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The IPCC sea level numbers

Filed under: — stefan @ 27 March 2007 - (Português)

The sea level rise numbers published in the new IPCC report (the Fourth Assessment Report, AR4) have already caused considerable confusion. Many media articles and weblogs suggested there is good news on the sea level issue, with future sea level rise expected to be a lot less compared to the previous IPCC report (the Third Assessment Report, TAR). Some articles reported that IPCC had reduced its sea level projection from 88 cm to 59 cm (35 inches to 23 inches) , some even said it was reduced from 88 cm to 43 cm (17 inches), and there were several other versions as well (see “Broad Irony”). These statements are not correct and the new range up to 59 cm is not the full story. Here I will try to clarify what IPCC actually said and how these numbers were derived. (But if you want to skip the details, you can go straight to the critique or the bottom line).

What does IPCC say?

The Summary for Policy Makers (SPM) released last month provides the following table of sea level rise projections:

Sea Level Rise
(m at 2090-2099 relative to 1980-1999)
Case Model-based range
excluding future rapid dynamical
changes in ice flow
B1 scenario 0.18 – 0.38
A1T scenario 0.20 – 0.45
B2 scenario 0.20 – 0.43
A1B scenario 0.21 – 0.48
A2 scenario 0.23 – 0.51
A1FI scenario 0.26 – 0.59

It is this table on which the often-cited range of 18 to 59 cm is based. The accompanying text reads:

• Model-based projections of global average sea level rise at the end of the 21st century (2090-2099) are shown in Table SPM-3. For each scenario, the midpoint of the range in Table SPM-3 is within 10% of the TAR model average for 2090-2099. The ranges are narrower than in the TAR mainly because of improved information about some uncertainties in the projected contributions15. {10.6}.

Footnote 15: TAR projections were made for 2100, whereas projections in this Report are for 2090-2099. The TAR would have had similar ranges to those in Table SPM-3 if it had treated the uncertainties in the same way.

• Models used to date do not include uncertainties in climate-carbon cycle feedback nor do they include the full effects of changes in ice sheet flow, because a basis in published literature is lacking. The projections include a contribution due to increased ice flow from Greenland and Antarctica at the rates observed for 1993-2003, but these flow rates could increase or decrease in the future. For example, if this contribution were to grow linearly with global average temperature change, the upper ranges of sea level rise for SRES scenarios shown in Table SPM-3 would increase by 0.1 m to 0.2 m. Larger values cannot be excluded, but understanding of these effects is too limited to assess their likelihood or provide a best estimate or an upper bound for sea level rise. {10.6}

• If radiative forcing were to be stabilized in 2100 at A1B levels, thermal expansion alone would lead to 0.3 to 0.8 m of sea level rise by 2300 (relative to 1980–1999). Thermal expansion would continue for many centuries, due to the time required to transport heat into the deep ocean. {10.7}

• Contraction of the Greenland ice sheet is projected to continue to contribute to sea level rise after 2100. Current models suggest ice mass losses increase with temperature more rapidly than gains due to precipitation and that the surface mass balance becomes negative at a global average warming (relative to pre-industrial values) in excess of 1.9 to 4.6°C. If a negative surface mass balance were sustained for millennia, that would lead to virtually complete elimination of the Greenland ice sheet and a resulting contribution to sea level rise of about 7 m. The corresponding future temperatures in Greenland are comparable to those inferred for the last interglacial period 125,000 years ago, when paleoclimatic information suggests reductions of polar land ice extent and 4 to 6 m of sea level rise. {6.4, 10.7}

• Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude. {4.6, 10.7}

• Current global model studies project that the Antarctic ice sheet will remain too cold for widespread surface melting and is expected to gain in mass due to increased snowfall. However, net loss of ice mass could occur if dynamical ice discharge dominates the ice sheet mass balance. {10.7}

• Both past and future anthropogenic carbon dioxide emissions will continue to contribute to warming and sea level rise for more than a millennium, due to the timescales required for removal of this gas from the atmosphere. {7.3, 10.3}

(The above quotes document everything the SPM says about future sea level rise. The numbers in wavy brackets refer to the chapters of the full report, to be released in May.)

What is included in these sea level numbers?

Let us have a look at how these numbers were derived. They are made up of four components: thermal expansion, glaciers and ice caps (those exclude the Greenland and Antarctic ice sheets), ice sheet surface mass balance, and ice sheet dynamical imbalance.

1. Thermal expansion (warmer ocean water takes up more space) is computed from coupled climate models. These include ocean circulation models and can thus estimate where and how fast the surface warming penetrates into the ocean depths.

2. The contribution from glaciers and ice caps (not including Greenland and Antarctica), on the other hand, is computed from a simple empirical formula linking global mean temperature to mass loss (equivalent to a rate of sea level rise), based on observed data from 1963 to 2003. This takes into account that glaciers slowly disappear and therefore stop contributing – the total amount of glacier ice left is actually only enough to raise sea level by 15-37 cm.

3. The contribution from the two major ice sheets is split into two parts. What is called surface mass balance refers simply to snowfall minus surface ablation (ablation is melting plus sublimation). This is computed from an ice sheet surface mass balance model, with the snowfall amounts and temperatures derived from a high-resolution atmospheric circulation model. This is not the same as the coupled models used for the IPCC temperature projections, so results from this model are scaled to mimic different coupled models and different climate scenarios. (A fine point: this surface mass balance does include some “slow” changes in ice flow, but this is a minor contribution.)

4. Finally, there is another way how ice sheets can contribute to sea level rise: rather than melting at the surface, they can start to flow more rapidly. This is in fact increasingly observed around the edges of Greenland and Antarctica in recent years: outlet glaciers and ice streams that drain the ice sheets have greatly accelerated their flow. Numerous processes contribute to this, including the removal of buttressing ice shelves (i.e., ice tongues floating on water but in places anchored on islands or underwater rocks) or the lubrication of the ice sheet base by meltwater trickling down from the surface through cracks. These processes cannot yet be properly modelled, but observations suggest that they have contributed 0 – 0.7 mm/year to sea level rise during the period 1993-2003. The projections in the table given above assume that this contribution simply remains constant until the end of this century.

As an example, take the A1FI scenario – this is the warmest and therefore defines the upper limits of the sea level range. The “best” estimates for this scenario are 28 cm for thermal expansion, 12 cm for glaciers and -3 cm for the ice sheet mass balance – note the IPCC still assumes that Antarctica gains more mass in this manner than Greenland loses. Added to this is a term according to (4) simply based on the assumption that the accelerated ice flow observed 1993-2003 remains constant ever after, adding another 3 cm by the year 2095. In total, this adds up to 40 cm, with an ice sheet contribution of zero. (Another fine point: This is slightly less than the central estimate of 43 cm for the A1FI scenario that was reported in the media, taken from earlier drafts of the SPM, because those 43 cm was not the sum of the individual best estimates for the different contributing factors, but rather it was the mid-point of the uncertainty range, which is slightly higher as some uncertainties are skewed towards high values.)

How do the new numbers compare to the previous report?

Sea level rise as observed (from Church and White 2006) shown in red up to the year 2001, together with the IPCC (2001) scenarios for 1990-2100. See second figure below for a zoom into the period of overlap.

The TAR showed sea level rise curves for a range of emission scenarios (shown in the Figure above together with the new observational record of Church and White 2006). The range was based on simulations with a simple model (the MAGICC model) tuned to mimic the behaviour of a range of different complex climate models (e.g. in terms of different climate sensitivities ranging from 1.7 to 4.2 ºC), combined with simple equations for the glacier and ice sheet mass balances (“degree-days scheme”). This model-based range is shown as the grey band (labelled “Several models all SRES envelope” in the original Figure 5 of the TAR SPM) and ranged from 21 to 70 cm, while the central estimate for each emission scenario is shown as a coloured dashed line. The largest central estimate of sea level rise is for the A1FI scenario (purple, 49 cm).
In addition, the dashed grey lines indicate additional uncertainty in ice sheet behaviour. These lines were labelled “All SRES envelope including land ice uncertainty” in the TAR SPM and extended the range up to 88 cm, adding 18 cm at the top end. One has to delve deeply into the appendix of Chapter 11 of the TAR to find out what these extra 18 cm entail: they include a “mass balance uncertainty” and an “ice dynamic uncertainty”, where the latter is simply assumed to be 10% of the total computed mass loss of the Greenland ice sheet. Note that such an ice dynamic uncertainty was only included for Greenland but not for Antarctica; instability of the West Antarctic Ice Sheet, a scenario considered “very unlikely” in the TAR, was explicitly not included in the upper limit of 88 cm.

As we mentioned in our post on the release of the SPM, it is apples and oranges to say that IPCC reduced the upper sea level limit from 88 cm to 59 cm, as the former included “ice dynamic uncertainty” (albeit only for Greenland, as rapid ice flow changes in Antarctica were considered too unlikely to bother at the time), while the latter discusses this ice flow uncertainty separately in the text, stating it could add 10 cm, 20 cm or even more to the 59 cm in the table.

So is it better to compare the model-based range 21 – 70 cm from the TAR to the 18 – 59 cm from the AR4? Even that is apples and oranges. For one, TAR cites the rise up to the year 2100, the AR4 up to the period 2090-2099, thus missing the last 5 years (or 5.5 years, but let’s not get too pedantic) of sea level rise. For 2095, the TAR projection reduces from 70 cm to 65 cm (the central estimate for A1FI reduces from 49 cm to 46 cm). Also, the TAR range is a 95% confidence interval, the AR4 range a narrower 90% confidence interval. Giving the TAR numbers also as 90% ranges shaves another 3 cm off the top end.

Sounds complicated? There are some more technical differences… but I will spare you those. The Paris IPCC meeting actually discussed the request from some delegates to provide a direct comparison of the AR4 and TAR numbers, but declined to do this in detail for being too complicated. The result was the two statements:

The TAR would have had similar ranges to those in Table SPM-3 if it had treated the uncertainties in the same way.


For each scenario, the midpoint of the range in Table SPM-3 is within 10% of the TAR model average for 2090-2099.

(In fact delegates were told by the IPCC authors in Paris that with the new AR4 models, the central estimate for each scenario is slightly higher that with the old models, if numbers are reported in a comparable manner.)

The bottom line is thus that the methods have significantly improved (which is the reason behind all those methodological changes), but the expectation of how much sea level will rise in the coming century has not significantly changed. The biggest change is that ice sheet dynamics look more uncertain now than at the time of the TAR, which is why this uncertainty is not included any more in the cited range but discussed separately in the text.

Critique – Could these numbers underestimate future sea level rise?

There’s a number of issues worth discussing about these sea level numbers.

The first is the treatment of potential rapid changes in ice flow (item 4 on the list above). The AR4 notes that the ice sheets have been losing mass recently (the analysis period is 1993-2003). Greenland has contributed +0.14 to +0.28 mm/year of sea level rise over this period, while for Antarctica the uncertainty range is -0.14 to +0.55 mm/year. It is noted that the mass loss of Antarctica is mostly or entirely due to recent changes in ice flow. The question then is: how much will this process contribute to future sea level rise? The honest answer is: we don’t know. As the SPM states, by the year 2095 it could be 10 cm. Or 20 cm. Or more. Or less.

The IPCC included one guess into the “model-based range” provided in the table: it took half of the Greenland mass loss and the whole Antarctic mass loss for 1993-2003, and assumed this would remain constant ever after until 2100. This assumption in my view has no scientific basis, as the ice-flow is almost certainly highly variable in time. The report itself states that this ice loss is due to a recent acceleration of flow, and that in 2005 it was already higher, and that in future the numbers could be several times higher – or they could be lower. Adding such an ill-founded number into the “model-based” range degrades the much more reliable estimates for thermal expansion, mountain glaciers and mass balance. Even worse: to numbers with error estimates, it adds a number without proper error estimate (the observational uncertainty for 1993-2003 is included, but who would claim this is an error estimation for future ice flow changes?). And then it presents only the combined error margins – you will notice that no central estimate is provided in the above table. If I had presented this as an error calculation in a first-semester physics assignment, I doubt I would have gotten away with it. The German delegation in Paris (of which I was a member) therefore suggested taking this ice-flow estimate out of the tabulated range. The numbers would have become slightly lower, but this approach would not have mixed up very different levels of uncertainty, and it would have been clear what is included in the table and what is not (namely ice flow changes), rather than attempting to partially include ice flow changes. The ice flow changes could have been discussed in the text – stating there that at the 1993-2003 rate, this term would contribute 3 cm by 2095, but it is bound to change and could turn out to be 10 cm or 20 cm or more. However, we found no support for this proposal, which would not have changed the science in any way but improved the clarity of presentation.

As it is now, because of the complex and opaque way of combining the errors, even I could not tell you by how much the upper limit of 59 cm would be reduced if the questionable ice flow estimate was taken out, and one of the reasons provided by the IPCC authors for not adopting our proposal was that the numbers could not be calculated quickly.

A second problem with the above range is that the models used to derive this projection significantly underestimate past sea level rise. We tried in vain to get this mentioned in the SPM, so you have to go to the main report to find this information. The AR4 states that for the period 1961-2003, the models on average give a rise of 1.2 mm/year, while the data show 1.8 mm/year, i.e. a 50% faster rise. This is despite using observed ice sheet mass loss (0.19 mm/year) in the “modelled” number in this comparison, otherwise the discrepancy would be even larger – the ice sheet models predict that the ice sheets gain mass due to global warming. The comparison looks somewhat better for the period 1993-2003, where the “models” give a rise of 2.6 mm/year while the data give 3.1 mm/year. But again the “models” estimate includes an observed ice sheet mass loss term of 0.41 mm/year whereas ice sheet models give a mass gain of 0.1 mm/year for this period; considering this, observed rise is again 50% faster than the best model estimate for this period. This underestimation carries over from the TAR models (see Rahmstorf et al. 2007 and the Figure below) – this is not surprising, since the new models give essentially the same results as the old models, as discussed above.

Comparison of the 2001 IPCC sea-level scenarios (starting in 1990) and observed data: the Church and White (2006) data based primarily on tide gauges (annual, red) and the satellite altimeter data (updated from Cazenave and Nerem 2004, 3-month data spacing, blue, up to mid-2006) are shown with their trend lines. Note that the observed sea level rise tends to follow the uppermost dashed line of the IPCC scenarios, namely the one “including land ice uncertainty”, see first Figure.

We therefore see that sea level appears to be rising about 50% faster than models suggest – consistently for the 1961-2003 and the 1993-2003 periods, and for the TAR models and the AR4 models. This could have a number of different reasons, and the discrepancy could be considered not significant given the error ranges of observations and models. It is no proof that models underestimate future sea level rise. But it is at least a plausible possibility that the models may underestimate future rise.

A third issue worth mentioning is that of carbon cycle feedback. The temperature projections provided in table SPM-3 of the Summary for Policy Makers range from 1.1 to 6.4 ºC warming and include carbon cycle feedback. The sea level range, however, is based on scenarios that exclude this feedback and thus only range up to 4.5 5.2 ºC. This could easily be misunderstood, as in table SPM-3 the temperature ranges including carbon cycle feedback are shown right next to the sea level ranges, but the latter actually apply to a smaller temperature range. As a rough estimate, I suggest that for a 6.4 ºC warming scenario, of the order of 20 15 cm would have to be added to the 59 cm defining the upper end of the sea level range.

A final point is the regional aspects. Planners of coastal defences need to be aware that sea level rise will not be the same everywhere. The AR4 shows a map of regional sea level changes, which shows that e.g. European coasts can expect a rise by 5-15 cm more than the global mean rise – that is a model average, not including an uncertainty range. The pattern in this map is remarkably similar to that expected from a slowdown in thermohaline circulation (see Levermann et al. 2005) so probably it is dominated by this effect. In addition, some land areas are rising and some are subsiding in response to the end of the last Ice Age or due to local anthropogenic processes (e.g. groundwater withdrawal), which local planners need to account for.

The bottom line

The main conclusion of this analysis is that sea level uncertainty is not smaller now than it was at the time of the TAR, and that quoting the 18-59 cm range of sea level rise, as many media articles have done, is not telling the full story. 59 cm is unfortunately not the “worst case”. It does not include the full ice sheet uncertainty, which could add 20 cm or even more. It does not cover the full “likely” temperature range given in the AR4 (up to 6.4 ºC) – correcting for that could again roughly add 20 15 cm. It does not account for the fact that past sea level rise is underestimated by the models for reasons that are unclear. Considering these issues, a sea level rise exceeding one metre can in my view by no means ruled out. In a completely different analysis, based only on a simple correlation of observed sea level rise and temperature, I came to a similar conclusion. As stated in that paper, my point here is not that I predict that sea level rise will be higher than IPCC suggests, or that the IPCC estimates for sea level are wrong in any way. My point is that in terms of a risk assessment, the uncertainty range that one needs to consider is in my view substantially larger than 18-59 cm.

A final thought: this discussion has all been about sea level rise until the year 2095. Sea level rise does not end there, as the quotes from the SPM at the beginning of this article show. Over several centuries, without serious mitigation efforts we may expect several meters of sea level rise. The Advisory Council on Global Change of the German government (disclosure: I’m a member of this body) in its recent special report on the oceans has proposed to limit long-term sea level rise to a maximum of one meter, as a guard-rail to guide climate policy. But that’s another story.

Update: I was just informed by one of the IPCC authors that the temperature scenarios without carbon cycle feedback range up to 5.2 ºC, not 4.5 ºC as I had assumed. This number is not found in the IPCC report; I had tried to interpret it from a graph, but not accurately enough. My apologies! The numbers in the text above that had to be corrected are marked by strikethrough font. -stefan

308 Responses to “The IPCC sea level numbers”

  1. 51
    tom says:

    18. Well, that’s a very confident view of the accuracy of predictive models. Not in any way justified,IMO , but confident.
    {see 2006 hurricane season}

    19. How accurate can we expect temperature recreations to be using proxies form 100,0000 years ago ??

  2. 52
    mark says:

    Too many faults in the ice core data, and the whole “it`s our fault” argument to know where to start.
    1) temperature jumps preceding co2 level jumps. I checked out the page on this site trying to explain the 800 year lag, except that they couldn`t explain it. They gave a circular argument saying that something kick started the temperature jump, which led to the co2 rise… but they don`t know what. Exactly, they don`t know.
    2) temperature drops often precede co2 drops as well. so if the co2 “feedback” is in effect, how does the temperature suddenly drop? and if the co2 did suddenly drop before the temperature, why in the heck would it when according to this site, co2 rise leads to runaway temperature and co2 rise due to this feedback effect.
    3) the dramatic rise in co2 levels started before the industrial revolution.
    4) the circular nature of the conclusions raised upon analysis of the data on this site, ie, the non-existence of any possible explanation for the initial lag between temperature and co2 rise before the alleged “feedback” effect comes into place, show that they are nothing more than pure assumptions based on the hypothesis that humans are at fault for the most recent rise, itself based on non-existent evidence. the conclusion that humans are at fault is just plain unsubstantiated, illogical and unscientific.

    If the sea rises, move. It will be gradual enough for most of the “developed” world to cope with. It`s strange I didn`t see any names on this thred even remotely Bangladeshi, and yet there are people who seem worried that they are going to wake up the next morning with their house submerged. Instead of wasting our time and effort saving the people who need it least, we should be spending it helping those who need it whether there`s indeed an ocean rise or not.

    Now, if this feedback effect is actually on the mark, and it`s runaway train from now on, it`s really too late for us to do anything except start the bidding on Mount Everest for the sake of our offspring 1000 years down the road. so let`s stop the arguments about how many decimeters and how much sloping will affect the rise. I appreciate the honest sentiments about saving the planet for our offspring, but if the earth says it`s time, it`s time. we should say so long, and thanks for the memories.

  3. 53
    Lynn Vincentnathan says:

    Re #23 & 14, there is also the problem of trees dying near the coast, as the salt water underground seeps further inland — which has been happening at an accelerated rate in India, Florida, Caribbean islands & elsewhere (places that have been slightly sinking, even without sea rise compounding this). And I think (not sure) that the saltwater also is bad for underground cement structures near the coast.

  4. 54
    John L. McCormick says:

    RE # 50

    Thank you George for putting those facts on the table.

    Stefan had me worried there for a minute.

  5. 55
    llewelly says:

    tom (51):

    18. Well, that’s a very confident view of the accuracy of predictive models. Not in any way justified,IMO , but confident.
    {see 2006 hurricane season}

    Global climate models are judged by testing how well they simulate the
    recent past – not by comparing them to the very different forecast
    models of peripherally related discipline (seasonal hurricane
    forecasting) . The AR4 SPM includes information on how well GCMs
    simulate the recent past – see page 11.

    In addition, although GCMs have improved greatly since 1988, Eli’s
    discussion of Jim Hansen’s 1988 GCM-based forecast, and comparison to
    how things actually turned out, is quite informative; even 19 years
    ago, NASA’s climate scientists were able to accurately forecast
    global temperatures up to the present.

    The methods used for seasonal hurricane forecasting are substantially different
    from those used for global climate models; they are very different
    problems. Your logic is akin to someone vacationing in Key West, who,
    upon hearing the NHC has forecast a major hurricane will pass nearby
    (or landfall) in 48 hours, insists that the NHC’s forecast should be
    ignored because the University of Florida has a lousy football
    team. Just as the association between UFL and NHC is not justification
    for judging the quality of NHC’s forecasts by the quality of UFL’s
    football team, the fact that seasonal hurricane forecasters and
    climate scientists attend some of the same conferences is not
    justification for judging global climate models by the success of
    William Gray’s (or TSR’s) statistical hurricane season forecasting.

  6. 56
    David Graves says:

    Yo, Tom, regular civilian readers of RC (yours truly fall into this catgory) would know that, as with many things, one must remeber all the pieces of the assumptions involved. The weak ENSO of 2006-07 altered the pattern of Northern Hemisphere mid-latitude westerlies, and *the models predict lower Atlantic hurricane intensity*, which is what was observed.

  7. 57
    yartrebo says:

    Re #50:
    “- The poles go through (nearly) 6 months of darkness each year. With no sunlight in the winter, and with the sun at a low angle in the sky in the summer, it is always going to be cold at the poles and any melting in the summer is going to freeze back in the winter;”

    History and climate physics disagree with you on this one. Heat transport and a thick greenhouse gas blanket were enough to make the South Pole a temperate region at one point during the Mesozoic Era.

    Even if the average temperature is below 0C, ice sheets will still melt if there is any more than minimal melting in the summer, because the melt water does not stay on the surface but runs off, usually to the base of the ice sheet.

    “- The interior of Greenland is below sea level. This is important if one is thinking of all of the ice melting or one is thinking of the glaciers “sliding off” into the sea.”

    There is plenty of ice above sea level in Greenland, and isostatic rebound will work to bring most areas that are under sea level back above sea level.

  8. 58
    yartrebo says:

    Re #53:

    Salt water intrusion (at least around here, in NYC) is usually caused by humans pumping too much groundwater, though a sea level rise could only hurt.

    As far as concrete goes, it is unaffected by salt and actually strengthened by water.

  9. 59
    Hank Roberts says:

    I sure wish we had a parallel topic in which people had to provide cites and the cites were checked first, before posting their opinions and beliefs and asserting them as scientific fact.

    Even reading RC every day, it’s increasingly hard to know who’s making things up.

    The information’s getting awful thin compared to the pretend stuff, and we’re seeing more double-team (one person posts bogosity, another comes along and posts thanks and gratitude for the ‘facts’).

    Don’t let this site get taken over, please. Ask some new reader to go through a topic like this and naively rate each post for superficial credibility. See if I’m right.

  10. 60
    John L. McCormick says:

    RE #59

    Hank, my apology for posting #54.

    That was my snarky reply; not a note of gratitude. I will be more careful next time.

    And, I am in total agreement with your concern RC might be overtaken by trolls and diversionists.

  11. 61
    Andrew Sipocz says:

    Stefan: Are there groups of scientists coring around the edges of Greenland trying to determine the minimum extent and thickness of the ice sheet during the last interglacial? A reconstruction of Greenland’s minimum ice sheet thickness would spark a lot of intuitive questions and maybe some answers about ice sheet dynamics.

  12. 62
    Dick Veldkamp says:

    Re #60 (John McCormick)

    I thought your reply was quite funny actually – a little snarkiness should be OK.

  13. 63

    [[ The poles go through (nearly) 6 months of darkness each year. With no sunlight in the winter, and with the sun at a low angle in the sky in the summer, it is always going to be cold at the poles and any melting in the summer is going to freeze back in the winter;]]

    That’s an elegant theory, but since the extent of the ice caps has indeed been shrinking (we can measure it from satellites), your theory doesn’t seem to match the evidence.

  14. 64

    [[1) temperature jumps preceding co2 level jumps. I checked out the page on this site trying to explain the 800 year lag, except that they couldn`t explain it. They gave a circular argument saying that something kick started the temperature jump, which led to the co2 rise… but they don`t know what. Exactly, they don`t know.]]

    But they do know. Do a Google search on “Milankovic cycles.”

  15. 65
    Richard Ordway says:

    #59. Hank wrote:

    [[I sure wish we had a parallel topic in which people had to provide cites and the cites were checked first]]

    I agree Hank. Something needs to be done about this website. This website is being taken over by histrionics and questionable experts and is defeating its original purpose which is to inform the public.

    Now, the public must be more confused than ever by the postings on this website.

    I can’t tell who is trolling, who is legit and who is obfuscating the truth.

    Probably, I am partly responsible, as you pointed out, by responding to trolls and leading the posts off track myself as well…and who even knows if my background is legit… or anyone else’s for that matter?

    We certainly know that some posters and groups of posters, by their repeated rants, are trying to destroy the continutiy of the threads…sucessfully I might add.

    What is happening now, is defeating the original purpose of this website.

    I read that this website was designed to be a link between climate science and the public…right now it is turning into a link between pseudo science (from the wierd public posts) and idealogues.

    Personally, I almost never posted to this site until I read that Gavin posted something such as…”OK readers, help me out” for responding to the myriad of seemingly legit, endlessly repeated public questions.

    Moderators…please take this seriously. Let’s not destroy this hugely sucessful open link between legitimate scientists and the legitimate public.

    Do whatever you have to do…it seems the community does not have to ability to sucessfully control the disrupters.

  16. 66

    Interesting article – particularly as a study by researchers at Columbia University, the City University of New York and the International Institute for Environment and Development has just found that around one-tenth of the world’s population lives within 10 metres of sea level. And in China and Bangladesh, the share of population in such zones is growing fast. See for more details.

  17. 67
    Ike Solem says:

    Measurements from Greenland are that between 2003 and 2005, 172 cubic kilometers (41 cubic miles) of ice was lost, three times as much as was added ( ). This kind of ice sheet response was not predicted by earlier models, and the direct gravity satellite observations (Grace) seem very reliable. (Previous studies were attacked by the likes of, who claimed that Greenland was actually gaining mass).

    This might not affect the equilibrium climate sensitivity, but will affect the transient climate sensitivity – the rate of response to forcing. It’s also a good example of how science can miss important variables, as well as of how a rapidly warming world might behave very differently than a world locked into a glacial/interglacial cycle will. A decade ago, it was widely agreed that the West Antarctic and Greenland ice sheets were stable and would melt slowly, like stationary ice cubes.

    With respect to the other missing variable in the IPCC reports, it seems that carbon cycle feedbacks will affect the equilibrium climate sensitivity to anthropogenic CO2 by increasing the ‘non-anthropogenic’ atmospheric CO2 levels. The IPCC, by not including the recent data, must be underestimating both the transient and equilibrium climate sensitivity. This raises the question: when does the IPCC plan to issue an updated report that does include ice sheet dynamics and carbon cycle feedbacks? Six years from now?

    The other issue is that the IPCC should extend their predictions to at least 2200, not just to 2100. If there are scientific reasons for not doing so, is it safe for observers to assume that the consensus among climate scientists is that there is little basis for trusting the models past 2100?

  18. 68
    John Lang says:

    The northern sea ice figures are available from The Cryosphere Today which has all the satellite pictures archived for every day back to 1979.

    Here is a graph of the daily sea ice area back to 1979.

  19. 69
    yartrebo says:

    Re #44:
    “What is the maximum sea level rise before the ocean boils?”

    The entire ocean cannot boil away, at least in the short term. Heating the ocean and atmosphere to +374C/221atm would boil about 75% (by mass) of the ocean, at which point there would no longer be a distinction between liquid and gaseous water.

    As far as what temperature and corresponding pressure below +374C gives the highest water level, I got as follows (very approximate):
    temperature: +180C
    pressure: 11 atm (10 atm H20, 1 atm other gases)
    height: 380m above present sea level

    At higher temperatures, water evaporation outpaces thermal expansion, and at lower temperatures, thermal expansion is dominant. I calculated that the 70m rise from melting ice becomes about 80m because of thermal expansion, assumed that 70% of the Earth’s surface is ocean, and used 5.15 * 10^18 kg and 1.4 * 10^21 kg as the masses of the dry atmosphere and hydrosphere respectively. I also assumed that the entire atmosphere is saturated and the same temperature throughout. If it isn’t fully saturated (unlikely since that would cause the pressure to drop and more H20 to rise from lower parts of the atmosphere) or if it is cooler at higher altitudes (quite likely), the maximum sea level will be higher and attained at higher temperatures and pressures.

  20. 70
    Ron Taylor says:

    I agree with those expressing concern about the trends in some of the recent posts on RC. I suggest that it is not censorship to apply a little stronger filter. You could, for example, filter out posts like #52, which is simply a restatement of incorrect and misleading talking points that have already been addressed several times. It is not necessary to allow these statements to be repeated over and over until some begin to attach significance to them. (Any lie repeated often enough….)

  21. 71
    Phillip Shaw says:

    Re #59 & #65

    Writing only as a layreader and infrequent commentor, I believe you may be overestimating the influence of the denier-trolls and underestimating the discernment of the average readers. This is a very credible and robust forum due, in my opinion, both to the excellent articles and to the excellent posts by folks such as yourselves. You provide good answers to the honest questions and needed rebuttals to the nonsense. And I’ve learned a lot from the additional links provided (Lynn’s link in #20 was great). I worry that too strong a reaction would hurt the forum more than it helps.

    My suggestion would be to add a metric next to each commentor’s name. The value would initialize at zero and any of the contributors could increment it by +1 for a good post and by -1 for a FUD post. I’d cap the values at +- 10 to keep people from frequent posting just to boost their numbers. I suspect that the trolls would drop off when their scores hits about -5 (though they may return under another name). Just a suggestion.

    Best Regards,

  22. 72
    George K says:

    Regarding my points at #50 about sea level rise over the past 8,000 years, this chart is from NASA – GISS from just this January.

  23. 73

    #59, Hank I disagree to a certain extent, let people write what they think so feedback may make them think some more.

    About Arctic ice sheet, it will likely never completely disappear during the long night, surely is on its way to vanish during summer time, the biggest factor accelerating its demise is not temperature but dominant winds, if they change in favour of dumping more old sea ice than usual on a continuous basis, then even with temperatures still quite cold, the ice sheet will appear semi-annually. There are a few obstacles to this, the biggest one being tidal effects, especially during full and new moons, which literally push the ice against the Canadian Archipelago, there is the Arctic Ocean Gyre which favours dumping of ice from the Russian side of the Pole to the North Atlantic, but equally favours packing the ice again on the Archipelago West Coast. The only way for it to disappear completely if for the winds to go off kilter, intead of a near permanent High pressure at the Pole during winter, the near permanent High pressure should stay year round, dumping more ice when it shouldn’t, during the summer. A near permanent high pressure during the summer would also allow more sunshine to add more heat to the open water of the Arctic Ocean. Its up to the models to come up with the possibility of such a scenario. I think it likely, having seen for myself the power of mere winds on the most huge ice packs.

  24. 74
    Steve Bloom says:

    Re #s 59/65: The moderators already exercise some degree of control. Rants do tend to get removed. Posts like #50 that are composed of denialist talking points but are politely phrased should be let through IMHO so long as they get refuted. My impression is that they do get refuted, although that probably ceases to be done consistently in the latter portions of some of the longer comment sections. OTOH, I suspect the majority of readers, especially the less informed ones, tend to read just the main posts and don’t even get to the comments, and certainly not to the bitter end. (Does the software track whether people enter the comment sections?)

    I do think that the site could use some sort of short “caveat emptor” blurb at the top of the comments making it clear that the non-green comments should not be taken as the gospel truth. To increase the number of those green comments without creating an extra burden for the existing moderators, perhaps some of the regular climate scientist commenters could be given moderator privileges to the extent needed to highlight their own comments in green. Possibly that would also encourage such folks to make more comments, which would be helpful.

    Finally, I think it would be good to have a brief “RC User’s Guide” prominently linked just below the banner. This would help keep first-time readers from feeling so intimidated, which I think is the biggest drawback of the site. If there’s any interest, I would help put it together.

  25. 75
    Steve Bloom says:

    While we’re on the subject of user friendliness, I want to compliment Stefan on the structure of this post: A short “abstract” with a hyperlink to the conclusion (“bottom line”), making the material equally accessible to users with different levels of knowledge (or interest in this particular subject).

    [Response: That hyperlink was actually Gavin’s idea, so he should be praised… -stefan]

  26. 76
    Ray Ladbury says:

    Re 50,70: George, you should have read your source:

    “Twentieth century sea level trends, however, are substantially higher that those of the last few thousand years. The current phase of accelerated sea level rise appears to have begun in the mid/late 19th century to early 20th century, based on coastal sediments from a number of localities. Twentieth century global sea level, as determined from tide gauges in coastal harbors, has been increasing by 1.7-1.8 mm/yr, apparently related to the recent climatic warming trend. Most of this rise comes from warming of the world’s oceans and melting of mountain glaciers, which have receded dramatically in many places especially during the last few decades. Since 1993, an even higher sea level trend of about 2.8 mm/yr has been measured from the TOPEX/POSEIDON satellite altimeter. Analysis of longer tide-gauge records (1870-2004) also suggests a possible late 20th century acceleration in global sea level.

    Recent observations of Greenland and the West Antarctic Ice Sheet raise concerns for the future. Satellites detect a thinning of parts of the Greenland Ice Sheet at lower elevations, and glaciers are disgorging ice into the ocean more rapidly, adding 0.23 to 0.57 mm/yr to the sea within the last decade. The West Antarctic Ice Sheet is also showing some signs of thinning. Either ice sheet, if melted completely, contains enough ice to raise sea level by 5-7 m. A global temperature rise of 2-5°C might destabilize Greenland irreversibly. Such a temperature rise lies within the range of several future climate projections for the 21st century. However, any significant meltdown would take many centuries. Furthermore, even with possible future accelerated discharge from the West Antarctic Ice Sheet, it highly unlikely that annual rates of sea level rise would exceed those of the major post-glacial meltwater pulses.”

    The main point about the models’ predictions of sea-level rise are that they tend to underpredict what is actually seen. This raises the concern level rather than lowering it. Sea-level rise gets a lot of attention because it is a virtual certainty, and will have somewhat predictable consequences. It is far from the only possible effect, though, of climate change. Again, focusing on the uncertainties is not particularly profitable. The systematic errors all run only one way, and it is against those arguing for complacency.

  27. 77
    Daniel C. Goodwin says:

    Thank you for this excellent summary, Stephan. Your last point is particularly well-taken: sea-level rise seems to be popularly expected to magically cease as of 2100. Discussions of adaptation strategies such as building sea-walls (where the concrete therein is a CO2 emissions source, of course) apparently originate from this statistical fantasy.

    A more important question than where the sea-level will be in x years is when we can reasonably expect thermal equilibrium to be achieved, when it stops rising. At what point are the ice sheets committed to total meltdown, when carbon-cycle feedbacks are properly taken into account? Have “scientific reticence” or an irrational faith our own viability as a species hindered our ability to contemplate and calculate through that “tipping point”?

  28. 78
    Rick Brown says:

    Re #70 Re # 50 [sea level rise of 1 -3 mm(per year?) for last 8,000 years]

    The NASA-GISS article you provide a link to says: “Over the past few thousand years, the rate of sea level rise remained fairly low, probably not exceeding a few tenths of a millimeter per year.” And: “Twentieth century sea level trends, however, are substantially higher that those of the last few thousand years. “

  29. 79
    J.C.H says:

    “- Sea level has been rising at 1mm to 3mm every since the large continental glaciers melted after the last ice age – ie for the last 9,000 years. …”

    Just guessing by eye, that graph shows sea level was around 39 meters lower 9,000 years ago, and that by 5,000 years ago sea level rise had erased all the seaside condos in that 39-meter range. From 5,000 years ago until now, there has been a significant slowdown in sea level rise versus the first 15,000 years graphed.

    I think maybe you left out a word. 1mm to 3mm every what and for how long?

  30. 80
    Hank Roberts says:

    >50, 70
    The confusion is that stating an overall “per year” rate doesn’t represent the data accurately.

    Rate of change is important.

    The article and chart linked in #70 point out that there were several large pulses; it’s not a steady rate of change so a “per year over 9000 years” hides the variability:

    “Twentieth century global sea level …. Since 1993, an even higher sea level trend of about 2.8 mm/yr has been measured from the TOPEX/POSEIDON satellite altimeter. Analysis of longer tide-gauge records (1870-2004) also suggests a possible late 20th century acceleration in global sea level.”

  31. 81
    Dick Veldkamp says:

    Re #58 Sea ice (John Lang)

    John, interesting links.

    Strangely enough the first graph (the sinusoidal one) is not showing much ice loss, while the other graphs are. I suppose it must be a visual illusion (after all, all graphs must be consistent). Doubtless teh real numbers are found with statistical procedures rather than from the graphs.

  32. 82
    tom root says:

    And BTW, who needs models to tell us that heat melts ice, though my Sunfrost frig seems to take forever to defrost. But once that ice gets to a certain point, big chunks just start dropping down, one after another, kaboom! kaboom!

    Comment by Lynn Vincentnathan � 27 Mar 2007

    Another common example: ice and snow on steeply pitched roof on a warming day will suddenly, without warning, come down in a rush. When certain undetectable thresholds are crossed gravity becomes stronger than what ever friction was holding the ice in place. No models necessary or even possible.

  33. 83
    Dave Rado says:

    Re. #45, see Stefan’s response to #47. Contrary to popular belief, many climatologists – and especially the IPCC, because it has to get a consensus – play it safe much too much.

  34. 84
    Dave Rado says:

    Re. #36, it’s the rapidly increasing weight of evidence all pointing in the same direction, rather than any new theory, that is the reason for the increasing levels of certainty. This evidence is summarised quite well here and in the links from that page.

  35. 85
    Ray Ladbury says:

    Re 40, 86 and others wrt conservatism of scientists. This goes to show the absolute falsehood of Crichton’s assertions about scientific consensus. The need to reach consensus in science means that scientific conclusions are almost always conservative rather than alarmist. Not every threat considered by the IPCC will necessarily be realized, but many are virtual certainties, many will likely be much worse than the IPCC predicted.

  36. 86
    George K says:

    When one is discussing sea level rise, (as in other types of data) I believe one should discuss the entire record.

    The simple fact is that sea level has been generally rising since the ice age was over and even after the large continental glaciers completely melted. Maybe it only increased a few tenths of a mm for periods of time, maybe it was faster than that at other times.

    Sea level has risen by about 10 metres since 8,000 years ago. That translates into 1.25 mm per year.

    Sea level has risen about 2 metres over the last 5,000 years. That translates into 0.4 mm per year.

    Since 1900, sea level has risen about 20 cm. That translates into 2.0 mm per year.

    I think it is important to recognize that sea level rise is not a brand-new phenomenon.

    Here are two other charts showing sea level rise over the recent past.

  37. 87
    John L. McCormick says:

    RE # 88

    [I think it is important to recognize that sea level rise is not a brand-new phenomenon.]


    Now, for the sake of good common sense, you should agree the topic relates to measured increasing deglaciation and thermal expansion of the oceans. Neither are brand new phenomena.

    But, this measured increasing rate of sea level in a world of 6.6 billion inhabitants (ten percent of whom reside on coastal plains) IS A BRAND NEW PHENOMENON.

    Give us some indication that has any meaning.

  38. 88
    Gerry Beauregard says:

    I’m curious what the predictions are like for beyond 2090-2099. While the time-scale for the predictions is “long-term” in comparison with a human lifetime (I’ll certainly be dead by 2090-2099), in many endeavors, people implicitely use much longer time frames.

    In Singapore, where I currently live, property is typically sold either on a 99 yr lease or freehold (i.e. in perpetuity). If people totally discounted anything beyond 2099, the sale price for a freehold property would be the same as for an equivalent 99-yr leasehold property. That’s not the case, however. Freehold property sells at a substantial premium over leasehold. Why? Because people expect that property to still be usable, and hence have value, 99 yrs from now.

    If there’s some non-negligible chance that the groundfloor of some beautiful new seafront freehold apartment is going to be underwater in, say, 125 yrs, the price of that apartment shouldn’t be substantially higher than that of an equivalent 99-yr leasehold apartment.

    After the IPCC FAR SPM was released, someone from the Singapore government (minister of the environment?) said something to the effect that Singaporeans shouldn’t worry because reclaimed land in Singapore is built up to 1.25m above sea level, way above the “worst case” sea level rise of 59cm. Hmm… perhaps worth the gamble for 99 yr leasehold property perhaps, but probably not for freehold, given that sea level rise is likely to continue long after 2099.

  39. 89
    Ray Ladbury says:

    Re 86: George K, it is a bit disingenuous to look at average rates of sea-level rise. The rate was quite rapid during the early part of your 8000 year period, and we know why–the melting of continental glaciers. The rate then slowed to a crawl, and we know why–the continental glaciers were mostly melted and there existed a rough equilibrium between Winter snowfall and Summer melt and a relatively stable climate. Now the rate of rise is again rapid–and we know there must be a reason. Why? Because things do not just happen. The planet does not just cyclically warm and cool. There are drivers that force it to do so, and if the behavior of the planet changes, that means one or more of the drivers must also be changing.
    Now, as John points out: The situation during this rapid rise is different than in previous epochs of rapidly rising sea level. In the interim, all the infrastructure of civilization has been developed, and human population has increased to 6.6 billion. I would call this a not insignificant difference.

  40. 90
    Philippe Chantreau says:

    I’m looking forward to the progress made in ice dynamics. The Greenland ice sheets are getting more and more cracks through which more and more liquid water flows, possibly creating lakes, or going to the bottom to lubricate the moving sheets. I can’t help to wonder what would be the results of a few accidents like what happened in the French Alps in 1892, but each multiplied by a 100 factor, taking place on each side of the globe. Antarctica is already harboring immense pockets of liquid water. In the St Gervais occurence, a 90000 cubic meters ice plug was pushed out by a 200000 cubic meters volume of liquid water. I just can’t help to wonder what potential exists in Greenland and Antarctica for similar, much larger events. I’m also wondering how we’re monitoring the amount of precipitation over the oceans and how that has changed.

  41. 91
    Mark A. York says:

    In Douglas Preston’s fine novel Tyrannosaur Canyon he describes the American West in that time before the asteroid hit. There were no ice caps, the whole place was tropical and the coastline was over in New Mexico someplace. That should be the take home lesson, not Crichton’s modern Jurrasic Park concoction and subsequent disconnect.

  42. 92
    Blair Dowden says:

    Re: Stefan’s response to #44:

    If you melt all ice on Earth, sea level would rise about 70 metres. Last time this happened was in the Eocene, about 40 million years ago, when climate was about 4 ºC warmer than now.

    I don’t think this is fully accurate. My understanding (from Wikipidia and other sources) is that 4º is the low end temperature of the Eocene, which ranged up to 6º warmer than today. Please correct me if I am wrong about this. Also, polar ice did not melt in the Eocene, instead ice sheets began to form at the end of it. Your statement implies that 4º of warming will (eventually) melt all the polar ice caps, while my understanding is it would take considerably more than this. Still, the 25 to 35 meters of sea level rise from the Pliocene (3 millin years ago) when temperatures were only about 3º is enough reason for concern.

  43. 93
    Mark A. York says:

    Interesting blog post on consensus at Scientific American. This is my answer.

    “god help you if you walk into a pub and act full of yourself, as many climate scientists do.”

    Really? Because the climate scientists I deal with have the patience of Job. They explain the same things over and over while fending off the same barrage of insults from people so ignorant of the facts they can’t accept any scientific explanation. And they won’t. No, I think you’re wrong. Mollycoddling the public, and let’s identify who they are: conservative or leaning that way politically is not the answer. The answer for them is education not geared to what they want to hear by a messagemaker.

    So far the Nicholson line applies. They can’t handle the truth.

  44. 94
    Dan says:

    re: 93. Agreed. Unfortunately, there are a large number of skeptics or deniers who refuse to acknowledge or even want to be educated about the subject. These laymen actually think they know more than people who have spent decades studing climate science. And then they refuse to admit when they are wrong. It is quite astonishing and sad that people actually “think” that way.

  45. 95
    pete best says:

    Hansens speaks of the potential for “rapid non linear collapse” on his recent talks on fossil fuel depletion and climate change here in the UK. Apparantly the number of ice quakes on Greenland has escalated quite rapidly and could indicate something serious going on.

    Is there any scientific rationale behind Mr Hansen reasoning ?

  46. 96
    AndrewM says:

    An archived Webcast is now available of a presentation that took place yesterday (March 28th) at the University of Texas at Austin.

    Description from the site:

    Secrets of the West Antarctic Ice Sheet: A Panel Discussion
    About Global Warming, Climate Change and Rising Sea Levels

    Webcast panel of some of the world’s leading polar ice experts on area of major uncertainty facing climate scientists. Panel will take questions from the Web and a live audience.

    Presentation doesn’t start until about 13 minutes into the Webcast. If you haven’t a fast enough connection to receive this, here’s part of what Reuters has to say:

    A Texas-sized piece of the Antarctic ice sheet is thinning, possibly due to global warming, and could cause the world’s oceans to rise significantly, polar ice experts said on Wednesday.

    They said “surprisingly rapid changes” were occurring in Antarctica’s Amundsen Sea Embayment, which faces the southern Pacific Ocean, but that more study was needed to know how fast it was melting and how much it could cause the sea level to rise.

    The warning came in a joint statement issued at the end of a conference of U.S. and European polar ice experts at the University of Texas in Austin.

    The scientists blamed the melting ice on changing winds around Antarctica that they said were causing warmer waters to flow beneath ice shelves.

  47. 97
    CobblyWorlds says:

    Hello Pete,

    “Is there any scientific rationale behind Mr Hansen reasoning ? ”

    I’ve not read the Hansen paper linked to above on this page, but don’t see how fossil fuel depletion is an issue that can limit emissions. We’ve used but a fraction of the available extractable reserves so far and there’s massive reserves of coal left.

    Re Greenland:
    You do worse than looking at Luckman et al “Rapid and synchronous ice-dynamic changes in East Greenland”

    Or google “basal lubrication”.

  48. 98
    J.C.H says:

    There is a documentary about some guys who went to Greenland and recovered a WW2 fighter plane that was resting on the ice where it made its emergency landing in 1942. After 50 years, they had to burrow down through 268 feet of new ice to reach the fighter on the surface of the 1942 ice cap.

    Few structures can just continue to get taller and taller forever, but especially not one in which the edges are being eroded by melting and erosion. It’s no different than the snowmen and snow forts and snow dams we built as kids. Snow is a wonderful building material, but it has its limits. In the 3rd grade we backed up an astonishing amount of water during recess by blocking a stream of melt water on our playground with a hastilly made snowbank. We watched in amazement from our classroom window when our engineering feat experienced a sudden and catastrophic failure – a huge wave of water swept out into the street. No adult saw it, but after school we rushed out and looked at the water line on the sides of the parked cars.

    So yeah, the 3rd grader in me thinks Hansen’s reasoning has a future.

  49. 99
    pete best says:

    Re 97, Dr Hanson has been talking in the USA recently with an article called “Lets call the coal thing off” in which he advocates just like he did in the UK talk that we can use all of the remaining Oil and Gas reserves but existing coal fired power stations and technology must not be built and old power stations phased out by 2050. Fortunately for the world (and Dr Hansen) Sen Kerry seems to think along the same lines, however getting this into legislation is another matter as the USA’s fossil fuel companies are powerful and unless clean coal technology can come along soon then the 159 new coal fired power plants that the USA intends to build will not have this technology fitted or retrofitted. This is a major issue for climate change, if the USA builds these power stations as they last for 60 years then the world would be committing itself to the 1 more degree rise that Dr Hansen seems very worried about. (choose enter site to skip the advertising to get to the article)

    Dr Hansen not only thinks that the issue of climate change is urgent he wants soemthing done about it sharpish. I believe that Dr Hansen also is attributed to have said 1 more degree celsius and we are done for.

    On ice dynamics etc I await consensus on the issues of rapid non linear climate change or other related system change. Back in 2005 the UK held an meeting on abrupt climate change and many thought that this material would make it onto the new IPCC report, however it appears to not have made it so I was just asking what RC thought of this possibility? However in the main I would suggest that RC in the main agrees with the IPCC assessment.

  50. 100
    tom says:

    Irony in 93 and 94.
    As a “laymen” who has worked very hard on understanding the subject, you two come across as smug, condescending know-it-alls.
    I enjoy reading this blog, it’s very informative and I have changed my views based on what I read. But I do not believe the science on AGW is anywhere close to being as conclusive as you two imply.