Neven, I think many of us have moments like that. Someone like Paul Gilding has had them. He now says: http://paulgilding.com/the-great-disruption "The coming decades will see loss, suffering, and conflict as our planetary overdraft is paid; however, they will also bring out the best humanity can offer: compassion, innovation, resilience, and adaptability... The crisis represents a rare chance to replace our addiction to growth with an ethic of sustainability, and it’s already happening. It’s also an unmatched business opportunity: Old industries will collapse while new companies will literally reshape our economy. In the aftermath of the Great Disruption, we will measure “growth” in a new way. It will mean not quantity of stuff but quality and happiness of life. Yes, there is life after shopping." What doesn't kill us, can hopefully make us stronger :)

The complete Nature-article on MWP 1A is here and explains the uncertainty margins: http://sciences.blogs.liberation.fr/files/deschamps12nature-1.pdf An earlier one gave a range of 290-500 years for a pulse of 14-24 meters: http://oceanography.dal.ca/publications/files/Kienast_et_al._2003_Geol.pdf It seems the uncertainty is gradually narrowing down, even if there's still quite some debate on the sources/causes.

Andrew, Nice overview, thanks. On Meltwater Pulse 1A there seems to exist some debate as to the probable sources and speed, with most estimates ranging from 3-5 m/century, but maybe even up to 10 m/century, if this Wikipedia entry is correct: http://en.wikipedia.org/wiki/Meltwater_pulse_1A Maybe that's where your 10 m/century came from? Also see for example: http://www.nature.com/nature/journal/v483/n7391/full/483549a.html The dating here seems to be extremely exact (duration of the pulse of 340 years). Don't know how reliable that is.

Kevin, Goelzer et al would only be relatively comforting compared to for example Meehl et al. What I appreciate about Meehl's figure 3 is that they show explicitly a risk of much higher sea level than the models. But how well-founded is the estimation of that risk? Goelzer et al do acknowledge uncertainty, but don't give an estimation of the potential magnitude of that uncertainty, perhaps because they consider that impossible. Still, indeed, that may give the larger public a false sense of relative comfort as far as the potential risk of SLR is concerned. Lennart

Rob, I think models are the best we have, if they have a minimum match with reality. We still have to judge that quality by some form of expert judgment or common sense, which still seems something better than wild speculation. Science also implies recognizing the limits of our knowledge, so the risk of putting too much trust in not yet fully enough developed models needs to be compensated by let's call it expert judgement. To me that's what Wadhams and Maslowski seem to provide, apparently to the annoyance of some of their colleagues?

Andrew, Seems fair in general. However, that 10 m/century is new to me. You have sources? As far as I know, about 4-5 m/century during Meltwater Pulse 1A seems to have been the maximum speed in the past. But I'm just a layman, so may have missed something. Lennart

On modelling ice volume (and sea level) further into the future see this new article by Goelzer et al: http://iopscience.iop.org/1748-9326/7/4/045401/article As compared to this recent one by Meehl et al: http://www.nature.com/nclimate/journal/v2/n8/fig_tab/nclimate1529_F3.html Goelzer et al project until the year 3000; Meehl et al until 2300. The first expect less than 2 meters of SLR at most by 2300 and about 1 meter/century by the end of the millennium. The second seem to think almost 3 meters is likely under BAU by 2300, but don't exclude a risk of up to 12 meters and about 5-6 meters/century by then. How seriously should we take this risk? And how serious should we take Goelzer et al?

Even an average linear decrease of 400 km3/yr from now on would bring us below 1000 km3 in 6 years and to practically zero by 2020. Over the past 25 years the linear decrease seems to have been about 500 km3/yr on average. Over the past 10 years it seems to have been about 800 km3/yr. If that speed continues it would be about 4 years until the Arctic is ice free for the first time, and 3 yrs for the first time below 1000 km3. It seems Wadhams and Maslowski have a good chance of being right on the spot with their 2015/2016 projections.

Geoff, Wadhams and Maslowski are supported I think by scientists like Jim Hansen in arguing that the/most sea ice models (amongst others) are not good enough yet to be trusted too much, to put it mildly. But Hansen also argues that many scientists suffer from a certain reticence and fear of being outcast as too 'alarmist'. Erring on the cautious side could in this case however be more risky than erring on the supposedly 'alarmist' side. So I think you have a strong case towards the politicians and the more people (and scientists) tell them so the better.

skywatcher, I agree with you that we should start from current science, so let's pick up again at the discussion between Pfeffer and Hansen. Pfeffer et al assume it's very unlikely, but maybe not impossible, that all GIS glaciers can speed up quickly to on average the speed of Jakobshavn Glacier in 2004-2005 (12.6 km/yr): http://courses.washington.edu/ess203/RESOURCES/READING/pfeffer_sealevel_science_2008_with_suppl_info.pdf They assume the same for the most important marine glaciers on AIS. For surface melting they assume continued acceleration at present day rates of change. Total SLR by 2100 could then be about 2 meters. Hansen & Sato think this may be too cautious an estimate (pp.22-23): http://arxiv.org/ftp/arxiv/papers/1105/1105.0968.pdf "The kinematic constraint may have relevance to the Greenland ice sheet, although the assumptions of Pfeffer at al. (2008) are questionable even for Greenland. They assume that ice streams this century will disgorge ice no faster than the fastest rate observed in recent decades. That assumption is dubious, given the huge climate change that will occur under BAU scenarios, which have a positive (warming) climate forcing that is increasing at a rate dwarfing any known natural forcing. BAU scenarios lead to CO2 levels higher than any since 32 My ago, when Antarctica glaciated. By mid-century most of Greenland would be experiencing summer melting in a longer melt season. Also some Greenland ice stream outlets are in valleys with bedrock below sea level. As the terminus of an ice stream retreats inland, glacier sidewalls can collapse, creating a wider pathway for disgorging ice. The main flaw with the kinematic constraint concept is the geology of Antarctica, where large portions of the ice sheet are buttressed by ice shelves that are unlikely to survive BAU climate scenarios. West Antarctica's Pine Island Glacier (PIG) illustrates nonlinear processes already coming into play. The floating ice shelf at PIG's terminus has been thinning in the past two decades as the ocean around Antarctica warms (Shepherd et al., 2004; Jenkins et al., 2010). Thus the grounding line of the glacier has moved inland by 30 km into deeper water, allowing potentially unstable ice sheet retreat. PIG's rate of mass loss has accelerated almost continuously for the past decade (Wingham et al., 2009) and may account for about half of the mass loss of the West Antarctic ice sheet, which is of the order of 100 km3 per year (Sasgen et al., 2010). PIG and neighboring glaciers in the Amundsen Sea sector of West Antarctica, which are also accelerating, contain enough ice to contribute 1-2 m to sea level. Most of the West Antarctic ice sheet, with at least 5 m of sea level, and about a third of the East Antarctic ice sheet, with another 15-20 m of sea level, are grounded below sea level. This more vulnerable ice may have been the source of the 25 ± 10 m sea level rise of the Pliocene (Dowsett et al., 1990, 1994). If human-made global warming reaches Pliocene levels this century, as expected under BAU scenarios, these greater volumes of ice will surely begin to contribute to sea level change. Indeed, satellite gravity and radar interferometry data reveal that the Totten Glacier of East Antarctica, which fronts a large ice mass grounded below sea level, is already beginning to lose mass (Rignot et al., 2008)." To me the key part seems to be that Hansen & Sato say the current "climate forcing is increasing at a rate dwarfing any known natural forcing", and this will continue under BAU. During interglacials the fastest SLR seems to have been about 2.5 meters/century, according to Rohling et al (2008): http://www.nature.com/ngeo/journal/v1/n1/pdf/ngeo.2007.28.pdf So if Hansen & Sato are right that the climate forcing this century is, or will be, much stronger than during previous interglacials, then it would seem to be reasonable that SLR could be much faster than 2.5 meters as well, if not already this century, then at least in the next ones. So what do we know about the climate forcings, now and during previous interglacials? The initial forcing now is mainly CO2. Back then it seems to have been stronger orbital summer insolation on the Northern Hemisphere. Both apparently cause strong albedo feedbacks. Hansen & Sato seem to argue the current CO2 forcing is an order of magnitude higher than the orbital forcing during the Eemian. If that's correct, wouldn't they have a strong argument for expecting more SLR than Pfeffer et al think likely? So does anyone know if the current forcing really is so much stronger than the one during the Eemian?

Jim, Agreed that an average doesn't show variations on shorter time scales. But to follow your extreme suggestion, what process could cause 2.5 meters of SLR in one year, implying a stable sea level the other 99 years?

Chuck, Interesting, hadn't seen that mentioned before. Another positive feedback by lowering albedo? Or otherwise? Lennart

Jim, Do you have evidence of more than 2.5 meters of SLR per century during an interglacial? Do you think it could be more than 5-6 meters/century in the coming centuries? What do you think of the potentially limiting negative feedback that Hansen calls the 'iceberg cooling effect'? Could that be overcome by a change in ocean currents, as you seem to propose? Lennart

skywatcher, I agree that in the short term in most places other problems than SLR will be more urgent for adaptation. My concern is that the risk of SLR is being grossly underestimated, even in Holland where I live. Meltwater Pulse 1A had a rate of SLR of 4-5 meters/century, as far as I know, maybe as a consequence of circumstances that do not apply in the future. But who knows? The main argument Hansen offers for his higher projections is that the current climate forcing is much stronger than the initial forcing during earlier interglacials. I'm still not clear on the accuracy and implications of that argument, so would love to hear others on that. Lennart

skywatcher, Based on your knowledge of glacier processes, what would you consider reasonable maximum rates of SLR in the coming centuries, and why? Hansen seems to think 5-6 meters/century would be possible under BAU. Rohling suggests 2.5 meters/century was the maximum rate during the Eemian. What is your estimate? To me this question seems quite urgent for proposing and deciding on mitigation policies in the coming years/decade.

Russell, Would that 0.6 meters for WAIS break-off imply this has happened before during interglacials like the Eemian and Holsteinian? Sea levels seem to have been about 8-10 meters higher then at maximum, with a maximum rate of SLR of maybe 2.5 meters/century (during the Eemian). Lennart

Hansen & Sato write about the 'iceberg cooling effect' (p.24): http://arxiv.org/ftp/arxiv/papers/1105/1105.0968.pdf "Exponential change cannot continue indefinitely. The negative feedback terminating exponential growth of ice loss is probably regional cooling due to the thermal and fresh-water effects of melting icebergs. Temporary cooling occurs as icebergs and cold fresh glacial melt-water are added to the Southern Ocean and the North Atlantic Ocean." They then show results from some model experiments (p.25): "By 2065, when the sea level rise (from ice melt) is 60 cm relative to 2010, the cold fresh-water reduces global mean warming (relative to 1880) from 1.86°C to 1.47°C. By 2080, when sea level rise is 1.4 m, global warming is reduced from 2.19°C to 0.89°C." So this may suggest Hansen & Sato think about 80 cm of SLR in about 15 years is the maximum rate of SLR, or about 5-6 meters per century, if this rate could be sustained for so long. It would mean about 2.5 meters of SLR around 2100. That would be from GIS and AIS together, so maybe about 1 meter from GIS at the most during this century. The next two centuries GIS could then maybe contribute 2-3 meters/century, which would mean (almost)complete disintegration by about 2300. This seems in line with Werther's idea about the possible collapse of GIS. The process might very well be along the line of M. Owens' scenario, for all I know, but then a bit slower than he suspects possible. It would be interesting to know what scientists like Hansen and Pfeffer think of these speculations.

Jim, I don't know if that could happen, but even if it did, how could it make GIS collapse in 40 years? That would mean at least 5 meters of SLR by GIS alone, so let's say about a meter per decade. How could the ice from GIS melt of flow fast enough into the ocean? Even Hansen seems to think that's physically impossible, since it would mean about all the extra energy in the earth system would be used for melting ice from GIS and there would be a negative feedback thru an 'iceberg cooling effect'. I agree that faster collapse than the mainstream thinks seems possible, or even likely, but even then there must be some constraints, kinematic, energetic or otherwise. Is Hansen missing something here? Or do I misread him?

Jim, you have to help me here: what do you expect to happen then? How will it make the ice of GIS melt or flow into the ocean fast enough to make sea level rise 5-6 meters in 40 years?

M. Owen: "Until GIS is gone in something like 40 years from today". How could that possibly happen in the real world? I follow Jim Hansen in his concern that GIS and WAIS could disintegrate faster than mainstream glaciology thinks likely, but even Hansen does not seem to think GIS could collapse in something like 40 years. If I understand Hansen correctly he thinks that BAU could lead to maybe 40 meters of SLR in the coming 800 years, so about 5 meters/century, with maybe 2-3 meters until 2100. This risk seems also included in this recent estimate by Meehl et al, by extrapolating past 2300: http://www.nature.com/nclimate/journal/v2/n8/fig_tab/nclimate1529_F3.html My impression is that the relatively slow SLR in the past and missing feedbacks in climate models have deceived glaciologists into thinking it cannot go faster than that. Hansen argues that the current and future forcings are likely much stronger than those in the past, so SLR will probably also be much larger than before. There will be constraints, but these are probably wider than the mainstream thinks. How they could be wide enough to make complete collapse of GIS possible in 40 years, I can't imagine.

Al, I interpret Levermann et al to say that Pfeffer et al are probably correct on GIS, but not on WAIS, or at least the Amundsen sector, so mainly PIG and Thwaites. Pfeffer et al assume an average speed of 14.6 km/yr for PIG/Thwaites in their upper limit of 2 m SLR in 2100, with an almost 40 cm contribution of these glaciers to total SLR. They have been accelerating quite of bit over the past decade and higher speeds than 14.6 km/yr have already been observed for short periods in some other glaciers (up to almost 40 km/yr). Since the acceleration seems to have only just begun, and PIG/Thwaites seem quite unstable, I'm not sure that Pfeffer et al have shown convincingly that 40 cm is their maximum contribution until 2100. But I agree that Hansen & Sato and Levermann and others haven't shown the opposite either.

Going from sea ice models to ice sheet models, this paper by Levermann et al (2012) comments on the kinematic constraints paper by Pfeffer et al (2008: http://www.pik-potsdam.de/~anders/publications/levermann_bamber12.pdf On p.855 they say: "An estimate of a maximum contribution to global SLR from WAIS using the same approach as for GIS (Pfeffer et al. 2008) is questionable since outlet glaciers are less constrained by topography in Antarctica compared to Greenland and thus discharge is potentially quicker than on Greenland." In other words, 2 meters maximum SLR by 2100 may still be an under-estimate, on the same argument that Hansen & Sato use in their paleo-paper. All this not based on ice sheet models as such, since they're judged not good enough yet to be used for such projections.

360 Gt of ice = 360 km3 of water = 1 mm SLR, according to: http://climatesanity.wordpress.com/conversion-factors-for-ice-and-water-mass-and-volume/

"Scientists are either going to face up to adopting the precautionary principle and the need to promote the worst-case scenario (as every military strategist does as standard procedure) or they are liable to be more of a hindrance than a help as the mega-feedbacks take off. If one starts from the premis that the purpose of science is the service of humanity, it could be argued that the failure to adopt the precautionary princple as the basis of prediction is essentially unscientific." I fully agree. Besides Hansen's warning of the risk of multi-meter SLR even this century, the most extreme worst-case scenario I've seen so far is this recent article by Meehl et al: http://www.nature.com/nclimate/journal/v2/n8/fig_tab/nclimate1529_F3.html They show a risk with BAU of up to 12 meters of SLR around the year 2300, with almost 2 meters around 2100 and about 6 meters around 2200. I've still no idea if Al Rodger's calculations are right. It would be nice to know Hansen's reply to those. I've asked him, but so far Hansen hasn't responded. Maybe if others would do the same we could ask him if we could post his reply here?

"From current observations, summer arctic ice disappears at approx. 390-400ppm. GIS presumably disappears somewhere between these two: 400 and 450ppm." Those paleo-data seem to indicate that 400 ppm (Pagani et al) was not crossed the past few million years, or even the past 15 million yrs (Tripati et al), with sea level 15-25 meters higher and no GIS and WAIS, or at least much smaller. Hansen himself thinks 350 ppm (crossed in about 1988) was the point when Arctic sea ice started its self-reinforcing decline, or maybe even earlier. Because of the inertia in the system we're seeing the effects of that today, with much more in the pipeline. To have prevented the Arctic decline, and the polar amplification as a result of that, we should have probably kept the concentration at least below 350 ppm.