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How much will sea level rise?

Filed under: — group @ 4 September 2008 - (Español) (Italian)

… is the question people have been putting a lot of thought into since the IPCC AR4 report came out. We analysed what was in the report quite carefully at the time and pointed out that the allowance for dynamic ice sheet processes was very uncertain, and actually precluded setting a upper limit on what might be expected. The numbers that appeared in some headlines (up to 59 cm by 2100) did not take that uncertainty into account.

In a more recent paper, our own Stefan Rahmstorf used a simple regression model to suggest that sea level rise (SLR) could reach 0.5 to 1.4 meters above 1990 levels by 2100, but this did not consider individual processes like dynamic ice sheet changes, being only based on how global sea level has been linked to global warming over the past 120 years. As Stefan discussed, any non-linear or threshold behavior of ice sheets could lead to sea level rising faster than this estimate. Thus, otherwise quite conservative voices have been stressing the ‘unknown unknown’ nature of this problem and suggesting that, based on paleo-data (for instance), it was really hard to rule out sea level rises measured in feet, and not in inches. (Note too, the SLR is very much a lagging indicator, and will continue for centuries past the time that atmospheric temperatures have stabilised).

The first paper to really try and assess the future limits on dynamic ice sheet loss appeared in Science this week. Pfeffer et al looked at the exit glaciers for Greenland and West Antarctica and made some back of the envelope calculations of how quickly the ice sheets could dynamically drain.

Good news: they rule out more than 2 meters of sea level coming from Greenland alone in the next century. This is however more than anyone has ever suggested and would be comparable to the amount that disappeared at the Eemian (125,000 years ago) (see this post for more on that).

Bad news: they can’t rule out up to 2 meters in total.

In summary, they estimate that including dynamic ice sheet processes gives projected SLR at 2100 somewhere in the 80 cm to 2 meter range, and suggest that 80 cm should be the ‘default’ value. This is remarkable in a number of ways – first, these are the highest estimates of sea level rise by 2100 that has been published in the literature to date, and secondly, while they don’t take into account the full uncertainty in other aspects of sea level rise considered by IPCC, their numbers are significantly higher in any case. And this week the Dutch ‘Delta Commission‘ published its estimate of sea level rise that the Dutch need to plan for (p111): 55 to 110 cm globally and a bit more for Holland, based on a large number of scientists’ input. [Clarifying update: this is meant to be a “high end estimate”.]

Lest readers think this is no big deal, the estimates for the number of people who would be affected by 1 meter of sea level rise is more than 100 million – mainly in Asia. Of some recent relevance is the fact that the storm surge caused by Gustav in New Orleans was within 1 foot of the top of the levees. Another 3 ft caused by global sea level rise would have put a lot more water into the ‘bowl’.

Thus better estimates of sea level rise from ice sheets remain a high priority for the climate community. More sophisticated models and deeper understanding are coming along and hopefully those results will be out soon.

We were going to leave it at that, but we’ve just seen the initial media coverage where this result is being spun as a downgrading of predictions! (exemplified by this Reuters piece, drawing mainly from the U. Colorado press release). This is completely backwards. We stress that no-one (and we mean no-one) has published an informed estimate of more than 2 meters of sea level rise by 2100. Tellingly, the statement in the paper that suggests otherwise has no reference.

There have certainly been incorrect assertions and headlines implying that 20 ft of sea level by 2100 was expected, but they are mostly based on a confusion of a transient rise with the eventual sea level rise which might take hundreds to thousands of years. And before someone gets up to say Al Gore, we’ll point out preemptively that he made no prediction for 2100 or any other timescale. The nearest thing I can find is Jim Hansen who states that “it [is] almost inconceivable that BAU climate change would not yield a sea level change of the order of meters on the century timescale”. But that is neither a specific prediction for 2100, nor necessarily one that is out of line with the Pfeffer et al’s bounds.

Thus, this media reporting stands as a classic example of how scientists get caught up trying to counter supposed myths but end up perpetuating others, and miss an opportunity to actually educate the public. The problem is not that people think that we will get 6 meters of sea level rise this century, it’s that they don’t think there’ll be anything to speak of. Headlines like that in the Reuters piece (or National Geographic) are therefore doing a fundamental disservice to the public understanding of the problem.

Update: Marc Roberts sends along this cartoon illustrating the problem… (click for full size).

386 Responses to “How much will sea level rise?”

  1. 101

    Rod, I see now that Stefan Rahmstorf’s paper was put on-line. It’s even simpler than I thought it would be. Obviously the problem with the approach outlined by me is, that warming “in the pipeline” is not observable, and using model computations to construct it introduces model uncertainty… then it is a good question if the method is anymore worth applying as compared to direct model simulations. Still it might be a useful consistency check.

  2. 102
    Peter Ellis says:

    The issue with WAIS and for a significant chunk of Greenland is that in extremis, it might not stop at all (until all of that ice is spread out and floating).

    Nope, sorry, still can’t grasp this. You can’t float a significant chunk of Greenland until you get to the point where melting that chunk doesn’t cause any sea level rise. That’s what floating is (modulo fractional changes due to differing density of saline/fresh water, but that’s not what we’re talking about).

    Say we have a shallowish tray a couple of inches deep, with your 1-foot cube of ice sitting in the tray. That corresponds to the Greenland ice sheet, sitting in a depression in the bedrock. There are a couple of cracks in the tray rim where ice is oozing out. The tray is sitting in water, which is gradually rising up the sides as the sea level increases.

    As the sea level rises closer to the rim of the tray, I can buy that it might make the streams of ice move faster. I think your point about shear is simply a restatement of my “less friction with the bedrock” in other words – it’s the friction with the ground that causes shear.

    As sea level continues to rise, it will eventually overtop the rim of the tray – i.e. the grounding line of the glacier moves far enough back that it reaches the part of the ice sheet that is below sea level. That will not magically float the huge cube of ice in that tray. You just cannot float the Greenland ice sheet until you have already attained the 7m sea level rise you’d get from melting it down! It doesn’t make physical sense.

    [Response: You are assuming that the ice sheet retains the same shape. It doesn’t. Imagine your ice cube suddenly becoming much thinner – now all of it is floating, and everything that was above sea level (minus 0.1 times the amount below sea level initially) has added to the water being displaced. (Note that we are really talking about WAIS, only a small part of Greenland is drained in such a manner). – gavin]

  3. 103

    …and Rod, I committed a thinko above. It is true that both the temperature “in the pipeline” and the sea level rate “in the pipeline” go asymptotically to zero under constant forcing — but in completely different ways.

    The temperature change “in the pipeline” is about half of the total temperature change, and most of this comes out over decades, only a small fraction over centuries.

    For sea level rise, however, as for the volume warming of the ocean, the amount “in the pipeline” is almost all of it, and it will take many centuries to fully come out. Like warming up the deep ocean, and melting continental ice sheets do.

    What this means is that Stefan Rahmstorf’s approximation (temperature anomaly == imbalance) isn’t really an approximation at all, but pretty realistic: any sea level rise that we start now by elevating global mean temperature will go on “forever”, many many centuries, before levelling out.

  4. 104
    David B. Benson says:

    Ryan T (77) — Here is the peak of the Eemian from Petit et al. analysis of the Vostok ice core (ybp, temperature above present, i.e., 1950 CE)

    128309 2.78
    128357 3.23
    128405 3.16
    128453 3.08
    128501 3.06
    128549 2.71

    so in Antarctica, at least, it was warmer then than now. (Technical note, these dates are now known to be off by about 5000 years.)

  5. 105
    Hank Roberts says:

    For both Gavin and Peter Ellis — I asked some of these same questions about the stability of the very deep ice in an older thread over at William’s site earlier, starting with this post here:

    In subsequent posts there I’ve quoted and cited what I did find when I tried my usual amateur frenzied reading on the subject. There’s quite a bit, and a lot of change in the literature. Go forward from anything I cited, it’ll be outdated already.

  6. 106
    pete best says:

    OFF TOPIC – BBC 3 part series on the history of Climate Change. Sunday GMT 9:00 pm

    I am already alarmed by the title as it appears to be telling the story of global story as if scientists (all of them) got it wrong!!


  7. 107
    Peter Ellis says:

    You are assuming that the ice sheet retains the same shape. It doesn’t. Imagine your ice cube suddenly becoming much thinner

    Hmm, but it can’t become much thinner without also becoming much wider, so yet again I think we’re approaching the same point from a different angle – which is that the sea level rise and grounding effects are about speed of outflow rather than any kind on magical non-physical “prying up” of a grounded chunk of ice.

    [Response: It would obviously get wider too, but yes, there is no magic here. – gavin]

  8. 108
  9. 109

    Good post, minor quibble: I’d call dynamic sea level rise, a known issue of unknown quantity, a “known unknown”, not an “unknown unknown”. Something that completely surprises us, say the Antarctic ozone hole prior to its discovery, would fall in the latter category.

    Eli Rabett points out a few other known unknowns:

    Their potential effect ranges from neutral to negative – none of them have the potential to make things better than expected.

  10. 110
    Rich Hilt says:

    Why do all the fancy calculations. Where were the sea levels when Greenland was green (Middle Ages)? London was around. There were people writing in China/Japan. What does the historic records say.

    [Response: Greenlannd hasn’t been green in 3 million years or so (maybe it was greener 125K or 400K BP). Sea level was lower then. – gavin]

  11. 111
    Roger Pielke, Jr. says:

    Response to #78:

    You can see my discussion of this issue here:

    Stefan is correct that the IPCC 1990 did not include important effects in its predictions (as my piece mentions). That did not stop the IPCC from calling its 1990 conditional predictions “best estimates”. Stefan is also right when he says that projections are not predictions, but this is true only when looking to the future (i.e., when the relevant emissions pathway is unknown). Looking backwards it is possible to accurately identify the emissions pathway and thus select among the conditional predictions the one that best matches. If you don’t like the word prediction then use “best estimate” as used by the IPCC. The point still holds.

    As far as discounting those predictions from models that that fail to incorporate all known forcings, then of course nothing would qualify as a prediction, since no model is comprehensive. ;-)

    The data shows that sea level rise is difficult to predict over the short term (since 1990), for the longer term, time will tell. But the lack of stability in published sea level predictions suggest that this is a really hard problem. Pointing that out based on experience to date would seem perfectly fair.

  12. 112
    catman306 says:

    “I’d note that coastal areas may become economically uninhabitable long before they become, literally, physically uninhabitable. I am thinking for example of the American southeast — Florida and the Gulf coast region — which will be impacted not only by rising sea levels but also by more large, powerful hurricanes. Well before it becomes physically impossible for people to inhabit these places, it can become economically impossible to inhabit them, given the enormous cost of not only holding back or retreating from the gradually rising sea level, but of repeatedly and frequently rebuilding in the aftermath of huge, powerful, massively destructive storms.” Secular Animist

    Here’s where insurance companies and the governments can make hurricane and flood insurance near the rising coastlines illegal or prohibitively expensive. The super rich can have their 1000 acre coastal estate and insure it themselves. Everyone else, easily replaced summer shacks or less.

  13. 113

    Good news: they rule out more than 2 meters of sea level coming from *Greenland alone* in the next century.

    Can someone tell me what we’re missing here?

    Until that is resolved, I’m still going with Hansen’s speculation.

    We are in the midst of the fastest global warming ever recorded in the paleo record short of an asteroid impact. *Greenland Alone*?

    Surely somebody has got an agenda here. Maybe somebody who has actually read the paper can tell us why any such *back of the envelope* calculations are meaningless here, and why we should wait for Cryosat II and some radar measurements to straighten this out.

  14. 114
    Hank Roberts says:

    Peter, do look at the evidence, rather than relying on logic. You could start with the papers I quoted at the link above, or try this one word search, and read some of the literature:
    You’re saying you don’t believe what has been described is possible. Consider it.

  15. 115
    Rod B says:

    Martin, thanks for the input. I think I have it now. The units [mm/yr/degree] are still misleading/inappropriate. Though I have no better suggestion; mm/degree would be just as misleading/inappropriate. How ’bout “mm/degree/some-time” :-?

  16. 116
    Andrew says:

    Re: 110 and others
    Much of the low lying Gulf of Mexico coast in Mexico, Texas, Louisiana and Florida is lightly inhabitated and consists of very large conservation areas. This includes the Laguna Madre (Mexico and Texas), Padre Island (longest barrier island in the world), half a million acres of wildlife reserves in Texas and much larger wetland areas in Louisiana and Florida and the Florida Everglades. It isn’t rich people’s homes that will be lost in the initial stage of AGW driven sea level rise, but rather some of the continent’s most important natural lands. There’ll be a lot fewer birds in NA unless something drastic is done; now.

  17. 117
    Denny Gee says:

    Louisiana is particularly vulnerable. Not only are the oceans rising, but the land is sinking! Oil is being pulled out from underneath, and as a consequence the ground level goes down.

  18. 118
    james says:

    Could somebody please provide me with proof that global sea water is not rising at an average of 3mm per year during the last 1000 years and is currently rising at the same rate. Thanks

    [Response: Recent sea level rise. – gavin]

  19. 119
    Hank Roberts says:

    New in Geophysical Research Letters:

    Wright, A. P., M. J. Siegert, A. M. Le Brocq, and D. B. Gore (2008),
    High sensitivity of subglacial hydrological pathways in Antarctica to small ice-sheet changes, Geophys. Res. Lett., 35, L17504, 6 September 2008

    Walker, R. T., T. K. Dupont, B. R. Parizek, and R. B. Alley (2008),
    Effects of basal-melting distribution on the retreat of ice-shelf grounding lines, Geophys. Res. Lett., 35, L17503, 5 September 2008

    Geirsdóttir, Á., G. H. Miller, N. J. Wattrus, H. Björnsson, and K. Thors (2008),
    Stabilization of glaciers terminating in closed water bodies: Evidence and broader implications, Geophys. Res. Lett., 35, L17502, 3 September 2008

    Haas, C., A. Pfaffling, S. Hendricks, L. Rabenstein, J. Etienne, and I. Rigor (2008),
    Reduced ice thickness in Arctic Transpolar Drift favors rapid ice retreat, Geophys. Res. Lett., 35, L17501, 3 September 2008

  20. 120
    Chris Dudley says:

    Gavin in #63,

    The time-scale of an infrastucture investment is important. In the case of the storm drains in NYC, 50 years could make sense. But, the most important decisions we are making right now are the ones that need to extend beyond 2100. A coastal highway could be abandoned but a nuclear plant requires cleanup as part of its decommissioning. Current plans for new coastal plants anticipate 60 year operating lifetimes which presumably would be extended by 30 years just as our 40 year plants are extended by 20 years. So, a plant that starts operating in 2023 will want to close finally in 2113. Allowing 30 years to cool down before decommisioning and a 20 year decommisioning process, that puts our concern about sea level rise out to 2163. So, decisions that are being made today need to be informed about what to expect well beyond 2100. British Energy, when considering the problem, only looked at low estimates of sea level rise, but perhaps, since nuclear power requires the furthest planning horizon, they should be taking a larger safety factor.

  21. 121

    Rod, all units are misleading if you don’t read up on the context. ;-)

    The unit mm/degree (or rather, m/degree) would be appropriate for equilibrium uplift (reached after a very long time).

  22. 122
    Guy says:

    #111 – Roger, your linked article there seems to cover things fairly, and my take on what you are saying there is that “differences between estimates / predictions need to be explained better and we need a lot more research”, which sounds absolutely fine (and of course this new paper is an important step in addressing this acknowledged problem in terms of research). But to this layman’s eyes your blog strikes the opposite tone when you state, re the previous IPCC estimates / predictions, “Rahmstorf et al. interpretation of the results is little more than spin”. I don’t have access to their full paper, but on the basis of your own wider explanation, their abstract seems entirely correct. 1990 is evidently only an partial estimate, not a full prediction, and so therefore cannot be fairly compared like-for-like with the others (you concede this point in the data for 2007, so why not in 1990 as well?) The only meaningingful comparisons with predictions that can be made are those from 1996 and 2001, both of which attempted to include all relevent forcings, and both of which have underestimated observation, the contention of their paper.

    So forgive if I have misunderstood, but… which is it? Spin or valid differences which require explanation? On the one hand you seem to be engaged in a useful attempt to clairfy differences in complex data, on the other you seem to infer that there are bumbling scientists who just keep changing their minds all the time and try to cover it up. And, particularly to the layman, it is sentances like “little more than spin” that leap out and create the impression that there are dark forces at work, which certainly appears not to be true. Perhaps a little more linguistic care is called for?

    [Response: The paper you want to have access to is here. Don’t know what might be spin about it, but then I don’t read Roger’s website, so I wouldn’t know. Stefan

  23. 123
    Nigel Williams says:

    Mike Tabony #23; Thanks Mike, methinks no one wants to talk about the gorilla in the room.

    But Hansen does:

    “More ominous tipping points loom. West Antarctic and Greenland ice sheets are vulnerable to even small additional warming. These two-mile-thick behemoths respond slowly at first, but if disintegration gets well underway it will become unstoppable. Debate among scientists is only about how much sea level would rise by a given date. In my opinion, if emissions follow a business-as-usual scenario, sea level rise of at least two meters is likely this century. Hundreds of millions of people would become refugees. No stable shoreline would be reestablished in any time frame that humanity can conceive.”
    Dr James Hansen: Global Warming Twenty Years Later: Tipping Points Near. 2008.

    Business as Usual?! Oh to be so blessed.

  24. 124
    Geert Jan van Oldenborgh says:

    Note that the Dutch Deltacommittee estimate is a *plausible upper bound*, not a best estimate. The “most plausible” estimates of sea level rise for the Netherlands in 2100 are 35cm to 85cm, based on the same scientific input as used for the 120cm “plausible upper bound” that was published last week. See for a list of differences.

  25. 125

    Andrew W posts:

    Paul Barton Levenson, a link for you:

    “It’s enough to make Miami, Jacksonville, and many other coastal communities around the world uninhabitable, along with much of Florida and Bangladesh.”

    Only if nobody does anything for a hundred years.

    And how much do you think they will have to spend to rescue all those cities and all those areas? It won’t be free. And since there will be more violent weather along coastlines, it won’t stay put very well, either. For instance, I think it’s futile to keep rebuilding New Orleans.

  26. 126
    Mark says:

    Roger #111, what makes you believe that the information left out will make one specific significant change? What makes you believe that there is no other left out information that will not change YOUR appraisal of what will happen.

    If you cannot say, then maybe the only thing that can be done is to assume that these unmodelled activities will add some change either positive or negative to the predictions made without their inclusion.

    We could call them “error estimates”.

  27. 127
    Edouard says:


    I don’t understand what this discussion is all about? Everyone makes mistakes. But the people who wrote this paper didn’t make any! Being from Europe, I know why Mr Rahmstorf could make his statement “our own Stefan Rahmstorf used a simple regression model to suggest that sea level rise (SLR) could reach 0.5 to 1.4 meters above 1990 levels by 2100,”, which is 0.9 meters above IPCC highest probable “estimates” from their last report.

    In european discussions people were told, that they would knowingly desinform people, when they explain, that sea level would only rise by 0.3 to 0.5 meters until 2100. This information had just been released by the IPCC. Why have those people been insulted by people who “discuss” the same way that Gavin does here on realclimate?

    Who is right? The 2000 scientists of the IPCC or Mr Rahmstorf with his “very new and own” estimate?

    I can’t believe what I read again here in this thread! Is climate science the science of insulting people who just reproduce what they have read from climate scientists? Or is climate science the science of never making any mistake in at least 20 years?

    Everyone who reads this thread must think that every kind of science is just a joke. The longer you follow the scientific debate about climate change the more you must respect people like Roger Pielke! Sorry, Mr Gavin, this is not science any more!

    Regards, Edouard

    [Response: I have no idea what point you are making – we did not criticise the Pfeffer et al paper in any respect, and I fail to see where anyone has been insulted. – gavin]

  28. 128
    dhogaza says:

    Stefan is correct that the IPCC 1990 did not include important effects in its predictions (as my piece mentions). That did not stop the IPCC from calling its 1990 conditional predictions “best estimates”

    You say this pejoratively. Why, Roger? (I can guess). If it’s the best science can do at the time – we’re talking eighteen years ago – then indeed, it’s a best estimate, and there’s no reason for you to sneer at their being called that.

  29. 129
    Brian Carter says:

    “Yes. but do the math – to get rid of 1 m of sea level from 70% of the globe is a lot of water. You would have to flood a lot more than Death Valley and the Dead Sea. Something more like Montana/Ontario and Quebec to recreate the paleo-Lake Agassiz for instance. – gavin]”

    There is a surprisingly big area of the continents that are below sea level, for instance the Qattara Depression in Egypt, and vast areas in central Asia. I estimate very roughly that these areas could cope with about a metre of sea level rise. They also supply an opportunity. They could possibly be filled by pipelines operating as syphons, and the flow could be used to generate power. The flow would not cease as the depressions fill as there would be evaporation to replace. Furthermore the potential reservoir size will increase somewhat as the weight of water depresses the bed of the reservoirs, though I haven’t attempted to calculate that. Finally, the new inland seas could supply new food sources.

  30. 130
    Ecotretas says:

    The Sea Level has been higher in Portugal many centuries ago. It is well documented. Unfortunately, countries like the USA, which have much less history, tend to lack knowledge in this domain. As my history teacher told me long ago, you have to know some history so you can understand the present and predict the future!

    [Response: I’m going to guess that this was either due to isostatic rebound from the removal of the European ice sheets after the last ice age or due to tectonic uplift. It was not a global signal. – gavin]

  31. 131
    Ricki says:

    Further to 87 on Ref. 63, 73, 120…design levels for infrastructure

    120 raises the issue for high risk structures such as nuclear power stations. Normally these are designed for increased actions (eg, often a maximum credible earthquake is used for design). Hence, it would be unaceptable to place such a structure in a floodable area.

    Haveing read the Nature article (I cannot access the paper) I would revise my suggestions (87) to

    For major infrastructure design or planning purposes
    2050: 1.5m x factor of 2 = 3m
    2100: 4m x factor of 3 = 12m

    Critical infrastructure such as power plants or major highways should just not be located near the ocean unless it is acceptable to abandon them to the sea (if that eventuates).

    Again revision would be required depending on the emergence of new information (tipping points, etc) and the degree of international effort to reduce emissions.

    Am I being to pesimistic in the numbers? Comments please?

  32. 132

    There’s a very small piece about (what appears to be) the Pfeffer et al Science paper in the Dutch newspaper “Volkskrant” (in the science section; it’s a quality newspaper and (I believe) regarded as slightly left leaning). It also mentions that sealevel will “only” rise by 2 meters at most by 2100, ie it refers to the estimate as a lowering compared to previous estimates.

    On another note, and not surprisingly, the Dutch deltacommission is being attacked in Dutch media for using upper limits as their estimates of sealevel rise, even though the KNMI has been heavily involved and points out that the difference is explicable. In my reading, it has most to do with the higher estimate of the mechanically enhanced ice loss, to reflect the most current measurements and insights.

  33. 133
    CobblyWorlds says:

    Pete Best #106
    Earth: The Climate Wars BBC2(UK) Sun 7 Sep, 9:00 pm – 10:00 pm

    I have posted a reply on “Are Geologists Different” Thread (around post 277), as it’s more relevant there. There’s also a link that should ease online viewing after the broadcast.

    #127, dhogaza,

  34. 134
    Mike Tabony says:

    Mr. Levenson (#125), I agree completely concerning the rebuilding of New Orleans. Author Michael Tidwell “Bayou Farewell” said it best after Katrina in 2005. At a news conference his summation (as best as I can remember it) was “If you’re not going to rebuild the marshlands surrounding the city, don’t spend a dollar on rebuilding New Orleans, and if you’re not going to solve the global warming problem, don’t spend a dollar trying to rebuild the marshlands.”

    This must extend much further than New Orleans however. The amount of wealth (capital) that will be destroyed by the rising seas worldwide is a number that no one has even come close to figuring to my knowledge. We can guess that the wealth destroyed by a two meter rise will be greater than that destroyed in all the wars in human history by a significant multiple. How will that wealth be replaced in a world struggling with a warming climate, rapidly shrinking glacial water supply, less readily available energy resources, changing agricultural patterns, encroaching coastlines, oceanic acidification, desertification, and the loss of biodiversity that supports our necessary ecosystems? In my uninformed opinion it cannot and will not be replaced in the foreseeable future, centuries, perhaps millenia.

    We should be planning way beyond the 50 years planning regime mentioned in an earlier post, as even a coastal highway takes material resources and, at present, fossil fuel energy and waste products to construct. We will not get close to controlling our future until our mindset accepts these things and we begin to act accordingly. And as Hansen is so right to point out, at a certain point (which may be closer than we’ll presently acknowledge), nature will take us out of the decision with positive environmental feedback.

    New Orleans and those other coastal cities that cannot withstand a 2 meter sea level rise (probably already in the pipeline during the next 150-200 years) should be slowly but steadily abandoned, starting today. Do we build or reconstruct a city for 50 years? Hopefully, much of the material resources (wealth, capital) in a condemned city’s structures can be recycled but that has to be planned too, before it is literally too late. After a Katrina, much, if not most, of the material is unusable rubbish.

    Finally, if we are not going to immediately and significantly address global warming, we should be starting to look at those cities and areas that cannot withstand a 4 meter sea level rise.

  35. 135
    Mike Tabony says:

    Nigel, (#’s 22 & 123), your gorilla is just about too big to get our minds around. The question about what will stop the melting before 80 meters of SLR still stands as far as I know. What is the present scientific thought on this? Has anyone determined a sea level rise per temperature degree increase ratio that makes any sense and explains why it stops at any point before 80 meters?

    This question must at some point be answered if humanity and a fair amount of the rest of the biosphere is going to have any long-term future. So we’re thinking several millenia here, that’s still a blink in geologic time. An effort to figure this out is not a lot to ask from the guilty party, before we finish committing the crime of knowingly carbonizing the atmosphere to the point of runaway global warming.

  36. 136
    Roger Pielke, Jr. says:

    Guy (#122)-

    Thanks, the reason for my calling Rahmstorf et al. “spin” was this following statement that they made in their paper:

    “Previous projections, as summarized by IPCC, have not exaggerated but may in some respects even have underestimated the change, in particular for sea level.”

    This is untrue. The 1990 IPCC certainly made “projections” that have “exaggerated” sea level rise to date (I would use a different phrase that that used by Rahmstorf et al., probably “overstated to date”). To ignore the 1990 IPCC sea level rise projections (even if to explain them away as now irrelevant) when discussing “previous projections” is misleading at best.

    [Response: Our paper compares the data to the scenarios of the IPCC Third Assessment Report, and nothing else. This is abundantly clear when you read it, and this is what we are discussing in the sentence that you call “untrue” above. We did not repeat the words “IPCC Third Assessment Report” in every single sentence, but refer in short to “IPCC”, because we were not expecting people to maliciously pick sentences out of their context and deliberately misinterpret them so that they can call them “untrue”. Stefan]

    Mark (#126)- Stefan is the one who is worried about incomplete models, so your question is best put to him. My work simply compared “best estimates” with observations. All models are of course incomplete.

    dhogaza (#127)- You are reading in too much — there is no pejorative intent. The IPCC’s 1990 “best estimates” are the IPCC’s “best estimates” from that time, and thus the basis for comparing with experience. All we can do is see how things have turned out. I am sure that 20 years from now we’ll look back on the AR4 as similarly simplistic. That is how science works.

  37. 137
    pete best says:

    Question off topic:

    Is Prof/Dr Ruddiman right about his GHG theory or humans preventing an ice age 10,000 – 5,000 years ago ?

    I thought this was not currently established scientific fact so why is it making the news ?

  38. 138
    Rod B says:

    Martin (121), agreed

  39. 139
    David B. Benson says:

    Mike Tabony (131) — New Orleans and other gulf coast communities are already being slowly and steadily abandoned.

    pete best (133) — The Holocene climatic optimum in central Greenland was about 7800 ybp. From orbital forcing and the behavior of the previous interglacials, one wouldd expect, on average, cooling since about then. Indeed, that has occurred, up to the present, defined as 1950 CE. W.F. Ruddiman is suggesting that without agriculture, it would be quite a bit cooler in the northrn hemisphere, but far from an ‘ice age’.

    As I understand it, the next attempt at a stade (massive ice sheets) is due to start before another 20,000 years of, baring AGW, gradual cooling. For more information on this see David Archer’s “The Long Thaw” or also his papers linakable from his publications web page.

  40. 140
    FhnuZoag says:

    Why do the researchers set 80cm – the low end of their confidence interval – as a ‘default’ value? Surely it would be meaningful for them to give as a point estimate something like a maximum likelihood estimate? And for policymakers, surely the more important value is not the minimum the sea level could rise but rather the maximum that it is reasonably possible for the sea to reach?

  41. 141
    pete best says:

    Re #140, I believe that the IPCC would not speculate on unknown issues such as ice sheet melt due to the unknown nature of the threat posed, ie there is not enough good physics of ice sheet disintegration. Instead they concentrated on thermal expansion of the oceans and run off from rivers and possibly glaciers.

  42. 142
    David B. Benson says:

    Probably do not need to be overly concerned about barrier islands: “Nature’s Effects on Barrier Islands”:

    Estuaries may well suffer due to upstream human developments which won’t be so far upstream anymore:

    [Capcha agrees, stating “shacks SS”]

  43. 143
    Hank Roberts says:

    > This is untrue. The 1990 IPCC certainly made “projections” that have “exaggerated” sea level rise to date

    Roger, are you saying the more recent IPCC reports are more accurate than the one from 1990?
    Is this a problem?

    The word “projections” is certainly a political/bloggish hot button term.


    You have to page down quite a while to get past people quoting you and find

    Journal of Climate Volume 15, Issue 20 (October 2002) pp. 2945–2952
    Reasons for Larger Warming Projections in the IPCC Third Assessment Report
    T. M. L. Wigley

    “… In terms of total radiative forcing over 1990–2100, if TAR gas cycle and forcing science is used in both cases to isolate the effect of emissions differences, the net effect of these differences is that B1-IMA forcing exceeds IS92c by around 0.5 W m−2 while A1C-MI exceeds IS92e by more than 2 W m−2 (Table 2 ). Changes in aerosol forcing are the main reason for these differences in total forcing….”

  44. 144
    Nigel Williams says:

    Hi Mike #135! Kinda like we’re having a private conversation here.. how are the kids?!

    Funny that in a thread with the title How Much Will Sea Levels Rise the entire discussion stops at the year 2100. Is that simply because most of the participants in the dialogue believe (hope!) they will be dead of old age by then?

    To my mind (and the mind of every movie gangster) nothing focuses the mind more than the impending death of your grand children – of your own creations. Maybe that’s why the gorilla is so hard to deal with.

    It must be (hey it IS) dispiriting for anybody involved in infrastructure (like for me involved in planning transport systems) to know that when my grand children look at most of my work they will say: “Why did he do this? He KNEW the sea was going to inundate this work! Why didn’t he build it someplace safe – like at 100 metres above sea level where we wouldn’t have had to re-build it?”

  45. 145
    Hank Roberts says:

    oh, wait, wrong Pielke:
    Climate Science: Roger Pielke Sr. …
    May 21, 2008 … Thus the value of global warming of the last 4 years fails to agree with the IPCC projections

  46. 146
    Ricki says:

    There are a number of reasons to focus on 2050 and 2100. First, these are basically the 50 and 100 year life times for structures. These numbers are used in many building codes and standards for the expected life of ‘normal’ structures and ‘important’ structures. Policy makers will use these for convenience.

    Also, most of the projections relate to 2100 and it is expected that the projections will change over time as we gather more information. Thus we need an agreed time point to use as a measure.

    I must agree though that the projected changes after that time have to be set out in the light of taking action to stop them.

    If we don’t recognize the danger we cannot respond to it.

    I will add to my last by including:

    2200: 8m? x 4 = 32m ???

    (Factor of 4 very difficult to settle on as there is so much uncertainty — what is the actual rate, are there tipping points, what can our race do to stop it or modify it once started, etc… should it be 5… or 6?)

    This looks surprisingly like an approach towards the magical 80m doesn’t it! But remember, this is a risk based strategy for planning and would be used to locate difficult to replace infrastructure from a planning point of view.

  47. 147
    Nagraj Adve says:

    In various articles, for instance, Hansen, James et al. 2007. “Climate Change and Trace Gases”, Philosophical Transactions of the Royal Society 365: 1925-54, and Hansen, James. 2007. “Climate Catastrophe”, New Scientist, 28 July: 30-34, James Hansen has been writing about the rise this century as being much larger. He even mentions it could be the better part of 5 metres by 2100.
    I’m intrigued to know what you think of his rather grim forecast.
    Nagraj Adve

  48. 148
    Charles Sifers says:

    Has anyone actually done any real research into the science here? I didn’t think so. According to the results of the Maldives Sea Level Project (,
    there has been no sea level change in the last 30 years.

    This is real data, not some fantasy based on a model.

    Science is about repeatable data, not fantasy. [edit]

    [Response: Morner is, let us say, somewhat on hiis own on this. Recent global sea level rise is incontrovertible. – gavin]

  49. 149

    Hassling yourself over engineering a city for 1 meter versus 2 meters of sea level rise indicates that you are making a fundamental mistake. You shouldn’t have built a city that close to the water in the first place. In the present place, you should be moving the city, except for the harbor itself, to a new location at least 100 meters above sea level. You should be reversing the past culture of building in the valleys and leaving the hilltops natural. It is OK to cultivate the valleys, but the cities should be on the hilltops. Ancient history, when there were so few people and they had so little technology that we built a cabin next to the farm, no longer applies. My house is 600 feet above sea level in the heartland. I suggest you follow my example, or build new cities in the badlands at higher altitude.
    There is no reason to put a nuclear power plant anywhere near sea level. De commissioning is not as difficult as somebody above suggests.
    Reference: “The Long Summer” by Brian Fagan and “Collapse, How Societies Choose to Fail or Succeed” by Jared Diamond and Al Gore’s speech at the Democratic National Convention last month. Sea level rise is all moot anyway if civilization collapses in 30 years. Sea level rise will be only one aggravating factor in the collapse. Famine is the usual cause of the collapse of civilization. Only 1 person in 10,000 will survive, making planning for cities in 2050 pointless.

  50. 150
    Guy says:

    #116 Roger – I’m still struggling to see what your problem is. You say in your own article why comparing the 1990 figures is not like-for-like with the other years. So why include them in the Rahmstorf et al paper? To include them would have been misleading, surely, not the other way around? (and a layman’s shot, the reason why the paper made it happily through peer review.)

    It seems that you agree with all the underlying points here, but still characterise the Rahmstorf paper as “spin”. I can well see that there is a case to be made that there should have been a reference to why 1990 was omitted, but – and this is the critical point – since those reasons are robust (and you agree with them it seems), it is surely unfair to call the paper “spin”. The authors do not attempt a feeble / inaccurate explanation, which would indeed have been spin, they simply omitted essentially irrelevant data, which is basic science.

    I must confesss, I can well understand why Stefan does not read your blog when you throw allegations like “spin” around where it seems quite unfounded. You agree that anthropogenic climate change is real and its effects potentially extremely serious – surely it is beholden on you to not engage in this kind of misinformation in communication? By all means call for clarification and more research, but don’t stoop to unreasonable allegations – we can all ill-afford it.