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How much will sea level rise?

Filed under: — group @ 4 September 2008 - (Español) (Italian)

… 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).


386 Responses to “How much will sea level rise?”

  1. 301
    Richard Wakefield says:

    Lennart Says:
    12 September 2008 at 10:20 AM

    How much risk are we willing to take with the lives of our children and their children? I’m 37 now. My sister is 27 and had her first child last week. My niece will hopefully be 70 in 2078. Her children could be 70 by 2105. What kind of world will we leave for them?

    Then you had better have a serious look at peak oil. By the time your niece is 50 we will be a much less energy depleted world, according to the peak oilers. For example, at China’s current energy consumption rate of growth, within 15 years they will require all the world’s current production of oil — 86 million barrels per day, leaving nothing for everyone else. Watch the video series http://www.chrismartenson.com/peak_oil. Matt Simmons, who accepts AGW, says that peak oil is the far sooner and far worse risk than AGW.

  2. 302
    Hank Roberts says:

    Richard C. wrote

    Well, the field scientists share your concern. This is an interview, with reference to a 2005 paper.

    http://www.mnforsustain.org/climate_change_greenland_glaciers_tulaczyk_1105.htm

    —-excerpt_follows—–

    …Many fiords, the channels carved by glaciers flowing into the sea, are deep with a shallow lip in front. Once the glacier floats off this shallow pinning point, it retreats into deeper water, making further disintegration likely. Reduced friction between ice and rock at the glacier bed can also increase glacier speed. Fiords often widen inland, causing the glacier to grate less heavily at the fiord walls and move faster as it retreats. And ice crystals in fast-moving glaciers can realign, further reducing friction, Howat said.

    The Helheim glacier’s speedup has already propagated 12.5 miles up the glacier. The center of the Greenland ice sheet is only 150 miles inland, and the researchers worry that the effects of the glacier’s retreat will continue to move inland, ultimately decreasing the thickness of the whole ice sheet.

    “If other glaciers in Greenland are responding like Helheim, it could easily cut in half the time it will take to destroy the Greenland ice sheet,” Howat said. “This is a process we thought was only happening in Antarctica, and now we’re seeing that it happens really fast in Greenland.”

    ——end excerpt——-

    As a reminder, this was the state of knowledge 75 years ago.

    http://www.jstor.org/pss/519888

    It’s comparisons like this about what we learn over time that make the Fermi Paradox interesting.

  3. 303
    Dan Robinson says:

    I’m wondering if any scientists have considered the following positive feedback factors in their climate prediction models.

    Just as water absorbs more visible sunlight than land, and turns it into heat, water absorbs more than land, to various degrees. As sea levels rise, the ratio of water to land surface increases, thereby decreasing the albedo of the earth in general, I wonder by what percent. I’ve seen no mention of these factors.

    Please copy responses to danrob@efn.org also.

  4. 304
    David B. Benson says:

    Lennart (294 & 295) — The comments from J. Harper are about the paper which is the main topic of this thread. This paper appears to discount the possiblity of much contribution from WAIS; this seems sensible until the ozone hole closes. After that, possibly James Hansen is correct.

    The is a bludgeonly simple way to remediate the problem; all it takes is about 1–2% of the world’s gross product, that’s $670–1340 billion per year. Just grow lots of biomass, use pyrolisys to produce biochar and sequester the biochar underground. If an average of 14 billion tonnes of carbon can be sequestered this way every year, in 100 years the CO2 concentration will be back to near an acceptable 280 ppm. (Obviously many variations on this idea are possible, and probably necessary due to peak oil and soon, peak coal.)

  5. 305
    Hank Roberts says:

    > pyrolysis, biochar

    I haven’t yet found a description of an actual built (or designed) processing station for doing this. Pointer welcome.

    Some numbers:
    http://pubs.acs.org/cgi-bin/abstract.cgi/esthag/2008/42/i11/abs/es071361i.htm

    Some practical advice from a real farmer:
    (In the context of cellulosic ethanol, but same applies to biochar):
    http://littlebloginthebigwoods.blogspot.com/2007/10/fuelish-fantasies.html

    —-excerpt——-

    Each of these processing plants will be surrounded by… switchgrass. Millions of acres, altogether.

    Close your eyes, and see it: hundreds of thousands of contiguous acres- of DRY grass. 8 feet tall.

    It has to be big, mature, and dry- for any of the fantasy to work. It turns out that if you cut it when it’s green, you seriously weaken the roots- no crop next year. And if you cut it when it’s green- you’ll either have to use it right now, or spend energy drying it, so it won’t rot.

    Are those huge fields of dry grass going to burn? Yes, they will…

    Will such fields always burn? Of course not. Will they burn often enough to make the whole proposition uneconomic? YES.
    ———end_excerpt———

  6. 306
    David B. Benson says:

    Hank Roberts (300) — Start here:

    http://terrapreta.bioenergylists.org/

    but also note that torrification is a form of mild pyrolisys and searching on that term and also torrified wood will find many upon many references; there are at least three torrification reactors in The Netherlands producing torrified wood.

    For slow (faster than mild) and fast pyrolysis, this review

    http://terrapreta.bioenergylists.org/node/578

    is quite useful.

    I opine that Greenpa is a bit fixated on the fire problem; you and I have been through that before here on RealClimate. If an actual problem, grow something else other than switchgrass.

    [Captcha chimbs in with "works consequences".]

  7. 307
    Nigel Williams says:

    In December 1991 the top 10 metres fell of the summit of Mount Cook – the highest (3750m) peak of New Zealand. 10 million cubic metres of rock and ice slid into the valley. With the benefit of hind-sight it is likely that the peak was acting as a thermometer – detecting the mean temperature of the local environment.

    As temperatures rose the tensile strength of the ice holding the peak together reduced to the point where it could no longer support its own weight – and off it came.

    So even in 1991 the climate was changing the properties of alpine ice. Not necessarily melting it but definitely changing the way it behaves – its dynamics.

    Its these sorts of changes which are in my view most likely to lead to an acceleration in ice sheet loss – shifting still frozen ice below the melt line unpredictably fast.

    The persistent dislocate between global mean temperature and AGW forcings can only be the result of sinks like ice and water continuing to absorb energy. With every gram of ice that melts there is more energy to apply to the rest. So as the sinks go all the ‘in the pipeline’ temperature rise will come with a rush.

  8. 308
    Wili says:

    I would like to second Dan’s (#303) concern about whether potential feedbacks have been taken into account.

    Besides the one he mentioned, what about changing albedo of the surface ice. Even small amounts of black soot and other dark particles on the surface and embedded in the ice will concentrate more and more over time on the surface as the ice around it melts. Higher concentrations of dark particles means faster melting which means yet higher concentrations of dark particles…

    We see this process every spring in Minnesota until the last ice and snow is almost totally black on the surface. How much of a factor is this likely to be, and has it been factored into the calculations?

    Also, with the loss of the overburden, the ground below will start to rise. Places that are now flat will become convex. Even a tiny change here could have large impacts. A marble on a flat table can remain stationary, but tilt the table even a tiny amount and the marble quickly rolls off. And of course this is a feed back too–the faster the overburden slides off, the faster the rise of the land below which further accelerates the slide….

    And these rises don’t always happen gradually, but sometimes happen suddenly, essentially as earthquakes. Surely these would also hasten the collapse of the ice and the rates of flow to the sea.

    I imagine that it would be difficult to model these, but if they are completely left out just because they are hard to model, wouldn’t that make the models quite consistently under-predict the rate of ice loss?

    [Response: This is hard to model, but not impossible Hansen and Nazarenko (2004) made a first attempt and more sophisticated treatments are coming along as well. It remains to be seen whether current ice losses are related to that since black carbon in the northern mid-latitudes has decreased in recent decades (big increase in the tropics though). - gavin]

  9. 309
    Wili says:

    Wow, thanks for the fast reply. I’ll check out that ’04 article.

    So are you admitting that most of the models are not accounting for such feedbacks and so we should not be so certain about a 2 meter limit by 2100?

    As I was making my earlier post, it occurred to me that, though current black carbon deposits are important for getting the melt going now, what will really accelerate the feedback is the concentration levels of dark matter in the older ice as it melts.

    So from what you said, I would expect a fair amount of acceleration from this source as melt reaches those sootier years, then dropping to not as much acceleration as it reaches ice from less-sooty pre-industrial times.

    But I’m probably missing something obvious here.

    While we’re on feedbacks, how worried should we be about recent reports of methane levels 1000 times background levels in parts of the Arctic Ocean. If the trigger has been pulled on the “clathrate gun,” are all bets off for rates of melt of the Greenland ice?

  10. 310
    Aaron Lewis says:

    Hank #297
    CALFED Delta contains all three kinds of projects at or below sea level. So what is the design life of a levee to protect threatened and endangered species? How much can we expect the sea level to rise within that design life?

    I am afraid that we are going to spend $40 billion on engineering/construction using the IPCC SLR due to thermal expansion value, and in 20 years, the levees will wash out killing all the little critters and contaminating the drinking water sources for 20 million people with salt water. I hear from other sources that scoping studies for CALFED habitat protection work are being proposed based on the IPCC sea level rise from thermal expansion value.

    Somebody (with the backing of a federal agency) needs to stand up and say, “Sea level rise will not exceed XXX meters under any conditions in the next 50 years. And, this is the value to use for critical engineering.” This one project gives having such a number a value of maybe $20 billion, right now. Add in the value of other costal infrastructure projects being currently planned; and, having a good number for SLR is currently probably worth a trillion dollars right now.

    My guess is the IPCC understating SLR will cost the global economy trillions of dollars over the next 50 years. It is probably the single most expensive gloss ever made in any report, anywhere, anytime.

  11. 311
    Hank Roberts says:

    Wili, a cite or link please to the “recent reports” you refer to?
    I tried some searches for 2008 with Google Scholar and couldn’t find it.

  12. 312
    David B. Benson says:

    Wili (309) — The research in the paper which is the subject of this thread did not use GCMs. They used observed maximum rates to conclude that 2 meters in 2100 CE was the maximum they could foresee.

    The paper does not appear to consider what will happen in Antarctica when the so-called ozone hole begins to close. I opine that greater ice losses will then occur.

  13. 313
    Guy says:

    Aaron Lewis said: My guess is the IPCC understating SLR will cost the global economy trillions of dollars over the next 50 years. It is probably the single most expensive gloss ever made in any report, anywhere, anytime.

    I have no idea what you are talking about. The IPCC specifically stated in the FAR that the sea level projections excluded dynamic ice-scheet processes. If you are saying that people have taken those figures to mean ALL potential rise, then that is their fault for not reading the report properly, and not that of the IPCC. No gloss.

    Personally I think you will never find a scientist predicting 99.9999% certainty. Well, they could for an 80m rise! The new report (which is the theoretical subject of this thread) says 2m per century, but the discussion here reveals this is still a little tentative, as it is thought that geologically rises of up to 5m have been possible in a century (again, not certain though). The science is advancing as fast as possible… more time, money, resources will improve the accuracy of phenomenally complex problem analysis, but currently is looks like you are asking for absolute certainty (and 0.0001% counts as 0 in my book) where there is none.

  14. 314
    Brian Dodge says:

    re 295 Richard Wakefield “Does this take in account additions each winter?”
    Yes; Howat et.al. are measuring ice mass balance, the difference between annual loss and accumulation. Also, if a volume of ice equal to that melted annually in the Arctic ocean melted from the Greenland ice sheet, very little of it would be replaced by snowfall (at current rates), and there isn’t underlying ocean water to re-freeze.

    re 311 Hank Roberts – methane releases in the Arctic -

    http://www.cosis.net/abstracts/EGU2007/01071/EGU2007-J-01071.pdf?PHPSESSID=e
    “However, our recent study in the Laptev Sea and the East-Siberian Sea (LESS) showed that CH4 supersaturation of surface water reached up to 10,000 %, implying that strong air-to-sea fluxes must occur at times.”

    http://westerstrand.blogspot.com/2008/09/methane-hot-topic.html
    “At around 110 degrees easterly longitude, when we where wrestling with drift ice in western Laptev Sea, we discovered two new areas where methane concentrations in both the water and in the air above clearly exceeded the normal methane concentration in Arctic. A few days later a new area in the eastern Laptev Sea was discovered at 133 easterly longitudes.”

    http://www.nature.com/nature/journal/v443/n7107/full/nature05040.html
    “Here we report a new method of measuring ebullition and use it to quantify methane emissions from two thaw lakes in North Siberia. We show that ebullition accounts for 95 per cent of methane emissions from these lakes, and that methane flux from thaw lakes in our study region may be five times higher than previously estimated.”

    The more I read about the intersection of climate science with public policy, the more I come to believe that:
    1. instead of trusting the models we are instead going to perform the experiment.
    2. ideological denialists won’t believe the results of the experiment either.
    3. the fact that we don’t get a “do-over” if we don’t like the results isn’t fully appreciated.

  15. 315
    Guy says:

    I am an ardent supporter of the work of Real Climate, and that of its primary contributors. But recently I’ve become aware that something is very wrong with this picture, and imho it needs to be urgently addressed. The problem can be summarised in two words – James Hansen. In the media, he is referred to as the world’s leading climate scientist (http://www.independent.co.uk/environment/climate-change/cleared-jury-decides-that-threat-of-global-warming-justifies-breaking-the-law-925561.html among others), so his words carry particular weight in public.

    The discussion on this thread is a perfect example. I am a layman, but I an incredibly confused as to how other climate scientists perceive his words on the subject. On the one hand, he is never directly contradicted, but on the other no-one really fully endorses his bold statements either. Given the stakes, I cannot work out if this is (frankly) cowardly timidity protecting scientific reputation, or if there is something genuinely wrong with either his method or logical deductions arising from the science.

    Take a look at his (highly readable) article penned for the UK’s (non peer-reviewed) New Scientist – http://pubs.giss.nasa.gov/docs/2007/2007_Hansen_2.pdf. This predates his own peer-reviewed http://arxiv.org/abs/0804.1126, discussed by Real Climate at http://www.realclimate.org/index.php/archives/2008/04/target-co2/. I have so many questions about – and problems with – all of this I scarcely know where to begin. This really needs its own article here, massively expanded on the April “Target CO2″ one. But for starters, consider these questions…

    1. As Lennary says in #294 (no inline response), Hansen asserts in the New Scientist article that as recently as “about 14,000 years ago, sea level rose approximately 20 metres in 400 years, or about 1 metre every 20 years.” This goes far beyond Gavin’s Hansen citation in the original article (although it doesn’t make a specific 2100 prediction, clearly it shows that it might be possible). Is this correct? Is it quite possibly correct?

    2. Hansen specifically suggests in his Target Atmospheric CO2 paper that the maximum figure should be 350ppm. In Gavin’s Real Climate response (Target CO2) the figure of 350ppm was not even mentioned, despite this being the focus of the abstract – indeed Gavin concludes “So what does this mean for the future? In the short term, not much”. When I asked about this, (#18 in Target CO2), Gavin replied that “I don’t get hung up on a precise target”, but agreeing that CO2 levels need to be brought down as fast as possible. Surely policymakers need a target based on best-available science? Worse, many targets now are based on an apparently catastrophic 450ppm or even higher. Look at the supporters of the site http://www.350.org, meant to spread the word about 350ppm – hardly any major NGOs support it yet, let alone governments. If the figure is the best science we have, this must change urgently. Surely it is beholden on Real Climate contributors to unambiguously state – at the very least – the maximum level that can be thought of as having a reasonable probability of avoiding runaway climate change (based on best current knowledge, knowing it may be revised in future)?

    3. Speaking of runaway climate change… no matter what the specific rate of SLR, Hansen et al write in the Target Atmospheric CO2 paper that “Continued growth of greenhouse gas emissions, for just another decade, practically eliminates the possibility of near-term return of atmospheric composition beneath the tipping level for catastrophic effects”. I can’t think of a more alarming sentence in all peer-reviewed science. Do Real Climate contributors agree with his words or not?

    4. No-one has demonstrated that Hansen has received any practical support from his seemngly-lone campaign to get governments to agree to a 350ppm target. In New Sceintist, he writes “Reticence is fine for the IPCC. Individual scientists also can choose to stay within a comfort zone, and not worry that they may say something that proves to be slightly wrong. But perhaps we should consider our legacy from a broader perspective. Do we not know enough to say more? Using the fact that a glacier on Greenland slowed after speeding up as “proof” that reticence is appropriate is little different from the common misconception that a cold weather snap disproves global warming”. Is it not time that other climatologists join his attempts to convince governments, even if it is just to release specifically supportive statements?

    I am a huge supporter of Real Climate, it is far and away the best resource we have for bringing scientists directly to the public. Frogive me if I have missed anything else that has already directly looked at this, but the James Hansen problem must be addressed urgently – in detail and at length.

  16. 316
    Aaron Lewis says:

    Re # 313
    Guy,
    I do not look for certainty, I look for rational risk management and economic analysis. And, Climate Science is being ever so reticent.

    What SLR numbers made it into the IPCC FAR Summary for Policy Makers? What SLR number in any part of the IPCC FAR is of any use to an economist doing an analysis of the cost of global warming and the cost of curbing GHG emissions?

    Engineers and economists ARE using the thermal expansion number. What are they supposed to use? The Thermal Expansion Number is the only number offered. I saw that number in a proposal going to BLM for CALFED Delta work. The organization that wrote that proposal is one of the smartest science and environmental consulting firms in the world. The Thermal Expansion Number is being used where it should not be used. Taxpayers are going to pay for this 3 times: once for doing the work with the wrong numbe; once for fixing the damage when it fails; and, once for doing it right.

    IPCC should have given a probability distribution curve. That is, they should have said, “considering all factors our best guess is XXX meters by 2050 and YYY meters by 2100 with a 5% chance that it will be as low as QQQ meters and as high as ZZZ meter; and if you doing critical public safety engineering then use ###. All the IPCC AR4 said is that it will be more than AAA. The IPCC is a global organization and should consider the public safety of large human populations and endangered species around the world.

    Pfeffer et al raises the bar, but still does not address all factors, and therefore should not be suggesting a total number, because that incomplete number is going to get used in engineering and economic analysis, and the resulting designs and analysis will be wrong. The error will propagate. The numbers in Pfeffer are not of a suitable quality for human safety engineering. If the Pfeffer values are going to be used, then a whopping big safety factor needs to be applied.

    The IPCC passage (and sidebar) on sea level rise reminds me of Feynman’s comments about his Brazilian students. It is as if they have memorized every word of the science text, but have no concept of reality. IPCC was mostly written by scientists. When I look for literature on brittle compressive failure of ice, I find it was written by engineers. This is the failure mode that could move ice into the oceans rapidly, so why don’t I see any engineers in the IPCC author list?

  17. 317
    sidd says:

    Guy wrote on 15 September 2008 at 4:01 AM:

    ‘”about 14,000 years ago, sea level rose approximately 20 metres in 400 years, or about 1 metre every 20 years.” ‘

    this is correct. i think this is melt water pulse 1A (MWP 1A)

    http://www4.ncsu.edu/~jpliu/sealevel/
    http://www.sciencemag.org/cgi/content/abstract/299/5613/1709?siteid=sci&ijkey=1Y7ijVfKZOpRU&kytype=ref

  18. 318
    Hank Roberts says:

    > brittle compressive failure
    What’s going to compress the ice? What’s your source for claiming this is what happens to the icecaps?

    Citation. Sources.
    Engineers do it, scientists do it, you can too.

  19. 319
    David B. Benson says:

    Guy (315) — During meltwater pulse 1A, surely several proglacial lakes dumped their waters into the oceans. Here is one which did so repeatedly during that time:

    http://en.wikipedia.org/wiki/Missoula_Floods

    Another possiblity, recently properly sized, is near the Ural Mountains; unfortunately I don’t have a link. Another is the Glacial Baltic Lake:

    http://en.wikipedia.org/wiki/Baltic_Ice_Lake

    although possibly forming too late to participate in Meltwater pulse 1A.

    The general period, in the northern hemisphere:

    http://en.wikipedia.org/wiki/B%C3%B8lling_Oscillation

    The paper which is the main thrust of this thread indicates that such extreme changes in sea level are most, most unlikely thiis century. In particular, there are no large proglacial lakes to suddenly drain.

    Next century? Well, that depends upon how fact Antarctica warms and the behavior of WAIS to the high sea level and great warmth. My understanding of Dr. Hansen’s warning is that, based on the paleorecord, with high levels of CO2 indeed Antarctic ice sheets will, over centuries, melt to a fraction of the current size.

  20. 320
    Mauri Pelto says:

    Aaron where do you come up with this stuff: brittle compressive failure of ice? Glacier ice failing due rain? This is not even close to the reality of glacier physics. You are not in a position of understanding glacier behavior well enough to comment on Pfeffer et al. work.

  21. 321
    David B. Benson says:

    Here is a report of Professor John Schellnhuber, director of the Potsdam Institute for Climate Impact Research stating that “only a return to pre-industrial levels of CO2 would be enough to guarantee a safe future for the planet.”:

    http://www.guardian.co.uk/environment/2008/sep/15/climatechange.carbonemissions

    which seems stronger, but consistent with, what I take to be Dr. James Hansen’s position.

  22. 322
    Hank Roberts says:

    Aaron, you’re wishing for more certainty than the science offers.
    > IPCC should have given a probability
    > distribution curve.

    What they said was, they don’t know, past expectations were underestimates, and had been
    http://scholar.google.com/scholar?hl=en&lr=&safe=off&q=IPCC+FAR+sea+level&btnG=Search

    and, as they said at the time, it was too soon to know more.

    This is as good as it gets, sometimes.
    _________________________________________
    ReCaptcha for this posting is: Lawyers arriving

    Ya think?

  23. 323
    David B. Benson says:

    Here is a news article in which James Hansen explains some of his recent position and outlines a plan:

    http://www.independent.co.uk/environment/climate-change/phase-out-coal-and-burn-trees-instead-urges-leading-scientist-929889.html

    but I opine we had better start growing the trees now.

  24. 324
    Ray Ladbury says:

    Guy and Aaron, One can look at the science as a policy maker and ask 2 questions:
    1) How bad will it get if CO2 gets to X ppmv (x=450, 500,…)?
    2) How high can CO2 get before I have a high probability of serious consequences?

    The models are not to the point where they can answer question 1 definitively–in part because we don’t know what tipping points we might encounter. To answer the latter question we can appeal to paleoclimate records and the remarkable stability of the last 10000 years and say 350 ppmv is probably safe. This is more an engineering answer, akin to how much weight a bridge can take.

  25. 325
    Hank Roberts says:

    Aaron, what Mauri said — where do you get the info you’re reporting? Sources count for more than opinions.

    If you can find out what the plans for the current Oakland side replacement for the Bay Bridge assume about length of service and sea level change, I’d be curious.

    I look at it and wonder, what were they not thinking, to built it so low over such a long stretch?

    Then I look at the Port of Oakland, and Oakland Airport, and think, right. Unimaginable it could be underwater. So they don’t imagine it.

  26. 326
    Steve Bloom says:

    These new results are very much consistent with Hansen’s concerns. The paper itself isn’t on-line as yet, although I assume it will be in a couple of days. The Mongabay article:

    Earth already committed to 2.4-dgree C rise from climate change
    Jeremy Hance, mongabay.com
    September 15, 2008

    Air pollution masking full impact of global warming

    As of 2005 the Earth was already committed to rise of global mean temperatures by 2.4°C (4.3°F), concludes a new study published in the journal Proceedings of the National Academy of Sciences (PNAS). The conclusion is significant because the Intergovernmental Panel on Climate Change (IPCC) has warned that a rise in global temperature by 1 to 3°C will lead to catastrophic consequences, including “widespread loss of biodiversity, widespread deglaciation of the Greenland Ice Sheet, and a major reduction of area and volume of Hindu-Kush-Himalaya-Tibetan glaciers, which provide the head-waters for most major river systems of Asia.” These glaciers, predicted to shrink considerably in the next few decades, provide food and water to over two billion people.

    V. Ramanathan and Y. Feng of the Scripps Institution of Oceanography at the University of California at San Diego argue that due to unique conditions in the Arctic, mean global temperature must be doubled to accurately reflect changes there. With a committed raise of nearly 5°C (9°F), the already diminishing sea-ice will continue to abate at alarming rates and the Greenland Ice Sheet may begin to crumble under climatic pressures. The researchers estimate the long-term exposure (thousands of years) of the Greenland Ice Sheet to a minimum warming between 1.9–4.6°C will lead to a complete melt of Greenland. Such a melt would raise sea levels by seven meters (23 feet).

    Given the dire projections, Ramanathan and Feng warn that time is running out. Unless tough mitigation policies on greenhouse gases are put in place, the authors say the Earth will be locked into a rise of 3°C by 2030. They write that “CO2 mitigation polices are extremely critical if we want to limit further increases in the committed warming.”

    Air pollution masking climate change

    Ramanathan and Feng also explore a question often raised by skeptics: “Why hasn’t the planet yet felt the full force of climate change?” So far, the planet has experienced a mean warming of 0.76°C since the late 1800s.

    The scientists conclude that a variety of factors are masking the full effect of climate change; one of the most significant of which is air pollution. Some types of air pollution send aerosols that reflect light like a mirror, brightening the planet and thereby cooling it. The pollutants ability to mask rising temperatures has been estimated at 47 percent. However, as nations clean up their skies, the masking effects of such pollutants disappear causing the Earth to undergo sudden warming. The researchers state that this relationship between dwindling air pollutants and higher temperatures can already be seen in Europe.

    Considering the many negative aspects for health and environment of air pollution, the researchers do not recommend that nations should forgo policies that clean-up air pollution. But they say countries and international organizations should be aware of the potential rise in temperature due to such actions.

    The authors suggest better models are needed to provide nations with more accurate predications of the relationship between air pollutants and greenhouse gases. “This is not easy and the costs may be substantial for developing such models and the associated observing systems, but,” the scientists conclude, “we do not have much choice.”

    V. Ramanathan and Y. Feng (2008). On avoiding dangerous anthropogenic interference with the climate system: Formidable challenges ahead. PNAS Online Early Edition for the week of September 15-19, 2008.

  27. 327
    sidd says:

    just read 2 nice papers

    Howat et al, Geophysical Research Letters, v35, L17505 (2008)
    “Rates of southeast Greenland ice volume loss from combined ICESat and Aster observations”
    estimate is 108 Km^3/yr ona average for the period 2002-2005, small glaciers melting much faster than the big ones

    and this one was in the ‘in press’ section of the GRL
    Wouters et al. “GRACE observes small-scale mass loss in Greenland”
    this paper is very nice. i feel like singing

    “Amazing GRACE, how sweet that data, for a wretch like me,
    I was lost , but now am found, was blind, but now can see”

    nyhoo

    Table 2 gives the mass loss from 2003-2007 as 39,111,229,134,270 Gigaton
    each year
    abstract sez 179+/-25Gt/yr corresponding to 0.5+/-0.1mm/yr sea level rise

    very nice pictures

  28. 328
    Hank Roberts says:

    > sea level rise
    I took three minutes poking around in Scholar and found this:

    Journal of the Geological Society; 2007; v. 164; issue.5; p. 1059-1063;
    DOI: 10.1144/0016-76492006-164
    © 2007 Geological Society of London
    Original Article
    Relative sea-level changes from NE Ireland during the last glacial termination
    A.M. McCabe1, J.A.G. Cooper1 & J.T. Kelley2

    1 1School of Environmental Science, University of Ulster, Coleraine BT52 1SA, UK (e-mail: m.mccabe@ulster.ac.uk)
    2 2Department of Earth Sciences, University of Maine, Orono, ME 04469-5790, USA

    Deglacial sea-level records from NE Ireland between 21 and 11 cal. ka bp record marine transgressions and sensitive lithospheric responses to ice loading. The sawtooth sea-level curve contains four intervals characterized by: (1) strong net uplift, subaerial channelling and a global meltwater pulse (21–19 cal. ka bp); (2) ice loading, isostatic depression and high relative sea level (19–17.5 cal. ka bp); (3) ice loading, renewed isostatic depression and high relative sea level for >1000 years (17–14.5 cal. ka bp); (4) catastrophic ice wastage, rapid uplift and lowstand (14.5–13 cal. ka bp). Geophysical models do not capture these fluctuations.

  29. 329
    Hank Roberts says:

    Interesting batch of papers here, none of which I’d heard of. Google Scholar for a string for it to locate the link. It’s from a PDF titled:
    SESSION NO. 42, 1:30 PM
    Saturday, July 26, 2003
    T13. Re-assessing the Role of Meltwater Processes
    During Quaternary Glaciations (Posters)
    (Commission on Glaciation)

    Just one example, from p.3 — far more detail about how ice melts in bulk and causes floods, than I’d come across before, is common in these papers. And no doubt there’s more since this:

    42-14 BTH 68 Kozlowski, Andrew L. [54359]
    THE ORIGIN OF THE CENTRAL KALAMAZOO RIVER VALLEY: RELATIONSHIPS BETWEEN OUTBURST FLOODS, TUNNEL CHANNELS, ICE DYNAMICS AND REGIONAL DRAINAGE HISTORY IN SOUTHWESTERN MICHIGAN

    “… At the time of valley formation by the westward flowing outburst the Lake Michigan lobe had retreated at least 35 km to the west to the Lake Border moraine (14,000 ka B.P.) or possibly beyond. With the Lake Michigan lobe absent to impede flow path, drainage proceeded southwesterly until draining into Lake Chicago near St. Joseph, Michigan. The source of the outburst appears to have been a system of tunnel channels beneath the Saginaw lobe. Along the KRV, the meltwater flowed beyond the extent of the subglacial channels and became subaerial. Segments believed to be tunnel channels display convex-up flow paths and contain eskers ….

  30. 330
    Guy says:

    Thanks to so many of you for helpful links on these questions. From an abstract scan, it looks like MWP 1A is largely driven by Antarctica, with some additional rise attributed to the Laurentide and Fennoscandian ice sheets. According to Gavin’s summary above (again, I can only read the abstract), the recent paper limits Greenland alone to 2m, despite considering both Greenland and Antarctica… does this not mean that we still have a potential to hit this kind of level? I appreciate the lack of proglacial lakes, but can we be sure the Antarctica melt will be slow? Several people have commented on the effects of the closing ozone hole (imagine the ironic cry of the deniers…) and other processes (discussed in #316).

    My basic layman’s logic is… if it has happened before, it can happen again. We have been surprised so many times by observation with regard to ice collapse…

    The Guardian report in #321 doesn’t help the feeling of impending doom, especially the current projections from the Tyndall Centre of our eventual CO2 concentration levels… 650ppm! Even drastic action only gets us to 450ppm. Presumably (hopefully) this drastic action does not include Hanson’s ultra-drastic “leave fossil fuels in the ground” scenario, and the more I read the more that looks like sense.

    However, there is still near-silence on this issue from Real Climate. I have asked from a response from the Real Climate contributors for a response to Hansen’s contentions five times now (in posts #180 #227 #249 #277 #315) over a period of 7 days, with a few supporting posts by others too. By nature I don’t buy conspiracy theories, but I am beginning to get a little suspicious here that at least there is an issue in academia that we mortals are not privvy too. I think (hope!) I’m asking genuine, reasonable questions… why do some respondants get rewarded by loads of answers to rude, ignorant questions while the elephant in the room keeps getting ignored?!

  31. 331
    Steve Bloom says:

    To be fair to Aaron, it’s perhaps a bit confusing that rain often gets mentioned in reports of melting glaciers. Aaron, the point is that if rain is falling on a glacier it’s a strong indication that the immediate environment is above freezing.

    Re the idea of “brittle compressive failure” leading to a bulk loss of ice, I suspect you didn’t grow up in a cold climate. That’s not how melting ice behaves. Melting feeding back on itself and leading to a loss of structural integrity (calving, collapse due to undermining, etc.) is probably a better way to think of it. Consider what happened with the jokulhlaup decribed above.

  32. 332
    Steve Bloom says:

    Mauri, please don’t forget my question above about the possible implications of that jokulhlaup frictional melting for the Pfeffer et al results.

    I also notice that there is a new GRL paper from Howat et al finding considerable inland thinning in southeast Greenland amd that the smaller glaciers are responsible for most of it. Both of those observations sound as if they may conflict with Pfeffer et al.

    The Howat et al press release:

    Small glaciers — not large — account for most of Greenland’s recent loss of ice, study shows

    COLUMBUS, Ohio – The recent dramatic melting and breakup of a few huge Greenland glaciers have fueled public concerns over the impact of global climate change, but that isn’t the island’s biggest problem.

    A new study shows that the dozens of much smaller outflow glaciers dotting Greenland’s coast together account for three times more loss from the island’s ice sheet than the amount coming from their huge relatives.

    In a study just published in the journal Geophysical Research Letters, scientists at Ohio State University reported that nearly 75 percent of the loss of Greenland ice can be traced back to small coastal glaciers.

    Ian Howat, an assistant professor of earth sciences and researcher with Ohio State’s Byrd Polar Research Center, said their discovery came through combining the best from two remote sensing techniques. It provides perhaps the best estimate so far of the loss to Greenland’s ice cap, he says.

    Aside from Antarctica, Greenland has more ice than anywhere else on earth. The ice cap covers four-fifths of the island’s surface, is 1,491 miles (2,400 kilometers) long and 683 miles (1,100 kilometers) wide, and can reach 1.8 miles (3 kilometers) deep at its thickest point.

    As global temperatures rise, coastal glaciers flow more quickly to the sea, with massive chunks breaking off at the margins and forming icebergs. And while some of the largest Greenland glaciers – such as the Jakobshavn and Petermann glaciers on the northern coast – are being closely monitored, most others are not.

    Howat and his colleagues concentrated on the southeastern region of Greenland, an area covering about one-fifth of the island’s 656,373 square miles (1.7 million square kilometers). They found that while two of the largest glaciers in that area – Kangerdlugssuaq and Helheim – contribute more to the total ice loss than any other single glaciers, the 30 or so smaller glaciers there contributed 72 percent of the total ice lost.

    “We were able to see for the first time that there is widespread thinning at the margin of the Greenland ice sheet throughout this region.

    “We’re talking about the region that is within 62 miles (100 kilometers) from the ice edge. That whole area is thinning rapidly,” he said.

    Howat says that all of the glaciers are changing within just a few years and that the accelerated loss just spreads up deeper into the ice sheet.

    To reach their conclusions, the researchers turned to two ground-observing satellites. One of them, ICESAT (Ice, Cloud, and land Elevation Satellite), does a good job of gauging the ice over vast expanses which were mostly flat.

    On the other hand, ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) does a better job at seeing changes at the steeper, less-flat margins of the ice sheet, Howat said.

    “We simply merged those data sets to give us for the first time a picture of ice elevation change – the rate at which the ice is either going up or down – at a very high (656-foot or 200-meter) resolution.

    “They are a perfect match for each other,” Howat said.

    “What we found is the entire strip of ice over the southeast margin, all of these glaciers, accelerated and they are just pulling the entire ice sheet with it,” he said.

    Howat said that their results show that such new findings don’t necessarily require new types of satellites. “These aren’t very advanced techniques or satellites. Our work shows that by combining satellite data in the right way, we can get a much better picture of what’s going on,” Howat said.

  33. 333
    Steve Bloom says:

    Mauri, visiting Howat’s web page I noticed this related in-press paper. Title/abstract:

    Synchronous retreat and acceleration of southeast Greenland outlet glaciers 2000–06: ice dynamics and coupling to climate

    A large portion of the recent increase in the rate of mass loss from the Greenland ice sheet is from increased outlet glacier discharge along its southeastern margin. While previous investigations of the region’s two largest glaciers suggest that acceleration is a dynamic response to thinning and retreat of the calving front, it is unknown whether this mechanism can explain regional acceleration and what forcing is responsible for initiating rapid thinning and retreat. We examine seasonal and interannual changes in ice-front position, surface elevation and low-speed for 32 glaciers along the southeastern coast between 2000 and 2006. While substantial seasonality in front position and speed are apparent, nearly all the observed glaciers show net retreat, thinning and acceleration, with speedup corresponding to retreat. The ratio of retreat to the along-flow stress-coupling length is proportional to the relative increase in speed, consistent with typical ice flow and sliding laws. This affirms that speedup results from loss of resistive stress at the front during retreat, which leads to along-flow stress transfer. Large retreats were often preceded by the formation of a flat or reverse-sloped surface near the front, indicating that subsequent retreats were influenced by the reversed bed slope. Many retreats began with an increase in thinning rates near the front in the summer of 2003, a year of record high coastal-air and sea-surface temperatures. This anomaly was driven in part by recent warming, suggesting that episodes of speedup and retreat may become more common in a warmer climate.

  34. 334
    Mauri Pelto says:

    Steve: I have tried to avoid the Howat paper which I have reviewed. A post has been submitted to Gavin on this topic exploring this paper. It notes that the thinning and acceleration is mainly associated with marine terminating outlet glaciers, and the cause of the acceleration is you guessed it, not more meltwater delivery to the base. I do not find the Pfeffer paper very important overall, it does not advance our understanding of the ice sheets behavior. It merely applies some of our understanding to setting realistic bounds on sea level rise. The Howat paper and several other recent papers are far more important, if less interesting to the media.

  35. 335
    Hank Roberts says:

    Guy, read a bit more and put the pieces together; nobody’s ignoring what you keep asking, it’s all being discussed right here, right now, as fast as people can do it. People have day jobs; people have to read actual references, not newspaper articles, and think before commenting in their own areas of expertise, and may well be writing papers instead of blogging on the most important questions. That’s how it works.

  36. 336
    Lennart says:

    So if I understand correctly Pfeffer e.a. do not take alle possible (feedback) mechanisms into account which could possibly cause the ice of GIS and WAIS to melt faster than currently projected? Peter Cox e.a., for example, think carbon cycle feedbacks may cause substantial additional warming:
    http://www.naturalnews.com/022481.html

    which Canadell e.a. already seem to be observing:
    http://www.pnas.org/content/104/47/18866.abstract

    Not to speak of albedo, methane and other possible feedbacks. And how about various possible ice melt mechanisms?

    In addition to Hansen and Schellnhuber it seems also Martin Parry has openly called for further-than-mainstream cuts in carbon emissions:
    http://www.nature.com/climate/2008/0806/full/climate.2008.50.html

    He called for 80% cuts in 2050 relative to 1990, which would be about 85% cuts relative to 2000, which is also the upper limit in IPCC’s AR4, Synthesis Report, Table SPM.6; still not enough though to have a good chance of limiting warming to less than 2 centigrade, it seems:
    http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_spm.pdf

    So 100% cuts by 2050 seem to be necessary to limit the risk of dangerous climate change as much as humanly possible. Is this even remotely feasible, politically and economically? The G8 have said something about 50% cuts. Oxfam last week published a report about climate change and human rights, in which they plead for at least 80% cuts in 2050 relative to 1990 (based on Meinshausen, in Schellnhuber 2005, and Parry):
    http://www.oxfam.org/en/pressroom/pressrelease/2008-09-09/human-rights-must-be-put-heart-global-climate-change-fight

    The strongest cases I’ve seen so far for 100% cuts by 2050 have been made by Spratt and Sutton in Climate Code Red (based on Hansen, but also on someone like Ken Caldeira):
    http://www.climatecodered.net/

    and by Lester Brown in Plan B 3.0:
    http://www.earth-policy.org/Books/PB3/index.htm

    So are we prepared for a global mobilization compared to the World War II mobilizations? How many RC-contributors would support such a mobilization? Nicholas Stern (almost) seems to realize this is what’s necessary, but he also seems to think it’s politically not feasible, so he’s a little more diplomatic about it. Even environmental organizations seem to be reticent to not scare politicians and general public too much (see also Climate Code Red). So me too is very curious how the people of RC think about all this. Are Hansen, Schellnhuber and others too worried or not?

  37. 337
    David B. Benson says:

    Ray Ladbury (324) — I contend that 350 ppm is still much too high for the long run; try 280 ppm.

    Guy (330) — This is the first time I’ve read the hypothesis that MWP 1A was largely due to Antarctica. Can you provide some links?

  38. 338
    David B. Benson says:

    ” The Antarctic Ice Sheet during the Last Glacial Maximum and its subsequent retreat history: a review”

    http://www.ingentaconnect.com/content/els/02773791/2002/00000021/00000001/art00083

    may be behind a paywall for interested readers. Here are relevant quotations from the conclusions:

    “2. Initial retreat of the WAIS appears to have occurred between 15,000 and 12,000 yr and continues into the Holocene. There was significant retreat of the ice sheet in the Ross Sea, and possibly Weddell Sea, after 7000 yr BP.

    3. During at least the final stages of the glacial maximum, the expanded WAIS was flowing across an extensive deforming bed. This implies gentle profiles, high rates of flow and discharge, and possibly rapid retreat. Thus, the LGM configuration may have changed during the LGM.”

    “5. The Antarctic Peninsula housed much greater volumes of ice during the LGM than was previously thought, and therefore made a greater contribution to the post-glacial sea-level rise.”

    Figure 13 indicates that Antarctic Peninsula ice retreat began 14–12 kybp, depending upon location.

  39. 339
    Steve Bloom says:

    Re #330: Guy, perhaps it’s because I’ve been following this stuff longer than you, but I have to say that I don’t think Hansen feels (or has a reason to feel) unsupported by other scientists. It seems clear that he’s on a campaign to get more scientists to speak out, and I would say there’s evidence of considerable progress along those lines. Certainly there are many scientists who don’t feel comfortable with taking on such a role or even with seeing others do so, but I don’t take that as any particular reflection on Hansen.

    Re this site’s coverage of Hansen’s work, bearing in mind that his views and results already get more of a public airing than those of any other climate scientist and that the limited time of the RC authors means that plenty of deserving topics never get written about at all, I see no particular need to change things. Also, in case you don’t know this, Hansen is also Gavin’s boss, which relationship would tend to reduce the effectiveness of any posts lauding the former’s work. Finally, if you search around the climate scientist blogosphere you’ll notice no lack of nice things being said about Hansen.

  40. 340
    Steve Bloom says:

    Re #334: Thanks, Mauri, I’ll look forward to seeing that.

    Just one specific two-part question about the jokulhlaup frictional melting if you don’t mind: Was the potential scale of this effect known about before this observation, and is there any major implication for modeling ice sheet behavior? Of course I’m happy to wait for the answer if this will be addressed in the forthcoming post.

  41. 341
    David B. Benson says:

    “Ice-sheet extent of the Antarctic Peninsula region during the Last Glacial Maximum (LGM)—Insights from glacial geomorphology”:

    http://www.earthscape.org/r2/gsab/v117i11/i.html

    Last sentence of abstract: “This also suggests the peninsula area contributed to global sea-level rise associated with meltwater pulse (MWP) 1a.”

  42. 342
    Hank Roberts says:

    Steve, this may help (teaser only, sorry)
    http://www.nature.com/nature/journal/v445/n7130/full/445830a.html
    Nature 445, 830-831 (22 February 2007) | doi:10.1038/445830a; Published online 21 February 2007

    Glaciology: Lubricating lakes
    Jack Kohler
    Abstract

    More than 150 subglacial lakes have been discovered beneath the Antarctic ice sheet. The four most recent additions, found right at the start of fast flow in a large ice stream, suggest that the lakes influence ice dynamics.

    Antarctica is well known as the coldest place on Earth, but it is also surprisingly wet. Whereas the upper part of the ice is cold, geothermal heat and the insulating effect of the overlying ice combine towards its base to bring extensive areas of ice at the bed to its melting point.

  43. 343
    suthisa says:

    Thanx for the pointer to an excellent paper. I have heard about Niagra Falls eroding many feet in a single day, long ago. Is it possible that glaciers can enlarge their exits to the ocean through erosion ?

  44. 344
    Hank Roberts says:

    P.S., Steve, guess how hot it can get under a moving glacier?

    Okay, got a temperature in mind? Sure?

    Now look:
    http://jgs.geoscienceworld.org/cgi/content/abstract/163/3/417

  45. 345
    Guy says:

    Thanks again for eveyone’s input, I really do appreciate it!

    #336 – I was referring to the discussed paper here – http://www.sciencemag.org/cgi/content/abstract/299/5613/1709?siteid=sci&ijkey=1Y7ijVfKZOpRU&kytype=ref. “With the use of a climate model of intermediate complexity, we demonstrate that with mwp-1A originating from the Antarctic Ice Sheet, consistent with recent sea-level fingerprinting inferences, the strength of North Atlantic Deep Water (NADW) formation increases, thereby warming the North Atlantic region and providing an explanation for the onset of the Bølling-Allerød warm interval.” (ps – thanks for the #337 ref, David)

    #335 – I know, I feel quite bad about repeatedly asking for clarification of busy, working scientists. However, since I first asked over a week ago there have been a great many replies since then on other subjects, some of which seem like pure distractions. And as shown most fully in #315, these are really fundamental questions that I cannot see Real Climate’s stance meshing well with that of Hansen’s – in this very thread they seem quite at variance. Maybe I have missed something, but I’d love to be shown what!

    The target 350ppm figure seems utterly crucial to me. This is what Hansen is proposing to world leaders, and being widely reported in the media. It is the basis of Hansen’s April paper, which Real Climate has discussed… but with no mention of that target. I need to know if this figure is supported by others – not just as a “safety first” figure, but as a reasonable maximum figure. Policymakers are still gunning for 450ppm… is this a recipe for disaster or not? From Real Climate at the moment, I still have no idea. If anyone else has any specific pointers to RC utterances on the subject, please let me know!

  46. 346
    Ricki says:

    I think the point being made by Guy and Aaron (315, 316) is that we need some form of risk based best guess on which we can make planning decisions.

    As they point out, the current engineering and regulator community are using the IPCC numbers (in some cases modified a bit by recent research). For example, currently, Australian regulators are using 0.91m as the 2100 year number! This is less than half of the 2m being discussed here.

    268 gets the point. Engineers need a reliable estimate for safety. Planning has to be approached in a similar manner. Eg, different design events are used for different risk structures — ‘normal’ buildings are designed for 0.002 annual probability of exceedance, while ‘post-disaster recovery’ buildings (such as hospitals, major bridges) are designed for 0.0004 (1 in 2500).

    If scientists (like RC) cannot give us an up to date re-assessment of SLR (say on a 6 monthly basis with an added factor to account for uncertainty) for use in planning, then as has been pointed out above, billions of $ will be spent on projects that will be drowned.

    Will no-one state their best guess SLR by 2050/2100 and add a factor for uncertainty?

    (see 131 above)

  47. 347
    Ray Ladbury says:

    David Benson (#336): I think one could reasonably argue for levels as high as maybe 330 ppmv, but no higher as an “engineering” level. It is a moot point, as I see little hope of holding things below 450-500 ppmv.

  48. 348
    Nick Gotts says:

    #336 (Lennart)”So are we prepared for a global mobilization compared to the World War II mobilizations?”

    That’s certainly what’s needed – only for decades rather than years, and without an enemy to focus on. Not easy to imagine. Which is why I’m hoping for nature to put on a series of showy (but minimally destructive) demonstrations that the problem can’t be ignored, or postponed for the next government to deal with.

  49. 349
    Hank Roberts says:

    > If scientists (like RC) cannot give us an up to date re-assessment
    > of SLR (say on a 6 monthly basis … then … billions of $ will
    > be spent on projects that will be drowned.

    But only dollars, not euros or other currencies, to the extent the precautionary principle is applied. It is applied except in the US for most purposes these days, and increasingly so.
    —————————————————-

    ReCaptcha: Great MARKETS
    I swear that AI seems to be waving hands and saying “pick me” …

  50. 350
    David B. Benson says:

    Guy (345) & Ricki (347) — Amateurs rush in where experts fear to tread.

    So I’ll rush in. First of all, the Pfeffer et al. paper makes clear that they find 80 cm in 2100 CE most likely; 2 meters is simply an almost risk-free upper bound. So simply double these for 2200 CE except that this does not take into account the possibilty of WAIS melting more in that next century.

    As for MWP 1A rapid surge and the prior, similar rise around 19,000 ybp, it does seem that Antarctica was a major participent. However, with the sea lowstand of about 135 m below present, the Antarctic Pennisula ice sheet extended all the way to the outer edge of the continental shelf; that’s a considerable mass of ice on a fairly gentle slope (which becomes even more gentle at the inner edge of the continental shelf). I opine this meant tidewatr conditions over areas much more extensive than at present. Put another way, the ice in Antarctica is now essentially up into the mountains (except for part of WAIS). Given the steep slopes, each meter rise in sea level exposes little ice to tidewater conditions. My amateur conclusion is that a repeat of MWP 1A is not possible under current condtions.

    However, you might care to look for papers which consider the stability of WAIS.

    Ray Ladbury (347) — At 330 ppm Greenland will slowly waste away. Holding the increase to 450 ppm is going to be tough to do; even tougher may be convincing everybody to begin sequestering carbon to bring the level back down in about a century.


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