancies (> 100%) observed in terms of ice sheet contribution to sea level when cumulated over time. Despite the recent important improvements of marine ice-sheet models in their ability to compute steady state configurations, our results question the capacity of these models to compute short-term reliable sea-level rise projections.”

  28. 228
    Michael says:

    I recall posting a question on this board a few years ago asking whether the collapse of the Larsen B ice shelf came as a surprise to the experts. The responses indicated that the Larsen B event took the experts by surprise.

    Little hope perhaps that the concept of abrupt SLR may be viewed with the importance it deserves, because a consensus-driven agreement process would never admit to such processes.

    The concept is, in my view, very important because we have created nuclear infrastructure and systems at current sea level at coastlines whose life times are comparable to the timescales of SLR-induced impact. We have already witnessed ocean-mediated impact on one installation (Fukishima) and that is already stretching industrial capability to maintain semblances of safety in the Asia-Pacific and other regions.

    The topic of abrupt SLR goes to the heart of energy policy for a safe planet.

  29. 229
    AbruptSLR says:

    As a follow-up to my post about Drouet et al 2013’s Marine Ice Sheet Model Intercomparison Project’s finding that even for a relatively simple perturbation as the removal of buttressing support of an ice shelf on an adjoining marine glacier (for example consider the case of the SW Tributary marine glacier discussed by MacGregor et al 2013, cited in my post #172, where the next major calving of the PIIS could accelerate the ice flow velocities of the SE Tributary glacier to the point of destabilizing the eastern shear margin of the Thwaites Glacier), currently cannot be modeled, over the short-term, to within a discrepancy of 100% between different current marine glacier models. To this simple example presented by Drouet et al (2013), I would like to note that there are very many other ways to destabilize ASE sector marine glaciers (including the Thwaites, and Pine Island, Glaciers), including (but not limited to):

    (1) Increased advection of increasingly warmer CDW (causing accelerated grounding line retreat) due to such factors as: (i) increased El Nino frequency and intensity both with increasing global warming and the coming end of the current El Nino hiatus period; (ii) increased local upwelling and increased volume of CDW (circumpolar deep water) both due to increasing global warming; and (iii) increased saline pumping action due to increasing volume of ice mass loss.

    (2) Increasing formation of shear zone crevasses near the calving front of the marine glaciers [see Lampkin et al (2013) in my previous post], which may, or may not, fill with surface melt water depending on weather conditions (note that surface ice melt does current occur in the coastal region of the ASE during the peak of some austral summers, and that as the WAIS has one of the fastest rates of surface warming on Earth, it is safe to assume that the frequency of surface melting in this area will increase at a non-linear rate through the end of this century); which will clearly accelerate the rate of calving in these marine glaciers.

    (3) The basal friction in the Thwaites Glacier gateway region is believed to be one of the key factors currently limiting the further acceleration of ice mass loss from Thwaites Glacier; however, as: (i) the distribution of basal friction in the Thwaites Glacier gateway is not well known, this factor believed to limit ice flow acceleration may not be as effective in the future as is currently believed to be the case; (ii) as the ice in the gateway progressively thins it will eventually float over the top of the regions of relatively high basal friction, so eventually this limitation will be removed; and (iii) as the ice flow velocity increases the associated increase in basal melt water (due to increased melting of the glacial ice by internal and basal friction) will serve to lubricate and warm the basal ice thus reducing the ability of the basal friction to restrain further ice acceleration.

    (4) There is a prominent trough located on the western side of the Thwaites gateway (adjoining the Thwaites Ice Tongue, see following Tinto & Bell 2011 reference) that can serve to guide warm CDW directly into the heart of the gateway, once the ice in the trough has thinned sufficiently to maintain a subglacial cavity/void in the trough, which could then accelerate grounding line retreat in this area in a 2D (as opposed to the current 1D) manner. Tinto, K. J. and R. E. Bell (2011), “Progressive unpinning of Thwaites Glacier from newly identified offshore ridge – constraints from aerogravity”, Geophys. Res. Lett., doi:10.1029/2011GL049026.

    (5) Future changes in local coastal wind and associated local ocean current patterns due to global warming, could acceleration the advection of warm CDW into the ASE, thus inducing accelerated groundling line retreat for the affected marine glaciers.

    (6) Increases in tides/storm surge/storm tide and in local/regional SLR in the ASE due to accelerated ice mass loss from Greenland; should accelerate calving of the fronts of the ASE marine glaciers.

    (7) Parallel effect to Thwaites to the Jakobshavn Effect (see Habermann et al 2013, Walter et al 2012, and Van De Veen et al 2011) as soon as the Thwaites groundling line retreats to the lip of the Byrd Subglacial Basin, BSB. (i) Changing basal conditions during the speed-up of Jakobshavn Isbræ, Greenland, M. Habermann, M. Truffer, and D. Maxwell, The Cryosphere Discuss., 7, 2153–2190, 2013,, doi:10.5194/tcd-7-2153-2013; (ii) Oceanic mechanical forcing of a marine-terminating Greenland glacier, Jacob I. WALTER, Jason E. BOX, Slawek TULACZYK, Emily E. BRODSKY, Ian M. HOWAT, Yushin AHN, and Abel BROWN; Annals of Glaciology 53(60) 2012 doi: 10.3189/2012AoG60A083; (iii) Van Der Veen, C. J., Plummer, J., and Stearns, L.: Controls on the recent speed-up of Jakobshavn Isbræ, West Greenland, J. Glaciol., 57, 770–782, 2011. 2155, d o i : 10.3189/002214311797409776.

  30. 230

    Would another factor in the acceleration in the break down of the West Antarctic Ice Sheet be simply the ongoing rise in sea level from other sources. Then the rise in sea level from the West Antarctic ice shelf itself will accelerate its own break down.

  31. 231

    You mention that the rise in sea level could threaten island nations. This presumably is referring to coral atoll islands. While sea warming and acidification could certainly destroy atolls by killing corals, is it not true that sea level rise by itself is unlikely to do so and in this respect, the health of the coral atolls, short of warming or acidification, is in the hands of the local inhabitants.

  32. 232
    perwis says:

    AbruptSLR #222:

    I am sorry, but I think it is you that are confused by the Lempert et al (2012) study.

    This is how I understand their methodology:

    1. They create a model (p 7) of future annual mean sea level z(t)=a+bt+ct^2+c*(t-t*), where the term a is the sea level anomaly at time zero (2011), b is a constant rate (mm/year), and c is an acceleration term (mm/year2), c* is the rate of abrupt sea level rise and t* is the year abrupt rise begins.

    The first three terms represent “the effects of relatively well-understood processes, such as thermal expansion of the oceans due to rising temperatures and the melting of small glaciers” (p 7), while the fourth term “represents currently poorly understood and poorly constrained processes; for example, potentially abrupt changes in the dynamics of ice flow (e.g., Alley et al. 2007), which is approximate with a step- function increase in the rate of sea level rise c* (mm/year) that occurs after some time t*.” (p 7)

    2. They construct two “extended scenarios” of local SLR in year 2100 (one is based on Pfeffer et al 2008 and one is based on CO-CAT 2010). These are represented as probability density functions in Figure 7. (The reason the extended scenario that is based on Pfeffer et al 2008 is ca 500 mm higher than Pfeffer et al 2008 is that they introduce uncertainty in the thermosteric component of -230 to +200 mm (Pfeffer has no uncertainty in that) and uncertainty from local circulations effects of -300 to +300 mm. They say that the resulting scenarios “range is also roughly consistent with the divergence of the simple parabolic fit to the local (PoLA) and global (Jevrejeva et al. 2006) observations extrapolated to the year 2100 (results not shown).” (p 19).

    3. They make a “rejection sampling approach to approximate or emulate the resulting expert assessment for the projected sea level rise in the year 2100” (p 19). I am not sure how they do this exactly (elsewhere they talk about a “quasi-random Latin Hypercube sample” p 23).

    Anyhow, the end result is a ” joint distributions for c* and t*” (Figure 7 right pane) as well as a “a joint distribution for the parameters a, b, and c, which is largely uncorrelated with that for c* and t*, and thus consistent with that used in Section 3.1.1.” (note on page 19).

    4. We get an indication of what the results of c* and t* are by looking at Figure 8, which I take show all the different variations of the c* and t* parameters that are consistent with the two extended scenarios. The colored lines show the decision-relevant conditions they are interested in.

    We don’t know what the other parameters (a,b,c) are, only that combinations of the parameters c* and t* on the red line means that the (a,b,c,) parameters are consistent with Section 3.1.1 and that this means “roughly 500 mm contribution from well-understood processes” (p 17) and ca 1400 mm of abrupt SLR by 2100 (coming from combinations of c* and t* at the red line).

    To conclude: if I am correct in my interpretation of their paper, they do not derive a scenario of 3.2 m GMSLR by 2100, as AbruptSLR says above. Instead they assume the two “extended scenarios” for local SLR shown in Figure 7 (suggesting a global mean SLR of 2.2 m and 1.7 m respectively), which they then use to deduce the policy-relevant parameters for abrupt sea level change (c* and t*).

  33. 233

    #231–A more sensible link than I expected, but I don’t think that we can assume the conclusion is correct. There is a severe potential issue with rate of change.

    It’s also a good example of the probable synergy between climate change impacts and other anthropogenic ones.

  34. 234
    AbruptSLR says:

    William #230,

    Due to the gravitational “fingerprint” effect, when the WAIS loses ice mass, the local sea level drops; however, currently the GIS is losing ice mass faster than the AIS so the sea level around Antartica is currently neither going up or down (but this will likely change in the future).

    perwis #232,

    I have e-mailed one of the co-authors of the POLA SLR study, to see whether he is willing to resolve the correct intrepretation of their study.