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Climate Hustle

Experts say the IPCC underestimated future sea level rise

Posted on 4 December 2013 by John Abraham

It looks like past IPCC predictions of sea level rise were too conservative; things are worse than we thought. That is the takeaway message from a new study out in Quaternary Science Reviews and from updates to the IPCC report itself. The new study, which is also discussed in depth on RealClimate, tries to determine what our sea levels will be in the future. What they found isn't pretty.

Predicting of sea level rise is a challenging business. While we have good information about what has happened in the past, we still have trouble looking into the future. So, what do we know? Well it is clear that sea levels began to rise about 100 years ago. This rise coincided with increasing global temperatures.

What causes sea level to rise? Really three things. First, water expands as it heats. Second, glaciers melt and water flows to the oceans. Third, the large ice caps on Greenland and Antarctica can melt and the liquid water enters the ocean; often the water transfer is added by calving at the ice fronts which result in icebergs that float into the ocean. In the past, much of the sea level rise was related to the first cause (thermal expansion). Now, however, more and more sea level rise is being caused by melting ice.

But this is all the past. What we really want to know is, how much will sea level rise in the future? There are a number of ways to predict the future. First, we can look at the deep past and see how sea level changed with Earth temperature long ago.

A second way to predict the future is through computational models. These models are computer programs which create a virtual-reality of the Earth. These virtual reality models are very useful because they allow scientists to play "what if" scenarios; but, they have their weaknesses as well. One of their weaknesses is that they don't necessarily capture all of the phenomena which cause sea level rise. It is believed by most scientists that the computer programs are too conservative.

How does this all relate to the current study? Well the authors took a different approach. They decided to ask the scientists themselves. What do they think sea level rise will be by 2100 and 2300 under different greenhouse gas scenarios? The authors found 360 sea-level experts through a literature survey. They then worked to find contact information for these scientists and finally, they sent a questionnaire. After receiving 90 expert judgments from 18 countries, the results were tallied. So, what do experts think?

Sea level rise over the period 2000–2100 for high and low warming scenarios. The ranges show the average numbers given across all the experts. For comparison we see the NOAA projections of December 2012 (dashed lines) and the new IPCC projections (bars on the right). 

Sea level rise over the period 2000–2100 for high and low warming scenarios. The ranges show the average numbers given across all the experts. For comparison we see the NOAA projections of December 2012 (dashed lines) and the new IPCC projections (bars on the right).

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Comments

Comments 1 to 24:

  1. This study is also discussed at Carbonbrief.

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  2. Great work. Abraham is the best. Has a similar study been done about top climatologists estimations of levels of CO2 and global temperatures? I have seen a few articles recently that suggest most climatologists think we are headed for at least 4 degrees C by the end of the century, but these weren't accompanied by specific polls, iirc. For example:

    http://www.climatecodered.org/2013/11/parts-of-australia-reaching-threshold.html

    "Mark Maslin, professor of climatology at University College in London,...said: 'We are already planning for a 4°C world because that is where we are heading. I do not know of any scientists who do not believe that...'"

     

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  3. You would expect IPCC to give the average expert judgement. On the other hand a bit too conservative estimates are expected according to Brysse et al 2012. Perhaps I'm going out of topic, but just mentioning some other apparently "too conservative" assessments and perspectives in the latest report:

    - The climate sensitivity interval assessed only as 'likely' with the lower end at 1.5 despite very very little supporting the lucky case of <1.5.

    - The many places mentioning the 'hiatus' and 'hiatus period 1998–2012'. There is even an entire chapter called "Climate Models and the Hiatus in Global-Mean Surface Warming of the Past 15 Years". But there is no good scientific reason to pick a single period starting in an extreme El Nino like that.

    Quite disappointing that the authors couldn't fully keep the scientific integrity vs the media debate including a lot of "sceptic" propaganda. But on the other hand that was predicted by Lewandowsky. At the recent AGU Chapman conference he mentioned that psychological theory suggests that the scientists would be affected:

    "There are several known psychological and cognitive variables which suggest that denialist discourse should seep into the scientific community".

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  4. John Abraham writes:

    "According to the best case scenario (humans take very aggressive action to reduce greenhouse gases), the experts think sea level rise will likely be about 0.4–0.6 meters (1.3–2.0 feet) by 2100 and 0.6–1.0 meters (2.0–3.3 feet) by 2300. According to the more likely higher emission scenario, the results are 0.7–1.2 meters (2.3–3.9 feet) by 2100 and 2.0–3.0 meters (6.5–9.8 feet) by 2300."

    This is what these experts as a group think likely. But what do they think is possible? What could be the worst case?

    According to one of the authors, Stefan Rahmstorf, about half of these experts think there's a 5% chance that SLR by 2300 could be more than 4 meters in a worst-case scenario (whereas 3.8 meters seems to be about the worst-case according to IPCC AR5, chapter 13, figure 13.13).

    Four of this half think there's a 5% chance it could be more than 9 meters. Three of these four think it could be more than 10 meters. Two of these three think it could be more than 12 meters. And one of these two thinks it could be even more than 15 meters by 2300 (see Rahmstorf's inline response to my comment 9 at the RealClimate post).

    So should citizens and policy makers make decisions based on the likely range, or on the worst-case in the possible range?

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  5. lennartvdl

    Decisions should be based on worse than worst case scenarios! There really is no indication that the abysmal efforts to mitigate climate change will change anytime soon, so we have to assume BAU. On that basis, we then have to assume an accelerating increase in positive feedbacks, not just from continued increase in anthropogenic CO2 and other GHGs, but also release of CO2 from the oceans as they warm; release of more CO2 and methane from melting permafrost; reduced albedo as the Arctic continues its death spiral; and reduced CO2 uptake from biomass such as Amazonia as extreme weather (changes in precipitation) adds to destruction caused by human activities. I for one cannot help but feel we're rapidly approaching a tipping point, if we haven’t done so already, that will result in runaway climate change.

    It will take a gargantuan and unprecedented global effort to slow down, let alone halt or reverse, climate change . For example, to replace the estimated 18.3 trillion kWh of electricity production by fossil fuels in 2025 (US IEA) would require the installation of about 4 million Megawatts of non FF electricity production. Assuming an optimistic 50% capacity factor, that’s about 6500 London arrays (the largest offshore wind farm in the world), or 1200 Hinkley nuclear power stations. And that will reduce CO2 emissions by a ‘mere’ 30% by 2025, which by itself will slow down the onset of runaway climate change, not prevent it.

    Although there’s hope that world leaders will wake up before it’s too late, it behoves policy makers to plan for worse than worst case – adaptation may be all we’re left with.

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  6. Worse than worst case is pretty hard to estimate, I think only situation during PETM comes close of the current imbalance on the energy budget, and as that happened on earth without ice sheets, there is no good precedent to take clues of what's going to happen on grand scale. Sure the geomorphology constrains glacier flows somewhat, but surges of glacial ice from most glaciers having a marine outlet - say in ten year intervals - might result on further increases in the IPCC numbers.
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  7. Phil,

    I agree if we're talking about the IPCC ranges. I was thinking of the expert judgement ranges, however, assuming that the experts take into account the potential feedbacks that the IPCC models ignore.

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  8. "water expands as it heats" in the posting can be misleading because water colder than 4C contracts as it heats (until 4C) and average ocean temperature is a tad above 3C according to sparse and somewhat contradictory information I've found. However, the topmost few hundred metres, with the significant +ve thermal expansion coefficient in lower latitudes, still has most of its heat remaining to be added to reach balance with the surface layer (I compute roughly 80% of heat still be added in the topmost 300 metres) so I think there's some decades of thermal expansion yet to come.

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  9. GrindupBaker,

    Ocean water is more complicated.  The coefficient of expansion changes with salinity and pressure.  In the Arctic at the surface, the maximun density is about -2C.  Unfortunately, I do not have a reference for the values,but they are complicated.  Without a clear reference table it is best to not speculate on expansion from temperature increase.

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  10. lennartvdl @4, in general you should base your policy on the expected value of your actions.  With regard to sea level rise, this means determining your range of policy options and finding the costs (including the mitigation and adaption costs) of that policy over the range of possible outcomes; multiplying the cost times the probability of each outcome for each policy option; taking the sum of the products for each policy option; then choosing policy with the lowest integrated product (ie, expected value).  Because costs for sea level rise increase with the size of the rise, and with the rapidity of the rise, the expected value of greater rises will be greater than equally probable, but lower rises.  Therefore they will be given more weight.  However, the worst possible outcomes will have such a low probability that their expected value will be small.  Thus, policy should be weighted towards avoiding higher sea level rises, but not premised doing everything to avoid the worst possible but low probability outcome.

    Of course, if the worst possible outcome represents an existential threat to (in reverse order of severity), our civilization, our species, or our biosphere, that changes the maths and we should be basing our policy on avoiding the worst possible plausible outcome.  In this case, however, global warming does not threaten the biosphere (although it does threaten mass extinction); it may but is unlikely to be to threaten our species extinction, and has a plausible risk of threatening our civilization.  Sea level rise by itself, however, does not threaten any of the three, even with multi-meter per century rises.  

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  11. Tom @10,

    I think you're right. And adding the risk of large and fast SLR to other severe climate risks probably does change the maths into making even relatively limited global warming a civilizational threat, that we better insure ourselves against, as far as we still can.

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  12. lennartvdl @11, obviously adding sea level rise to the generalized threat from global warming does increase the risks; but the overall risks from sea level rise itself are sufficiently small that even quite rapid (2 m/century) sea level rise does not greatly increase the risk.  Indeed, were it the only risk, or were the other risks minor, I would say the correct response to the risk from sea level rise would be adaption.  Unfortunately the threats are a package deal, and the threats from other aspects of global warming are sufficient to make mitigation a necessity. 

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  13. Tom @12,

    I doubt you're right there. How are countries going to adapt to SLR of say 2-4 meters/century after 2100 (assuming they've managed to adapt until then)? The rich countries may succeed in adaptation for a while, but in the poor countries mass migration seems a more likely result, which would probably have strong effects in the rich countries as well (assuming there will still be rich and poor countries by then). It will be very costly to keep adapting to rapidly and continually changing shorelines, and retreat may be the only option at a certain point. Only if you assume the world will get richer and richer over the coming centuries, can I imagine such SLR would not be a major problem. But I don't think we can assume that, because there seem to be strong limits to such growth.

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  14. lennartvdl @13, I highly recommend that you spend some time playing with a sea level rise mapper such as the one at flood.firetreen.net  If you do, you will notice several things.

    1)  Even with 60 meters of sea level rise, only a very small fraction of the Earth's land surface is flooded.  Bear in mind that the map simply shades all land under a given elevation, so that it shows extensive flooding north of the Caspian sea in Russia and Khazakstan where sea level is a function of rainfall and irrigation in the area rather than the melting of ice caps, and hence will never show flooding to that extent.  Indeed, the Caspian Sea is likely to show a decline in sea level as the area becomes more arid.

    Because only a small fraction of the Earth's land area is affected, even by such large sea level rises, sea level rise is a major problem almost exclusively for some low lying coastal areas (Bangladesh, Florida, Denmark) and for small island states whose islands are coral atolls rather than volcanic.

    2)  Even for those areas which have a major problem, they represent just a small portion of the Earth's population (2.2% for Bangladesh, probably around 5% altogether).  So, even if the problem areas need to be abandoned entirely, while that will cause significant short term suffering, within a generation they will be just as other residents in the land to which they emigrate and the problem will cease as a distinct problem.  Of course, the sea level rise is likely to be slow meaning the problems arising from emigration will continue in the long term, but be far smaller and easier to cope with at any given time.

    3)  If you look at a 4 meter map, you will sea that even at 4 meters - double the highest one century estimate shown above for BAU, and nearly four times the median estimate - Dahka remains unflooded, as does the majority of Bangladesh.  Sea level rise at any reasonable estimate, therefore, is not a threat that will destroy even Bangladesh in a century; although it will increase harm in Bangladesh from riverine flooding and cyclones. 

    4)  Moving on from the map, Dutch land reclamation from the sea began in the thirteenth century.  That makes it clear that protecting land from the sea does not need massive, modern technology and nor need it be beyond the means of even the relatively poor.  Indeed, all else being equal, deltas will naturally be reclaimed from the effects of sea level rise by the deposition of additional sediment from the rivers that formed them.  All else is not equal, of course, and changes in flow patterns in the hramaputra and Ganges rivers due to glacial melt, and potential greater aridity may be more of a threat to Bangladesh than sea level rise.  But that Bangladesh will lose land to sea level rise is not a foregone conclusion by any means, and significant efforts to prevent that can be achieved just by local labour.  The same can be said of most other delta areas.  (Some, however, have porous soils that prevent protection of the land by sea walls so not all land can be saved.  I believe Florida to be in this situation, but don't quote me on that.)

    5)  For those areas where land has a moderate slope from the shoreline, sea level rise will involved moving a part of their infrastructure back from the sea on a periodic basis, but a a slow rate relative to the pace of infrastructure construction in many parts of the world.  It represents an additional cost, but not a large additional cost relative to normal production.

    All of this is not to say that sea level rise will not have costs (it will); or to say that it will not cause significant suffering for some people (it will).  It will not, however, cause the sorts of costs and suffering that are likely to bring a civilization to its knees.  Its costs will be far less, in relative terms, than those of WWII or the cold war; and massively less in proportion to disasters that have previously struck our civilization such as the Black Death.  As a threat to our civilization, sea level rise ranks significantly behind overfishing, and well behind some of the other potential, but not certain consequences of global warming including ocean acidification, ocean annoxia, and reduced food yield.

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  15. I would have said that disruption to the hydrological cycle is by far the biggest threat posed by climate change. The implications of Dai 2010 are scary but dont seem grab attention the way sealevel rise does.

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  16. Tom @14,

    I've played with such maps and that hasn't comforted me. Again, for the coming century you may well be right, as also suggested in this paper by Nicholls et al (2010):
    http://rsta.royalsocietypublishing.org/content/369/1934/161.full

    They estimate up to 187 million people forced to relocate during this century in the worst-case (2.4% of the current global population). They also speak about adaptation 'pessimists' and 'optimists':
    "The ‘pessimists’ seem to take it as read that adaptation will either fail or people will not even try to adapt. In contrast, the ‘optimists’ appear overly confident that benefit–cost approaches describe human behaviour in response to threats such as sea-level rise."

    Beyond 2100 it seems less clear to me that adaptation will be the preferred option, or even technically or economically possible, if SLR by that time is so fast that cities/countries will need to adapt almost continuously (and not only to SLR of course). Miami/Florida for example seems almost impossible to defend, even in this century, due to its porous geological foundation:
    http://thinkprogress.org/climate/2013/06/23/2199031/scientist-miami-as-we-know-it-today-is-doomed-its-not-a-question-of-if-its-a-question-of-when/

    Meanwhile probably some cities/countries will adapt in time to prevent big disasters from happening, but others will likely be too late, so disasters wil probably be bigger and more frequent.

    Also protection costs will rise disproportionately with higher sea level, according to Nicholls et al:
    "[A] dike required in response to a 2m rise in sea level is assumed to be four times the cost of that required for a 1m rise in sea level."

    Since we can't simply assume the world will become richer and richer, due to both climate change and resource depletion, these rising adaptation costs will at some point become an unbearable burden on public budgets. The less we invest in mitigation the sooner we will probably reach this point.

    But at the same time: the less we invest in adaptation, the more and bigger disasters will happen. So we need to invest more now, in both mitigation and adaptation, to prevent costs from becoming unbearable further into the future.

    In Holland, where I live, planners now assume a worst-case of 60 cm in 2100 and 120 cm in 2200, based on IPCC 2007. They know it could be worse, but at this point we don't insure ourselves against that risk. The Dutch Delta Committee considers 130 cm by 2100 and 4 m by 2200 as the worst-case (including subsidence). They think we could adapt to such rises.

    Our Environmental Assessment Agency and Delft University think we can adapt to 1.5 meter/century for at least four centuries. They consider this as a worst-case, even though the main adviser to the Delta Committee say there's a significant risk of more than 1.5 meter/century.

    So, returning to the question, is SLR by itself a civilizational threat? Maybe not, but perhaps I'm more of a pessimist than you are. I agree that other threats may be bigger, certainly during this century. But like I said, SLR adds to these other risks, and may be very significant in itself in coming centuries.

    Also, the world is much fuller now than during the Black Death, and we have nuclear weapons now, which is a big civilizational threat in itself. Still, you may be right that civilization is more resilient than I fear, and that even billions of deaths will not hurt civilization much, unless maybe caused by nuclear war. We should minimize such risks, however, so including the risk of SLR, in my mind.

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  17. Tom,

    I looked  at the sea level rise map you linked here.  I looked at Miami since I live in Florida.  Your map showed little damage in Miami at a 3 meter sea level rise even though I know that the land side of Miami is only one meter high and the entire area should be inunduated. I found this map from Climate Central (which only shows the USA).  It shows that Miami is only a handful of islands after 3 meters of sea level rise.  I found this UNEP report which shows a lot of Bangladesh (15 millon people's houses) underwater after 1 meter of sea level rise while your link shows only a few dots of water.  Substantial parts of Bangladesh will be gone with even 1 meter of sea level rise, much less 4 meters.  Reading the background on your link it says the elevation is from satalite data and shows the tops of the trees and buildings.  This does not seem very useful to me.

    It is my experience that these maps underestimate the damage of sea level rise.  They show inundated area, but  I cannot live in a house that is 10 cm above sea level.  Any surge would flood the house.  For this reason .25 meters of sea level rise appears to have no damage, because a minimum of .25 meters is required everywhere to avoid floods.   If sea level rises .25 meters the people who live .5 meters above sea level will have to move because they will flood with any storm surge.  Usually people move after disasters like hurricanes.  This is affecting Miami now as low lying areas flood from high tides they used to be resistant to. In addition, areas like Miami are cut off from the mainland and will become useless islands with no fresh water before they are inundated.

    The deltas of the world have a greatly disproportionate amount of the good farming land.  If the Nile delta (covered in the UNEP link above) is flooded, the farmers cannot simply move into the desert and expect to be able to raise crops like they formerly did.  Virtually all the deltas are low-lying.

    Sea level rise will not wipe out civilization, but even 1 meter of rise will displace millions of people worldwide, and the food source for many more.  This will not happen tomorrow, but it is an important problem.

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  18. Michael Sweet @17, the UNEP report shows 15 million (10% of the population) effected, not inundated.  It also shows 17,000 km2 (11.5%) inundated.  The percentage inundates will be lower than the 10% because the coastal area effected is not, for the most part a high population area (for Bangladesh).  I am not entirely sure what is meant by "effected" given that a larger area has already been effected by increased salinity, and the entire country will be effected by increased riverine flood levels with higher sea levels.  Presumably it means that part of the population whose land is either inundated or becomes vulnerable to storm surge.

    That quibble aside, I do not disagree with anything you say.  Bear in mind that I am responding to the claim that:


    "And adding the risk of large and fast SLR to other severe climate risks probably does change the maths into making even relatively limited global warming a civilizational threat, that we better insure ourselves against, as far as we still can."


    My point has not been that sea level rise is costless, or even low cost.  It is that relative to other effects of global warming it is a minor cost, and that given that tackling global warming will be expensive, the costs from sea level rise would not justify it alone.  Therefore sea level rise is not likely to cause a level of global warming that does not threaten our civilization to become a threat to that civilization.  In this it is unlike threats from ocean acidification, anoxia, ecosystem collapse and simple ongoing temperature increases which within two centuries with BAU could rise to levels which make the tropics seasonally uninhabitable.  Thus, while from Bangladesh's perspective (and Florida's), sea level rise is the most imminent and dangerous threat from global warming, this is not so globally where it ranks well down the list.

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  19. Tom,

    I do not think that that we substantially disagree.  Reasonable people may have different concerns when looking at projections many years in the future.

    It is surreal when I suggest that having the homes of several hundred million people destroyed, along with a considerable percentage of the best farmland in the world, is a problem.  You return that we have to keep our eye on the big problems, not the smaller ones and I have to agree. What kind of a discussion is that?  Hopefully Hansen's message  will start to sink in and something will be done.

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  20. michael sweet @19, agreed on all points.

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  21. To return to the paper itself: in the conclusion it says that 13 experts (out of 82) think there's about a 17% chance that SLR by 2100 could be more than 2 meters in the RCP 8.5 scenario. So it seems about 16% of the experts think there's a substantial risk of 2 meters or more in the worst-case.

    Figure 2 of the paper shows that 5 out of 82 experts think there's about a 5% chance that SLR by 2100 could be 3 meters or more in this BAU scenario, with the highest estimate about 7 meters. So it seems about 6% of the experts think there's some risk of more than 3 meters of SLR by 2100 in the worst-case.

    But even in the strong mitigation scenario of RCP 3 three experts out of 84 think there's about a 5% chance of more than about 1.5 meters of SLR by 2100., with the highest estimate around 2 meters. So it seems almost 4% of the experts think that even in the best case there's some risk of more than 1.5 meters of SLR by 2100.

    About 50% of the experts think that in this best case there's about a 17% chance of more than 0.6 meters of SLR by 2100 and about a 5% chance of more than 0.7 meters. So even in the best case there seems to be a significant risk of more than 60 cm of SLR around 2100.

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  22. Some more results from figure 2:

    - 7 out 72 experts think there's about a 5% chance of 7 meters or more by 2300 under BAU, with the highest estimate about 15 meters.

    - 5 out of 72 experts think there's about a 17% chance of 6 meters or more by 2300 under BAU, with a highest estimate of almost 10 meters.

    - 3 out 74 experts think there's about a 17% chance of 3 meters or more by 2300 even in the best case, with 3.5 meters as highest estimate.

    - 3 out of 74 experts think there's about a 5% chance of 4 meters or more by 2300 in the best case, with almost 8 meters as the highest estimate. 

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  23. lennartvdl @21, I will concentrate to the upper likely estimate (83% confidence bound) for rcp 8.5 in 2100, as the range of opinions is shown by this graph from Real Climate:

     

     As can be seen, there is a long tail of opinion with the highest value at 6 meters.  So, how should this guide us as policy makers (or voters for policy makers).  Well, first we recognize that we are not expert, so that our independant judgement based on the evidence we look at is likely to be far more unreliable than that of the experts, both because we have less information, and because we have less experience in the field, leading us to be prone to "rooky mistakes".  Second, we recognise that scientists, as scientists are also prone to mistakes; but that they are self correcting at an institutional level.  (Ideally they would be self correcting at an individual level, but the history of science shows that to be an ideal rather than a universal reality.)  Therefore we focus on the areas of agreement among scientists rather than the outliers.

    At that stage, we notice that while there is a long tail, the number of experts in the tail from 2 meters up (14) is less than the number of experts who are lowballing the risk (17 with an estimate of 0.5 meters).  The two groups more or less cancel each other out, and we are left with the two central groups (1 - 1.5 meters).

    Third, we are cautious, but not overly so in our estimate.  Therefore we take the upper bound of the concensus estimates.  Hence 1.5 meters.

    As an actual policy maker, I would do that more formally, or at least I would get my appropriate staff to do so.  I would get them to develop a Probability Density Function of the expert estimates.  Further, I would get them to develop cost/benefit evaluations for my nation for a range of adaption responses relative to the range of estimates.  I would then attempt to begin the adaption program with the best expected utility given the PDF from the expert elicitation (withing budget).  As cost increase substantially with sea level rise, this would result in an adaption response targeted for the upper range of the PDF, but within the main body of upper estimates (ie, most likely 1-1.5 meters).  It would be higher for nations like Bangladesh and and lower for nations like Australia; higher for states like Florida, and lower for states like Oregon.  I would also need to normalize the sea level rise values relative to expected local rise for a given global rise, as the sea level rise is not the same in all areas.

    The concentration you show on upper estimates by the most extremely pessimistic experts is not a rational response to sea level rise.  It ignores most of the evidence, and all of the costs of responding, which are substantial.

    Finally, I have focussed on adaption because mitigation must have the same policy in response to all the risks of global warming plus OA.  As sea level rise is a minor portion of that risk (while still substantial) it has little effect on assessing the need for mitigation.  We should be mitigating anyway, even if we thought sea level rise was going to peak at 100 cm, and the ideal level of mitigation would only rise slightly with sea level rises of 3 meters by 2100. 

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  24. Tom @23,

    Thanks for the nice sketch of your risk approach, which I think is quite reasonable. I do have some questions though.

    How do you/we best estimate the potential (moral and economic) risks, costs and benefits of (not) investing in adaptation vs mitigation? Or put differently: what would be your preferred adaptation and mitigation policy goals?

    For example: should we use the risk of 1.5 meters of SLR by 2100 as a worst-case in adaptation planning, as long as there’s no strong mitigation policy in place? And if so, on what grounds should we ignore the risk estimate of the circa 17% of experts who think there’s a substantial risk of 1.75 meters or more by 2100 under BAU? We have to draw a line somewhere, but why should that be the line?

    And what should be our mitigation goal? Should it be the 6% per year global CO2 emissions reductions that Hansen et al recently argued for? What (economic or other) risks would such a goal/policy entail? Or should it be the 50% reductions by 2050 that UNFCCC still has its goal (on paper at least)? What risks would remain with that less ambitious goal, for SLR and other effects, and how do you/we weigh all those risks?

    For example: it seems even with strong mitigation there’s at least some risk of more than 1.5 meters of SLR by 2100 and more than 4 meters by 2300. With less mitigation the risks will be higher. How much risk should we as global society accept, and what does this imply for the risks that specific countries, regions and cities should accept, such as small island states, Bangladesh and others?

    I hope you could explain some more where your reasoning would lead us, and am certainly willing to explore and discuss where for example Hansen’s prescription could lead us, if your advice would be substantially different from his.

    Thanks again for your input, Lennart

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