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Sea level in the 5th IPCC report

Filed under: — stefan @ October 15th, 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", Surv Geophys, 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", J. Geophys. Res., 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", Clim Dyn, vol. 39, pp. 861-875, 2012.
  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. 1
    Scott Belyea says:

    I’m a moderately informed layperson, I suppose. I found this to be one of the best IPCC summary pieces I’ve seen.

    (Mind you, I did have an initial misreading of a graph label as “Antarctic ice-sheet rabid dynamics,” and that did seem a bit of an emotional reaction.)


  2. 2
    Lennart van der Linde says:

    “By 2300, for unmitigated emissions IPCC projects between 1 and more than 3 meters of rise.”

    But what is the worst-case by 2300, based on a worst-case of about 1.5m by 2100, as argued by Levermann and Grinsted? That would imply about 3 cm/yr of SLR by 2100, and probably some further acceleration during the next century, right? So would it be reasonable to say the worst-case for 2100-2300 is about 4 meter/century, so almost 10 meters of total SLR by 2300?

    This would be about the worst-case estimate by Meehl et al (2012), based on semi-empirical models:

    IPCC could not reach consensus on the value of the semi-empirical models. But should policy makers, and citizens, only use science on which a great deal of consensus exist? Or should they use a risk and precautionary approach and conclude that a worst-case of about 10 meters by 2300 cannot confidently by excluded by science at this point? Would 10 meters by 2300 as a worst-case be a scientifically sound conclusion?

  3. 3

    I’d like to add that there is no consensus on the skill of current generation process based models of sea level rise. I think it is safe to say that there is disagreement over the relative levels of confidence should be assigned to current process based vs semi empirical approaches. Both approaches should IMO have been included in the reported uncertainty ranges. I frankly think that this would also much closer represent the level of uncertainty there is in the sea level community. See e.g. the comparison between AR5, SEMs and “ice sheet experts” here:

  4. 4
    Lennart van der Linde says:

    An important addition by Anders Levermann is his statement that worst-case estimates should get lower as science progresses, not go up:

    “In the case of sea level, society might want to know what is science’s best guess for the future rise, but for any practical purposes of coastal protection it is the worst case that is relevant. What is the upper limit of sea-level rise? An upper limit is different from a best guess and has at least two peculiar properties that are trivial but important. First, for all practical purposes, the upper limit cannot be exceeded. That means that if you build costal protection with respect to this upper limit, then you are safe, independent of scientific uncertainty or socio-economic scenarios. Second, an estimate of an upper limit is getting lower the more information is available — i.e., the more our scientific insight deepens. You start with the highest number available and then seek scientific evidence that allows dismissing this value and pushing the number down until you find no further reason to decrease it. Then you have your upper limit and you are safe.”

  5. 5
    Christian says:

    Very helpful, thanks.

  6. 6
    Hank Roberts says:

    this one by Paul Voosen has a high paywall, sorry to say.

    [Response: Sorry - when I saw it it wasn't behind a paywall. Now it is for me, too. stefan]

  7. 7

    Hi Stefan,

    Thanks for this thorough and interesting post. It’s nice to see that the SLR work in AR5 is not as egregiously conservative as in AR4, but it still very much sounds like there is a hyper aversion to Type I errors which leads to these still likely conservative low ranges (in particular, the unwillingness to explore the upper tails of the distribution).

    As the IPCC discussion evolves this fall and winter about the future of the report, some folks have suggested smaller, quicker, more targeted reports (along the lines of the Special Report on Extremes, SREX). I’d be curious to hear your and the RC folks thoughts. First, what’s your sense on this as a viable and useful option? Second, would a Special Report on SLR be a useful targeted report? It seems like the science is evolving fast, the policy relevance is high, and it needs to be explored in more detail and with a more balanced risk-management perspective…

    Best regards,

    Bill Anderegg

  8. 8

    Very useful. I reposted it to Facebook just now, adding the figures in feet; more people (of the type we wish to influence) understand feet much better!

  9. 9

    This is what I posted to Facebook just a few minutes ago. JB

  10. 10

    Sorry — The key IPCC 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. For high emissions IPCC predicts a global rise by 52-98 cm (that’s 5 to 10 meters; a meter equals 3 + feet) by the year 2100, which will threaten the survival of coastal cities and entire island nations.Even with aggressive emissions reductions, a rise by 28-61 cm 3 to 6 meters; or 5 to 18 feet, is predicted. Goodbye Galveston.

  11. 11

    I also posted “Also — “aggressive emission reduction is just not going to happen. So plan for 15 to 30 feet. OH BTW, it gets higher after that; I have not yet seen an estimate of where it will level off. Perhaps someone here has such an estimate?”

  12. 12
    questions says:

    Nice paper. Do we know for which element (steric or water transfer from ice) and in which proportion climate models tend to underestimate the historical SLR , when their results are compared to climatologies? Thanks.

  13. 13
    prokaryotes says:

    That means that if you build costal protection with respect to this upper limit, then you are safe

    But the upper limit might be higher than the upper limit estimate? A lot could and will change since the entire planets gravitational field will readjust, which means more seismic activity. And SLR is likely to be exponential, hence why it accelerates. Coastal protection needs to account for higher storm surges, affected by storm strength, storm size and uneven SLR distribution and re-configuring of ocean flow.

    Even if you have coastal protection in place, a tsunami could easily roll over it.

    But the coastal levee, 10 meters tall, 3 meters wide and running for 2.4 kilometers, crumbled in the March 11 tsunami.

    Some survivors said residents may have placed too much faith in the levee, known locally as “the Great Wall,” leading to a higher number of dead and missing people.

    “The tsunami’s height was twice that of the levee,” said Giichi Kobayashi, a 76-year-old fisherman in the district, his face stiffening.


    Each degree of global warming might ultimately raise global sea levels by more than 2 meters

  14. 14
    Hank Roberts says:

    > 52-98 cm (that’s 5 to 10 meters; a meter equals 3 + feet)
    > by the year 2100

    NO! WRONG!

  15. 15
  16. 16
    Benjamin Tennen says:

    RE: John Burgeson – “52-98 cm (that’s 5 to 10 meters; a meter equals 3+ feet)”

    52-98 centimeters = 0.52 – 0.98 meters
    1 meter = 3.28084 feet

  17. 17
    Dave Berry says:

    John Burgeson: 52-98cm is half to one meter. You have misplaced a decimal point.

  18. 18
    paulina says:

    Thanks for writing this.

    I don’t understand how adding [a] the upper limit of the estimate of the MBAIS collapse contribution (several decimeters, so let’s say .5 m), to [b] the upper limit of the “likely” range, provides a “worst case” estimate (if that is what is done in the Levermann/e360 quote/link above).

    Don’t you FIRST have to, somehow, assess [c] the upper limit of the *very* likely, or *extremely* likely, range (w/o collapse contributions) and THEN add [a] the upper limit of the collapse contribution (ie, and THEN add the .5 m) ?

    That is, wouldn’t you need to do something similar to what Aslak Grinsted did (as in the Grinsted/glaciology link above), but with some kind of different (component-based) assessment of [c] (ie, not a sigma-based derivation that starts from the interpreted-as/treated-as “likely” ranges)?

    Finally, either way, an assessment like this would give the worst case based, at least for some components, on RCP8.5, the worst of the scenarios, but not in itself a “worst case,” right?

    Grateful for corrections, clarifications, comments, answers. Thanks.

  19. 19
    John L says:

    From the linked table by #3 Aslak Grindsted, it seems like the FAR gave the most realistic estimate of the sea level rise, even better than AR5, according to the “ice sheet experts” as well as taking into the account the bias in leaving the semi-empirical models out.

    Could it be the ESLD effect getting stronger due to greater mass media pressure not to be alarmist?

  20. 20
    Missoula says:

    This all begs the question, what is a reasonable upper-limit for the year 2100? >1m? >2m?

  21. 21
    KR says:

    Do the AR5 regional sea level rise estimates take into account gravitational effects of ice cap reductions? As per the discussion in Jerry Mitrovica’s video?

    [Response: Of course, Stefan ]

  22. 22
    sidd says:

    1)The ANDRILL results seem to give a timescale of 1kyr for WAIS collapse. I recall a comment by Bindschadler stating that 1Kyr should be seen as an upper limit. And I recall Mercer’s warnings from many years ago.


    2)Gregoire depiction of saddle collapse (eg 67N Greenland)
    3)albedo forcing as Box has illustrated
    4)EAIS may be less stable than thought

    Every millimeter of SLR from Greenland is accentuated at the other end of the world in Antarctica, as Mitrovica has pointed out, and every millimeter lifts the ice shelves stabilizing the Antarctic glaciers. Last year Greenland melt at 574 Gton (GRACE) was a millimeter and a half averaged globally, more down south. And as the southern ocean warms, the circumpolar winds tighten their girdle, intensifying the Ekman rise of warm circumpolar deep water melting into the vasty deeps beneath Antarctica.

    Do you feel lucky ?


  23. 23
    Non-Scientist says:

    Sadly, the average person will believe it when and only when they see it.

    That will be someone living in 2100, most of whom will not remember 2050 let alone 2000. To their eyes the new sea level will be normal. *

    *And no one of consequence will be interested in stories of those over 75 recalling when they were 15.

    [Response: You should be aware, though, that in 2100 there will not just be a “new sea level” but more importantly a rapidly rising sea level – at the highest end of the IPCC scenarios at a rate of 16 cm per decade, which is about equal to what we had in the 20 th Century – not per decade but for the whole century. stefan

  24. 24
    Jon Kirwan says:

    I was able to get past the paywall at The Chronicle for Higher Education. Seemed like a good synopsis of the characters. But I wouldn’t know any better, either way.

    Here’s a quote from near the bottom of that article and ascribed to John A. Church, who served as one of the coordinating lead authors, with Peter U. Clark being the other…

    “I daresay we will be attacked by both sides,” he said, “for having too high numbers and having too low numbers.”

    What is left unsaid is that the attacks from one side for having too high numbers will be coming from denialists. The attacks from the other side for having too low numbers will be coming from those far more informed about the topic and disappointed by what didn’t make it into the report.

    The summary there makes it sound as though Dr. Church was suggesting they captured the right middle road of informed knowledge. But from reading this page and many of the various links therein (such as Aslak Grinsted’s), I find Jason Box’s comment (quoted in this page’s article) all the more pointed: “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].”

  25. 25
    Douglas McClean says:

    Could someone explain what figure 3′s legend means by the annotation “No Antarctic PGs”?

    [Response: PG=Peripheral Glaciers. These are excluded in those analyses to avoid double counting according to the IPCC figure caption. (And, no I don't really understand this). The source paper is Marzeion et al. (2012a) (#OA) so you might get a clearer answer there. - gavin]

    [Further Response: See comment below for more details]

  26. 26
    Brennan says:

    Peer review in action, posts 14-17.

  27. 27
    Hank Roberts says:

    Brennan, peer review’s meant to be done _before_ publication (grin).

    Has anyone checked whether John B has fixed that error?

    John Burgeson says:
    15 Oct 2013 at 11:33 AM
    This is what I posted to Facebook

    Good example of why peer review is done — and was needed. It’s a low threshold to catch obvious errors before they go out.

  28. 28
    Lennart van der Linde says:

    Also see the full paper by Meehl et al (2012):

    About their worst-case estimates for 2100 and 2300 they say:
    “[T]he semi-empirical method… indicates greater increases than the IPCC AR4 example, with sea-level rise of nearly 115 cm and 145 cm by 2100 in RCP4.5 and RCP8.5, respectively, with an eventual increase approaching 440 cm and 960 cm for RCP4.5 and RCP8.5, respectively, by 2300. These values inform the upper range of the shading in Fig. 3 that encompasses the larger estimates. But the limit of the higher end of the shading is depicted as being indistinct to reflect that these are only estimates. There is no real way of knowing if these higher total sea-level rise values are credible, or if higher or lower values are more likely.”

    So even this worst-case may not be a worst-case, but it seems we simply cannot really know at this point.

  29. 29
    wili says:

    It is amazing to me that there can be so much uncertainty about what would seem to be an extremely simple, well understood type of physical change–the melting of ice.

    But as we have seen with the wildly inaccurate (all on the conservative side) models for Arctic sea ice melt, modeling the seemingly simple process of ice melting in a warming world turns out to be not at all simple or straight forward.

    And if this process of water changing state, which is pretty much just a process of physics and a bit of chemistry, is so very easy to get wrong–specifically, is so easy to model too conservatively so the models predict wrongly that it will be a very slow process when in fact it seems to be a much faster process–how confident can we be that other models and estimates of processes that involve multiple feedbacks that include chemical and biological interactions as well as physical ones aren’t even more wildly inaccurate on the ‘conservative’ side?

  30. 30
    Martin Manning says:

    Thanks to Stefan for this summary of what currently seems to be the biggest problem in climate science. John Church, Peter Clark and the other 12 lead authors of Chapter 13 have clearly been struggling to deal with a complex problem. But they are in a situation where for the science it is sometimes better to be a bit conservative, and then proved wrong, rather than exaggerate the problem, and then become totally discredited. That is not risk management in the normal sense, but it has come up in some earlier IPCC assessments.

    However, while semi-empirical models are being recognised in the report, more could be done to merge these in with process based models. For example, some other areas of science are using ways of combining aleatory and epistemic forms of uncertainty to produce fuzzy intervals or developments of the Dempster-Shafer approaches which deal with ranges going from what is ‘belief’ (e.g current process models) to what is ‘plausible’ (e.g semi-empirical).

    Recognition of these different forms of uncertainty is shown a bit subtly by Chapter 13 giving 5-95% ranges from the models and then saying that these are ‘likely’, i.e only believed to be 66% of the total range. The synthesis in section 13.8 also says that neither a ‘very likely’ range nor upper bound can be defined. And 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.

    While the IPCC approach to uncertainty could be improved, getting coastal planning to take account of even 0.5 m SLR seems an even bigger challenge. So Figure 13.25 which shows the very large multiplication factors that this causes for coastal flooding could actually be the best wake up call coming out of the chapter.

    [Response: Thanks for your thoughts, Martin. Coastal planning: here in Germany the ongoing process of upgrading our coastal defences is based on the assumption of up to 1.5 meters of sea-level rise. Any volunteer expert willing to write a post on those multiplication factors in Fig. 13.25, showing how even for 0.5 m SLR the frequency of flooding events of a given height increases drastically in many locations? This is am important issue which I did not cover above.

    p.s. A few questions to you as IPCC veteran: why is underestimating a risk not so bad but overestimating it destroys our credibility? What is the cause of this asymmetry? And isn't that exactly the kind of thinking that leads to "erring on the side of least drama"? And shouldn't we as scientists in the public interest be more concerned about the risk to coastal cities than about the risk to ourselves and our reputations? - stefan]

  31. 31
    Roger Lambert says:

    Hi Stephan

    Would you feel comfortable providing an analysis of the impressive discrepancy between the IPCC estimates and those of Hanson and Sato, who state that more than 5 meters SLR is likely by 2100? Are there mistaken assumptions or missing feedbacks at play? Thanks.

    [Response: I haven't seen any scientifically credible basis for those 5 meters. -stefan]

  32. 32
    Alex says:

    What’s the rough breakdown of sea level rise attribution for human GHG emissions and emergence from the little ice age?

  33. 33
    Ray Ladbury says:

    Huh? That doesn’t even make any sense. The LIA ended over a century and a half ago. For warming to occur there has to be an energy source. Mr. Sun ain’t it.

  34. 34
    Hank Roberts says:

    > 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.

    > 32, Alex, Little Ice Age
    The Little Ice Age and the Medieval Warming were named by people experiencing the local climate in parts of Europe. The people experiencing them assumed they were global climate changes.

    You’ll find bloggers passionately arguing that they must have been global, because [Anything But The IPCC], and since no evidence of them appears in global sea level records, that must prove climate change doesn’t affect sea level.

    But absence of evidence is not evidence of absence.

    A great many people have looked at the paleo record at many locations, and more such records — each for a particular site — are being published.

    A global event would have happened everywhere at the same time. So far that pattern hasn’t been found, near as I can tell. Instead local events happened that didn’t coincide.

    Check your source if you read that either warm period or “little ice age” was global — is it a blogger’s claim? Be skeptical. It’s part of a PR story, if so.

    Or was it a science journal? Weigh the evidence. Tell us where you found it.

    Life is full of little surprises.

  35. 35
    Hank Roberts says:

    The Hansen “Fig.1″ link I tried to post broke.
    That should point to

    Ten years ago almost believed the Antarctic could be unstable, and few thought that the geology we could see left behind when the continental ice caps melted indicated any kind of rapid event. Some few thought meltwater could go right through glaciers year after year, build up, and burst out. One glaciologist said that glacial ice would close any openings during each winter, not get riddled by holes that would persist.

    You can find some of that accumulated in the comments over several years at

    I was, there, reading and asking questions. You can see I was not as worried then as Hansen (grin) but more concerned than the ice experts quoted there were, then.

    The inline replies at that link are appropriately skeptical, as the scientific evidence began to accumulate. Drumlins, for example — did the ones found all over form slow, or fast? That has long been debated. Then someone watched one form — rapidly. So, if drumlins, found everywhere the ice caps melted, are evidence of fast change then — hmmmmm.

    Scientists don’t yet have evidence to say continental ice could change fast — but have long believed it wasn’t likely.

    Nevertheless, it moves.

  36. 36
    Dan H. says:

    It is not quick so simple as the “melting of ice.” Predictions of an ice-free Arctic ranged from 2013 (Maslowski) to 2058 (Liu, et. al.) to 2100 (Jahn and Hlland). Much of this uncertainty is related to the complex interaction between the Arctic air and water leading to the absorption of energy in the summer and radiation in the winter. Predicting the melting of the Greenland and Antarctic glaciers is much more complex.

    Add to the previous factor the IPCC projected 21st century temperature rise of 0.3 – 4.8C (depending on the RCP interval selected), and it becomes surprising that the predicted sea level rise is contained in such a small interval. All this does not even take into account plate tectonics.

  37. 37
    Doug Bostrom says:

    Stefan: “Any volunteer expert willing to write a post on those multiplication factors in Fig. 13.25, showing how even for 0.5 m SLR the frequency of flooding events of a given height increases drastically in many locations?”

    Barging in (sorry!). I’m not an expert in anything but wringing unwanted flood water out of the lower floor of my house; I can offer only anecdotal information.

    A sea level rise of “only” 0.5m can be quite drastic indeed. Why? Because there’s an extraordinary difference between a house with no water on the floor versus one with “just” a centimeter inside where it’s not supposed to be, when one takes into account such things as sharing a house with water or leftover deposits of silt on the floor. Indeed it hardly matters whether there’s half a meter or only a centimeter of wrongly placed water in a home; it turns out that any interior flooding of whatever amount effectively makes a house useless for its intended purpose.

    In our case we don’t have a problem with the ocean but our story may nonetheless be helpful. Our own house was constructed in a context where it coexisted happily with the outdoors– just barely– similarly to many homes in immediate vertical proximity to water. It turns out that only a small statistical difference one way or another in rainfall intensity is what allows our house to be livable or not. We’re lucky that we can engineer our way out of our problem, for the time being.

    There are other ways to make our own luck, including doing what we can to improve our statistics whether they be frequency and size of storm surges or the style of rainfall we receive. Remarkably enough we do seem to have that power but it turns out to be an unfortunate “skill.”

  38. 38
    Lennart van der Linde says:

    @Roger Lambert #31,

    Where do Hansen & Sato say 5m of SLR by 2100 is likely?

    As I understand them they think 2-3 meters by 2100 may be possible under BAU. And they point out that with a 10-year doubling time of the ice sheet contribution to SLR you would get about 5m around 2100. It seems Hansen doesn’t rule this out entirely, but I don’t think he has said it’s likely.

    He and Sato do point to a plausible negative iceberg cooling feedback which may limit the worst-case SLR by 2100 to about 2.5m and the rate of rise to about 5-6m per century in the next centuries. This is pretty close to what some semi-empirical models seem to find as a worst-case.

  39. 39
    Hank Roberts says:

    > Hansen … where …

    Dang, the blog software here persists in breaking this link:

    Try copying it into another browser window and it’ll work.

  40. 40
    Jim Larsen says:

    29 wili says, “if this process of water changing state…is so easy to model too conservatively…how confident can we be that other models and estimates of processes that involve multiple feedbacks that include chemical and biological interactions as well as physical ones aren’t even more wildly inaccurate on the ‘conservative’ side?”

    Because the models work better with temperature than melt. Paleodata and modern data support the temperature data but not the melt data. This could result in erroneous temperature data as albedo changes, but I’d amateurishly think that the end result for temperature would be closer to reality than the melt predictions.

  41. 41

    sea rising is a natural phenomena – sediments washed from the hills into the -#b] places like lake Chad, Aral sea getting dry = less water on the land = more water into the sea.

    movements of the tectonic plates makes one part of island, or continent to rise other part / parts to sink – that isn’t sea rising:

  42. 42
    Non-Scientist says:

    “You should be aware, though, that in 2100 there will not just be a “new sea level” but more importantly a rapidly rising sea level – at the highest end of the IPCC scenarios at a rate of 16 cm per decade, which is about equal to what we had in the 20 th Century – not per decade but for the whole century. stefan

    1. Tx for the info. If (probably when) SLR=16cm/decade average people near the coast will finally notice.
    2. They can notice, then declare it’s God’s Will and must not be opposed ;-)

  43. 43
    Alex says:

    @Ray Ladbury

    Huh? That doesn’t even make any sense. The LIA ended over a century and a half ago. For warming to occur there has to be an energy source. Mr. Sun ain’t it.

    The LIA represents a roughly one degree C increase in global temperature that has not fallen since. After reading your comment, I built a 1-D, transient model of the Greenland ice sheet. With a step change in temperature at the surface of the ice sheet, and assuming a constant thickness of 2km, the time required for the mid-point of the ice sheet to reflect only 50% absorption of the energy reflecting the temperature increase is… 159.5 years.

    So, you see, my question makes sense. I’m still waiting for an estimate.

  44. 44
    John Mashey says:

    Thanks for the commentary.

    A few years ago I attended a great 1-day session on Preparing for Sea Level Rise in the Bay Area – A Local Government Forum.

    It started with talks by scientist on what was known and unknown, then on to strategies for dealing with such, in an area that has a large amount of valuable infrastructure right at sea level, including Silicon Valley and 3 airports.

    But planning for SLR is very different and much uglier than just planning to deal with extreme storms, and they split us into groups, gave us descriptions of imaginary towns on the Bay, and said Plan. Among other awkwardnesses:
    a) Sewage plants like to be near sea-level, but below where people live.
    b) In the long term, it might be most-effective to pick a line to defend a ways up the hills … but oddly, those who own property below tend to think otherwise.
    c) One can build dikes, but when it rains, the water does not flow uphill from areas now below sea level. This is different from a storm that temporarily floods some area and then drains away.
    d) One of the main roads is Rt 101, and it is just barely above high tide.

    The encouraging part of the experience was spending a day with town planners who passionately cared what their towns would look like in 50-100 years, wanted to understand the science and talk about strategies. I would feel better if I though the SF Bay Area was necessarily a representative place.

  45. 45
    wili says:

    Jim Larsen at #40 said: “Because the models work better with temperature than melt. Paleodata and modern data support the temperature data but not the melt data.”

    Thanks, Jim. I hadn’t thought of that.

  46. 46
    Marco says:

    @41 from stefanthedenier is there to provide us with some comedy relief, right?

  47. 47
    sidd says:

    This might belong in the Unforced variations thread, but I was hoping someone reading this thread might recall the reference.

    I am trying to recall a recent paper arguing for fast sea level rise at the end of the Eemian. Unfortunately all i find in my currently available archive with is the 2000 paper in Chemical Geology by McCulloch and Esat, which is, amazingly enough, available freely through a search at

    Chemical Geology 169 (2000) 107–129
    “The coral record of last interglacial sea levels and sea surface temperatures”
    Malcolm T. McCulloch) , Tezer Esat

    From a site in the Huon Peninsula in Western Australia, through clever isotopic dating, they deduced that:

    “During the penultimate deglaciation, there was much more rapid and sustained increase in sea level compared to the last deglaciation. The final 80 m of sea level rise took only ~2 ka during the penultimate deglaciation …”

    I am almost sure that there is fairly recent work documenting rapid transgression at the end of the Eemian, (perhaps from Barbados or NE coast of N. America ?)

    Would be glad for leads.


  48. 48
    Jim Eaton says:

    stefanthedenier: Wow! I didn’t realize there was one website with so many scientific inaccuracies in one place! If I were so inclined, I could spend weeks refuting each and every one of the supposed “facts” on this site. But I am not so inclined. I have a life.

  49. 49
    Doug Bostrom says:

    stefanthedenier: “Aral sea getting dry” as a natural phenomenon.

    Priceless, truly.

    “Formerly one of the four largest lakes in the world with an area of 68,000 square kilometres (26,300 sq mi), the Aral Sea has been steadily shrinking since the 1960s after the rivers that fed it were diverted by Soviet irrigation projects.”

    STD is getting his anthropocene all jumbled.

  50. 50
    perwis says:


    I suspect this is the paper you are looking for:

    O’Leary, M. J., Hearty, P. J., Thompson, W. G., Raymo, M. E., Mitrovica, J. X., & Webster, J. M. (2013). Ice sheet collapse following a prolonged period of stable sea level during the last interglacial. Nature Geoscience, 6(9), 796–800. doi:10.1038/ngeo1890

    Aslak Grinsted has a nice compilation of some other recent papers on evidence for WAIS collapse during the last interglacial:

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