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2000 Years of Sea Level (+updates)

Filed under: — stefan @ 20 June 2011 - (Deutsch)

A group of colleagues have succeeded in producing the first continuous proxy record of sea level for the past 2000 years. According to this reconstruction, 20th-Century sea-level rise on the U.S. Atlantic coast is faster than at any time in the past two millennia.

Good data on past sea levels is hard to come by. Reconstructing the huge rise at the end of the last glacial (120 meters) is not too bad, because a few meters uncertainty in sea level or a few centuries in dating don’t matter all that much. But to trace the subtle variations of the last millennia requires more precise methods.

Andrew Kemp, Ben Horton and Jeff Donnelly have developed such a method. They use sediments in salt marshes along the coast, which get regularly flooded by tides. When sea level rises the salt marsh grows upwards, because it traps sediments. The sediment layers accumulating in this way can be examined and dated. Their altitude as it depends on age already provides a rough sea level history.

How is sea level reconstructed?

But then comes the laborious detail. Although on average the sediment buildup follows sea level, it sometimes lags behind when sea level rises rapidly, or catches up when sea level rises more slowly. Therefore we want to know how high, relative to mean sea level, the salt marsh was located at any given time. To determine this, we can exploit the fact that each level within the tidal range is characterized by a particular set of organisms that live there. This can be analyzed e.g. from the tiny shells of foraminifera (or forams for short) found in the sediment. For this purpose, the species and numbers of forams need to be determined under the microscope for each centimeter of sediment.



The foram Trochammina inflata under the microscope

To get a continuous record of good resolution, we need a site with a rapid, continuous sea level rise. Kemp and colleagues used salt marshes in North Carolina, where the land has steadily sunk by about two meters in the past two millennia due to glacial isostatic adjustment. Thus a roughly 2.5 meters long sediment core is obtained. The effect of land subsidence later needs to be subtracted out in order to obtain the sea level rise proper.



Ben Horton and Reide CorbettPostdocs Andy Kemp and Simon Engelhart in the field at Tump Sand Point, one of the study sites [Sorry for photo mix-up]

How did sea level evolve?

The graph shows how sea level changed over the past 2000 years. There are four phases:

  • Stable sea level from 200 BC until 1000 AD
  • A 400-year rise by about 6 cm per century up to 1400 AD
  • Another stable period from 1400 AD up to the late 19th C
  • A rapid rise by about 20 cm since.



Sea level evolution in North Carolina from proxy data (blue curve with uncertainty range). Local land subsidence is already removed. The green curve shows measurements from a nearby tide gauge a reconstruction based on tide gauges from around the world (Jevrejeva et al. 2006, 2008). The red curve shows results from a simple model connecting global temperature with sea level. For the last millennium the sea level curve follows what can be expected from temperature – the two independent reconstructions thus mutually reinforce each other by their consistency. Before 1000 AD there is a discrepancy: warm temperatures in the reconstruction used would lead to rising sea level, but the sea level reconstruction is flat. However, temperature data from before 1000 AD are sparse and less reliable, and lowering temperatures in this period by only 0.2ºC removes the discrepancy. Thus, a possible explanation for the discrepancy is that the temperature reconstruction is a little too warm before 1000 AD.

These data are valid for North Carolina, where they are also in agreement with a local tide gauge (green)(Fig. 2 in the paper). But they also agree with another proxy data set from Massachusetts. Sea level changes along the US Atlantic coast do not need to fully coincide with global mean sea level, however. Even though the level rises uniformly if I fill water into my bath tub, the ocean has a number of mechanisms by which local sea level can deviate from global sea level. One of these mechanisms can also occur in the tub: the water can “slosh around”, in the oceans on multidecadal time scales. And there are some other factors as well, like changing ocean currents or changes in the gravitational field (due to melting continental ice). In the paper these factors are estimated and it is concluded that the North Carolina curve should be within about 10 cm of global mean sea level.

Connection to climate

I was involved in this study, together with Martin Vermeer and Mike Mann, to analyse the connection of the sea level data with climate evolution. We used a simple semi-empirical model, which basically assumes that the rate of sea level rise is proportional to temperature. In other words: the warmer it gets, the faster the sea level rises. This connection has already been established for the past 130 years, but it also works well for the past millennium (red curve). There is a discrepancy before 1000 AD (see figure caption).

According to this model, the rise after about 1000 AD is due to the warm medieval temperatures and the stable sea level after 1400 AD is a consequence of the cooler “Little Ice Age” period. Then follows a steep rise associated with modern global warming. Modern tide gauge and satellite measurements indicate that sea level rise has accelerated further within the 20th Century.

Reference: Kemp et al., Climate related sea-level variations over the past two millennia, PNAS 2011.

Further info on sea level: see the PIK sea level pages.

Update 21. June. I’d like to clarify two issues that have come up in discussion of our paper.

The first is: what can we learn from this for future sea level? A proper answer has to be given in another paper, but we can note now that the model fit to the new proxy data is highly consistent with the fit we obtained in 2009 to the tide gauge data. Hence it implies almost the same future projections as in our 2009 paper (75-190 cm by 2100).

The second issue is a misunderstanding of our Fig. 4D. The blue and green curves shown there, labelled Rahmstorf (2007) and Vermeer & Rahmstorf (2009), do not show sea level predictions we made in those earlier papers. They show new predictions (or rather hindcasts) with the model equations used in those two papers, for comparison with the more sophisticated equation used in the present paper. As we write in the paper: “These two models were designed to describe only the short-term response, but are in good agreement with reconstructed sea level for the past 700 y.” The former means we never used them to compute long-range hindcasts – they are merely shown here for comparison purposes, so that readers can see what difference the additional term in Eq. 2 actually makes. And the good agreement for the past 700 years was quite a surprise to me – I did not expect these simple models to hold up for such a long time scale.

Update 2 (June 23)

People have asked whether the use of the Tiljander proxies in the Mann et al (2008) EIV surface temperature reconstructions matters for the conclusions of this or any related studies. The answer, as provided previously in the literature (see this reply to a comment in PNAS) is no.

The impact of whether or not these proxies are used was demonstrated to be minimal for the Northern Hemisphere land+ocean EIV reconstruction featured in Mann et al (2008) [see Figure S7b of the Supplementary Information of that article, which compares the reconstruction both with and without 7 potential ‘problem proxies’, that include the Tiljander proxies; a similar comparison was also made in Figure S8 of the Supplementary Information for the followup article by Mann et al (2009)] . The same holds for the specific global mean EIV temperature reconstruction used in the present study as shown in the graph below (interestingly, eliminating the proxies in question actually makes the reconstruction overall slightly cooler prior to AD 1000, which–as noted in the article–would actually bring the semi-empirical sea level estimate into closer agreement with the sea level reconstruction prior to AD 1000).



Comparison of Mann et al (2008) global mean (land+ocean) temperature reconstruction with and without the 7 proxy records discussed in the text [shown in both cases is the low-frequency (>20 year timescale) component of the reconstruction]. Reconstruction is based on calibration against the HadCRUT3 series using the global proxy network
.

References:
Mann, M.E., Zhang, Z., Hughes, M.K., Bradley, R.S., Miller, S.K., Rutherford, S., Proxy-Based Reconstructions of Hemispheric and Global Surface Temperature Variations over the Past Two Millennia, Proc. Natl. Acad. Sci., 105, 13252-13257, 2008
Mann, M.E., Zhang, Z., Rutherford, S., Bradley, R.S., Hughes, M.K., Shindell, D., Ammann, C., Faluvegi, G., Ni, F., Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly, Science, 326, 1256-1260, 2009


67 Responses to “2000 Years of Sea Level (+updates)”

  1. 1

    Very interesting indeed. On the graph, the increase in SLR appears to set in somewhere around 1800, which seems a bit early for big anthropogenic effects–though it’s a bit hard to be definite due to the scale and image size. Can you say more about that? (It’s a question the ‘usual suspects’ would raise, to be sure.)

    [Response: Actually, the sea level reconstruction (blue curve) doesn’t show an increase until later (roughly 1900). Its the semi-empirical model (red curve), that shows an increase since around 1800. Since the semi-empirical model has been driven by the global temperature reconstruction, it is by definition consistent with it. The global temperature reconstruction does in fact show a minimum in temperature at about 1800 (clearer if you look at the relevant figure of the paper itself, i.e. Figure 2a), and hence a warming since then. – mike]

    Also, a slight typo–the second sentence of the “How Did Sea Level Evolve?” section should read “There *are* four phases.”

    [Response: Fixed. Thanks. – mike]

  2. 2
    Thomas says:

    It may sound a bit nitpicking, but water sloshing in a bathtub is a gravity wave, water height in the ocean varying on decadal or longer scales must be totally different physics.

    [Response: Stefan’s analogy here, as I interpret it, is reasonable. There is a strong regional pattern of sea level variation on multidecadal and longer timescales associated with e.g. the differential gravitational pull of growing or decaying continental ice sheets. See e.g. Mitrovica et al (2009). – mike]

    [Response: You’re right about the gravity waves – it was intended as a simple analogy that can’t be carried too far. The ocean is a stratified fluid on a rotating sphere, and the water motions are rather more complex than those in a bath tub. -Stefan]

    I presume North Caroline is far enough south to not be affected by postglacial rebound, but Massachusetts IIRC is.

  3. 3
    sidd says:

    Thanks for the nice article. Would you care to explain further (or direct me to a reference) the bit about determining the height relative to mean sea level

    ” exploit the fact that each level within the tidal range is characterized by a particular set of organisms that live there.”

    I have become rather pessimistic about the fate of the large ice sheets of late. If West Antarctica were to deliver something similar to MWP1B this century, how much warning would we have before sea levels rose at a couple inches a year for a hundred years ? (Although I suspect it would be more like 0.5 inches a year for a decade, followed by 10 inches a year for a couple decades and then decline.)

    Or have we had all the warning we are going to get ?

    sidd

  4. 4
    Edward Greisch says:

    Thanks for the article and the link to the pnas article. I see another hockey stick.

  5. 5
    CArsten says:

    Nice read, but Stefan can you comment on the criticism appearing in spiegel.de today (http://www.spiegel.de/wissenschaft/natur/0,1518,769424,00.html) regarding that only this US site matches the data of the past ~120yrs? Their main claim is neglecting other non matching site data will make this research not looking very solid…

    [Response: spiegel.de got a number of things badly wrong; I responded to those this morning at KlimaLounge. Regarding your specific question: our Fig. 3 tries to give an overview over previous reconstructions, regardless of their quality. You will see that a couple of them are “all over the place” (i.e. the ones from Israel and the Cook Islands). They do not only mismatch the other data sets but are implausible in themselves, in requiring huge sea level jumps over short time periods. Apart from these two, the only one that does not match the new data, within stated uncertainties, is the one from Iceland. This warrants further investigation whether this is a local effect.
    (p.s. This response applies to the full time series, not just the past 120 years. For the past 120 years the proxy reconstruction nicely matches the Jevrejeva et al. (2006) and Church&White (2006) global tide gauge reconstructions – see above and Fig. 6 of paper.) -Stefan]

  6. 6
    Davos says:

    “To get a continuous record of good resolution, we need a site with a rapid, continuous sea level rise. Kemp and colleagues used salt marshes in North Carolina, where the land has steadily sunk by about two meters in the past two millennia due to glacial isostatic adjustment. Thus a roughly 2.5 meters long sediment core is obtained. The effect of land subsidence later needs to be subtracted out in order to obtain the sea level rise proper.”

    How did you already know that this location was a site of ‘rapid, continuous, sea level rise’ if this is the sort of thing you were actually trying to test for with your study? And, if you are to “effect of land subsidence”, how do you know what this component is relative to either the post-glacial rebound or the rise in sea-level itself? It seems like at least 2 of the three need to be known in order to identify the magnitude of the 3rd. Is this study working off of assumed or expected/modeled values for certain variables in order to complete the reconstruction?

  7. 7
    Nick Barnes says:

    Thomas@2: glacial isostatic rebound in North Carolina is essential to this research (the land there is sinking at ~1mm per year, which means that there is a steady background sedimentation rate). See the post and also the article.

  8. 8
    Paul S says:

    Very interesting piece. Would the steady rise after 1000AD imply that the global average temperature may have been at a fairly stable “high” until about 1400, rather than dropping off after a short medieval peak?

    On that note is there any possibility of using this and similar sea level reconstructions in multi-proxy temperature reconstructions?

  9. 9
    chris says:

    fascinating research. The link between global temperature and rate of sea level change provides a brilliant opportunity for cross-validation of these two parameters over the last several millenia (one might add-in the relationship between atmospheric [CO2] and Earth temperature in the period before any significant human impact on [CO2]). As these data sets expand (paleo-sea level/paleo-temperature) there’s every chance we can home-in on some really self-consistent interpretations of temp/sea level/greenhouse gas relationships going back several millenia which we be extraordinarily useful as targets for modelling and as predictors of future scenarios.

    I have a couple of questions about the PNAS study:

    (i) Why does the “equilibrium temperature” (the temperature at which sea level change is zero) vary with time (e.g. Fig 4c). Does the pattern of change (warming raises the equilibrium temperature, cooling decreases it), indicate a negative feedback on sea level change (e.g. as land ice melts it requires a little warmer temperature to continue to melt further land ice…and vice versa??).

    (ii) The sea level hindcasts in Fig S6a. This is really a Grinsted et al question, but how were the Moberg (2005) and Jones and Mann (2004) temperature reconstructions normalised to give a global (rather than N. hemispheric) temperature that is required for a valid temperature-sea level hindcast?

  10. 10

    Thanks–yes, Figure 2A does allow one to see the detail much more clearly.

    The summary graphics of other proxy reconstructions around the world are also worth the time to take a look!

  11. 11
    Martin Vermeer says:

    chris #7,

    (i) We found it necessary to introduce a component of sea level change that equilibrates on a short time scale (order several centuries) in addition to a “secular” component. This short-term component is essentially what Grinsted et al. used (but they left the secular component out). But, while they defined an “equilibrium sea level” Seq, as a function of temperature, we chose to do it the other way around (which is mathematically equivalent): define an “equilibrium temperature” T0(t) corresponding to a given sea level. You have to make it time dependent so that it, and the corresponding component of sea level, can equilibrate. This is expressed in Eq.2b in the paper. But basically your understanding is right: there is a component of the cryosphere that responds in this way, on a relatively short time scale tau, which is one of the parameters coming out of the hindcast.

    (ii) You would have to ask the authors… my impression is that they just took the NH reconstructions as if they were global. For this figure we just re-plotted their curves from data on Aslak Grinsted’s web site.

  12. 12
    J. Bob says:

    Some time ago I started looking at sea levels, using tidal gauge info, to see how it correlated to temperature. The So. & East coast of the US was chosen, since that seemed to have the least seismic activity, including uplift. I would have preferred the East coast of S. America, but the data records were not as good as US records. Looking at the records, noted in the figure below:

    http://www.imagenerd.com/uploads/n_amer_s_e_composite_3-MKDwk.jpg

    seven stations were selected, to form a composite anomaly. These included Galveston, Pensacola, Key West, Charleston, Baltimore, Atlantic City & New York. The composite was filtered with a 10 yr. Fourier filter, and compared a Trend line:
    http://www.imagenerd.com/uploads/us_se_tidal_composite-EZEn2.gif

    It was noted that, after ~1915, the trend line held fairly close to the filtered composite, in spite of increasing CO2. The HadCRUT3 global anomaly was also included as a comparison

    An addition, some long term records were evaluated, comparing temperature to CO2. These were from stations that began recording prior to 1800:
    Central England – 1659-2010
    Debilt Netherlands – 1706 – 2010
    UPPSALA (LÄN)Swed. – 1722-2010
    BERLIN (TEMPELHOF), Ger – 1701-2010
    PARIS (14E PARC MONTSOURIS) Fr, 1757-2010
    GENEVE (NASA), Switz. – 1753-2010
    BASEL (BINNINGEN) Swiz.- 1755-2010
    PRAHA (KLEM.-RUZYNE) Czech – 1775-2010
    STOCKH (GML-LAN) Sw – 1756-2010
    BUDAPEST (Hungary) – 1780-2009
    HOHENPEISSENBERG, Ger – 1781-2010
    MUNCHEN, (RIEM FLUGHAFEN ), Ger – 1781-2010
    EDINBURGH (SCOTLAND), GB- 1785- 1993
    WROCLAW (SOUTH WEST), Pol – 1792-2010

    CEL & Debilt were from:
    http://hadobs.metoffice.com/hadcet/data/download.html
    http://climexp.knmi.nl/data/ilabrijn.dat

    The rest were from the Rimfrost site:
    http://www.rimfrost.no/

    The anomaly of each site was computed (1969-1999 base) & a composite average was formed for each year. The data set was then filtered with a 50 yr. Fourier Convolution filter, and compared to the CO2 Mauna Loa & Law Dome (DE08 & De08-2) ice core data.

    The result is shown below:
    http://www.imagenerd.com/uploads/co2_temp_1650-2010-6OeMR.gif

    Since all the long term temperature data was taken from central & western Europe, the HadCRUT3_NH anomaly was also included.

    A few items noted were:
    On a long term basis, there was little CO2 change, while Europe went through some temperature swings, comparable to the present.

    While the Ave14 & HadCRUT3_NH seem to follow each other (especially the post 1900 rise, 1940 dip & subsequent rise), CO2 seems to have little correlation.

    Ave14 seems to lead the HadCRUT3_NH curve by about 10 years, so we may be in for a NH dip, or are already in it.

  13. 13
    JCH says:

    I’ll probably ask this the wrong way, but is Mitrovica’s theory – paper cited by mike’s response @ 2 – showing up in the observations?

  14. 14
    Martin Vermeer says:

    JCH #13: unfortunately not. We actually looked at this; see the Supplementary Material, where we did a sensitivity analysis. Using Mitrovica’s numbers, one finds that in North Carolina, the observed variations in sea level may be, due to this ‘fingerprint effect’, as little as 83% of the global variations; but, dividing the NC record by 0.83 didn’t noticably affect the quality of the semi-empirical model fit. So no, you cannot see this (yet) in the observational data.

  15. 15
    Jan Sjoerd de Vries says:

    From the link given on the 120m sea level rise after the last ice age it is easily derived that the maximum sea level rise was typically 150cm / century for several thousands of years. The sea level rise for the 21st century mentioned in the update to this paper is also app. 150cm, although given with a large uncertainty.
    A simple question, maybe: Is this a coincidence, or is there another, physical explanation?

  16. 16
    Nick Verge says:

    From the article:

    “To determine this, we can exploit the fact that each level within the tidal range is characterized by a particular set of organisms that live there. This can be analyzed e.g. from the tiny shells of foraminifera (or forams for short) found in the sediment. For this purpose, the species and numbers of forams need to be determined under the microscope for each centimeter of sediment.”

    This research relies on being able to determine palaeo-waterdepth of a saltmarsh very precisely, and here lies the rub.

    Different species of foram do indeed live at different water depths, in this case in a salt marsh. However, once they die their test can be transported by water, wind and biological activity (other critters) to different water depths, sometimes shallower, from that at which they lived and are then deposited Moreover, foram tests are relatively robust, they survive in sediments and so if earlier sediments are eroded, and the resulting sediment redeposited, any contained forams will be too. Failure to identify contamination of the sedimentary record by allochtonous foram tests, will lead to false water depth estimations for the sediments that host the forams.

    I expect the authors of this research have taken all these complications into account, but this is not stated in this article. I dont disagree with their findings, but would like to have greater confidence and reassurance that they indicate is what the authors interpret them to indicate.

  17. 17

    Going back a little further than 2000 years of sea level proxy analysis, here is a very recent paper that some of the readers here might find interesting, and going back a little further here is another quite interesting result. Interesting in that we haven’t quite yet figured out how that water could have gotten from Lake Agassiz into the Champlain Sea, since at the time the route seems to have been securely blocked by the Laurentide ice. Curious.

  18. 18
    Gilles says:

    two questions :

    – is it right to say that this study doesn’t show any significative influence of anthropogenic, post -1970 warming on SLR, since the SLR reacts mainly with a very large time constant and averages the temperature over a time much longer than 40 years ? so that it is not really justified to associate the current rate with anthropogenic influence, but rather to the exit of LIA ?

    [Response: If you’re interested in what happens in the 20th Century, then this proxy study is not the way to go but rather you should look at the tide gauge data. These are consistent with the proxies (see our graph) but of course more accurate and with higher time resolution, and we have a global set. Thus you should look at the Vermeer & Rahmstorf (2009) study linked above, which correlates the tide gauge record with global mean temperature since 1880 and shows that the modern acceleration of sea level rise is closely related to modern global warming.]

    – if the model is so sensitive that the pre-1000 discrepancy could be removed by only a 0.2 °C change, what is the reliability of the agreement between 1000 and 1500 ? i don’t think that temperatures were known to this accuracy, so is the agreement just adjusted by a convenient choice of the “right” temperature reconstructions ?

    [Response: There was little (or even no) choice here since we needed a global land + ocean reconstruction (i.e. not just northern hemisphere, and not just land – both of these would have been useless). In any case we have not tried any other temperature reconstruction. -Stefan]

  19. 19
    CM says:

    The auditors are sharpening their, uh, teaspoons. If you can stand the bile, comments over at ClimateAudit about the “preferential treatment” of this paper are good for a laugh. I think McIntyre manages to

    – get the affiliation, degree level, and supervisor of the lead author (tendentiously) wrong (as many of his commenters already pointed out);
    – make a false comparison between distinct PNAS submission routes, as Lindzen, enjoying the member’s privilege of sending a “contributed” paper could not also enjoy the pre-arranged editor that is an option for “direct submissions” (h/t Nick Stokes);
    – misstate the PNAS quarantine period for reviewers and editors due to previous co-authorship (two years, not four);
    – rail about unavailable data (foram dataset used to develop transfer functions) without even following a simple reference to the previously published work of the lead author (h/t R. Norvegicus); and possibly
    – suggest that a downweighting of the proxy data applies to the sea-level reconstruction, when it actually relates to the fit to that reconstruction of a model based on reconstructed temperatures (? – not sure I got that right, though, and would appreciate if someone could clarify).

    As far as I can tell, he’s right that the data from the foram assemblages used in the reconstruction isn’t available on-line – will it be?

    And about that weighing factor: is it important, is it subjective, and if not, can the rationale be explained to Bears of Very Little Statistical Brain like yours truly?

  20. 20
    Slioch says:

    Just in case you have a libel lawyer handy who is looking for some work:

    http://news.scotsman.com/letters/Letter-Warming-39scam39.6789573.jp

  21. 21
    Martin Vermeer says:

    CM:

    - suggest that a downweighting of the proxy data applies to the sea-level reconstruction, when it actually relates to the fit to that reconstruction of a model based on reconstructed temperatures (? – not sure I got that right, though, and would appreciate if someone could clarify).

    Yes, it only relates to the fit.

    And about that weighing factor: is it important, is it subjective, and if not, can the rationale be explained to Bears of Very Little Statistical Brain like yours truly?

    The reconstructed sea levels as provided by Andy Kemp have their own uncertainties, which we used in the computation; but as so often with these things, those formal uncertainties do not capture everything that is there in the way of error sources(*). This shows up in the Bayesian fit as all ensemble members getting very small posterior likelihood values.

    Furthermore, looking at the original sea level point data it probably contains a bit of auto-correlation. So we have less independent sea level data to work with than we think we do.

    Both circumstances can be accounted for — and yes, this is a bit subjective — by re-scaling the assumed uncertainties for the sea level data. You can play with this for yourself as the code we used should be with the paper (haven’t checked).

    (*) You see the same in geodesy: the per-km error of levelling as estimated from back-and-forth between benchmarks is always smaller than from loop closures, which again is smaller than from network adjustment. Same for triangulation networks and even modern GPS networks: the prior uncertainty of the observations is never the whole story.

  22. 22
    Alex Harvey says:

    Dear Prof. Rahmstorf,

    This has two parts.

    1)

    I assume you have seen responses in the German media at Der Spiegel Online (translated from German by GWPF
    http://thegwpf.org/science-news/3267-new-sea-level-study-divides-climate-researchers-.html).

    [Response: Is that a legal, authorized translation? It contains a section on supposed “deviations from previous studies” that is not in the English version of the article published by Der Spiegel (and which was corrected in the German version, because there are no such previous studies by Mike or me, as the original article falsely claimed.) -Stefan]

    The translation quotes you as saying, “The new study confirms our model of sea level rise – the data from the past sharpens our view in the future”.

    Then it continues, “But other experts doubt exactly this claim. They see a major problem of the new study in the fact that it is ultimately based only on the finding from the coast of North Carolina. That could be too limited for a statement regarding global developments.”

    I am surprised that no one has commented here on this objection or preempted it with discussion?

    [Response: That is discussed in great detail in the paper, but also in the post above. We estimate that the NC data should track the global mean sea level to within +/- 10 cm (on the time scales we resolve), and so far nobody has challenged this estimate. Specifically, Jens Schröter, quoted in the Spiegel article, has confirmed to me that he also finds this a reasonable estimate. -Stefan]

    2)

    The study has also been criticised on various blogs for using [edit] Tiljander lakebed sediment data series [edit]
    I don’t claim to understand the objection but given that it has spread like wildfire on blogs I am surprised that no one has mentioned it here.

    Is this objection valid?

    [Response: No. Just more of the usual deception from dishonest mud-slingers. More on that in short order. -Mike]

    Cheers,
    Alex Harvey

  23. 23
    Ray Ladbury says:

    Re: #18

    Is it right to say that Gilles is constructing his talking points so that they justify his preconceived notions?

    Is it true that he will torture any result within an inch of absolute falsehood just to support those same notions?

  24. 24
    ferd berple says:

    200+ years ago the British Admiralty undertook one of the largest scientific studies ever conducted. They charted the oceans of the earth in such exquisite detail that except for minor corrections, these chart are still in widespread use today.

    For obvious safety reasons, included in these charts are tens of thousands of “drying rocks”. Rocks that were at the time underwater at high tide, but uncovered or awash at low tide.

    As such, all that one needs to do to confirm sea level rise is to take a British Admiralty chart for your area down to the beach at low low tide. The year the chart was drawn will be noted in the legend. Most likely yours was drawn before the industrial age, before CO2 could have been an issue.

    Look for any cross marked with 4 dots on the chart. Now look at the ocean. Depending on the scale, the rock may be drawn further offshore than it is in reality, for safety reasons to make it visible on the chart. Allow for this when looking at the chart.

    Most mariner’s can already tell you what you will find. The same British Admiralty charts from 200+ years ago, except for minor corrections are still in widespread use today.

  25. 25
    CM says:

    Martin (#21), thanks.

  26. 26

    “. . . included in these charts are tens of thousands of “drying rocks”.”

    Which means there are tens of thousands of data points to evaluate. “Your local beach” is hardly enough to confirm anything.

    To do that, you’d have to:

    1) identify when each rock was charted;
    2) identify local tidal extremes both when charted and now;
    3) inspect each rock, verifying its vertical position vis a vis tidal extremes now;
    4) determine if the rock is affected by subsidence or uplift, applying a correction to #3 as needed;
    5) analyze the data to determine probable sea level rise.

    (This would be a rather involved process, given the ‘messiness’ of the data set. How would you do it–probabilistic approach based upon proportion of rocks which don’t dry anymore? Infer mean SLR from the tidal magnitudes of ‘non-drying rocks’ versus the time baseline?)

    Seems quite a bit easier to go with the tidal gauge record; it has its complications, but at least you are starting with reasonably direct measurements of sea level.

  27. 27
    CM says:

    PS. I don’t see any code with your paper on the PNAS site, though.

  28. 28
    CM says:

    Ferd #24,

    Sea level rise over the 20th century was 17 cm, give or take a bit. Tidal ranges tend to be quite a bit more.

  29. 29

    24 ferd berple

    You seem to speak with certainty, but ‘low low tide’ varies a lot from day to day. So we might expect a lot of hype, both positive and negative from the test you suggest.

  30. 30

    I am curious about the dribbly dotted red line. I would have expected it to follow the medieval global warming period from 1000 AD back to 500 AD, or is this no longer believed to be credible? Also, I thought temperatures started cooling beginning about 1100 AD and continued dropping until around 1500 AD.

    [Response: You are confused about the relationship between temperature and sea level in the semi-empirical model. It is the rate of SLR change that is proportional to the temperature. The reason you see such a sharp upward slope in the dotted red line between AD 500 and 1000 is that the relatively warm conditions indicated by the temperature reconstruction is suggesting a fast rate of SLR increase. By contrast, the actual SLR estimate during that same interval is relatively flat, suggesting that temperatures were not as warm as indicated by the temperature reconstruction. That is why, as stated in the paper, the sea level reconstruction appears to suggest that temperatures during AD 500-1000 were not as warm as indicated in the M08 reconstruction (about 0.2C or so cooler on average). -Mike]

    Also, can there be a little more said about the simple model relating atmospheric surface temperatures to sea level?

  31. 31
    grypo says:

    It appears McIntyre is continuing with the teaspoons, CM. This time he has a powerpoint presentation that may or may not have anything to do with the new paper (I dunno), which may or may not be comparable to the sea level numbers, and an assumption about downweighing (who knows!) He went far enough to know when the pdf might have been created. He’s a real sleuth. I also noticed the PNAS time limit for referees mistake. He actually relied on “free-market energy blog” for that information which came from a letter sent by the PNAS to member in 2008. Oops! He obviously doesn’t care about accuracy anymore, or even the appearance of having it. U Penn, Penn St? So Mann’s graduate student got preferential treatment from the PNAS by being ‘given’ a “prearranged editor” within the “prohibited” window of CoI rules. Yup. Too bad none of that is at all accurate.

    [Response: Hmmm. Given that no graduate student of mine, to my knowledge, had any involvement with this paper at all, its hard to see how said imagined graduate student could have received any hypothetical ‘preferential treatment’ let alone any treatment at all. Very curious indeed. -Mike]

  32. 32
    Deep Climate says:

    #31

    That’s a mixup between U Penn and PSU no doubt, combined with McIntyre’s rather skewed world view. As it happens, I’ve just posted the following comment at my Open Thread # 10.

    =============================
    Says Steve McIntyre, commenting on the new sea level reconstruction paper by Kemp et al at PNAS, “Climate Related sea-level variations over the past two millennia”:

    A few days ago, PNAS published Kemp et al 2011, a submission by one of Mann’s graduate students. …

    It was certainly generous of PNAS to give a “prearranged editor” to a submission by a graduate student at Penn State.

    Only problem is, Andrew Kemp received his PhD at the University of Pennsylvania, not Penn State, and his PhD advisors included Ben Horton and three others, but not Michael Mann.

    It makes one wonder what else McIntyre got wrong.

    http://deepclimate.org/2011/05/20/open-thread-10/#comment-9340

  33. 33
    CM says:

    (Off-topic but totally cool paleo-temperature reconstruction: Body Temperatures of Dinosaurs Measured for First Time)

  34. 34
    deston says:

    isostatics are one thing. subduction is another. isnt the the east coast sinking, aside from glacial isostatics>tectonic subduction? and maybe more direct to the study- would nutrient flow also affect mash growth- ie, eutrophication- and re…sult in bigger, ‘higher’? deposits? does sea level rise vs. variance of avaialble nutrient loads have different deposit profiles? I see stuff on west coast marshes that wouldnt grow, or grow bigger, if not for the golf course nitrogen drift….how does one correct for change in available nutuients to the salt marsh resultingfrom ag outflows? #freakology

  35. 35
    ferd berple says:

    “low low tide’ varies a lot from day to day”

    [edit. Ferd a.k.a. “Greg Elliott”, take it elsewhere]

  36. 36
    Sean says:

    I see code and data is online, and I hope that is an arguement avoided.

  37. 37
    Davos says:

    “[Response: You are confused about the relationship between temperature and sea level in the semi-empirical model. It is the rate of SLR change that is proportional to the temperature. The reason you see such a sharp upward slope in the dotted red line between AD 500 and 1000 is that the relatively warm conditions indicated by the temperature reconstruction is suggesting a fast rate of SLR increase. By contrast, the actual SLR estimate during that same interval is relatively flat, suggesting that temperatures were not as warm as indicated by the temperature reconstruction. That is why, as stated in the paper, the sea level reconstruction appears to suggest that temperatures during AD 500-1000 were not as warm as indicated in the M08 reconstruction (about 0.2C or so cooler on average). -Mike]”

    What is the basis that makes the suggestion that the temperatures were not as warm as indicated during AD500-1000 more valid than perhaps the M08 reconstruction suggesting that the SLR estimate instead is incorrect?

    [Response: That’s a very fair point. Both alternatives, in my view, are equally viable. Future work will hopefully better pin this down. -Mike]

  38. 38
    KR says:

    I did not see the code or data on the PNAS site (aside from the supplemental), although I’ll admit I’m not completely familiar with PNAS. Where would this code and data be located?

  39. 39
    Martin Vermeer says:

    Davos #37, my perspective differs a little from Mike’s here. I do not consider it likely that the “knee” at 1000AD and the downturn before that are realistic. It would imply or suggest that sea level around year zero would be a metre or more below present — something that seems to be ruled out by the classical Roman fish tanks result, and so large that, if true, we would probably know about it. Also, we see no similar “knee” in any of the other sea level curves in Figure 3 that appear good enough to make such a statement — Massachusetts, Louisiana.

    There are alternatives to our proposition of a -0.2K temperature offset — which seems a bit large against the apparent quality of this reconstruction, even this far back — but they would be speculative at this point.

  40. 40
    Martin Vermeer says:

    KR #38: there would be an obvious link in the main text, and it’s not there. We’re working on it ;-)

  41. 41
    Gilles says:

    #23 Ray : I don’t think Stefan’s answer really contradicts my remarks ; that the conclusion that “20th-Century sea-level rise on the U.S. Atlantic coast is faster than at any time in the past two millennia.” has nothing to do with anthropogenic influence since it would already have been deduced from pre-1950 data, for instance. So if it proves anything, it’s only that natural variability CAN produce such variations in the SLR. And that the comparison with a model based on temperatures relies on inaccurate temperature reconstructions, and fails to reproduce the data at some point.

  42. 42
    Ray Ladbury says:

    Gilles, thank you for proving my point. Yes, if you take any single datum, you might be able to explain it by natural variability. If you take all the data…not so much. It is why you focus only on cherrypicked results interpreted in isolation of physics and the rest of the evidence.

  43. 43
    Hank Roberts says:

    >fred berple … British Admiralty

    Previously:
    “Accumulated sealevel rise since Bligh is less than 30cm
    – wave height on calm day.”
    http://www.realclimate.org/?comments_popup=7897#comment-208046

  44. 44
    JCH says:

    One of the SLR papers that is making the rounds is Unal and Ghil, 1995, which found sea level rose an average of 1.62 mm/yr (+ or- .38) between 1807 and 1988 (averaged over 181 years.)

    Does that paper have a number for just the 19-Century years?

  45. 45
    folbec says:

    out of topic for this tread but interesting :
    “Why volcanism isn’t the source of increasing carbon dioxide emissions”

    http://bigthink.com/ideas/38998

    One year of volcanic activity releases the same CO2 as the following human source:

    A state such as Florida, Michigan or Ohio.
    ~13 times less than land use changes (3.4 gigatons)
    ~11.5 times less than light-duty vehicles (3.0 gigatons)
    ~5.3 times less than concrete production (1.4 gigatons)
    ~2 dozen 1000 MW coal-fired power plants (2% of the world’s coal-fired electrical generation)
    Or, roughly the same CO2 emissions as Pakistan, Kazakhstan, Poland or South Africa.

  46. 46
    Terry says:

    Mike #37. “It is the rate of SLR change that is proportional to the temperature” How can that be from a purely physical point of view. Surely one would normally expect a linear relationship of SLR and temp.

    [Response: Ummm, no. One quantity represents an approximate integral of the other. That this is true empirically is obvious from a comparison of the historical observations. It isn’t that hard to review this sort of stuff on your own. Read for example the referenced papers by Rahmstorf, Rahmstorf and Vermeer, etc. -Mike]

  47. 47
    Noureddine says:

    The big question arises about the maps of the famous seas: the Red Sea southern straight, the Black Sea at the Bosphorus, The mediterranean at Gibraltar. How about the Bering sea? Your findings will shed light on historical migrations issues.

  48. 48
    M says:

    Terry #46: “Surely one would normally expect a linear relationship of SLR and temp.” I think the key point here is the difference between equilibrium and transient: in equilibrium, the rate of change of SLR will, by definition, be zero, and this state can be true at any number of temperatures. However, in the transient the rate of SLR will be roughly proportional to the difference between the current temperature and the temperature at which the current SLR would indeed be in equilibrium…

    (mind you, I’m a little surprised that this relationship holds over a period when there is both decreasing and increasing absolute SLR)

  49. 49
    l. david cooke says:

    Hey All;

    This point may have already been made; however, I think it is important enough to reitterate.

    Though examiniation of old tidal gauges are good, many were painted wood and metal with a life of between 7-15 years. If anyone is truly interested in how much the N. Atlantic region has risen in the last 50-75 years they only need to inspect bridge pillars and concrete seawalls in protected bays, lagoons, marshes, river mouths and marinas scattered up and down the southern US East Coastal region. Going further, if anyone has an interest in surface acidification they can also inspect these same submerged concrete structures for calcium errosion.

    Generally, though the biota may change or the waters in these areas be subject to various changes in pollutants, generally the waterlines are clearly indicated and samples in the top 1/3rd can be compared to a basin control either at the “basin bottom” or via underground cores.

    That there are thousands of potential sites, the potential for robust evidence is overwhelming. This evidence would certainly reduce the breadth of concern over conflicting references.

    Similar to Dr. Mann’s tree ring analysis of the ’70’s (Popular Science ran the initial popular press article, if I recall correctly.) it should be possible to extend the modern era evidence via coastal chalk and limestone deposits. I’ll leave that up to the experts to discuss.

    [Response: Hmmm. During the 70s I was mostly focused on t-ball practice and comic book collecting. I think you must have someone somewhat…well..more senior than me in mind here. -Mike]

    Cheers!
    Dave Cooke

  50. 50
    Septic Matthew says:

    36, Sean: I see code and data is online, and I hope that is an arguement avoided.

    Where?


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