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How Many Cans?

Filed under: — david @ 22 March 2014

XKCD, the brilliant and hilarious on-line comic, attempts to answer the question

How much CO2 is contained in the world’s stock of bottled fizzy drinks? How much soda would be needed to bring atmospheric CO2 back to preindustrial levels?

The answer is, enough to cover the Earth with 10 layers of soda cans. However, the comic misses a factor of about two, which would arise from the ocean. The oceans have been taking up carbon throughout the industrial era, as have some parts of the land surface biosphere. The ocean contains about half of the carbon we’ve ever released from fossil fuels. We’ve also cut down a lot of trees, which has been more-or-less compensated for by uptake into other parts of the land biosphere. So as a fraction of our total carbon footprint (fuels + trees) the oceans contain about a third.

At any rate, the oceans are acting as a CO2 buffer, meaning that it’s absorbing CO2 as it tries to limit the change to the atmospheric concentration. If we suddenly pulled atmospheric CO2 back down to 280 ppm (by putting it all in cans of soda perhaps), the oceans would work in the opposite direction, to buffer our present-day higher concentration by giving up CO2. The land biosphere is kind of a loose cannon in the carbon cycle, hard to predict what it will do.

Ten layers of soda cans covering the whole earth sounds like a lot. But most of a soda can is soda, rather than CO2. Here’s another statistic: If the CO2 in the atmosphere were to freeze out as dry ice depositing on the ground, the dry ice layer would only be about 7 millimeters thick. I guess cans of soda pop might not be the most efficient or economical means of CO2 sequestration. For a better option, look to saline aquifers, which are porous geological formations containing salty water that no one would want to drink or irrigate with anyway. CO2 at high pressure forms a liquid, then ultimately reacts with igneous rocks to form CaCO3.

Further Reading

Tans, Pieter. An accounting of the observed increase in oceanic and atmospheric CO2 and
an outlook for the Future. Oceanography 22(4) 26-35, 2009

Carbon dioxide capture and storage IPCC Report, 2005


23 Responses to “How Many Cans?”

  1. 1
    Andrew says:

    I think the most compact reasonable form of carbon would be a block of graphite. If I didn’t slip any zeros, a 1km cube of graphite is about 2.26 Gigatons of Carbon, which would use about 7.5 Gigatons of CO2 to create. Well, just make one of these every year or so and we’re done. Graphite is pretty stable, so we can just put it um, over there somewhere.

  2. 2
    Fred Magyar says:

    Perhaps the climate scientists should team up with Artist and photographer Christopher Jordan and come up with a really good image to get the message across.

    Here’s an example of his work:

    Running the Numbers II: Portraits of global mass culture
    (2009 – Current)

    http://www.chrisjordan.com/gallery/rtn2/#insatiable

  3. 3
    Russell says:

    If you want to make the CO2 problem concrete, skip the fizz and go directly to the solid state.

  4. 4
    Steve Fish says:

    To continue this fun. I think that a pop can contains something over two volumes (of the liquid) of CO2. So, considering that the can is a cylinder, assume two volumes. Cans are about 5in tall so 10 layers X 2 for the oceans X 2 for the CO2 = 16ft 8in or a little over 5 meters at sea level. So how far into this layer can IR penetrate before photons interact with a molecule of CO2? What is the probability that a photon could pass clear through?

    Steve

  5. 5
    john byatt says:

    Any drinking water left after that?

  6. 6
    Andrew says:

    If I have managed the zeros correctly, a cubic kilometer of graphite is 2.66 Gigatons, which would correspond to about 10 Gigatons of carbon dioxide. The earth’s atmosphere is about 20% oxygen, so the extra 27 Gigatons of oxygen can presumably be safely vented. Thus, all that is required each year is to extract a cubic kilometer of graphite from the atmosphere; there is no need to cover the Earth in soft drinks.

    You can put the cube, um, over there.

  7. 7
    Steve Metzler says:

    Any drinking water left after that?

    Realists. There’s always one. Pfft :-)

  8. 8
    paulina says:

    @fred #1 I love that idea!

    Another idea (for a school art-science project, for instance): collecting empty cans and combining them (before eventually recycling them) in a temporary earthwork, 20 layers high (as per the RC amendment), making visible a fragment of the xkcd soda sequestration land-cover.

  9. 9
    Dale Power says:

    I don’t know that this is a helpful image right now. Okay, I’m not a climate scientist, and I think I do get what is being said, but giving deniers a way to show that there is almost no CO2 in the atmosphere is a poor plan! Really, I read this with my mouth hanging half open, wondering if I hit the wrong blog today. One of the major problems that science is having at the moment is explaining to people predisposed to disbelieve what is being said that something as “minor” as CO2 is an actual issue as it is.

    Instead, why not stack those soda cans to the moon? People seem to like that image well enough.

    [Response: OTOH, something as "minor" as CO2 is much easier for humans to change. Or even someday clean up. The concentration is what it is -- I personally don't see that information as playing into climate change denial. David]

  10. 10

    Mm, a worldwide stack 20 soda cans high doesn’t seem all that ‘minor’ to me.

    Obviously, MM did V.

  11. 11
    Alastair McDonald says:

    OTOH, something as “minor” as CO2 is much easier for humans to change. Or even someday clean up.

    I don’t think so. It has taken two centuries of industrial production to put it there. Do you think it will be any easier to remove it?

  12. 12
    Alastair McDonald says:

    David,

    You say “OTOH, something as “minor” as CO2 is much easier for humans to change. Or even someday clean up.”

    I don’t think so!

    It has taken two centuries of industrial production to put it there. Do you really think it will be any easier to remove it?

    [Response: easier (than if the CO2 would fill up 10 layers of soda cans on the whole surface of the Earth)]

    Moreover, we have used up a large faction of the fossil fuels in producing that CO2. Where are we going to get the power to remove it? Don’t forget there is a second law of thermodynamics which says that entropy increases. How are you going to push all that entropy back in the can now it has been released?

  13. 13
    Laurie Dougherty says:

    I keep hoping someone will figure out how to extract carbon from CO2 in such a way as to produce more carbon fiber therefore bringing the price of carbon fiber bicycles down to what I can afford.

  14. 14
    john byatt says:

    Realists. There’s always one. Pfft :-)

    But I don’t like soda

  15. 15
    Russell says:

    David, while covering the whole 508 million km2 surface of the Earth remains a tall order, human action and agriculture has thus far physically altered or displaced fully half its land area. The industrial revoution has merely accelerated this transformation- with enough generations, even digging sticks and shepard’s crooks can transform continents.

  16. 16
    David Miller says:

    Russell, I love that idea. If only it didn’t sublimate so quickly it would make a better demonstration.

    20 tons would be a block of dry ice about 13 cubic meters; two high, two wide, and a bit over 3 deep; driving a car into would do quite a number on the car indeed.

  17. 17
    Russell says:

    David: the proposed workaround is to cover a hollow wooden step pyramid six shelves high with a facade of dry ice blocks like the one shown .

    In warm weather they should take about 72 hourd to sublime. This incidentally eliminates the hazards of structural collapse a solid pile of evaporating blocks would pose.

    Another art of the proposed installation is a thermal camera imaging how warm people dissappear when they go behind the infrared opaque plume of CO2 rich air on the pyramid’s lee side .

  18. 18
    Fred Magyar says:

    @ Laurie Dougherty,

    Nature already produce something that is a great carbon sink, is a fiber, has the tensile strength of steel and can be used to

    make a fantastic bicycle, it’s called bamboo. I have two friends in Brazil who already make this happen one is an agronomist who is a (specia-list) in bamboo plantations and has a business called Bambu Carbono Zero, the other is an industrial designer,his name is Flavio Deslandes and he produces Bambucicletas. I am currently building bamboo bicycles with his frames in South Florida.

    BTW could the moderators please do something about permitting the use of the word (S P E C I A L I S T) I’m really not trying to spam this site or sell drugs for erec-tile dysfunction >;-)

  19. 19
    G.R.L. Cowan says:

    I think the most compact reasonable form of carbon would be a block of graphite.

    That is not a reasonable form in this context.

    Moreover, we have used up a large faction of the fossil fuels in producing that CO2. Where are we going to get the power to remove it? Don’t forget there is a second law of thermodynamics which says that entropy increases. How are you going to push all that entropy back in the can now it has been released?

    There are two encouraging facts. One, enhanced weathering is an established way in which an additional increase in entropy can be driven by the removal of CO2 from the atmosphere. The CO2′s entropy decreases, but the removal releases heat, and this heat release, divided by the ~300-K temperature at which it occurs, amounts to an entropy increase that more than compensates.

    Two, the relatively small amount of energy it takes to catalyse this spontaneous process can be supplied by carbon-free energy that is enormously more abundant than fossil fuel energy.

    This enormous abundance is evident in the satellite view at http://goo.gl/maps/AZ7gK . It contains a tar-sands mine, easy to spot, and a zero-carbon fuel mine, larger in output than the tar operation, and impossible to spot.

    The most compact reasonable form to store carbon in, after extracting the excess from the atmosphere, is magnesite, MgCO3.

  20. 20
    Ben H. says:

    One thing that’s always bothered me about the sequestration concept is that what happens if the CO2 is suddenly released due to an accident of some sort? Isn’t there a danger of another Lake Nyos event (in 1986)? (Note that the CO2 released there had somehow been stored naturally.) Evidently, there was another event in Lake Monoun in 1984. I’m not saying that sequestration isn’t worth exploring, but I just think we ought to be cautious.

  21. 21
    Andrew says:

    Graphite is 6 times more carbon per unit weight than magnesite and nearly as dense; it’s was to synthesize at room temperature if you don’t mind squeezing it, uh, firmly, and can cope with the occasional unpredictable explosion….

    http://www.minsocam.org/ammin/AM38/AM38_50.pdf

  22. 22
    Russell says:

    21.

    To popularize carbon capture, government need only subsidize its conversion into covalent crystals large enough to cut and polish

  23. 23
    arch stanton says:

    Re: carbon sequestration –

    Sometimes I wonder if burying large quantities of “plastic” in landfills is such a bad thing.


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