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Comments 51 to 62 out of 62:

51. Stevo at 13:49 PM on 21 October, 2011
    adelady, sorry, I was wearing my ironic/sarcastic hat when I wrote about ending up on the beachfront. In the case of Brisbane we are already at the stage where we are running out of land into which to accommodate our expanding population. I cannot see any action saving the homelands of many Pacific atoll dwellers and delivery of practical aid to these people is happenning way too slowly. Inevitably this will lead to displacement of these populations. How effectively do we expect we will prepare for and cope with SLR changes to our major cities, let alone the myriad other challenges posed by a changing climate when so far we have failed to supply enough sand bags to Kirrabati, and the "it isn't happenning" strain of denialism is still flourishing in this country?
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52. scaddenp at 13:56 PM on 21 October, 2011
    Careful with those maps. Something above the sealevel might look okay but if it is on soft sediment, then it can still succumb to coastal erosion. Deltas, dunes, and dune barriers over former estuaries would be examples. Sealevel rise that effect longshore sediment transport can cause a switch in coastal processes from prograding to aggrading. If you already have issues with coastal erosion, then expect it to get a lot worse in next 100 years.
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53. Riduna at 14:30 PM on 21 October, 2011
    One should question the utility and verity of a model which ignores “what is happening with the climate, but … focuses exclusively on sea levels. Others have noted the obvious, that one can not purport to model future trends in SLR, be they in respect of 100 or 500 years without taking into account those factors most responsible for it, namely the rate at which land based ice is expected to melt and the causes for that melting. It is overly simplistic to assume that anthropogenic emission of greenhouse gases will be either the sole or even the major cause of ice melting over the next 100 years. Nor can one ignore the unprecedented warming of the Arctic and Southern Oceans and the effects this is having. It is a strange model which apparently ignores the potential for rapid increase in and massive loss of polar ice. Hansen and Sato 2011 hold the view that loss of this ice will double each decade this century, that in consequence SLR this century will be non-linear and, starting from a low base, will continue rising relatively slowly in the first half of this century but much more rapidly and with disastrous consequences in the second half – particularly during the last decades of the century. Their conclusions are consistent with those of Shakhova et al 2010 who draw attention to the dangers posed by the melting of permafrost, particularly on the seabed of the Siberian Central Continental Shelf, producing massive release of methane. They note that this is now occurring and warn that emission of 3.5 gigatonnes of methane, about 0.1% of Siberian methane deposits, would be sufficient to cause abrupt climate change. Antarctica is already being affected by warm ocean currents causing near collapse of the PIG and Thwaites glaciers, draining almost 30% of the WAIS, with capacity to cause SLR of ~1.5 metres. These currents have the potential to cause break-up of WAIS and EAIS shelves, causing major increase in the rate of glacier discharge. These events are now occurring and will continue for the rest of this century. Loss of mass from the Greenland ice sheet is, as predicted by Hansen, more than decadaly doubling, accelerating and shows no signs of slowing. Nor could it, since the Arctic Ocean is warming so rapidly that it is predicted to be seasonally ice free within 20 years with consequential loss of albedo and increase in the rate of future warming. Given these considerations, I find it extraordinary that the model used by Aslak Grinsted should be regarded as of any reliability in predicting SLR over the next 50 years, let alone the next 500. In this regard it should be treated, as is the IPCC 4AR, as very conservative because it largely ignores the causes and reality of polar ice loss – in other words, what is already happening.
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54. jyyh at 19:26 PM on 21 October, 2011
    Is this the new lowest bound? anyway nice to see sigmoidal curves on a projection for they may be found also on the ice core record.
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55. Hyperactive Hydrologist at 00:16 AM on 22 October, 2011
    I still feel catastrophic collapse of the Greenland ice sheet is unlikely. If you look at a topographic bedrock map of Greenland the central region is lower than the surrounding perimeter especially on the eastern side where there is a mountain range. I think once the glaciers retreat from the sea we could see a slow down in ice loss. The central region, which contains the majority of the ice, should be relatively stable. However, Antarctica shows a very different picture,especially for the Western Antarctic Ice sheet. Much of this region is actually below sea level and a large increase in ocean warming could potentially destabilise the ice sheet.
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56. John Hartz at 02:11 AM on 22 October, 2011
    @Agnostic #53: "The journey of 1,000 miles begins with the first step."
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57. Riduna at 08:18 AM on 22 October, 2011
    John - All very true but why go back to step 1 when we are already well down the road?
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58. Glenn Tamblyn at 18:58 PM on 22 October, 2011
    Hyperactive Hydrologist. Catastrophic collapse is unlikely. On what time scale? If a couple of 1000 years of temps maybe 0.5 C warmer than now removed 1/2 or more of the GIS, what will the rate of loss be with temps 2-4 C warmer than now? How much of decline is movement towards the coast, how much is melting in-situ? Today? What will this balance be in 50 years?, 100? 200?
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59. Hyperactive Hydrologist at 20:02 PM on 22 October, 2011
    Glenn, I'm not disputing rapid ice loss and subsequent rise in sea level. I'm saying perhaps Greenland is more stable and resilient to complete collapse compared with the Western Antarctica (WAIS) based on topography. In-situ melting of a large ice sheet will be much slower than an ice sheet losing mass from glacial carving and in-situ melting. WAIS has the potential to suffer from a mechanism of sea water intrusion and consequently an increase in basal lubrication. For me the real danger of rapid or even catastrophic sea level rise comes from the WAIS. The key is how much of an impact will 2-4oC of warming have on Antarctica.
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60. Rob Painting at 20:20 PM on 22 October, 2011
    Hyperactive Hydrologist - yup, that seems to be the conclusion of a couple of recent papers - the West Antarctic Ice Sheet may have been the main contributor to sea level rise in recent (ish) warm periods. See SkS post: Rising Oceans - Too Late to Turn the Tide? and The Role of Ocean Thermal Expansion in Last Interglacial Sea Level Rise - McKay (2011) the study referenced.
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61. Daniel Bailey at 01:02 AM on 23 October, 2011
    Hyperactive Hydrologist shares my concerns about the WAIS being more of a threat to SLR than does the GIS. Once the restraining influence of the PIG & Thwaites linchpin are removed the risk of ice sheet dynamic decomposition of the WAIS escalate. Melt rates are a non-issue compared to that. I've been planning on writing a post on this for some time; perhaps the time has come to put aside malaise and laziness and actually do it.
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62. John Hartz at 04:29 AM on 23 October, 2011
    @Agnostic #57: The issue you have raised is directly addressed in "The Next 500 Years of Sea Level Rise" by Michael Lemonick posted on Climate Central on Oct 19. Lemonick had posed that very same question to Aslak Grinsted.
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