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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. 101
    David B. Benson says:

    Anders (85) and Lawerence Brown (99) — To me, relative X means the ratios of two Xs. In this case it seems to be the ratio of two probabilites, so any extended real number.

  2. 102
    David B. Benson says:

    (101) Oops! Any non-negative exended real number.

  3. 103
    Timothy Chase says:

    Charles Muller (#93) wrote:

    #83 Timothy

    Thank your your precisions and references. If I summarize, the main issue seems the long term feedbacks due to carbon cycle evolution under warming conditions. According to Friedlingstein 2006, the 11 models of CMPIP-M-4 presently conclude to a positive feedback, with (A2 SRES) +20-200 ppm CO2 and +0,1-1,5°C for 2100. These additional feedbacks are not still accounted by GCM models, at least those used in IPCC 2007 for equilibrium climate sensitivity.

    Agreed. And obviously there is a great deal of a difference between 20-200 ppm and what would be required to result in another doubling. However, no one that I am aware of is projecting that all “slow” feedbacks will have reached equilibrium by 2100. Presumably the slow feedbacks could take a millenia or more. Then again they may be a little faster than we expect – and so far the evidence to date would suggest that they are.

    Charles Muller (#93) wrote:

    What I still miss is, for climate sensitivity at 2xCO2 (540 ppm) we’re discussing here, how you “jump” from a best estimate of 3°C to 6°C. The A2 SRES used by Friedlingstein 2006 go far over the doubling (856 ppm for CO2, but also 3731 ppb for CH4, and, with all other GHGs + negative forcing integrated, a 8,07 W/m2 forcing very different of the 3,7 W/m2 for the sole 2xCO2 used to estimate CS). In spite of this, the worse estimate from CMPIP-M-4 is “just” +1,5°C (I ignore the mean value of the distribution).

    If I were basing the claim that 3 C fast feedbacks translates into 6 C once you include slow feedbacks simply on the basis of the carbon cycle and as I pointed out above claiming that the entire system would reach equilibrium, that might be problematic. But I am not claiming that the equilibrium of slow feedbacks will be achieved by 2100, nor am I claiming that only feedbacks from the carbon cycle will be involved.

    There are other feedbacks.

    Offhand, some of the more significant slow feedbacks will come from the ice sheets. This includes both Greenland and the West Antarctic Peninsula. It may even include parts of the EAIS. And of course it includes the glaciers, such as those associated with the Tibetean Plateau. All of these have albedo effects and all are “slow” feedbacks which are considerably more sensitive than we expected – given a variety of feedbacks which we hadn’t even been aware of before.

    Additionally, I do not know precisely what aspects of the carbon cycle have been included. Which models are considering feedback from the biosphere? Ocean? Probably most. How about the permafrost? Have they included the yedoma layer which we didn’t even know about until recently? This is a little more uncertain but may be quite significant.

    Have they included shallow water methane hydrates? I presume some have. Did they include the fact that we are discovering them at much shallower depths than we previously expected them? Perhaps. What about the cracks in the ocean floor which expose deeper deposits to changes in ocean temperature? We hadn’t expected those, either.

    And as you have stated, the coupled modeling which incorporates aspects of the carbon cycle is still in its infancy.

    In any case, I am no expert in this area. Jim Hansen is. But I wouldn’t want you to simply accept his authority in this area, either. There is the paleoclimate record that exists with respect to Antarctica. And presumably it shows that the “slow” feedbacks which are not included in the Charney climate sensitivity roughly doubles the effect of the “fast” feedbacks which are.

    However, both the “slow” and “fast” feedbacks will actually be working at the same time, and the “slow” feedbacks which we are seeing are moving more quickly than has been expected. Likewise, the paleoclimate record that Hansen refers to suggests that the “slow” feedbacks can move surprisingly fast.

    I trust the climate models, but I trust the paleoclimate record more.

  4. 104

    Re 101 Thanks David. What I’m curious about is what the numerator and denominator represent. I’ve tried to access the “Summary For Policy Makers” section of the IPCC report without sucess. My version of Acrobat reader(5.0) isn’t bringing the pdf version up . The relative aspect may have something to do with the fact that different versions of different models were used in constructing the graph.

  5. 105
    J.C.H. says:

    96. What’s that supposed to mean? Just because somebody quotes a statement like that to make a politically motivated point, anything supporting the iris theory (whatever one thinks about it) must be bad science?

    Comment by henning — 29 October 2007 @ 12:04 PM

    henning, I intended no colorization of the science in any way. I’m unqualified to do that. The quote is from the scientific paper.

    Does the following reflect nominal support?

    “Lindzen hypothesized cirrus clouds and associated moisture work in opposition to surface temperature changes. The data seemed to indicate that when the Earth’s surface warms, clouds open up to allow heat to escape. A cooling surface, in turn, causes clouds to close and trap heat.

    This elegant, self-regulatory, atmospheric mechanism was soon attacked for being based on limited data and the inability of other researchers to identify the effect in other cloud and temperature data sets.

    New Data Support Theory

    But the new research from the University of Alabama-Huntsville supports the validity of the iris effect. …”

  6. 106
    Timothy Chase says:

    Charles Muller (#94) wrote:

    [Re:] #92 Spencer el al 2007 paper doesn’t really support the precise mechanism proposed by Lindzen for Iris effect, …

    Agreed.

    Actually more the opposite (if their analysis were correct) since the clouds under consideration have a higher greenhouse effect associated with them, not a higher albedo effect. Likewise, such cloud coverage is presumably reduced, not increased, entirely counter to what Lindzen expected.

    Charles Muller (#94) wrote:

    … but more simply observes a strong TOA negative correction associated with warming events at 20°S-20°N (that is : in the 2000-2005 period of observation, the most significative warming episodes of the surface + low troposphere – 40 days or more – leads to a negative SW+LW cloud forcing at the top of the atmosphere).

    Additionally this is precisely the same area where we see a clear sky super greenhouse effect, so even if cloud coverage were reduced as a negative feedback to the greenhouse effect, the clear sky greenhouse effect itself will still be considerably stronger over these latitudes, not weaker.

    I quote:

    At sea surface temperatures (SSTs) larger than 300 Kelvin, the clear sky water vapor greenhouse effect was found to increase with SST at a rate of 13 to 15 watts per square meter per Kelvin. Satellite measurements of infrared radiances and SSTs indicate that almost 52 percent of the tropical oceans between 20 N and 20 S are affected during all seasons….

    Satellite studies (8–10) have found that for clear skies and SSTs above 298 K, the spatial variation of Ga with SST, dGa/d(SST), exceeds the rate of increase of sea surface emission, ds(SST)4/d(SST) = 4σ(SST)3. For a tropical SST of 300 K, 4σ(SST)3 ~ 6.1 W m-2 K-1. This effect, termed the “super greenhouse effect” (11), occurs in both hemispheres during all seasons. It is also observed for interannual variations of Ga with SST during the El Nino in the tropical Pacific (12). Observations in the tropical Atlantic ocean (11) show that the clear sky downwelling infrared flux incident on the surface (Fa-) also increases faster than the surface emission with increasing SST. The net result is further warming of the surface, which in turn induces additional heating of the atmosphere column above.

    Direct radiometric observations of the water vapor greenhouse effect over the equatorial Pacific Ocean
    F.P.J. Valero, W.D. Collins, P. Pilewskie, A. Bucholtz, and P.J. Flatau
    Science, 274(5307), 1773-1776, 21 March 1997

    Gavin (inline to 94) wrote:

    Spencer et al has nothing to do with the iris effect despite their claims. Their correlations are based on a dynamic mode of variability (the Madden-Julian Oscillation) which has nothing to do with any SST forced response in the clouds. It’s just a bad analogy (rather like using the day-night contrast to estimate climate sensitivity).

    So let me get this straight.

    Spencer et al (2007) is cited as evidence for the iris effect of Lindzen in order to conclude that the negative feedbacks to the greenhouse effect due to clouds will be substantial.

    However,

    1. The mechanism which they claim to have identified is actually the opposite of what Lindzen described, where he claimed that clouds would increase as the result of the greenhouse effect and their albedo effect would hold down temperatures, but in the tropics the clouds that Spencer et al were dealing with presumably become fewer in number.
    2. The clouds actually have a stronger positive feedback component to them, resulting in an increased greenhouse effect.
    3. The effect according to Spencer et al isn’t necessarily all that significant as they describe it as being “nominal.”
    4. Even if clouds were decreasing there would be the clear sky super greenhouse effect where the rate at which downwelling thermal radiation grows relative to increasing temperatures is actually higher in the tropics than the rate at which surface thermal radiation emissions increase.
    5. Their analysis is flawed in the sense that it is actually based upon a poor analogy in which they are not measuring the actual trend of clouds, but periodic behavior associated with an with a climate oscillation (the Madden-Julian) and has nothing to do with the response of clouds to sea surface temperatures (SST).

    I can see why it so often referenced in the skeptic community…

    Quite illuminating – although I suppose it shouldn’t be all that surprising.

    Thank you both.

  7. 107
    Pascal says:

    re#103

    timothy

    ” and the “slow” feedbacks which we are seeing are moving more quickly than has been expected.”

    can you explain this sentence or have you a good proof of that?

    I’m not sure that, for example, both Antarctica peninsula and Greenland ice sheets are rushing into oceans.

    but maybe can you see an anormal increasing of sea level?

    What else about the “slow feedbacks”?

    My posts are moderated (why?, maybe God knows it) so I hope this one can be read.

  8. 108
    Hank Roberts says:

    Pascal –

    Floating ice changes albedo when it melts, not sea level.
    Dr. Bitz’s thread here on Arctic sea ice melt described models of a fairly pessimistic expectation, last spring. Reality was worse this year.

  9. 109

    I spoke too soon. I have been able to bring up the Summary For Policymakers of the IPCC Report, containing SPM 6. with projected surface temperature changes. I haven’t figured out what the probabilities are relative to, though there’s plenty of interesting material included.

  10. 110
    Timothy Chase says:

    Pascal (#107) wrote:

    timothy [wrote]

    ” and the “slow” feedbacks which we are seeing are moving more quickly than has been expected.”

    can you explain this sentence or have you a good proof of that?

    I’m not sure that, for example, both Antarctica peninsula and Greenland ice sheets are rushing into oceans.

    Charles Muller (CM), Phil Scadden (PS) and I (TC) were discussing the distinction between “fast feebacks” and “slow feedbacks.” Apparently when people refer to climate sensitivity as being ~3 C, this is simply according to the Charney analysis from the 1970s which includes only what are called “slow feedbacks” such as the amplification of the greenhouse effect by water vapor and the albedo effect due to sea ice.

    It excludes feedbacks which “change the boundary conditions” since according to Charney analysis such changes in the boundary conditions would themselves be regarded as “forcings.” The latter set of feedbacks, which include those due to ice sheet instability and the carbon cycle, are normally refered to as “slow feedbacks.” However, given what we are seeing in terms of current climate trends and the paleoclimate record, such a name would seem to be more a matter of wishful thinking than an apt description of the processes involved.

    When Charney analysis speaks of the climate system achieving equilibrium, this only takes into account the “fast feedbacks.” The actual equilibrium, which may occur much later, must include both the “slow” and the “fast” feedbacks. As such, when Charney analysis gives us a climate sensitivity of 3 C this is refering to the unrealistic theoretical construct of the fast feedback equilibrium.

    According to Hansen, the actual long-term climate sensitivity is presumably closer to 6 C. This latter figure comes from the analysis of the paleoclimate record for Antarctica. And it would appear that the projections by the IPCC have been almost entirely limited to those which take into account the “fast” feedbacks, not the “slow” feedbacks.

    Anyway, I believe the full thread for this discussion has been:

    CM #58, TC #70, CM #72, PS #73, TC #74, TC #75, CM #79, TC #83, CM #93, TC #103

    Pascal (#107) wrote:

    I’m not sure that, for example, both Antarctica peninsula and Greenland ice sheets are rushing into oceans.

    Just wait for the positive feedback between the two as the rise in sea level from both starts raising the coastal glaciers off their foundations.

    Pascal (#107) wrote:

    but maybe can you see an anormal increasing of sea level?

    There is more uncertainty there than you might think. Apparently gyres may have been reducing the rise in sea level along the coasts as the climate warms even though the overall sea level was rising — and much of the sea level rise that we attributed to earlier in the twentieth century may actually have happened later in the twentieth century. We may not know for a while though.

    Pascal (#107) wrote:

    My posts are moderated (why?, maybe God knows it) so I hope this one can be read.

    Actually I suspect the blog is having hiccups. Not that big a problem usually, but just this week I had three rather longish posts not go up and I honestly don’t think the moderators had anything to do with it. But in retrospect not that great a loss.

  11. 111
    David B. Benson says:

    Lawerence Brown (104) — Your final sentence appears to me to explain the notion of relative probablity about as well as what is in the SPM.

  12. 112
    Timothy Chase says:

    Pascal (#107) wrote:

    I’m not sure that, for example, both Antarctica peninsula and Greenland ice sheets are rushing into oceans.

    Sorry, I misread this sentence.

    Well, as one example, within the past decade we have seen the Greenland’s melt (as measured in terms of volume) double in the decade 1996-2006.

    Please see:

    Changes in the Velocity Structure of the Greenland Ice Sheet
    Eric Rignot and Pannir Kanagaratnam
    Science 17 February 2006: Vol. 311. no. 5763, pp. 986 – 990
    DOI: 10.1126/science.1121381
    http://www.sciencemag.org/cgi/content/abstract/311/5763/986

    Likewise, icequakes of Greenland tripled over a decade’s time, we have seen ice sheets along Antarctica disintegrate, after the disintegration of the ice sheets we have seen glaciers increase their speed of descent towards the ocean by as much as a factor of 10. Granted, it is “slow” right now, but the melting has been increasing quite substantially, and whereas the IPCC had been speaking in the neighborhood of a sea level increase of 50 cm, figures between one to two meters are becoming common as the result of the observed higher rates since, and with the nonlinear processes and resulting positive feedback, Jim Hansen has suggested that a sea level doubling per decade and increase of several meters (up to 5 m) by the end of the century is more realistic.

    Additionally, we appear to be seeing positive feedback in the carbon cycle from both the South Ocean and the vegetation (for the latter, during the warmer, drier years), positive feedback which we did not expect to see for several decades.

    If you would like more references, I can certainly look these up later this evening, but all of this has been covered before and is well-known. And frankly after the the discussion which you were responding to, I am a little referenced out at the moment.

  13. 113
    Timothy Chase says:

    Pascal (#107) wrote:

    I’m not sure that, for example, both Antarctica peninsula and Greenland ice sheets are rushing into oceans.

    Sorry, I misread this sentence.

    Well, as one example, within the past decade we have seen the Greenland’s melt (as measured in terms of volume) double in the decade 1996-2006.

    Please see:

    Changes in the Velocity Structure of the Greenland Ice Sheet
    Eric Rignot and Pannir Kanagaratnam
    Science 17 February 2006: Vol. 311. no. 5763, pp. 986 – 990
    DOI: 10.1126/science.1121381
    http://www.sciencemag.org/cgi/content/abstract/311/5763/986

    Likewise, icequakes of Greenland tripled over a decade’s time, we have seen ice sheets along Antarctica disintegrate, after the disintegration of the ice sheets we have seen glaciers increase their speed of descent towards the ocean by as much as a factor of 10. Granted, it is “slow” right now, but the melting has been increasing quite substantially, and whereas the IPCC had been speaking in the neighborhood of a sea level increase of 50 cm, figures between one to two meters are becoming common as the result of observed changes, and with the nonlinear processes and resulting positive feedback, Jim Hansen has suggested that a sea level doubling per decade and increase of several meters (up to 5 m) by the end of the century is more realistic.

    Additionally, we appear to be seeing positive feedback in the carbon cycle from both the South Ocean and the vegetation (for the latter, during the warmer, drier years), positive feedback which we did not expect to see for several decades.

    If you would like more references, I can certainly look these up later this evening, but all of this has been covered before and is well-known. And frankly after the the discussion which you were responding to, I am a little referenced out at the moment.

  14. 114
    Imran Can says:

    Great to see a paper addressing the uncertainty range. I always wondered about the IPCC 2001 report which showed the predicted global temperatures for 2000-2100 (p34 of the summary for policy makers). I am wondering why the current(2007) global temperatures (rolling average) are below the entire envelope of scenarios given in that graph. Was the uncertainty range underestimated on the low side ? I just have a concern that we reallly ought be be putting forward ranges that are wide enough so that 5-10 years forward from a projection date we are actually inside the range. Otherwise it doesn’t give a lot of credibility to the longer term more alarming view.

  15. 115
    Charles Muller says:

    #94 Gavin comment

    “Their correlations are based on a dynamic mode of variability (the Madden-Julian Oscillation) which has nothing to do with any SST forced response in the clouds.”

    Well, you may certainly be right that Madden-Julian Oscillations is the key point, but I’d just like to undertand why. As you do not critic Spencer atl al. 2007 measurements, I suppose there’re right on this point.

    Spencer et al. 2007 choose the peak tropospheric T anomalies in 20°S-20°N (so called “ISOs”). Fig 2A shows that there are (small) positive SST anomalies during these ISOs and (clear) water vapour positive anomalies, at least at the peak of the warming phase of ISOs.

    So, in what way these factors (higher T tropo, higher SST, higher evaporation) differ substantially from a GHGs forced warming (except nominally that they are called “Madden-Julian Oscillations”) ?

    By advance thanks.

  16. 116
    David Warkentin says:

    Anders (85), Lawrence (99, 104), David (101, 108):
    According to the original Figure 10.28 in chapter 10 of AR4 WG1 (p.808), the units are 1/(deg C) – this is characteristic of such plots, which are probability density functions. The total area under each curve is 1; for a given model, the probability of the mean surface temperature change falling between any two temperatures is the area bounded on left and right by those limits, below by the horizontal axis, and above by the curve corresponding to that model. For example – for the curve that looks like a bell curve with a peak of about 0.8 1/C around 3.3 C, you would say that the model gives the probability of the 2090-2099 temperature change falling in the range 3.25-3.35 C as about 0.8 x (3.35-3.25) = 0.08, or 8%.

  17. 117
    Patrick 027 says:

    Re 106 – Actually, the ‘iris effect’ mechanism is supposed to involve a shrinking of high cloud area (if it is true), see also http://earthobservatory.nasa.gov/Newsroom/NasaNews/2002/200201167312.html .

  18. 118
    Ellis says:

    Gavin, in your response to comment #41 you stated,”…especially since the winds were even more favorable for ice export in the early 90’s. It wasn’t as warm back then…. – gavin.”
    Now having read the papers you suggested and following them through other papers and links I am uncertain of your meaning in regards to the 90′s as compared to now. It is pretty obvious that the early 90′s positive AO dwarfs the 2006 positve AO and 2004/2005 negative AO, however, the fact that the winds were going in the opposite direction then, seems to indicate to me that they were not favorable to ice export.

    http://www.arctic.noaa.gov/reportcard/ocean.html

    “The sea level time series correlates relatively well with the AO index and with the inverse of the sea level atmospheric pressure (SLP) at the North Pole. Consistent with these influences, sea level dropped significantly after 1990 and reached a minimum in 1996-1997 when the circulation regime changed from cyclonic to anticyclonic.”

    Just to make my position clear, its pretty obvious the arctic is warming, but all that you see here at RC from commentors is that the ice melting is proof of global warming and the disasters that will ensue, when in fact warmth was but a small factor in this years arctic ice minimum.

    As to the specific papers you cited, Miller et al 2006 states,”Recent changes in the magnitude of the annular patterns have been interpreted as the signature of anthropogenic forcing by changes in the concentration of greenhouse gases (GHGs) or else stratospheric ozone [Shindell et al., 1999; Fyfe et al.,1999; Kushner et al., 2001; Kindem and Christiansen,2001; Sexton, 2001; Gillett and Thompson, 2003; Shindell and Schmidt, 2004; Arblaster and Meehl, 2006].” This seems at odds with Shindell et al 1999, which states,”Polar ozone depletion is a candidate for driving observed stratospheric trends2,20,21, which are correlated with the AO index. However, the simulation SG, where ozone changes are absent, exhibits the same increase in the AO index as SO, where polar ozone chemistry is calculated through an interactive photochemical parameterization22. To identify the effect of ozone forcing, EOFs of wintertime SLP (for simulation SO) were recalculated for two separate periods: December and January versus March and April. Although ozone forcing is largest during the latter period, the multi-decadal trend in the AO index, evident in Fig. 2b, is present only in the December to January variability. This result, along with the nearly identical AO trends in models SO and SG, suggests that ozone forcing is not necessary to increase the AO index or to strengthen the stratospheric polar vortex.”

    So perhaps you could clarify, is it the ozone or is it the ghg’s? And how do you represent stratospheric cooling in the models without having ozone changes?

    Thanks again.

    [Response: I don't follow you at all. Positive AO phase is favorable to ice export from the Arctic, presumably we can agree on that. Last winter's +ve phase was therefore a contributory factor to the loss of multi-year ice this year. However, more and more sustained +ve phases occured previously (in the early 90s) and yet the summer minimum did not get anywhere close to this years values. Therefore, while dynamics probably added to the situation, it is not responsible for the bulk of the trend - which is almost certainly thermal in origin. With respect to potential forcings, strat ozone depletion is a strong candidate for the change in the Southern hemisphere (Thompson and Solomon, 2002) where strat ozone depletion has been strongest and the change in the AAO more significant. In the Arctic (which is in a different regime than in the south), GHGs appear to have an effect, but whether it has yet been demonstrated in the real world is ambiguous. - gavin]

  19. 119
    Lynn Vincentnathan says:

    RE #107 & “slow” feedbacks moving more quickly…

    It’s the “geological” mindset or perspective. Sort of like a 1000 years in our time is like a blink of God’s or a geologist’s eye.

    Quickly in geological time could mean years, decades, or even centuries. As for slow, well…

    We will have time to get out of the way of sea rise as it happens, even if it happens much more quickly than expected, and even just by walking away from it. It won’t happen as fast as a sea surge or hurricane.

    However, the tipping points are approaching and once they are passed it may take a long time (in our layperson’s scheme of things) for bad things to happen, but we will have very likely ensured that they will happen — if not to us, then to our progeny.

    For instantce, hydrogen sulfide outgassing is a possibility and that would rapidly kill off a whole lot of life.

    My understanding is if we get to a 3C increase, that will more or less guarantee we will eventually (on a geological time scale) reach 6C (due to these other “slow” feedbacks mentioned in this discussion), and the possibility of really, really bad things happening.

  20. 120
    Timothy Chase says:

    Patrick 027 (#116) wrote:

    Re 106 – Actually, the ‘iris effect’ mechanism is supposed to involve a shrinking of high cloud area (if it is true), see also http://earthobservatory.nasa.gov/Newsroom/NasaNews/2002/200201167312.html .

    Thank you for the correction. I thought I had read somewhere that he was relying upon albedo, but if Spencer’s study were right, it would actually conform to Lindzen’s original (albeit poorly named) hypothesis.

  21. 121
    Timothy Chase says:

    I had written (#103):

    However, both the “slow” and “fast” feedbacks will actually be working at the same time, and the “slow” feedbacks which we are seeing are moving more quickly than has been expected. Likewise, the paleoclimate record that Hansen refers to suggests that the “slow” feedbacks can move surprisingly fast.

    Pascal quoted the following…

    … and the “slow” feedbacks which we are seeing are moving more quickly than has been expected.

    … then Pascal (#107) wrote:

    can you explain this sentence or have you a good proof of that?

    I’m not sure that, for example, both Antarctica peninsula and Greenland ice sheets are rushing into oceans.

    Ok. Let’s set aside the carbon cycle for the moment and just focus on Greenland and Antarctica…

    Greenland

    Previously when refering to the following paper:

    Changes in the Velocity Structure of the Greenland Ice Sheet
    Eric Rignot and Pannir Kanagaratnam
    Science 17 February 2006: Vol. 311. no. 5763, pp. 986 – 990
    DOI: 10.1126/science.1121381
    http://www.sciencemag.org/cgi/content/abstract/311/5763/986

    … I stated:

    Well, as one example, within the past decade we have seen the Greenland’s melt (as measured in terms of volume) double in the decade 1996-2006.

    However, it has actually more than doubled, going from 90 to 220 cubic kilometers per year.

    From the abstract:

    Using satellite radar interferometry observations of Greenland, we detected widespread glacier acceleration below 66- north between 1996 and 2000, which rapidly expanded to 70- north in 2005. Accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year. As more glaciers accelerate farther north, the contribution of Greenland to sea-level rise will continue to increase.

    Rignot et al (2006)

    In the main text, they describe glaciers which have continued to have roughly constant mass balance loss, then neighboring glaciers which have suddenly sped up, with one going from a mass loss of 5 to 36 cubic kilometers in nine years.

    Please see:

    In central east Greenland, no flow change is detected on Daugaard-Jensen (Fig. 2E) and Vestfjord glaciers (area 9) in 1996 to 2005. The 3.7-km/year frontal speed of Daugaard- Jensen is identical to that measured in 1969 (10), and the glacier is in balance. Immediately south, Kangerdlugssuaq Glacier has been stable in speed since 1962, but was thinning and losing mass in 1996 (11). The glacier must have longitudinally stretched the 1-km thick ice to thin it by 250 m. The acceleration increased the mass loss from 5 km3 ice/year in 1996 (12) to 36 km ice/year in 2005 (Table 1), which is 6% of Greenland’s total accumulation.

    Rignot et al (2006)

    Likewise, according to another paper (Eckstrom et al, 2006) icequakes in Greenland have more than tripled in one decade’s time.

    Recent evidence suggests that ice sheets and their outlet glaciers can respond very quickly to changes in climate, primarily through dynamic mechanisms affecting glacier flow (12, 15). The seasonal signal and temporal increase apparent in our results are consistent with a dynamic response to climate warming driven by an increase in surface melting and the supply of meltwater to the glacier base. The number of events detected at each outlet glacier using the global seismic network is relatively small, and it is therefore difficult to draw robust conclusions about behavior at any single glacier. However, both the seasonal and temporal patterns reported here are observed for independent subsets of the data corresponding to east and west Greenland. The increase in number of glacial earthquakes over time thus appears to be a response to large-scale processes affecting the entire ice sheet. We note also that a part of the increase in the number of glacial earthquakes in west Greenland is due to the occurrence of more than two dozen of these earthquakes in 2000 to 2005 at the northwest Greenland glaciers, where only one event (in 1995) had previously been observed.

    Seasonality and Increasing Frequency of Greenland Glacial Earthquakes
    Goran Ekstrom, Meredith Nettles, Victor C. Tsai
    Science Vol 311, 24 Mar 2006

    Ekstrom et al (2006) provide two charts, one of which shows that the glacial earthquakes (sometimes refered to as “icequakes”) have more than tripled between 1995 and 2005. These are reproduced by Tamino:

    Ice Quakes more than tripled in Greenland between 1995 and 2005.
    http://tamino.wordpress.com/2007/03/30/greenland-tremors/

    Incidentally, the numerical data (including latitude, logitude, direction, time lag, and surface wave magnitude) is given in:

    Analysis of glacial earthquakes
    published 14 April 2007
    Victor C. Tsai and Goran Ekstrom
    Journal of Geophysical Research, Vol. 112,
    F03S22, doi:10.1029/2006JF000596, 2007

    Interestingly, they note that there is a characteristic size to the quakes, so while they should remain of roughly the same magnitude, their number can be expected to increase. Elsewhere it has been noted that melting is increasingly moving northward and towards the center of Greenland.

    Antarctica

    Overall we know that the mass balance of Antarctica is decreasing, with most of this occuring in the West Antarctic Peninsula.

    I quote:

    Abstract: Using measurements of time-variable gravity from the Gravity Recovery and Climate Experiment satellites, we determined mass variations of the Antarctic ice sheet during 2002–2005. We found that the mass of the ice sheet decreased significantly, at a rate of 152 +/- 80 cubic kilometers of ice per year, which is equivalent to 0.4 +/- 0.2 millimeters of global sea-level rise per year. Most of this mass loss came from the West Antarctic Ice Sheet.

    Measurements of Time-Variable Gravity Show Mass Loss in Antarctica
    Isabella Velicogna and John Wahr
    Science 311, 1754 (2006)

    Only a few years before they thought that the mass balance of Antarctica would increase for a while due to increased precipitation in the interior. It might have been at the time but this has changed. They are showing some very slight growth in EAIS as the result of precipitation increase it would appear, but this is quite small in comparison to the melt loss in WAIS. Additionally, while in the past melting has been limited to the West Antarctic Peninsula, it is now moving inland and large areas of melt have been detected as far south as 85 degrees, within about 500 km of the south pole.

    Please see:

    NASA Finds Vast Regions of West Antarctica Melted in Recent Past
    05.15.07
    http://www.nasa.gov/vision/earth/lookingatearth/arctic-20070515.html

    … for a satellite image map of the melt areas. As mentioned in the text, this was quite unprecedented.

    Then of course you have the collapse of several major ice sheets.

    When the ice sheets go, the glaciers which they buttress pick up speed. In a study of eight Antarctic glaciers, they found that speeds of descent increased by as much as a factor of eight after the loss of the Larsen B ice sheet.

    Please see:

    Interferometric synthetic-aperture radar data collected by ERS-1/2 and Radarsat-1 satellites show that Antarctic Peninsula glaciers sped up significantly following the collapse of Larsen B ice shelf in 2002. Hektoria, Green and Evans glaciers accelerated eightfold between 2000 and 2003 and decelerated moderately in 2003. Jorum and Crane glaciers accelerated twofold in early 2003 and threefold by the end of 2003. In contrast, Flask and Leppard glaciers, further south, did not accelerate as they are still buttressed by an ice shelf.

    Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B ice shelf
    E. Rignot, G. Casassa, P. Gogineni, W. Krabill, A. Rivera, and R. Thomas
    Geophysical Research Letters, Vol. 31, L18401, doi:10.1029/2004GL020697, 2004

    They conclude with a warning that the phenomena may spread beyond the reaches of the West Antarctic Peninsula to the main continent itself.

    Please see:

    Further south, glaciers drain larger reservoirs of ice, and the
    thinning of Larsen C [Shepherd et al., 2003] may trigger an even larger contribution to sea level. These observations are also particularly relevant to the evolution of ice streams and glaciers draining West Antarctica. Although the climate conditions of the Antarctic continent are colder and drier than in the Peninsula, ice shelf thinning could be caused by a warmer ocean instead of warmer air temperatures. As ice shelves thin and eventually break up, the large continental glaciers draining West Antarctica could accelerate and precipitate the ice sheet into a state of negative mass balance. This is in fact what we believe is happening in the Amundsen Sea sector of West Antarctica [Rignot et al., 2004].

    ibid.

    Glacier retreat is widespread along the West Antarctic Peninsula – and it is sweeping further south. Here is something from a non-technical article.

    The first comprehensive study of glaciers on Antarctic Peninsula has uncovered widespread glacier retreat and suggests that recent climate change on the peninsula is responsible.

    Eighty-seven percent of the 244 marine glaciers have retreated over the last 50 years, a new study says. The widespread glacier retreat began at the northern, warmer tip of the Antarctic Peninsula. As atmospheric temperatures rose along the peninsula — more than 2.5 degrees Celsius (4.5 degrees Fahrenheit) in the last 50 years — the trend of retreat moved south toward colder mainland Antarctica.

    Glaciers from Antarctic Peninsula in Widespread Retreat, Science Study Says
    April 21, 2005
    http://earthobservatory.nasa.gov/Newsroom/MediaAlerts/2005/2005042118836.html

    The technical paper is here:

    Retreating Glacier Fronts on the Antarctic Peninsula over the Past Half-Century
    A. J. Cook, A. J. Fox, D. G. Vaughan, J. G. Ferrigno
    Science 22 April 2005:
    Vol. 308. no. 5721, pp. 541 – 544
    DOI: 10.1126/science.1104235
    http://www.sciencemag.org/cgi/content/abstract/308/5721/541

    What we are seeing is accelerating and it is spreading. “Slow” feedbacks which are proving surprisingly nimble. Of course the paleoclimate record says that things can move a great deal more quickly – but we can save that for later.

  22. 122
    Charles Muller says:

    #119 (or #118 if 112-113 doubling is corrected)
    Lynn wrote :
    It’s the “geological” mindset or perspective. Sort of like a 1000 years in our time is like a blink of God’s or a geologist’s eye.
    Quickly in geological time could mean years, decades, or even centuries. As for slow, well…

    You underscore a problem: accounting for “slow” feedbacks suppose that a climate model or a carbon cycle model run for centuries. But is there any sense for such an exercise? Let’s consider the biological pump: how can we reasonably constrain adaptative processes on a century or millenia time scale, especially for oceanic fauna and flora with a high reproduction (so mutation) rate? So, Timothy (#103) may be right to look after paeloclimate rather than climate models projections on that point.

    On a more theoretical point of view, we’re discussing here the climate sensitivity as defined by IPCC (equilibrium surface temperatture after a CO2 doubling – 3,7 W/m2 – and all feedbacks integrated). The problem in the “slow” feedback analysis is that it seems a never-ending runaway : there are positive feedbacks (ice melting, carbon pump saturation) ; which imply less albedo, more CO2 ; which imply new positive feedbacks (more ice melting, more carbon pump saturation)… and so on. But of course, that would be an absurd reasoning (a 0,5 W/m2 initial forcing would be enough to engage Venus-like runaway!). I suppose that for a 3,7 W/m2 forcing, the additional energy of forcing+feedbacks is used for faster processes (melting ice, evaporation, warming of subsurface oceanic layers, etc.) and the new equilibrium is reach on a quite short timescale. Any information about that (how long are the GCMs runs when they evaluate 2xCO2 CS?)

  23. 123
    Pascal says:

    Timothy

    re (#103)

    thanks for your responses and links.

    Be sure I don’t contest measurements and studies of ice-sheet melting in Greenland and Antarctica.
    I don’t contest also the relatively recent decreasing of CO2 ocean sink.
    I just ask why we don’t see a perceptible acceleration of sea level increasing (you answer on this point) and, for CO2, why we don’t see a greater slope on CO2 concentration curve.

    CO2 emissions are more and more important, the sink is decreasing, and the slope is the same.

    How can we explain this?

  24. 124
    petefontana says:

    Hank Roberts, first my apologies. I often post before thinking.

    This was the December 6, 2015 “tipping point” reference. I’m sure I never “heard this” anywhere and shouldn’t have implied that I did.

    I added 10 years to the AGU date. It was a calculation based on the available data. Perhaps it is in error.
    http://www.columbia.edu/~jeh1/keeling_talk_and_slides.pdf

    Although, other people are sort of using the same timeframe.

    http://www.ens-newswire.com/ens/jun2007/2007-06-01-01.asp

    Oddly enough, I am a great admirer of Dr. Hansen. Much of what he says is undeniably true. But are any of us perfect?

  25. 125
    Charles Muller says:

    #109 Timothy

    I don’t really understand the “clear sky super greenhouse effect” from Valero et al 1997, or in what way this cancel an Iris or Iris-like effect. Clear sky recent observations suggest a positive feedback, that is an increase of specific humidity in upper troposphere (recent works of Allan, Soden, Wong, etc.) and a consequent diminution of OLR. But Spencer et al 2007 take account of all sky conditions (cloudy and uncloudy – see fig 2-D) when they estimate the SW+LW radiative budget.

  26. 126
    Charles Muller says:

    #107 (Pascal) #112 (Timothy)

    If continental ice melting (Greenland, Antarctic, terrestrial glaciers) is accelerating and if warming of the 0-700 m (and deeper) oceanic layer is still on, you shoud observe a higher rate of sea-level rise. But it is not the case according to Topex-Jason-Poseidon. I think that’s what Pascal has in mind. Lombard et Cazenave 2005 have showed that 20th century sea level thermosteric evolution is still dominated by decadal oscilations, with eventually negative values, even in the second part of the century. But are we in such a negative oscillation (if so, Levitus or Lyman should have observed a cooling) ?

  27. 127
    Eachran says:

    Group, thanks for the explanation. Everything understood.

    The GIMBI index seems to be rising quite rapidly.

    Not a bad thing, I would have thought. Soemthing about something concentrating the mind.

  28. 128
    Nigel Williams says:

    Lyn, it’s not just a matter of walking away from the water’s edge. It’s more a matter of having somewhere useful to walk towards. A place that provides enough food and other necessities.

    Cycle thru
    http://cegis.usgs.gov/video/sealevel_world.avi
    a few times to see how far we have to run, and what’s left at 80m of sea level rise.

    All the high quality coastal soils are gone, and the land that’s left may be drought-ridden or utterly unsuitable for agricultural activities.

    We cant afford to rebuild our global infrastructure twice in a millennia, so we have to plan for the worst from the outset. And it will be bad.

  29. 129
    Hank Roberts says:

    > rate of sea level rise

    What? No change? Looks likely it’s real, though hard to quantify yet.
    The satellites are very new and recent data overlapping old data, the whole idea that the planet is a sphere has been tossed as mass concentrations affecting sea level are mapped. It’s ‘arguable’ but that’s the least worrisome thing you can say.

    http://www.realclimate.org/images/sealevel_2.jpg
    http://www.grida.no/climate/vital/19.htm
    http://www.oceansatlas.org/unatlas/projectmanager/atlas_cd/cds_upload/1084987425690_MillerSLRsmall001.jpg

  30. 130
    Aaron Lewis says:

    Re 128

    My house is above the 80M high tide mark, but it is not built for the the weather that we are likly to have with global warming. So it will require a retrofit for the new climate.

    However, some part of all of our utilities would be flooded. Power generators. Power substations. Natural gas pumping and control stations. Rail roads (for food). Air ports. Sea Ports. Roads. Drinking water systems. Fuel refineries.

    Just because you are above the high tide mark does not mean you are OK.

  31. 131
    Lynn Vincentnathan says:

    Re #122 & it “may be right to look after paeloclimate rather than climate models projections on that point [of slow feedbacks]…The problem in the “slow” feedback analysis is that it seems a never-ending runaway…But of course, that would be an absurd reasoning (a 0,5 W/m2 initial forcing would be enough to engage Venus-like runaway!).”

    From what RC folks have told me, not so. The Venus-like “PERMANENT RUNAWAY” won’t happen until the sun becomes a big, hot, red ball some billions of years from now.

    However, the paleoclimate info seems to indicate we could go into a “REGULAR RUNAWAY” scenario, that eventually on a geological timescale stops and reverses itself. Some refer to this as “hysteresis” — going out of some bounds, then coming back. For instance the end-Permian mass extinction conditions eventually stopped and the world returned to a life-hospitable state….or we wouldn’t be here to talk about it. My knowledge is limited, but I think such an event could last about 100,000 or 200,000 years. It’s interesting that a portion of our today’s CO2 emissions could be in the atmosphere up to 100,000 years (see: http://www.realclimate.org/index.php/archives/2005/03/how-long-will-global-warming-last ).

    Also, my understanding is we would have to reach about a 3C increase in warming to reach a tipping point on this (at least I hope the tipping point is not at a lower warming level, and hope it is at a much higher level). So I’m not sure where you got the idea that a “0,5 W/m2 initial forcing” would be enough to set us on a hysteresis path, much less a permanent runaway path (maybe that was enough for Venus?).

    I refer to the Venus-like “permanent runaway” as a special case of “runaway” in general, since this term is a metaphor based on runaway horses (or trains), and we know they eventually stop (I got 3 broken ribs and a punctured lung as a kid to prove it). It’s a good layperson’s anthropocentric term, because we are mainly concerned about a situation running away from us (getting out of our control).

    My understanding is that at this point we can control the situation somewhat; reducing our emissions should eventually reduce or stop the warming trend. Once it is out of our control we’re pretty much in for a terrible ride that will amount to much more harm than broken ribs, and no matter how much of our GHG emissions we reduce, there will be virtually nothing we can do about it — except maybe with huge reductions slightly reduce the severity of the harms.

    Skip placing hope in geo-engineering; by that time we will probably be too poor and fighting with each other over rapidly diminishing resources to implement anything, even if some genius were to actually come up with a workable solution without worse-than-the-cure side effects.

    Best bet is to reduce GHGs now ASAP AMAP.

  32. 132
    Pekka J. Kostamo says:

    RE 122: There is no such problem – there will be no Venus runaway. The positive feedback only operates until a resource runs dry. That is, until all the continental ice has melted, and/or all the permafrost in Alaska, Canada and Siberia has melted. The system then seeks new equilibra and the runaway is ended. The composition and sensitivity of these resources is not well known, hence the end results are unknowable for the time being. Except estimates of sea level rise, of course.

    A few thousand boreholes, properly measured, might throw some light on this issue.

  33. 133
    Lynn Vincentnathan says:

    Thanks Nigel (#128), and here is a set of world maps showing what the world would look like with a 100 meter rise: http://resumbrae.com/archive/warming/100meter.html

    Could you or anyone tell me how much warming there would have to be (and how long it would take, given the lag time) to reach, say, a 30M sea rise? 60M sea rise? Off-hand, wild guessimates welcome (think possibilities, not high necessarily probabilities).

    I’ve started writing a fictional story about this and just want some ballpark ideas. While I don’t mind erring a bit on the side of going beyond what conservative estimates scientists might have (it IS a sci-fi story, afterall), I don’t want to extremely overshoot the science, like WATERWORLD or DAY AFTER TOMORROW did.

    With my limited knowledge, I’m just guessing that if we do reach 6C warming by 2100 or 2200 that at least by 2000 years from now the sea would have risen 60M. And if we reach just shy of 3C warming (or whatever the tipping point is) by 2100 or 2200 (but do not trigger limited runaway warming or hysteresis), that by at least 2000 years from now the sea level would have risen about 30M.

    Anyone??

  34. 134
    Nick Barnes says:

    Timothy Chase @121: please take care to use the terms “ice sheet” and “ice shelf” correctly. Ice shelves float; ice sheets don’t. We haven’t lost any ice sheets. Ice shelves such as Larsen B, on the other hand, have gone (and shrunk).

  35. 135
    sidd says:

    Pascal Says at 30 October 2007, 2:26 AM
    “I just ask why we don’t see a perceptible acceleration of sea level increasing”

    There is some evidence of just that:
    Abdalati, “Ice Sheets, Glaciers and Rising Seas” (a search for the title should get you to the presentation) has a graph of the Jason/Topex/Poseidon data. also in Lombard et al,Earth and Planetary Science Letters, 254, 2007
    In Fig. 6. You can easily see that the sea level rise has increased from 2 mm/yr in the middle 90s to to 4mm/yr today.For those who want to play with the data: see
    http://podaac.jpl.nasa.gov/DATA_PRODUCT/OST/index.html#jason

    Charles Muller Says at 30 October 2007, 3:49 AM
    “Lombard et Cazenave 2005 have showed that 20th century sea level thermosteric evolution is still dominated by decadal oscilations, with eventually negative values, even in the second part of the century.”

    You may wish to see a newer Lombard reference cited above.

    Another interesting paper is Meier et al., Science, 317, 2007, wherein they point out that glaciers (as opposed to Greenland Ice sheet and Antarctica) have dominated the eustatic contribution in this century. They propose that this dominance will continue for most of this century. I fear they are wrong. They have taken the acceleration in melting of the ice sheets to be a constant, and extrapolated into the future century, Hansen has proposed a much more threatening scenario where the rate of icesheet disintegration increases exponentially, doubling every decade. I note that the rate of acceration of melting of Greenland in the Meier reference does lead to doubling of the melt rate in about a decade (although of course they assume that subsequent doublings will take longer). We shall soon see whether Meier or Hansen are closer to the truth.

    sidd

  36. 136
    henning says:

    With all these apocalyptic scenarios one should keep in mind that mankind will probably not want to afford burning all the fossil fuel in existence but stop doing so at some point, purely because better, cheaper technology becomes available. We should also believe in the future far enough to assume, that climate science will progress and with it our options to actively do something about it. If 20th century technology could/can ruin the planet and 20th century science could detect it (phase 1), 21st century technology will be able to correct it and 21st century science will be able to make sure its done properly (phase 2). Does that mean we shouldn’t worry? Of course not – after all we are in the 21st and science is already telling us what to do about it. So from where I’m standing, phase 2 has begun – and thats a good thing.

  37. 137
    Timothy Chase says:

    infinite feedback?

    Charles Muller (#122) wrote:

    On a more theoretical point of view, we’re discussing here the climate sensitivity as defined by IPCC (equilibrium surface temperatture after a CO2 doubling – 3,7 W/m2 – and all feedbacks integrated). The problem in the “slow” feedback analysis is that it seems a never-ending runaway : there are positive feedbacks (ice melting, carbon pump saturation) ; which imply less albedo, more CO2 ; which imply new positive feedbacks (more ice melting, more carbon pump saturation)… and so on. But of course, that would be an absurd reasoning (a 0,5 W/m2 initial forcing would be enough to engage Venus-like runaway!).

    Consider the sum (1/2)^1+(1/2)^2+(1/2)^3+…

    It is an infinite sum in the sense that there is an infinite number of terms. However, it approaches but never exceeds 1. This is the case with the kind of feedbacks that we are talking about here. If you just look at amplification of CO2′s greenhouse effect by water vapor, the rise in temperature due to CO2 will result in a certain amount of additional water vapor.

    The water vapor from this first increase in temperature will have its own greenhouse effect, raising the temperature further but by a smaller amount. This additional rise in temperature will result in still more water vapor which will raise the temperature still more, but by a smaller amount.

    And so it continues. But in the end, all of the water vapor adds somewhat less than 1.8 C to the original 1.2 C for a CO2 doubling in the fast feedbacks. (Amplification due to the albedo effect from sea ice will have its share of the pie, too.)

    *

    What applies in the case of the “fast” feedback from water vapor or sea ice applies in the case of the “slow” feedback from the carbon cycle and ice sheets. Although one may analyse it in terms of an “infinite sum” of incremental feedbacks, each additional increment will be smaller, and the total feedback will be finite.

  38. 138
    Dave Rado says:

    Re. all the posts about Spencer et al 2007, I hope RC will do an article about it, as it’s widely cited on the blogosphere, and it would be nice to have a thorough RC article about it to link to.

  39. 139
    J.C.H. says:

    Lynn, rent WarterWorld, which was apparently mistakenly shown in some theaters that were supposed to be showing AIT. The announcer says all of the world’s ice has melted. As he’s saying this, a graphic of the globe shows the Arctic ice rapidly melting as seas rapidly rise and cover all the continents. The hero, who looks to me oddly like a person who never saw AIT, drinks his own pee and grows gills.

    Kneel at the altar of adaptation. Who needs mitigation?

    A lot can be learned from Dave Rado’s post above: 52. He corrals most of the world’s ice.

    Nick, once parts of the ice sheet are bergs in the ocean, it floats, right! To get people like me thinking through this, once in the drink, has it essentially raised sea level as much as it ever will?

    It snows on Greenland. It’s an amazingly tall chunk of ice. Frank Lloyd Wright did not design it. No engineering firm put it together so it could grow taller forever. It’s a structure, like the bridge to St. Paul. With the edges thinning, or disappearing altogether, how tall can it get?

  40. 140
    Timothy Chase says:

    Nick Barnes (#134) wrote:

    Timothy Chase @121: please take care to use the terms “ice sheet” and “ice shelf” correctly. Ice shelves float; ice sheets don’t. We haven’t lost any ice sheets. Ice shelves such as Larsen B, on the other hand, have gone (and shrunk).

    Thank you, Nick. I am not sure how long it would have taken for me to pick up on the difference, but now that I know the difference I will try to keep it in mind from now on.

  41. 141
    Dave Rado says:

    Re. henning, #136, see See #59 and #74 ( especially point (c)) of the "Gee-Whiz Geoengineering" thread.

  42. 142
    Martin Vermeer says:

    #132: “There is no such problem – there will be no Venus runaway. The positive feedback only operates until a resource runs dry.”

    True, but this is what happened on Venus too: the process stopped when all crustal rocks had been cooked of their CO2.

    (And in principle reversible: all it takes is suspending a parasol at Venus’ Lagrange point. It will take a long time for the cooling to propagate down to the surface, and all the CO2 and water to rain out and react with the surface rock…)

  43. 143
    henning says:

    @Dave 137
    You make my point. The problem has been identified – even quantified. And the fact that something like the IPCC exists is an indicator for politicians having acknowledged the problem. Of course things should move faster and more decisive (although at least here in Germany some people already claim things move too fast) but politics always tends to be notoriously slow on the uptake. At least we’ve started doing something and the combined social “forcings” of political pressure and ever rising cost of fossil energy move in the right direction. If we keep this up, there’s no reason to assume we’ll end up with a 70m sealevel rise.

  44. 144
    David B. Benson says:

    Lynn Vincentnathan (133) — One way to provide estimates is to look at the sea stand rise from LGM to the so-called Holcence Climatic Optimum. That is, roughly, an S-shaped curve over about 10,000 years, a rise of at least 120 meters and involved about 6C warming. For the purposes of your story, however, it is the maximum rates in melt-water pulse 1A times which might be useful to you. About four meters per century, according to Wikipedia.

    However, with the warming imposed as a forcing, I suppose you might up that to one meter per decade?

  45. 145
    Hank Roberts says:

    Hmmmm.
    http://www.space.com/scienceastronomy/solarsystem/venus_oceans_020516.html

    (This is entertainment science, not journal science, I didn’t backtrack to see if this is based on anything published)

    —–excerpt follows———

    “How did Earth manage to hold onto all of its water, while Venus apparently lost all of its water?”

    … studying an unusually warm pool of water in the Pacific Ocean northeast of Australia, as well as the atmosphere above it….

    At sea surface temperatures above 80 Fahrenheit (27 C), evaporation loads the atmosphere with a critical amount of water vapor … a controlling factor seems to keep the same thing from happening on Earth.

    Here, sea surface temperatures never reach more than about 87 Fahrenheit (30.5 C), and so the runaway phenomenon does not occur.

    But the scientists are not sure why it does not occur.

    “What is limiting this effect over the warm pool of the Pacific?” asked Richard Young, another member of the research team.

    —– end excerpt——-

  46. 146
    Dave Rado says:

    Re. 134:

    Timothy Chase @121: please take care to use the terms “ice sheet” and “ice shelf” correctly. Ice shelves float; ice sheets don’t.

    In case any laymen reading this are confused about why the melting of ice shelves contribute directly to rising sea levels even though ice shelves float, it is because they are anchored to the shore ; so they don’t float freely in the same sense that pack ice and icebergs float. However, because they are partly submerged, their direct contribution to sea level rise is much smaller than the contribution made by the melting of an equivalent volume of (land-based) ice sheets.

    However, as Timothy explained in #121, in addition to the direct sea level rise that occurs when ice shelves melt, there is a much larger secondary effect, in that ice shelves act as a brake, greatly reducing the rate of flow of the glaciers behind them from the land to the sea; and when ice shelves melt, the rate of glacier flow increases quite rapidly.

  47. 147
    Timothy Chase says:

    Pascal (#123) wrote:

    re (#103)

    thanks for your responses and links.

    Not a problem — although a little bit of work! ;-)

    Pascal (#123) wrote:

    Be sure I don’t contest measurements and studies of ice-sheet melting in Greenland and Antarctica.
    I don’t contest also the relatively recent decreasing of CO2 ocean sink.
    I just ask why we don’t see a perceptible acceleration of sea level increasing (you answer on this point) and, for CO2, why we don’t see a greater slope on CO2 concentration curve.

    CO2 emissions are more and more important, the sink is decreasing, and the slope is the same.

    How can we explain this?

    Water level? Hank responds to this in #129, so I will focus on carbon dioxide.

    You ask:

    I just ask why we don’t see a perceptible acceleration of sea level increasing (you answer on this point) and, for CO2, why we don’t see a greater slope on CO2 concentration curve.

    Perhaps because we have seen just such an acceleration.

    Here is a non-technical article…

    Unexpected Growth In Atmospheric Carbon Dioxide
    ScienceDaily (Oct. 23, 2007)
    http://www.sciencedaily.com/releases/2007/10/071022171932.htm

    … and a quote from the article it is based on:

    Growth in Atmospheric CO2. Global average atmospheric CO2 rose from 280 ppm at the start of the industrial revolution (~1,750) to 381 ppm in 2006. The present concentration is the highest during the last 650,000 years (5, 6) and probably during the last 20 million years (7). The growth rate of global average atmospheric CO2 for 2000–2006 was 1.93 ppm y^-1 [or 4.1 petagrams of carbon (PgC) y^-1, Table 1]. This rate is the highest since the beginning of continuous monitoring in 1959 and is a significant increase over growth rates in earlier decades: the average growth rates for the 1980s and the 1990s were 1.58 and 1.49 ppm y^-1, respectively (Fig. 1).

    OPEN ACCESS: Carbon sink slowdown contributing to rapid growth in atmospheric CO2
    Josep G. Canadell, Corinne Le Quere, Michael R. Raupach, Christopher B. Field, Erik T. Buitenhuis, Philippe Ciais, Thomas J. Conway, Nathan P. Gillett, R. A. Houghton, and Gregg Marland
    Proceedings of the National Academy of Sciences , October 2007.
    http://www.pnas.org/cgi/content/abstract/0702737104v1

    For those who are interested, the technical article above goes into an analysis of the contributions of rising emissions and declining sinks to the increased rate at which CO2 levels are rising.

  48. 148
    Aaron Lewis says:

    Re 135
    Try defrosting your freezer. It starts as a drip. Then it is a good trickle. Then there are small pieces of ice falling off. Then there are large pices of ice falling. There is always this sudden transition from, “Oh,Gee! this is going to take all day” to “Wow, that did not take as long as I expected.

    I fear the same for the ice sheets.

  49. 149
    Charles Muller says:

    #Sidd 135 Thank you for the reference.

    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.

    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 ? 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).

    #Dave 138 I agree with you, a paper on Spencer et al 2007 woud be welcome here.

    #Timothy 137 I agree, ice melting and carbon saturation are finite feedbacks.

  50. 150
    Aaron Lewis says:

    Re 139
    Cold ice is very strong. Very cold ice is stronger than some kinds of concrete. Ice makes a great foundation if you can keep it cold.

    Ice above 28 F starts to melt under high pressure. Ice above 28F is quite weak. It can flow and deform under your body weight (200 lb/ inch ^2) in a matter of minutes.

    If the ice has a significant amount of water moving through it, then it is unpredictably fragile. Columns of water in moulins can impose significant forces on ice, and in the event of a collapse, the resulting flow is a slurry of ice and water. Consider what happened when such a collapses occurred on Mt Blanc. http://query.nytimes.com/gst/abstract.html?res=9D04E4D81F39E233A25750C1A9619C94639ED7CF

    Temperature is critical in talking about the mechanical properties of ice.

    http://skua.gps.caltech.edu/hermann/ice.htm

    http://www.tms.org/pubs/journals/JOM/9902/Schulson-9902.html


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