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Ups and downs of sea level projections

Filed under: — stefan @ 31 August 2009

By Stefan Rahmstorf and Martin Vermeer

The scientific sea level discussion has moved a long way since the last IPCC report was published in 2007 (see our post back then). The Copenhagen Synthesis Report recently concluded that “The updated estimates of the future global mean sea level rise are about double the IPCC projections from 2007″. New Scientist last month ran a nice article on the state of the science, very much in the same vein. But now Mark Siddall, Thomas Stocker and Peter Clark have countered this trend in an article in Nature Geoscience, projecting a global rise of only 7 to 82 cm from 2000 to the end of this century.

Coastal erosion: Like the Dominican Republic, many island nations are particularly vulnerable to sea level rise. (c) S.R.
Coastal erosion: Like the Dominican Republic, many island nations are
particularly vulnerable to sea level rise. (Photo: S.R.)

Semi-empirical sea level models

Siddall et al. use a semi-empirical approach similar to the one Stefan proposed in Science in 2007 (let’s call that R07) and to Grinsted et al. (2009), which we discussed here. What are the similarities and where do the differences come from?

For short time scales and small temperature changes everything becomes linear and the two new approaches are mathematically equivalent to R07 (see footnote 1). They can all be described by the simple equation:

dS/dt = a ΔT(t) + b     (Eq 1)

dS/dt is the rate of change of sea level S, ΔT is the warming above some baseline temperature, and a and b are constants. The baseline temperature can be chosen arbitrarily since any constant temperature offset can be absorbed into b. This becomes clear with an example: Assume you want to compute sea level rise from 1900-2000, using as input a temperature time series like the global GISS data. A clever choice of baseline temperature would then be the temperature around 1900 (averaged over 20 years or so, we’re not interested in weather variability here). Then you can integrate the equation from 1900 to 2000 to get sea level relative to 1900:

S(t) = a ∫ΔT(t’) dt’ + b t     (Eq 2)

There are two contributions to 20th C sea level rise: one from the warming in the 20th Century (let’s call this the “new rise”), and a sea level rise that results from any climate changes prior to 1900, at a rate b that was already present in 1900 (let’s call this the “old rise”). This rate is constant for 1900-2000 since the response time scale of sea level is implicitly assumed to be very long in Eq. 1. A simple matlab/octave code is provided below (2).

If you’re only interested in the total rise for 1900-2000, the temperature integral over the GISS data set is 25 ºC years, which is just another way of saying that the mean temperature of the 20th Century was 0.25 ºC above the 1900 baseline. The sea level rise over the 20th Century is thus:

S(1900-2000) = 25 a + 100 b     (Eq. 3)

Compared to Eq. 1, both new studies introduce an element of non-linearity. In the approach of Grinsted et al, sea level rise may flatten off (as compared to what Eq 1 gives) already on time scales of a century, since they look at a single equilibration time scale τ for sea level with estimates ranging from 200 years to 1200 years. It is a valid idea that part of sea level rise responds on such time scales, but this is unlikely to be the full story given the long response time of big ice sheets.

Siddall et al. in contrast find a time scale of 2900 years, but introduce a non-linearity in the equilibrium response of sea level to temperature (see their curve in Fig. 1 and footnote 3 below): it flattens off strongly for warm temperatures. The reason for both the long time scale and the shape of their equilibrium curve is that this curve is dominated by ice volume changes. The flattening at the warm end is because sea level has little scope to rise much further once the Earth has run out of ice. However, their model is constructed so that this equilibrium curve determines the rate of sea level rise right from the beginning of melting, when the shortage of ice arising later should not play a role yet. Hence, we consider this nonlinearity, which is partly responsible for the lower future projections compared to R07, physically unrealistic. In contrast, there are some good reasons for the assumption of linearity (see below).

Comparison of model parameters

But back to the linear case and Eq. 1: how do the parameter choices compare? a is a (more or less) universal constant linking sea level to temperature changes, one could call it the sea level sensitivity. b is more situation-specific in that it depends both on the chosen temperature baseline and the time history of previous climate changes, so one has to be very careful when comparing b between different models.

For R07, and referenced to a baseline temperature for the year 1900, we get a = 0.34 cm/ºC/year and b = 0.077 cm/year. Corresponding values of Grinsted et al. are shown in the table (thanks to Aslak for giving those to us!).

For Siddall et al, a = s/τ where s is the slope of their sea level curve, which near present temperatures is 4.8 meters per ºC and τ is the response the time scale. Thus a = 0.17 cm/ºC/year and b = 0.04 cm /year (see table). The latter can be concluded from the fact that their 19th Century sea level rise, with flat temperatures (ΔT(t) = 0) is 4 cm. Thus, in the model of Siddall et al, sea level (near the present climate) is only half as sensitive to warming as in R07. This is a second reason why their projection is lower than R07.


a [cm/ºC/year]

[cm /year]

“new rise” [cm] (25a)

“old rise” [cm] (100b)


total model rise [cm]








Grinsted et al “historical”







Grinsted et al “Moberg”







Siddall et al






8.3 (?) 7.9

Performance for 20th Century sea level rise

For the 20th Century we can compute the “new” sea level rise due to 20th Century warming and the “old” rise due to earlier climate changes from Eq. 3. The results are shown in the table. From Grinsted et al, we show two versions fitted to different data sets, one only to “historical” data using the Jevrejeva et al. (2006) sea level from 1850, and one using the Moberg et al. (2006) temperature reconstruction with the extended Amsterdam sea level record starting in the year 1700.

First note that “old” and “new” rise are of similar magnitude for the 20th Century because of the small average warming of 0.25 ºC. But it is the a-term in Eq. (2) that matters for the future, since with future warming the temperature integral becomes many times larger. It is thus important to realise that the total 20th Century rise is not a useful data constraint on a, because one can get this right for any value of a as long as b is chosen accordingly. To constrain the value of a – which dominates the 21st Century projections — one needs to look at the “new rise”. How much has sea level rise accelerated over the 20th Century, in response to rising temperatures? That determines how much it will accelerate in future when warming continues.

The Rahmstorf model and the Grinsted “historical” case are by definition in excellent agreement with 20th Century data (and get similar values of a) since they have been tuned to those. The main difference arises from the differences between the two sea level data sets used: Church and White (2006) by Rahmstorf, Jevrejeva et al. (2006) by Grinsted et al. Since the “historical” case of Grinsted et al. finds a ~1200-year response time scale, these two models are almost fully equivalent on a century time scale (e-100/1200=0.92) and give nearly the same results. The total model rise in the last column is just 1.5 percent less than that based on the linear Eq. 3 because of that finite response time scale.

For the Grinsted “Moberg” case the response time scale is only ~210 years, hence our linear approximation becomes bad already on a century time scale (e-100/210=0.62, the total rise is 15% less than the linear estimate), which is why we give the linear estimates only in brackets for comparison here.

The rise predicted by Siddall et al is much lower. That is not surprising, since their parameters were fitted to the slow changes of the big ice sheets (time scale τ=2900 years) and don’t “see” the early response caused by thermal expansion and mountain glaciers, which makes up most of the 20th Century sea level rise. What is surprising, though, is that Siddall et al. in their paper claim that their parameter values reproduce 20th Century sea level rise. This appears to be a calculation error (4); this will be resolved in the peer-reviewed literature. Our values in the above table are computed correctly (in our understanding) using the same parameters as used by the authors in generating their Fig.3. Their model with the parameters fitted to glacial-interglacial data thus underestimates 20th Century sea level rise by a factor of two.

Frosty legacy: We cannot afford to lose even a few percent of the land ice on Earth, which in total would be enough to raise global sea levels by 65 meters. (Calving front in Svalbard, (c) S.R.)

Frosty legacy: We cannot afford to lose even a few percent of the land ice on Earth, which in total would be enough to raise global sea levels by 65 meters. (Calving front in Svalbard, photo by S.R.)

Future projections

It thus looks like R07 and Grinsted et al. both reproduce 20th Century sea level rise and both get similar projections for the 21st Century. Siddall et al. get much lower projections but also strongly under-estimate 20th Century sea level rise. We suspect this will hold more generally: it would seem hard to reproduce the 20th Century evolution (including acceleration) but then get very different results for the 21st Century, with the basic semi-empirical approach common to these three papers.

In fact, the lower part of their 7-82 cm range appears to be rather implausible. At the current rate, 7 cm of sea level rise since 2000 will be reached already in 2020 (see graph). And Eq. 1 guarantees one thing for any positive value of a: if the 21st Century is warmer than the 20th, then sea level must rise faster. In fact the ratio of new sea level rise in the 21st Century to new sea level rise in the 20th Century according to Eq. 2 is not dependent on a or b and is simply equal to the ratio of the century-mean temperatures, T21/T20 (both measured again relative to the 1900 baseline). For the “coldest” IPCC-scenario (1.1 ºC warming for 2000-2100) this ratio is 1.3 ºC / 0.25 ºC = 5.2. Thus even in the most optimistic IPCC case, the linear semi-empirical approach predicts about five times the “new” sea level rise found for the 20th Century, regardless of parameter uncertainty. In our view, when presenting numbers to the public scientists need to be equally cautious about erring on the low as they are on the high side. For society, after all, under-estimating global warming is likely the greater danger.

Does the world have to be linear?

How do we know that the relationship between temperature rise and sea level rate is linear, also for the several degrees to be expected, when the 20th century has only given us a foretaste of 0.7 degrees? The short answer is: we don’t.

A slightly longer answer is this. First we need to distinguish two things: linearity in temperature (at a given point in time, and all else being equal), and linearity as the system evolves over time. The two are conflated in the real world, because temperature is increasing over time.

Linearity in temperature is a very reasonable assumption often used by glaciologists. It is based on a heat flow argument: the global temperature anomaly represents a heat flow imbalance. Some of the excess heat will go into slowly warming the deep ocean, some will be used to melt land ice, a tiny little bit will hang around in the atmosphere to be picked up by the surface station network. If the anomaly is 2 ºC, the heat flow imbalance should be double that caused by a 1 ºC anomaly. That idea is supported by the fact that the warming pattern basically stays the same: a 4 ºC global warming scenario basically has the same spatial pattern as a 2 ºC global warming scenario, only the numbers are twice as big (cf. Figure SMP6 of the IPCC report). It’s the same for the heating requirement of your house: if the temperature difference to the outside is twice as big, it will lose twice the amount of heat and you need twice the heating power to keep it warm. It’s this “linearity in temperature” assumption that the Siddall et al. approach rejects.

Linearity over time is quite a different matter. There are many reasons why this cannot hold indefinitely, even though it seems to work well for the past 120 years at least. R07 already discusses this and mentions that glaciers will simply run out of ice after some time. Grinsted et al. took this into account by a finite time scale. We agree with this approach – we merely have some reservations about whether it can be done with a single time scale, and whether the data they used really allow to constrain this time scale. And there are arguments (e.g. by Jim Hansen) that over time the ice loss may be faster than the linear approach suggests, once the ice gets wet and soft and starts sliding. So ultimately we do not know how much longer the system will behave in an approximately linear fashion, and we do not know yet whether the real sea level rise will then be slower or faster than suggested by the linear approach of Eq. 1.

Getting soft? Meltwater on the Greenland Ice Sheet. Photo by Ian Joughin.
Getting soft? Meltwater lake and streams on the Greenland Ice Sheet near 68ºN at 1000 meters altitude. Photo by Ian Joughin.

Can paleoclimatic data help us?

Is there hope that, with a modified method, we may successfully constrain sea level rise in the 21st Century from paleoclimatic data? Let us spell out what the question is: How will sea level in the present climate state respond on a century time scale to a rapid global warming? We highlight three aspects here.

Present climate state. It is likely that a different climate state (e.g. the glacial with its huge northern ice sheets) has a very different sea level sensitivity than the present. Siddall et al. tried to account for that with their equilibrium sea level curve – but we think the final equilibrium state does not contain the required information about the initial transient sensitivity.

Century time scale. Sea level responds on various time scales – years for the ocean mixed layer thermal expansion, decades for mountain glaciers, centuries for deep ocean expansion, and millennia for big ice sheets. Tuning a model to data dominated by a particular time scale – e.g. the multi-century time scale of Grinsted et al. or the multi-millennia time scale of Siddall et al. – does not mean the results carry over to a shorter time scale of interest.

Global warming. We need to know how sea level – oceans, mountain glaciers, big ice sheets all taken together – responds to a globally near-uniform forcing (like greenhouse gas or solar activity changes). Glacial-interglacial climate changes are forced by big and highly regional and seasonal orbital insolation changes and do not provide this information. Siddall et al use a local temperature curve from Greenland and assume there is a constant conversion factor to global-mean temperature that applies across the ages and across different mechanisms of climate change. This problem is not discussed much in the paper; it is implicit in their non-dimensional temperature, which is normalised by the glacial-holocene temperature difference. Their best guess for this is 4.2 ºC (as an aside, our published best guess is 5.8 ºC, well outside the uncertainty range considered by Siddall et al). But is a 20-degree change in Greenland temperature simply equivalent to a 4.2-degree global change? And how does local temperature translate into a global temperature for Dansgaard-Oeschger events, which are generally assumed to be caused by ocean circulation changes and lead to a temperature seesaw effect between northern and southern hemisphere? What if we used their amplitude to normalise temperature – given their imprint on global mean temperature is approximately zero?

Overall, we find these problems extremely daunting. For a good constraint for the 21st Century, one would need sufficiently accurate paleoclimatic data that reflect a sea level rise (a drop would not do – ice melts much faster than it grows) on a century time scale in response to a global forcing, preferably from a climate state similar to ours – notably with a similar distribution of ice on the planet. If anyone is aware of suitable data, we’d be most interested to hear about them!

Update (8 Sept): We have now received the computer code of Siddall et al (thanks to Mark for sending it). It confirms our analysis above. The code effectively assumes that the warming over each century applies for the whole century. I.e., the time step for the 20th Century assumes the whole century was 0.74 ºC warmer than 1900, rather than just an average of 0.25 ºC warmer as discussed above. When this is corrected, the 20th Century rise reduces from 15 cm to 8 cm in the model (consistent with our linear estimate given above). The 21st Century projections ranging from 32-48 cm in their Table 1 (best estimates) reduce to 24-32 cm.

Martin Vermeer is a geodesist at the Helsinki University of Technology in Finland.


(1) Siddall et al. use two steps. First they determine an equilibrium sea level for each temperature (their Eq 1, and shown in their Fig. 1). Second, they assume an exponential approach of sea level to this equilibrium value in their Eq. 2, which (slightly simplified, for the case of rising sea level) reads:

dS/dt = (Se(T) – S(t)) / τ.

Here S is the current sea level (a function of time t), Se the equilibrium sea level (a function of temperature T), and τ the time scale over which this equilibrium is approached (which they find to be 2900 years).
Now imagine the temperature rises. Then Se(T) increases, causing a rise in sea level dS/dt. If you only look at short time scales like 100 years (a tiny fraction of those 2900 years response time), S(t) can be considered constant, so the equation simplifies to

dS/dt = Se(T)/ τ + constant.

Now Se(T) is a non-linear function, but for small temperature changes (like 1 ºC) this can be approximated well by a linear dependence Se(T) = s * T + constant. Which gives us

dS/dt = s/τ * T + constant, i.e. Eq (1) in the main post above.

R07 on the other hand used:
dS/dt = a * (T – T0), which is also Eq. (1) above.
Note that a = s/τ and b = –a*T0 in our notation.

(2) Here is a very basic matlab/octave script that computes a sea level curve from a given temperature curve according to Eq. 2 above. The full matlab script used in R07, including the data files, is available as supporting online material from Science

% Semi-empirical sea level model - very basic version
T1900=mean(tempg(11:30)); T=tempg-T1900;

a=0.34; % sea level sensitivity parameter [cm/degree/year]
b=0.077; % note this value depends on a and on the temperature
% baseline, here the mean 1890-1909

% rate of rise - here you need to put in an annual temperature time series T
% with same baseline as chosen for fitting b!
dSdt = a*T + b;

% integrate this to get sea level over the period covered by the temperature series
S = cumsum(dSdt); plot(S);

(3) Here is a matlab/octave script to compute the equilibrium sea level curve of Siddall et al. Note the parameters differ in some cases from those given in the paper – we obtained the correct ones from Mark Siddall.

% Siddall et al equilibrium sea level curve, their Fig. 1, NGRIP scenario
A = 15.436083479092469;
b = 0.012630000000000;
c = 0.760400212014386;
d = -73.952809369848552;

% Equilibrium sea level curve
Se=A*asinh((Tdash+c)/b) + d;
% Tangent at current temperature
Se0= A*asinh((0+c)/b) + d;
Te=dSe*Tdash + Se0;
plot(Tdash, Se, 'b', Tdash, Te, 'c', Tdash, 0.0*Se, 'k', [0 0], [-150 40], 'k')
xlabel('Dimensionless temperature')
ylabel('Equilibrium sea level (m)')
fprintf(1, 'Slope: %f m/K, Sensitivity: %f cm/K/year, zero offset: %f m\n\n', dSe/4.2, 100*dSe/4.2/2900, Se0);

(4) We did not yet receive the code at the time of writing, but based on correspondence with the authors conclude that for their values in Fig. 3 and table 1, Siddall et al. integrated sea level with 100-year time steps with a highly inaccurate numerical method, thus greatly overestimating the a-term. In their supporting online information they show a different calculation for the 20th Century with annual time steps (their Fig. 5SI). This is numerically correct, giving an a-term of about 4 cm, but uses a different value of b close to 0.12 cm/year to obtain the correct total 20th Century rise.


Church, J. A. & White, N. J. A 20th century acceleration in global sea-level rise. Geophysical Research Letters 33, L01602 (2006).

Grinsted, A., Moore, J. C. & Jevrejeva, S. Reconstructing sea level from paleo and projected temperatures 200 to 2100 ad. Climate Dynamics (2009).

Jevrejeva, S., Grinsted, A., Moore, J. C. & Holgate, S. Nonlinear trends and multiyear cycles in sea level records. Journal of Geophysical Research 111 (2006).

Moberg, A., Sonechkin, D. M., Holmgren, K., Datsenko, N. M. & Karlen, W. Highly variably Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433, 613-617 (2005).

Rahmstorf, S. A semi-empirical approach to projecting future sea-level rise. Science 315, 368-370 (2007).

Rahmstorf, S. Response to comments on “A semi-empirical approach to projecting future sea-level rise”. Science 317 (2007).

Siddall, M., Stocker, T. F. & Clark, P. U. Constraints on future sea-level rise from past sea-level change. Nature Geoscience (advance online publication, 26 July 2009).

415 Responses to “Ups and downs of sea level projections”

  1. 201
    David B. Benson says:

    Somewhat off-topic, but I think important.
    Irrigated afforestation of the Sahara and Australian Outback to end global warming
    (The pdf is avvailable to all.)

  2. 202
    Richard Steckis says:


    So we destroy whole ecosystems to what end? Deserts are ecosystems.

  3. 203
    Hank Roberts says:

    Nicholas blogs suspicion at 7 September 2009 at 3:28 PM

    > I still haven’t seen a reference from anyone showing that the IPCC “knew”
    > that their sea level estimates were low. I suspect the obvious ….

    You should read more at this site, Nicholas; you could cite to sources rather than to your suspicions, which in this are unsupported.

    The IPCC did not give a ‘middle of the range’ estimate as you suspect.
    Try these, to start to catch up; you can follow the references forward:

  4. 204
    Hank Roberts says:

    And following this forward, you’ll find this among much else:

    Volume 107, Issues 1-2, 1 June 2009, Pages 3-9
    Coastal vulnerability related to sea-level rise
    Statistical analysis of recent Mediterranean Sea-level data

    “… From 1990 onward the sea level recorded in most Mediterranean tide gauges indicates a rise in sea level at a rate 5–10 times higher than the 20th century mean rate.

    While this trend should be regarded with some caution because of the relatively short data series, the trend observed coincides with the increase in global land and marine surface temperatures during the 1990s, and it is important to note that no other short sub-period in the data sets shows such an extreme trend.”

    “… There is now better understanding that the risk of additional contributions to sea level rise from melting of both the Greenland and possibly Antarctic ice sheets may be larger than projected by ice sheet models assessed in the AR4, and that several meters of additional sea level rise could occur on century time scales (2, 7, 27, 28). Such risk arises in part from ice dynamical processes apparent in observations since the TAR but not fully included in ice sheet models assessed in AR4. New insights also come from recent paleoclimate studies (29)….”

    NN, you really didn’t know about any of this??

  5. 205
    Jim Eager says:

    Nicolas Nierenberg (194), the only way that someone could not have seen the IPCC’s own explicit statement that the IPCC knew that their sea level estimates were low would be for them to deliberately not read it:

    Summary for Policymakers, p.14, quote:
    “Models used to date do not include uncertainties in climate-carbon cycle feedback nor do they include the full effects of changes in ice sheet fl ow, because a basis in published literature is lacking. The projections include a contribution due to increased ice fl ow from Greenland and Antarctica at the rates observed for 1993 to 2003, but these fl ow rates could increase or decrease in the future. For example, if this contribution were to grow linearly with global average temperature change, the upper ranges of sea level rise for SRES scenarios shown in Table SPM.3 would increase by 0.1 to 0.2 m. Larger values cannot be excluded, but understanding of these effects is too limited to assess their likelihood or provide a best estimate or an upper bound for sea level rise.”

  6. 206
    Rod B says:

    Mark (190), that sounds about right!

  7. 207
    Chris Dudley says:

    Richard (#184),

    Really, no more should be said about the green jobs “study” than the White House said: We’re importing turbines from Spain.

    But, one can read further:

  8. 208
    Crazy Bill says:

    Re refilling aquifers. How about the crazy idea of pumping water up onto East Antarctic and letting it freeze – it should stay there for a while.

    On the other hand, it might take a largish new power station or two to push enough water up the mountain…

  9. 209

    #197 Alastair:

    > How soon can we get fast reactor technology on line?

    What about by 1985? or by 1965?

    Hey, this is old technology… there are safety and cost issues, like there are with nuclear in general, even more so. If however the world decides to go 100% nuclear, this will be the technology chosen. And that’s what any calculation of fuel availability should be based on.

    …and I see you didn’t comment on the 35 million tonnes, or the sea water ;-)

  10. 210

    #209 Martin,

    I can go earlier than that. The first reactor at Dounreay became critical in 1958. The last reactor was taken of-line in 1998. decommissioning is going to take over 300 years and cost £2.9 billion (approx. $5 billion.)

    If fast breeders are such a good idea why have they all been abandoned?

    As far as the uranium in sea water is concerned, how much energy is it going to take to extract and refine it? Just pumping a billion tons of water, onto land where the extraction site is built, for each ton of uranium extracted (assuming 100% efficient extraction) is going to use up a lot of energy.

    Meanwhile, the Greenland ice sheet will continue to melt, sea levels continue to rise and the coastal extraction plant will be flooded.

    Be realistic!

  11. 211

    simon: The fast breeder reactor (FBR) is designed to breed fuel by producing more fissile material than it consumes. Depletion of uranium reserves therefore not a problem.

    BPL: Then you’ve got an economy with large amounts of plutonium being transported all over the country. That should make Al Qaeda happy…

  12. 212
    Mark says:

    There are also political issues.

    When you’ve threatened invading another sovereign country for doing something LESS likely to have military applications, it’s quite hard to justify making fast breeders.

  13. 213
    Mark says:

    “Mark — please withdraw the statement attributed to Al Gore.”

    How can I withdraw the statement?

    He showed Florida flooded if some of the West Antartic Ice Sheet melted. I can’t remember off the top of my head the time or percent or the level.

    If I ***hadn’t*** said that I didn’t know these things in the statement, then maybe I would have something to withdraw.

    But I did say it and with the provisos given, the statement doesn’t warrant withdrawal.

  14. 214
    Sekerob says:

    Martin, Did you visit the BraveNewWorld site of Barry Brooks and familiarize with the IFR concept? Wiki’s, so reliable and quick, here a wiki on that topic

    I think 90% or more suffer from China Syndromitus when it comes to nuclear. The French continue to smile from ear to ear and have not stopped their research and development and glad they this. Guess, half of Italy would be having rolling power periods without them, no one willing to compromise on their energy needs. In 2003 we were very close already.

    Choices have to be made NOW, yet nobody willing.

    [Response: Not every thread has to end up with another discussion about nuclear power. This is OT. – gavin]

  15. 215


    > If fast breeders are such a good idea why have they all been abandoned?

    If nuclear fission is such a good idea, why aren’t we generating all our electricity using it?

  16. 216
    Mark says:

    Because we haven’t got economical fusion generators, Martin.

    Not the same thing.

  17. 217
    Jkiesel says:

    If I want to read discussions about nuclear energy I can read another blog. But if I want to read about climate change and sea level rise I HAVE to go to another blog???

    Please keep on topic: Ups and downs of sea level projections

  18. 218

    From Al Gore’s movie:

    “If Greenland broke up and melted, or if half of Greenland and half of West Antarctica broke up and melted, this is what would happen to the sea level in Florida… (video graphics of effect of ocean rise of that magnitude)”.

    No time scale was given.

  19. 219
    Ray Ladbury says:

    Folks, do we really want to get bogged down on the nuclear power thing again? It may turn out that as we face unpalatable options in the future, nukes may be one of the least bad. However, we have a whole helluva lot more we can do in terms of conservation and renewables before we have to confront those unpalatable options. At the very least, increased conservation is one thing we can all (well maybe not Simon) agree on.

  20. 220
    Lawrence Coleman says:

    Thanks for your follow up comments wili and mark. I would assume it’s because the scientific community doesn’t yet fully understand the chain of consequences that crossing these tipping points will bring. The IPCC is shit scared of the ‘I told you so’ bleating by the denyalists as Mark mentioned that they wont even acknowlege the possibility of an uncontrollable chain recation. For polititions and leaders of industry to act NOW we need another public megaphone apart from the IPCC. One good analogy of the effect of tipping points being breached is this…imagine the world’s climate as a suspension bridge and the many hundreds of steel cables holding the bridge up as the climate tipping points. If the bridge is under constant pressure such as a force 5 hurricane (CO2, greenhouse gasses in the atmosphere)…and 1 cable snaps (1 tipping point is breached) then all the others are under additional strain as well as the structure not being as well soon another one breaks (another tipping point is crossed)..and then another and very soon after that another one goes and so on..very soon the entire bridge is occillating wildely twisting and buckling leading quickly to it’s imminent destruction. I just thought that would be an accurate analogy to keep in mind for those analogy nuts out there..haha!

  21. 221
    franz mair says:

    hi stefan,

    i had a look at this page:

    the global see level rise, shown in this figures, depends mostely on the indian ozean. atlantic and pacific looks flat or decreasing.

    now, what is right and what is wrong?

    [Response: The trends in the three basins look very similar to me. Or are you just talking about the last 4 years? Then you’re not talking about climate trends but internal variability. -stefan]

  22. 222

    > Martin, Did you visit the BraveNewWorld site of Barry Brooks and familiarize with the IFR concept?

    Sekerob, yes. It didn’t help. I’m not new to the subject my friend ;-)

  23. 223

    #208 Not-so-crazy Bill: energy wise, it could work. Barring calculation errors I get 100 GW for pumping all of current sea level rise up 3000 m… I’m sure there are other valid reasons why this couldn’t possibly work :-)

  24. 224
    tamino says:

    We could discuss (endlessly!) the pros and cons of nuclear power, the rate at which solar and wind power can be brought online, methods for storing renewable energy, etc. etc. etc. It makes me think of two doctors arguing over whether lung cancer is better treated with radiation or chemotherapy.

    That’s all well and good, but the overriding action both doctors agree on is: QUIT SMOKING.

    Step 1: pass the ACES bill through the senate, forge a strong agreement on greenhouse gas emissions reduction in Copenhagen.

    Then let’s argue about step 2.

  25. 225
    Richard Steckis says:

    #217 J.P. Reisman

    Greenland or even half of it cannot break up and melt. The Greenland Ice Sheet lies in a basin. Therefore it cannot physically break up, it cannot physically slide into the ocean and the thickness of the cap would require at least 1000 years of above zero temperature for it to fully melt.

    Therefore Al Gore was talking a load of bull regardless of whether he gave a time scale or not.

  26. 226

    #219 Lawrence Coleman

    The IPCC is governed by mandate and procedures, it has nothing to do with being scared. They don’t do the science per se, they are more the procedure by which it is reviewed, vetted and presented.

    Among other things I learned at WCC3 was the reality that the IPCC founding basis (under the cover) was not a political agenda to promote crazy ideas of human caused global warming. I spoke with several people who were there at the beginning 30 years ago and one was clear in saying that it was formed (in the mind of some of the lead states) to put down the crazy idea that humans were causing cliamte change.

    Unfortunately critical examination of the science took it the other way…, oh well.

  27. 227
  28. 228
    stevenc says:

    “The projections include a contribution due to increased ice fl ow from Greenland and Antarctica at the rates observed for 1993 to 2003, but these fl ow rates could increase or decrease in the future.”

    Jim, this reads to me like they are saying their estimates could be too low or too high. I can’t read it in such a way that it becomes an explicit statement that they knew their estimates to be too low.

  29. 229
    Mark says:

    “I would assume it’s because the scientific community doesn’t yet fully understand the chain of consequences that crossing these tipping points will bring.”

    No, if the artic ice sheet melts, there are many things that are known and can be modelled.

    The effect of albedo changes and rock weathering or biological activity changes are second or third order changes that depend on things we cannot currently measure. E.g. what IS under all that ice. We’ve looked in a few spots, but that’s hardly solid evidence of whats hiding under there.

    The biggest problem is working out when they will happen, since these are not climate forcings and are truly chaotic like weather events and not amenable to computational analysis to result in a definitive forecast.

    We can figure that blood loss and shock will likely kill a patient if they don’t get to emergency within 30 minutes, but whether they can travel the 12 miles to the hospital in time depends on the traffic, not the patient.

  30. 230
    Mark says:

    “For polititions and leaders of industry to act NOW we need another public megaphone apart from the IPCC.”

    Al Gore.

    But then again, the denialists just go “ignore him, he’s fat!”.


    And one reason is that anyone who says what the worst case scenario is, or mentions something like tipping points is shouted down by the monied shills as an “alarmist”.

    And that can’t be fixed by finding more people to talk about tipping points et al. It requires the shills shut the f… up. Which they won’t while someone is willing to pay them to hear them deny AGW.

  31. 231
    Mark says:

    JPR: “No time scale was given.”


    The movie said nothing about the time. The models said that this could *possibly* happen in 2100 or 2050 or whatever.

    But what did the denialists do (and get!)?

    They said “Al says that the GIS will melt and Florida will flood! This isn’t TRUE!!!!”. And a judge agreed and said that he SHOULD have been more explicit.

    a) Why?
    b) Denialists then tout that a judge says Al was wrong. And he’s fat. Which isn’t the case.

    Which is why I think that tipping points and sudden shifts like methane releases from tundra etc aren’t being done: the denialists have already shown they WILL represent this as unscientific and non-scientific judges WILL agree with them and then the denialists WILL misrepresent the judge’s ruling to make it look like a lie.

  32. 232
    Rod B says:

    Jim Eager (205), how you get “…the IPCC knew that their sea level estimates were low…” from your quote from the IPCC report is inexplicable.

  33. 233
    Mark says:

    “> he even said that “***IF*** it occurs by 2050, then…”.”

    OK, so if that was withdrawn Hank, what changes? It can be read as worse for Al.

  34. 234
    Mark says:

    stevenc: “Jim, this reads to me like they are saying their estimates could be too low or too high.”


    An average dice roll will be 3.5 spots.

    YOU WILL NEVER SEE 3.5 SPOTS. So the acutal average you get will be something close to 3.5.

  35. 235
    Mark says:

    Richar Skeksis: “Greenland or even half of it cannot break up and melt. The Greenland Ice Sheet lies in a basin.”

    Is the basin over a mile deep?

    Because the GIS is well over 3km deep at its thickest.

    Now, when you have a big block of ice (2 ft thick) on a plate (that is a basin shape, isn’t it: the sides are always higher than the centre), will that plate overflow with water when the ice melts???

  36. 236
    stevenc says:

    true mark, doesn’t pertain to the point at all but true none the less

  37. 237
    Mark says:

    lawrence 220, your analogy is fairly good. It can even be taken to what happens with the ONE CABLE.

    All fractures start as a small break in the semi-crystalline metallic structure. But all metallic structures have breaks in the semi-crystalline structure. It’s when you get ductility in metals and work-hardening. Which site of lowered structural integrity goes first is inherently intractable under major stress.

  38. 238
    Rod B says:

    Mark (230), they must have a wrong address for me; I haven’t received a skeptic check in a long time — actually ever. Though I seldom use the “alarmist” tag. Do you think I should start, in order to get my money? BTW, who mails the check?

  39. 239
    Hank Roberts says:

    Mark says: 8 September 2009 at 10:26 AM
    “> he even said that “***IF*** it occurs by 2050, then…”.”
    OK, so if that was withdrawn Hank, what changes?

    Your reputation for accurate citation and quotation would be restored.

  40. 240
    Mark says:

    “true mark, doesn’t pertain to the point at all but true none the less”


    So a prediction of 3.5 for an average dice roll will be either too low or too high, won’t it.

    So why does that not faze you whilst the IPCC saying the exact same thing does?

    THAT is the point.

    Why so scared of that truth of statistics and not of the other? Bipartisanship?

  41. 241
    Mark says:

    Rod B(238) so you admit to denialism?

  42. 242
    walter crain says:

    hi guys…sorry to burst in like this. what i am i to make of this?

    the reality of AGH is already a “hard sell” (because the denialists are so freakin’ good at PR). if/when temps “moderate” due to NAO it will be an even harder sell…

    gavin, how’s PROJECT JIM coming along?

  43. 243

    #225 Richard Steckis

    Don’t forget that chunks of Greenland will likely rise as the ice melts.

    Besides, Al Gore was obviously being illustrative. Look at the overall context.

  44. 244
    Hank Roberts says:

    Rod, how can you fail to see what the IPCC said?
    Which of those words are you focusing on?

  45. 245

    #228 stevenc

    You need more context, read the entire report. Not just a single passage.

    Look, then leap.

  46. 246
    wili says:

    Thanks to all for the discussion and links, but most of the linked sites are at least a year old, and it sounds to me as though we now have a good bit more data about how rapidly methane releases are accelerating with concurring increases in atmospheric methane.

    Since this is potentially such a huge new factor in gw, perhaps it would be worth a lead post sometime soon (if I may be so presumptuous as to make such a suggestion)?

    A point raised above that I hadn’t thought of (and one more pertinent to the current thread) is whether massive release of seabed methane hydrate would (temporarily) l o w e r sea level as the water rushes down to fill the space evacuated by the escaping gas. Or, short of a sudden, massive release (apparently a quite unlikely prospect), could the steady and increasing release of this methane offset whatever sea level rise is caused by heat expansion and melting ice sheets and glaciers?

  47. 247
    Jim Eager says:

    stevenc (228), try reading the quoted statement in the context of what was known at the time that the Fourth Assessment Report was released in 2007 that was not known at the May, 2006 cut-off for examining the “current” science:

    Estimates of ice flow based on rates observed for 1993 to 2003 were already known to be too low. Why? Because empirical observations between 2003 and release of the report in 2007 showed that the rate had in fact increased, hence the caveat.

  48. 248
    Martin Vermeer says:

    Richard Steckis #225 and Mark #235, you both look silly pretending to understand how continental ice sheets behave. Ice is ice, but when you have cubic kilometres of it you get very different physics from the ice cube in your whisky glass.
    Better men than you are scratching their heads over it. Until you grasp that, please stick to the whisky.

  49. 249
    Martin Vermeer says:

    Jim, this reads to me like they are saying their estimates could be
    too low or too high. I can’t read it in such a way that it becomes
    an explicit statement that they knew their estimates to be too low.

    It’s as explicit as it ever gets in a political compromise text that they had a dark suspicion… why would they speculate on a proportionality with global temperatures? Why saying “Larger values cannot be excluded”, but not “Smaller values cannot be excluded”? Why the disclaimer about an upper bound? Their stated ignorance on ice flow was obviously unsymmetric.

  50. 250
    Hank Roberts says:


    Estimate for total methane hydrate: 100,000 to 1,000,000 trillion cubic feet.

    “A cubic meter of hydrate yields about 160 cubic meters of methane at standard temperature and pressure and about 0.87 cubic meter of water”

    “volume of seawater 1.37 billion cubic kilometers”

    1 cubic kilometer = 3.53146667 × 10e10 cubic foot