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Comments 51 to 74 out of 74:

51. barry1487 at 13:57 PM on 8 February, 2012
    Steve, a 5 meter rise by 2100 is a worst case potential discussed in the paper. Uncertainties and negative feedbacks that mitigate against this potential are clearly discussed by Hansen and Sato. The graph is a simple model of an ice-sheet loss doubling regime derived from paleo-records, and is not meant to represent an actual scenario for the year-by-year rate of sea level rise acceleration (Hansen says this explicitly in the 2007 paper the graph is modelled from). As mentioned above, the more likely realization of such a potential would come with short periods of massive inundations rather than a smooth rate increase. Hansen is not saying that he believes the 5 meter rise by 2100 is the likeliest outcome, just that it is not implausible and shouldn't be discounted. His general theme in this and other papers is that the official projections, like those in the IPCC, are too conservative presenting upper bounds. And even though we should be wary of focussing on short-term data, as you have done, it's worth pointing out that global sea level rise (and Arctic sea-ice decline) have so far been underestimated by the IPCC. Hansen may have a point; and he is aware of the limits of and arguments against his thesis, and has called for more study to test it. Whether consciously or not, you have misrepresented his argument here. Here's the monograph underpinning the graph that's got your attention: Hansen (2007): Scientific reticence and sea level rise Read the whole thing for comprehension and context. It's not long.
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52. GillianB at 17:16 PM on 8 February, 2012
    Thanks for this. I didn't realise that sea levels would vary with El Nino and La Nina. It makes sense.
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53. Sascha Tavere at 10:14 AM on 9 February, 2012
    (novice question) "Summer melt on West Antarctica has received less attention than on Greenland, but it is more important" says M. Mann in his reticence paper. Because of ocean temperatures or are the other or more reasons?
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54. skywatcher at 10:56 AM on 9 February, 2012
    Though I've not read the paper, I'd suggest it is that it is because, of these two ice sheets, the West Antarctic Ice Sheet (almost a physically separate entity from the East Antarctic Ice sheet because of the Transantarctic Mountains) has a great deal more potential for rapid melt/collapse than the Greenland Ice Sheet. Both contain broadly comparable potential sea level rise components, but Greenland has a lot of mountains round the perimeter to buffer the ice sheet, which the WAIS does not have. The WAIS is also grounded well below sea level, and so warmer sea water can readily melt it from the base (Greenland also, but sea water access is mostly limited to the narrow fjords). If the WAIS begins to go, there's a lot less to stop it, and a consequently rapid sea level rise. Having said that, I think Greenland has more melting on its surface, and an observed flow acceleration already, so is not immune to significant melt!
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55. Steve Case at 12:47 PM on 9 February, 2012
    Hi Barry, nice to see you here. #52 Barry
    That's right, Dr. Hansen is not saying the 5 meter rise is the likeliest outcome, he just says it's plausible. I've run the numbers on his 5 meter every ten years rate doubling scenario and have determined that sea level would be going up at a rate of nearly one millimeter per day by 2100. Further comment is not necessary, it stands as its own testimony.
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56. skywatcher at 13:32 PM on 9 February, 2012
    Steve Case, your argument from incredulity is just that. Just because you have a hard time imagining the collapse of an ice sheet, and the resulting pulse of sea level rise, does not negate the plausibility of it happening. Meltwater Pulse 1A had 20m SLR in ~200 years, a mean of 0.26mm/day for two hundred years. Assuming a sigmoidal curve for the pulse, which is certainly reasonable, the peak sea level rise was quite possibly close to, or even larger than, 1mm/day. This is a matter of palaeoclimatic record, and we presently have two ice sheets (Greenland and West Antarctica) that have been known to be a great deal smaller in global conditions with this much CO2 in the atmosphere.
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57. Sascha Tavere at 14:22 PM on 9 February, 2012
    Skywatcher #55, thanks Mean annual rate of ice mass loss Greenland: 200-300 Gt Mean annual rate of ice mass loss Antarctica: 70-210 Gt Permafrost warming: 0.5°C to 2° Freshening of part of the Arctic Ocean: 2006–08 increase 8400 ± 2000 km3 http://www.aoml.noaa.gov/phod/docs/Goni_etal_2011.pdf I am prepared to worry about either or both but I still don't quite see why Antarctica should be favoured eventhough the WAIS is indeed perhaps more prone to melt at its base. Is there evidence (yet)? In favour of "Artic worries" might be the the permafrost melt run-offs draining into Arctic waters. Am I being obtuse?
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58. skywatcher at 15:27 PM on 9 February, 2012
    #58: West Antarctica is buttressed by two great ice shelves, the Ross and the Ronne-Filchner. As we have already seen on the Antarctic Peninsula, ice shelves can spectacularly collapse in very short spaces of time, leading to great armadas of icebergs (Larsen A & B, Wilkins - video). If the main ice shelves go, sea levels will suddenly rise much more quickly as the grounded glaciers that are buttressed by the ice shelves accelerate. An interesting paper on a past example driven by warming seas. Ultimately, gradual acceleration is most likely with Greenland (and is observed), while West Antarctica has the potential for more sudden acceleration, but isn't presently losing mass so fast.
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59. KR at 16:09 PM on 9 February, 2012
    skywatcher - Agreed; Greenland is a prime candidate for steady increases in ice loss/sea level rise, while Antarctica is a risk for jumps in sea level due to grounded glaciers cutting loose. Either way, folks (are you listening, Steve Case?) rises on the order of 1mm/day are entirely possible, given that warming is something like an order of magnitude faster than anything in the paleo record. And hence, so are the possibilities of sea rise rates. Personally, I'm not encouraging coastal real estate investments for anyone I know...
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60. Sascha Tavere at 21:43 PM on 9 February, 2012
    Skywatcher/KR Understood ... collaps means exactly that. Nothing gently timeconsumig about it. (I got waylaid by language, like: economic collapse) Thank you for clearinng the head. It isn't a story easily told to Monckton enthousiasts though (there are still a few left).
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61. Steve Case at 01:41 AM on 10 February, 2012
    #60 KR at 16:09 Yes, I read you post.
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62. Sascha Tavere at 02:28 AM on 10 February, 2012
    @ Steve Case #62 I am always interested in "contrarian" opinions ... they sometimes offer a glimpse of hope in - what I consider - a bleak picture. Do you not agree with the figures or with the conclusion? Would you be kind enough to tell me why or refer me to something to read which reflects your view?
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63. Steve Case at 03:19 AM on 10 February, 2012
    #63 Sascha @ 02:28 As Skywatcher says, I think the rate of sea level rise increasing more than 100 times the current value is incredible, "argument from incredulity" he says. I like to go to the source of these sorts of claims and run the numbers. I'm always in fear of getting the sign wrong or getting decimal places wrong or going off half-cocked. Somewhere up thread it was pointed out that Dr. Hansen's curve isn't an asymptote but sigmoid or geometric. They were right. Eventually it has to level off. I can't go on forever. Considering that the geometric nature of that curve hasn't started yet, see my post #2 on this thread; I wonder when it will start. I would think anyone would.
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64. David Stoney at 04:38 AM on 10 February, 2012
    Thanks for an informative post and many useful comments. For what it may be worth, the term "collapse" should probably be approached with caution. Living near the southeast US coast and being uncertain about what the term might mean with regards to WAIS, I asked Robert Binschadler to help me out. He allowed that a "collapse" of WAIS might give rise to a 2 m/century SLR. Here's an interview with Binschadler Here's what's happened more recently at Pine Island Glacier Not comforting...
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65. skywatcher at 08:59 AM on 10 February, 2012
    Steve Case, the current ice contribution to total sea level is relatively small, and so you won't see a geometric curve in the total SLR, such as the 20-year figures shown above, as it is hidden by other components. Accelerations in ice mass loss (such as observed, e.g. Veliconga et al 2009) will come to dominate the SLR budget if the accelerations in mass loss of the ice sheets continue. As it is, the SLR curve for the whole 20th Century shows acceleration too, not visible on a 20-year plot. Quite how fast the acceleration continues, once ice loss comes to dominate, is the matter of question, not whether the acceleration has started. David Stoney, thanks for the links. Always worth listening to what Bindschadler has to say about Antarctica, the first link is particularly informative.
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66. Steve Case at 10:41 AM on 10 February, 2012
    #66 Skywatcher @ 8:59 According to the Church and White paper I've seen, the big jump in acceleration occurred in about 1925 for what reason I don't know.
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67. KR at 11:30 AM on 10 February, 2012
    Steve Case - The Church and White 2006 paper, if that is what you are referring to, does show a jump in acceleration around ~1930. "The slopes are 0.71 ± 0.40 and 1.84 ± 0.19 mm yr^-1 respectively, respectively, implying an acceleration of 0.017 ± 0.007 mm yr^-2 (95%)" They relate this to the more rapid rise in temperatures (with a 20 year lag) beginning in the early 1900's. From their conclusions: "From 1993, the rates of rise estimated from tide gauge and altimeter data (after correction for GIA effects) are about 3 mm yr^-1, faster than the quadratic (about 2.3 mm yr^-1) at this time." (the quadratic for the entire 20th century). There are ~10 year +/- variations in the rate of sea level rise, but it's definitely accelerating if you look at all of the data, rising faster than quadratic in rate.
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68. skywatcher at 11:40 AM on 10 February, 2012
    ...Except that Church and White (2011) themselves estimate an accelerating sea level rise:

    The linear trend from 1900 to 2009 is 1.7 ± 0.2 mm year-1 and from 1961 to 2009 is 1.9 ± 0.4 mm year-1. However, there are significant departures from a linear trend. We estimate an acceleration in GMSL by fitting a quadratic to the time series, taking account of the time variable uncertainty estimates. From 1880 to 2009, the acceleration (twice the quadratic coefficient) is 0.009 ± 0.003 mm year-2 (one standard deviation). This estimate is slightly less than but not significantly different from the (one standard deviation) estimate of Church and White (2006) of 0.013 ± 0.003 mm year-2, but still significantly different from zero at the 95% level.

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69. Sascha Tavere at 20:06 PM on 10 February, 2012
    In a laboratory setting (constant temperature, no wind) how does ice melt? Any acceleration? (Please tell me to go away if I'm too ignorant. I'll still read your comments trying to understand.)
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70. David Lewis at 13:52 PM on 12 February, 2012
    Hansen, in a 2005 editorial essay cited Kienast et.al.2003 and said "In melt-water pulse 1A... sea level rose about 20m in approximately 400 years" which "is an average of 1m of sea level rise every 20 years". Which is about 0.14 mm per day. This would be a catastrophic rate for people living near the sea. He noted that the ice sheets disintegrating at the time "were at lower latitudes than the ice that remains today and the period of rapid ice sheet disintegration was undoubtedly preceded by a period in which the ice was preconditioned for collapse". But, he points out climate forcing today far exceeds the forcing that drove melt-water pulse 1A. And, he said, sea level rise doesn't have to add up to 20 m to obligate scientists who expect it to happen to warn that an event anything like it would "wreak havoc" on civilization. Hansen has said in the past that if the planetary energy imbalance is about 1 W/m2 and all that energy was going into melting ice it would raise sea level by about 1 m per decade. Obviously, an armada of icebergs shooting off Antarctica wouldn't have to melt to raise sea level. Pfeffer told a story at the AGU this year about how the sea level rise threat isn't being fully grasped even by those he would have thought must be exposed enough to the data to get it. He said MOMA in NYC exhibited a model showing a New York that by 2100 had protected itself from 2m of sea level rise. He asked the designers, what was the plan for the ongoing steady or possibly accelerating rise after that - how was their design going to cope with another meter and another one after that? Blank stare.
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71. Enginerd at 13:28 PM on 15 February, 2012
    For what it's worth, the latest data point for sea level was just posted---and it is the highest observation in the period of satellite record. One data point, I know, but we may indeed be heading out of the "pothole". [This is my first time posting a hyperlink using HTML. Forgive me in advance if I botched it...]
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72. Tom Curtis at 13:31 PM on 15 February, 2012
    Enginerd @72, it's a bit hard picking out that blue dot siting astride the end of the trend line. But yes, it's there, and the highest to date.
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73. Steve Case at 22:49 PM on 20 February, 2012
    Enginerd at 13:28 PM on 15 February, 2012 Tom Curtis at 13:31 PM on 15 February, 2012 Here's the latest from the CU Sea Level Research Group CU Sea Level Research Group It was updated a week ago.
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74. Rob Painting at 05:25 AM on 21 February, 2012
    It will be interesting to see how much sea level rises over the next few months as conditions in the tropical Pacific move toward neutral. There seems to be greater volatility in the sea level response, but it may be temporary.
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