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The certainty of uncertainty

Filed under: — group @ 26 October 2007

A paper on climate sensitivity today in Science will no doubt see a great deal of press in the next few weeks. In “Why is climate sensitivity so unpredictable?”, Gerard Roe and Marcia Baker explore the origin of the range of climate sensitivities typically cited in the literature. In particular they seek to explain the characteristic shape of the distribution of estimated climate sensitivities. This distribution includes a long tail towards values much higher than the standard 2-4.5 degrees C change in temperature (for a doubling of CO2) commonly referred to.

In essence, what Roe and Baker show is that this characteristic shape arises from the non-linear relationship between the strength of climate feedbacks (f) and the resulting temperature response (deltaT), which is proportional to 1/(1-f). They show that this places a strong constraint on our ability to determine a specific “true” value of climate sensitivity, S. These results could well be taken to suggest that climate sensitivity is so uncertain as to be effectively unknowable. This would be quite wrong.

The IPCC Summary For Policymakers shows the graph below for a business-as-usual carbon emissions scenario, comparing temperatures in the 1980s with temperatures in the 2020s (orange) and 2090s (red). The latter period is roughly when CO2 will have doubled under this scenario. The resulting global temperature changes cluster between 2 and 5 degrees C, but with a non-zero probability of a small negative temperature change and long tail suggesting somewhat higher probabilities of a very high temperature change (up to 8 degrees is shown).

We have very strong evidence for the middle range of climate sensitivities cited by the IPCC. But what Roe and Baker emphasize is that ruling out very high sensitivites is very difficult because even the relatively small feedbacks, if they are highly uncertain, can have a very large impact on our ability to determine S.

Paleoclimate data do provide a means to constrain the tail on the distribution and perhaps to show the likelihood of large values of S is lower than Roe and Baker’s calculations suggest. In particular, Annan and Hargreaves (2006) used a Bayesian statistical approach that combines information from both 20th century observations and from last glacial maximum data to produce an estimate of climate sensitivity that is much better constrained than by either set of observations alone (see our post on this, here). Their result is a mean value of deltaT close to 3ºC, and a high probability that the sensitivity is less than 4.5ºC, for a doubling of CO2 above pre-industrial levels. Thus, we emphasize that Roe and Baker’s result do not really tell us that, for example, 11°C of global warming in the next century entury is any likelier than we have suggested previously.

On the other hand, there is a counterpoint to such a comforting result. Roe and Baker note that the extreme warmth of the Eocene — something that has stymied climate modelers — could in principle be explained by not-very-dramatic changes in the strengths of the feedbacks, again because small changes in f can produce dramatic change in S. The boundary conditions for Eocene climate remain too poorly known to include in a formal calculation of climate sensitivity, but at the very least the extreme climate of this time suggests that we cannot readily cut the tail off the probability distribution of S.

It would be wrong to think that climate scientists have been ignorant of the non-linear nature of feedbacks on climate sensitivity. Several papers dating back a couple of decades show essentially the same result (for example, Hansen et al., 1984; Schlesinger, 1988; see below for full citations). But Roe and Baker’s paper is probably the most succinct and accessible treatment of the subject to date, and is a timely reminder of some very basic points that are not always appreciated. For example, it is often assumed that the tail on the distribution of climate sensitivity is due to the large uncertainty in some feedbacks, particularly clouds. Roe and Baker make it very clear that this is not the case. (The tail in S results from the probability distribution of the feedback strengths, and unless those uncertainties are distributed very, very differently than the Gaussian distribution assumed by Roe and Baker, the tail will remain). Furthermore, they point out that “uncertainty” in the feedbacks need not mean “lack of knowledge” but may also reflect the complexity of the feedback processes themselves. That is to say, because the strength of the feedbacks are themselves variable, the true climate sensitivity (not just our ability to know what it is) is inherently uncertain.

What will get the most discussion in the popular press, of course, are the policy implications of Roe and Baker’s paper. Myles Allen and David Frame take a stab at this in their Perspective.* Their chief point is that it is probably a bad idea to assign a specific threshold value for CO2 concentrations in the atmosphere, above which “dangerous interference in the climate system” may result. For example, 450 ppm is an oft-cited threshold since this keeps deltaT below 2°C using standard climate sensitivities. But the skewed nature of the distribution of possible sensitivities means that it is much more likely that 450 ppm will give us more than 4.5°C of global warming rather than less than 2°.

Allen and Frame suggest that the way to address this is though an adaptive climate change policy, in which there are movable CO2 concentration targets that can be revised downwards if future observations suggest that the climate sensitivity is indeed greater than the middle IPCC range. We agree that adaptive policies are needed. There is no point in continuing to pursue a 450 ppm stabilization goal in the eventuality that temperatures have already exceeded the expected 2 deg C. More reductions would be called for. Similarly, if temperature rises more slowly than expected, that would buy time. However, in our view, Allen and Frame’s discussion turns the precautionary principle on its head by implying that downward revision can always be done later, after more data are in. But a good adaptive strategy depends on nimble action and forward thinking — both of which are typically in short supply. If reactions to a worse-than-expected climate change are delayed, they make an overshoot of any temperature target very likely, and corrective action very expensive. Thus conservative strategies would seem in order, which probably implies initial targets of much lower than 450 ppm, and still subject to further revision.

The bottom line is that climate sensitivity is uncertain, but we can pretty much rule out low values that would imply there is nothing to worry about. The possibility of high values will be much harder to rule out. This is something policy makers should recognize and confront.


Hansen, J.E., et al., in Climate Processes and Climate Sensitivity, J. E. Hansen, T. Takahashi, Eds. (Geophysical Monograph 29, American Geophysical Union, Washington, DC, 1984), pp. 130–163.
Schlesinger, M.E., 1988: Quantitative analysis of feedbacks in climate model simulations of CO2-induced warming. In Physically-Based Modelling and Simulation of Climate and Climatic Change, M. E. Schlesinger, Ed., NATO Advanced Study Institute Series, Kluwer, Dordrecht, 653-736.
*See also the news article in Nature. And our congratulations to Myles Allen and his colleagues who won the Euro Prix award for their climateprediction.net work.

251 Responses to “The certainty of uncertainty”

  1. 151
    Pascal says:

    Timothy

    I know this study:

    Unexpected Growth In Atmospheric Carbon Dioxide
    ScienceDaily (Oct. 23, 2007)

    I’m speaking about the recent years and not about the difference between last decade and 2000-2006

    In the same study, there is the fact that CO2 emissions of 2006 were the highest never seen.

    look at this link and tell me if you see any acceleration.

    http://www.esrl.noaa.gov/gmd/ccgg/trends/

    for the 4 last years there is even a deceleration of 0.08 ppm/y.y

    for sea level you know this link

    http://sealevel.colorado.edu/current/sl_noib_ns_global.jpg

    for the last years I don’t see and calculate any acceleration.

    on the contrary there is a very slight deceleration.
    you can get the data and make the calculation by yourself.

    But, to avoid any confusion, I recall that my present interest is in the recent years and not in the last decades (more climatologically representative)

    In the same way we can ask ourselves why there is an ocean cooling since 2003.

    Maybe, surely, it is the “variability” but, maybe also, there is another phenomenon in ocean mixing.
    If my calculations are good, ocean is enough cold (3.5°C average) to absorb 1600 years of CO2 doubling RF.
    Of course we’ll not go so far as there.

  2. 152
    pete best says:

    With regard to the certainty of uncertainty I read an article in the UK newspaper the Independent the other day stating that sinks were becomming sources decades earlier than expected. Is this possibly true?

  3. 153
    Timothy Chase says:

    Charles Muller (#149) wrote:

    You can find publications of LEGOS (including Lombard et al. 2007) at this page:
    http://www.legos.obs-mip.fr/fr/equipes/gohs/publis

    The Fig. 6 (T-P and Jason from 1993 to 2006) shows no particular acceleration for the sea leve rise in the most recent years 2002-2006.

    Not a large one, but we aren’t exactly expecting a large one as of yet. But looking at the diagram, despite the seasonal variation would seem to be positive trend in the mass component for sea level.

    I am assuming that you aren’t simply looking at the endpoints in the trendlines but are considering the slope of the relevant trendlines? If not, it is worth keeping in mind the fact that there is a statistical slight-of-hand where one cherry-picks the endpoints, claiming that there is no trend when despite the choice of endpoints statistical analysis shows quite the opposite.

    But to make an accurate determination of the trend we would need the actual numbers – then possibly run them through Excel.

    Yet there clearly isn’t a great deal of change in the mass component to sea level rise. However, I would keep in mind the fact that over a decade’s time, we have seen more than a doubling of the rate of loss of mass balance in Greenland, a tripling in icequakes, the warming of the West Antarctic Peninsula resulting in the acceleration of glaciers, the accelerating loss of global glacier mass balance, etc. If things continued at their current rate there wouldn’t be much of a problem for a very long time. But things are changing.

    Charles Muller (#149) wrote:

    The review of Sheperd and Wingham 2007 gives a best estimate of 0,35 mm/yr for present contribution of Greenland + Antarctica to sea level rise.
    http://www.sciencemag.org/cgi/content/abstract/315/5818/1529

    A doubling / decade of this rate (0,35mm/yr) would give approx. 3,58 m for 2100 (plus thermosteric). Well, isn’t it a bit… pessimistic ?

    Would you rather count on… wishful thinking?

    Charles Muller (#149) wrote:

    I read some Hansen papers about “dangerous climate change”, but his comparison (notably) with Eemian didn’t convince me (beyong global mean temperature of the two periods, there was a huge solar forcing on Greenland during the thermal maximum of Eemian).

    Yes, I believe that the huge solar forcing was the forcing responsible for the high temperatures at the time. (CO2 levels were roughly preindustrial 275 ppm, although I believe the peak was 290 ppm.) So to say that “it was the temperatures and the solar forcing” is in a certain sense double-counting. Today we have the high temperatures contributing to ice melt and we have the huge longwave forcing due to higher levels of carbon dioxide. But I wouldn’t count both as if they are independent of one-another.

    And while the albedo of ice is around 0.4 to sunlight, I believe it is closer to aphalt in the infrared spectrum. So in our time ice is experiencing a similar forcing, but with more longwave flux, less shortwave – although we also have the dubious benefit of anthropogenic black carbon emissions. After having dropped nearly to preindustrial levels, they are climbing again as the result of the economic development of China.

  4. 154
    Nick Barnes says:

    #146: my understanding is that ice shelf breakup does contribute to eustatic sea-level rise, as you say, but only a little, and less so for larger ice shelves (the anchoring is more distant). There is also a small contribution to sea-level because of the different density of fresh and salt water. I think, though can’t cite, that both of these effects are small besides the albedo and braking effects.

    More on ice sheets vs ice shelves. From the point of view of an amateur such as myself, there are essentially three ice sheets in the world.

    1. The Greenland Ice Sheet (GIS): 3 million cubic kilometres, averaging 2km thick, covering the great majority of the interior of Greenland. On low-lying land (around sea-level), mostly surrounded by mountains, draining to the sea by various large glaciers. Subject of some specific concern about global warming because of large temperature rises predicted for the arctic, and because of some arctic-specific feedback effects (e.g. the albedo feedback following loss of arctic sea ice). The IPCC says it’ll take a thousand years to melt. Some scientists are increasingly more concerned and less conservative and raise real concerns about major melting before 2100.

    2. The East Antarctic Ice Sheet (EAIS): 25 million cubic kilometres, on various terrain, mostly separated from the sea and from the WAIS by various mountain ranges. The true deep-freeze. Unlikely to melt much any century soon, unless some huge non-linear feedbacks take hold. May grow a small amount with global warming, due to increased precipitation. Let’s cross our fingers and ignore this one, because if it melts then sea-level rises by 60 metres and that’s beyond bad.

    3. The West Antarctic Ice Sheet (WAIS): about 3 million cubic kilometres, averaging around 3 km thick. This is different from the other two in a couple of ways. Firstly, the bedrock it sits on is mostly below sea-level (very far below sea-level in places). Secondly, as well as being drained by a number of glaciers, it also meets the sea in a pair of enormous ice shelves: the Ross shelf and the Ronne shelf. These are akin to enormous glaciers: hundreds of metres thick, floating on the sea, formed mostly by precipitation, steadily flowing out to sea and breaking up into bergs. There are essentially no ice shelves in the arctic (where bergs come from regular glaciers), and none in the antarctic which compare to these two. You can think of the WAIS as a branch of the southern ocean which is filled with ice, and the R&R shelves as the floating boundaries.

    This marine nature of the WAIS causes some concern, because it means that it might be affected by sea-level rise from: if the GIS lets rip, we really don’t know what might happen to the WAIS. The models we have for ice sheet dynamics are quite new, and we have no way of testing them. Also, the stability of the ice shelves is doubtful and without them the ice sheet would probably flow much more rapidly into the sea, finding a new equilibrium after losing a great deal of mass.

    Apart from these last concerns, the WAIS is much less worrying than the GIS, because the huge thermal inertia and albedo effect of the EAIS, the antarctic continent itself, and the large amount of antarctic sea ice in the southern winter, all act to reduce the degree of warming for the WAIS (whereas the GIS is the victim of various unfortunate circumstances which amplify warming there).

    Ice shelves, especially the really large R&R shelves, are of interest for a couple of reasons. If we lose them rapidly then there would be a small rapid sea-level rise, major rapid local effects to do with huge quantities of meltwater, and an albedo feedback effect. But also the shelves are understood to have a braking effect on the glaciers and ice sheets which feed them. Without the ice shelf, ice will flow into the sea more rapidly. We’ve seen this in glaciers after the loss of the Larsen A and B ice shelves (relatively small shelves on the Antarctic Peninsula), and we’ve seen a similar effect in Greenland, where the floating end of the glacier, and the fjord choked with calved bergs, could apparently perform a similar braking function, now lost for several rapidly-retreating glaciers.

    A bit more terminology: “eustatic” sea-level changes are due to changes in the amount of water (e.g. the Greenland ice sheet melting, or growing due to precipitation). “steric” sea-level changes are due to changes in temperature or salinity (water expands as it gets warmer). Most of the sea-level rise we saw in the 20th century was steric: the world got warmer, the sea-level rose. Almost all of the firmly predictable rise for the 21st century is steric: the “18-59cm” number the denialists love to quote from the IPCC is steric rise. The great unknown, which the recent Hansen paper suggests at several metres, is the 21st century eustatic rise, due primarily to ice sheet melting (also melting of polar and mountain glaciers, and of ice shelves).

  5. 155
    Ray Ladbury says:

    re 149. Charles Muller, I would be reluctant to do a linear extrapolation for a system with several known positive feedbacks. I certainly don’t think we’ll melt all of Antarctica or even Greenland, but we know from the paleo record very significant melting and sea level rise are possible once the warming epoch gets under way. I would say this is one area where we have a lot of unknown unknowns.

  6. 156
    Dave Rado says:

    Re. henning, #143 (in which you refer to my post #141), I’m not sure why you think that simply avoiding a 70m sea level rise would be something to celebrate. As the IPCC WGII report makes clear, even a 59cm sea level rise would be very serious, especially when combined with the many other expected impacts of climate change such a huge rise in:

    a) the frequency and intensity of extreme weather events;

    b) large scale semi-permanent and permanent droughts in many regions; large scale flooding in others; and in some regions a pattern of severe droughts followed by severe flooding;

    c) large scale destruction of many important ecosystems;

    d) mass extinctions;

    e) a huge increase in the incidence of some human diseases such as cholera; and so on.

    As the UNEP report quoted in Timothy’s post points out, “irreversible damage to the world’s climate will be likely unless greenhouse gas emissions drop to below 50% of their 1990 levels before 2050.” (Actually this is conservative – a 60-80% cut is more likely to be required). And as far as our “moving in the right direction” (as you put it) is concerned, there is no evidence that the steps currently being taken or planned by governments will even stop worldwide emissions from continuing to grow (at best they may reduce the rate of growth slightly) – let alone achieving the required cut.

    It is true as you imply that Germany seems to be pulling its weight in this regard; but unfortunately, in terms of firm action (as opposed to talk), most other countries are not. And even Germany is bigger on targets than they are on providing any evidence that they know how they will achieve them.

    The UNEP report states that there has so far been “a remarkable lack of urgency” in the response [to climate change], which the report characterised as “woefully inadequate”. And so far all the signs are that the rate of worldwide emissions growth is rapidly increasing, at just the time when it needs to be drastically reducing.

  7. 157
    Nick Barnes says:

    It’s worth adding: one suspected dynamic of WAIS melting is quite different from GIS melting: massive calving. If either Ross or Ronne ice shelf largely breaks up, they will be replaced by open water and sea ice: the WAIS would be directly exposed to the sea. Remember that the base of the WAIS is on bedrock far below sea-level. In this eventuality, scientists believe that pieces of the ice sheet, possibly very large pieces of it, may simply break off and float away.

    This can’t happen to the GIS or the EAIS, which are not below sea-level.

  8. 158
    Lynn Vincentnathan says:

    #136 re us weaning ourselves from fossil fuels once “better, cheaper technology becomes available” …

    The better, cheaper tech is already here, and some has been around for over 2000 years (such as passive solar – which profligate us here in the U.S. haven’t taken up). Windmills have also been around a long time and are now very advanced – some are small & quiet & can be mounted on a roof. Then there’s solar, and electric vehicles, and geothermal tech.

    There are reasons corporations don’t like these as much as oil & coal, which bring in more profits, partly from garnering hefty tax-breaks and subsidiies (& I’m not even counting the military expenses of ensuring our supply and flow — which would make a gallon a gasoline skyrocket in price). For instance, EVs have very low maintenance costs, and car companies make a pretty penny off their maintenance side on I.C.E. cars. What’s good for big corps is not necessarily good for the people living on earth, but they are the ones who have the power and set the agenda. While the people pay for the product and for the harms caused by the product.

    Fossil fuel prices are held artificially low, but we are paying for them in full plus, if not fully at the pump, then on April 15th and in higher health/environment costs, even if we drive EVs and have wind-powered electricity.

  9. 159
    Nigel Williams says:

    Lynn 133. Thanks. A couple of observations. From what I can gather 80m is all ice melted, and that will happen as an S curve where we are currently at 4mm per year cf 2mm per year rise just a wee while ago. This will move up through some big numbers (possibly approaching half a metre a year at worst) as the ice sheets fall to bits, then the annual rate will slacken again as less ice is left to melt. Finally its all gone at 80m.

    But the sad bit after that is that then there is thermal expansion to follow, which will add a few more metres but will occur over a much longer time as the deep ocean warms. So from today until the ice caps are gone and the thermal expansion is done beaches will never stabilise, and so coastal fisheries, shell fish areas and estuaries are gone for quite a while. The coast will just be horrible freshly eroded hill sides and coastal waters full of sediment for thousands of years. These conditions are quite fatal for fish and shell fish spawning, so bang go the world’s major fisheries.

    The second exercise is to move beyond the small scale models of 80m high tide and Google Earth parts of the world like central Russia in detail, looking at elevations. It’s quite shocking to see how far inland the 80m high tide mark is as you run up the river valleys. It looks like much of the area between the Arctic Ocean and the Caspian Sea – Black Sea area will just be a vast archipelago of messy ridges sticking out of a new ocean, even though these areas appear on the large scale maps as being nicely above high tide. The same no doubt applies elsewhere, like India, China, central USA and Australia. The implications for civilisation are most disturbing, and there is no way back.

  10. 160
    Lynn Vincentnathan says:

    #158, Thanks Nigel. Now how long would that take to get to, say, a 60m rise, once we’ve reached 6C warming — IF we do reach that, which could happen soonest by 2100, but more likely in a century later, IF we don’t mitigate drastically. I’m thinking many centuries after than for that much ice to melt and the deep oceans to warm.

    In addition to land loss and harm to sea life, there would also be methane clathrates melting, maybe even hydrogen sulfide outgassing at some point.

    It’s like we all a bunch of naughty children playing with matches on a sea of kerosene, and our parents are nowhere to be found.

  11. 161
    J.S. McIntyre says:

    Open question (spurred by the 5.6 temblor some 15 miles from my home this evening)…

    As the West Antarctica and Greenland Ice Sheets decay, melt or however you care to describe it, this also means there will be an eventuallt lessening of weight upon the earth’s crust. And I realize for this to be a significant effect it may take a loonngg time, but…

    …will the release of weight trigger quakes, and will the effect be local, or more global?

  12. 162
    Nigel Williams says:

    Lynn 160. Have a look at Hansen on this:
    http://pubs.giss.nasa.gov/docs/2005/2005_Hansen.pdf
    He discusses 1 degree, 2 degree and ice/water/air interaction very helpfully, and discusses the critical time constants you are looking at.

    For me; I look at the WAIS and Greenland – the ice they hold entails 15m of sea level rise. With the recent trends and observations of conditions there I can only feel that they are going to loose at least 10% of their mass over the next few decades. 10% = 1.5m = Disaster.

  13. 163
    sidd says:

    Charles Muller Says 30 October 2007 at 3:37 PM
    “The Fig. 6 (T-P and Jason from 1993 to 2006) shows no particular acceleration for the sea leve rise in the most recent years 2002-2006.”

    please see e.g. :
    http://www.laseagrant.org/forum/docs/03-20-07/WaleedAbdalati.pdf

    on page 10 you will see a graph that may illustrate my point that sea level rise has accelerated.

    if you would look at the data
    http://podaac.jpl.nasa.gov/DATA_PRODUCT/OST/index.html#jason
    and perhaps try your own curve fits, you will see that the rate from 1993 to 1996 was 2 mm/yr and you may see from the Lombard (2007) reference that the 2003-2006 rise is 4mm/yr.

    “A doubling / decade of this rate (0,35mm/yr) would give approx. 3,58 m for 2100 (plus thermosteric). Well, isn’t it a bit… pessimistic ?”

    Quite.

    But i will say this. When i see articles like Velicogna and Wahr, Nature, 443, 2006, informing me that, in the Greenland Ice Sheet (GIS)
    “The rate of ice loss increased by 250 per cent between the periods April 2002 to April 2004 and May 2004 to April 2006”
    it does darken my views. if the doubling time for mass loss in GIS is on the order of years, rather than millenia, what credence may i place on estimates for WAIS and EAIS ?

    i submit that the downside is too big, this wager is too rich for my blood.

    and what of the generations to follow ?

    sidd

  14. 164
    Timothy Chase says:

    pete best (#151) wrote:

    With regard to the certainty of uncertainty I read an article in the UK newspaper the Independent the other day stating that sinks were becomming sources decades earlier than expected. Is this possibly true?

    I am not sure that I have heard that story as of yet. However, in a certain sense pretty much all carbon sinks are also carbon sources. The ocean releases carbon dioxide in warm weather, absorbs in the cold. Plants take up carbon dioxide as they grow and release it upon death. But a net carbon sink may become a net carbon emitter – or it may be weakened so that it is no longer acting all that effectively as a carbon sink.

    And the major carbon sinks appear to be weakening….

    With the South Ocean we are seeing a significant decline in its ability to absorb our emissions. However, the mechanism behind this is a little different from what had been expected. Instead of it simply becoming more saturated it, changing atmospheric circulation is resulting in surface winds that cause upwelling of rich organic material from below which then releases both carbon dioxide and methane.

    Please see:

    The CO2 flux variability from the longest inversion correlates with the Southern Annular Mode (SAM), an index of the dominant mode of atmospheric variability in the Southern Ocean. Here we use the SAM definition of Marshall (2003; 19) based on the difference in mean sea level pressure between 40ºS and 65ºS, which is entirely based on observations and fully independent of our inversion. The correlation of the monthly mean anomalies is small (r=+0.22) but significant at the 99% level (16,18). The positive correlation indicates that the ocean outgasses CO2 compared to its mean state when the SAM is positive, i.e. when the winds are intensified South of 45ºS (20), and suggests that wind-driven upwelling and associated ventilation of the sub-surface waters rich in carbon dominates the variability in CO2 flux (18).

    Saturation of the Southern Ocean CO2 Sink Due to Recent Climate Change
    Corinne Le Quéré, et al
    Science, Vol 316 22 June 2007

    Open Access from Global Carbon Project page:
    http://www.globalcarbonproject.org/products/SO_ScienceMay07.htm

    The following mechanism is what had actually been expected – and it is occuring rapidly in the North Sea…

    New observations from the North Sea, a NW European shelf sea, show that between 2001 and 2005 the CO2 partial pressure (pCO2) in surface waters rose by 22 matm, thus faster than atmospheric pCO2, which in the same period rose approximately 11 matm. The surprisingly rapid decline in air-sea partial pressure difference (ΔpCO2) is primarily a response to an elevated water column inventory of dissolved inorganic carbon (DIC), which, in turn, reflects mostly anthropogenic CO2 input rather than natural interannual variability. The resulting decline in the buffering capacity of the inorganic carbonate system (increasing Revelle factor) sets up a theoretically predicted feedback loop whereby the invasion of anthropogenic CO2 reduces the ocean’s ability to uptake additional CO2. Model simulations for the North Atlantic Ocean and thermodynamic principles reveal that this feedback should be stronger, at present, in colder midlatitude and subpolar waters because of the lower present-day buffer capacity and elevated DIC levels driven either by northward advected surface water and/or excess local air-sea CO2 uptake. This buffer capacity feedback mechanism helps to explain at least part of the observed trend of decreasing air-sea ΔpCO2 over time as reported in several other recent North Atlantic studies.

    Rapid decline of the CO2 buffering capacity in the North Sea and implications for the North Atlantic Ocean
    Helmuth Thomas, et al
    Global Biogeochemical Cycles, Vol 21, GB4001, doi:10.1029/2006GB002825, 2007

    In the main text the authors note that similar observations have been made in the North Pacific Ocean (2006) and parts of the North Atlantic Ocean (2007).

    There is also a fair amount of evidence that plants are weakening as a carbon sink. However, if so, this would appear to be a function of stress due to heat and drought.

    Please see:

    In general, we find that the remarkable feature of the 2002-2003 anomaly seems to be that climate fluctuations, not only related to El Nino and occurring across all latitudes, acted together to create an unusually strong outgasing of CO2 of the terrestrial biosphere. Further research will be required to investigate if this fluctuation carries features of projected future climate change and the CO2 growth rate anomaly has been a first indicator of a developing positive feedback between climate warming and the global carbon cycle.

    Impact of terrestrial biosphere carbon exchanges on the anomalous CO2 increase in 2002-2003
    Knorr, et al
    Geophysical Research Letters, Vol. 34 (5 May 2007), L09703.
    http://www.agu.org/pubs/crossref/2007/2006GL029019.shtml
    (subscription or purchase)

    Each instance of a carbon sink weakening is an instance of positive feedback from the carbon cycle. With respect to the South Ocean and plants, I believe we weren’t expecting them to weaken significantly for several decades. In time, as the temperature rises, even the oceans may become net emitters as the warmer upper layers lose their capacity to hold the carbon dioxide which they have already absorbed.

  15. 165
    Ike Solem says:

    Regarding the estimated linear feedbacks used in the paper:

    “Researchers (17, 18) estimated mean and SD of feedback factors calculated from two different suites of climate models. First, Colman (17) found a mean and SD of (0.11, 0.06) for the albedo feedback factor; (0.17, 0.11) for the cloud feedback factor; and (0.42, 0.06) for the water vapor and lapse rate feedbacks combined.”

    “Second, Soden and Held (18) found a mean and SD of (0.09, 0.02) for the albedo feedback factor; (0.22, 0.12) for the cloud feedback factor; and (0.31, 0.04) for the water vapor and lapse rate feedbacks combined. The water vapor and lapse rate feedbacks are typically combined because models show a strong negative correlation between the two. Although the combined feedback for water vapor and lapse rate has the largest magnitude, the greatest contributor to uncertainty is the cloud feedback.”

    17. R. Colman, Clim. Dyn. 20, 865 (2003).
    18. B. J. Soden, I. M. Held, J. Clim. 19, 3354 (2006).

    And the nonlinear feedbacks:

    “The first nonlinearity we consider is that at higher temperatures the T4 dependence of the Stefan-Boltzmann equation means the climate system is able to more effectively compensate for radiation perturbations than at lower temperatures. The second nonlinearity is that the water vapor feedback depends on the moisture content of the air, which via the Clausius- Clapeyron relation is a nonlinear function of temperature. Physically, the Stefan-Boltzmann feedback becomes more negative and the water vapor feedback becomes less positive as the temperature increases.”

    By focusing soley on the equilibrium climate sensitivity, the authors do miss a lot of features important to people about the overall climate system – for example, what’s the equilibrium sensitivity of the carbon cycle to the temperature change brought about by 2X CO2? What are the chances of exhausting carbon sinks? (That would mean that a 50% reduction in CO2 emissions would have no effect on the rate of growth of atmospheric CO2). What are the chances that existing net carbon sinks could turn into net sources of CO2 to the atmosphere (for example, wildfires turn biomass into atmospheric CO2)? At what temperature do such effects kick in?

    Taking a “wait-and-see” approach to such questions seems like a bad idea.

  16. 166
    Timothy Chase says:

    J.S. McIntyre (#161) wrote:

    Open question (spurred by the 5.6 temblor some 15 miles from my home this evening)…

    As the West Antarctica and Greenland Ice Sheets decay, melt or however you care to describe it, this also means there will be an eventuallt lessening of weight upon the earth’s crust. And I realize for this to be a significant effect it may take a loonngg time, but…

    …will the release of weight trigger quakes, and will the effect be local, or more global?

    Yes, we can probably expect more conventional earthquakes (as opposed to glacial “earthquakes”), but it should be on what are essentially “short” geological timescales. And it should be local.

    Please see the inline to Climate Insensitivity, comment 92

    Response: I am happy to be able to correct you that tectonic rebound from the Greenland ice sheet won’t have impacts on earthquakes around the world. Big earthquakes are due to processes much deeper in the earth’s crust, and much more localized. It is, on the other hand, rather likely that rising sea levels will help to destabilize the Antarctic ice sheet. On what timescale, however, remains quite uncertain. –eric

    PS

    We aren’t noticing the effect as of yet, either. Greenland’s conventional earthquakes aren’t showing any trend despite the loss of glacial mass balance. So this is one thing we probably won’t have to worry about all that much any time soon.

    [Hmmm. I have been the bearer of good news — and the world hasn’t come to an end. But there could be some sort of lagtime…. I will give it another twenty-four hours just to make sure.]

  17. 167
    henning says:

    @Dave Rado 156
    I never said it was reason to celebrate. My point is that we actually have started doing something – and evidence suggests that we’ll do more in the future. You can’t shut down global economy from one day to the next. It takes some time to adjust, to invent new technologies and bring them to market. I’ve read the IPCC reports (well most of them) cover to cover and the current debate in here about signs everywhere indicating that AR4 means nothing and everything will be much worse and much sooner and totally irreversible does not convince me. If that was true, we should demand our money back from the IPCC and rather spend it on a couple of additional life-boats.
    The Problem with Roe and Baker is, that it opens the gates for utter exaggeration. Soon Climate Scientists will have to defend their findings not only against the “it doesn’t exist” folks but even more so against the “in reality it is much worse and you just didn’t calculate your feedbacks scary enough” crowd. We should come back to the modeled basis and not start always quoting the top end of the error bar plus a generous Roe/Baker factor – that won’t help speeding things up – rather produce the notion of too-late-anyway.

  18. 168
    Pascal says:

    #163

    sidd

    there is a contradiction between the fact that is pointed in this link:

    http://earthobservatory.nasa.gov/Study/Greenland/greenland5.html

    “The second and potentially greater source of the discrepancy is time. Different studies used data from different years. Scientists now know there was a substantial jump in ice loss from 2002 through 2004 compared to previous years. ”

    and the fact that we don’t see any acceleration of sea level increasing in the last ten years.

    look at this graph (built with Univ of Colorado data)

    http://www.host-img.com/visitors/1193819840.jpg

    where can we see a jump since 2002?

  19. 169
    Eachran says:

    OK, I have read all and very interesting it is too.

    I especially dont like the long tail and its implications.

    Some of you seem to be going down the GIMBI (Group (RC) Ice Mass Balance Index) route.

    So to get you started, I say this : 1m sea level rise before 2050.

    You could use 1m by 2050 as the benchmark and calculate the GIMBI from there : thus by trending (you dont have to use straight line) sea level rise to that date and valuing every additional piece of new information as it happens the trend will be affected and therefore GIMBI. So if a chunk of the Greenland ice sheet unexpectedly falls into the water and increases sea level then GIMBI will tend to be in excess of 1. An alternative way of looking at the index is by observing how much closer or further away is the 1m benchmark.

    A simpler way of doing a GIMBI is to ask a team of experts their best guess, and to take an average.

    Dont be shy, when I was working in industry we (all experts) used to have best guesses at exchange rates and interest rates and the like for a Christmas sweepstake the next year : we were always wrong and sometimes disastrously so.

    But all you experts, we laypeople need to know what it is that you really believe. None of this, if and but and maybe stuff : that is for the peer reviewed journals.

    It is a bit of fun too, amidst all the gloom.

  20. 170
    catman306 says:

    NIck Barnes, are you sure that the Greenland Ice Sheet is resting on rock that is at or above sea level? How much above sea level?

  21. 171
    Nick Barnes says:

    catman306 @170: I stand corrected, thank you. The bedrock under the central GIS is depressed by the weight of the ice, and forms a shallow basin reaching 300m below sea-level (cf 2500m under the WAIS). Without the ice, the basin would probably rebound, but we don’t know how far. The basin is surrounded by higher ground. I was right to state that the WAIS is different from the GIS and EAIS: the WAIS meets the sea in the R&R ice shelves, the GIS only feeds the sea through glaciers.

  22. 172
    Imran Can says:

    #159,#160
    I think you guys have been watching a little too much Waterworld …. why don’t you compare the IPCC 2001 global temperature rise predictions to current global means (year averages or rolling averages – whatever you want). I think you’ll find the 2007 world is way below the ENTIRE ENVELOPE of the scenarios – which doesn’t say much about the IPCC’s ability to understand the uncertainty on the lowside (topical). And I think you’ll sleep better at night.

    Imran
    PS You can find the graph on p34 of the Summary for Policy Makers.

  23. 173
    Timothy Chase says:

    Eachran (#169) wrote:

    Some of you seem to be going down the GIMBI (Group (RC) Ice Mass Balance Index) route.

    So to get you started, I say this : 1m sea level rise before 2050….

    A simpler way of doing a GIMBI is to ask a team of experts their best guess, and to take an average.

    Currently the IPCC is consistently underestimating the level of sea level rise. Even prior to any large feedback involving the ice sheets or carbon cycle, the actual rise in sea-level continues to be at the top edge of the envelope of the IPCC’s predictions.

    Please see:

    Rates of sea-level rise calculated with climate and ice sheet models are generally lower than observed rates. Since 1990, observed sea level has followed the uppermost uncertainty limit of the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR), which was constructed by assuming the highest emission scenario combined with the highest climate sensitivity and adding an ad hoc amount of sea-level rise for “ice sheet uncertainty” (1).

    A Semi-Empirical Approach to Projecting Future Sea-Level Rise
    Stefan Rahmstorf
    Science 19 January 2007:
    Vol. 315. no. 5810, pp. 368 – 370
    DOI: 10.1126/science.1135456
    http://www.sciencemag.org/cgi/content/abstract/1135456

    We can do better.

    Please see:

    We can explore the consequences of this semiempirical relationship for future sea levels (Fig. 4), using the range of 21st century temperature scenarios of the IPCC (1) as input into Eq. 2. These scenarios, which span a range of temperature increase from 1.4° to 5.8°C between 1990 and 2100, lead to a best estimate of sea-level rise of 55 to 125 cm over this period. By including the statistical error of the fit shown in Fig. 2 (one SD), the range is extended from 50 to 140 cm. These numbers are significantly higher than the modelbased estimates of the IPCC for the same set of temperature scenarios, which gave a range from 21 to 70 cm (or from9 to 88 cm, if the ad hoc term for ice sheet uncertainty is included). These semiempirical scenarios smoothly join with the observed trend in 1990 and are in good agreement with it during the period of overlap.

    ibid.

    This approximation (very) closely tracks sea-level rise from 1880 to 2000 by assuming that the rate at which height increases is a strict linear function of the temperature with a straight averaging of the calculated rate for a period from 15 years before to the point in time for which height is being calculated (i.e., the embedding period).

    Now this won’t take into account strong positive feedback from the ice sheets or the carbon cycle. Neither will be linear and both have a great deal of uncertainty attatched. This will lead to an underestimation – in the long term. But it also doesn’t take into account the fact that we will be running out of glaciers – which appear to be responsible for more melt than that due to glaciers. This will lead to overestimation. And to some extent the two uncertainties can be expected to compensate for one another, at least in the short term.

    Incidentally, no need to look at the center of the range — as the uncertainty given is largely a function of our bad behavior.

    PS

    I gave Hank and Stefan a hard time when Hank brought this up before. Not sure that they know, but that was me being tongue-in-cheek. I like it. Big improvement.

  24. 174
    Lynn Vincentnathan says:

    Well, Imran (#172), that’s just what I wanted to avoid, WATERWORLD science. Where did they get all that water? Not from this planet! (I love sci-fi, but not when it extremely violates the laws of physics or common sense.)

    What MIGHT (possibility, not nec high probability) it look like in 1000 years if much of earth’s cryosphere melts & waters expand if we reach 6C warming is the question. Presumably there is a maximum amount of sea rise, unless those pesky Gorkians from Planet Gork insist on dumping more water on earth to gorkiform it for their inhabitation.

  25. 175
    Timothy Chase says:

    Correction to #173

    In the second to last paragraph:

    Now this won’t take into account strong positive feedback from the ice sheets or the carbon cycle. Neither will be linear and both have a great deal of uncertainty attatched. This will lead to an underestimation – in the long term. But it also doesn’t take into account the fact that we will be running out of glaciers – which appear to be responsible for more melt than that due to ice sheets. This will lead to overestimation. And to some extent the two uncertainties can be expected to compensate for one another, at least in the short term.

    The italicized sentence should be “But it also doesn’t take into account the fact that we will be running out of glaciers – which currently appear to be responsible for more melt than that due to ice sheets.”

  26. 176
    dean_1230 says:

    I’m going to work through a thought-experiment… let me know where i screw up:

    First, let’s assume that the vast majority of the WAIS is below sea level and that the percentage above sea level is negligible (anyone know the accuracy of that assumption?). let’s assume that the entire WAIS calves (which, i assume to mean that it breaks away from the continent and floats off into the ocean). That, according to #154, adds 3 million sq km. of ice to the ocean. Almost instantaneously, the sea level will fall (the shelf was displacing its volume, now it’s floating and is displacing it’s weight).

    If that’s correct, then melting the WAIS will lower the sea level, not raise it. the mitigating factor is the aforementioned assumption: how much of the WAIS is currently below sea level?

    Hence we may only need to address the GIS and EAIS as potentially raising the sea level. the EAIS is basically unchanged (some studies say it’s growing, some say its shrinking). The GIS is shrinking. the net result over the last decade or so is basically no change to the sea level.

    which leads me to wonder if all the fears being generated from a 60m sea level rise is simply an emotional argument aimed at trying to scare us…

    and when people resort to emotional arguments on a scientific subject in order to get me to do something, i immediately grab my wallet, because it’s going to cost me.

  27. 177
    catman306 says:

    Nick Barnes, because the GIS is a bowl, with it’s bottom below sea level, melt water and heat flow to the bottom, not into the oceans. That heat will probably destabilize the ice mass faster than predicted. The glaciers would flow fast like a river. There are grooves cut into the sea bottom where ice has done this in the past, probably without much warning.

  28. 178
    Timothy Chase says:

    sidd (#163) wrote:

    if you would look at the data
    http://podaac.jpl.nasa.gov/DATA_PRODUCT/OST/index.html#jason
    and perhaps try your own curve fits, you will see that the rate from 1993 to 1996 was 2 mm/yr and you may see from the Lombard (2007) reference that the 2003-2006 rise is 4mm/yr.

    Pascal (#168) responded:

    #163 sidd
    there is a contradiction between the fact that is pointed in this link:

    http://earthobservatory.nasa.gov/Study/Greenland/greenland5.html

    “The second and potentially greater source of the discrepancy is time. Different studies used data from different years. Scientists now know there was a substantial jump in ice loss from 2002 through 2004 compared to previous years.”

    and the fact that we don’t see any acceleration of sea level increasing in the last ten years….

    Two problems.

    The statement by NASA is regarding Greenland. The chart is a chart of the rate of sea level rise. The majority of sea level rise is still due to thermal expansion. Likewise you are omitting the embedding period from your analysis. (See #173)

    Oh, and sidd was comparing 1993-6 to 2003-6. 2 mm/y vs 4 mm/y. The rate has accelerated.

  29. 179
    J.S. McIntyre says:

    re 157, 170, 171

    NIck Barnes, are you sure that the Greenland Ice Sheet is resting on rock that is at or above sea level?

    =================

    Being one of those members of the peanut gallery wanting specifics, I looked the GIS up just to get an idea of how thick it was. From Wiki:

    “The Greenland Ice Sheet is a vast body of ice covering roughly 80% of the surface of Greenland. It is the second largest ice body in the world, after the Antarctic Ice Sheet. The ice sheet is almost 2,400 kilometres long in a north-south direction, and its greatest width is 1,100 kilometres at a latitude of 77° N, near its northern margin.

    “The mean altitude of the ice is 2,135 metres. [1] The ice sheet covers 1.71 million km², or roughly 80% of the surface of Greenland. The thickness is generally more than 2 km (see picture) and over 3 km at its thickest point. It is not the only ice mass of Greenland – isolated glaciers and small ice caps cover between 76,000 and 100,000 square kilometres around the periphery.”

    That’s quite a bit of ice. Makes me think of Lex Luthor’s obsession with beachfront property….

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

  30. 180
    J.C.H. says:

    There is a graphic on this website that does a good job of showing that GreenLand is changing quickly in more than one way:

    http://earthobservatory.nasa.gov/Library/ICESat/

    Change in Ice Sheet Thickness

  31. 181
    Nick Barnes says:

    catman306 @ 174: an interesting conjecture. As I said, we don’t have a good understanding of the ice sheet dynamics, and what you say is plausible at first sight. A few minor points:

    – the central surface of the GIS is colder than the periphery, and the basement of any ice-sheet is well-insulated from the surface. Would melt water reach the central basement of the GIS still carrying much heat? We’d need to model it.

    – I don’t have a topo map of the Greenland bedrock. Anyone?

    – I don’t have a map of the glaciers around the GIS and the ice streams within it. Anyone? If there is a pronounced basin within the GIS, ice streams will not flow to the sea. Presumably this means the GIS will melt in-place??

    – Possibly interesting paper here. Any AGU subscribers who can peruse it and report back? http://www.agu.org/pubs/crossref/2001/2001JD900087.shtml

    – At least some melt water flows along rivers inside (not underneath) the sheet (ref: moulin studies in Greenland in the last couple of years). This is a bit like cave formation in limestone rocks.

    – the WAIS has some interesting ‘fold’ features which suggest rapid flow in the past (ref: Google WAIS fold).

  32. 182
    SecularAnimist says:

    I must say that this particular thread is particularly depressing. Just to note a few points that are touched on by the most recent comments:

    1. Empirically observed effects of anthropogenic warming, eg. melting and sea level rise, are consistently more rapid and more extreme than predicted by models.

    2. Positive feedbacks that either reduce the ability of the Earth system to absorb excess CO2 (saturation or weakening of carbon sinks), or cause the Earth system to emit more CO2 and methane (eg. thawing permafrost), are being observed much sooner than expected.

    3. There are growing indications that the Earth’s biosphere is already suffering from the effects of warming, eg. oceanic phytoplankton, a very high rate of species extinction, and severe damage to entire bioregional ecosystems (eg. the Amazon) that may ported their collapse.

    4. Anthropogenic emissions of CO2 are increasing, and accelerating, and current proposals for reducing them present no plausible scenario in which emissions will actually peak and decline in anywhere near the time frame that is required to avoid what are generally considered “dangerous” levels of CO2 (although points 1-3 above suggest that the current levels are more dangerous than has been generally believed).

    It is very hard to imagine a realistic happy ending to this story.

  33. 183
    catman306 says:

    Nick Barnes- Thanks for finding my conjecture interesting. Every educated person knows that the world wasn’t always as it was on the day they were born. The ice will melt. People who frequent this site might just find out the reason, the when, and how fast.

  34. 184
    Hank Roberts says:

    Tim Chase — there’s a noun wrong in this bit:

    > we will be running out of glaciers – which appear
    > to be responsible for more melt than that due to glaciers.

    Re this

    > PS I gave Hank and Stefan a hard time … being tongue-in-cheek.

    It’s hard to tell at best what the writer was feeling, there’s no emotional tone in ASCII except the reader’s projection, and as you said in a slightly different context “

  35. 185
    catman306 says:

    SecularAnimist – Without disagreeing with your four points, it’s possible that the next dominant species will see evidence of our passing and ponder the reasons. They’ll see the mistakes homo sapiens made, and vow not to repeat them. THAT will be the happy ending.

  36. 186
    sidd says:

    dean_1230 Says at 31 October 2007, 9:17 AM:
    “First, let’s assume that the vast majority of the WAIS is below sea level and that the percentage above sea level is negligible (anyone know the accuracy of that assumption?)…Almost instantaneously, the sea level will fall (the shelf was displacing its volume, now it’s floating and is displacing it’s weight)”

    this is inaccurate. WAIS would, if melted, contribute as much as greenland to sea level rise.

    Nick Barnes Says, at 31 October 2007,10:17 AM:
    “I don’t have a topo map of the Greenland bedrock. Anyone?”

    here is one i made a while ago from nsidc data:
    http://membrane.com/sidd/greenland.html

    “I don’t have a map of the glaciers around the GIS and the ice streams within it. Anyone? ”

    please see:Rignot et al., Science, 311, 2006.

    For similar map of Antarctica, Rignot et al. Science, 297, 2002.
    For Antarctica without ice
    http://commons.wikimedia.org/wiki/Image:AntarcticaRockSurface.jpg
    ICESAT rendition of Antarctic ice surface:
    http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16758

    sidd

    sidd

  37. 187
    Pascal says:

    re #178

    Timothy

    I quote:

    “Oh, and sidd was comparing 1993-6 to 2003-6. 2 mm/y vs 4 mm/y. The rate has accelerated.”

    the sea level rise was 2.2 mm/y in 1993-1996 and 3.3 mm/y in 2003-2006.

    So we must not exagerate.

    And If I “choose” the 4 years precedent period (1999-2002) I find 4.6mm/y.
    So what is your opinion between 1999-2002 and 2003-2006?

    My position is always that there is no acceleration in the last ten years.
    It’s a fact.

    But rather to quibble about this evidence, it’s more interesting, I think, to ask oneself: why?

    My assumption is that the ocean cooling, found by Lyman(2006) for 2003-2005, is real, even if the amplitude of this cooling is too high.
    You can observe, after the rectification of Lyman 2006, that there is, at best, a stabilization of 0-750m temperature anomaly.
    I don’t repeat, for the Xth time, that SST are decreasing since 2003, but it’s also a fact.

    This little cooling or stabilization compensates the loss of ice of our prefered ice sheets.

    Another question is: why such an ocean cooling?

  38. 188
    Andrew Sipocz says:

    Well, somehow these threads always turn to sea level rise predictions. How quickly will Greenland’s land surface rebound as it loses its ice? How quickly does the ocean basin respond to increased water weights (ocean basin deepening)? Do these crust/mantle processes occur on similar time frames or does rebound outstrip basin deepening? Can we consider the ocean basin volume as static for the purpose of sea level change prediction because the speed of water inputs will far outstrip the speed of mantle movements? And I’m totally confused on this point: is there a loss of ocean basin volume as the below sea level bed of Greenland rises up and displaces it? Maybe just answer this: Are the geologic models that predict the wishy washy flows of the earth’s mantle accurate or are there additional degrees of uncertainty in sea level rise estimates due to unknown continental and oceanic crust/mantle reaction times and magnitudes? You do have to be a rocket scientist to get this all down. BTW: Timothy translated Dr. Hansen’s possible “several” meters of potential sea level rise as 5 meters. I choose to believe several translates as 4 meters as my home and another 40,000 plus homes in Galveston County will only be under during spring tides at 4 meters whereas 5 puts the home in the big pool all the time.

  39. 189
    B Buckner says:

    RE 188: I live on the coast of Maine, where during the last glaciation there was a two mile thick layer of ice. The ice melted faster than the geostatic rebound and the area was subsequently submerged under about 200 feet of ocean. The land eventually rose and we are now about 50 feet above sea level.

  40. 190
    sidd says:

    Pascal Says at 31 October 2007, 1:00 PM:

    “the sea level rise was 2.2 mm/y in 1993-1996 and 3.3 mm/y in 2003-2006”
    1)what are the error bars on this result ?
    2)may i ask where these numbers from? the graph you posted earlier has no attribution.

    i use data from
    http://podaac.jpl.nasa.gov/DATA_PRODUCT/OST/index.html#jason. i obtain 2 mm/yr +/-0.2 mm/yr for 1993-6 and 4 +/- 0.2 mm for the 2003-6 period. the latter estimate agrees with Abdalati, Pg 10 http://www.laseagrant.org/forum/docs/03-20-07/WaleedAbdalati.pdf
    as well as with Lombard et al.Lombard et al,Earth and Planetary Science Letters, 254, 2007. With regard to the period 2003-2006 i quote directly from page 200 of the Lombard eta al. “…the Jason-1 sea level curve rises by 4.0+/-0.4 mm/yr. ” For the period 2002-2006 they go on to give a value of 3.1 +/- 0.2 mm/yr. which is closer to your estimate for 2003-2006. So it seems that your source is not quite in agreement with mine, and so i would be most interested to see the data or publication from which your graph derives.

    sidd

  41. 191
    henning says:

    Is climate science becoming a gloom contest now? My understanding is, that Greenland and Antarctica iceshields are actually warmest at their base where they’re being warmed by the earth. Sliding ice is a geothermal effect – never an atmospherical and has nothing to do with global warming. Most of the shield itself is way below freezing point and won’t melt in a couple of millenia. Melting occurs on the edges and any percipitation in the center regions will accumulate and cause the shield to grow. I think its false to assume, that meltwater from the edges or the surface can penetrate deep into (and under) the very cold ice body and cause it to just swim away.

  42. 192
    Eachran says:

    OK, so it looks that we have a number of candidates for estimating GIMBI.

    It really is not a bad measure to keep the mind concentrated and for the layman it is a number which requires no thought.

    So TimothyChase wants to re-base 2050 to 1,5m : no problem with that, but for GIMBI we need to have a nice, good looking number, like 1000.

    So 1000 equals 1,5m. OK

    What about the Group, or Mr Hansen for that matter?

    The Group could put the GIMBI on the home page so all can see it with their coffee and croissants in the morning.

    Incidentally I am not at all depressed about the future : worried but not depressed.

  43. 193
    Aaron Lewis says:

    RE 160
    We do not need 6C rise in atmospheric temperatures to trigger ice sheet collapse. All we need is enough heat in the oceans, and a mechanism to transfer heat from the ocean to the ice. Rain works.

    Last summer, stationary “cyclonic” storms setup on the edge of the Arctic Sea ice (July) and the east coast of Greenland (June), apparently driven by the temperature differential between the open seawater and the ice. (They are clearly visible on the satellite photos, but I have not seen anything about them by weathermen and climate experts.) Thus, there is enough heat in seawater adjacent to NH ice to drive heat transfer mechanisms. Less ice on the Arctic Ocean is likely to intensify these mechanisms.

    The first question is: How long would it take to transfer enough energy to the Greenland Ice Sheet – not to melt it, but to weaken it so that sections of it collapse under their own weight, and potential energy is converted to kinetic energy? That is a very different question from the IPCC assumptions of Greenland Ice sheet melting in place. (Last summer’s ice surge observations may not “prove” anything to the scientist, but they suggest things to the prudent engineer.)

    In Hansen, 2005, he talks about the use of the word “explosive” in relationship to ice sheet collapse. I would disagree. “Explosive” implies material moving at supersonic speeds. Ice collapse is a sub-sonic process. However, since ice collapse is driven by water with a potentially large head (pressure), material can reach terminal velocity very rapidly.

    It is worth looking at the Lake Missoula floods in Washington State for what can occur. While the Missoula floods are normally presented as a failure of an ice dam on the Clark River, the nature of ice dams on rivers results in Lake Missoula having a significant sub-glacial character. (See the USACOE pages on river ice.) And, similar channels in Antarctica show that discharges from sub-glacial lakes can have similar erosion features. “Potholes” is worth walking, and easier to get to than Antarctica.

    The Lake Missoula floods moved large volumes of ice, hundreds of miles across a downgrade of only 2 or 3 %. And, they occurred not once, but time after time. Was it explosive? No! It only went 40 miles per hour.

    Certainly the basement rock in Greenland is relatively flat. However,the thickness of the ice sheet itself provides enough height to provide potential energy for at least a partial collapse process. I am aware of modeling of the profile of the Greenland Ice Sheet that suggests that the ice will melt to a shape that is stable. However, these models seem to ignore the formation of moulins and more importantly, rain. They seem to assume that there will be ice on the Arctic Ocean. That is no longer a valid assumption for a risk assessment.

    The second questions is: When will ocean water with a temperature of above 0C contact the base of the WAIS? Again the ice sheet does not need to melt in place for sea level rise to occur. All we need is enough heat to soften the foundation of the ice, so that the ice collapses into the sea under its own weight. Much less heat is required to soften the foundation, than to melt the body of the ice. Therefore, less time is required to transfer the heat.

    The IPCC Summary for Policy Makers provides unwarranted comfort.

  44. 194
    Hank Roberts says:

    Henning, what’s your source for your information? Why do you believe it’s true? You’re using almost exactly the words just published by the inimitable Benny Peiser in his newsletter. Have you read his source?

  45. 195
    Timothy Chase says:

    Hank Roberts (#184) wrote:

    Tim Chase — there’s a noun wrong in this bit:
    (#173)
    “we will be running out of glaciers – which appear to be responsible for more melt than that due to glaciers.”

    Fixed it in 175:

    The italicized sentence should be “But it also doesn’t take into account the fact that we will be running out of glaciers – which currently appear to be responsible for more melt than that due to ice sheets.”

    Hank Roberts (#184) wrote:

    Re this

    “PS I gave Hank and Stefan a hard time … being tongue-in-cheek.”

    It’s hard to tell at best what the writer was feeling, there’s no emotional tone in ASCII except the reader’s projection, and as you said in a slightly different context “

    Humour. (I’m told mine is dry.) Affection. (But I prefer not to be obvious.)

    Despite the fact that this is all text, I like to think that I can read people and get to know their characters — and know when they are special human beings. And that matters to me.

  46. 196
    Pascal says:

    sidd

    re#190

    my source is Univ of Colorado

    http://sealevel.colorado.edu/results.php

    and for the data this file:

    http://sealevel.colorado.edu/current/sl_noib_ns_global.txt

  47. 197
    Hank Roberts says:

    Here is:

    ” a computer model depicting changes in the Antarctic ice sheet since the peak of the last ice age nearly 20,000 years ago. The West Antarctic ice sheet has lost nearly two-thirds of its mass during this period, a volume sufficient to raise sea level 33 feet (10 m).”
    http://www.mos.org/soti/icecore/
    http://www.mos.org/soti/icecore/images/AntChange2.mov

  48. 198
    Hank Roberts says:

    People will tell you the Antarctic ice can melt only on the edge.
    This popped up just recently in Benny Peiser’s newsletter and is making the rounds.

    You can look this stuff up if you’re practicing to become skeptical about what people tell you.

    For example, from tossing a few likely search terms at Google and perusing the first few dozen hits:
    http://www.jpl.nasa.gov/releases/2001/borehole.html

    —–excerpt——-
    March 16, 2001
    ICE PROBE REVEALS FIRST-EVER IMAGES DEEP WITHIN ANTARCTIC STREAMS

    Scientists have had their first inside look at ice layers, frozen debris and a surprising channel of water deep beneath an Antarctic ice stream, thanks to an ice probe designed by NASA’s Jet Propulsion Laboratory, Pasadena, Calif.

    Plunged more than 1,200 meters (more than 3,900 feet) down four boreholes drilled in the West Antarctic ice sheet …..
    ….
    Equipped with two cameras and lights, JPL’s ice probe revealed what appears to be a basal water system, or series of water channels at the base of the ice stream. In places, this water-filled cavity measured approximately 1.4 meters deep (4.6 feet). Based on previous calculations, researchers expected the depth of a water basal cavity to be only in the millimeter range…..
    ——end excerpt——-

    There’s more to be found if you check what people tell you.

  49. 199
    Hank Roberts says:

    See also:
    http://skua.gps.caltech.edu/hermann/upc/pressrelease.html

    “UNEXPECTED BASAL CONDITIONS UNDER ANTARCTIC ICE STREAM C DISCOVERED WITH A NEW BOREHOLE VIDEO PROBE

    Barclay Kamb and Hermann Engelhardt (Caltech),
    Frank Carsey, Lonne Lane, and Alberto Behar (JPL)

    In a recent study of Anarcticas Ice Stream C, a joint NSF-NASA research team has discovered a surprising gap between the base of the ice stream and the rock below. The gap is up to 1.4 m (60 inches) wide, and is filled with water at a pressure nearly high enough to lift the ice stream off its bed. The discovery is significant in relation to the mechanism for rapid movement of the ice streams, which are huge, fast-flowing ice currents within the slow-moving ice sheet that covers most of Antarctica. The mechanism for their rapid movement is being intensively studied because of the possibility that rapid ice-stream flow may cause the ice sheet to disintegrate, resulting in a disastrous rise in world-wide sea level. …”

  50. 200
    David B. Benson says:

    Aaron Lewis (139) — Well stated. A minor point is that the maximum flow rate of the Bretz (Missoula, Spokane) floods was almost 130 km/h (80 mph), from the Wikipedia article.

    With regard to sea stand rise with 3C global warming, I opine that 15 meters, plus whatever contribution from the melting of the Patagonia ice caps, Tibetan and other glaciers, is it. I doubt this would be significantly higher even at 6C global warming as the EAIS is under extremely cold air so that the warming would have little effect. But I’m only an amateur at this.