Comments 1 to 21:

1. nigelj at 06:08 AM on 6 January, 2019

   Regarding the article published in the magazine Science: "A Terrifying Sea-Level Prediction Now Looks Far Less Likely. But experts warn that our overall picture of sea-level rise looks far scarier today than it did even five years ago."

   This is a very good article, but the title is so badly worded and immediately creates the impression sea level is not an issue. I know the second part of the title is a cavet that hints at problems, but the impression is made in the first sentence, that there's no problem. Please just stop this naive journalism.

   It's only when you read half way through the article that you find they are still predicting about 5 metres of sea level rise by year 2300, which is huge, and not so far into the future. This is buried away and would be easily missed. Sigh.

   Meltwater pulse 1a was 16,000 years ago and associated with destabilisation of the antarctic and about 2 metres of sea level rise per cenury is attributed to this, spread over several centuries. I often quote this because its an event that happened, so is particularly pertinent. Modelling the future is challenging, but we have this known information from our past to consider as well.

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2. One Planet Only Forever at 06:17 AM on 6 January, 2019

   nigelj,

   I agree. The Bold main title gets used without the important context.

   It would have been better to reverse the two.

   Experts continue to warn that our overall picture of sea-level rise looks far scarier today than it did even five years ago - However, a recent more terrifying sea-level prediction now appears to be far less likely.

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3. Evan at 09:55 AM on 6 January, 2019

   nigelj, am I missing something. What I've read Meltware Pulse 1A was associated with about 5m/100 years for 4-500 years. Or are you saying that 2m/century is the amount associated with Meltwater Pulse 1A that was on top of the background declatiation rate top give a total of 5m/century?

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4. nigelj at 10:37 AM on 6 January, 2019

   Evan @3

   I agree melwater pulse 1a is associated with 5 metres per century, but read the wikipedia article. Only half this is at most is attributed to destabilisation of the antarctic, the remainder to the melting of ice sheets over north america which do not exist anymore, so my conclusion is a destabilising antarctic as discussed in the article above would perhaps cause 2 metres of sea level rise per century. Which would be catastrophic.

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5. Evan at 01:39 AM on 7 January, 2019

   Thanks for the clarification nigelj@4.

   I assume you've seen Fig. 1 in the paper by Alley et. al. (2005) where they present a graph of sea level vs. CO₂ concentration. Using a climate sensitivity of 3C/doubling CO2, their graph works out to about 12m/1C warming, at least for the first 2C of warming or so. If we assume that we stabilize at 2C, that implies 24m of sea-level rise. At 2m/century that's 1200 years.

   The engineer in me says that given how rapidly we've warmed the planet, that we will get a sizable chunk of that 24m much, much sooner than using 2m over 1200 years. I'm not disagreeing with your analysis nigelj, but rather noting that we might get a quick pulse that then settles down into a long tail that approximates 2m/century, because we are causing a warming pulse that represents more of an impulse to the system than what we expect happened during deglaciation cycles.

   And of course this assumes that we stabilize at 2C.

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6. nigelj at 05:02 AM on 7 January, 2019

   Evan @5, as far as I can figure out The IPCC currently predict 1 metre of sea level rise per century (worst case scenario) spread out over many centuries and based on medium to high climate sensitivity. If we burn all fossil fuels we are talking 30 metres plus of sea level rise but over a lengthy period.

   But some of us think they are being too conservative and that it could be more than 1 metre per century at least for a period of time. Maybe not by 2100 but soon after this

   Anyway  I would think 2 metres per century for maybe 5 centuries for example 'would' fall into the categorisation of a quick pulse! And it would be devastating for infrastructure.

   I was simply trying to get a handle on what has happened in the past known with some certainty, and that is 2 metres per century as far as I can tell. 

   However I would definitely agree we cannot rule out more than 2 metres. J Hansen has written a paper somewhere finding that 5 metres per century is possible based on physics and modelling, but many worst case factors have to coincide for this to happen.

   I do not know nearly enough physics really, but I know warming is looking like a quadratic curve, so you would expect melting of ice and expansion of sea water to follow this, and this suggests about 1 metre of sea level rise per century over many centuries. However the wild card is ice sheet destabilisation, where glaciers speed up, or the face of ice sheets starts to collapse. This looks like it would cause a step change in a quadratic curve. Glaciers would however come up against limiting facor of friction. The article suggests the face of ice sheets could collapse rapidly from undercutting and the possibility of 4 metres per century for a couple of centuries at least perhaps until things reach a new equilibrium.

   2 metres per century. 4 metres per century. All bad.

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7. Evan at 06:25 AM on 7 January, 2019

   nigelj@6, thanks for your thoughts. I've seen a lot of this material and agree with your analysis. We are talking varying shades of bad.

   Have you seen Richard Alley's short video clip where he says (not predicting) that 15-20' this century is possible? Here is an article in Rolling Stones that contains Richard Alley's video clip.

   Needless to say Richard Alley knows this stuff better than any of us, and he is a seasoned scientist, not given to making exaggerated claims. His message is sobering. But we all agree, even if he is wrong, it is still time to prepare. So here is a feel-good PBS story about a town that is taking the right steps, right now, to prepare for what is coming.

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8. Riduna at 13:22 PM on 7 January, 2019

   Evan

   In 2018 I wrote an article There Will Be Consequences in which I cited work by Dr. Hansen and some leading glaciologists showing that under certain conditions mass loss from the polar ice caps could produce sea level rise of >5m. within a century.

   The likelihood of multi-metre sea level rise this century is seen as inevitable by many experts in the field. While some may reject that conclusion, they must do so in the light of increasing loss of ice from the Greenland Ice Shelf, Dr Rignots assessment of Antarctic instability and Hansen et al 2016.

   We may naïvely take comfort from those who espouse sea level rise of <1m. this century (5AR) but the evidence is compelling that without sustained reduction in greenhouse gas loading and emissions, we are very likely to be assured of multi-metre sea level rise in the latter part of this century.

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9. Evan at 14:00 PM on 7 January, 2019

   Riduna@8, thanks for the links. I take it seriously when people like James Hansen and Richard Alley talk multi-meter sea-level rise this century. nigelj was citing a background rate that is low multimeter due to historical precedence, and I realize that some of the best researchers are talking 5m or more. I cannot see that anything we will do will change the conclusion that it is time to start building coastal defenses where it makes sense, and time to start retreating where it does not make sense. In that light I found it interesting that there are already places where the retreat has started, even if climate change is not specifically identifed as the reason for the retreat.

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10. nigelj at 16:11 PM on 7 January, 2019

    Evan @9, thank's for the video link. Given this is a climate expert talking about more than 5 metres sea level rise per century, I agree he cannot be simply dismissed. Shame the video was so short.

    Basically this is how I have approached the sea level rise  issue, fwiw. I have been aware for quite some time  of claims of 5 metres sea level rise per century versus the 1 metre IPCC estimate. Its hard for me to know who to believe short of cracking open a textbook or two. My own instincts fwiw have always been that the IPCC have been too conservative in their estimates, but I try to avoid confirmation bias, and catastrophic mindset thinking, so I looked at the historical record for guidance.  This is something that is happened, it does not rely on " ifs, buts and maybes" and pages of differential equations.

    The historical record does have periods of multi metre sea level rise per century, associated with ice sheet destabilsation. They appear to be around 2 - 3 metres per century for several centuries. So that is what I would see as a very strong possibility. 

    Here's another thought. I design infrastructure. Obviously the 200mm sea level rise last century is not too problematic. Its unlikely to devastate anything and is easy to design for and it was reasonably constant. Get up towards 500mm  and we have serious problems. Florida is already in this territory. It threatens existing infrastructure and making planning for the future difficult. Much land would just have to be put off limits.

    At one metre and even assuming its constant over time, building is a big problem. It would be absurd building foundations to cope. Huge areas of land would simply have to be put off limits for development.

    Now we have this scenario that it "could" be more than  one metre, who knows, perhaps two metres, or more than five metres. I dont even think it matters too much which, because a) it is all going to lead to huge loss of coastal land and b) the unpredicatbility of it all makes design impossible. 

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11. Evan at 01:13 AM on 8 January, 2019

    nigelj@10 Here is another perspective. The way we deal with hurricanes is that we try to get everybody back from the shore and out of the way. We let the hurricane do its damage, then we clean up the mess and move everybody back (I mean no disrespect for those who suffer during hurricanes, just trying to summarize).

    We seem to take comfort if we can convince ourselves that the sea level rise due to ice loss will be slow. How do we do that with sea level rise that occurs over millenia? It would almost be better if sea level rise happened fast (like a Hurricane) and we got it over with quickly so that we had something like a single, focused event to deal with. Slow sea level rise might actually be harder to deal with than rapid sea level rise.

    I think the only thing that matters is continually monitoring all of the world's ice so that we can improve ice-loss forecasting. We need to budget for this from now on just as we budget for tsunami and hurricane monitoring systems.

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12. johnthepainter at 04:42 AM on 8 January, 2019

    What I'm wondering is how we can reconcile the conclusion of this study with the following one, reported December 18: "Discovery of recent Antarctic ice sheet collapse raises fears of a new global flood" https://www.sciencemag.org/news/2018/12/discovery-recent-antarctic-ice-sheet-collapse-raises-fears-new-global-flood?fbclid=IwAR2jMO7SgRVG4Hy7vTY3zaqi66sI2543COyiAiaIB2ej3vUvyMS5W3PgRgM

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13. nigelj at 04:50 AM on 8 January, 2019

    Evan @11 yes true it would be better to get the worst of it out of the way relatively quickly, provided we  knew the exact extent of sea level rise so we could rebuild behind  the danger zone confidently. But we will never have that certainty. The most likely outcome is we will get the worst of both worlds, long slow sea level rise with unpredictable shorter periods of rapid sea level rise embedded in this. 

    I dont envy the people trying to understand it and model it. Its not as if we can build full size glaciers and do experiments with them.

    Anyway all I know is our climate agency did graphics of the impact of 500 and 1000mm on our coastal cities and the impacts were larger than I suspected. It's hard judging sea level rise just by visually looking at a beach, its so easy to underestimate it. Cheers.

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14. nigelj at 05:05 AM on 8 January, 2019

    johnthepainter @12 the original article didn't say the antarctic wouldn't collapse, it just shifted the time frame forward from by the end of this century to around 2200 - 2300. 

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15. Lachlan at 12:25 PM on 10 January, 2019

    nigelj @1

    I liked the title.  When I read it, I thought "This is something to show to people who claim all realists are alarmists".  Your concerns about misreading it are valid, but must be weighed against the benefit.

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16. Lachlan at 12:35 PM on 10 January, 2019

    Evan @11,

    I agree that it would be nice to be able to "get it over with" and rebuild, but I think that having slow increase is far, far better.  If 90% of a city is intact, then it can relocate the other 10% to higher ground.  If 10% of a city is intact, it is really hard to rebuild the other 90%.

    If a change happens over a millenium, think how many cities have the same borders a they did 1000 years ago. 

    The only drawback I see in gradual seal level rise is the "boiling frog" problem, where people don't accept the rise and keep rebuilding on the same sites after damage.

    As nigelj says, the worst case is to have rapid bursts on a long-lived underlying trend, and that the greatest technical problem is unpredictability.  (The greatest actual problem is denialism.)

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17. Evan at 16:04 PM on 10 January, 2019

    Lachian@16 Yes, I agree with your analysis/comments and those of nigelj noting that the non-smoothness of the rise will also be a big problem. My point is to add perspective: slow does not mean "no problem". However slow sea-level rise will be long term, it will likely start with a big pulse and then slow down. Note that Meltwater Pulse 1A occurred rather early in the sea-level rise associated with the last deglaciation.

    I am sure that together with "orderly" rebuilding of the 10% to which you refer, there will be many genuine disasters and cities where people just leave after a major storm which, on top of sea level rise, becomes too much to cope with. There will no doubt be a smorgasbord of damage and adaptation scenarios. The main point is that it is already time to start an orderly retreat from areas already being impacted.

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18. nigelj at 04:47 AM on 11 January, 2019

    Lachlan @16, you are possibly right about the title. But the first half of the article dealt with the findings that the terrifying prediction was unlikely this century,  and we only found out it was merely postponed half way down the article. Many people only read titles or the first couple of paragraphs of articles. A summary would have helped.

    Evan @17 

    I think there is something in your theory of a pulse and a long tail. It looks like it could happen. If we knew that the period of multi metre sea level rise was confined to for example 2200 - 2500 we could plan for this and build behind the danger zone. Then adapting to a long tail at perhaps 1 metre per century would be feasible, although ugly. The trouble is can we be sure rapid sea level rise would be confined to a specific time period? Im not sure enough is known about how ice sheets will respond. Nevertheless, if a pulse was to eventuate we would be very motivated to find out.

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19. Riduna at 15:34 PM on 11 January, 2019

    It is wishful thinking to assume that multi metre sea level rise will not occur this century and equally unrealistic to believe that if only 10% of a city is inundated by sea level rise (SLR), the rest of it will remain habitable. Further, the only thing which can be inferred from Pulse 1A is that SLR of ~5m/y can be sustained for centuries by rapid melting of an ice sheet.

    There is growing evidence that ocean surface (0 – 6,000m) heat absorption has been significantly underestimated. The most likely effects of ocean temperature rise are thermal expansion of seawater and more rapid degradation of Antarctic and Greenlandcvoastal ice reasting on the seabe. Both will result in more rapid SLR and increased instability of the ice sheets, potentially leading to further SLR acceleration.

    If the inundated 10% of a city (why only 10%?) contains major infrastructure (port facilities) and industrial facilities (factories) the other 90% may be habitable but offer little or no employment to the inhabitants. Moreover, adjacent coastal flooding may destroy transport infrastructure making it impossible to produce goods and services or support inhabitants with goods produced elsewhere.

    Even with SLR of only 2m., it should be expected that an effective SLR of >4m. could be created by storm surges – and it should be expected that with rising sea surface temperature and atmospheric moisture, the severity and incidence of storms will increase significantly, as will the destruction they cause. It is also likely that flooding of coastal lands this century is no longer avoidable.

    Retreat may be possible.  Clinging on is not.

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20. nigelj at 17:53 PM on 11 January, 2019

    Riduna, I think multi metre sea level rise this century is possible, but somewhat unlikely, because it would require more than just melting ice, it would require physical destabilisation and destruction of ice sheets which requires strong local warming right around the antarctic and greenland oceans, considerably more than presently, and all over the next couple of decades surely. How would that happen? Seems unlikely to me by 2100, but very possible by 2200 as warming accelerates, if we do nothing.

    I'm not minimising the problem, just thinking how would it happen? It is pretty much just as bad if its by 2200 anyway. Hell, 1 metre is very serious. I think its important to talk about dangerous but realistic, evidence based defensible scenarios or the public will dismiss climate science as inflated scaremongering.

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21. Evan at 23:34 PM on 11 January, 2019

    For anybody interested and reading this post, I suggest Googling James Hansen, Richard Alley, and Eric Rignot together with "sea level rise" and read the reports and watch the videos that come up. All of these respected scientists regularly talk about multi-meter sea-level rise this century.

    Scientists routinely say that Earth is responding faster than anyone thought possible and faster than the models predicred. A good example is melt in the Arctic, which is occurring faster than the models have predicted.

    We only have one chance to prepare before rapid sea-level rise really kicks in. That time is now.

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