Prof. Zeng and list: 1. I concur with your objections. But I have another - which is that neither the present article or your own recent discussions on this list use the word "Biochar" - which I think has great promise, and certainly has received as much public notice as any of the other discussed options. My reason for preferring Biochar over burial (or combustion, or BECs, etc) are the outyear advantages to soil productivity after a single application. You are correct that less can go into the soil with Biochar than with burial. But that advantage is overcome when one captures (a carbon neutral advantage) the energy in the pyrolysis gases and when one counts hundreds of years of added sequestration when the biomass (in charcoal form) is placed in active soil. I do not understand why Biochar is not part of your (and the Caldeira--Keith) list of comparison topics.
2. Let's further explore the phrase: "it would take approximately 2.5 acres of crop land to remove the CO2 emission from just one U.S. resident". I like your argument some, but think more needs to be said. At [http://en.wikipedia.org/wiki/List_of_countries_by_carbon_dioxide_emissions_per_capita] we see about 20 tons CO2 per US capita per year, so Caldeira-Keith are thinking about 8 tons CO2 per acre-yr. Preferring metric and more biomass-oriented units (using factors of 3.67 tons CO2 per ton C and 2.47 acres per hectare and ignoring a ton/tonne difference) the 8 ton number turns into .88 tons C/ha-yr. Let's call it 1 ton C/ha-yr, since I am not sure where land disturbances fit in for Caldeira-Keith. This is certainly not the best that is being done around the world. In fact it is less than the world average NPP (60 Gt C/yr) divided by 13+ Gha [or about 4.5 tC/ha-yr}. So maybe Caldeira/Keith are restricting themselves to some sort of ag waste value. I do not believe we should be limiting ourselves to wastes or NPP at all when we are talking about bio-sequestration in a serious way. Sugar cane is sometimes quoted as up to 80 tons/ha-yr (same as 8 kg/m2-yr). I have seen future world average productivity values above 10 times the Caldeira-Keith value (as we start applying modern techniques to above average species in above average locations). I notice below that you also note a factor of ten discrepancy. 3. But also, let's consider other items in this calculation, such as whether we should be using 20 tons CO2 per capita per year and its 2.5 acre equivalent. Clearly the former number must decline a lot if we are serious about sequestration. There should be nothing magic about 20 tons CO2/capita per year. A more pertinent number comes from our per capita carbon legacy value and the time period over which we in the US should be removing that legacy atmospheric carbon. Suppose we assume for ease that we will shoot for the Caldeira-Keith target of 1 hectare per capita. Since the US has an area of about 1 gigahectare, and an average future population of 400 million, then we have available 2.5 hectares per capita - about 250% of the Caldeira-Keith value. If we can sequester an average of 1 ton carbon per hectare-yr, then (given the US land total) this is 1 gigaton C/yr. The world total might be ten times higher (given our roughly 8% US share of available global land). Like you, I don't know why this should be considered "impractical", as we would then be sequestering more than we are now emitting. 4. See a few further comments below. Ron ----- Original Message ----- From: "Ning Zeng" <z...@atmos.umd.edu> To: "geoengineering" <geoengineering@googlegroups.com> Sent: Wednesday, September 29, 2010 7:58:36 AM Subject: [geo] Re: The Need for Climate Engineering Research, in IS&T, by Caldeira and Keith Hi Ken, David: This is a really nice article! However, I take issue with a couple of statements on biological methods: 1. You said "biological systems are relatively inefficient in their ability to capture CO2. It is estimated that it would take approximately 2.5 acres of crop land to remove the CO2 emission from just one U.S. resident—an impractical requirement" I assume you are comparing with engineered chemical removal. But this is an unfair comparison because the two methods are limited by very different factors: biological by land availability, chemical methods by cost-effectiveness which we don't really know yet at large scale. Thus you are comparing the limitation of one method with the (presumed) strength of another method. Also, photosynthesis is not efficient at converting solar energy, but it is actually fairly efficient at capturing CO2, especially if you think plants are self- sustaining. [RWL: Agreed - and there are plenty of legal, energy, technical, and economic issues associated with placing CO2 underground. It shouldn't happen if biosequestration is cheaper and of similar magnitude. In addition, the transport of biomass to where both biopower is needed and deep underground sequestration is possible must eliminate a lot of the available biomass. Your burial approach and Biochar do not suffer from this limitation. Also combustion, BECS (and your approach) do not allow for some portion to go toward biofuels, whereas Biochar does, and will be needed. The fact that '2.5 acres of crop removes one US person's CO2 emission' does not automatically lead to the 'impractical' conclusion. If you consider that there are 50 million km2 (5 giga hectare) of forest land, at 1 tC/ha/y sequestration rate (10% of typical forest NPP), the forests can store 5 GtC/y, compared to the current 8 GtC/y fossil fuel emissions. Obviously, other practical factors will likely lower the number, but even if we can do 1 GtC/y, that would be substantial, and it's not 'impractical'. [RWL: Biochar, unlike your tree burial approach, can operate equally well with almost all ag, animal and urban wastes. I emphasize also that NPP is the wrong metric - much old-growth forest land has zero NPP. In addition, part of the needed sequestration can be accomplished through REDD+, with end-of-life service as input to Biochar (or your) operations. We should not concur that 1 Gt C/yr of sequestration is anywhere near an upper limit. ] 2. "however, current evidence suggests that it would make more environmental sense not to bury biomass but to use it in place of coal in electric power plants, which are notorious CO2 emitters. " As dissussed earlier in this group, I don't think we can conclude "to burn" is obviously better than 'to bury', or vice versa. Indeed, in last few years, US forest service as well as venture capitalists have tried to use forest biomass for fuel, and discovered that the cost of transportation and processing makes it uncompetitive in most cases. This of course does not mean it can not work at a sufficiently high carbon price. RWL: There is approximately as much current worldwide use of biomass for energy as from nuclear or hydropower. Biomass is by no means uneconomic when viewed globally . Here we should be talking of what is the least cost for providing a new service of CO2 removal; I think the answer will prove to be Biochar. With (the un-mentionned) Biochar, there is plentiful additional no-cost sequestration as microbe and fungus populations increase, much less new above and below ground biomass - for centuries or millenia. There are strong indications that much less nitrogen will need to be applied (carbon neutral side) and that much N2O release can be avoided (carbon negative side). I believe we will shortly be able to prove that these out-year impacts (for hundreds of years likely) will greatly exceed the first-year-only comparative values for any of the four Caldeira-Keith listed CDR options (including burial). But I agree with your basic statement - nothing has been yet proven for burn vs bury; I just ask that your and their comparison list be expanded to include Biochar. ] There is a tantalizing possibility that biomass 'storage' (burial or above-ground storage) can be done at relatively low cost now because it involves only harvest and storage. The stored wood can serve as a biomass/bioenergy reserve should other use such as direct burning or cellulosic ethanol becomes practical at large scale. [RWL: I concur that there is advantage in this "storage" aspect of your approach. But the same can be said of above-ground simple air storage for later use as Biochar. The disadvantage of temporary burial is in not gaining the big productivity advantage of using the char in the ground as soon as possible - to achieve the important out-year advantages. Both your burial and the Biochar schemes can fit well with using biomass as a dispatchable backup for solar and wind systems - but your scheme (unless used with Biochar) removes the longevity advantage you are claiming. You must given thought to the degradation of buried or stored biomass over time; I have not. Maybe not a problem, but must be less of a storage problem for Biochar options where storage should normally be less than a year. I hope we can continue this comparison of biosequestration CDR systems. I like your approach next-best to Biochar. I believe there is still plenty to talk about on this list - if we together have successfully argued against the 2.5 acre limitation argument claimed in the Caldeira-Keith article. And I hope to hear why the word Biochar is not mentioned along with the other bio options. Biochar is distinctly different from no-till, BECs, burial, and anything else bio-oriented - because of these continuing out-year advantages. (As proven by "Terra Preta" soils, which I saw up close near Manaus, Brazil less than two weeks ago. ] To those on this list who remember Peter Read - this too-long response is an attempt to keep his dream alive. Ron Sincerely, -Ning Zeng On Sep 28, 11:13 am, Ken Caldeira <kcalde...@carnegie.stanford.edu> wrote: > http://www.issues.org/27.1/caldeira.html > > *Issues in Science and Technology, Fall 2010** > * > > *The Need for Climate Engineering Research* > > KEN CALDEIRA > > DAVID W. KEITH > -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to geoengineer...@googlegroups.com. To unsubscribe from this group, send email to geoengineering+unsubscr...@googlegroups.com. 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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