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How much will sea level rise?

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… 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).

Reader Interactions

386 Responses to "How much will sea level rise?"

  2. David Benson: “At 330 ppm Greenland will slowly waste away.” I’ll take slowly, especially since it might give us time to adapt. What really concerns me is the probability that we could trigger a massive “natural” ghg release and render any mitigation moot. I think that at 330 ppmv, that probability is small. Of course 280-300 is more desirable, but I don’t see people rushing to sign up for that goal.

  3. David, where on earth do you get the assumption that you can just double the SLR of 2100 to get the SLR for 2200? This is the kind of nonsense that leads to real disaster. And you are totally ignoring the effect of sea level rise on the WAISDce shelves.

    Wow! That is really all I can think to say.

  5. Thanks David (349). I appreciate your effort to make a stand.

    Would all of the RC scientists also feel confident taking 2m for 2100 to be a prudent upper limit without adding further ‘uncertainty’ factors? Imagine the siting of major highways or rail or a new suburb.

    What I am trying to get a handle on is how to approach planning for development as opposed to building of single buildings.

    For example, if an existing suburb was sited in an area say 2 to 4m above sea level where the setback was some distance from the shore (to avoid coastal erosion) should new houses be allowed? (Scenario A)

    The other side of the coin is to say, for the same location, if there was no suburb already there, should a new suburb be allowed? (Scenario B)

    So if 2m was to be taken as the 2100 target, new houses might be OK for an existing suburb as they may be expected to last well into the next century. Whereas, a new suburb should not be allowed as it would be in the wrong place for the longer term.

  7. Rick, what about the size of the swell?

    If your annual event is a 2m swell then when you were 4m above the sea level, you’re safe from it. When the sea moves 2m higher, you’re not.

    Worse, climate change means that the greater energy in the system will increase the size and severity of storms.

    Your 10-year 4m swell event may become an annual occurrence.

  8. I despair. What is the science-based target now?!!! Lennart’s excellent post on #336 cites other papers saying 450ppm is far too dangerous a target. And, once again, Real Climate contributors are totally silent.

    I am trying to get a UK-baesd initiative off the ground, but at the moment do not have a target of any kind I am comfortable with a) working and b) getting widespread scientific support. What hope of any action – anywhere – without one? At this point, it really is no use citing the IPCC reports when so many climate scientits (including a very public, very vocal and very qualified Hansen) suggest that these targets will do no good whatsoever. I’m not concerned about what is politically acheivable yet – I need to know where we can aim first, and then figure out if we have any chance of getting there afterwards (though I am aware that Hansen says 350ppm is definitely do-able). Any other method of proceeding seems irrational to me.

    What does it take to get an RC response to this? So many other responsants have been helpful, but we really need something meaty directly from the experts in the field. This is now my 7th attempting at getting a response over 9 days – not even a “we’re working on it”, a link to a previous response or a reason why there hasn’t been a response. Help! Don’t you understand why this target is so important?! Or if it is unimportant, or totally un-knowable, please explain why!

    I despair…

  9. But are you not saying it is the increasing storm surges that current coastal dwellers, their children and their grandchildren should be more worried about?

  10. Ron Taylor (353) — Pfeffer et al. consider WAIS to be stable (at least up until 2100 CE). So until somebody finds a paper suggesting otherwise, 2 meters, possible up to 4 meters by 2200 CE seems to account for potential WAIS melt.

    Ricki (355) — Planning for how far in the future? If we don’t reverse the growth of CO2, the sea level will continue to rise for many, many centuries.

    Guy (358) — Joe Romm (ClimateProgress; physics PhD) is pushing for 420 ppm, tops. As you know Jim Hansen has proposed ‘less than 350 ppm’. I agree with both, but go further in stating than the sooner we get back to about 290 ppm, the better.

    sidd (357) — Thank you for the link to the map and I stand corrected. However, EAIS is certainly going to remain stable, at least for a long time. The part of WAIS grounded below sea level is potentially worrisome.

  11. Thanks for all of the comments and Real Climate’s patience, justice, and fortitude in allowing me to post my points.

    The problem is that most people thinking about the behavior of the GIS have assumed substantial sea ice, and therefore a cold, dry Arctic. Based on water temperature changes detected by buoys, in Feb., 2002 I predicted a sudden and rapid decline in Arctic Sea Ice. These predictions were ridiculed. I understand and accept ridicule. Nevertheless, for the last 5 years, I have been thinking of the implications of a warmer, moister Arctic that has less sea ice, and therefore absorbs more solar radiation.

    Bindschadler et seq. (http://yosemite.epa.gov/oar/globalwarming.nsf/content/ResourceCenterPublicationsProbability.html ) do not address how the strength of ice varies with temperature, nor do they address any possibility of an ice free Arctic ocean as a source of latent heat. The combination of (latent) heat from the Arctic/North Atlantic and the non-linear function of ice strength with respect to temperature means that GIS collapse is possibly enormously more rapid that anything considered by Bindschadler et seq.

    We can project loss of sea ice, and resulting available latent heat. Any good engineering library has resources with equations of ice strength under various conditions that can be solved by any student with a computer. So, it is not reasonable to say that this is beyond science. It may not be a calculation that has been published. It may not be a calculation that some science agency manager wants to see published, but it is not beyond science. This is the Lake Missoula Model, (a watershed dammed by ice) and that is science. The result is the flow of a mix of solid ice and water into the ocean at high rates of speed. Such flows can scour any gap they need through any type of rock – in a few hours. The real question is, “Do you accept the possibility of an ice free Arctic Ocean in the near future?” An ice free Arctic Ocean has implications.

    The assumption that summit ice will be protected from the warmth at lower altitudes is also not correct. We have that assumption because the summit ice that we normally see sits on rock (i.e., the glaciers in the Alps, Cascades, & etc.). However, in Greenland, the ice massifs are supported by ice, not rock. Latent heat can/will eat the GIS from the bottom up.

    How likely is this concept of high speed water/ ice flows? Get out your geologist alter ego and go look at places where we know ice sheets retreated. Many of these locations have evidence of high speed, high volume ice/water flows. That makes me think that my physics is correct. I do not appeal to authority or citations, because I cannot find any peer reviewed literature that is contemporaneous with previous ice sheet retreats. 

  12. Of course the storm surge, tide, wave action, etc would have to be taken into account. This adds a bit of complexity, but is in any case regularly assessed by coastal engineers.

    The increase in storm intensity is less predictable than general sea level rise as there may be local and regional climate effects. In any case I believe this to be a lesser effect than the rise itself as the coastal erosion will be driven by the SLR even though the erosion will largly take place during storm events.

    On the issue of how far ahead we look — I do not think we can look further than 100 to 150 years as far as infrastructure planning is concerned except that caution should be exercised for extremem hazard situations such as nuclear plants. These are generally dealt with by regulators on a case by case basis anyway.

    For normal infrastructure planning, structures will be expected to last for 70 to 120 years before we replace them. Also, in the next 20 years it will become clear both how big the climate change impact will indeed be and how much effort the human race is willing (or able) to expend to counter it.

    Therefore, we can limit ourselves to planning for the next 100 to 150 years for the time being.

  14. For those interested in considerable detail:

    Climate and Sea Level Change: Observations, Projections and Implications
    By R. A. Warrick, E. M. Barrow, T. M. L. Wigley
    Published by Cambridge University Press, 1993
    ISBN 052139516X, 9780521395168
    424 pages

  15. some information about antarctic and greenland glaciers, with maps of elevation changes and marine glaciers may be found here:

    Shepherd and Wingham, “Recent Sea-Level Contributions of the Antarctic and Greenland Ice Sheets” (2007), Science, v315, pp1529-1532

  18. OK… I gave up reading them all in the 200s…

    1. Richard C ain’t me (and our names are backwards, I’m more spacey!) And Richard Wakefield… please try to live up to your Richardness. Your deliberate suppression of any logical talent you might have embarrasses your fellow Richards. I’ll dedicate this post to you, in the hope that you’ll take off those blinders:

    The two ice sheets in question, Greenland and WAIS, share common features – they are way below sea level and bowl shaped. They got that way to a certain extent by their very mass. The “pry” verb Peter is questioning is about tides and only applies at the grounding line. In fact, we can (mostly) ignore everything except the grounding line. Beyond it, it’s just floating ice. Twice a day the grounding line moves. Tide comes in, the ice lifts, and the grounding line retreats. Tide goes out, the grounding line advances. Beyond the grounding line, ice randomly breaks off, floats towards the equator, and melts. Everything beyond the grounding line is secondary – it affects flow, but flow might actually slow the retreat! Yep, it looks bad on the surface, but for t+50y, it’s a different story, since the ONLY important result is grounding line advance or retreat and increased flow tends to advance the grounding line. Kind of like La Nina – which warms the planet by cooling the surface – the cooler the surface, the more radiation imbalance there is and the faster the planet warms! That we live at the surface/atmosphere boundary is just anthro-bias. Ask a deep water fish whether La Nina is a warming or cooling event.

    Since the ice sheet is bowl-shaped, the grounding line by definition is the lowest part of the system. Thus, the heaviest water will travel there. Meltwater is salt-free, and so light, while warmer seawater will displace it. The meltwater will travel outward, until it freezes. Thus, the edges of the ice shelf are thicker than the grounded edge. So we have a thermohaline pump, with heat from the ocean eating the grounding line competing with the ice flow from the centre of the ice sheet. In WAIS, the ocean is “winning” at a rate of 120 metres per year. Since the distance between Ross and Ronne is about 1000 km, 1000/.24 = 4000 years to complete instability. Add in any sort of acceleration factor, and a few centuries becomes the best estimate. Greenland is NO different from WAIS except it is in a warmer environment and has significantly better mountain protection – almost 360 degrees. But once the grounding line get past the mountains, Greenland’s situation deteriorates phenomenally. The Gulf Stream’s NW offshoots will infiltrate southeastern Greenland (note the GRACE ice loss maps), break off icebergs, and flush them to the tropics. There, they will cool the surface, which, as per La Nina, will warm the planetary system by increasing radiative imbalance. GIS has less than 100 years of life left. Folks laughed when I said ice-free Arctic Ocean in 2020. Seems I was a tad optimistic 15 years ago. I’m probably optimistic when I say ice-free Greenland Sea in 2100. The key thought-process is that ice is *there* or *not* at a single temperature, and ocean currents are big. Look at Sidd’s map of Greenland: http://membrane.com/sidd/greenland.html
    Once currents can travel between the two big mountain passes in southern Greenland, it’s game over.

  19. Gavin
    Your statement of a 30C°/3km lapse rate in Greenland seems confused. The dry rate is 10C°/km but the average environmental lapse rate is closer to 6.5C°/km. Your point, however, is well taken
    woody

  20. woody,

    You’re right that the mean tropospheric lapse rate is about 6.5 K/km, but that’s a global average, and depends on the water vapor cycle and the release of latent heat. Greenland may be cold enough that there’s very little water vapor above it (cf the Clausius-Clapeyron law), and thus the lapse rate may be closer to the adiabatic 9.77 K/km.

  22. Aaron, just for chuckles, the Telegraph article commits the classical blooper of converting 4 degrees C temperature rise to 39.2 degrees F… “according to scientists”. Yeah, sure ;-)

  23. #371 Aaron. Yes it’s real science – see #314. My layperson opinion is that it probably only relevant to Greenland melting in the context of additional total global warming, since greenhouse gases are relatively well mixed over short time constants in the atmosphere and between N & S hemispheres; although, a continuous and large release over a summer could boost CH4 concentrations and greenhouse effect in the northern hemisphere.

  24. Re Aaron @371, oh it’s real alright. Increasing methane off-gassing is one of the oft’ mentioned potential tipping points.
    It may be time to start being afraid. Very afraid.

    Captcha: wheat First

    (We can’t eat oil.)

  25. 371 Aaron, it’s just an expected carbon feedback which means that unless we can get to 350ppm (or 300) by a few years ago, staying below 1000 (CO2 equivalent) will be durn difficult. This means that slowing human CO2 emissions is now silly, as 2ppm isn’t squat in the new world. Take a match and light the straw in a barn, then try using a squirt gun to put it out. Woulda worked for the match… None of this is news. Permafrost temperatures have been tracked for a long time. They’re about to go above 0C in large swaths of Alaska and Siberia. Arctic ocean temps have been studiously recorded. “Gee, it’s getting rather warm.” Clathrates form to the extent current temperature allows, and melt from the bottom up as things warm. The melted clathrate expands ~160 times in volume, so pressure builds up, the thinning clathrates above break, and a huge natural gas leak forms. (Though pressure release drops temperatures, so the leaks can re-seal if not too bad.) Again, not even close to news. We KNEW all this many years ago.

    The Deniers are now right, even though they are totally wrong. It is silly to reduce CO2 emissions since building levees and dikes and moving ports and cities takes concrete and lots of fossil fuel. We might as well emit the CO2 and save a couple billion lives instead of the 500 million predicted by Lovelock. Letting folks divert it to Monster SUV or military use isn’t wise, though.

    Notice that the melt zones on Greenland now stretch from coast to coast, and center on the two big gaps in the mountains. Soon, the oceans will invade, burrow through the sheet and start flushing. I bet Pfeiffer et al didn’t count on Siberia joining the game, so they got crap results. (If I’m wrong, let me know) When one considers the ENTIRE system, instead of just one’s own tiny specialty, then completely different results appear. Since it takes a couple years to come up with results, folks doing the science use outdated assumptions with regard to systemic conditions. So the game of catch-up for related disciplines never gets better. (Though they do their best. This isn’t an insult, just a condition resulting from the tremendous pace in change of climate and knowledge thereof)

    The alternative is to start brimstoning the atmosphere, but Russia disagrees, and has nukes and an attitude.

  26. 371 Aaron – or anyone.

    http://nsidc.org/data/google_earth/images/permafrost.jpg
    This map shows a very extensive undersea permafrost in the Arctic ocean covering almost all the shallow continental shelves. I believe it is there since the last ice age – but how come it has been maintained for so long? Can that be just because of an extraordinarily slow response, or is there some process maintaining it? How would that process be changing now?

  27. Richard C., Actually, there is every reason still to try and hold CO2 as low as possible. Even if (and it is still if) we can’t keep things from getting bad, it is quite another thing to contend that we can’t make them worse. We will do much better at both mitigation and remediation if we keep the changes from occurring rapidly on a human scale.

  28. 376 Pekka, grand map. The sea north of Siberia used to be covered with ice. Now it’s ice-free, and the ocean is only 10 to 25 metres deep http://mappery.com/map-name/Arctic-Ocean-Bathymetric-Map The rivers have increased their flow, too. Clathrates are only stable as long as the regime doesn’t change. If one assumes that Mann’s hockey stick is anywhere accurate, then any clathrates that would have released would probably have done so long ago. (They probably did!) Those that survived up until now were stable at Hockey shaft conditions. But since Siberia and the Arctic Ocean have warmed tremendously, the zone of stability has shifted. Everything outside the new zone will thaw and release. That will further warm the planet, and so the arctic, which will further destabilize clathrates. We’re just along for the ride…

    http://nsidc.org/images/arcticseaicenews/20080924_Figure3.jpg Another good map, which compares the health of the arctic ice cap between 2007 and 2008. As you can see, much of the ice in 2008 is first year ice (age is a good proxy for thickness and strength). First year ice is full of salt and so especially weak. Instead of the small recovery extent suggests, 2008 is significantly worse than 2007, and it sets up a catastrophic 2009.

  29. 3rd series of attempts to post this … Captchad! (Sounds like a Florida election official) This one has to work, Captcha: Estimate kindled

    73 Aaron asked, “The question stands. What is the value for SLR that you are 99.9999% sure will not be exceeded in the next 100 years?” 6 metres for GIS, 6 metres for WAIS, 10 metres for EAIS, and 2 metres for thermal expansion and glaciers and fudge. Then the converse, 0.4 metre for GIS, 0.4 metre for WAIS, -0.3 for EAIS and fudge. So, 0.5 metre

    [Response: Are you using a < symbol? use the html instead &lt; – gavin]

  30. Thanks, Gavin… So, 0.5 metre < rise by 2108 < 24 metres. (Unless mankind does something to change the game, of course.)

    88 Iblis, I did a quick guesstimate of the major depression volume, and it was a piddly 5cm of sea level mitigation or so. (Don’t quote me!)

    181 Bruce – Yep, the difference in CO2 levels between the Eemian and today is key, since increased CO2 increases polar temps, so it takes a lower global temperature to affect polar ice (though the estimates of Eemian temperatures could be in error for exactly the same reason). Add in the much higher CH4 levels today, and the Eemian isn’t so grand a model.

    377 Ray, we’ll know pretty soon. If human emissions get dwarfed, then it’s a new ball game. Yep, keeping CO2 as low as practical would still be right, but the CO2 still needs to be spewed. Moving people out of harm’s way takes a lot of energy, and the only developed energy source we have is carbon. It will take every bit we’ve got and more, so burning dinosaurs even harder will be needed, unless we’re willing to do extensive triage and write off quite a few billion people. If this does come to pass, it will test the US’s adherence to their core concept, that “All humans are created equal.” Is it Animal Farm? “But some humans are more equal than others.”

    Yep, I used the word “now” in a deliberately vague and somewhat wrong context for emphasis and was tongue-in-cheek too (even said Deniers were RIGHT!). So knowing the methane data that has just come in, what is a good guesstimate on the odds that we’ve already initiated a methane release which will ramp up enough to take us over the edge regardless of any reasonable CO2 emissions scenario? I’d say 80%.

  31. This was released yesterday about the Wouters et al., GRL paper (‘GRACE observes small-scale mass loss in Greenland’). Basically their findings are that Greenland currently accounts for 0.5 mm/yr sea level rise. Their seems to be a speed up in the last few years too (record loss in 2007), although the authors mention that more observations are necessary to come to reliable predictions for the future:

    An accurate picture of ice loss in Greenland

    Resarchers from TU Delft joined forces with the Center for Space Research (CSR) in Austin, Texas, USA, to develop a method for creating an accurate picture of Greenland’s shrinking ice cap. On the strength of this method, it is now estimated that Greenland is accountable for a half millimetre-rise in the global sea level per year. These findings will be published in the scientific journal Geophysical Research Letters in early October.

    The research was based on data from the German-American GRACE (Gravity Recovery and Climate Experiment) satellites, two satellites that have been orbiting the earth behind each other since mid-2002. Deviations in the earth’s gravitational field cause fluctuations in the distance between the satellites, which is measured to a precision of a millionth of a metre. As gravity is directly related to mass, these data can be used to plot changes in the earth’s water balance, such as the disappearance of the ice caps. Satellite data of this kind are ideal for measuring areas such as Greenland, where the extreme conditions make local measurements very difficult. With this in mind, researchers from TU Delft and the CSR devised a method that would create a more accurate picture of the changes taking place in Greenland than had previously been possible.

    Sea level rise
    Greenland lost an average of 195 cubic kilometres of ice per year between 2003 and 2008, which is enough to cause an annual increase in the global sea level of half a millimetre, or 5 cm over the course of the next century. A report recently published by the Dutch Delta commission estimated that the melting ice cap in Greenland would cause the sea level to rise by 13 to 22 cm by 2100. But these two figures do not necessarily contradict each other: whereas the first two years of the study showed a loss of 131 cubic kilometres of ice per year, during the last two years this figure had risen to 222 cubic kilometres per year, an increase of 70 percent. This sharp increase was mainly caused by the extremely warm summer of 2007, when more than 350 cubic metres of ice melted in just two months. However, it is not yet clear whether the ice will continue to melt at this rate during the next few years, as ice loss varies greatly from summer to summer. Long-term observations are needed to compile a reliable estimate of Greenland’s contribution to the rising sea level during the next century.

    Patterns
    The method used also enables scientists to plot the loss of mass per region, thereby providing new insight into the patterns of ice loss. For example, for the first time since measurements were started, the extremely warm summer of 2007 saw a decrease in the ice mass at high altitudes (above 2,000 metres). It also became clear that the ice loss is advancing towards the North of Greenland, particularly on the west coast. The areas around Greenland, particularly Iceland, Spitsbergen and the northern islands of Canada, seem to be particularly badly affected. A follow-up study will focus on the influence of these smaller glaciers on the sea level.

    More information
    Wouters, B., D. Chambers, and E. J. O. Schrama (2008), GRACE observes small-scale mass loss in Greenland, Geophys. Res. Lett., doi:10.1029/2008GL034816, in press. http://www.agu.org/journals/pip/gl/2008GL034816-pip.pdf

  32. 361 Aaron, grand post. Note that Greenland’s hip, from the bay near Tasiilaq to the Jakobshavn at Ilulissat, is under 600km wide. The “eating from the bottom up” of ice you mentioned will focus there. Lower ice gets eaten by the ocean and warm moist air, and higher ice either fractures and dumps, or flows into the terminus. This temporarily helps replace or protect the low ice from further erosion. Since the base of the sheet is far underwater, it’s a wet-melt system. Exponential melt is likely once the triple-whammy of an ice-free arctic ocean (can you imagine what the summer rainfall will be in Greenland?), increased ocean temperature, and melting past the mountain passes occurs. Tides will work to clean-up ice debris at the passes between the mountains, such as at Jakobshavn and Tasiilaq, so ice flow will be fastest there, and the ice will slump. This will attract melt and rainwater on the surface, and moulins below. Combine the three forces and a groove will begin to show, getting deeper and more fragile until the smallest of paths is cut through Greenland’s hip and melt-rate becomes essentially unlimited as currents replace tides and outflow as the clean-up crew. That’s 2×300 km to 6 metres of sea level rise.

    Then again, Greenland’s hip didn’t fracture and and the ice sheet mostly survived in all the other interglacials. Of course, that was with 1/3 less CO2, 2/3rds less CH4, and a whole lot less ice-darkening soot than today. Greenland’s temperature at the hip seems to be a bit higher today than during the warmest part of the Eemian (http://www.worldclimatereport.com/index.php/2007/10/16/greenland-climate-now-vs-then-part-i-temperatures/ shows Jakobshavn’s temp rise, http://www.uni-mainz.de/FB/Geo/Geologie/sedi/Deklim/ppt_xls/Hamburg/eem_web_kaspar_cubasch_english.pdf shows an Eemian simulation), and today still has summer ice in the arctic ocean to melt, so another temperature spike is all but certain. The Eemian’s warm period was pretty short, too. Our Anthropocene era comes on the tail of the Holocene, which was 10,000 years long, so Greenland has had a long time to prep for the meltdown. Interesting times ahead.

  33. In reply to

    Post 10
    Andrew Says:
    4 September 2008 at 11:35 PM

    Hi Andrew , i dont know if you are aware of the palma volcano that is just off of the west african coast ?

    A snipet …..

    NARRATOR: The big question is where and when the next large volcanic island collapse will occur. Because these landslides all happened in the ancient past no-one has ever witnessed one. They are so rare scientists cannot be sure what the precursors will be, but of all the large volcanic islands around the world one in particular shows disturbing signs of instability. If this island collapses it would create a mega-tsunami that would race across the Atlantic and hit the east coast of the United States. Every city on the shoreline would be destroyed. From New York in the north to Miami in the south. The wave would wreak havoc for as much as 20 kilometres inland. The origin of this wave would be thousands of miles away. This mega-tsunami would come from a volcanic island off the coast of North Africa, from one of the Canary Islands. It would come from the island of La Palma. La Palma is one of the western-most islands in the Canaries. 80,000 people live here making their living from farming and tourism. There are also two volcanoes on the island, one extinct, one active. In the early 1990s a British geologist travelled to the island to study the active volcano called the Cumbre Vieja.

    Please find the whole convo at

    http://www.bbc.co.uk/science/horizon/2000/mega_tsunami_transcript.shtml

    Reading it is very alarming .

  35. I have a question, which came to me this week as I was in the public viewing gallery when Dr James Hanson testified on carbon emissions to the UK select committee on climate change, to which I still haven’t received a satisfactory answer.

    My question is, whether building a dam across the Strait of Gilbraltar, which is only 14.2km across prevent rising sea levels in the Mediterranean and would it also help global warming by stopping the hotter Mediterranean sea from warming global sea levels?

    I know that thousands of years ago the Mediterranean was naturally dammed by an ice wall (and the sea evaporated). Building a man-made dam would be technically feasible and excessive evaporation could be avoided by simply opening the dam now and then. It would certainly help the countries that have a Mediterranean coastline avoid rising sea levels. I was wondering whether cutting the hotter Medterranean off from the rest of the world’s oceans would also have a cooling effect on the world’s oceans through oceanic currents? No doubt there are other massive environmental implications of such a project that could render it unworkable. Just a thought.

  36. #385 Brian

    Well here’s my shot in answering…

    >My question is, whether building a dam across the Strait of Gilbraltar, which is only 14.2km across

    pretty long and on a small quake zone, but maybe.

    >prevent rising sea levels in the Mediterranean and would it also help global warming by stopping the hotter Mediterranean sea from warming global sea levels?

    plus the additional effect of hotter Mediterranean evaporating more rapidly, giving rise to water vapor -> eventually clouds… increasing turbidity in the area… I can’t work this out.

    >I know that thousands of years ago the Mediterranean was naturally dammed by an ice wall (and the sea evaporated).

    You’re probably talking of ‘Messinian salinity crisis’ (but my source is Wikipedia :-) ), I’ve understood that was caused by a series of earthquakes blocking the Gibraltar.

    >Building a man-made dam would be technically feasible and excessive evaporation could be avoided by simply opening the dam now and then.

    No need for this as there is Suez Canal, I don’t know if it is wide enough for the flow.

    >It would certainly help the countries that have a Mediterranean coastline avoid rising sea levels.

    Agreed. No reason to believe someone would terrorize it.

    >I was wondering whether cutting the hotter Medterranean off from the rest of the world’s oceans would also have a cooling effect on the world’s oceans through oceanic currents? No doubt there are other massive environmental implications of such a project that could render it unworkable.

    See at least “Lessepsian migration”, “eutrophication” can happen on a large scale as in the Baltic Sea.