Your article attempts to calculate the macro-economic costs of air capture by taking different estimates of the ‘individual’ costs from the literature. A question which would need to be addressed as well concern the energetic costs of the process: If capturing a ton of CO2 costs more energy than the production of that energy emits to the atmosphere, than it’s not a useful strategy to decrease the carbon loading of the atmosphere; to the contrary. (That would still be the case even if the energy comes from renewable sources; those energy sources could better be used to replace fossil energy than to capture carbon out of the air.)

There are slightly conflicting results out there regarding the energy costs of air capture, and of course it depends on the process used (and on how ‘inclusive’ one calculates the energy costs). According to the IPCC report the process currently produces as much CO2 from coal-generated energy usage as it strips from the air, so it is not (yet) at the stage of useful deployment [IPCC WG3, 2007]. Bachiocchi et al (2006) conclude that it is not (yet) energy efficient, while Zeman (2007) comes to the opposite conclusion. The recent Nature Climate Crunch special (2009) quoted some numbers from Roger Aines (Livermore Nat Lab) that enable a more macroscopic view: Taking 0.25 Gt carbon out of the air per year would cost 900,000 GWh of energy per year according to their assessment. That means that capturing 1 ton of carbon costs 3.6 MWh, or that 0.28 ton C is captured per MWh of invested electricity costs. (I’m assuming they’re talking about tons of C, not CO2, which would be a factor of 3.6 more.)

I compare that to the emission rate of electricity generation from http://www.eia.doe.gov/cneaf/electricity/page/co2_report/co2report.html. Here, the emissions from coal are roughly 2 pounds of CO2 per kWh, which equals 0.28 t C per MWh, the same as the energy costs of capturing the same CO2 (in line with the IPCC statement cited above).

That implies that if coal fired energy is used to capture CO2, we’re not changing the carbon content of the atmosphere by a single bit, and that if cleaner energy is used, we may as well use it to replace coal generated energy instead, since that would decrease the CO2 content of the atmosphere by the same amount as using it to capture CO2 from the air.

The bottom line is that the energy costs of air capture needs to be reduced before it could be considered to play a role in mitigating climate change. Now the expectation is of course for that to happen as a consequence of more R&D, and for the relative CO2 emissions (CO2/kWh) of energy generation (including coal fired power plants) to decrease.

I believe this to be relevant to your paper, and to any estimation of monetary costs of air capture, since the costs per ton of CO2 needs to include these energy costs. At the current break-even point (assuming the numbers above to be correct), these costs reach infinity. And the real monetary costs will not be nearly as low as found when only the captured CO2 is taken into account, and not the required CO2 from fuelling the process.

That said, if and when the energy costs are low enough, it may be a useful technology to use to help averting dangerous climate change.

Bart

David S. Goldberg, Taro Takahashi, and Angela L. Slagle
Carbon dioxide sequestration in deep-sea basalt
Proceedings of the National Academy of Sciences, 2008 105:9920-9925; doi:10.1073/pnas.0804397105

Do you reckon we could marry these two ideas? Can we think of a local CO2 sequestering plant that could either bottle CO2 (or CO2 rich air that is less expensive to produce) or pipe it to local greenhouse farms to boost yield? Of course this still “leaves” the carbon hanging around after you harvest, but that could be taken care of in some other way (?).

There are also other technical issues such as insulating the greenhouses.

Information may be obtained at:

NEGATIVE FOOTPRINT http://sites.google.com/site/negativefootprint/
Atmospheric Carbon Dioxide Removal:
http://sites.google.com/site/maketheworldgreenorg/
Electric Car: http://sites.google.com/site/jreed4301electriccar/

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VP6-4VKDH06-1&_user=918210&_coverDate=02%2F11%2F2009&_rdoc=2&_fmt=high&_orig=browse&_srch=doc-info(%23toc%236198%239999%23999999999%2399999%23FLA%23display%23Articles)&_cdi=6198&_sort=d&_docanchor=&_ct=33&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=768e34359f3f6782ea259894b0efc053

http://www.iht.com/articles/2009/01/30/america/forest.1-419181.php

I’ve just skimmed portions of David MacKay’s book. I agree with Len Ornstein that it seems wonderful. Especially so since it’s provided for free. (I’ll have to look for a “tip jar” tonight.)

One comment I have regarding MacKay’s Figure 31.6 (on “ocean nourishment”). He makes the 120 areas separated from one another (even though I know of no practical reason why they would need to be separated). This has the visual effect of making the total area seem bigger. Also, he cuts off the figure just at the limit of the areas of nourishment (not showing the rest of the Atlantic Ocean). This also has the effect of making the areas look larger.

His central point that the total ocean area involved is a significant fraction of the land area of a country is well-taken. But it’s also important to remember that the earth is more than 2/3rds ocean.

P.S. Just FYI, his calculation of the ocean area needing nourishment to remove the United Kingdom’s CO2 emissions is 120 areas, each 900 square kilometers. That’s a total of 108,000 square kilometers. The land area of the United Kingdom is approximately 240,000 square kilometers (or about twice as large). And the total area of the oceans of the world is 361,000,000 square kilometers…or approximately 3,300 times larger than the area needed to absorb the United Kingdom’s CO2 emissions.