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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. 151
    Joel Shore says:

    Re #44 (Edward Greisch):

    It might be worth pointing out something that I think you might be confused about: Note that with the earth being like 70% ocean, the average sea shore slope is a very small correction to any calculation of sea level rise. For example, even if the sea levels rose enough to increase the area of the oceans to 74% of the earth’s surface (which is, of course, I think a large overestimate of anything that is going to happen), you would get less than a 6% error in the predicted sea level rise by not accounting for this (i.e., because the surface area of ocean has increased by ~6%…In the area where the ocean already was, the increased volume is given by the surface area times the increase in sea level. In the new area, it is some fraction of this since the depth of the ocean in this new area ranges from a value of 0 to a value of the sea level increase).

  2. 152
    t.r. says:

    Geography is another such field.

  3. 153
    pat neuman says:

    re 96 108

    It seems funny that what seems to be happening in Minnesota seems to be happening in Antarctica’s Amundsen Sea Embayment and other places.

    They said “surprisingly rapid changes” were occurring in Antarctica’s Amundsen Sea Embayment,

    For example, ice fisherman are often surprised by patches of thin or open water that they drop into on occasion even after some seemingly Minnesota style cold spells.

    They’re caught off guard by huge blocks of ice jamming and flooding their homes as rapid thaw of snow on frozen ground occurs even though the winter has been milder than a Minnesota style normal.

    They don’t seem to know or care much about the interactions between snow cover, albedo, ice thickness and how their lake surfaces freeze and thaws.

    The winter of 2006-2007 was a tricky one ya know. Little or no snow cover until early in March. Air temperatures in February were below averages but with sunny skies. The ice got thick in some parts but not others. The sun penetrated the snow free lake surfaces until the snow came. Warm mid March weather got rid of the snow on top allowing the sun back in to warm the water beneath the ice and eliminate it. Thus much of the lake ice was elimated by waters below the ice that didn’t really get very cold, Minnesota style.

    The seemingly funny part is Minnesotans need to be told about the unpredictable danger in driving or snowmobiling out on the lakes, over and over again every year, yet they go out anyway (rarely catch anything worthwhile – mostly bottle bass).

    It seems most humans needs to be told again and again about the consequences of greenhouse gas emissions but global warming doesn’t seem to bother them much so they won’t change. They’ll take their chances which are not as good as they used to be.

    March, 2007 daily high and low temperature plots at Minneapolis are at:

    Plots showing surprisingly abrupt jumps in the frequency of record high low air temperatures and abrupt falls in the frequency of record minimum low temperatures at Minneapolis and Park Falls 2 South, Minnesoa are at:

  4. 154
    Mike says:

    Re 145: Would just like to clarify, I personally am not using these arguments; I am quoting the arguments of someone else, and am trying to address his criticisms. I have looked at the data on this site, but have only recently begun educating myself on the topic, so I’m seeking some clarification. For example, I understand that the ice core data from antartica extends back 700,000 years; my question was, is it 700,000 years of data altogether, or are there gaps? Now I know that there are no gaps. Your comments regarding corroborating lines of evidence are very interesting; I have read about tree rings and carbon dating, but tend to find more about WHAT the results say, rather than HOW they were done. If I tell him “uranium dating provides evidence” I expect he will reply “those tests can be wrong.” But now I can argue that the process involves multiple labs, samples, and different kinds of tests; the results are simply much more reliable. So thanks for the info!

  5. 155
    Ike Solem says:

    Lynn, that’s a great approach. After all, the rise of human civilization over the past six thousand years relied on a fairly stable climate for agricultural production, as I understand it. This is a very rich topic for making the link between human civilization and climate – there are the cultures that lived in the Sahara before it dried out, there are the Anasazi ruins of the American Southwest, the collapse of Mayan civilization due to drought, and the southward expansion of the Scandinavians due to regional cooling.

    However, the contrarian claim is that such climatic fluctuations are ‘normal’ and thus the current warming trend is also ‘normal’. The counter to this is to explain that climate is sensitive to many different variables, from orbital solar forcing over long timescales to volcanic eruptions over short timescales. By focusing on the fact that climate is sensitive to forcings, you can show students how the climate is also sensitive to anthropogenic CO2 forcing, which is greater than any current ‘natural’ forcing.

    Thirty second soundbite: “Our climate is sensitive and variable, as the paleoclimate record and the record of human civilization shows. Human civilization is very dependent on a stable climate regime, as history shows. By adding CO2 to the atmosphere, we are creating a strong influence that will (and is) having a stong effect on the sensitive climate system; the effects are sure to impact human civilization and we therefore have to take action to slow the rate of global warming, and that means drastically reducing fossil fuel use.”

  6. 156

    I think the denialist trolls on these threads are quite valuable. I read one of the denialists and try to phrase my response, then I can scroll down and see if I got it right.

    My own feeling is that we need to be able to refute, civilly, courteously, and briefly, the denialist arguments. To be able to do that you need to be aware of the argument you’re hearing and have an answer ready, and have a high degree of confidence that your answer is right. Practicing on the troll comments in the threads is a good way to be ready.

  7. 157
    Michael says:

    Re 113. It is not obvious that one discipline is more advanced in its use of tools than the other. It seems to me that all disciplines have trouble in describing, simulating and modelling systems which are far-from-equilibrium. Specifically, there are numerous systems that appear, for example, to exhibit self-organised criticality (SOC) and yet very little is understood about SOC. It is not surprising, for example, that the IPCC does not venture to properly describe the melting of the poles because that would be well beyond current methodologies and theoretical frameworks. How, for example, would one be able to describe the formation and dynamics of the large-scale vortices that are bouncing around the antarctic circulatory system and their (resonant) interaction with, for example, the west antarctic ice sheet. Such an example fits within the scope of the nonlinear dynamical processes, dominated by ice sheet disintegration, which James Hansen refers to in the above-linked paper Scientific Reticence and sea level rise. Turning to economics, similar issues apply. If economies are to deliver carbon downsizing on the scale required over the timescales required (eg 90% reduction over about 20 years), the process is nothing other than an economic phase transition. It is hard enough describing simple phase transitions in physics, let alone the phase transitions in process and triggering on the global economic scale. Is the global economy about to enter a state of SOC as it responds to the challenge of climate change? Perhaps. Will global corporations go through a process of SOC as they carbon downsize, synchronise, transition, and accelerate to the carbon lock-down phases? Perhaps. Will antarctica, and specifically the west ice sheet, enter a state of SOC? Perhaps … the climatological experts on RC could provide some comment on the third term which is at the heart of James Hansens paper on Scientific Reticence and his suggestion that paleoclimate ice sheet models do not generally contain the physics of ice streams … or realistic interactions with the ocean … and that sea level rises during this century may be an order of magnitude greater than the IPCC story.

  8. 158
    S. Molnar says:

    Re #137 (Hank Roberts, according to the current number scheme): Who else is talking about it? Why, RealClimate, that’s who. I’m also talking about it, but that fact seems not to impress people for some obscure reason.

  9. 159
    Philippe Chantreau says:

    Serial catowner, I’m not sure that trying to reply to denialists is necessarily a worthy undertaking. If you’re the same serialcatowner who posts in Seattle PI, I share a lot of your concerns, but some denialists are so entrenched in their opinions and so irrational, you’ll never have them acknowledge even the hardest facts. RC is best to actually foster your understanding of the underlying science. Rather than trying to oppose denialists, it is best if you can disseminate that understanding to whatever portion of the general public you have access to, where there is often no established opinion and a limited comprehension of the issues/underlying science.

  10. 160
    Jack Roesler says:

    This report of findings from Antarctica just yesterday indicates that a huge ice sheet is thinning much quicker than predicted. If it melts, or slips into the ocean(could that happen?), the seas would rise 20 ft. The question is, when could this happen?

  11. 161
    Hank Roberts says:

    >137, 144
    thank you Adam for the pointer. I’ve also found mention at Eli Rabett’s blog of Hansen’s presentations.

  12. 162
    Mark A. York says:

    Lynn thanks. Ike Solem makes a great point following up. Thefatc we cam into exoesuence in one of these geological and orbital blips should give anyone pause. Of course the Crichton crowd still thinks Dinoasaurs and humans lived together so…

  13. 163
    Lynn Vincentnathan says:

    RE #102, “Sea level … now rising at 3 mm per year? … 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.”

    [My post on this failed, so here goes again.]

    The problem with this thinking is that there are many other harms from GW, not just sea rise, which seems not to be the worst of the problems. Severe drought, heat killing the crops, & loss of glaciers on which a large chunk of humanity depend for irrigation & drinking water are perhaps more serious problems from GW.

    So even if sea rise does not seem to be a serious problem (and others have pointed out it is a serious problem), there are plenty of other serious problems in store.

    We need a holistic approach. While analyzing a narrow aspect of GW, such as sea rise, we need to keep in the back of our minds all the other effects of GW, and all the other problems arising from the same measures that cause GW (e.g., energy production), such as acid rain and local pollution, to name a few.

    And the non-environmental problems relating to failure to mitigate GW, such as wealth loss due to inefficiency & lack of conservation.

    There are health issues related to our inactive lifestyle. When feasible it is good for the health and spirits to offset a bit of our driving by walking or cycling. Eating low on the food chain is also good for our health, as well as mitigating GW & many other environmental problems.

    Of course there are benefits from measures that cause GW, such as a high lifestyle, but either we choose to tighten our belts in wise ways now — which would be good for our health & pocketbooks — or we (mainly our descendents) may be thrust into a harmful, debilitating type of poverty. Perhaps we need to think about what we really want. Is it money & things, or is it happiness, love, health, & comfort, which we think those things will buy — but we get so caught up in the shopping spree that we forget our original goals.

    I know people who live far from work and drive many miles each day, only because the realtor showed them homes far away (there were comparable, even better homes closer). If feasible (& other requirements can be met), there are many benefits to living closer to work/shops/schools, aside from reduced environmental harms — more time with family, less stress, less health problems from car fumes.

    Mitigating GW is a win-win-win-win situation, v. failing to mitigate, a lose-lose-lose-BIG LOSE situation.

    We cannot simply focus on one narrow aspect of GW, such as sea rise, figure we can get out of the way in time not to get our feet wet, then dismiss the problem of GW altogether.

  14. 164

    [[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.”

    You know why they keep repeating that “cause unknown”? because Jeff Severinghaus of Scripps Institution of Oceanography did a completely crappy job of explaining the 800-year lag issue, and RealClimate compounded its error in not cleaning it up and explaining it by linking to it off its analysis of tGGWS. It doesn’t mention Milankovich cycles by name, nor explain C02 absorption by colder vs. warmer water. It’s deeply confusing.

    For instance:

    Some (currently unknown) process causes Antarctica and the surrounding ocean to warm. This process also causes CO2 to start rising, about 800 years later.Then CO2 further warms the whole planet, because of its heat-trapping properties.[My emphasis]

    A normal reader of that would conclude he was saying this:

    Gee, we have NO IDEA AT ALL why ice ages stop, however, we DO know that that same mysterious process we know absolutely nothing about increases C02 800 years later, at which point the mysterious process slows down and C02 takes over. Which looks like hand waving. IS hand-waving, frankly. It implies by omission much more uncertainty than there is about Milankovich cycles.

    What he should have said:

    The argument that a graph showing a correlation between temperature and C02 with a lag for the latter shows changing T causes changing C02 is flawed for a number of reasons. I’ll give as a counterexample a model of T/C02 feedback over the 5,000 years it takes for the Earth to warm up from a glaciation. That fits the ice-core derived data well, including even the 800-year lag at the end of the glaciation. In other words, I can easily give a counter-example in an artificially minimalist case where we don’t even have to quibble over what warmed the ice up to begin with that matches the data, therefore, logically, your claim that the data shows change in T causes change in C02, period, is invalid.

    I can’t even speculate* on why Severinghaus chose to use “currently unkown.” The reason that correlation graphs are not generally not sufficient evidence of causation is fundamentally the possibility that a third factor is causative both of the correlated variables. I’d be intrigued as to what possible observations we could have that would change the causes of the ice warming up from “currently unkown” to “currently known.”

    *Okay, I’ll GUESS from this:

    “Changes in the amount of summer sunshine, due to changes in the Earth’s orbit around the sun that happen every 21,000 years, have long been known to affect the comings and goings of ice ages. Atlantic ocean circulation slowdowns are thought to warm Antarctica, also.”

    that the point was supposed to be something like, “…whatever the relative contributions of the known factors turns out to be.”

  15. 165

    64. Barton Paul Levenson:

    They, and we, do know. However, unfortunately, on RC there’s an explanation of that 800-year lag talking about warming from a glaciation that starts with the Antarctic warming for “cause unkown” which same “process” causes C02 to rise 800 years later. Moreover, people in comments on RC are repeating that “cause unknown.” So it’s “our” fault, not just INCLUDING RC, but in fact, ESPECIALLY RC’s fault, frankly. And it should be fixed ASAP.

  16. 166
    bob says:

    This is off-topic, but I really wish somebody from RealClimate (or someone as knowledgeable) would take apart Margaret Wente’s columns on climate change in the Globe & Mail ( — the woman is a mine of disinformation.

  17. 167
    FatBoy says:

    To a non scientist it certainly seems that there is an accelerated rise is sea level height and whilst this may seem small over a decadal period, beyond that it will have a significant impact on waterside communities, particularly in the third world who are least able to adapt to sea level rise. Interestingly no one seems to take responsibility. Even wealthy pro AGW seem quite happy to go on with their lifestyle and travel choices and offset thier “carbon foot print”. However this approach to me is similar to suggesting being a paedophile is OK as long as I pay for a thirld world school or other childrens project. Wrong is Wrong.

  18. 168
    George K says:

    CO2 only changes from 180 ppm to 280 ppm from the height of the ice age to the height of the interglacial.

    Is that enough to contribute to the temperature change of 5C? How much does it contribute?

  19. 169
    tamino says:

    Re: #167 (George K)

    The increase in CO2 contributes about 2 deg.C to the net warming from glacial to interglacial. The other 3 deg.C is mostly from reduced albedo, caused by wasting away of the ice sheets.

  20. 170

    very very nice blog.thank you for your informations…

  21. 171
    David B. Benson says:

    Re #167: George K — About half.

  22. 172
    Michael says:

    Re 163. Sea level rise is a big problem. It is the problem that is making governments and corporations sit up and consider what action needs to be taken. If the IPCC sea level rise numbers are wrong or deficient, there are urgent implications for protection of infrastructures on coasts. These infrastructures have multiple mappings into global supply chains and national and regional infrastructures. If we use James Hansens terminology in his Scientific Reticence and Sea Level Rise paper (linked above), there are three terms to consider. Terms 1 relates to thermal expansion of ocean water and term 2 relates to melting of alpine glaciers. For the sake of simplicity (superposition and coupling arguments etc), these are the linear terms. Term 3 is the nonlinear term and relates to ice sheet disintegration. If I understand the situation correctly, the IPCC has broadly focussed on terms 1 and 2 and so the sea level rise number is at most 1 meter, ingoring all the fiddle factors.

    By their nature committees and groups seeking consensus and groupthink don’t do nonlinear because nonlinear doesn’t easily sit with consensus or groupthink. But James Hansen suggests the nonlinear bit could give about 10 meters. If the IPCC did consider the nonlinear effects but just lost the report because it was put in the wrong filing cabinet then they should come clean. If they didn’t do the nonlinear bit, they should also come clean. What the world needs is for climate experts to step up to the line and suggest what they think the missing third term might be. Upper and lower bounds would be OK. Statements that they don’t know would be OK. To ignore it as an unspeakable is surely not acceptable because that misleads many into thinking sea level rise = term 1 + term 2 + zero and so BAU is probably OK. At least if there were a distribution of experts estimates and bounds for the third term then others who are risk mapping the effects (eg on global supply chains, on infrastructure) can apply relevant bounds.

    James Hansen has written that he thinks the sea level rise by 2100 may be about 10 meters. What do other climate change experts think? What do climate change experts expect the component of sea level rise due to nonlinearity to be?

    How should the world’s public interpret the experts silence about the missing third term?

  23. 173

    Actually he said five meters. This is a ballpark figure that assumes thermal expansion, meltwater runoff and the collapse of West Antactica, derived by simple decadal doubling of the 1 cm/decade increase in the ice sheet contribution of global sea level rise observed today. Thermal expansion may increase that figure.

    Since we thought we were looking at a 1.8 mm/year increase, and it turns out the actual increase may be 3.2 mm/year, there may be some cause for concern. But what the hell, it’s only water.

  24. 174
    Steve Bloom says:

    Re #172 (Michael): Did Hansen actually say 10 meters by 2100? Where? I had the impression that he was actually being somewhat “reticent” about stating a worst-case for 2100, probably because it would just be a guess.

  25. 175
    Ike Solem says:

    The sea level rise numbers reported in the IPCC seem to be estimates of the transient climate response. It seems that they should have also included an equilibrium sea level response, after the estimations of the equilibrium temperature sensitivity of the climate models. The issue of the ice sheets is discussed at (section 4.0):

    “With the effect of ice sheets included, the total rise could be larger by a substantial factor. However, projections of the contribution of the ice sheets to future sea level rise are not presented here due to the difficulties of performing a credible calculation. For reference, the volume of ice in the Greenland and Antarctica ice sheets is equivalent to a sea level rise of 7 and 73 meters, respectively”

    What this seems to mean is that there is little scientific basis for claims that there is a low-percentage chance of abrupt climate change based on model runs that ignore the ice sheet response. 95% confidence that there is little chance of abrupt climate change is unsubstantiated if you are basing that on models that ignore ice sheet dynamics and carbon cycle feedback effects. Nobody knows. If you don’t know if a gun had a bullet in it or not, would you point it at your head and pull the trigger?

    On the other hand, rapid melting of the ice sheets might have a temporary cooling affect on the upper ocean temperatures, if the results of Lyman et al are correct (RC: Ocean heat content: latest numbers). They estimated a cooling upper ocean with no reduction in sea level due to thermal contraction, pointing toward increased meltwater flowing into the oceans. This negative feedback would reduce the possibility of drastically abrupt climate change…perhaps. A better system for monitoring ocean heat content and transport is needed, and NASA and NOAA should cooperate on this… though under the current situation (political comissars overseeing these institutions) it seems unlikely. Maybe Congress could provide some kind of earmarked funds?

    The only legitimate scientific discussions over anthropogenic climate change are how fast will it happen, and how far will it go. The fact that it is happening has already been settled.

  26. 176
    Tavita says:

    Some of you may find this link to “Disappearing Islands” a page concerning sea-level rise and various island nations of interest.

    And you all may have additional links that will help to improve the link above. I suggested RC to them and they added it.

    (Note: When using the Safari browser the site above automatically reloads as soon as it is loaded. At least with Safari one has to actively stop it using the stop loading button once it has fully load. With my old Netscape browser there was no problem.)

  27. 177
    Jim says:

    I was under the impression that the Antartic Ice sheet is gaining more mass than it loses. At least every paper I can find is saying that. Why do you say it is losing mass Stefan? I know the Antartic pensisula is warming, but the continent as a whole is getting colder not warmer and the ice sheet as a whole is growing not retreating. Is there a study that states otherwise? (I posted something similar a while back and Ray Pierre agreed with me at the time, however he postulated it would change in 10 years.)

  28. 178

    I was under the impression that the Antartic Ice sheet is gaining more mass than it loses

    That’s odd that you say that, because this result has been out there for a while now. Do you read science papers? They are easy to find :

  29. 179
    pat neuman says:

    “A 3 degree Celsius global average warming would lead to a sea level rise of 80 feet,” said Hansen.

    … “Equilibrium Sea Level Rise for ~3 C Warming (25+- 10 m = 80 feet) Implies the Potential fo Us to Lose Control” …

    As I see it, although we may see a … “3 degree Celsius global average warming” … an … “Equilibrium Sea Level Rise for ~3 C Warming … may be delayed quite a bit by thawing, break-up and heat-expansion factors and other uncertainty factors.

  30. 180
    Hank Roberts says:

    Jim, would you be willing to explain your approach to finding things out? How did you search for information about Antarctica and come to the conclusion that you gave? I’m sincerely curious — did you ask your librarian, or use Google, or rely on websites you trust?

    I’m very interested in how people try to find out about climate change — and particularly, how people look for information as you say you did, and not only don’t find the current science, but find information somehow that leads them to wrong answers.

    It’s not an easy process, and learning how it goes wrong is sometimes very helpful. How’d you look?

  31. 181
    Michael says:

    Apologies. In the Scientific Reticence paper Hansen wrote “That time constant yields sea level rise of the order of 5 m this century.”

    Re 179. What might be an estimate for the time constant in the equilibriation delay, eg if WAIS disintegrated? Non-equilibriated levels would presumably be somewhat uneven and so ports / coastal plant would be exposed to different levels of risk in different regions. Has modelling been done of the equilibriation process assuming distinct (eg high risk) ice sheet disintegration events?

  32. 182
    OldManOnFire says:

    I’ve been contemplating how to get our lethargic minds away from American Idol and to pay attention for a few nanoseconds to the discussion on potential and threatening global climate changes??

    Guys, if you were to discover that with possible draught conditions from warming climates, that hops, barley, and grapes will no longer grow, you can assume there won’t be any more beer and wine!! We will never get laid again!

    Perhaps if you discovered that tobacco plants are not draught tolerant, you might deduct that there won’t be any more cigarettes and cigars!! There will be lots of weeds but not the right kind of ‘weed’!

    And ladies, as well as metrosexuals, what if the cacao tree can no longer flourish? Do you know what this means? It means no more chocolate!!

    Disasters in the Middle East, and anything else that idiot-Bush causes, will pale in comparison to a life without beer, cigarettes, and chocolate! Get the point??

    Now…regarding CO2 emissions that we hear about 24/7, I realized most people cannot grasp this concept because CO2 is invisible! There’s a solution for everything! I believe our federal government should ‘require’, for a one-month period each year, that all oil and oil-based fuels are to be laced with a harmless black dye! This will allow each and every one of us to visibly see CO2 emissions!! Just imagine every car, motorcycle, truck, construction equipment, farm implement, train, bus, factory, airplane, military vehicle, power plant, lawnmower, and weedeater spewing black soot??!!

    If this does not heighten the discussion about CO2 emissions, then gawd help us…

    I know some reading this post are in a ground hog-day funk reading and re-reading paragraph number two above, but for the rest of us, I ask that we at least gain a better understanding of global climate changes, and that we get personally involved in the discussion.

    How about it…a CO2 Awareness Month????

  33. 183
    Almuth Ernsting says:

    Last autumn, it was reported that the Arctic winter sea ice had declined far more rapidly than before during the 2004/05 and 2005/06 winters. Is there a figure for the overall sea ice anomaly for the winter just past, or does anybody know when this is likely to be published? Many thanks!

  34. 184
    Ron R. says:

    O.T. It’s looking to be the driest rainfall year on record for California central coast.

  35. 185
    Lynn Vincentnathan says:

    Re#172 & 163, I totally agree sea rise is a very serious issue (I should have made that clearer). My main point was that it is only one of many problems GW is bringing us, and that we need a holistic perspective, at least to keep in mind, while looking at single aspects of GW. Otherwise the contrarians will pick GW problems off one-by-one.

    Like, “Species X is going extinct; well, we can live with that. Species Y is going extinct? Too bad, but no problem.” Without considering that a lot of species are going extinct or threatened with extinction due to GW (& other human-caused problems) — and we have no idea how much we can rip apart the web of live without it seriously harming us, maybe a lot, but maybe just a little & it will come back to kick us in the face.

    And species loss is just one of many problem due to GW (& to our other environmental harms). Polar bears might make good poster species for some people, but others couldn’t care less. A poster with all the species that are going extinct, and with all the problems from GW might be more appropriate. Of course, then people might be overwhelmed and stymied — but at least it would be more honest.

  36. 186
    Jim says:

    Oh I don’t know,

    maybe this one
    and the FAQs on this site (Written in 04 though)

    I did some more quick digging.
    One year 2005 they say it is growing the nex they say it is losing 2006. Just never can tell these days.


  37. 187
    Jim says:

    Re 180.

    I read alot, but it is mostly online texts. Doesn’t everyone use google?


  38. 188
    tamino says:

    Posts 182-184 (by Pumuker) are spam, and should be removed. How did this get through in the first place?

  39. 189
    George K says:

    You can find all the daily data on polar ice back to 1979 at this site. The 2006 summer ice minimum was higher than 2005 and 8 of the last 16 years including 1990. 1999 was the lowest level.

    In terms of antarctic ice mass, those measurments from the GRACE satellite (which use gravity measurement) are not very accurate. There are significant variations in gravity levels by season, by day and significant modeling is required to produce the results. In addition, Grace doesn’t have very good resolution (400 kms.)

  40. 190

    I read alot, but it is mostly online texts

    Then why do you continue to spread your FUD here?

    #188 – More FUD.

  41. 191
    Hank Roberts says:

    Jim, consider going to a school or library and asking a reference librarian for help. They know how to tell wheat from chaff, needles from haystacks, science from public relations.

    Online, look using Google Scholar instead of Google.
    Compare the results you get — you’ll see how different even Google’s weak filter for garbage makes the results.

    Make sure the search you use will look for articles in peer-reviewed science; look at the footnotes and read them to see if they’re being properly used.

    There’s a whole lot of public relations/political/bogus fake info available.

    The TCS site is the latter; they’ll rush stuff out like the above when science is being published that threatens their publisher’s interests (you do know who owns that place don’t you, George K? Know how to look it up?).

    “Opinions Differ on Shape of Earth” headlines happen because of places like TCS.

  42. 192
    Chuck Booth says:

    Re #186 That summary states, “…the interior of the East Antarctic ice sheet is actually gaining mass”- Note the word “interior.”

    If you had checked out the research article to which it referred, you would have seen this:

    Science 24 June 2005:
    Vol. 308. no. 5730, pp. 1898 – 1901

    (Originally published in Science Express on 19 May 2005)

    Snowfall-Driven Growth in East Antarctic Ice Sheet Mitigates Recent Sea-Level Rise
    Curt H. Davis, Yonghong Li, Joseph R. McConnell, Markus M. Frey, Edward Hanna

    Satellite radar altimetry measurements indicate that the East Antarctic ice-sheet interior north of 81.6°S increased in mass by 45 ± 7 billion metric tons per year from 1992 to 2003. Comparisons with contemporaneous meteorological model snowfall estimates suggest that the gain in mass was associated with increased precipitation. A gain of this magnitude is enough to slow sea-level rise by 0.12 ± 0.02 millimeters per year.

    Free abstract available at:

    It doesn’t say the entire ice sheet is gaining mass, only a portion of the interior, meaning the overall loss of ice is occurring more slowly than previously thought.

  43. 193
    Almuth Ernsting says:

    Re 188:
    Thanks – the Northern Hemisphere sea ice anomaly is on this page: .

    Looks like we’ve got the greatest Arctic sea ice anomaly on record right now.

  44. 194
    Ray Ladbury says:

    Sorry, George and Jim, but the net result, known for over a year, is a net loss of Antarctic ice. See for example:

    This includes results from ICESAT as well as GRACE, and other techniques.

  45. 195
    Blair Dowden says:

    “A 3 degree Celsius global average warming would lead to a sea level rise of 80 feet.”

    This figure is based on estimated sea levels three million years ago (the Pliocene), when global average temperature was about 3 degrees warmer than today. But the Pliocene was a time of cooling and ice cap formation, while the near future is a time of warming and ice cap melting. An ice cap creates its own regional climate by reducing local temperature. For example, if you removed all of the ice from Greenland, a new ice cap would not form at today’s temperatures. This implies that three degrees of warming will leave a much larger ice cap than the Pliocene started with, so sea level increases may not be nearly so high.

    It is also noted that sea levels in the previous interglacial period (the Eemian) were 4 to 6 meters higher than today, with global average temperature only about one degree higher. But the ice reacts to local temperature, not the global average. Conditions during the Eemian were very different than the predicted greenhouse warming. Solar forcing in the polar regions during the critical spring period was more than 60 watts per meter above normal due to the Milankovitch orbital changes. Compare this with 3.7 W/m2 of direct forcing from the doubling of carbon dioxide levels expected in the 21st century. It is quite possible that this was the cause of the extensive melting in the polar regions, and the one or two degrees of global temperature rise was mainly a side effect.

    The critical factor for sea level rise from melting ice caps is the time required for the ice cap to reach equilibrium at a certain temperature. This could be from a few hundred to a few thousand years. Hansen comes in at the low end. If the equilibrium time is longer than the pulse from greenhouse warming, the full melting potential may never be reached.

    So while I believe that sea level rise from global warming will be a serious problem over the time frame of the next few centuries, the total amounts have been overstated.

  46. 196
    Michael says:

    On page 34 of the report entitled “The Future Oceans – Warming Up, Rising High, Turning Sour” from the German Advisory Council on Global Change at it is written:

    “rates for sea-level rise of up to 5m per century are documented, and these probably do not represent an upper limit”

    it is also written on that page

    “the assumption that ice masses would have melted significantly more rapidly with faster warming is quite plausible.”

    Page 34 of the above-quoted report also states “Thus climate history shows that a much more rapid rise than that expected by the IPCC for the 21st century is possible”.

    In his article in the New York Review July 13 2006, James Hansen, who is not an author of the German report, quoted above, states “The Earth’s history reveals cases in which sea level, once ice sheets began to collapse, rose one meter (1.1 yards) every twenty years for centuries.”

    Climate change experts, can you please provide answers to the following:

    Q1. What is special about the warming conditions in the 21st century to say that a 5 meter rise may or may not occur?

    Q2. Has the IPCC discounted the prior historical rises and if so on what basis?

    Q3. With regard to the first quote, what might an “upper limit” be?

    Q4. With regard to the quote from Hansen’s article, what phenomena might be taken to construe the point at which “ice sheets began to collapse”, and have such phenomena yet been observed?

    Q5. When the Larsen B ice shelf collapsed, were you taken by surprise and had you prior to the event thought that such a thing would not happen so quickly?

  47. 197
    Hank Roberts says:

    Note, read for content, don’t rely on sequence numbers. Here, numbers after 182 changed (subtract three from what they were briefly) when three spams that got by were deleted by the moderators.

    Sequence numbers sometimes increase; posts appear in timestamp order; if one’s held by moderators longer than later ones, it will then pop into the sequence later, causing all the posts “below” it to be renumbered).

    Happens on all web forums.

    Re “fud” — Jim, George, no shame being fooled the first time, but you have to develop real skepticism when reading claims, to tell the science from the “fear, uncertainty, and doubt” garbage being used to confuse people about the science.

    Say where you get your ideas; check whether you’re being fooled by someone with a political or commercial public relations website. There are more of them than there are science websites. They have more money to fool people. Read carefully.

  48. 198
    Michael says:

    Re 188. Lynn, I share your concerns and did not mean to detract from the broader and very important issues you were raising. The web of life is creaking under the pressure of GW and causing great harm, and yet so many choose not to see it, let alone care.

  49. 199
    Blair Dowden says:

    Re #199: Michael, I am not an expert, but I will try to respond to your questions. As for what is special about warming conditions in the 21st century, I would like to suggest that what is special is the conditions under which a sea level rise of 5 meters per century actually occurred.

    Twenty thousand years ago the northern part of North America was covered by an ice cap larger than that on Antarctica today. Unlike the Antarctica and Greenland of today, this ice cap was centered well below the Arctic Circle. That made it much more sensitive to a warming event. In addition, it was a temporary ice cap, in contrast to the permanent ice cover of today’s ice caps. The weight of the ice caused the ground to sink, leading to warming of the bottom of the ice because it was closer to the center of the Earth, and lowering the altitude of the top of the ice, also adding to its warming.

    So you had this huge, unstable ice mass almost collapsing under its own weight, which melted very rapidly when changes in orbital parameters caused some warming. I do not think it is valid to extrapolate its melting rate to the situation in the 21st century.

    I think this answers your Q2, which is why the IPCC has discounted it. As for Q3, no one really knows what the maximum melting rate will be, but we have every reason to expect it will be considerably less than that during the recovery from the last ice age.

    While sea level rise due to global warming is a real concern, I argue that the 5 meters per century is another overstatement of the issue based on faulty extrapolation from the past, similar to that which I talked about in my post #198.

  50. 200
    Arvella Oliver says:

    **I hope I’m not violating any rules by recommending a book; if I am, I hope the moderators will delete this. No, I don’t know the author (darn it) nor am I benefitting in any way from this post**

    “Something New Under the Sun: An Environmental History of the Twentieth-Century World” by J.R. McNeill (Norton & Co., 2000).

    Lynn, I think you and your students would like this book. It’s pretty much as the title describes: a history of humanity’s interaction with the environment. It has extensive notes and a massive bibliography (40 pages! I’d say McNeill’s done his homework). It’s a fairly easy read, and the organization is excellent. Also, McNeill manages a fairly objective tone throughout.

    As an historian I think it’s important for people to understand how we got ourselves into this global warming mess. I think it helps us get a grip on what’s happening now and pushes us to imagine the future.