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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. 301
    Mark says:

    PS the reason why it’s a “mess” is because you have made the mess.

    What is your problem with the statement that the IPCC estimate of SLR is going to be low?


    PPS stop with the BS about “just the quote”. Since the quote was about the IPCC SLR predictions and the model is the source of those predictions, but not included in the quote, hence making it impossible to answer using “just the quote, nothing else”.

  2. 302
    Lawrence Coleman says:

    This is a little off topic but I’ve been thinking of ways to retrofit modern airliners to use hydrogen fuel cells as their power source. The only major parts that need retrofitting are the engines and the fuel tanks.
    What about plug-in hydrogen fuel cells straight into the wings and fuselage where the fuel reservoir is curently. When a plane need refueling a truck with a modified forklift slots the standardized size hydrogen fuel cells straight into both wings..maybe 5-7 cells per wing and 10 say in the fuselage. Question..which you guys may be able to does the power/weight ratio of hydo-fuel cells stack up against av-gas/ kerosene? Would the thrust on take-off be comparable to kerosene? How will the extreme cold of higher altitudes (minus 50-60C) effect the cells? How would the flying range compare? What type of engine would best utilize this fuel source? My idea could be completely unfeasible or it might just have a glimmer of hope..we’ll see.
    Thanks guys!

  3. 303
    Rod B says:

    Mark (301) says, “…What is your problem with the statement that the IPCC estimate of SLR is going to be low?…”

    Because in the (not my) referenced quote they didn’t. It ain’t rocket science. Not even climate science. But hang in there; I’m rooting for you! ;-)

  4. 304
    simon abingdon says:

    #300 Maybe they’d soak into the ground.

  5. 305
    Hank Roberts says:

    > the insight offered, both about the issue at hand and about finding
    > things if you know they’re out there. It’s those people I do this for.

    Well said and well done, Martin.
    Many thanks for your persistence and summing up.

  6. 306
    Hank Roberts says:

    Lawrence, the search term you’re looking for: “energy density”
    Google is your friend, or would like to be.
    Short answer: no way, nohow, you can work it out for yourself.

  7. 307
  8. 308
    Hank Roberts says:

    PS, hat tip to the inimitable H.E. Taylor’s compendium,8816,1920435,00.html
    More bad news on sea level rise

    PS — did you know there’s this widget?
    This is the picture:
    This is the link:

    As featured here, upper right hand corner of the page:

    Get one today for your own website

  9. 309
    stevenc says:

    We also evaluate the contribution of rapid dynamical
    changes under two alternative assumptions (see, e.g., Alley et
    al., 2005b). First, the present imbalance might be a rapid shortterm
    adjustment, which will diminish during coming decades.
    We take an e-folding time of 100 years, on the basis of an
    idealised model study (Payne et al., 2004). This assumption
    reduces the sea level rise in Table 10.7 by 0.02 m. Second,
    the present imbalance might be a response to recent climate
    change, perhaps through oceanic or surface warming (Section No models are available for such a link, so we assume
    that the imbalance might scale up with global average surface
    temperature change, which we take as a measure of the magnitude
    of climate change (see Appendix 10.A). This assumption adds
    0.1 to 0.2 m to the estimated upper bound for sea level rise
    depending on the scenario (Table 10.7). During 2090 to 2099,
    the rate of scaled-up antarctic discharge roughly balances the
    increased rate of antarctic accumulation (SMB). The central
    estimate for the increased antarctic discharge under the SRES
    scenario A1FI is about 1.3 mm yr–1, a factor of 5 to 10 greater
    than in recent years, and similar to the order-of-magnitude
    upper limit of Section It must be emphasized that we
    cannot assess the likelihood of any of these three alternatives,
    which are presented as illustrative. The state of understanding
    prevents a best estimate from being made.

    page 821 AR4

    There are competing hypotheses. One would increase the sea level projections. They don’t know which one is correct. Perhaps many have an opinion on which one is correct but that is not expressed by the IPCC which clearly states it does not know. Conclusion: the IPCC did not at the time of publication know that their projections were too low. What they know now I cannot say. It is not a matter of probability. It is a matter of which model is correct or at least most correct.

    This is my interpretation of what is said as posted here and it fits well with my interpretation of the excerpt we previously discussed.

  10. 310
    Hank Roberts says:

    The Wall Street Journal, SCIENCE JOURNAL SEPTEMBER 11, 2009
    New York City Prepares for Era of High Seas
    As Ocean Levels Rise, Storm Surges Become More Worrisome for Coastal Cities, Especially Those in the Northeastern U.S.

    “… Sea level may rise faster near New York than at most other densely populated ports due to local effects of gravity, water density and ocean currents, four new forecasts of melting ice sheets suggest. Already, the ocean level world-wide is rising twice as fast as predicted two years ago….”

  11. 311
    CTG says:

    Re 304: “Maybe they’d soak into the ground.”

    Simon, the correct sequence is: 1) engage brain; 2) open mouth.

    Not the other way round.

  12. 312
    llewelly says:

    Lawrence Coleman, #302:

    … how does the power/weight ratio of hydo-fuel cells stack up against av-gas/ kerosene?

    Hydrogen has a very good energy to mass ratio: 143 MJ/kg . Much better than kerosene (42.8 MJ/kg) or av-gas (46.4 MJ/kg).

    The problems with hydrogen are:
    (a) it is not an energy source. There is no large source of readily available natural hydrogen that can simply be gathered up for a lower energy cost than the energy it contains. That’s the real advantage of coal, oil, and nuclear over hydrogen. Mining and processing coal, oil, or uranium requires a lot less energy than the energy contained in the coal, oil, or uranium. In the same vein, the energy needed to build and maintain a solar power plant is far less than the energy it can gather from sunlight. Hydrogen is not like those energy sources. Instead – energy from some other source must be converted into hydrogen. Because CH4, oil, and coal are already in chemical form, like hydrogen, it’s much more efficient to produce hydrogen from CH4, oil, or coal than from solar, wind, or nuclear. But hydrogen production from fossil fuels produces about as much CO2 as burning the fuels directly.
    (b) Hydrogen has a terrible energy/volume ratio. It requires a huge fuel tanks, which greatly increase the surface area of any aircraft, and complicate the profile design, which both in turn increase drag. Past hydrogen powered aircraft have always suffered severely due to hydrogen’s terrible energy/volume ratio.
    (c) Hydrogen is really, really hard to store and transport. A tank that will securely hold O2, N2, CO2, and many other gasses will leak H2 like a sieve. Many otherwise impermeable metal tank walls are quite permeable to H2. Essentially all normal welds are permeable to H2.

    I strongly recommend Joe Rohm’s book The Hype About Hydrogen .

  13. 313

    > Conclusion: the IPCC did not at the time of publication know that their
    > projections were too low.

    Nope… they knew but refused to speak out. It follows logically from the numbers… the only situation where their projections would not be too low would be if the “rapid shortterm adjustment” hypothesis were close to 100% certain; an extreme claim.

  14. 314

    Oops… the previous went off too early. What I wanted to add was that that’s my understanding of what I read. Admittedly this part of the IPCC report is particularly poorly written. But we knew that.

    About it not being a matter of probability: no, not for the scientists, but a policy maker will put subjective probabilities on it; he/she has to. So the IPCC afforded themselves a luxury that their clients don’t have.

  15. 315

    #309 stevenc

    To reiterate what Marten Vermeer has stated. Nit picking the statement does not change the overarching understanding. From a scientific point of view nit picking is great when considered in relevant context.

    Form a policy standpoint, probability is an important factor in decision making. While risks need to be quantified for action assessment, the overarching probability rules the day. We are going to warm and there will be regional impacts on food production from precipitation changes. This will alter the economy worldwide and it is a negative, not a positive.

    Worldwide, we will see the monthly budget for food purchases going up. Crop loss form drought and flooding will be key factors here.

    This will force many businesses to lose sales. Everything will change. And it does not stop there, it just keeps warming. Add it up. Connect the dots. Big picture is critical at this point in time. Nit picking the lines and ignoring the reality is a good way to destroy economic capacity.

    If you truly hate economic capacity, by all means, promote the insane idea that it won’t be so bad.

  16. 316
    stevenc says:

    Martin, I would doubt anyone that believes they are 100% correct on a complicated process so my instinct tells me that you are correct and the SLR is probably understated. I would join Hank in expressing appreciation for the time you take here in helping those of us from unrelated fields and hope you continue to do so. Perhaps once more data has been attained the IPCC will feel more comfortable making a more definitive statement.

  17. 317
    Mark says:

    Rod states in 303: “Because in the (not my) referenced quote they didn’t.”

    Yes they did.

    They said that their model didn’t include ice melt from land ice.

    What do YOU think that would mean? How would it be possible that missing that out (which they said they missed out) for that to result in an OVER estimate of the likely sea level rise???

  18. 318
    Mark says:

    PS if all you go on is your referenced quote, then all you have to do to be “right” is to miss out anything that would have informed an intelligent listener, Rod.

  19. 319
    Chris Dudley says:

    Hank (#310),

    I’m sure that the readership of the WSJ which has kept the paper profitable will be pleased that the city considers storm surge flooding to be equivalent to a snow storm.

    ‘For Mr. Aggarwala, any changes in climate are best countered by incremental adjustments as science and circumstances demand. “If we have to shut the stock exchange for a day because water is running down Wall Street, that’s not unprecedented,” Mr. Aggarwala says. “A major snowstorm can do that. The key challenge is how quickly we can recover.”‘

    One can’t help but feel that this relaxed attitude, encouraged by the denialism of the WSJ itself, is a lot like saying an earthquake could take down the twin towers so lets go slow on preventing terrorism.

    How quickly will sea level rise reverse if we just wait to run out of fossil fuels? Wall Street will never recover. Prevention is what is needed. Terminating fossil fuel use is the first step, a step that Wall Street has it in its power to take.

  20. 320
    stevenc says:

    John, you have made an argument over accuracy in citation into an argument over accuracy in results. They are not the same thing but I can understand how viewing it that way might be one’s first reaction. This is why I have tried to state as clearly as I could that I have no opinion on sea level rise projections.

  21. 321
    Rod B says:

    Mark, I’ve done all I wish to, can, or will do to help your reading comprehension. Though it’s simple: you (and the others) read into written words what you wish is there or what you think should have been there, or what, in your (and other’s) view, the authors should have written, or what you infer they really meant to write, not what was actually written. If you claim the quote (205) says the IPCC has absolutely underestimated SLR because they definitely underestimated ice flow or melt (e.g.), you’re hallucinating.

  22. 322
    Jim Eager says:

    Mark, it does not help that you keep writing that the “(IPCC’s) model didn’t include ice melt from land ice.” It did, but at the average rate for the period 1993 to 2003. The problem is that rate was already outdated well before the report was released.

  23. 323
    Jim Eager says:

    Wow. I wasn’t aware that I needed “protecting.”

    Let’s take a look at some facts and the timeline.

    First, the cut off for the IPCC’s review of the published science was May, 2006. That means no data, observations or analysis from after 2005 would have been incorporated into the IPCC’s analysis and projections. Second, the estimates of sea level rise from the melting of the Greenland and Antarctic ice caps was based on the trend from 1993 to 2003, which pushes the knowledge base and analysis back a further two years. That means that the the IPCC projections were based on data and analysis that was up to four years behind the current science when the report was published in early 2007.

    The WGI authors were obviously well aware of this lag, which is precisely why they wrote the caveat that I quoted from the WGI Summary for Policy Makers (@205). As others have pointed out, that quote was greatly expanded upon in sections 10.6 and 10.7 of the full WGI report, which are laced with many more caveats, some of them quite specific, such as:

    “The TAR concluded that accelerated sea level rise caused by rapid dynamic response of the ice sheets to climate change is very unlikely during the 21st century (Church et al., 2001). However, new evidence of recent rapid changes in the Antarctic Peninsula, West Antarctica and Greenland (see Section has again raised the possibility of larger dynamical changes in the future than are projected by state-of-the-art continental models, such as cited above, because these models do not incorporate all the processes responsible for the rapid marginal thinning currently taking place (Box 4.1; Alley et al., 2005a; Vaughan, 2007).” Section

    Further accelerations in ice flow of the kind recently observed in some Greenland outlet glaciers and West Antarctic ice streams could increase the ice sheet contributions substantially, but quantitative projections cannot be made with confi dence (see Section” Section 10.6.5

    How many explicit caveats that the WGI authors did not have high confidence in their own projections of future sea level would you like? Feel free to read through sections 10.6 and 10.7 looking for more.

    So, what happened to the understanding of ice dynamics and the cryosphere between 2003 and early 2007, when the Fourth Asesment report was released to warrant so many caveats?

    I suggest that you take a look at what papers were published on the subject in that timeframe and shortly afterward showing increases in rate of movement, melt and mass loss in Greenland and Antarctica, not to mention papers showing accelerated ice melt and breakup in the paleorecord, or 2005’s eclipsing of 1998 as the warmest year in the temperature record. Although published too late to be included in the analysis and projections of the report, the authors of Sections 10.6 & 10.5 would certainly have been well aware of the current research prior to release of the report, and thus knew that their projections, based on outdated data, observations and assumptions, were almost certainly too low, and thus insisted on inserting the caveats that they did.

    You’ can look up the papers yourself, though, because I have better things to do than help someone who is just trying to pull our collective chain.

  24. 324
    simon abingdon says:

    #311 Lighten up, CTG.

  25. 325
    Ray Ladbury says:

    #322, Wise up, Simon.

  26. 326

    #320 stevenc

    “I have no opinion on sea level rise projections.”

    Why not?

  27. 327
    William says:

    #319 Chris Dudley
    Terminating Fossil fuel use is the first step? You’ve got it reversed, the first step is to replace fossil fuel electric generation with nuclear and alternative sources. It’s only that you can even think of eliminating fossil fuel use. Realistically, fossil fuels will still be in heavy use 50 years from now. It’s too plentiful and cheap and there is too much available infrastructure for it’s development and transport. Also, you cannot think for a minute that countries with huge reserves are just going to let them sit in the ground and allow their economies to dry up. If the US or China does not use it then it will be sold to other countries that will. We should hit 450 PPM by 2040 or 2050.

  28. 328
    Hank Roberts says:

    Rod, there’s always someone making mistakes and making confident statements about their misunderstanding. You can always find someone picking one sentence from something and posting it as _the_ fact.

    If you want an argument, stay with responding to people who do that stuff and don’t cite their sources. You’ll be among others who are equally interested in having arguments, trumping points, and being successful arguing.

    Meanwhile there’s work to be done.

    If you’re trying to understand the science, look at the references.
    You can always find someone’s opinion to argue with.

    To understand the science you have to start with reading it. Not someone’s recollection of what they read about it.

    Jim Eager did a good job just above here of summarizing and pointing to the material needed.

    If you want to be a simon, you can just pick arguments and make fun of the discussion. Your choice, to the extent the moderators allow you to do so.

  29. 329
    simon abingdon says:

    An example, “IFRs use virtually all of the energy content in the uranium fuel whereas a traditional light water reactor uses less than 1% of that energy content. This means that breeder reactors can power the energy needs of the planet for at least 10,000 years.[citation needed]” (From Wikipedia).

    Is this nonsense? Do you dismiss this out of hand? Is Barry Brooks an idiot? Will not sea-level rise be mitigated by embracing nuclear power? Why does Gavin dismiss such a discussion as OT? Why is Ray so opposed to nuclear power as a solution to global warming problems? And why would a scientist have anything whatever to do with any political agenda?

  30. 330
    simon abingdon says:

    Sorry, Brook.

  31. 331
    stevenc says:

    John, because sea level rise is a very complicated issue. For instance take groundwater. Shah 2000 estimated the mining of groundwater to be 750-800km^3 per year. This is a very large amount of water. I don’t know what percentage of this water makes it to the ocean. I don’t know if the mining of groundwater will increase or decrease. Recent trends have been that the use of groundwater is leveling off. Logic says that as the population and wealth of the population increases then water usage will increase also. One would hope that we would conserve our groundwater. History says we won’t. How can I predict such a thing?

  32. 332
    Rod B says:

    Hank, that was all that I was doing and all that I ever claimed I was doing. If one wants to pick a credible argument with me it’s best to make it over something I actually said as opposed to the argument they want to have.

  33. 333
    Hank Roberts says:

    So can we quit talking about nukes, and leave off parsing that one English sentence now? Please? There’s a topic here, with science worth learning.

  34. 334
  35. 335
    Martin Vermeer says:

    Jim #323: no, you obviously don’t need protection…
    Don’t ever stop hallucinating ;-)

  36. 336

    Martin, would that there were more valiant writers like yourself.

  37. 337

    #331 stevenc

    But you do know that sea level is rising, and that rise rate trend recently accelerated, right?

  38. 338
    stevenc says:

    John, yes of course I am aware the sea level rise has recently accelerated. Ever see the disclaimer on mutual fund advertisments?

  39. 339
    Ray Ladbury says:

    Simon Abingdon muses:

    ” Why does Gavin dismiss such a discussion as OT?”

    Because it is.

    And then: “Why is Ray so opposed to nuclear power as a solution to global warming problems?”

    Who says I am? I have questions about whether it CAN BE a solution given the technical issues and the time frame. It is also not a renewable resource and means we will yet again have to replace an energy infrastructure in the future when the U-235 and Th-233 run out

    And finally, Simon queries: “And why would a scientist have anything whatever to do with any political agenda?”

    Oh, bite me! Are you serious? Do you seriously want to deprive scientists of political rights and opinions? Do you really not understand that individual scientists can be political, but that the process of science keeps the political agendas from overly influencing the science? Good lord, Man. Have you ever even taken a science class? Have you ever known a scientist?

  40. 340

    #339 Ray Ladbury

    Re: simon Monckton comment: “And why would a scientist have anything whatever to do with any political agenda?”

    Maybe in simons world scientists should not have voting rights. Hard to imagine why you would want scientists trained in logic and scientific method and reasoning… I mean you really don’t want people that study and research to vote now, do you?

    Think of what that would lead to? Scary I tell you.

    Much easier to control things if you don’t have scientists muddling up the ballot box.

    So, simon, can you give us a list of the people that should be allowed to vote so we can suggest it to our congress?

  41. 341
    Rod B says:

    Hank, I agree this debate has long worn out its welcome. But it is not OT and IMO fairly significant. The thread is about projections of sea level rise. Jim (205) made the assertion, “…the IPCC’s own explicit statement that the IPCC knew that their sea level estimates were low…” That is a solid claim with virtually no reservations. But that claim can not follow logically from the IPCC quote in 205. I agree Jim’s follow-up post (323) is a good and fairly complete summary of the IPCC’s position. But it still does not support the original claim. Some excerpts: “…However, new evidence….has again raised the possibility…”; “…the authors of Sections 10.6 & 10.5 would certainly have been well aware of the current research…” which is a presumption of Jim, reasonably founded no doubt, but a presumption none-the-less. If you read all of the precise words in 323 or its references, you still can not credibly claim that, “…the IPCC knew that their sea level estimates were low…”

    For a thread that is discussing the evidence of what the sea level might or might not do because of this or that, those exaggerative conclusions, even if innocuously meant, detracts from the credibility of the science. I think that is relevant and important.

  42. 342
    simon abingdon says:

    #339 (Ray) and #340 (JPR)

    I was obviously talking about the scientist qua scientist.

    And on topic may I say that we have not a clue about what is under several kilometres of Greenland or Antarctic icecap and what their response to an increase of temperature might be.

    Moreover sea level rise or fall is to me obviously almost exclusively due to thermal expansion/contraction. Given the vast volume of the oceans, the thermal sluggishness of water and the imponderable nature of the resulting hysteresis effects (to say nothing of known large-scale circulations), any associated theories must be at present necessarily no more than speculative and indeed be most probably nugatory.

  43. 343
    Hank Roberts says:

    > we have not a clue about what is under several kilometres of Greenland
    > or Antarctic icecap and what their response to an increase of temperature
    > might be.

    They probably won’t be at all pleased, once they awaken.

  44. 344
    David B. Benson says:

    simon abingdon (342) — What is the function of the National Academy of Sciences’s research arm, the National Research Council? What about NOAA’s fisheries office?

  45. 345
    Ray Ladbury says:

    Simon @342. Let me summarize your point:

    You say YOU don’t have a clue what’s going on, so everybody else must be equally clueless. ‘Bout got it?

    Well, I’ll buy the premise, at least.

  46. 346
    Hank Roberts says:

    Pine Island Glacier —

    “… the average rate of volume loss quadrupled from 2.6 ± 0.3 km3 yr−1 in 1995 to 10.1 ± 0.3 km3 yr−1 in 2006. The region of lightly grounded ice at the glacier terminus is extending upstream, and the changes inland are consistent with the effects of a prolonged disturbance to the ice flow, such as the effects of ocean-driven melting. If the acceleration continues at its present rate, the main trunk of PIG will be afloat within some 100 years, six times sooner than anticipated.”

  47. 347
    simon abingdon says:

    #345 (Ray)

    Only ad hominems left now.

    #346 (Hank)

    Just to get a sense of proportion:

    Antarctic 13,720,000 sq km max 5 km deep Greenland 1,755,637 sq km max 3 km deep
    Pine Island 175,000 sq km approx 2 km deep

    I’m sure you’ve read reports that Antarctic ice may now be increasing. For example:

    Extrapolate all you like. The evidence still doesn’t convince.

  48. 348
    Martin Vermeer says:

    Hank #343, Thanks!! I had forgotten the fun of reading real sci-fi… an impressive pastiche of Lovecraft and Frederick Pohl…

  49. 349
    CM says:

    Hank (#343), you beat me to it. The classical background to Stross’s creepy tale is here:

    BTW, note the lament in the first chapter that a “relatively obscure” man such as the narrator, who moreover as a geologist is not, “in the strictest sense,” a speci-alist in the fields primarily concerned, has “little chance of making an impression where matters of a wildly bizarre or highly controversial nature are concerned.” Sound familiar? :-)

  50. 350
    wili says:

    Hank, thanks for the stats, the links, and the mathematics problem. In an effort to steer the topic back toward sea level, perhaps you or others could help me a bit with crunching numbers. With helf from a friend, this is what we go so far using the figures you provided:

    3.53146667*10e10=35,314,666,700=35.3 Billion cubic feet per cubic km.

    1 Trillion cu feet= 28,316,846,712 cu meters

    Take the high figure of 1 Quintillion cubic feet aka 1,000,000 Trillion cubic feet and convert to meters=
    28,316,846,712,000,000 meters cubed

    Multiply that figure by .13= 3,681,190,072,560,000 cubic meters of water voided by the Methane escape.
    3,681,190,072,560,000/1,000,000,000 cu meters per cubic km=3,681,190 km cubed of water volume reduction.

    Divide that by 1,370,000,000 km cubed for the entire ocean and you get .002687 or less that a point 3% drop in ocean volume by extracting all the methane in the clathrates.

    The average depth of the sea floor is 3,720,000 mm below mean sea level, so a drop of .002687 would be 9995.64mm or 9.99564 Meters, aka 32 feet 9&1/2 inches.

    So, in the extremely unlikely (and horrific) event that all the clathrates were released at once, there would be a considerable drop in sea level if these are rigt. Even if less than one percent a year was released, it could offset a considerable portion of the projected rise, at least.

    But I’m thinking we must have gone wrong somewhere with our math. Would it be too much to ask for some smart poster here to point out where we went wrong?