Andrew(s) and list Thanks very much for a very complete response. See below.
On Dec 21, 2013, at 11:49 AM, Andrew MacDougall <[email protected]> wrote: > Hello Dr. Larson, > > Thank you for your interest in my research. To answer your questions: > > “a) that only as much carbon can be removed from the atmosphere as was > inserted (no explanation on why this limit), and” > > This evidently was not clear enough in the paper but there is no limit to > how much carbon is removed from the atmosphere using CDR. The model is > prescribed the atmospheric CO2 concentration at a given date and diagnoses > net human emissions from the carbon cycle. The totals for removal are seen > in table 1. In all cases more carbon needed to be removed that had > originally been emitted to the atmosphere to return atmospheric CO2 to 280 > ppm. [RWLa: My apologies. I was stuck on the word “mirror”. And getting back exactly to 350 pp (not 280 ppm) levels. I did read Table 1, but didn’t express myself well. You did. My comment assumed that the predicted future arctic CO2 was similar to fossil CO2. > > "b) is to be taken out as a “mirror” of the way it was inserted (and > I am not understanding details of the mirror, but this seems highly > unrealistic), and" > > The mirror is an intentional simplification. There are off course an > infinite number of possible pathways to remove CO2. The path actually > taken would likely depend on technology, desirable cooling rate, and the > industrial capacity of a possibly damaged and impoverished civilization. I > selected mirrored scenarios because they are simple but less idealized > than either instant removal of CO2 or linear removal of CO2. [RWL: Understood and agreed. There has to be a realistic ramp-up (sigmoid?) phase that will be the hardest to justify. My motivation here was to say we are in an emergency situation and need to move as fast as possible. The simplification of mirroring makes the task look impossible to those who won’t read your paper carefully. > > "c) requires going back to 280 ppm (not the usual 350 ppm)" > > I initially did conduct unpublished model runs where I returned > atmospheric CO2 concentration to 350 ppm. However, I found the models to > difficult to evaluate as the climate was never in equilibrium with 350 > ppm. To evaluate the differences between the final climate and the > Holocene climate I needed to use the pre-industrial climate as a known > baseline. Also note that in the simulations that where published the > Greenland ice sheet does not restabilize until 280 ppm is reached > (although this could be a hysteretic effect). [RWL: Agreed that modeling the end game will not be easy. If we ever get serious about CDR, there will be more difficulty stopping gracefully than starting. But if we can stop fossil use, presumably we can start that up again relatively painlessly. I hope any others who have performed simulations like you will also comment on the best ways to model a close-out. It was the very far-off dates associated with 280 and mirroring that I was worried about. I am fearful of those objecting to CDR saying it is useless, when they see terms like the years 2800 and 3000. The fact that there are “infinite number of possible pathways” should be great news to doctoral candidates and faculty advisors. So yours is a helpful first step. > > " d) looking at very high out year peaks, that are inconsistent with > the concept of CDR." > > I do not see how high peaks are inconsistent with the concept of CDR. CDR > is a technology that can be deployed at whatever time that society deems > that it needs to be deployed (and the society has the resources to deploy > the technology). [RWLd1: I come from a position (like Jim Hansen, 350.org, etc) that we should have started long ago. I feel that the economics are so much better if we start early, that these larger RCP scenarios may imply to some that waiting is better. If the second is appreciably more costly than the lowest, then (to save modelers like you time and energy), we can concentrate on the lower. But I presume the higher numbered RCPs are not that much more difficult. I am just hoping to put most emphasis on the lower ones. If anyone sees any merit in waiting in the belief that the RCP 6.0 and 8.5 scenarios make sense, I hope to hear why. > Note also that the new IPCC scenarios the Representative Concentration > Pathways prescribe concentration pathways not emissions pathways. It is > possible the follow any of the RCPs while deploying technology to capture > a significant fraction of anthropogenic emissions. One could for example > remove emissions such that an emission total that would otherwise lead to > RCP 8.5 instead lead to RCP 6.0. [RWLd2: I guess I follow this. This is also saying that the fossil-phaseout pattern is not any easier to predict than the CDR introduction pathways (the options are doubly infinite). The words “mitigation” and “reduction” are needed - as well as “removal”. What is important to me is that there is recognition (which you have demonstrated) that our present situation is not at all “irreversible” - a term we hear too often. > > “ b) Use annual removals that emphasize speed and are reasonably least > cost - not a “mirror” of anything.” > > One must also consider the induced rate of atmospheric cooling. Much of > the damage from climate change comes from the “change”. Agriculture and > ecosystems will need time to readapt to the cooled climate. [RWLb'1: Andrew: I think you are here raising an important point - that we perhaps shouldn’t have too rapid a drawdown. You show changes as large as -20 Gt C/yr for the highest RCP. Should we worry if the lowest had that (almost unimaginable) rate - but for a much shorter period? I have been imagining a peak of -10 Gt C/yr for a biochar scenario with an average that is perhaps half as much. I have read nothing along this different “peak” line. > > “So, I hope that Dr. MacDougall or others will extend his analyses in > these directions.” > > At present I do not have any plans to continue research in this area. In > fact this paper evolved from a side projected focused on “can we fix this” > that was never approved by my committee. However, a graduate student > supervised by Dr. Karen Zickfeld of Simon Fraser University is working on > a similar project (with the same ESM) but focusing of economic modelling > of negative emission scenarios. [RWLb'2: I urge your committee to authorize more along these lines. And same for Dr. Zickfeld’s student. I was impressed that you had material on MOC(sv) and several other non-standard variables in Figure 2. The arctic ice values look like they are out of date (given September zeroing predicted only a few years hence). > > Once again thank you for your interest in my work and good luck developing > biochar tech. [RWLb’3: The biochar community urgently needs your help. Biochar is the only CDR approach with long-lived “knock-on” impacts that I am pretty sure your model is not capturing, but could. Example - how much increase was there in today's 2000- 2500 Gt of soil carbon in your model? Did the present day 60 Gt C/yr of terrestrial flow change at all? > > P.S. It is Mr. MacDougall. [RWL: So you received a deserved compliment. Keep up the good work - but lets go beyond the idea of “mirroring”. Ron > > Sincerely: > -Andrew MacDougall, MSc > PhD Candidate > University of Victoria > Victoria BC Canada > > > >> Andrew and list >> >> 1. I have been meaning to comment on this article by Dr. Andrew >> MacDougall, that you cited in October. I was reminded by it being >> referenced in the last Science issue (Dec. 6, p 1149) I received. The >> article is NOT behind a paywall. It is at: >> http://onlinelibrary.wiley.com/store/10.1002/2013GL057467/asset/grl51021.pdf?v=1&t=hp8v0lqy&s=27b2334b807e010482b6e4fa3f41a8703797473f >> <snip the remainder of mine as not now being needed> -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/geoengineering. For more options, visit https://groups.google.com/groups/opt_out.
