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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. 51
    Hank Roberts says:

    > Alex … LIA represents a roughly
    > one degree C increase in global temperature

    According to whom, Alex? Why do you believe this? What source are you relying on for the, er, global statement?

    I think I found the one recent paper suggesting that. It’s a claim based on a record from one site. Just one location.

    It’s been made much of at a few blogs arguing that changing temperature doesn’t affect sea level. That’s not what it actually says. But have you looked beyond that one paper? Morano’s really not a good source, if he’s where you’re getting this idea. If not, where?

    Rate of change matters.

    Tell us if this is your own conclusion from logic, or something you read — and if so, where — and we* might be able to help find more useful information.
    *we being frequent amateur readers like me.
    Failing that, if you give some citation to what you’re relying on, the real scientists here will be able to discuss it.

  2. 52
    perwis says:

    Martin Manning and Stefan Rahmstorf #30:

    Stefan, you say in your post that “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%, ”

    Martin, you say that “while for planning purposes that raises the question as to whether more of the residual 34% of the full range is on the high side or low side of the ‘likely’ range, this section does say that coastal planning needs to be considered in a risk management framework.”

    We have at least two different interperations here on how to interpret the IPCC “likely” range here? Which is to correct interpretation? Martin’s way or Stefan’s way?

    The IPCC is not very clear on this. All that is stated int the SPM and the IPCC Uncertainty Guidance Note (Mastrandrea et al 2010), is that “Likely” means “66-100% probability”.

    What speaks for Martin’s interpretation is that the lower bound is much more uncertain. The IPCC Sea level chapter authors says that “The time-mean rate of GMSL rise during the 21st century is very likely to exceed the rate of 2.0 [1.7–2.3] mm yr–1 observed during 1971–2010” (p 13-53), which suggests that the lower bound is not so uncertain, while they say that “there is currently insufficient evidence to evaluate the probability of specific levels above the likely range.”

    What speaks for Stefan’s interpretation is that this is that the IPCC authors may already have taken this into consideration when assigning the “likely” interval. I have not yet found any specific mention of this in the report, other than the following from Chapter 1:
    “In the WGI contribution to the AR5, uncertainty is quantified using 90% uncertainty intervals unless otherwise stated. The 90% uncertainty interval, reported in square brackets, is expected to have a 90% likelihood of covering the value that is being estimated. The value that is being estimated has a 5% likelihood of exceeding the upper endpoint of the uncertainty interval, and the value has a 5% likelihood of being less than that the lower endpoint of the uncertainty interval. A best estimate of that value is also given where available. Uncertainty intervals are not necessarily symmetric about the corresponding best estimate.”

    Does anyone know for sure how the uncertainty intervalls should be interpreted? Have I missed something?

  3. 53
    Hank Roberts says:

    Also for Alex: if you’re relying on Asafoku at Pielke Sr., cited to Asafoku’s paper “On the recovery from the Little Ice Age” in Natural Science, vol.2, No.11, 1211-1224 (2010): — do read about the publisher.

    So there, just guessing, I’ve found two places Alex might be getting this (not counting Morano’s). Pretty weak notion so far. Of course, they laughed at Galileo ….

  4. 54
    Lennart van der Linde says:

    Blanchon et al (2009), also referred to by Grinsted, seems to be the most outspoken paper on possibly very fast SLR during the Eemian:

    But it seems unclear how strong their evidence is. They infer a rate of SLR of more than 36 mm/yr for a total jump in SLR of about 3 meters in less than a century. I can’t see if they found direct evidence for that and how certain their evidence is.

    Maybe others here know more?

  5. 55
    Lennart van der Linde says:

    Sorry, I meant ‘about 2-3 meters’ in less than a century, not ‘about 3 meters’.

  6. 56
    Hank Roberts says:

    > I can’t see if they found direct evidence
    Are you looking at that PDF, at the supporting material section?
    What’s lacking? Did you look at any of the 75 papers citing that one?

  7. 57
    perwis says:

    A typo in my comment above: it should say “the UPPER bound is much more uncertain”.

    I would be grateful if someone could point me to a reference that explains in more detail what the IPCC authors intend with the uncertainty intervals. I would guess this is not the first time people have been confused about this.

  8. 58
    tokodave says:

    #36 Dan H. All this does not even take into account plate tectonics.

    I’m a geologist. Please explain how plate tectonics are relevant to a discussion of sea level rise in the next 100 years? Are you confusing isostatic rebound here?

  9. 59
    Hank Roberts says:

    Perwis, as a bystander, I found this somewhat helpful; it’s written for the authors:

    Climatic Change
    October 2011, Volume 108, Issue 4, pp 675-691, Open Access
    DOI 10.1007/s10584-011-0178-6

    The IPCC AR5 guidance note on consistent treatment of uncertainties: a common approach across the working groups

    A goal for the Fifth Assessment Report, which is currently under development, is the application of a common framework with associated calibrated uncertainty language that can be used to characterize findings of the assessment process. A guidance note for authors of the Fifth Assessment Report has been developed that describes this common approach and language, building upon the guidance employed in past Assessment Reports. Here, we introduce the main features of this guidance note, with a focus on how it has been designed for use by author teams. We also provide perspectives on considerations and challenges relevant to the application of this guidance in the contribution of each Working Group to the Fifth Assessment Report.

    The “Related Content” link takes you to more recent specific items

  10. 60
    sidd says:

    Mr. perwis: Thanx for the reference to O’Leary et al., which uses data from Western Australia also. I was actually thinking of Blanchon (2009) referred to by Mr. Linde, although I note that the 36mm/yr in the supplementary discussion refers to the Holocene and not the Eemian. They do have an interesting discussion of SLR at the end of the LIG, arguing for a faster than millenial rise, occurring on an ecological timescale.

    “However a millennial-scale interval is inconsistent with paleoecological data which requires that back-stepping was more rapid and occurred on ecological time-scales, as outlined in the following argument:

    i. The lower-crest shows no evidence of coral-community adjustment prior to its demise. This means that cessation of reef growth was ecologically rapid. Nor was the surface of the lower crest subsequently re-colonized by corals, only by a cap of coralline algae. This not only means thatreef-crest corals likely suffered a mass mortality event43, but that the environmental change was permanent. In other words, reef-crest demise and shift to another marine environment occurred rapidly on an ecological time-scale.

    ii. Coral communities in distal areas of the patch-reef complex also died rapidly with no time for adjustment. They were subsequently eroded and re-colonised by a sediment-tolerant community. The erosional break, however, prevents any assessment of the timing of that community shift.

    iii. But coral communities in proximal areas of the patch-reef complex were not eroded and show a continuous transition into the new environment (lower-tract lagoon to crest of upper tract) that occurred on a cm scale (see sections D4 and E2). This confirms that there was an ecologically rapid shift to a reef-crest environment ie, it occurred over the life-span of one or two generations of coral. (Note that this rapid transition was not caused by progradation of the upper-reef crest ovethe patch-reef complex because facies seaward and landward of the crest are composed of mixed assemblages). In other words, reef demise and back-stepping must have occurred on an ecological time-scale.”

  11. 61
    Lennart van der Linde says:

    @Hank #56,

    As Sidd (#60) notes Blanchon et al 2009 seem to mainly refer to Meltwater Pulse 1A as indirect evidence for a rapid Eemian SLR of more than 36 mm/yr. But since I’m no expert I don’t really understand how strong this evidence is.

    For example: what is the ecological speed limit, so to speak, for coral reefs to be able to keep up with SLR? Is that 36 mm/yr or less? How certain are we of this limit?

    And should these regional rises still be corrected for gravitational effects to determine the globally averaged rise? How large are the uncertainties?

    Blanchon et al also refer to Rohling et al 2008 for other (uncertain) evidence of rapid SLR during the Eemian, but it seems Grant et al 2012 (including Rohling) regard Rohling et al 2008 as an overestimate. So could Blanchon et al 2009 also be an overestimate?

    On the other hand, what if they are right after all?

  12. 62
    Alex says:

    Hank Roberts, the issue of whether the cooling period between 1550 – 1850 was global or limited mainly to the northern hemisphere, as implied by NASA’s definition in the link below, wasn’t really my point, but it may affect the magnitude of the attributions I was interested in. My point was that it takes a long time, even hundreds of years, for energy to diffuse into the largest ice sheets on the planet. This would seem to be relevant to understanding the causes and dynamics of sea level rise.

  13. 63
    Ray Ladbury says:

    Your way of looking at this doesn’t make sense. If the planet is warming, it must be because there is more energy going in than leaving. The planet doesn’t just have some equilibrium temperature that it always goes back to. So, no, you don’t make sense.

  14. 64
    Hank Roberts says:

    > it takes … even hundreds of years

    That’s what we used to think.
    But why do you still think so?

    What we all learned years ago is what we think, unless we look into what’s new. Where are you looking for new information?

    What’s interesting is what we can find by looking.
    No one was looking at the underside of the big ice sheets, til recently. There was argument about the possibility there had been rapid change, but there was no evidence (other than argument about, say, the channeled scablands, and observations of jokuhlaupts in some places).

    Now, we have observations starting to come in.
    That’s where the surprises have all come from.

    That’s why people are concerned.

    If we just held with what you believe now — what everyone believed ten years ago — we wouldn’t worry.

    Now, we have some new observations to think about.
    I’m not arguing for the sudden collapse into little ice cubes and rush to the ocean notion.

    But I agree with Hansen — by the time we have evidence, it could well be too late to stop a rapid change, if one’s in the offing at our present rate of change.

    Cautionary principle — what do you think about that?

  15. 65
    Lennart van der Linde says:

    Grant et al 2012:

    Rohling et al 2008 (2007 online):

    Blanchon et al 2009 refer to Rohling et al 2008 (2007 online), which estimated a maximum rate of SLR during the Eemian of 2.5 meter/century about 123.5 kyr ago. Blanchon et al think their 2-3 meter/century rise was around 121 kyr ago.

    Grant et al (2012) don’t give an estimate for a maximum rate, but give a common rate of up to 0.7 meter/century during the Eemian. They don’t exclude higher rates though.

    IPCC AR5 WG1 chapter 5 says:
    “LIG sea level rise rates of between 1.1 and 2.6 m per century have been estimated based on a foraminiferal 18O record from the Red Sea (Rohling et al., 2008a). However, the original Red-Sea chronology was based on a short LIG duration of 124–119 ka, after Thompson and Goldstein (2005). The longer LIG duration of 130–116 ka indicated by the coral data (Stirling et al., 1998) reduces these rates to 0.4–0.9 m per century, and a revised chronology of the Red Sea sea level record adjusted to ages from Soreq Cave yields estimates of sea level rise rates of up to 0.7 m per century when sea level was above present level during the LIG (Grant et al., 2012).”

    Chapter 13 says:
    “For the time interval during the LIG in which GMSL was above present, there is high confidence that the maximum 1000-year average rate of GMSL rise associated with the sea level fluctuation exceeded 2 m kyr–1 but that it did not exceed 7 m kyr–1 (Chapter 5) (Kopp et al., 2013). Faster rates lasting less than a millennium cannot be ruled out by these data.”

    Kopp et al 2013:

    They note that the Eemian and today are quite different periods with respect to insolation (and GHG-forcing) in particular, so the question is what Eemian SLR can and cannot tell us exactly. Blanchon seems to think 5 meter/century SLR is possible in the coming centuries, maybe already starting in this one.

  16. 66
    Hank Roberts says:

    > hundreds of years … diffuse

    Ah, there’s your misunderstanding. You’re thinking about diffusion, and in that you’re right, diffusion of heat through solid ice is slow.

    The surprise in the last few years wasn’t about diffusion.

    DOI: 10.1002/jgrf.20079

  17. 67
    Anonymous Coward says:

    If you define LIA as “the cooling period between 1550 – 1850” you’re going to have a problem telling (in your words again) “human GHG emissions and emergence from the little ice age”.
    That’s because anthropogenic GHG emissions were already significant at that point due in large part to deforestation and CH4 emissions (coal use accounts for ~3GtC in the 1851-1875 period and much less before that).

    But if you could for the sake of the argument disentangle “human GHG emissions and emergence from the little ice age”, you would only have to feed whatever warming you attribute to a non-anthropogenic rebound from the LIA into a model such as this one to get its contribution to the sea level:

    You also state: “My point was that it takes a long time, even hundreds of years, for energy to diffuse into the largest ice sheets on the planet. This would seem to be relevant to understanding the causes and dynamics of sea level rise.”
    But it’s not the melting of the core of the largest ice sheets which is currently causing the sea level to rise! It’s melting at the margins, and not necessarily the margins of the largest ones. And ice has a way of flowing downslope to the margins of ice sheets.
    All your point means is that it would take hundreds of years for the sea level to rise tens of meters which is not what people are presently most concerened about (a few meters would be bad enough).
    Energy can be transmitted quite rapidly through ice that’s undergoing a lot of surface melting by the way. Look up “moulin”.

  18. 68
    sidd says:

    Alex writes:
    “My point was that it takes a long time, even hundreds of years, for energy to diffuse into the largest ice sheets on the planet …”

    I seem to recall a talk by Alley where he pointed out that (i am paraphrasing from memory) :
    “We used to think that it took thousands of years for the heat to get in the ice sheet. Now we see that it gets there in 5 minutes”

    while speaking of moulin formation in Greenland.

    Latent heat is a a wonderful thing, especially when one phase is mobile. Heat transfer is no longer a diffusive process, it becomes a percolative process , with percolation connectivity exploding as we near the transition temperature from below in the immobile phase. I am not yet aware of anyone reporting a critical exponent or power law for meltwater percolation through a kilometer chunk of ice. In fact I can’t recall if anyone ever did such percolation networks when the percolating fluid is the hi T phase of the substrate and substrate is near the critical point for fusion. Anyone know of something like that ?

    Put in the pressure dependent melting point and the gravity gradient and make it even more fun.

  19. 69
    Barry E Finch says:

    @perwis #57 I’m not certain this is your question but it is something not well understood I’ve noticed. There are uncertainties in projections between upper & lower bounds because these are simulation “models” not calculations. A calculation, which produces a single result, is not possible for the climate system, too complex. I’ve written simulation “models” for elevator systems, orders of magnitude simpler. There’s some randomness (the chaos theory) and the “models” must reflect that and run the 100 years simulation (they use 15-minute intervals) numerous times with the same basic known formulae of the climate processes but varying the specifics a tad to match reality, else they model only a reality in which a rain drop hits your roof “just so”. The mean gives the likely projection and the range shows how “flakey” the climate system is or isn’t to small changes in the overall patterns. A calculation, which produces a single result, cannot do this.

  20. 70
    Hank Roberts says:

    > Richard Alley
    Mentioned with a link, in the earlier topic here, with inline comments

  21. 71
    Barry says:

    @wili #29 Arctic & Antarctic scientist Dr. Dan Lubin discusses observations & models in 2008 at Global Warming and the Polar Regions Signs of Human Impact University of California Television (UCTV). As example, he discusses the net +3.4 wm-2 forcing due to aerosols (pollution, mostly Europe & Asia) constrained above the Arctic throughout winter by the Polar Night Jet. I expect there are many factors affecting heat quantities in this zone, likely not yet all known or quantified.

  22. 72
    wili says:

    Sidd, is this the Alley talk you were thinking of?

    I think what you were thinking of is what he says shortly after minute 19. The numbers he gives are “about ten thousand years” for regular conduction, “about ten minutes” for heat transfer through drainage of moulins.

  23. 73
    John L says:

    #52 perwis
    IPCC’s handling of uncertainties is at least very poorly communicated. As you point out even great scientists in the area like Manning and Rahmsdorf get confused.

    From the IPCC AR5 uncertainty guidance note:
    “A statement that
    an outcome is “likely” means that the probability of this
    outcome can range from ≥66% (fuzzy boundaries implied)
    to 100% probability.”

    I’m more and more convinced that the use of “likelihood intervals” like 66%-100% is a pure conceptual error. People misinterpret it because they are intuitively correct that it should be a single number. There is no point communicating how uncertain the experts are on their own judgement mixed up with the uncertainty in the determination of parameters. And they already have the concept of “confidence” for that! In addition the fixed set of intervals does not even allow to express this uncertainty in any meaningful way. It is also simply self-contradictory to have intervals like 66-100% overlapping 90-100% etc.

    It may be that they mixed up the intervals with approximations which correctly must be used. Like alternatives 90%, 95%, 99% etc.

    Just to clarify, this doesn’t necessarily mean that there is anything wrong with the assessments per se or the science behind it.

  24. 74
    Alex says:

    Ray, you are confused and need to read my posts more carefully. I never said that the earth or an ice sheet will revert to some equilibrium temperature. I asked how much of the currently observed sea level rise is attributed to recent anthropogenically-caused warming and how much to warming over the last several hundred years that is presumably not anthropogenically dominated. The response time of the largest masses to a warming atmosphere is the root of the question.

    Others didn’t seem to have a problem understanding that, and helped by pointing out some of the complexities involved in the melt process and how it might affect the timescale.

    I will add that the transport time from a warmed atmosphere to the ocean is the bigger term, and no less interesting. The mass is huge, depth can be deep, specific heat is large, but circulation will speed the process. These complexities are all likely part of the reason my question remains unanswered. But it is still a relevant one, and it most certainly “makes sense”.

  25. 75
    Hank Roberts says:

    > how much to warming over the last several hundred years
    > that is presumably not anthropogenically dominated

    What “warming over the last several hundred years that is presumably not anthropogenically dominated” do you refer to?

    Are you presuming that the underlying trend over several hundred years is an warming trend?

    Is there a journal article to that effect?

    My impression is that we’re in the typical slow cooling phase after the end of an ice age, but for human activity, so the “warming” you’d be assuming would be a negative number: and references there

  26. 76
    wili says:

    Alex said: “warming over the last several hundred years that is presumably not anthropogenically dominated”

    Why would you presume that there is any such thing as non-“anthropogenically dominated” net warming over the last several hundred years?

    We are (or would be without AGW), after all, in the late stages of an interglacial, so we should have expected things to be gradually cooling over the last hundred years, if we hadn’t started dumping massive amounts of extra C into that atmosphere.

    IIRC, there was a recent paper on this. I’ll see if I can track it down (unless one of our other intrepid paper-tracker-downers finds it first).

  27. 77
    Alex says:

    What “warming over the last several hundred years that is presumably not anthropogenically dominated” do you refer to?

    Why would you presume that there is any such thing as non-”anthropogenically dominated” net warming over the last several hundred years?

    The folks over at suggest that the warming after the 1550-1850 cooling period was a result of increased solar activity and decreased volcanic activity (or conversely, the LIA itself was a result of decreased solar activity and increased volcanic activity). They cite a half dozen or so papers there, if you wish to look them up.

  28. 78
    wili says:

    So you already knew the answer to the question before you asked it? Hmmm.

    Over the last eight thousand years, global temps have dropped on average about on tenth of a degree C every millennium. So in general, we might expect slight continued cooling over the last few centuries.

    But yes, things like volcanic activity can cause short term wobbles in this longer-term tragectory.

  29. 79
    Hank Roberts says:

    > 1550 to 1580
    That’s longer ago than the last several hundred years.

    The longer the span of time, more data points — it gets easier to decide if your chaces are good you’re detecting a trend.

    More so when the data points back year after year are all equally reliable, and of course they’re not whichever proxies and temperature series you pick from.

    Did they have papers about the time span you ask about?

    I wish we had a librarian here.

  30. 80
    Hank Roberts says:

    2 typos, should be: 1550-1850; chances

  31. 81
    guthrie says:

    One of the issues is surely that the artificial cut off point of 2100 confuses some people, and gives deniers opportunities. The further the graph goes on to show the likely equilibrium sea level, the better. That way communication of the dangers might be improved.

  32. 82
    Ray Ladbury says:

    Alex, your question presumes a long time constant for equilibration of the climate that is not supported by evidence. Moreover, were the equilibration time constant that large, it would in turn imply a high climate sensitivity. Sure you want to go there?

  33. 83
    Martin Manning says:

    Sorry for the delay in responding, but in response to perwis#52 my reading of recent papers suggests that estimates of the contribution to SLR from glaciers and thermal expansion in the AOGCMs seem to have fairly symmetric probability distributions. But estimates of the ice sheet contribution in recent papers on models (such as Bindschadler et al, 2013, J. Glaciology, 53:195) and reviews of expert opinion (such as Bamber & Aspinall, 2013, Nature Climate Change, 3:424) both show probability (or possibility) distributions more definitely skewed towards the upper end of the range.

    If there was a definite probability distribution for SLR coming from all of that then a ‘likely’ range can be defined so that the probabilities of being above or below it are each 17%. But when subjective judgments are being used to come up with fuzzy statistics based on a range of expert opinions and ‘likely’ just means a probability somewhere in the range 66 – 100% then it seems to be much less clear whether that range remains centered around some median value. Neither of the IPCC guidance notes on uncertainties has been very clear about how a ‘likely’ range should be defined when expert opinions on the median can differ. But when you look at the likelihoods given for different parts of the range for equilibrium climate sensitivity it seems that the ‘likely’ range tends to get centered on the mode rather than the median when aiming for some form of consensus. For a positively skewed distribution this brings the ‘likely’ range down.
    On Stefan’s point I agree that scientists should avoid a tendency to ‘err on the side of least drama’. But also social communication is becoming the major challenge. Where I live there is now a major controversy because the local government has reclassified about 1000 coastal properties as subject to erosion and storm surges and that seems headed for the courtroom because their property values have been affected. So there can be a lot of drama even when planning for something like 0.5 m SLR. I’m not worried about scientists risking their reputations so much as society refusing to listen to things they don’t want to hear. Better adaptation strategies start with acceptance of changing risks and development of community response. Part of that is to get recognition of thresholds for sustainability of coastal properties, such as loss of insurance cover, and then proactive discussions about the options for retreat or cost of sea walls etc. Perhaps it’s clearer to say that I still think we need to plan for SLR between 0.5 and 1.5 m over the next 100 years, but when talking to people with houses in the low elevation coastal zone I’d rather discuss what things would look like at 0.5 m and because that is going to happen sooner than we would like, we need to plan for it now.

  34. 84
    Alex says:

    So you already knew the answer to the question before you asked it? Hmmm.
    This is beginning to venture into the absurd. No, I still do not know the answer to my question: How much of the currently observed (~0.2m) sea level rise is attributable to warming during the naturally-dominated recovery from the Little Ice Age? Answering that question requires understanding the timescales of heat transfer from the atmosphere to the ocean and ice sheets. And yes, Hank, 1850 is within several hundred years, and also well within the time frame of simple heat conduction to the center of a thick ice sheet.

    The answer to my question may not be an easy one to come up with, but it is directly relevant to any projection of future sea level rise, such as the ones discussed in the body of this post. Therefore, the authors of that report section should presumably have an answer, or at least an estimate.

  35. 85
    Alex says:

    There’s a bug in the html transition from preview to post. Only the first line above should be italicized as a quote from wili.

  36. 86
    Hank Roberts says:

    Alex, are you assuming intact ice? Diffusion isn’t constraining temperature change because the ice isn’t simple.

    Are you assuming a change in sea level during the 1550-1850 cold spell? Do you assume a recovery must have happened?

    Point to something we can read, rather than just telling us your assumptions, logic, and reasoning, if you have sources.

    Seriously, move the conversation along, eh?

  37. 87
  38. 88
    sidd says:

    Re: Bindschadler et al., J. Glaciology 2013.

    I take it that this is the SeaRISE project paper ? This examines 10 ice sheet models, discovers that:

    “In most cases, the ice volume above floatation lost is linearly dependent on the strength of the forcing. Combinations of forcings can be closely approximated by linearly summing the contributions from single forcing experiments suggesting that non-linear feedbacks are modest.”

    Stipulated that the projections from the models can be linearized as described. But I fear that the models diverge from reality, especially for ice sheets on submarine beds. Has anyone actually come up with a model that matches the pacing and magnitude of previous WAIS collapse as shown in, say, ANDRILL data ? Or, closer to the present, a model that reconstructs putative ice sheet retreat resulting in MWP1A transgression ?


  39. 89
    Hank Roberts says:

    PS for Alec– here, a brief excerpt from the 2011 RC topic suggests to me that there’s not yet enough information to decide whether either temperature or sea level changed at the time — the uncertainty, for that time span, is greater than the small changes that would matter.

    I’m not trying to tell you you’re wrong; I’m trying to find out what you’re assuming, what you know, and why you know it.

    Mike and Martin were clear about what they did and didn’t know back in 2011, and what assumptions were made. That helps:

    What is the basis that makes the suggestion that the temperatures were not as warm as indicated during AD500-1000 more valid than perhaps the M08 reconstruction suggesting that the SLR estimate instead is incorrect?

    [Response: That’s a very fair point. Both alternatives, in my view, are equally viable. Future work will hopefully better pin this down. -Mike]
    Comment by Davos — 24 Jun 2011 @ 1:38 PM


    M #58:

    would a plausible physical explanation be that the deep ocean and ice sheets are still responding somewhat to the post-glacial temperature increase (eg, T-T0,0>0), but that the faster components of SLR like the surface oceans and glaciers were actually responding to the decrease in temperature since the early Holocene?

    Very perceptive, M. That is pretty much what I believe is happening.
    Comment by Martin Vermeer — 29 Jun 2011 @ 1:37 PM

    Just to say, I think Alec’s way oversimplifying the question and making assumptions for which I haven’t found support yet. Hoping Alec will look further at the 5th IPCC Report (the actual topic here) and see whether his question could be answered within the limits of the uncertainties. I think not.

  40. 90
    adelady says:

    The response time of the largest masses to a warming atmosphere is the root of the question.

    Alex, are you presuming that any warmth transmitted to ice must come from the atmosphere?

    I would have thought everyone was on board with the idea that the big effects on the margins of the icesheets (and on Arctic sea ice which doesn’t affect this SLR discussion) were from warmed water. Which undermines the undersides and the edges … which allows the ice to flow more quickly … which means more ice gets into contact with warmer water more quickly … which melts more ice.

  41. 91
    Roger Lambert says:

    Stefan and Hank, re:

    “> 31, Roger Lambert, Stefan

    Hansen answers that question the same as Stefan; Hansen also hasn’t seen any scientifically credible basis for those 5 meters:

    … what are the shapes of the ice sheet mass loss curves for Greenland and West Antarctica? Is there evidence that they may be exponential? It’s too early to tell, as shown by Fig. 1 above. The picture may begin to be clearer within the next several years. The problem is, by the time the data record is long enough to be convincing, it may be exceedingly difficult or impossible to prevent sea level rise of many meters.”

    Yet, Hanson and Sato in “Paleoclimate Implications for Human-Made Climate Change” say:

    “BAU scenarios result in global warming of the order of 3-6°C. It is this scenario for which we assert that multi-meter sea level rise on the century time scale are not only possible, but almost dead certain”


    “What about the intermediate scenario, EU2C? We have presented evidence in this paper that prior interglacial periods were less than 1°C warmer than the Holocene maximum. If we are correct in that conclusion, the EU2C scenario implies a sea level rise of many meters.”

    They seem to be arguing, from my layman’s perspective, that there is historical evidence to argue that ice sheet loss will be exponential. And that while our observations are, as yet, too premature to verify this exponential loss, they do seems to be arguing that it seems likely?

  42. 92
    Hank Roberts says:

    Roger, do you have the original text on that? Searching the quotes I found secondary sources/reviews with that quote taken from a book chapter:

    There’s a link to that page, but not the actual text, at

  43. 93
    Dan H. says:

    Alex, et. al.,
    You may to examine a brief synopsis of the research done on sea level rise over the past three centuries:

    The uncertanty in global measurements increases substantially prior to 1850. This may not answer your initial question, as it does not differentiate between natural and manmade.

  44. 94
    Lennart van der Linde says:

    @Roger #91,

    I think ‘on the century time scale’ means one to several centuries and ‘multi-meter’ means two or more meters, not necessarily five meters in this century.

    The EU2C scenario means many meters after several (or many) centuries (or millennia), as I understand it.

  45. 95
    John L says:

    #83 Martin Manning
    From your comment I do the interpretation that the likelihood interval is intended to capture the range of expert opinions. OK, then it makes relatively more sense. After all, in the end there is a group of experts that must agree about the formulation. But still, the use of “fuzzy statistics” in communication is very questionable (IMHO) and only risks leading to confusion and artificial levels of uncertainty. The “level of consensus” might better be separated as its own variable.

    For example, in the case of sea level rise an (arguably) equivalent clearer statement:

    “For the scenario RCP8.5 there is a 75% chance of a sea level rise of 52-98 cm to 2100 over 1986-2005. This assessment has medium confidence and low consensus”.

    The 75% is the average (or better, median?) expert judgement. Confidence is the belief in the model used to derive the numbers and corresponds to how much this judgement is predicted to change with further research. The level of consensus corresponds to the spread of expert opinions.

    [Response: I don’t think the likelihood statement in this case reflects a “range of expert opinions”. The way I understand it, a few of the IPCC sea level authors made a calculation of a model-based 90-percent uncertainty range. Then the authors said: because the range of model results included in this calculation probably does not reflect the full uncertainty of the problem, we call this 90% model range the assessed “likely” range. If it was constructed in this way, I would have to agree with Martin that this “likely” range can be asymmetric, i.e. with more uncertainty at the top end than the bottom end. So the probability of exceeding 98 cm is not 17% as I initially assumed but could be greater than that. -stefan]

  46. 96
    Hank Roberts says:

    > ‘on the century time scale’
    Good point by Lennart.
    It’s a poor scale that has only one tick on it.
    It has to mean it shows up on a scale of plenty of centuries.

  47. 97
    Keith Woollard says:

    Lennart@93, I would have to disagree. I don’t think anyone would class 2 as multi and I don’t think that was the intent of the writer. And the century time scale to me means a century.

  48. 98
    Hank Roberts says:

    > I would have to disagree

    Have you found a copy of the whole paper? What do the authors say?

  49. 99
    sidd says:

    After having recently ranted here about the lack of models describing MWP1A, I recall that I have previously ranted here about precisely such a model

    Gregoire(2012) doi:10.1038/nature11257

    describing saddle collapse of the N American sheets in last deglaciation.

    I shall watch 67N in Greenland carefully. And keep better track of my rants.


  50. 100
    Keith Woollard says:

    So let me see Hank, you and Lennart are allowed to guess at what the author means and I am not? I am reading the paragraph as a normal, sensible, educated English speaker