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Planetary Boundaries.

  1. Sep 23, 2009 #1
    Here is a special feature in Nature outlining the state of ten biophysical systems crucial for humanity to thrive on Earth.


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  3. Sep 23, 2009 #2


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    So, the IPCC value of 3C/CO2 doubling does not factor in the reduction of surface area of ice coverage or change in vegetation.
  4. Sep 23, 2009 #3


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    The part about threatening the viability of contemporary human societies is (IMO) over stated. There would have to be a significant shift of agriculture, likely global famines and wide spread suffering, but I'm confident that human society would continue to be viable, albeit smaller than it is currently.
  5. Sep 23, 2009 #4


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    Nice. It seems to be publically available as well.

    Note especially there is commentary provided from seven expert commentators; some of these are pretty critical of the original paper.

    In particular, my first reaction to the climate stuff is that if they want to look at long term effects, then they have to consider long term effects of the carbon cycle as well, which can be expected to draw a substantial amount of carbon back out of the atmosphere.

    The expert commentary on the climate aspect of "boundaries" is Myles Allen, whom we have just recently been talking about in the context of the effects of cummulative carbon emissions. He recently published a paper on this, and at was raised by joelupchurch in [post=2343330]msg #84[/post] of thread "Estimating the impact of CO2 on global mean temperature". The following discussion (and msg #83 for a closely related paper by Matthews et al) gives a bit of background.

    Allen's response as part of the this special issue is Planetary boundaries: Tangible targets are critical. Allen is very critical of the particular way in which this boundary is described, and this is all very closely associated with his recent paper. An extract of his response:
    The campaign to establish 350 parts per million (p.p.m.) as a long-term target carbon dioxide concentration has acquired considerable momentum despite relatively little support for this specific number in the scientific literature. [...] The problem is not that 350 p.p.m. is too high or too low a threshold, but that it misses the point. The actions required over the next couple of decades to avoid dangerous climate change are the same regardless of the long-term concentration we decide to aim for.

    There are all kinds of problems with looking at this extremely long term response, as the original paper appears to do. Allen goes in to some specifics of the problems.

    Cheers -- sylas

    PS. For Xnn:
    No, the conventional equilibrium sensitivity value certainly does factor in ice cover [strike]and vegetation[/strike]. The very high sensitivity values suggested by Rockström et al are specifically for past climate conditions. Equilibrium sensitivity is not strictly a constant, but can vary over long time scales. High sensitivity would have to include melting of the large land ice caps, I think, that is not a part of equilibrium sensitivity values now. Sea ice, and retreating glaciers, are all a part of the IPCC estimate. The original paper is incorrect or ambiguous to state that the present sensitivity value does not include decreases in the surface area of ice cover; it would have been better to say clearly that there are additional long term slow factors in surface cover which go beyond what is considered in the conventional value.

    Quoting Allen's response again:
    Rockström et al. acknowledge that the strength of feedbacks in the present-day climate suggests a most likely value for climate sensitivity of 3 °C, with a 'likely' (one-standard-error) uncertainty range of 2–4.5 °C. Yet they cite evidence from paleoclimate research (Open Atmos. Sci. J. 2, 217–231; 2009) that, in the past, additional feedbacks due to polar ice-sheet melting and poleward shifts in vegetation resulted in a climate sensitivity of 6 °C, with a 'likely' range of 4–8 °C.

    Allen goes on to argue that this is not really coherent; if they are justifying a 350ppm limit in which ice sheets won't melt, then they shouldn't be using high sensitivities that apply for melting of ice sheets.
    Last edited: Sep 23, 2009
  6. Sep 23, 2009 #5


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    So, did the Editors at Nature publish an error by allowing the statement that ice cover was not included?
  7. Sep 23, 2009 #6


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    I think the statement in the main paper is ambiguous. A straightforward reading gives a claim that is obviously false, and this should have been picked up, in my opinion. But it apparently was not.

    It is a useful reminder that it is perfectly normal and actually quite common for a published version of a paper to include errors. Peer review is not a way to ensure there are no errors. It's an initial hurdle for a presentation to get out to the wider scientific community, and it is generally after publication that you get the most substantive engagement of contrasting ideas. This special issue was intended to foster debate, and it has done so.

    This particular error is more of a badly phrased and misleading wording, rather than outright error. The problematic passage (my emphasis in bold), plus the first sentence of the following paragraph, is as follows:
    There are at least three reasons for our proposed climate boundary. First, current climate models may significantly underestimate the severity of long-term climate change for a given concentration of greenhouse gases12. Most models11 suggest that a doubling in atmospheric CO2 concentration will lead to a global temperature rise of about 3 °C (with a probable uncertainty range of 2–4.5 °C) once the climate has regained equilibrium. But these models do not include long-term reinforcing feedback processes that further warm the climate, such as decreases in the surface area of ice cover or changes in the distribution of vegetation. If these slow feedbacks are included, doubling CO2 levels gives an eventual temperature increase of 6 °C (with a probable uncertainty range of 4–8 °C). This would threaten the ecological life-support systems that have developed in the late Quaternary environment, and would severely challenge the viability of contemporary human societies.

    The second consideration is the stability of the large polar ice sheets. [...]

    One straightforward natural reading is that the models don't consider decreases in the surface area of ice cover at all; and that is false. The intended reading (IMO) is that some of the things omitted from existing models include additional long term decreases in the surface area of ice cover. It's quite likely (IMO again) that an expert reviewer might recognize the intended reading straight away without thinking of the more natural reading to someone not immersed in the literature on this.

    That existing models consider ice cover is explicit in many publications. A major reference is
    The ice cover feedback is one of the major feedbacks considered in models and part of the conventional sensitivity estimates, as described in that paper. The IPCC 4th AR looks at models in chapter 8, and sensitivity in section 8.6.3. The ice feedback is covered in subsection "Cryosphere Feedbacks", page 638.

    What Rockström et al are really talking about, I am pretty certain, is a possible long term loss of major land ice sheets over and above the changes in sea ice and glaciers that are more significant in the short to medium term. This is mentioned in the following paragraph as a "second consideration", but it is actually the same thing considered from another angle, I am pretty sure.

    It is worth emphasizing that Allen and Rockström et. al. are actually in strong agreement over the importance of climate change and the need for action. The difference is really about the nature of the boundary being described; whether it makes sense to think in terms of 350ppm as a level of concentration we must maintain, or whether it is better to identify a level of cummulative emissions that should not be exceeded. It's not a slam dunk for either side, but for what it is worth, at this point I am finding Allen's proposals persuasive.

    Cheers -- sylas
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