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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. 101
    Mike Burnett says:

    One sixth of the worlds population lies below 25 meters. The four countries most at risk: China, India, Bangladesh, and the US. In the US, 50 million people live below 25 meters, or 1/6 of our population. If you use the first order assumption that economic contribution is proportional to population, this would mean that $2 trillion of our economy is at risk.

  2. 102
    Floccina says:

    So could what you are saying be boiled down to this:
    Sea level had been rising 1 mm per year with out increased co2 but is now rising at 3 mm per year?
    So what was going to happen in 300 years will happen in 100 years.

    No wonder most economists say treat GW as a natural occurring phenomenon. People are extremely adaptable and we get more capable of adapting as technology increases. The longer we wait the easier the reaction.

  3. 103

    [[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.]]

    If 268 feet of ice was deposited in that time, wouldn’t ice depths fairly recently in geological time have extended to astronomical sizes? Is it possible the plane sank in the ice?

  4. 104

    [[No wonder most economists say treat GW as a natural occurring phenomenon.]]

    Who cares what economists have to say about climatology? Do climatologists go around researching markets?

  5. 105
    Ray Ladbury says:

    Re99: Ah, spoken like somebody who doesn’t understand the problem. I recommend checking out post 88 for a mild example of the economic implications of climate change. Much of humanity lives in regions less than a meter above sea level. And the real worry is not the 3 mm per year rise, but the fact that the rate of rise is accelerating–and we don’t fully understand why. In a world where population is expected to increase by ~50% before it stabilizes, a shrinking land mass is a concern.
    And sea level rise gets so much attention only because it is the most certain consequence of climate change. Others–failure of agriculture infrastructure, spread of tropical diseases, etc., while less certain, could have a greater impact.
    The science here is settled. It is well understood by the experts–even if not by the general public–and there is no controversy. What is needed now is a risk management and mitigation approach, where cost and probability of occurrence are both considered.

  6. 106
    Phillip Shaw says:

    Re #99:

    Flocinna, you wrote “People are extremely adaptable and we get more capable of adapting as technology increases. The longer we wait the easier the reaction.”. You seem to be an advocate of the “Don’t Worry, Be Happy” approach to dealing with AGW.

    The fallacy of that approach is that new technology doesn’t just happen. As an engineer with decades of technology development experience I know that new technology only comes into existence when countries, corporations, and individuals invest the necessary resources to develop and implement it. Identifying a need for a particular technology, such as carbon-neutral electrical generation, is easy. Turning it into a reality can be very difficult, and often take years longer than the initial planning.

    A good example of this is commercial nuclear fusion power generation. Billions of dollars, and thousands of man-years, have been spent since the mid-1960s trying to make the theory a reality. We’ve been “25 years” away from practical fusion power for the past 40 years. Anyone care to bet when the first fusion power plant will go on-line?

    The engineering challenges we face with AGW are mind-boggling. Even considering only the problems associated with sea level rise, it quickly becomes apparent that the sooner we get started, the less expensive and less disruptive the mitigation efforts will be. Take just one city, say Los Angeles, for example. How much money and time will it take to upgrade its airport, LAX, for a 4 – 6 meter sea level rise? How about its port facilities, water treatment plants, power plants, roads, wastewater plants, and so on? If Los Angeles spent $100,000,000 a year, every year, starting today, it would still take more than a generation to do all of the needed work.

    The sea level rise is coming. Ignoring it until it is lapping around your ankles makes no sense.

  7. 107
    Figen Mekik says:

    Humans may be adaptable, but I don’t agree with “the longer we wait, the easier the reaction.” Whether we are able to adjust to the changes global warming is bringing or not, basically we are messing with a system whose components and feedbacks we don’t fully understand. Not to mention, I don’t believe anyone in the world has the right to damage climate which is being shared by the whole world. And worse, we don’t have the right to damage climate for our children. We need to act now by reducing emissions and reversing the damage as quickly as possible.

  8. 108
    Ike Solem says:

    RE the West Antarctic Ice Sheet (#96, AndrewM), there’s a discussion of that at Thinning of West Antarctic Ice Sheet

    The consensus view of the workshop: Satellite observations show that both the grounded ice sheet and the floating ice shelves of the Amundsen Sea Embayment have thinned over the last decades. Ongoing thinning in the grounded ice sheet is already contributing to sea-level rise. The thinning of the ice has occurred because melting beneath the ice shelves has increased, reducing the friction holding back the grounded ice sheet and causing faster flow.

    Oceanic changes have caused the increased ice-shelf melting. The observed average warming of the global ocean has not yet notably affected the waters reaching the base of the ice shelves. However, recent changes in winds around Antarctica caused by human influence and/or natural variability may be changing ocean currents, moving warmer waters under the ice shelves.

    Our understanding of ice-sheet flow suggests the possibility that too much melting beneath ice shelves will lead to “runaway” thinning of the grounded ice sheet. Current understanding is too limited to know whether, when, or how rapidly this might happen, but discussions at the meeting included the possibility of several feet of sea-level rise over a few centuries from changes in this region.

    The evidence is in; the contrarians can’t explain the simultaneous warming of both poles as well as the melting of glaciers all over the world using ‘natural variability’ arguments. It’s time for the media who report on global warming to put the contrarian viewpoint in the category of ‘discredited science’.

  9. 109
    Charles Muller says:

    About Topex/Poseidon/Jason and tide gauge data : its seems strange that a new instrument (satellite) just coincide with a new trend (3-3,2 mm / y for 1993-2006 rather than 1,7-1,8 mm/yr for 1900-1992), even if there’s a well-known decadal variability (Cazenave 2005), with a low centennal acceleration (Church and White 2006). Should ancient measurements of sea-level be reevaluated, and if so, what would be the real present anomaly when compared to previous decades? Was there already an accelerating trend prior 1992, in the 1980s? Or is there a clear shift in the 1990s explaining the acceleration of the satellite-era?

    Thanks for information.

  10. 110
    Alexander Ac says:

    An interesting graph from New Scientist showing CO2 emissions in perspective:

    it seems, that we have troubles to stabilize the CO2 concentrations at 550 ppm…

    at the moment, I don’t see any convenient solution…
    Still, if the developed world is not able to reduce it’s co2 output meaningfully , HOW could we await this from developing nations in the future, which from historical point of view have emmited much less carbon than we did…

  11. 111
    jdwill says:

    Just how good are the measurements? You are talking about mm/year.

    1. How do you account for thermal expansion, is it regionalized?
    2. How are you accounting for tectonic plate shift? Is it not likely that the shape of the basins are changing at rates that are significant given the time scale and the mm/yr change rates?
    3. Is there a global calculation for H2O that accounts for ground absorbtion, evaporation cycles, and ice mass?

    If I have missed some links where the workup to this is shown, I would be grateful if you could share them.


    James Williams

  12. 112
    Chuck Booth says:

    Re #100
    I’ve always found that doing preventive maintenance on my house and car is less expensive than repairing the damage after the roof leaks because I was too cheap to replace some loose shingles, or the engine freezes up because I was too lazy to add oil. I don’t recall any exact figures, but I think some economists have estimated that a bit of preventive maintenance for our planet with regard to AGW (i.e., reducing GHG emission) will be less expensive than, say, rebuilding or relocating entire cities flooded by the rising sea level (and storm surges that go along with elevated sea level); I’ll pit these economists against your laissez faire economists any day.

  13. 113
    cbone says:

    Re: 101

    Who cares what economists have to say about climatology? Do climatologists go around researching markets?

    I think your comment typefies the relative vacuum that the climate sciences are sometimes percieved to operate in. Some have argued that climate scientists could learn a lot from the statisitcal data analysis methods of economists:

    Advanced statistical methods take into account the failure of standard assumptions and their effects on results, and offer new techniques to overcome the problems. However, these up-to-date methods have not been widely embraced in climate research. Development of advanced statistical methods in the fields of econometrics and geostatistics has now surpassed the methods still used in climate research.

  14. 114
    James says:

    Re #100: [People are extremely adaptable…]

    In this case, an appropriate adaptation would be to alter lifestyles so as not to cause further AGW and its related problems. But if you dare to suggest actually doing that, you’d better have ear protection ready, else you’ll be deafened by the squawking from the denialist camp.

    Hey, you suppose maybe some humans aren’t so adaptable after all?

  15. 115
    David B. Benson says:

    Re #104: James Williams — Tectonic plate shift is interesting in regard to ocean basin size. The main change is due to the Australio-indian plate shoving into Asia at a considerable rate. This means that the Indian Ocean becomes larger at about that rate. Despite this extra capacity, the oceans are rising…

  16. 116
    Ray Ladbury says:

    Re 104: Uh, James, it’s not all that complicated. To a first approximation, a certain amount of land is above sea level, and a certain amount below. The fastest plate moves at about 15 cm per year–and that’s in the pacific basin and so not going to have much effect. In brief answer to you questions
    1)Yes, they can take thermal expansion into account. That is only going to increase the sea level rise in any case.
    2)Not significant. There’s not a lot orogenesis going on these days, while arrangements of the continents will change over HUNDREDS OF MILLIONS of years, you won’t see a lot of change in elevations.
    3)Sorry, other than ice mass, I don’t see the relevance of this. There’s virtually no soil in Greenland or Antarctica under the glaciers, so not much ground absorption.

    What you are asking is what the models do.

  17. 117
    jdwill says:

    Re 105: David

    Right, but how are these shifts being measured? Are we able to use something like radar/sonar from satellites to map the ocean bottoms accurately and calculate volume?


  18. 118
    Hank Roberts says:

    >100, “most economists say”
    >101, “who cares what most economists say”

    A thought — rather than argue about the statement, how about providing a source for it? If it’s true that any economist says this, show us who and where by providing a cite or a link to the page.

    Here’s a good short discussion about that kind of claim.
    It’s often not useful to make, or respond to:

  19. 119
    jdwill says:

    Re 105: David

    Thanks. But, how is the volume calculated? Do we have the ability to map the ocean bottoms accurately using Sonar or some type of radar from satellites? Is there a link to a global project or series of projects that have done such a mapping?


    James Williams

  20. 120
    Tim McDermott says:

    Re #100
    So could what you are saying be boiled down to this:
    Sea level had been rising 1 mm per year with out increased co2 but is now rising at 3 mm per year?

    I think you misread, or didn’t follow the links. For the last two millennia, sea level has been rising at a few _tenths_ of a millimeter per year. This century, the sea will rise more than it has, cumulatively, since the time of Christ.

  21. 121
    pete best says:

    Re 110: I have read that article and yes it is going to be difficult with a world population of 9 billion come 2050 for us to stabalise a t550 ppm, however way before 2050 PEAK OIL and GAS would have occured and that means plenty of other issues for us to mull over such as where to get our energy from in the first place.

    Fossil fuels might be responsible for AGW but they are also responsible for nearly everything that modern humans have come to expect of life, progress and prosperity for all at least in principle.

    I await the cohesive energy strategy for a sustainable future myself and until I see it in print for thr entire world then I doubt that we will mitigate CO2 levels below 550 ppmv.

    Scary aint it

  22. 122
    jdwill says:

    BTW the site for some reason held and then later reposted one of my posts. Sorry for any repeats or misnumberings.

    #116. OK and thanks for as much as you answered, but how is this known? I am looking for some links to detail. Plus, are the plates limited to planar movement?; could they not be raising and lowering on the mantle? My essential question: is the 3D shape of the ocean floor being measured or studied? If so, where and how?

    The reason I am bringing this up is to help me understand what the level of uncertainty in sea level measurements might be.

  23. 123
    J.C.H says:

    There is post-glacial bounce, which appears to be in the model.

    NASA, NOAA, etc. – these guys have toys on top of toys on top of toys. They’ve measured this stuff ten ways to Sunday, and they will keep adding measurements and new measurement toys. NOAA has been doing this stuff almost since the American Revolution. When the 28th Marines and my father planted a flag on top of Mt. Suribachi, Iwo Jima, a crew of science guys from the NOAA were right there with them.

    NASA, NOAA, etc. are staffed by dedicated people and they’re exceptional scientists.

    In the news today:

  24. 124
    David B. Benson says:

    James Williams — The Indian plate is disappearing under Asia at 4 mm/yr, according to Wikipedia. I’ll leave you to find a suitable depth for the Indian Ocean, say about 200 km south of the continental shelf. But if it is 1 km deep there is a volume increase of 1 m/yr. Clearly not enough to matter…

    Wikipedia is always a good place to start. There are often links to more authoritative sources.

  25. 125
    sunbeam says:

    Something that always bugs me about this stuff, is when it is supposed to start to happen.

    Correct me if I’m wrong but wasn’t the average sea level rise for the past century 3mm a year? And isn’t that what we are measuring now?

    If data is in that contradicts my assumptions, please mention it.

    But if drastic changes are in store, or happening now as the news suggests to me, when are you going to see 10mm a year or higher in sea level rise, which is the bottom line? I’m aware sea level can “differ” depending where on the globe you are, but if Greenland or Anarctica is going to lose dramatic amounts of ice, it has to show up sometime.

  26. 126
    David B. Benson says:


    4 m^2/yr per meter along an east-west axis the width of the Indian portion of the plate.

    Still doesn’t matter.

  27. 127
    sunbeam says:

    Just want to add, since there doesn’t seem to be an edit button, that there is nothing special about the 10mm. I simply want to know when the sea level rise figure is going to have a dramatic rise. Also if my impression of the current rate is wrong, I’d like to know.

  28. 128
    jdwill says:

    #122 JCH

    Thanks, that article is exactly the sort of starting point I was looking for.

    James Williams

  29. 129
    J.C.H says:

    Here is another article in the news today:

  30. 130
    Phillip Shaw says:

    Re 103:


    The plane that was recovered from the Greenland glacier is one of six P-38Fs, plus a B-17, that crash landed on the snow in July 1942 after encountering bad weather while being ferried to Iceland on their way to England. All of the crews were recovered safely. The account of the location and recovery of the P-38 is told in the book The Lost Squadron.

    I believe that the present depth of the remaining wrecks of 268 feet is simply snow and ice accumulation since 1942. If I remember the story correctly the firn layer is about forty feet down. Since the snow was firm enough to land on I can’t think of any mechanism for the planes to sink into it. And consider that the 268 feet of ice and snow represent 300 feet or so of snowfall. Over the fifty years between the crash and the recovery that’s only six or seven feet of snow a year.

    The recover took fifteen years of dedicated work, 1977 – 1992. The recovered plane has been restored and is now flying on the airshow circuit as Glacier Girl. It is the only P-38 flying with its original engines. If you’d like to learn more just google Glacier Girl.

  31. 131
    David B. Benson says:

    Increase in ocean volume — According to Wikipedia, the ocean area in 362 million km. At an increase of 3 mm/yr, that is 1.086 billion m^3/yr. Suggested sources include

    melting in Alaska
    melting in Greenland
    melting of other glcaiers and ice caps, such as in Patagonia
    groundwater depletion
    thermal expansion

  32. 132
    Hank Roberts says:

    Search for “bathymetry” and urge your national government to fund it!

    Best done from space, e.g.:

    Don’t take this plan for more accurate measurement as a reason to imagine there might be some sudden and unnoticed change happening at the base of the ocean that’s raising water levels worldwide without anyone noticing any earthquake or other effect, please.

  33. 133

    Re #96:

    A statement from the panelists at the University of Texas meeting regarding the potential for severe effects from relatively rapid melting of the West Antarctic Ice Sheet is here:

    (Tech note: The webcast referred to in #96 unfortunately doesn’t seem to work on an Intel Macintosh. My old P4 powerbook seems to handle it.)

  34. 134
    Nigel Williams says:

    Re #57. Yartrebo, I guess the down-side of isostatic rebound of Greenland (and presumably the Antarctic too) is that it will increase the gradient towards the sea, which will of course speed up the loss of the remaining ice. Another positive feedback?

  35. 135
    Regina says:

    critique and bottom line links don’t work

  36. 136
    Mike says:

    So, to clarify my earlier question, the antarctic ice cores provide uninterrupted CO2 data for the past 700,000 years? There are no gaps in the record? I think the fellow I was debating with claimed the Vostok ice cores contained large gaps of thousands of years where all sorts of crazy stuff could have happened, ergo the past 150 years might be a natural phenomenon we have no record for. Is it possible to date the carbon reliably, to verify the record? And do ocean sediment samples verify the ice core data by comparing CO2 levels, or another method? Thanks!

  37. 137
    Hank Roberts says:

    Whoah! Who else besides Dr. Hansen is talking about this?
    Where’s the info from, anyone recognize it?

    found here:

  38. 138
    Swan says:

    I know you’ll appreciate this because the Bush administration has done so much to keep science information from getting out–

    Re: prosecutor-purgegate

    What explains the failure of the mainstream media to cover the purge scandal for so long, and so many other scandals? Do you think somebody just set up newspaper editors to cheat on their wives, and threatened to tell if the editors wouldn�t play ball when they come back some day and ask for something?

    It wouldn�t be that hard to do, when you think about it. People wouldn�t talk about it.

  39. 139
    Jeffrey Davis says:

    re: 136

    Natural variability is an abstraction. It refers to natural variability of the forces that produce this or that phenomenon. Things don’t happen randomly. If there’s a fluctuation, there’s something that caused the fluctuation. In climate, there are known vectors — sun, greenhouse gases, clouds, etc. We’ve currently got a huge spike in the greenhouse gas CO2. Unless you’ve got something else in mind, that’ll do it.

  40. 140

    [[I think the fellow I was debating with claimed the Vostok ice cores contained large gaps of thousands of years where all sorts of crazy stuff could have happened, ergo the past 150 years might be a natural phenomenon we have no record for.]]

    Driven by what?

  41. 141
    Andrew Dodds says:

    Re: 130

    Actually, the butial of objects like this is quite interesting..

    Aircraft like this are usually found much further down than normal accumulation would suggest – this is because of a couple of effects –

    (a) The aircraft is dark and so likely to warm up more in the sun, melting ice around and underneath it, and
    (b) The pressure of the aircraft will also lead to pressure-melting and gradual sinking of the aircraft.

  42. 142
    Henry Molvar says:

    Sea level rise, etc.

    See for one of the many possible consequences.

  43. 143
  44. 144
    Gerry Beauregard says:

    Re #88 (my own earlier post)

    “Singapore has already put measures in place in response to earlier studies by the IPCC, which was set up by the United Nations Environment Programme and the World Meteorological Organisation. All reclaimed land, for example, are [sic] currently designed to be about 125cm above the highest recorded tide. This is well above the sea-level rise of 59cm projected by the IPCC report.”

    Some people in Singapore (which has lots of reclaimed land) apparently believe that 59 cm is the upper limit for sea level rise, but in fact it could go up a bit more than that even this century – and of course still more after that. It’s pretty hard to raise the height of reclaimed land after roads and buildings have been built on it! And this is just one relatively minor example of the economic impact of climate change…

  45. 145
    pete best says:

    Re 137, I posted about it on post 99, he wants a moratorium on new coal fired power stations until clean coal technology is developed proven it would seem. Europe is also using more coal these days and we all know about China. Clean coal technology is vital to mitigate AGW, however I am doubtful that this will be met within the 10 year period that Dr Hansen required before we allegedly reach that 1 Degree tipping point he has talked about.

    Look at for information on energy services. It is all very interesting there and lots of articles speak of peak oil and gas, new vehicles and alternatives to assist us in our sustainable brave new world.

  46. 146
    Richard Ordway says:

    #136 Mike wrote: “ergo the past 150 years might be a natural phenomenon we have no record for…”

    This is a wierd comment. Please read this website. No serious scientist would just use one line of evidence…or their peers would destroy his or her career (maybe not… since Richard Lindzen, Fred Singer, Michaels and other frauds are doing this..just not in peer-reviewed literature).

    Many other lines of corroborating evidence exist…ocean bed cores, Carbon 12 to 13 ratios in rock, lake bed sample cores, fossil tea leaves (sorry, joke…tree leaves’ stomata (fossil holes that breathed in CO2), amber holes, precipitates that correspond to CO2, other leaf fossils vs animal fossils, and others (its too early in the morning).

    What do you take the scientific communtity for? Only idiots or political rantists would claim that the scientific community would use only one line of evidence.

    For instance, for tree rings, easily 40 tree cores from one area would be used to confirm the data.

    For ancient rock sample Uranium series dating…easily four independent labs would analyze ten or more samples using different techniques to date or do carbon 12/13 anlysis and test the results against each other.

    It is a pretty tough business and you are eventually scutinized pretty closely.

    If you have holes in your arguments, you will eventually be torn to shreds in the peer reviewed journals…if you are honest enough to do it…

    This ice core data body of evidence has been used, examined, refined and built up since the 1950s.


  47. 147
    stephan harrison says:

    Re 134. The gradient might be reduced as isostatic adjustment will have the greatest effect where the ice thins most….near the edges of the ice sheet.

  48. 148
    J.C.H says:

    During WW2 my uncle worked as a physicist under G.R. Irwin at the Naval Research Laboratory:

  49. 149
    Lynn Vincentnathan says:

    RE #93, thanks for the link (
    ). I skimmed over the entry.

    It’s good scientists are looking to the social & behavior aspects re global warming. As mentioned in my (not yet released) post, we need a holistic perspective (re all the problems involved with GW & with the measures that cause GW & a host of other problems, as well).

    And, here, I’ll mention that we need a holistic approach to the study of GW, which includes not only the physical sciences, but the social/behavioral sciences, & even the humanities, since people are causing the problem & people are being affected by the problem.

    My field, anthropology, is the only one I know of (except gerontology) that is simultaneously a physical science, social/behavioral science, and humanities, and it has a subfield in the subfield of sociocultural anthropology called environmental anthropology. So it is well positioned to get involved with GW studies, but to my knowledge it’s not been very involved (a few studies here & there). In fact, it was when I was working on a lecture for Intro to Anthro in 1990, on the final chapter, “The Future of Humanity,” that I seriously got into looking at GW, both for my students and for myself.

    In that lecture I would chalk in a chart (based on rough ideas, not stats) about human adaptibility — sort of an exponential (hocky-stick) curve — a long near-flat line of minor advances over a couple million years, then a slight upswing for the intro of agriculture 12K years ago, and a big upswing at the very end for the industrial revolution, which has even allowed us to go into many new environments (the sky, outer space & under the water), & have a rich lifestyle, etc.

    Then as I finish chalking in the exponential-like curve, I mention that there’s nothing inherent in this that means we’ll automatically keep progressing. In fact, we could cause ourselves so many problems (e.g., environmental, resource depletion), that we crash. And I draw a sharp broken downswing to the end of the curve.

    I’m gonna try harder to push GW in anthropology. If this is not one of the great human (anthro) issues, then what is?

  50. 150
    Andrew Simmons says:

    Ah, now this is something that’s been nagging at me for some time.

    …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.

    Clearly the difference between the outputs of different models and different scenarios diverges over the period of time being modeled. Clearly at some point, merely extrapolating the best, worst, and average trend lines (for both global mean temperatures and for sea-levels) stops being at all useful. For starters, off the top of my very lay layperson’s head, non-linearities and feedbacks that may not be apparent at decadal timescales will emerge. Secondly noise in the system and minor differences between models will be amplified to the point that they drown useful outputs. The scenarios seem likely to become increasingly inaccurate over time, as well — for instance, there are plenty of events beloved by catastrophists that could greatly reduce human population and/or CO2 emissions in a very short period (a new influenza pandemic for instance, or some sort of major earthquake or volcanic event that causes significant economic disruption, or… pick your favourite Horizon episode…

    So, I understand that there are good reasons for the IPCC charts to stop at 2100 (or 2090, or thereabouts)… but surely I’m not the only one to wonder what happens to those plots if the x axis is extended out another century or two? Even if they’re artificially restricted scenarios such as “CO2 emissions stabilise at 1990 levels for the next 300 years”? If anyone’s got pointers to vaguely credible sources such data, I’d love to know…