Climate sensitivity to a doubling of atmospheric CO2 is only a well defined term (if at all) if you state what time scales or processes you are including and what you mean by "a doubling of atmospheric CO2".
The higher sensitivity values seem relevant to time scales (i.e., > 10 kyr) where ice sheets and possible biogeochemical (e.g., CH4) feedbacks can kick in at full force. The lower sensitivity values seem relevant to shorter time time scales (i.e., < 100 yr) when ice sheet feedbacks are not relevant and we assume that CH4 feedbacks need not be considered. Furthermore, if you instantaneously double CO2 and then let some of the CO2 be absorbed by the ocean and biosphere you will get less of a climate response that if you maintain the doubled CO2 concentration. My take is that the longer-term response seems to be closer to 6 C whereas the shorter-term response (assuming no big methane feedback) seems closer to 3 C. (For my own subjective probability distribution under these assumptions, I would suggest something like an 80% probability of being in the range of 2/3 to 3/2 of these values , i.e., short-term in the absence of strong methane, probably something like 2 to 4.5 C for a CO2 doubling, and for the longer term, something like 4 to 9 C for a sustained CO2 doubling.) --- I am planning to give a semi-tutorial talk on this at AGU in the same session that Hansen is speaking in: *CONTROL ID: * 1209062 *TITLE: * Radiative Forcing and Climate Response: From Paleoclimate to Future Climate *PRESENTATION TYPE: * Assigned by Committee (Oral or Poster) [Invited] *CURRENT SECTION/FOCUS GROUP: * Paleoceanography and Paleoclimatology (PP) *CURRENT SESSION: * PP10. Earth System Sensitivity To Radiative Forcings: Lessons From Earth History *AUTHORS (FIRST NAME, LAST NAME): * Ken Caldeira1, Long Cao1 *INSTITUTIONS (ALL): * 1. Dept. of Global Ecology, Carnegie Institution, Stanford, CA, United States. *Title of Team: ABSTRACT BODY: * The concept of radiative forcing was introduced to allow comparison of climate effects of different greenhouse gases. In the classic view, radiative forcing is applied to the climate system and the climate responds to this forcing, approaching some equilibrium temperature change that is the product of the radiative forcing times the ‘climate sensitivity’ to radiative forcing. However, this classic view is oversimplified in several respects. Climate forcing and response often cannot be clearly separated. When carbon dioxide is added to the atmosphere, within days, the increased absorption of longwave radiation begins to warm the interior of the troposphere, affecting various tropospheric properties. Especially in the case of aerosols, it has been found that considering rapid tropospheric adjustment gives a better predictor of “equilibrium” climate change than does the classic definition of radiative forcing. Biogeochemistry also provides additional feedbacks on the climate system. It is generally thought that biogeochemistry helps diminish climate sensitivity to a carbon dioxide emission, since carbon dioxide tends to stimulate carbon dioxide uptake by land plants and the ocean. However, there is potential to destabilize carbon locked up in permafrost and at least some possibility to destabilize methane in continental shelf sediments. Furthermore, wetlands may provide a significant methane feedback. These and other possible biogeochemical feedbacks have the potential to greatly increase the sensitivity of the climate system to carbon dioxide emissions. As time scales extend out to millennia, the large ice sheets can begin to play an important role. In addition to affecting atmospheric flows by their sheer bulk, ice sheets tend to reflect a lot of energy to space. If carbon dioxide remains in the atmosphere long enough, there is potential to melt back the large ice sheets, which would add additional warming to the climate system. It is likely that these millennial time-scale feedbacks could double climate sensitivity over that estimated by century-scale models. The inclusion of these feedbacks may be one reason why paleoclimate studies seem to indicate a much higher climate sensitivity than do the current generation of climate models that focus on the physics of century-scale climate change. What is the relevance of “equilibrium” climate change on a dynamic planet? Each gas or aerosol has a different time evolution in the atmosphere, so the time evolution of the climate response to a methane release, an aerosol release, and a carbon dioxide release would be very different, even if they had the same initial radiative forcing (or radiative forcing integrated to some time horizon, as is done in Global Warming Potential calculations). Furthermore, the climate response to emissions of these radiatively active substances will depend, to some extent, on the state of the climate system into which these substances are introduced. Changes in continental positions and altitudes can affect snow and glacier feedbacks. Changes in ocean heat transport can affect cloud properties and the distribution of sea-ice. For many applications, it may be more fruitful to focus on the time-evolution of the climate response to emissions and abandon the concept of climate sensitivity to radiative forcing. On Sat, Aug 13, 2011 at 4:13 PM, <[email protected]> wrote: > John - Thanks for bringing this dialog back to climate sensitivity - which > obviously is a very key parameter for this list. You seem to have correctly > stated the present view of Dr. Hansen as being 3 degrees C per CO2 > doubling. But in your second citation to the work of Dr. Wasdell, Hansen's > view is stated as this being better defined as 6. Dr. Wasdell recommends > 7.8 (as you have reported) - and your citation [2) for his work gives 3 or > 4 additional experimental (not model) strong endorsements of this very large > sensitivity value. > > Can anyone else support the larger sensitivity values? Is the difference > only in the time domain? How does such a large fundamental difference get > settled? Will the final authority be the IPCC? > > Ron > > ------------------------------ > *From: *"John Nissen" <[email protected]> > *To: *"marty hoffert" <[email protected]> > *Cc: *[email protected], [email protected], > [email protected], [email protected], "Tyler Volk" < > [email protected]>, "David Wasdell" <[email protected]>, "Alan > Robock" <[email protected]>, "Stefan Rahmstorf" < > [email protected]>, "John Shepherd" <[email protected]>, > "Mark Lynas" <[email protected]> > *Sent: *Friday, August 12, 2011 7:53:53 AM > *Subject: *Re: [geo] Jamais Cascio-- on the problematic idea of 350 > > > > Hi Marty, > > I was just looking through some bulky emails to delete them, when I noticed > this one on climate sensitivity. You put 2.5 degrees warming for CO2 > doubling. It is interesting that the climate sensitivity has been > reappraised, e.g. by Hansen giving 3 degrees [1] and by Wasdell giving a > much higher figure of around 7.8 degrees [2]. > > Wasdell raises a fundamental point about the behaviour of the climate > system over recent Ice Ages when temperature has varied by 5 degrees, yet > CO2 has only varied between 180 ppm and 280 ppm. > > [quote from [2]] > * > That raises the fundamental “Emperor’s Clothes” question of climate > science: > > 8.5.1 “If the Charney sensitivity, supported by our modern computer models, > projects that a doubling of the concentration of atmospheric carbon-dioxide > leads to a temperature rise of 3ºC at equilibrium, then why, in the > empirically measured behaviour of the planetary system, does an increase of > only 56% in CO2 concentration (from 180 ppm to 280 ppm) lead to a 5ºC change > in temperature?” > * > [end quote] > > Now that assumes that the CO2 causes the warming (rather than the polar > amplification of Milankovitch signals, as I would suggest [4]). However I > think Wasdell is onto something. There does seem to have to been *systemic > optimism* in the climate science community about the effect of CO2 > emissions on global warming, and we are on course to reach 4 degrees or > more, even with the most drastic CO2 cuts one could imagine through a UNFCCC > path (especially given the political situation in US and China). > > Furthermore the potential contribution of Arctic methane to global warming > is being ignored. > > Furthermore, 1.5 degrees is now being accepted as a significantly safer > target than 2 degrees [3]. And there are calls for the CO2 level to be > quickly brought below 350 ppm, amid concerns about ocean acidification as > well as global warming. > > How much evidence does the scientific community need, before accepting the > requirement for geoengineering? Perhaps those who still say that > geoengineering is too risky (in relation to benefits) should answer this > question. There are some on this list! > > Cheers, > > John > > [1] > http://www.columbia.edu/~jeh1/mailings/2011/20110118_MilankovicPaper.pdf > > [2] http://www.apollo-gaia.org/Climate%20Sensitivity.pdf > > [3] > http://globalwarmingisreal.com/2011/06/03/unfccc-chief-says-two-degrees-is-not-enough/ > > [4] Nissen, J "Arctic sea ice thermostatic control of global temperature", > EGU 2011 (to be published) > > --- > > On 02/11/2009 17:16, Marty Hoffert wrote: > > All: > > David Keith & Andy Revkin are right that we should not be > over-interpreting every perhaps natural climatic fluctuation as evidence of > global warming however much that feeds into the human psychodrama. That in > no way detracts from the robust finding that the secular trend of rising > global mean temperature, polar cap melting and sea ice recession from > continued CO2 emissions is overwhelmingly supported by data. And given our > present fossil fuel based energy system, and independently estimated global > climate sensitivity, the prospect of CO2 growing to at least 450 ppm causing > future warming at or above 2 degrees Celsius later this century & persisting > into the next is likewise a near certainty. Why on Earth do some argue > about this still? > > Attached is a paper published in '92 by yours truly & Curt Covey in > Nature showing that for consistency with paleoclimate data from two very > different periods, the mid-Cretaceous and the last glacial maximum -- NOT > from climate models or the climate-change-irrelevant ERBE data in Lindzen's > latest GRL piece -- Lindzen's weak climate sensitivity estimate of 0.5 > degrees Celsius warming for CO2 doubling must be wrong. Past climate changes > are major constraints. We find a pale-ocalibrated climate sensitivity of ~ > 2.5 degrees Celsius including the usual fast feedbacks -- consistent with > the 3 degrees or so cited by Jim Hansen, a value roughly doubled in > glacial-interglacial transitions involving "slow" CO2 feedbacks; most likely > from ocean carbon pumps [see, e.g., Volk, T., and M.I. Hoffert (1985) > "Ocean Carbon Pumps: Analysis of Relative Strengths and Efficiencies in > Ocean-Driven Atmospheric CO2 Changes." In E.T. Sundquist and W.S. Broecker > (Eds.)* The Carbon Cycle and Atmospheric CO2: Natural Variations Archean > to Present*, pp. 99-110, Geophysical Monograph 32, American Geophysical > Union, Washington, DC.]. > > Were climate as insensitive as Lindzen claims, it would be virtually > impossible to explain the large warming (e.g., the mid- Cretaceous, the > Paleocene-Eocene Thermal Maximum, etc.) and cooling (e.g., the > quasi-periodic hundred-thousand-or-so year long glaciations of the past few > million years) excursions of past climates relative to now. This should > have, on the face of it, resolved the issue for good. Lindzen's > unwillingness and inability to deal with this in his response to us in*Nature > * (also attached) has, for most knowledgeable climate researchers, settled > the issue. > > But for ideologues with minds made up facts too distracting to their > conclusions are best ignored. Admittedly, Dick Lindzen is highly intelligent > and ingenious in constructing arguments reinforcing his self-delusion. But > in the end you can't fool Mother Nature. That's not even the worst of it. > The worst is that in the global warming "debate" still going on, issues > resolved decades ago, particularly the physics, logic and data supporting > humankind's global warming from the fossil fuel greenhouse, all of which > were legitimately & vigorously debated at the time in hundreds of > peer-reviewed papers, are, whether from scientific illiteracy or sheer > intellectual laziness, totally IGNORED by present-day climate change > deniers. You don't believe fossil fuel burning has, is, and will > increasingly cause global warming, by an amount which we can estimate pretty > well within some uncertainly bounds, which we can also estimate pretty > well? You want to argue that? Fine. But if you want scientific credibility > you should at least engage peer-reviewed climate science that's been out > there for decades. > > Have fun, > > Marty Hoffert > > [snip the cartoon] > > > I bring to your attention the following: > > > the latest Lindzen paper > > http://www.drroyspencer.com/Lindzen-and-Choi-GRL-2009.pdf Be warned this > is > > tough. > > A simple to understand over review is: > Climate Sensitivity Estimates: Heading Down, Way Down? (Richard Lindzen's > New Paper) by Chip Knappenberger > > Using the same climate science; (I am ultra kind to call it science, it is > not yet mature enough) Lindzen concludes that Hansen et al estimate a > sensitivity that is almost 10X too large; Lindzen claims a delta of +0.5 C > for CO2 concentration doubling. > > You are all intelligent fair human beings, but you carry your biases on > your > sleeves. Would you bet your reputation or your retirement funds on either > of > these gentlemen being right. I doubt it. I suggest you all might temper the > discussion here with a grain of uncertainty. However, I also understand > that > if Lindzen is right, geoengineering becomes of virtually no importance. > > -----Original Message----- > > From: [email protected] > [mailto:[email protected] <[email protected]>] > On Behalf Of John Gorman > Sent: Sunday, November 01, 2009 12:34 PM > To: [email protected]; [email protected] > Cc: geoengineering > Subject: [geo] Re: Jamais Cascio-- on the problematic idea of 350 > > > I think the point is as follows: > > If we fix the CO2 at say 2 times preindustrial then the net forcing will > not > instantly get the global temperatures up to their long term steady state. > The oceans will lag. > > It sounds as if you are thinking of the net forcing which has resulted in > the temperature rise for the land has gone away-but it hasn't in the case > you have quoted. The CO2 level will stay constant for centuries even if all > emissions stop today. > > There was a very recent post or link on this recently which quoted 3deg for > doubling of CO2 in the short term but 6deg long term. This link, which I > cant find just now, used data from some much earlier time when the doubling > had already resulted in almost completely ice free poles. This is > presumably > what will happen now if we get to doubling and don't do something about it. > The Arctic is melting now. It isn't going to stop melting just because we > stop increasing the temperature. So there is a temperature inertia or lag > due to ocean heat capacity but an even bigger one due to ice cap latent > heat of melting. > > john gorman > > > ----- Original Message ----- > From: "John Nissen" <[email protected]> <[email protected]> > To: <[email protected]> <[email protected]> > Cc: "geoengineering" > <[email protected]><[email protected]> > Sent: Sunday, November 01, 2009 1:56 PM > Subject: [geo] Re: Jamais Cascio-- on the problematic idea of 350 > > > Hi all, > > > > Jamais Cascio is described as an "environmental futurist" and has > > written a book with the title "Hacking the Earth" [1]. It's great that > > somebody is pointing out the implication of the 350 ppm target! > > > > But he shares a very common scientific belief which I do not understand: > > > > "(Even more troubling: even if we stopped all anthropogenic carbon > > sources immediately, we'd still see continued warming for at least > > decades, possibly longer, simply from the thermal inertia of the > > oceans. Absent a radical step, we're guaranteed to see at least > > another degree or two of warming, no matter what we do.)" > > > > > > With global warming, the land and atmosphere warm faster than the > > oceans. If emissions stopped overnight, the oceans would still be > > warming up, thus cooling the atmosphere, rather than warming it. Thus > > the thermal inertia of the oceans would drag down the global temperature > > (mean of surface temperatures over the globe). Am I right? > > > > Cheers, > > > > John > > > > [1] http://www.lulu.com/content/6048806 > > > > -- > > > > Dan Whaley wrote: > >> You will appreciate this one Greg... > >> > >> http://www.fastcompany.com/blog/jamais-cascio/open-future/350 > >> > >> 350 > >> BY Jamais CascioTue Oct 27, 2009 at 2:55 PM > >> 350 parts-per-million is the carbon limit. How will we get back there? > >> > >> 350.org > >> > >> It may be odd to focus a political movement on a relatively obscure > >> bit of science, but a world-wide push to limit concentration of > >> atmospheric carbon dioxide to 350 parts-per-million made a big splash > >> last week, with rallies and gatherings all over the planet focusing on > >> drilling this number into the public consciousness. The number comes > > >> from work done by (among others) NASA's James Hansen, looking for > >> potential climate "tipping points." 350ppm for CO2 is a safe limit-- > >> get too much beyond it, and the dangers multiply. > >> > >> It's an audacious goal, for reasons of both communication and science. > >> > >> In terms of communication, while a simple meme like "350" or "350ppm" > >> fits nicely on protest signs and bumper stickers, it's a term without > >> much context for the vast majority of the populace. In and of itself, > >> that's not a problem; however, it can make a visceral connection to > >> the concept more difficult. Activists adopting the 350 meme will need > >> to match rhetoric with education, to make the number meaningful. > >> Again, not impossible, but likely an ongoing challenge. > >> > >> The scientific audacity with the 350 meme comes from a single, simple > >> fact: current concentration of atmospheric CO2 is roughly 385ppm. That > >> is, we already exceed the 350 limit, and most climate scientists say > >> we'll be hard-pressed to keep from going over 450ppm by the middle of > >> the century. And carbon dioxide takes centuries to cycle out of the > >> atmosphere--even if we stopped all anthropogenic sources of CO2 right > >> this minute, we'd still see too-high concentrations for years to come. > >> > >> (Even more troubling: even if we stopped all anthropogenic carbon > >> sources immediately, we'd still see continued warming for at least > >> decades, possibly longer, simply from the thermal inertia of the > >> oceans. Absent a radical step, we're guaranteed to see at least > >> another degree or two of warming, no matter what we do.) > >> > >> If this sounds like I think the 350 movement is a bad idea... I don't. > >> I rather like the simplicity of the meme, and the target is--if > >> difficult--smart. It's not saying "let's keep things from getting too > >> much worse," it's saying "let's make things better." That's the kind > >> of goal I like. > >> > >> But getting back to 350ppm requires more than a rapid cessation of > >> anthropogenic sources of atmospheric carbon. It requires an > >> acceleration of the processes that cycle atmospheric CO2. Planting > > >> trees is an obvious step, but it's slow and actually doesn't do enough > >> alone. We'll also need to bring in more advanced carbon sequestration > >> techniques, such as bio-char. The combination of the two would likely > >> bring down atmospheric carbon levels, given enough time. > >> > >> Unfortunately, we may not have enough time. > >> > >> If efforts to eliminate carbon emissions continue to happen at a pace > >> most generously described as "leisurely," we will almost certainly > >> face a situation where we approach and even pass critical tipping > >> point concentrations. Ocean thermal inertia means that climate > >> benefits from emission cessation won't be seen for decades. There's a > >> very real scenario where finally get it right, both cutting out > >> anthropogenic emissions and sequestering megatons of carbon via plants > >> and bio-char ... and still face terrible environmental consequences, > >> simply because we didn't act fast enough. > >> > >> That's where we start to talk about much more radical, and potentially > >> dangerous, steps. Geoengineering to hold temperatures down is one; to > >> meet the 350ppm goal, we will likely also start looking at large-scale > >> methods to sequester carbon, such as with triggered algae blooms. > >> > >> 350ppm is an audacious goal, but one worth striving for. But its > >> challenge comes not just in the effort to eliminate anthropogenic > >> carbon emissions around the world--a massive endeavor alone--but also > >> in figuring out how to remove the extra carbon already there. I hope > >> that the 350 leaders have thought through the implications of what > >> that means. > >> > >> [Images: "Organized Spelling B" by Wade in Da Water on Flickr, > >> Creative Commons Licensed; "Summer Bloom in the Baltic Sea" by NASA > >> Visible Earth] > > > --~--~---------~--~----~------------~-------~--~----~ > You received this message because you are subscribed to the Google Groups > "geoengineering" group. > To post to this group, send email to [email protected] > To unsubscribe from this group, send email to > [email protected] > For more options, visit this group at > http://groups.google.com/group/geoengineering?hl=en > -~----------~----~----~----~------~----~------~--~--- > > -- > You received this message because you are subscribed to the Google Groups > "geoengineering" group. > To post to this group, send email to [email protected]. > To unsubscribe from this group, send email to > [email protected]. > For more options, visit this group at > http://groups.google.com/group/geoengineering?hl=en. > -- You received this message because you are subscribed to the Google Groups "geoengineering" group. 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