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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. 1
    sidd says:

    Thanx for the pointer to an excellent paper. I have heard about Niagra Falls eroding many feet in a single day, long ago. Is it possible that glaciers can enlarge their exits to the ocean through erosion ?

  2. 2
    Chris Colose says:

    Good (scary) post.

    I am wondering where I might find best estimates for how much a shifting pole-to-equator precipitation gradient would offset sea level rise from thermal expansion and glacier loss over the 21st century? Obviously we’re already including net growth in the interior of ice sheets, but I can’t imagine that the ratio of melt to growth remains constant with time.

  3. 3
    Nigel Williams says:

    We lay-folk cannot access the full text, but firstly how do the predictions take into account BAU and more importantly sensible recognition of BAL – Business as LIKELY?

    We are not going to get the inertia of combined Asia, India, Europe, UK, USA, Arabia, Africa and Australia to stop commissioning a 500MW coal fired power plant every three days for the foreseeable future. Unless a realistic BAL is recognised in the expected forcing then a modest 0.8m doesn’t tell us much.

    Secondly; Does the paper unambiguously declare the longer term outcome? Many people assume when scientists say that the sea is going to rise 0.8m by 2100 that that is the end of the matter, but of course its not. Unless we get CO2 back below 275ppm the ice is going to continue to melt and we are going to get +80 metres of rise, and most of the good infrastructure works in coastal cities around the world are merely serving to enhance the dive experience for future tour-boat operators.

    [Response: 80 meters is all ice gone, including East Antarctica – which would take us back to the Eocene. I think we can rule that out for the time being! – gavin]

  4. 4

    That doesn’t mean I’m going to buy property only 1 meter above sea level. Illinois is close enough to sea level.

  5. 5
    Danny Bloom says:

    How much will sea levels rise? From my reading and guesstimates,
    I would say 70 feet by the year 2500 A.D….but that is still a long way away,
    so no need to panic. Then again, it could happen much quicker if things go haywire in a way that we cannot predict now. I would be worried, very worried. Most people in the world just
    want to keep driving their cars to work, though, and go on living as if there
    is no tomorrow. You know what? Maybe there will be no tomorrow, if we keep this up. But give it another 500 years. No need to panic now. Ask Jimmy Lovelock.

  6. 6
    Nigel Williams says:

    .. well nothing to panic about until you start thinking about what is involved shifting half the worlds population, food production and industrial base up above the 80 metre line, at the same time as our primary energy source (oil) needed to accomplish this is past peak and never to be seen again. No worries! Yeah rite!

    [Response: 80 centimeters (not 80 meters), though that is already large enough! – gavin]

  7. 7
    Ellen Thomas says:

    For those of you who do not read Dutch: The Deltacommissie gives indeed 55-110 cm SLR for 2100 global, and the bit more for Holland (executive summary, p. 10, 2nd paragraph) is 0.65-1.30 m relative sea level rise (expected in Holland) by 2100, 2-4m by 2200.

  8. 8
    Eli Rabett says:

    Does the 80 cm include the volume expansion of sea water at higher T?

    [Response: Yes. 30 cm worth which is roughly what the models suggest for A1B. – gavin]

  9. 9

    From the draft of an article: ” Saying “Americans will never give up keeping their houses too cold in the summertime” sounds silly next to “Lots of children and older people will get sick and some will die if we don’t stop Global Warming.” ”

    Thanks to RealClimate for helping the rest of us understand climate, but doesn’t it make sense to be “alarmist” when alarming things keep happening “ahead of schedule?” Things icy are a good example, from arctic icecap extent to Greenland and Antarctic glacier dynamics.

    I think it is time to say that you have to be a bit alarmist to be a real conservative… as in, preserve the good that we’ve got, before it is too late.

    Or maybe I’m just cranky because I can’t go home to New Orleans for a few more days.

  10. 10
    Andrew says:

    One meter of sea level rise by 2100 would be catastrophic from my perspective on the upper Texas and Louisiana coasts. These areas provide the world with an “experiment” in sea level rise and what it means in terms of real world impacts. Subsidence from ground water withdrawal and oil and gas extraction have caused regional subsidence along these coasts ranging from 1′ to 15′. The USGS has a great publication discussing ground water extraction and subsidence. Robert Morton and others have documented the oil and gas induced subsidence and its affect on coastal wetlands and beaches in the Journal of Coastal Research and other publications. The Conrad Blucher Institute (on line) has developed apparent sea level rise rates for the Texas and SW Louisiana coasts (this accounts for both subsidence and eustatic sea level changes). Much of the area is already experiencing rise rates on the order of a meter or more per century due to the ongoing production of oil, gas and associated water causing collapse and consolidation of the producing rock strata.

    The bottom line from this experiment is that simply drawing a new coastline along a topographic contour is overly simplistic and greatly underestimates the damage to human infrastructure and the loss of coastal environments. Just look at the issue of wetland loss and increased storm damage or fishery losses in SW Louisiana. Thousands of acres of rice fields have been abandoned because they no longer have the needed elevation and slope to drain. Beaches and dunes are lost entirely. Wildlife refuges and Parks and being eroded away and converted to open water. Cities once protected by tens of miles of wetlands are now near open water and are subject to hurricane storm surge damage. The NW Gulf of Mexico coast is in crisis and this is before eustatic rise really kicks in.

    The overall effect from the ancillary coastal changes is to take the damage estimates and number of people affected by a future sea level rise scenario and multiply it by ten.

  11. 11
    sidd says:

    I was led through a reference from the paper by Pfeffer et al. to Rignot et al., Nat. Geosci.,v1, p106, specifically Fig.1, which shows mass imbalances across Antarctica. I note that the large imbalances in Pine Island and Thwaites glacier lead directly to the Byrd subpolar basin, and the small mass loss in Tottenham points to a similar, but shallower basin, as may be seen from

    A point of interest in Pfeffer et al., p1342
    “The aggregate cross sectional gate area of PIG and Thwaites is ca. 120Km^2”
    and refers to a private communication by Rignot. However I note that Rignot, in Phil. Trans. Roy. Soc. A, 2006, v364, pp1637-1655, notes that Thwaites is widenning and may double in width. As these (PIG,THW,TOT) glaciers retreat toward deeper basins inland, is it not the case that they would all widen?
    If so, would not the gate area increase ?

  12. 12
    Hank Roberts says:

    Odd — the words “… Previous projections of 20 feet or more of sea level rise by the end of the century …” appear twice — attributed both to a phone call with the author by the science news writer, and in direct quotes in the official press release. Is there anything like that claim in the published paper? Else this is odd spin.

    [Response: Yes. but unreferenced. – gavin]

  13. 13
    Ark says:

    In my opinion it is clearly the press release from the University of Colorado that is either immensely stupid or deliberately misleading, aimed at defusing the study’s results. It starts like this:

    Global Sea-Rise Levels By 2100 May Be Lower Than Some Predict, Says New CU-Boulder Study

    September 4, 2008

    While the disintegrating Columbia Glacier in Alaska is adding to ocean levels this century, the total global sea rise by 2100 may be lower than some are anticipating, according to a new University of Colorado at Boulder study. Photo by Tad Pfeffer/University of Colorado

    Despite projections by some scientists of global seas rising by 20 feet or more by the end of this century as a result of warming, a new University of Colorado at Boulder study concludes that global sea rise of much more than 6 feet is a near physical impossibility.

  14. 14

    Tipping Points considered? The initial rises would flood and melt permafrost areas releasing Co2 and Methane. Icky, icky, mess.

  15. 15
    Hugh says:

    Is this what you were looking for Gavin?

    Under BAU forcing in the 21st century, the sea level rise surely will be dominated by a third term: (3) ice sheet disintegration. This third term was small until the past few years, but it is has at least doubled in the past decade and is now close to 1 mm/year, based on the gravity satellite measurements discussed above. As a quantitative example, let us say that the ice sheet contribution is 1 cm for the decade 2005–15 and that it doubles each decade until the West Antarctic ice sheet is largely depleted. That time constant yields a sea level rise of the order of 5 m this century. Of course I cannot prove that my choice of a ten-year doubling time for nonlinear response is accurate, but I am confident that it provides a far better estimate than a linear response for the ice sheet component of sea level rise under BAU forcing.

    Hansen, J. (2007) Scientific reticence and sea level rise Environmental Research Letters 2 p.4

    [Response: That’s just a thought experiment for contrast to a linear response, not an informed projection. The quote from later on in that paper is cited above where he gives his expectation. – gavin]

  16. 16
    Vincent van der Goes says:

    In addition to the comment by Ellen Thomas #7:

    In Holland (at least the western part of it), most of the terrain is a mix of sand and peat bog. Since those lands have been transformed into polders, they have slowly been going down. The reason is that the peat bog dries and shrinks. This worsens our situation, and I fear Holland will finally lose to the battle with sea somewhere in the coming centuries. Long before that, silt creeping into the land and the growing risk of floods are going to be huge problems.

    However, the situation in Bangladesh is much worse, the poor people there cannot afford a multi-billion delta plan like Holland.

  17. 17
    Aaron Lewis says:

    Just now, it is raining in Greenland. Rain advects heat into the ice, even into the lower layers of ice that support huge weights of ice. As ice warms, it weakens. Warmed foundation ice is not strong enough to support the ice above it. At some point the foundation begins to collapse and the result is a ice/water debris flow with very high initial acceleration. The proper model is the Missoula floods. Such floods are only constrained by the volumes of water and ice in the system. Such water and ice floods can carve any outlets that they need. Look at the potholes in Washington State. Big holes carved into basalt by water flowing over it. The basalt was not even blocking the flow, and the water still carved big holes into it.

    There is a lot of ice in the Greenland system that only needs a lot of water to flush it into the ocean. With a warm North Atlantic and a seasonably open Arctic Ocean, the amount of rain on Greenland could be considerable even by British Columbian standards. Greenland could collect more water than the old Clark River watershed, and thereby move more ice to the ocean.

    Then, this article only addresses sea level rise from Greenland. If it is warm enough for Greenland to melt a bit, it is likely also warm enough for Antarctica to melt a similar amount. And, it is likely that permafrost will melt. That give us another meter of sea level rise, for a total of 5 meters of sea level rise. What would 5 meters of sea level rise do to the stability of the WAIS? Would that unpin any of it and set it sliding into the water?

    My best guess based on guesses about heat transfer by rain in Greenland is that we will see 3 meters of sea level rise in the next 30 years, and another 3 meters in the following decade, for a total of 6 meters in 40 years. Of course that is outrageous. However, 30 years ago, I would have said that there was not even a 1 in a million chance that it would be raining in Greenland on 9/4/2008. In the context of an ice-only system in Greenland, my estimate is silly. However, with open water on both sides resulting in large transfers of latent heat, I think the possibility of water/ice slurries must be considered.

    Before you jump up and tell me I am crazy, get out your science stuff; and go defrost a few freezers, making careful observations of the ice and melt water. Does it melt or does it fall out in chunks? What happens when a glacier gets to the ocean? It calves. The point is, ice near its melting point tends to fracture – suddenly – leaving two chunks of ice separated by a film of water. Such ice/water slurries tend to move downhill easily. For example, pieces of ice from the freezer tend to bounce out of the freezer onto the floor. How far does it fall, and how far from the freezer does it end up? And, In the case of Greenland, after it has been rained on for 20 years, we are talking about a lot of ice near its melting point.

    Who would like to step forth and tell me why it is likely to stop raining in Greenland?

    [Response: Rain on the flanks is not that uncommon, but enough rain on the bulk of the ice sheet to affect the surface mass balance as much as you suggest is not on. The ice sheet is 3km high, which given the lapse rate is ~30 C below the temperature at sea level – it simply can’t support large amounts of liquid water. As I mistakenly said to Mike below, going beyond the science is not helpful. – gavin]

  18. 18
    Slioch says:

    As far as human adaptation is concerned, it seems to me that a steady sea level rise of say one metre per century for five centuries would be more difficult to cope with than a one off rise of five metres in one century.

    At least in the latter case humanity would have some line in the sand above which to start trying to rebuild coastal cities and ports. A constantly shifting sea level, in whichever direction would be very difficult to cope with.

  19. 19
    Thomas says:

    H. von Storch does not fully agree with Rahmstorf. Is there any reply yet to that?

  20. 20
    JK says:

    I think it’s fair to say that in the paper you quote Hansen has gone a bit further than you suggest:

    ‘Under BAU forcing in the 21st century, the sea level rise surely will be dominated by a third term: (3) ice sheet disintegration. This third term was small until the past few years, but it is has at least doubled in the past decade and is now close to 1 mm/year, based on the gravity satellite measurements discussed above. As a quantitative example, let us say that the ice sheet contribution is 1 cm for the decade 2005–15 and that it doubles each decade until the West Antarctic ice sheet is largely depleted. That time constant yields a sea level rise of the order of 5 m this century. Of course I cannot prove that my choice of a ten-year doubling time for nonlinear response is accurate, but I am confident that it provides a far better estimate than a linear response for the ice sheet component of sea level rise under BAU forcing.’

    I say only a bit further because his estimate is obviously tentative, but the upper estimate of Pffer et al is less than half Hansen’s ballpark figure.

  21. 21
    Ricki says:

    What about the Nature report that it is not a practical situation to have more than 2m rise by 2100. Can you please read the paper and give us a report (us layfolk cannot access it anyway).

    In any case, I agree with Andrew at 10. The impacts will be huge. For example, large areas of many of the highest producing regions are within a couple of metres of sea level. Ganges delta, Nile delta, Southern Vietnam, areas around the China Sea, Florida, Netherlands, etc. look at and you can see what I mean.

    Our grandchildren may not know what it is like to walk on a sandy beach.

  22. 22
    Nigel Williams says:

    Gavin no, I do mean 80 metres. 80 metres is when its all gone, isn’t it. What magik event can occur in a post oil post BAL post 400ppm CO2 world to stop continuous melt from today thru 30cm thru 80cm thru 2m to all-gone-80 metres? Anything you can think of?

  23. 23
    Mike Tabony says:

    If none of the Realclimate guru’s want to correct Aaron Lewis’s errors (#17) maybe they had better start thinking greater sea level rise. My unscientific freezer melting sure sounds like his experience. Don’t get me wrong, a 1 meter (default plus a little more) rise by 2100 is bad enough but 3-5 meters by 2050 is catastrophic by anyone’s judgement.

    Where does Slioch (#18) think the human race will get the energy and material resources to rebuild the world’s ports? After a five meter SLR in one century mankind will be struggling to feed itself until the population adjusts significantly. Can we even consider what that stuggle will be like? It’s not likely to be pretty, that much we do know.

    Finally, will someone answer Nigel Williams (#22). What will stop the melting? That there may be few if any human observers by that time (80 meters) does not render it an uninteresting question.

    All of these questions stem from only one. When will the human race recognize it is rendering the planet uninhabitable with its addiction to fossil fuel? When will people stop turning the key and flipping the switch?

    [Response: But where are you getting the idea that sea level will rise 3 to 5 m by 2050? No-one serious has ever predicted that. The problems that exist with the projections grounded in some physics are serious enough without having to male up even scarier monsters. You might think it helps ‘up the ante’, but it doesn’t – it just allows people who don’t want to think that there is any problem the opportunity to paint all statements as alarmist nonsense. If you decry the abuse of science by the disinformation campaigns, you should try harder to follow the science (uncertainties and all) on these issues as well. – gavin]

    [Updated response: Sorry! I missed the comment above (#17) that you were referring to. So transfer all of the above response to #17. – gavin]

  24. 24

    Re #7 Ellen Thomas,

    The Deltacommissie based its estimates on the work of… Stefan Rahmstorf, thus no wonder that they see a higher rise than the estimates of the IPCC AR4.

    Btw. satellites show a declining ocean level since the end of 2006, partly by La Niña conditions, but still going on now (switch of PDO to negative?). How will that affect future projections?

    [Response: It’s worth pointing out that the list of authors also included Hans von Storch as well as many others. – gavin]

  25. 25

    Re #17 Aaron Lewis:

    When I visited Greenland in 2000, we had several days of rain, including in Ilulisat/Jacobshavn, the endpoint of the largest ice fjord of Greenland, be it in August, not early September.

    According to the temperature trends around Greenland, current temperatures are just reaching the temperatures in the period 1930-1945. Summer temperatures are even slightly below that period. See:

    [Response: But glacier retreat is much further along than in the 1930s (see here for instance) – and that’s the key for sea level rise. – gavin]

  26. 26
    Danny Bloom says:

    Ricki, above, no. 51 post, re: “Our grandchildren may not know what it is like to walk
    on a sandy beach.”

    Powerful thought, Ricki. But I think it will most likely be our grandchildren’s descendants in far distance future who will have that experience, maybe 30 generations hence. We are okay for the next 200 years, I think. But the problems that will come later are arising (no pun intended) now, and we need to tackle them now, if indeed they can be tackled. I think your projection is too, soon, but it is poetic one. And this discussion sometimes need poetry, too. Science and poetry combined. Thanks for that.

    Our greatgreat X 30 grandchildren might also know what it is to live in a Lovelock Retreat. A what, you asked? Google it and see.

  27. 27
    John Wilson says:

    SLR of 80 cm by 2100 above what level? (1990? pre-industrial? 2008?)

    [Response: Present-day. But I’ll check whether it’s specifically 2000 or 2008 – though I doubt the exact year is of much relevance given the uncertainty. – gavin]

  28. 28
    Mike Tabony says:

    In a minor but related question not really suited for the moderators of this blog, has anyone considered the economic affect of the growing belief of significant sea level rise among real estate buyers worldwide?

    Because of my belief in global warming and its effects, you could not sell me a piece of property within 3 meters of sea level. When this becomes a popular view, many trillions of dollars in real estate value are going to disappear. Poof! What are going to be the ramifications of this staggering loss of wealth to a significant percentage of the populace?

  29. 29
    Mike Tabony says:


    I only asked in #23 that you refute #17. I have no interest in “upping the ante”. (The losses we are facing in economic wellbeing and probably personal safety and liberty with the “default SLR” are already way more than I want to bet.) I’m originally from Louisiana and very well may live long enough to see the town I was conceived in be abandoned to the Gulf of Mexico.

    By the way I considered your “Don’t abuse the science” comment a cheap shot. Employ the science and tell me why post #17 is wrong. Please just don’t say “No one else has said that.” Someone is always the first person to say something whether that something is correct or not. Aaron Lewis’ statements are so out of the mainstream discovering “incorrectness” should be easy. Take your time, the floor is yours.

    Thanks for your time and devotion to this educational website.

    [Response: My bad – I missed the earlier comment. Apologies. – gavin]

  30. 30
    Bruce Tabor says:

    A Jim Hansen quote from the text of his recent testimony to congress:
    “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.”
    Page 2 of this document:

    Clearly the issue is not how much sea level will rise but how fast. Another Hansen paper, which I’m having trouble finding the link to at the moment is, “Target Atmospheric CO2: Where Should Humanity Aim?”. While it has been some time since I read it, the paper makes a good case based on paleoclimatinc evidence that a large proportion of the world’s ice is unstable in the long term even at current CO2 levels. Hence Hansen argues for a 350 ppm long term target.

    It is probably the case that most of the GIS and WAIS are unstable at current and projected CO2 levels. If CO2 reaches 450 ppm and remains there for decades, parts of the EAIS will become unstable too. Long term sea level rises of 12 to 25 metres are reasonable under those circumstances. But how fast? If the sea is rising at 0.2 metres or more a decade, coastal urban areas, ports and other infrastructure – not to mention the heavily populated agricultural delta regions of the planet – will have to contantly respond. The challenge for human society will be unprecedented.

  31. 31

    It will be interesting to discuss the assumptions underlying the Dutch projections of >1 m. SLR by 2100. I have seen the draft report, it will be published eventually as an annex to the Deltacommission report. Some info in Dutch on my blog.

  32. 32
    John L. McCormick says:

    RE #10

    Andrew, I looked into the coastal infrastructure in the Houston-Galveston ship channel and inland conditions where subsidence continues despite expensive efforts to reduce mining aquifers.

    Bottom line for me is steady loss of 42 percent of America’s petrochemcial industry.

    And, assuming much of the world’s petrochemical and oil refining capacity is also at or close to seaports, Houston will not suffer alone.

    Maybe, in the not too distant future, Algeria will supply the chemicals Houston cannot. That is not good news.

    John McCormick

  33. 33
    Jeffrey Davis says:

    Hansen’s paper last summer looked at 3 time scales — 10s, 100s, and 1000s of years — for the scary sea level rises and decided that millennial was out: the geological record showed that if the seas were to rise, they’d rise pretty fast. The most likely was a time scale of centuries, but that they couldn’t rule out decadal. Which isn’t alarmist, but is pretty scary.

  34. 34
    Hank Roberts says:

    5 meters = 16.4 feet, approximately. So he’s not talking about Hansen. The 20 foot number shows up two places: the famous Associated Press typographical error that was widely spread in the news and so hard to correct, and the “total melt of Antarctica” projections. (It took me multiple emails to our local SF Chronicle paper to get that mistake fixed on their website — with help from the science writer!. Editors had it repeated in the text and took out only one error each time we complained to them.)

    I think the university must be trying to lessen the impact of this paper, which would be expected to draw scary-new-large-estimate headlines, coming out right during the political conventions.

    Shorter press release:
    “Yes, we’ve found alligators, but no ichthyosaurs, while trying to drain the swamp — good news!”

    It’s an election year in the US. This makes everything weirder than usual.

  35. 35
    Kent Guy says:

    I think this whole discussion (and posts #21 and #26) raises a real problem. Things will be bad in this generation, and worse in the next. The issue is the really unliveable and cataclysmic stuff – onwards and upwards to the 80m “eventually” level – is still probably hundreds of years off. And people don’t respond on an emotional level to problems that are so distant – although our actions right now (and specifically, I’d contend, our decisions at Copenhagen in 2009) may or may not make that future almost inevitable.

    If our decisions right now kill billions in a few hundred years, are we any less culpable than if it were our own generation? Logically no, but I fear it won’t be seen this way…

  36. 36
    Hank Roberts says:

    Just to nail this down, the original error, based on error in Associated Press wire report, began thus as it appeared locally:
    ________excerpt follows________

    Melting ice could raise levels up to 20 feet by 2100, scientists say
    David Perlman, Chronicle Science Editor

    Friday, March 24, 2006
    Glaciers and ice sheets on opposite ends of the Earth are melting faster than previously thought and could cause sea levels around the world to rise as much as 13 to 20 feet by the end of the century, scientists are reporting today….
    …. The teams then compared that era to what might happen in this century …
    They concluded that … would lead to an average global temperature increase of at least 4 degrees Fahrenheit and a rise from today’s global sea level of 13 to 20 feet…..
    ————–end excerpt from original story as printed—————–

    Oops. (AP failed to state over what span of time, leaving the earlier “this century” mistakenly attached to the longer term number). Then they listed a long range of scientists as supporting the research, after grossly misstating the research papers.

    Belatedly as corrected:

    Current page:
    noted at the bottom says
    “This story has been corrected since it appeared in print editions.”

    Melting ice could raise levels up to 3 feet by 2100, scientists say
    David Perlman, Chronicle Science Editor
    Friday, March 24, 2006
    Glaciers and ice sheets on opposite ends of the Earth are melting faster than previously thought and could cause sea levels around the world to rise as much as three feet by the end of this century and 13 to 20 feet in coming centuries, scientists are reporting today. …

    My thanks to David Perlman who persisted until the Chron’s editors fixed the error not just in the headline but also, eventually, in the text of the article. Pity they only say corrections were made but not how and why it was corrected, eh?

  37. 37
    sidd says:

    Pfeffer et al. consider the Ross and Filchner-Ronne shelves to be stable for this century (p1342). Woul this assumption hold after, say, a 1M SLR ? ie
    would rising seas destabilize the shelves ?

  38. 38
    Peter Black says:

    For anyone who would like to explore what sea level rise would mean for the US coastline, check out this interactive map I made using the EPA’s data. It’s not the .8 meter-2meter estimate, but it’s close:

  39. 39
    Chris says:

    Sea level has potential to be a big problem; no doubt about that. But there are a couple of others that scare me more.

    Ocean acidification has the potential to cause a breakdown of large segments of that ecosystem. How many people depend on the ocean for food?

    Speaking of food, farmers will really struggle in a climate in flux. Rice, corn, wheat, etc., they all need specific and different amounts of rain and temperatures to grow well. Whatever changes are coming, there will be changes. It is not easy to switch from one thing to growing something else. Worldwide, there is not much of a food surplus now. Let us hope the changes don’t accelerate too much. How many people depend on farmers for food?

    Add in the decrease in cheap energy and food production becomes even more of a problem. People crib about $4 gas; $4 (or more) bread will cause a real ruckus.

  40. 40
  41. 41
    E Leader says:

    Complete layman here, with a history background, not science.

    Here’s my uneducated question – while I respect Gavin’s comments about not abusing the science, it seems to me that many measurable indicators of climate change are (to the extent I can tell) occurring/progressing/worsening faster than predicted by most models, whether we’re talking about atmospheric CO2 levels, arctic ice melting, glacial retreat, etc.

    Is it too much of a stretch to assume that sea levels will rise faster than currently predicted, largely because many of the factors that contribute to sea levels rising are occurring at faster-than-predicted, and possibly accelerating, rate?

  42. 42
    John Lang says:

    Jason/Topex measurements indicate sea level is increasing at 3.2 mms per year.

    In the last 15 years, sea level has increased by:

    – 1 inch

    If the trend continues for 100 years, sea level will increase by:

    – 1 foot

    Where do people get all these exagerated figures from?

    [Response: Current trends in sea level rise are larger (> 3mm/year), and estimates of future changes rely on more that the excel linear regression routine. Read the papers linked to above to get an idea. – gavin]

  43. 43
    Richard Ordway says:

    How is the “ozone-hole-repairing-itself-in-’50-years’-so-that-the-antarctic-warming-catches-up with-the-arctic-warming” figured into this? Is this still not figured in or is it?

  44. 44
    Geoff Beacon says:

    An informative piece and an interesting discussion. Sea level rise gets more attention than other aspects of climate change.

    Is that because it is the worst?

  45. 45
    Rod B says:

    A quicky Sandbox101 clarification: Stefan’s paper says 3.4mm/yr/degree. I do not understand the \per year\ part. If global surface temp goes up 1 degree, does the sea level rise 3.4mm per year for ever? Seems odd, but I don’t know.

    [Response: My paper is here, if you want to read it. The simple approach is only valid for the initial sea level response to large and rapid rise in global temperature, as sketched in Fig. 1 of my paper. Some people have misunderstood this. Obviously this rise will not continue forever but asymptotically approach a new equilibrium. And obviously this approach will not work for the small natural sea level fluctuations found in the preindustrial centuries, since these are ruled by different physics. You have to have a big rapid global warming that sticks out well from the natural variability for this approach to make any sense at all. stefan]

  46. 46
    SecularAnimist says:

    Geoff Beacon wrote: “Sea level rise gets more attention than other aspects of climate change. Is that because it is the worst?”

    I am more worried about drought. Even the worst-case scenarios of sea level rise suggest that it will take many years, perhaps decades, to have truly catastrophic effects (e.g. to displace tens of millions of people in coastal areas).

    On the other hand, another effect of global warming, namely massive, continent-wide, intense, persistent drought, could begin at any time and have catastrophic effects on agriculture, leading to widespread famine within a few years. Indeed there is evidence that such intense, chronic drought may already be kicking in with a vengeance in Africa and Australia, as well as parts of North America.

    Surely this is a more urgent and imminent danger than sea level rise, yet it doesn’t seem to get as much attention.

  47. 47
    Figen Mekik says:

    #44: Yes, because [1] one of the big problems with sea level rise is that there is very little one can do to stop it. How big of a wall are we going to build to hold off the water? How strong can we really make it? Just the eastern seaboard of the US has a daunting enough length, what about the rest of the world? And [2] as they mention in the post, sea level continues to rise for a significant period of time even after temperature rise is stopped or reduced.

  48. 48

    #49–“People crib about $4 gas; $4 (or more) bread will cause a real ruckus.” I want to validate everything about your post except the timeline: the good bread I used to buy regularly hit $4 last year. I still miss it. . .

  49. 49
    Timo Hämeranta says:

    It’s always advisable to read the paper first, but the authors public comments are informative, too:

    ScienceNOW Daily News 4 September 2008

    “They calculated how fast glaciers would have to flow in order to raise sea level by a given number of meters and then considered whether those flow rates were plausible or even physically possible. In Greenland, they calculated ice loss through specific rock-bounded “gates,” which are carved in the edges of the island. In West Antarctica, the gates are not well defined, so the team used approximations of how flow might respond to rising temperatures….estimates of several meters of sea level rise made by some other researchers are “physically untenable” because not enough ice could be pushed through the glacial gates, according to the paper’s authors. ”

    PhysOrg Sept 4, 2008

    “…global sea rise of much more than 6 feet is a near physical impossibility…

    Most of the marine-based ice in West Antarctica is held behind the Ross and Filcher-Ronne ice shelves, which Pfeffer’s team believes are unlikely to be removed by climate or oceanographic processes during the next century….

    Policymakers need to be able to predict sea level accurately if communities, cities and countries around the world are going to be able to plan effectively, Pfeffer said. “If we plan for 6 feet and only get 2 feet, for example, or visa versa, we could spend billions of dollars of resources solving the wrong problems.”

    Daily Telegraph 04/09/2008

    “…estimates of future rises remain hazy, mostly because there are many uncertainties, from the lack of data on what ice sheets did in the past to predict how they will react to warming, insufficient long-term satellite data to unpick the effects of natural climate change from that caused by man and a spottiness in the degree to which places such as Antarctica have warmed….

    “We simply don’t understand the physics of ice dynamics well enough to make accurate model predictions,” says Dr Harper. “There are just too many uncertainties…”

    Climatologists usually present possibilities and probabilities, here I’m pleased to see “a near physical impossibility.”

  50. 50

    Rod B #45 yes, a very good question. Note that Stefan calls this a “semi-empirical” approach, meaning that there isn’t much physics behind it.

    But there is this much physics behind it: both the melting of glaciers and the warming of ocean water is driven by the imbalance between incoming and outgoing heat energy. As a proxy for this you could take the difference between current mean temperature and the equilibrium mean temperature for current forcing. But as we well know, the latter is not very well known.

    So, instead, StefaSo, instead, Stefan takes current (realized, transient) mean temperature excess. In the simplified situation where the excess forcing grows exponentially, this quantity is proportional to the excess forcing, as well as to the above described heat imbalance. For the 20th century the assumption of exponentiality is roughly OK.

    Note that this is the same kind of idea that was behind Pat Frank’s writing, only there it was a fallacy ;-)