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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. 101
    Lennart van der Linde says:

    Hansen & Sato 2011 say:

    “We suggest that ice sheet mass loss, if warming continues unabated, will be characterized better by a doubling time for mass loss rate than by a linear trend. Satellite gravity data, though too brief to be conclusive, are consistent with a doubling time of 10 years or less, implying the possibility of multi-meter sea level rise this century.” (p.1)

    “Hansen (2007)suggested that a 10-year doubling time was plausible, and pointed out that such a doubling time, from a 1 mm per year ice sheet contribution to sea level in the decade 2005-2015, would lead to a cumulative 5 m sea level rise by 2095.” (p.22)

    “The eventual sea level rise due to expected global warming under BAU GHG scenarios is several tens of meters, as discussed at the beginning of this section. From the present discussion it seems that there is sufficient readily available ice to cause multi-meter sea level rise this century, if dynamic discharge of ice increases exponentially.” (p.23)

    “Exponential change cannot continue indefinitely. The negative feedback terminating exponential growth of ice loss is probably regional cooling due to the thermal and fresh-water effects of melting icebergs. Temporary cooling occurs as icebergs and cold fresh glacial melt-water are added to the Southern Ocean and the North Atlantic Ocean.” (p.24)

    “By 2065, when the sea level rise (from ice melt) is 60 cm relative to 2010, the cold freshwater reduces global mean warming (relative to 1880) from 1.86°C to 1.47°C. By 2080, when sea level rise is 1.4 m, global warming is reduced from 2.19°C to 0.89°C.” (p.25)

    “[B]urning all or most fossil fuels guarantees tens of meters of sea level rise, as we have shown that the eventual sea level response is about 20 meters of sea level for each degree Celsius of global warming. We suggest that ice sheet disintegration will be a nonlinear process, spurred by an increasing forcing and by amplifying feedbacks, which is better characterized by a doubling time for the rate of mass disintegration, rather than a linear rate of mass change. If the doubling time is as short as a decade, multi-meter sea level rise could occur this century.” (p.27)

    The negative iceberg cooling feedback ends the exponential growth of SLR, according to H&S. In their model example the rise from 2080-2100 would still be about a meter, if I understand correctly. So that would make SLR by 2100 about 2.5 meters in total.

    So what does multi-meter mean? Somewhere between 2-5 meters for this century, and many more for the next centuries, if H&S are correct.

  2. 102
    Martin Manning says:

    #95 John L
    Yes, your version of how to state the SLR projection for RCP8.5 would definitely be clearer. But would the lead authors agree? I don’t know.

    This is a long standing problem in climate science. Morgan & Keith (1995, Subjective judgements by climate experts. Environmental Science & Technology 29, 468-476) showed that for some basic questions a very careful way of determining the views of experts can lead to a bimodal distribution rather than just a broad range of views around a single mode value. This then raises questions about the usefulness of a median.

    Also I’m not just using the terms ‘fuzzy’ or ‘possibility’ in a loose sense as these are both being used quite extensively in other fields of science. There is a journal called ‘Fuzzy Sets and Systems’ and several journals cover ‘possibility theory’ – e.g. a good paper is: Dubois, 2006, Possibility theory and statistical reasoning. Computational Statistics & Data Analysis 51, 47-69.

    Uncertainties are here to stay, and the new estimates for SLR coming from models show that they are also increasing, so we need to get better at living with them.

  3. 103

    Here’s a scenario. Note that a lot of the Antarctic ice sheet is grounded well below sea level (over 2km down in parts) and that a lot of that below sea level ground extends right to the ocean. If a warming ocean starts to melt ice from underneath and it comes away in increasingly large chunks, all it takes is significant isostatic rebound to create a domino effect, and a very large chunk comes away. That means even more rebound, deep under water. That sort of thing tends to cause tsunamis.

    No one AFAIK has a model that can predict how this sequence of events could unfold. But perhaps we will see the experiment performed?

  4. 104

    #97, 98, 100–“So let me see Hank, you and Lennart are allowed to guess at what the author means and I am not? ”

    Keith, they didn’t say that; they presented their own interpretations for your consideration. You *could* choose to actually consider them.

  5. 105
    Lennart van der Linde says:

    So how plausible is exponential growth of ice mass loss from GIS and AIS? Why would CO2-emissions grow exponentially, but not ice mass loss? Hansen & Sato give the example of a 10-yr doubling time, but how about a 20- or 15-yr doubling time?

    Ice mass loss from the ice sheets over 2005-2010 was about 1 mm/yr of SLR (Shepherd et al 2012). So let’s say this will be the average over 2000-2020. A 20-yr doubling time implies about 64 cm of SLR-contribution by 2100, so about 1.2 m of total SLR (assuming about 20 cm from glaciers and about 35 cm from thermal expansion). The rate of SLR would by about 16 mm/yr from 2080-2100.

    A 15-yr doubling time, starting with about 1 mm/yr from 2005-2020, implies about 115 cm of SLR-contribution from the ice sheets by 2100, so about 1.7 m total SLR by 2100, and assuming about 4 cm/yr rise from 2095-2100. By 2200 and 2300 this could imply about 5.7 m and 9.7 m, assuming a constant rate of SLR of 4 cm/yr during this period and no strong kinematic constraints.

    A 14-yr doubling time, starting with circa 1 mm/yr from 2002-2016, implies about 178 cm of SLR-contribution from the ice sheets by 2100, so about 2.3 m total SLR by 2100, assuming about 5 cm/yr rise from 2086-2100. By 2200 and 2300 this could imply about 7.3 m and 12.3 m, assuming a constant rate of SLR of 5 cm/yr during this period and no strong kinematic constraints.

    A 14-yr doubling time implies about a 5%/yr acceleration in ice mass loss. How plausible is that, assuming CO2-emissions rise about 2-3%/yr? Maybe the acceleration in the rise of CO2-concentrations would be a better metric? An acceleration of 5%/yr would imply either strong continued use of fossil fuels and/or a strong positive feedback in lower carbon uptake by carbon sinks and/or higher release by sources. Or would rise in temperature/forcing/ocean heat uptake be the better metric, since also albedo changes in the Arctic, for example, could accelerate the warming and melting/disintegration of the ice sheets?

    Anyhow, to me this seems a worst-case scenario that we cannot rule out at this moment, so continued emissions pose a great risk to our common future (not to speak of possibly even more abrupt ice-mass changes and other risks of further warming).

  6. 106
    Hank Roberts says:

    > I am reading the paragraph as a
    > normal, sensible, educated English speaker

    How does that work for you when reading science papers?
    I don’t trust myself to know what an excerpt means, without the context — science papers usually define their terms.

    Let’s find out. It will be fun.

  7. 107
    Jim Larsen says:

    103 Philip M said, ” That sort of thing tends to cause tsunamis.”

    The ice shelves are already floating.

    “significant isostatic rebound”

    Ice deforms and adjusts far faster than isostatic rebound. We’re still experiencing rebound from the last ice age.

  8. 108
    Hank Roberts says:

    Back in 2001, we got our first, surprising look at the base of the icecap:

    Equipped with two cameras and lights, JPL’s ice probe revealed what appears to be a basal water system, or series of water channels at the base of the ice stream. In places, this water-filled cavity measured approximately 1.4 meters deep (4.6 feet). Based on previous calculations, researchers expected the depth of a water basal cavity to be only in the millimeter range.

    To the researchers’ surprise, they also found rock and other debris embedded in the ice much higher than expected. It was believed that frozen debris would be found no higher than two meters (almost seven feet) off the base of the ice stream. In contrast, the visual data shows frozen debris some 26 meters (85 feet) off the base, which has yet to be explained.

    A layering effect in the ice was also uncovered by the probe. Though not yet fully understood, it is thought that, upstream, ice and gravel have frozen onto the base of the ice sheet. With the ice streams constantly moving, water may slide under debris-laden layers, lifting them up, allowing the process to repeat.

    “The layered information will turn out to be very interesting,” said Carsey. “These layers tell us about processes upstream.” By analyzing these ice layers, researchers may learn how ice streams flow and stop flowing.

  9. 109
    Hank Roberts says:

    There’s newer information on deep ice probe work, e.g.
    lists for example
    Christoffersen P., Tulaczyk S., Carsey F., and Behar A., “A quantitative framework for interpretation of basal ice facies formed by ice accretion over subglacial sediment”, Journal of Geophysical Research, (in Press)

    and much else out there, e.g.
    Nature Geoscience 2, 585 – 588 (2009)
    Published online: 20 July 2009 | doi:10.1038/ngeo581
    Formation of mega-scale glacial lineations observed beneath a West Antarctic ice stream

    … a dynamic sedimentary system that undergoes significant change by erosion and deposition on decadal timescales.

    The assumption that Antarctica won’t change for thousands of years is as outdated as thinking continents don’t drift.

  10. 110
    Jim Larsen says:

    Here’s a nice article which discusses tides and ice streams/shelves. They found one ice stream (the Rurford) varies its speed by up to 20% every two weeks to coincide with spring and neap tides. Tidal forces far outweigh sea level rise and isostatic rebound.

  11. 111
    Jim Larsen says:


  12. 112
    Lennart van der Linde says:

    One more point about Hansen & Sato: 2100 is of course an arbitrary year; the eventual rise and magnitude of the rise in sea level are the most relevant variables for adaptation and mitigation considerations.

    With a 20-yr doubling time the rate of rise could be about 32 mm/yr from 2100-2120 and the total rise almost 2 m by 2120.

    It seems H&S think about 5-6 meter/century is the maximum rate of rise under BAU and that about 10 meter of long-term SLR is almost inevitable by now, even if we would somehow manage to return to 350 ppm CO2 within a few centuries.

    How long would it take to reach this maximum rate of rise and how long could that rate be sustained under different forcing scenarios? How could we adapt to such rates and magnitudes of SLR? How can we avoid the worst scenarios? How will IPCC AR5 answer these (and other) questions? How will humanity as a whole answer them?

  13. 113
    perwis says:

    Martin Manning #83:

    From a practical climate adaptation perspective it would be useful to have something like
    p(x>0.98 m)=y%, where x=GMSLR in year 2100 given RCP8.5

    But interpreting the IPCC definition of “likely”, then all we can say on the subjective judgments of the authors in the SLR chapter of the IPCC is:
    p(x>0.98 m)=anywhere between 0% to 34%

    Then, if we are risk-aversive we could choose to base our adaptation plans on p(x>0.98)=34% and if we are risk-takers we could choose p(x>98)=0%.

    Is this how those of us working with climate change adaptation should interpret the IPCC numbers? If not, why not?

    Finally, it is worth noting that the recent NOAA expert assessment provides a much more unequivocal statement:
    “We have very high confidence (>9 in 10 chance) that global mean sea level will rise at least 0.2 meters (8 inches) and no more than 2.0 meters (6.6 feet) by 2100.” (Parris et al 2012).

    Parris, A., P. Bromirski, V. Burkett, D. Cayan, M. Culver, J. Hall, R. Horton, K. Knuuti, R. Moss, J. Obeysekera, A. Sallenger, and J. Weiss. 2012. Global Sea Level Rise Scenarios for the US National Climate Assessment. NOAA Tech Memo OAR CPO-1. 37 pp.

  14. 114
    Lennart van der Linde says:

    Perwis #113,

    That NOAA Assessment is indeed very interesting compared to IPCC AR5. Would it be fair to say thay NOAA estimates the likelihood of more than 1 m of SLR by 2100 as about 50% under RCP8.5? And the chance of more than 2 m as about 5%?

    If so, it seems they give Hansen & Sato a 5% chance of being right. To ignore this chance, would be a significant risk for society to take, in my book, in view of the potential effects of such a large and fast SLR.

  15. 115
    gavin says:

    From one of the chapter authors, referring to comment #25:

    There are two elements to PGs in this caption. In the case of the “No Antarctic PGs,” this refers specifically to the glacier model results from Marzeion et al., where they used CRU data (“observed climate” in caption for Fig. 13.7), for which there are none for Antarctica, thus their modeled glacier contribution does not include Antarctic PGs.

    The “No Antarctic PGs” in the figure legend on panel (a) for the “Individual CMIP5 AOGCMs” and the “CMIP5 AOGCM mean” should not be there, and will be removed in published draft. It does belong with “Adjusted CMIP5 AOGCM mean” because the “adjustment” is the addition of the other terms (land water and glaciers except Antarctic PGs – see above) to the AOGCMs.

    Then the figure caption refers to the ice-sheet contributions since 1993, which are from Chapter 4’s (cryosphere) assessment of all methods of measuring changes in the mass of the ice sheets. Since some of these methods (i.e., GRACE) cannot resolve the PGs around the ice sheets, Chapter 4 included the PGs with the ice sheets in reporting the ice-sheet mass change. The dotted line on the figure is the “adjusted model mean” beginning in 1993 which is the OAGCM (thermal expansion) PLUS land water, glaciers, and the ice-sheet contributions.

    But since the PGs around Greenland and Antarctica are included in these ice-sheet contributions, the glacier contribution to the “adjusted model mean” since 1993 excludes the PGs.

    The figure caption was not very clear and will undergo some clarification for the published version.

  16. 116
    Tom Bond says:

    The IPCC AR 5 paper Summary for Policy Makers under Section 3 Observed Changes/B3 Cryosphere states;

    The average rate of ice loss from the Greenland ice sheet has very likely substantially increased from 34 [–6 to 74] Gt yr–1 over the period 1992–2001 to 215 [157 to 274] Gt yr–1 over the period 2002–2011.
    The average rate of ice loss from the Antarctic ice sheet has likely increased from 30 [–37 to 97] Gt yr–1 over the period 1992–2001 to 147 [72 to 221] Gt yr–1 over the period 2002–2011.

    In round terms this is an increase of polar ice melt mass loss from 60 Gt per year to 360Gt per year or a six-fold increase.

    With GHG emissions increasing exponentially why wouldn’t this ice melt rate continue to also rise exponentially?

    Coastal planning policy makers globally would be wise to monitor this ice melt closely as it will be a useful tool in determining the projected maximum credible sea level rise estimation for different time frames.

  17. 117
    wili says:

    “With GHG emissions increasing exponentially why wouldn’t this ice melt rate continue to also rise exponentially?”

    Good question. Even more so when you factor in feedbacks.

    So can some of our math wizzes calculate where a sextupling of ice cap melt every ten years would put us in 2100 wrt SLR?

  18. 118
    Ray Ladbury says:

    Regarding polar melting:

    Remember that as you move toward the poles, the ice will become more stable. At some point, you will have run up against ice that only sees a few months of sun per year. Runaway melt isn’t something that will keep me awake at nights.

  19. 119
    Sloop says:

    New ocean observing system to be deployed in North Atlantic subpolar region to comprehensively measure overturning circulation.

    This ocean observing system (an impressive ocean engineering achievement, once the team gets it to work as intended) may provide data to compel future changes to SLR projections for US/Canada seaboard. A slowing of northward Atlantic surface currents will lead to increased SLR along western Atlantic coast.

  20. 120
    wili says:

    Good point, Ray. I would be more comforted by it if sun were the only factor in the melt. Certainly the scarcity of sun at the north pole hasn’t seemed to keep things from what sure looks to me like a “runaway” (here meaning rapidly accelerating) melt of Arctic sea ice, especially when one looks at the volume numbers/graphs.

  21. 121
    Lennart van der Linde says:

    Ray, at what elevation do you live?

  22. 122

    This is a long standing problem in climate science. Morgan & Keith (1995, Subjective judgements by climate experts. Environmental Science & Technology 29, 468-476) showed that for some basic questions a very careful way of determining the views of experts can lead to a bimodal distribution rather than just a broad range of views around a single mode value. This then raises questions about the usefulness of a median.

  23. 123
    Patrick 027 says:

    re 103 Philip Machanick, 107 Jim Larsen

    regarding isostatic rebound – there is a plastic part and an elastic part. The elastic part is fast – presumably on the time scale that it would take seismic waves to travel relevant distances. The elastic part is about 30 % of the total (William F. Ruddiman, “Earth’s Climate: Past and Future”, W.H. Freeman and Company, New York, 2001, p218) …

    (PS some of the elastic part is relieved as the plastic part takes over, because plastic deformation would be responding to the force that supports elastic deformation; however the elastic component that matches compression at increased pressure must remain (isostatic adjustment reduces the pressure anomaly at depth – at a given location, and also following the material when there is sideways flow above)… and also, see next sentence).

    … However, the crust – or at least the upper part of it (?) – has some rigidity and can support (some) shear stresses indefinitely …

    (the lithosphere as a whole is harder to deform, but does; the asthenosphere is more plastic – due to greater dissolved H2O due to the history of relative lack of partial melting, rather than the greater partial melting (which would deplete the rock of H2O as melt escapes upward, leaving behind lithospheric rock, from what I’ve read (Karato, Shun-ichiro, “The Dynamic Structure of the Deep Earth”, Princeton University Press, Princeton, 2003))

    …- ie a load on the surface, or removal or addition thereof, won’t need or change the underlying crust with the same distribution – it will be spread laterally.

    (Though it is interesting to consider an ice sheet grounded below sea level, with the top of the ice removed – what if some ice was temporally stuck against the bottom, before it broke free… That’s the one thing missing from the movie “2012” – why didn’t they show those ~1 mi high tsunamis rolling over Antarctica – seriously, a missed opportunity for some adrenaline (and norepinephrine?)-charged CGI magic… but anyway…)

    I actually have wondered about what that means for sea level rise. If the oceans sank 30 % of their equilibrium response to the increased mass … well, okay, water is a little less than 1/3 density of (uppermost) mantle -uppermost mantle is about 3.3 kg/L ( Dott, Robert H. Jr., and Donald R. Prothero, “Evolution of the Earth” 5th ed., McGraw-Hill, Inc, St. Louis, 1994, p119), so the equilibrium response would be 2.3/3.3 ~= 70 % of the rise from volume increase without any isostatic adjustment (depressing the mantle by 1 m creates a negative mass anomaly equal to 3.3 m of H2O – this ignores elastic compression, which must remain), so the short term response would be ~ 91% of what it would be without adjustment) … but the water is being added all the way to the shifting coastline itself, so the edge of the continent is being weighed down too … but crustal rigidity would result in an increased downward slope to the sea, so maybe the net isostatic change at the coast tends to be zero, thus allowing the 91% rise in sea level to be realized there…????? (PS I only understand this stuff on an introductory level)…

    …well, the maps above show much greater regional variation than that in some places.

    (If the graph Fig. 10-10 B in Ruddiman p.217 is taken literally – which I’m not sure it’s meant to (it has a schematic feel to it, although there are tick marks and labels on one axis), it looks like isostatic response reaches 1/2 equilibrium value ~ 1500 years (eyeballing), ~4/5 @~5000 yrs (eyeballing), and nearly complete by 18000 – 19000 years.)

  24. 124
    Patrick 027 says:

    … oh, I think some of that stuff may vary with the spatial scale of the load/changes and presumably the oceanic crust will flex more easily than the continental crust just because it’s thinner (and often warmer) (and I think the oceanic lithosphere may be thinner too but I just put my books away so that’s it for now).

  25. 125
    Patrick 027 says:

    (PS some of the elastic part is relieved as the plastic part takes over, because plastic deformation would be responding to the force that supports elastic deformation; however the elastic component that matches compression at increased pressure must remain (isostatic adjustment reduces the pressure anomaly at depth – at a given location, and also following the material when there is sideways flow above)… and also, see next sentence).

    I forgot about the temporary shear strains that would also reduce (but spread) the pressure anomaly at depth initially, so I’m not sure exactly how the pressure anomaly varies over time, but that’s way off on a tangent.

  26. 126
    Brian Carter says:

    A complication is that (if I remember correctly) isostatic rebound has zero total volume change, so the area that has lost its load rises and the surrounding area sinks to compensate. If for example the Greenland ice melts and the area that was under it rises, the coastal area and the shelf will sink to compensate. Irrespective of what happens adjacent to Greenland, the global sea level rise will be reduced. No doubt this has been allowed for in the SLR calculations.

  27. 127
    AbruptSLR says:

    If you go to the following website:

    and then click on the button in the upper right-hand corner labeled: “Arctic Sea Ice Forum” and then click on the button labeled: “Antarctica” you will find almost a thousand posts referencing peer reviewed evidence that supports the position that the AR5 SLR projections do not adequately capture the risk of the Antarctic contribution to SLR both this century and into the future. While individually each one of the referred lines of evidence can be discounted, and/or ignored, cummulatively these lines of evidence clear indicate that the IPCC AR5 SLR projections will very likely be revised upward (probably several times) as more data is collected and trends for accelerating ice mass loss from the Antarctic Ice Sheet, AIS, and particularly from the West Antarctic Ice Sheet, WAIS, become better documented.

  28. 128

    Thanks to several who pointed out my terrible error in this thread earlier. I apologise with a very red face. Schoolboy error!

  29. 129

    “it will accelerate further in this century”

    It may accelerate, but it has certainly not accelerated during the past 2 decades. Before a brave may => will transformation is attempted, we need an explanation for the observed slight deceleration during the satellite era.

    CU Sea Level Research Group
    Deceleration is about 0.2 m/cy² on both datasets.

  30. 130
    Hank Roberts says:

    > observed slight deceleration
    You linked to the AVISO page; seems to me that it explains the wiggles you “observe” in the text rigt on the page. There’s nothing there about detecting a change in rate in that data.

    sea level trends patterns observed by satellite altimetry are transient features.

  31. 131
    L Hamilton says:


    Is the “deceleration” in sea level based on fitting a quadratic? What’s the justification for that model? If we don’t assume a model and just use lowess regression, the pattern of sea level change does not look anything like a quadratic, nor is there deceleration.

  32. 132

    Regardless of your view on whether human activity is contributing to climate change, I hope we can agree that the flaring of waste natural gas from North Dakota oil and gas fields is a terrible waste that should be quickly addressed. See nyt article at. see photo USA at night at

  33. 133
    AbruptSLR says:

    The following reference makes it very clear that over the last two decades, the measureable SLR has been accelerating:

    Calafat and Chambers; GRL, July 2013; doi: 10.1002/grl.50731

    Calafat, F. M., and D. P. Chambers (2013), Quantifying recent acceleration in sea level unrelated to internal climate variability, Geophys. Res. Lett., 40, 3661–3666, doi:10.1002/grl.50731.

    “Sea level observations suggest that the rate of sea level rise has accelerated during the last 20 years. However, the presence of considerable decadal-scale variability, especially on a regional scale, makes it difficult to assess whether the observed changes are due to natural or anthropogenic causes. Here we use a regression model with atmospheric pressure, wind, and climate indices as independent variables to quantify the contribution of internal climate variability to the sea level at nine tide gauges from around the world for the period 1920–2011. Removing this contribution reveals a statistically significant acceleration (0.022 ± 0.015 mm/yr2) between 1952 and 2011, which is unique over the whole period. Furthermore, we have found that the acceleration is increasing over time. This acceleration appears to be the result of increasing greenhouse gas concentrations, along with changes in volcanic forcing and tropospheric aerosol loading.”

  34. 134
    wili says:

    “0.022 ± 0.015 mm/yr2”

    ASLR, I’m guessing you have done the calculations on what such a rate of acceleration, if it persists, would mean for total SLR (even without sudden rises) for 2050, 2100…?

  35. 135
    Lennart van der Linde says:

    Wili #134,

    Calafat & Chambers 2013 say:
    “[S]ince 1973, SL accelerations have been increasing at a significant rate of 0.002mm/yr3 until reaching its present value of 0.022 ± 0.015mm/yr2 for the 60 year record centered around 1982… [T]he detected SL acceleration is likely not part of a natural cycle.”

    So let’s assume, conservatively (?), that average SLR is about 3 mm/yr in 2000 and that the average acceleration will be about 0.05 mm/yr2 for the period 2000-2020. Let’s further assume this acceleration doubles every 20 years (so the acceleration increases by about 3.5 %/yr2). Then average SLR would be about 4 mm/yr in 2020 and about 3.5 mm/yr in the period 2000-2020. So SLR over that period would be about 7 cm.

    The average acceleration would be about 0.1 mm/yr2 from 2020-2040, so by 2040 the average SLR would be about 6 mm/yr and the average SLR over this periode would be about 5 mm/yr. So total SLR over this period would be about 10 cm.

    Continuing this excercise would imply an average acceleration from 2040-2060 of about 0.2 mm/yr2, so average SLR would be about 10 mm/yr by 2060 and average SLR over this period would about 8 mm/yr, with a total SLR of about 16 cm.

    For 2060-2080 the average acceleration would be about 0.4 mm/yr2, so average SLR would be about 18 mm/yr by 2080 and average SLR over this period would be about 14 mm/yr. Total SLR from 2060-2080 would then be about 28 cm.

    The average acceleration would be about 0.8 mm/yr2 from 2080-2100, with average SLR about 34 mm/yr by 2100 and about 26 mm/yr for this whole period. Total SLR for this period would then be about 52 cm.

    Adding all this up would give about (7 + 10 + 16 + 28 + 52 cm =) 113 cm of SLR over this century, and a rise of 3-4 cm/decade by 2100, so maybe 3-4 meter/century, so total SLR could then be about 4-5 meters by 2200 and 7-9 meters by 2300, assuming no significant further acceleration (or deceleration).

  36. 136
    wili says:

    Thanks, LvdL, and sorry for my mathematical laziness. That fits my general impression that the current rates of acceleration will not yield more than a few inches/dozen centimeters within the next few decades, but then things start getting much worse quite quickly. Of course, this is all just curve fitting. I expect there could be some major discontinuities, possibly on both sides (as you say, acceleration or deceleration): Richard Alley, among others, has pointed out the possibility, at least, of much more abrupt SLR; Hansen has pointed out that at some point all that ice being dumped in the Ocean is likely to cool down the planet for a while, though he was mostly thinking about GIS loss to the Atlantic and subsequent cooling (or slowing down of warming?) of the Northern Hemisphere, iirc.

    If we are to go beyond mere mathematical projections–what (other) major feedbacks are likely to kick in that might affect this trajectory either way?

    (And thanks again for doing my maths for me.)

  37. 137
    AbruptSLR says:

    You cite Hansen’s most famous negative feedback factor of large amounts of ice cooling the local oceans (and then the associated atmosphere, another negative feedback would be that as the Hadley Cells continue to expand (north and south)the associated larger desert regions send dust into the Southern Ocean to promote plankton growth and associate sequestration of Carbon Dioxide.

    The number of possible positive feedback factors are much more numerous including: (a) an albedo flip as the polar sea ice and marine ice sheet (the WAIS) retreat faster than modelled; (b) a projected change of the Filchner-Ronne Ice Shelf, FRIS, from a cold ice shelf to a warm ice shelf due to projected changes in wind driven local ocean currents could promote a collapse of the FRIS; and some people believe that the Ross Ice Shelf, RIS, could follow soon thereafter; (c) increases in eustatic SLR from ice mass loss from the GIS could destabilize many of the coastal glaciers in both the WAIS and the EAIS; (d) degradation/loss of the permafrost could pump much more methane into the atmosphere than previously projected; (e) increases in the intensity and/or frequency of EL Nino events would clearly accelerate the rate of SLR; (f) once sufficient ice mass loss occurs from an ice sheet the local seismic activity would increase which would promote more ice mass loss; (g) a possible flip in the atmospheric circulation cells to a pattern associated with an equable climate; and probably most certainly (h) the warming of the Southern Ocean’s Circumpolar Deep Water, CDW, (associated with the current El Nino hiatus period) will certainly promote future acceleration of ice mass loss from both the WAIS and the EAIS due to the advection of warm water against the groundlines for key glaciers/ice sheets/ice shelves.


  38. 138
    AbruptSLR says:


    Not to sound like an alarmist but some other random negative feedback factors that I did not list previously include:
    (a) increase in wildfires due to climate change; (b) increase in the warming potential of methane due to changes in atmospheric chemistry as the methane concentration increases; (c) increasing polar storm activity increases ice mass loss, increases polar amplification and increases upwelling of warm CDW in the Southern Ocean; (d) the basal geothermal heating increases (particularly in the WAIS) as ice mass loss increases the upwell of magma behind the thin lithosphere; (e) increase in the rate of ice calving as glaciers (particularly in the WAIS) retreat down a negative slope on the seafloor; (f) increase in methane hydrate decomposition with global warming; (g) increased release of soil organic carbon (SOC) from terrestial sources (particularly forests) with global warming; (h)the cleaning up of air pollution (aerosols) particularly in China will increase radiative forcing; (i)global warming will likely decrease the absorption of carbon dioxide in both the ocean and terrestial sinks; (j) the albedo flip associate with vegetation growth in the Arctic (in addition of surface ice loss and carbon black deposits (particularly in Greenland)); (k) displacement of polar regional plankton with nano-plankton (that does not sink as well) due to regional ocean warming; (l) change in dimethylsulphite (DMS) due to ocean acidification; (m) increased surface ice melt in Antarctica leading to more ice mass loss (and more SLR); (n) albedo flip due to increased area of permanent land inundation (eg Florida, Louisiana, river deltas around the world); (o) increases in the global hydrolocial cycles; and (p) non-linear interactions between all of these negative feedback factors.

  39. 139
    wili says:

    ASLR wrote: “The number of possible positive feedback factors are much more numerous”

    That is my impression, too. May I add to your excellent list:

    –As the GIS melts, the elevation of its highest areas (and really all areas) drops down to levels that are warmer on average, thus melting them faster, lowering them faster…

    –melt pools on the surface of the ice sheet encourage biological activity that changes the albedo of that part of the ice sheet, thus forming more and bigger pools, more biological activity…

    –As ice melts, it can reveal the dust in older ice and concentrate it on the surface, thus changing albedo…

    –more of a forcing than a feedback, but there was an article a year or two ago iirc that claimed that higher CO2 levels actually directly weakened ice structures.

    Do we have any idea how much carbon is under these ice sheets that will be released as they melt back?

    I would love to hear that there are other powerful negative feedbacks to offset some of these.

  40. 140
    AbruptSLR says:

    The article at the following link indicates how chaos theory can be used to estimate the probabilities of dragon king events (such as ASLR):

  41. 141
    Hank Roberts says:

    > ASLR
    Nice list. Got citations for those items? It’d be most helpful to have pointers to your sources for each of them.

  42. 142
    Dan H. says:

    Extrapolating a 50-year acceleration trend over three centuries may not prove fruitful. Continuing your acceleration would yield 20-25 m by 2500, more than the entrire Greenland and West Antarctic ice sheets combined. C & C also noted that the acceleration existed only since 1952, and “is due to external forcing (anthropogenic and/or natural).” I think the critical wording in your post is “assuming no significant further acceleration or deceleration.”

  43. 143
  44. 144
    Lennart van der Linde says:

    Dan #142,

    You’re right that extrapolating several centuries on may be not prove fruitful, but then again it my prove more fruitful then assuming the process-based ice sheet models are mature enough to fully trust them.

    It does seem likely, not to say inevitable, that any acceleration will stop sooner or later. The question is when and why?

    Many experts seem to think we don’t understand the ice sheets well enough at this point to exclude such an extrapolated scenario, so from a risk- and rights-based perspective it seems unwise and wrong to simply follow the process-based models.

  45. 145
    Dan H. says:

    The lack of understanding is a huge stmbling block, and another reason not to extrapolate too far. The other issus is that C & C used nine tidal gauges from the U.S., Europe, and Australia for their analysis. Their acceleration and uncertainty calculations are for these sites, which varied substantially (one actually showed a sea level decline). Expanding their results to the entire ocean basin adds greater uncertainty to their calclations.

    Of course acceleration must stop – the icecaps have a limited supply of frozen water. As mentioned in C & C, the acceleration is dependent on the external forcings. Changes there will lead to further acceleration or deceleration.

  46. 146
    AbruptSLR says:


    Here is a reference on the increasing intensity of El Nino, with global warming:

    Other references can be found here:,13.0.html

    and here:,41.0.html

    and here:,70.0.html

  47. 147
    Hasis says:

    Dan H – “Continuing your acceleration would yield 20-25 m by 2500, more than the entrire Greenland and West Antarctic ice sheets combined.”

    EAIS not get a look in then?

  48. 148
    Hank Roberts says:

    > Abrupt
    Sorry, I wasn’t clear.

    Could you cite to primary sources — papers published in science journals?

    Often, newspaper and blog sources do mention where they found what they’re telling you. Sometimes it’s just another blog, copypasting. It’s always worth checking.

    If you take the time, and look up the original paper — you may get better information. Otherwise you end up repeating what someone on the Internet somewhere says it says.

    Gavin asks us readers to use DOI when citing sources. You’ll generally be able to provide that if you look for it.

    Not doubting your opinion — just wanted to ask if you’d looked into the claims yourself. It’s generally a good idea.

  49. 149
    Lennart van der Linde says:


    Paleo data suggest that the current CO2 level could already lead to 10-20 meters of SLR in the long term (e.g. Foster & Rohling 2013), not to speak of the effects of much higher CO2 levels. The question is how fast this could happen. It seems 3-5 meters per century cannot be excluded at this point, even if process based models don’t show such fast rises (yet?).

    The current and potential future forcing (globally) seems so much stronger than those in the past, that it would seems very risky to put too much trust in the current process based models. We don’t know enough, but what we do know, suggests to me that semi-empirical and expert extrapolations are very useful and needed complements to the process based models for informing mitigation and adaptation decisions.

  50. 150
    Lennart van der Linde says:


    For your information: ASLR posts all the original sources on the forum-pages he linked to. If your read them you find many papers supporting his statement here.