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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. 251
    Dick Veldkamp says:

    #208 (Bill) #223 (Martin) Can we store sea water in Antarctica?

    The question seems interesting enough to look at it a little more.

    It seems to me you have to take into account two things:
    1. Potential energy (you would have to lift the water over a km or so)
    2. Letent heat: you have to freeze all that sea water. Dumping sea water on ice would cool down the water and heat up the ice. If there’s too much water, you would obviously melt the existing ice, rather than freeze the water. The energy you have available for freezing is (say Antarctica is -50 C) 50 deg K times the heat capacity of the ice.

    I haven’t done the math. Probably you would also have to factor in input solar radiation, and radiation out to space.

    Then there may be the matter of destroying another ecosystem.

  2. 252

    #242 walter crain

    Sounds like a biased article and it sounds like Vickys statements are being leaned out of context and possibly Mojib’s as well.

    Either the author is unaware of the confusing nature of his subtitles and contexts or it is deliberate.

    There were a few sessions on decadal variability and there was a lot of discussion about the problems in that area. Oceanic cycles were on everyones mind. I do not recall Mojib Latif’s comments in those sessions but I was late to one.

    The fact that the author uses Vicky Pope’s statement in this piece is used merely to support the assertion regarding the we may be cooling meme (I doubt she said it in that context but was rather answering a different question). There is nothing wrong with her statement but it is used in a way so as to possibly mislead by inference.

    My limited knowledge of NAO is that it does have a chunk of the global signal but I don’t think it is so huge as to control the whole climate but rather is a part of the natural variability. As far as I can tell it is largely not predictable beyond a few years. So how can he say it is going to cool us down for 20 years?

    The article also contradicts itself. It says it’s going to cool for 20 years and then it says “more agree that the short-term prognosis for climate change is much less that once thought.”

    That’s a weird statement too since most everyone in climate science knows that short-term is largely not well understood?

    Latif’s statement, if he really said it, (notice there are no quotes there) that “the next few years a natural cooling trend would dominate over warming caused by humans” is actually ridiculous, unless of course the NAO is going to give us -1.6 W/m2, which of course is highly unlikely.

    The claim that “Another favourite climate nostrum was upturned when Pope warmed that the dramatic Arctic ice loss in recent summers was partly a product of natural cycles rather than global warming” is equally bizarre and further reveals the bias inference. Of course natural is still involved in the Arctic, to say otherwise would be just as silly.

    Then of course the author inserts a weather argument saying it’s less ice loss this year than 07/08. I guess he doesn’t know much about ice mass or climate trends, but climate does not seem to be the subject of his article, but rather the target of his bias.

    Fred Pearce, the author sums it up by saying “The world may badly want reliable forecasts of future climate. But such predictions are proving as elusive as the perfect weather forecast.”

    In other words, he still does not know the difference between weather and climate. How do these guys get climate reporting jobs in the first place?

    Personally, I doubt Vicky would approve of the bias in the article or the way her statements were used to support the idea of global cooling as inferred.

    UK Met did a poster presentation showing that 2040 will have 2003 like heat waves every other year. Does not sound like they are predicting much cooling to me.

  3. 253

    #242 walter crain

    Also, there is the consensus issue. If Mojib actually does think it will cool for 20 years, he is still outside the consensus. That does not make him wrong, but where’s the beef? In other words on what basis, since the ocean cycles are not that predictable, where does he get a 20 year prediction from?

    Vicky was right, we know more about the climate (30+ years) “In many ways we know more about what will happen in the 2050s than next year,” than we do about the weather.

  4. 254
    Bob Tisdale says:

    franz mair (221): Thanks for the link to my humble website. Personally, I enjoyed the obvious impacts of ENSO on sea level.

    It is unfortunate that the sea level data through the KNMI Climate Explorer isn’t up to date. I would have enjoyed extending those comparisons. And before anyone thinks I’m claiming that ENSO is responsible for all of the changes in sea level, read the post. I’ve qualified it.

  5. 255
    stevenc says:

    John, I don’t think I lept. My comment was that you cannot get an impression of an explicit knowledge of underestimating sea level rise from the statement presented. It was nothing more and nothing less.

    Jim, then you should have used a statement that explicitly backed your point and not one that appears to have so many reservations in it and clearly leaves both the upper and lower limits open to adjustment.

    Martin, I agree that they left the upper limit of sea level more open to adjustment then the lower limit. It still seems to fail the test of being an explicit statement of knowing they underestimated sea level. As far as politic of statements go; I assume if there was negotiation in how the statement was worded then there was disagreement among the scientists working on the topic. If there wasn’t then there would have been no need for negotiations.

    I personally have no opinion on the rate of ice flow. I am not well enough informed to have aquired one at this point.

  6. 256
    cce says:


    Page 829 says:
    “the EMICs in Figure 10.34 indicate 0.2 to 0.6 m °C–1 for their final steady state (year 3000) relative to 2000.”

    I’m glad you agree that all of this information is in Figure 10.34, which is the reason why I keep referring you to it.

  7. 257

    Re: response to comment 33 by Stefan

    Dear Stefan,

    The physics of heat exchange largely escapes me, but where I live, the more ground that is covered with snow, the less the sun is able to melt it. It is only when the ground is partly exposed that the sun can really get going on it. So, in your response:

    “(i) There was much more ice back then, hence a greater surface area on which any warming could act to melt ice – this argues that melting during deglaciation would have been faster than in future.”

    Faster? Maybe more ice was melting, but can you please explain how it would have melted faster?

    I can see that now maybe there is less ice to melt, but it seems that it will melt faster due to the more heat-absorbent albedos surrounding it.

  8. 258
    walter crain says:

    #252 John P. Reisman (OSS Foundation)

    thanks for your analysis. kind of surprising (to me) for a newscientist article to be so fast and loose with its qualifiers.

  9. 259
    Hank Roberts says:

    > It still seems to fail the test of being an explicit
    > statement of knowing they underestimated sea level

    But there was no such test. They said they couldn’t after their deadline come up with new numbers based on the new information that they acknowledged had come in so they warned about that.

    This business of looking back at the science and saying it was imperfect fails to understand how science works.

  10. 260
    Jim Eager says:

    Stevenc (254), take it up with the IPCC, because that’s how they chose to word their caveat, and it’s meaning is abundantly clear to anyone who was paying attention to how fast ice dynamics and flow rates in Greenland and Antarctica were changing between 2003 and 2007.

  11. 261
    stevenc says:

    Hank, it was a test of that statement when it was declared to be one.

  12. 262
    Hank Roberts says:

    Look, this stuff being copypasted is exactly what was cynically predicted — people now who are complaining that the IPCC or the scientists — years ago — didn’t warn them clearly and bluntly enough, early enough to do anything.

    Next we’ll hear them complain that why they can’t be responsible for the lack of preparation and if the IPCC had only been clearer of course they’d have done their duty to the future, if someone had only warned them.

    The same sort complaining now about insufficient warning were complaining about ‘alarmists’ and ‘so-cia-lists’ and conspiracies to pollute their precious bodily fluids. Some still are.

    The warnings were clear. This is the angry beast Wally Broecker warned about.

  13. 263
    Rod B says:

    Hank(244), et al: as quoted from the IPCC: “…do not include uncertainties …”, “…do [not] include the full effects of changes in ice sheet flow…”, “…but these flow rates could increase or decrease in the future….”, “…understanding of these effects is too limited to assess their likelihood or provide a best estimate…” [emphasis mine; quote is not]

    I have a very difficult (impossible) time reading into that, “…the IPCC knew that their sea level estimates were low…” Maybe it’s just me. Or maybe you guys read into it whatever you choose, eh?

  14. 264
    Martin Vermeer says:

    Rod B, you could start by acquiring an upper-class British accent and a fake title. And you still sound too scientifically curious at times, almost as if you cared about facts. Lose the attitude… don’t let this career opportunity pass you by ;-)

  15. 265
    Mark says:

    “Martin, I agree that they left the upper limit of sea level more open to adjustment then the lower limit. It still seems to fail the test of being an explicit statement of knowing they underestimated sea level.”

    How do you work that one in to it?

    When ice sheets melt and you have left it out of your future trends, what do you think REALITY (where ice sheets melt) will say with relation to your future trend model (where ice sheets DON’T melt)?

    Think it through.

  16. 266
    Mark says:

    #202: “So we destroy whole ecosystems to what end? Deserts are ecosystems.”

    As are tropical savannah. Which was what the desert was once.

  17. 267

    #254 stevenc

    I understand that sometimes a small step can seem like a giant leap and vice versa. Perspectives can be funny that way.

    You seem to be looking for the precision with regard to interpreting a single line. But in the context of the entire report, what was known then, and adding what we are learning… the idea of trying to nail down who was arguing over what in a single line in a report that is greater than the sum of that line… well…

    The whole is greater than the sum of its parts. Some can’t see the forest through the trees, and sometimes the lady doth protest too much. Of course that does not mean there is no forest, or there is no reason for a protestation, or, that there may be a problem with a particular part. Context will get you relevance every time.

    If it helps you understand, maybe I can clear this up for you. The estimate that sea level rise by 2100 will be 1.4 feet, is too low. How do I know? Well, I don’t. It’s about probability, not precision.

  18. 268
    CM says:

    John P. Reisman (re #252-253), walter crain,

    As you often remind us, John, context is key. The context of these speeches was a “climate services” program, so concerns over predictability on local and decadal scales were predictably more of an issue than when taking a long-term view of global warming.

    Regarding Fred Pearce’s qualifications, he has been one of New Scientist’s lead environment reporters for more than a decade. But here I don’t think his story reflects Latif’s speech very well.
    Latif’s Powerpoint and a recording of the session are online (38-MB MP3; Latif comes in at about 23’35” – 40’45”), so we don’t have to guess.

    Your corrections would be very welcome before I drop off a comment at the magazine’s site along the following lines:

    Latif showed the 20th century variability around a rising trend, and noted that it could happen that temperatures cooled for a decade or even two, and if that happened, people would say global warming had stopped. He did not predict that this would happen over the next decade or two as the article suggests. His comment on needing to “ask the nasty questions ourselves” was in the context of addressing model biases, not in the context of a decadal cooling.

    Latif felt the jury was still out as to the relative contribution of internal decadal variability to the recent warming. He went on to discuss the NAO as one of the important internal modes of the climate system exhibiting low-frequency variability. Putting two and two together, Pearce got “NAO cycles were probably responsible for some of the strong global warming seen in the past three decades”. However, Latif said nothing about an impact of the NAO on global temperature, as opposed to regional, and noting regional (Europe/extratropical northern hemisphere) warming effects of the recent NAM/NAO trend hardly breaks with “orthodoxy” (see AR4, last para of section It was also not explicitly said that the NAO “explained” the recent “recovery” of the Sahel region, but this at least seems to me a reasonable inference from the talk.

    In short, in his presentation Latif did suggest — in passing, and very off-hand — that as much as two decades of cooling could happen despite underlying AGW. That would probably upset a few bets. He did not predict “that in the next few years a natural cooling trend would dominate over warming caused by humans” and that the cooling would be down to changes in the NAO. Nor did he mention that the NAO was moving into a “colder phase”. If anything, his speech was about the difficulties attending such predictions and the need to improve our abilities to make them. Maybe he said something later in a discussion session that Pearce picked up on.

    Sheesh, there went my whole morning, and I haven’t even looked at what Vicky Pope said about the Arctic. Still, I’m pretty sure it did not overturn any “favourite climate nostrum“… Who’s copy-editing my favorite magazine these days?

  19. 269
    Mark says:

    “I have a very difficult (impossible) time reading into that, “…the IPCC knew that their sea level estimates were low…” Maybe it’s just me. Or maybe you guys read into it whatever you choose, eh?”

    Nope, it’s what you fail to read into it.

    No ice sheet melting in the future estimation.

    In the real world, ice sheets will melt if it gets warmer.

    Ice sheets melting will increase the water in the oceans, raising levels.

    Ergo, a forecast that doesn’t include melting will most likely be an underestimate.

  20. 270
    Ray Ladbury says:

    Rod @263, the section is a little difficult to parse. Let’s break it down.

    First, flow of ice sheets is bound to increase melting, since it increases the rate of flow to lower altitudes (warmer temperatures) and into the oceans (ditto). The current analysis doesn’t consider the contribution of ice sheet flow, and so is an underestimate of what is going on even now. That underestimate could increase if flow increases, or decrease if it decreases. It’s still an underestimate.

  21. 271
    stevenc says:

    I don’t really see the problem with my comments. I am not changing their meaning with my own opinion since I don’t have one. I am reading their words literally. If there are problems with their wording then it is those who are not reading their words literally and are reading their own opinions in between the lines that need to take it up with the IPCC, not I. It really is that simple.

    John, I’m not clear what your point is. I can see a forest and it has a lot of trees in it.

    Will the aquifers recharge or continue to be depleted?

    Will deforestation increase or decrease?

    Will temperatures rise faster or slower then IPCC predictions?

    Will droughts or ample rain predominate?

    What are the dynamics of ice melt?

    Will dam construction increase or decrease?

    Obviously I have some opinions on some of these but certainly not on all hence I have no opinion on sea level rise being over or under estimated. It doesn’t mean I can’t see the forest it just means I can’t identify all the trees.

  22. 272
    stevenc says:

    Ray, didn’t the models neglect to include ice sheet flow based on an expectation of increased precipitation balancing out the loss of ice mass? Has this hypothesis now been entirely tossed out based on observations or is it still in dispute over what the long term balance will be?

  23. 273
    Mark says:

    Where did you get this from stevenc? “Ray, didn’t the models neglect to include ice sheet flow based on an expectation of increased precipitation balancing out the loss of ice mass?”

    John, I’m not clear what your point is. I can see a forest and it has a lot of trees in it.”

    Yup. Read up on what “can’t see the forest for the trees” means.

    “Will the aquifers recharge or continue to be depleted?”

    How will they recharge? And what does that do to disprove AGW?

    “Will temperatures rise faster or slower then IPCC predictions?”

    Yes, one of those two. What does that do to disprove AGW?

    “Will droughts or ample rain predominate?”

    Define “ample”. Bangladesh will get super-ample rain (called flooding). But that rain won’t get further inland. China won’t have the snowmelt keeping the rivers full in summer and so will get droughts. So your query is stupid because it depends on where you live. And what does it do to disprove AGW?

    “What are the dynamics of ice melt?”

    They take energy to change phase and then becomes runny. And what does it do to disprove AGW?

    “Will dam construction increase or decrease?”

    What does this have to do with dicussions about AGW?

  24. 274
    Hank Roberts says:

    > didn’t the models neglect …
    Stevenc, nope, you’ve got the timing wrong on that one. The notion that the meltwater would fall as permanent snow on top of the icecaps was wishful thinking and by the last IPCC known not to be happening now, and not to have happened during the last big warming episodes.

    Nice dream while it lasted, but it didn’t last very long.
    Let’s see:

    No surprises there. The people still promoting that idea include McAudit and Marrowhasty; the other links discuss it as historical curiousity.

  25. 275
    Rod B says:

    Tenney Naumer (257), I’ll give you one thought, which is far less worthy than any from Stefan. The albedo/reflection off snow is greater than off ice, on the average.

  26. 276
    Hank Roberts says:

    CM — excellent debunk of the New Scientist piece. I hope that gets added to the Wiki and other collections for later reference.

    Good luck improving New Scientist’s science coverage. I used to try; I eventually got a nice letter from their editorial department pointing out that New Scientist is not a science magazine, it’s an entertainment/lifestyle niche publication, and they didn’t feel it was their job to be accurate but rather to attract people. In current terms they’re selling, you know, the controversy. Look at their “Darwin Wrong/Tree of Life” cover and issue a while back, which quite effectively attracted lots of attention by being not just wrong but prominently wrong.

    [Response: I think this is a little strong. The journalists do try hard (most of the time) to get it right, but they do have editorial pressures to be as exciting as possible while still being accurate. Unlike Nature or Science, they can't rely solely on academic subscriptions and so need to stand out on the newsstand alongside Cosmo, GQ and Newsweek. I don't envy them that task. Obviously, when they do mess up, it should be pointed out - but it is inevitable that they will get the balance wrong on occasion. Even if imperfect, having NS out there is better than not having it at all. - gavin]

  27. 277
    Hank Roberts says:

    PS, anyone who’s been reading New Scientist may find the kind of science writing they were hoping for in Science News. I’ve found it worth subscribing:

  28. 278

    #268 CM

    I think your assessment looks reasonable. IMO his mention of cooling for a couple decades seemed hypothetical for the purpose of illustrating what the denialists would do with it.

    ‘Climate Services’ was the general theme at the conference. Improving the decadal or inter-decadal can help regions understand what and when to plant so it is very important.

    I listened to the talk again and actually I was in the room then, I just didn’t remember anyone predicting cooling, so I did not make the connection.

    It’s pretty clear though that Fred Pearce took Mojib’s words out of context and that is unfortunate. If New Scientist is supposed to be reputable, they should post a correction.

    I like the way Mojib wrapped it up in saying: “I’m definitely not one of the skeptics, okay, and if my name was not ‘Mojib Latif’, my name would be ‘Global Warming’.”

    His concerns with what the press might do with his words was not unfounded.

  29. 279

    #271 stevenc

    My perspective regards the bridge between the science and the public perception in the debate. What I am saying is that generally, in the discussion, I think it would be helpful if we discuss things in the larger context and not get too caught up in the minutia without the larger context.

    In truth, I can’t identify all the trees either. So I just keep exploring. But some trees have risen above the others and their impact is quite clear.

  30. 280
    Rod B says:

    Ray, Hank, Mark, et al: what is more inexplicable than Jim Eager’s assertion from his IPCC quote(205), is your all’s convoluted almost paranoid defense of it. You’re throwing in statistics theory, how one should project into and interpret the IPCC’s quote, what the IPCC really meant (but didn’t say), a more thorough parsing, how to draw logical conclusions beyond the quoted words, and presumably some knowledge of code words used by the IPCC so that only insiders can know what they are really saying. Now, all of that might very well be true! But it has no bearing on the original extremely simple (as stevenc also points out a couple of times) charge — NONE.

    Jim simply presented a direct quote from the IPCC and then drew a definitive conclusion from that quote that logically can not be drawn from that quote. Period. End of point-counterpoint. Jim, IMO, simply misspoke, but it certainly was not a major sin (or even a minor one, really) and did not undercut AGW theory in the least. Why all of the fuss?

  31. 281
    Mark says:

    Rod B, no what is inexplicable is your inability to think.

    IPCC report ignored ice sheets from land melting.

    In the real world, they melt.

    You still can’t connect those two statements.


  32. 282
    Mark says:

    PS “You’re throwing in statistics theory,”

    No, throwing in “when you’ve left out something you KNOW will add to the problem, your estimate will be an underestimate”.

    Or do you have a theory that as ice melts from off land, the water runoff reduces ocean content..?

  33. 283
    Ray Ladbury says:

    Methinks thou dost protest too much. There is a whole helluva lot more reason to think SLR will be worse than IPCC projections than to think it will be better. That’s not paranoia or statistics. That’s physics.

    I merely pointed out that since the IPCC was not considering ice sheet flow, that this would only make the projections worse. Now, pray, where do you see paranoia in that.

  34. 284
    Ray Ladbury says:

    stevenc, I think the failure to include ice-sheet flow is mainly due to the unsatisfactory knowledge of how to model it more than any precipitation dynamics–though I’m more than happy for someone more knowledgable to set me straight here.

  35. 285
    Doug Bostrom says:

    Rod B 9 September 2009 at 10:47 AM

    “Why all of the fuss?”

    Rather perhaps “How all the fuss?”

    Going all meta, over at the Guardian George Monbiot has lamented over how degraded and pointless the Guardian’s “Comment Is Free” threads become.

    Monbiot’s frustration has a simple root cause. Comment should certainly be free, but limitless? What happens when a resource has no cost? One outcome is, it ends up being increasingly exploited for diminishing gain.

    The “Tragedy of climate commons” thread on this site is an ironically appropriate example of the “free beer as opposed to Free Speech” tragedy. Go there to find people engaging in boundless yet picayune nattering over matters beyond peripheral to climate change.

    It would be a rather simple tweak to impose a limited supply of comments available to any given poster ID over a given period of time. Having only a limited supply of bullets, perhaps we would choose our targets more carefully (and yes I’m guilty of firing wildly in all directions). Governance might include recognition of some kind (executed with scant effort if sensibly implemented) for contributions such as Hank Roberts’ which are so frequent and yet often include useful directions for inquiry.

  36. 286
    CM says:

    Hank (#276), thanks for that. I’d have liked to try to head off this meme before it spread, but with 200+ comments on the NS page already, all of them premised on the accuracy of the report, it’s probably a bit late.

    I don’t mean to dump on the journalist — Gavin (inline) is spot on. Nor on New Scientist, which I think has excellent coverage of environmental science issues in the multi-decadal mean, internal variability to be expected…. Though I certainly see what you mean by “selling the controversy”. (Hated the “Darwin was wrong” cover — ditto the Einstein one).

  37. 287
    Mark says:

    Ray, 284, I think (from what I’ve read) that the biggest problem is that you can either say “it will happen before 2200″ which isn’t a lot of help, or say “5% chance by 2050, 30% by 2075, …” which *was* done somewhere.

    But that helps not at all when planning what to do. The politics says that you don’t do as much as is needed *for 2100* by 2020 because money is involved and we’ll get the sack.

    Rather like the New Orleans levees: politics wouldn’t allow the levees to be rebuilt “this year” for a 20-year event protection. So “we’ll do it next year” (I wonder how many times it then went “Ok. Next year, then. Definite”…).

    When money/time/effort is spent on something that doesn’t give a ROI within the next year, it’s called all to hell.

    So there is, out there, a “what’s the likelihood of X” wrt ice sheet failures. I *have* read some of one.

    But they are left out of policy decisions because policy taken to combat them now would be “courageous” (in the words of Sir Humphrey in Yes Minister…).

    In the case of ice, it depends on which bit breaks and what that causes to break (as per steven’s analogy). Therefore predictions are highly uncertain and every “failed” prediction is waved like a flag.

    cf UK’s warm winter but with snow in london 3rd Feb therefore showing the climate change was a complete hoax.

  38. 288
    Hank Roberts says:

    > having NS out there is better … – gavin]
    I agree,Imy comment was overbilious. Thanks as always, Gavin.

  39. 289
    stevenc says:

    Ray and Hank, yes obviously I was mistaken on the ice flow and precipitation balancing and thus not being included in the projections. They actually did take the ice flow from 1993 to 2003 and used this as the basis for their projections. So the question is was this amount of ice flow above or below what the average will be in the future. To state that ice flow was not incorporated in the projections at all seems to be an error also.

  40. 290
    Martin Vermeer says:

    > Rod B, no what is inexplicable is your inability to think.
    Same words, more politely spoken. How do you explain statistical thinking to folks having not the faintest?
    Suffice to say, Jim Eager was perfectly right about the IPCC quote. Don’t take my word for it: use the source Luke. In Chapter 10, Table 10.7 you find the same sea level rise values (5%-95% percentiles) as are summarized in Table SPM.3, on which Jim’s quote is based.
    Only, Table 10.7 has an extra row, labeled “Scaled-up ice sheet discharge”. This is the IPCC “guesstimate”.
    Please note how mostly positive these values are… for the B1 scenario, they run from 0.00 to 0.09 m. For A1FI, from -0.01 to 0.17 m. Can you say “unsymmetric”? The IPCC didn’t even venture to guess where within this range reality will land… but for our argument it doesn’t matter, now does it.

  41. 291
    Hank Roberts says:

    Rod, you’re doing it again, trying to make a discussion about changes in the world into an argument about the interpretation of a sentence in an outdated document.

    You realize what you’re doing–adding distraction, away from material you’ve read and discussed and are trying to obfuscate.

    “… The ice flow changes could have been discussed in the text … one of the reasons provided by the IPCC authors for not adopting our proposal was that the numbers could not be calculated quickly.
    …. the models used to derive this projection significantly underestimate past sea level rise. We tried in vain to get this mentioned in the SPM, so you have to go to the main report to find this information….”

  42. 292
    simon abingdon says:

    #273 Mark “Read up on what “can’t see the forest for the trees” means.”

    Actually Mark, the original expression was “can’t see the wood for the trees”, which has two possible meanings. Either “can’t see the big picture” (forest/trees) or “is unaware of important details” (timber/trees). As JPR would say “context is the key”.

  43. 293
    Martin Vermeer says:

    Hank #291, thanks! I should have refered to that… OTOH, nothing like the source.

  44. 294
    CM says:

    Update: The New Scientist “cooling” piece has been reposted. The heading is even stronger, but at least “nostrum” has become “belief” — and there’s a clean slate for comments, so we can remove this OT discussion there.

  45. 295
    Rod B says:

    My, my, my, my! What a mess! You all responded to my critique of your responses by repeating exactly what I accused you of! Example: Ray says (and I’m not picking on Ray; it’s just clearer), “…There is a whole helluva lot more reason to think SLR will be worse than IPCC projections than to think it will be better….” That makes sense and I suspect is probably very true. However it has no relevance, bearing, connection or relationship with the point-counterpoint that kinda started with Jim’s 205 and probably deserved to end with my first retort. Or one maybe can find other parts of the IPCC report, e.g. Martin V, that might justify Jim’s assertion. But that also has no relevance, bearing, connection…. In fact virtually NONE of climate science has any relevance…

    I was going to agree with Ray that I protest too much. But actually I’m just keeping it going too much. You all are the guys that protest too much. My (and stevenc’s) retort over a small overstatement probably deserved no challenge, let alone a hubbub that is based on nothing related to the initial contention. Why all that protest? What’s the fear? Hank actually clears it up. While he is trying to accuse me of some wrongdoing, he’s actually right: my contention was ONLY “…about the interpretation of a sentence in an outdated document…” It had nothing to do and made no direct comment or implication “…about changes in the world…” Though he then goes on to support Jim’s quote by citing some other different quote.

  46. 296

    I do not understand why you able gentlemen let Rod B take up so much of your valuable time — he’s only been doing it for years now.

    In the meantime, would anyone happen to know when the 2009 figures for GrIS’s mass balance loss will be out this year? Hopefully, they will be publicized well in advance of Copenhagen.

  47. 297
    walter crain says:

    re: 268 “CM”: John P. Reisman (re #252-253), walter crain,
    (new scientist article about NAO)

    CM, John, you guys are awesome. thanks for the informed discussion of the facts AND your activism. “nostrum”… like you said, “sheesh”.


  48. 298
    Martin Vermeer says:

    Rod B, yes, I happily plead guilty to an immune system oversensitivity to mendacity. Let’s just agree to disagree on this… after all you’re the expert on out-of-context exegesis.
    Now can we get back to discussing fast breeders? ;-)

  49. 299

    Tenney, it started out with a question, repeated here, by Nicholas Nierenberg. Perhaps he honestly wanted to know (but was too lazy to find out); perhaps he honestly believed it wasn’t true. One always wants to assume honesty.

    I hope Nicholas is happy now. And I hope some among the readership appreciate 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.

  50. 300
    Mark says:

    “You all responded to my critique of your responses by repeating exactly what I accused you of!”

    There was no critique, Rod B.

    You keep complaining that there’s no reasons, but you HAVE NOT answered what your problem is with these two simple points:

    IPCC report ignored ice sheets from land melting.

    In the real world, they melt.

    So what do YOU think is the inevitable consequence of that?

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