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. 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'. 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. 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. Sincerely, -Ning Zeng On Sep 28, 11:13 am, Ken Caldeira <[email protected]> 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 [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.
