Comments 1 to 29:

1. JosHagelaars at 04:11 AM on 16 July, 2011
   It turns out that amount of weathering is sufficient to remove all CO2 from the atmosphere in 3500 years. Faster than I expected given the amount of carbon in the atmosphere. Do you have a reference or a link regarding this calculation, so I could read more about it ? Is the Urey reaction treated in another post ? Thanks in advance.
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2. Composer99 at 06:57 AM on 16 July, 2011
   So calcification releases CO₂ into the ocean (and eventually the atmosphere), while weathering removes CO₂ from the atmosphere and eventually deposits it into the oceans. I assume these two processes occur at dissimilar rates, since volcanism also releases CO₂ into the atmosphere, and the volcano-weathering interaction is Richard Alley's famous CO₂ thermostat.
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3. Patrick 027 at 08:25 AM on 16 July, 2011
   Re Composer99 - weathering also refers to the weathering of silicate rocks, such as CaSiO3. CO2 + CaSiO3 = CaCO3 + SiO2 (weathering + carbonate mineral formation, example of a net reaction) In intermediate steps additional CO2 can be taken up by the water which is then released upon formation of CaCO3, but the net reaction has a net uptake of CO2. This can balance geologic emission. Actually geologic emission includes some oxidation of organic C in rocks, and geologic sequestration includes some organic C burial. Meanwhile, carbonate minerals, so far as I know, become thermodynamically unstable at sufficiently high temperatures, and so their geological processing under some conditions should be able to feed the geologic emission of CO2. I think geologic emission may be somewhere around 0.2 Gt C per year, so this would replace an atmosphere's worth of CO2 in 3500 years if the atmosphere had 700 Gt of CO2. (I'm unsure about how much geologic emission is directly into the oceans, though, but that would still require some additional geochemical process in order for that CO2 to not be released into the air.)
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4. Patrick 027 at 08:28 AM on 16 July, 2011
   ... of course, if there were no weathering, CO2 would build up in the atmosphere and ocean, as higher CO2 partial pressure would increase the equilibrium C content of the ocean. So it would take longer than 3500 years for geologic emission at 0.2 Gt C /year to actually ~double atmospheric CO2.
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5. Doug Mackie at 09:07 AM on 16 July, 2011
   @JosHag: I am not trying to be flippant (when I am trying it is obvious). However, the very purpose of the Skeptical Science site and the purpose of this series of posts is to encourage people to think for themselves. A lot of what we have written so far does involve a little chemistry. We appreciate that not everyone has had this training so we have tried to keep it simple. However, this calculation involves no chemistry at all so we think it would be worthwhile having a go yourself. The calculation has been done carefully but in this case a first approximation or speherical cow calculation will give you an answer with the right order of magnitude. To do this calculation as a first approximation you need to know four things: 1) Equations 1 & 4 2) World annual river flow 3) World annual river bicarbonate concentration 4) Amount of carbon in the atmosphere. Have a go yourself and come back if you can't do it.
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6. Composer99 at 02:35 AM on 17 July, 2011
   Thanks for clearing that up, Patrick.
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7. Bob Lacatena at 04:16 AM on 17 July, 2011
   Doug:

   The oceans are not full of salts (NaCl and/or carbonate species etc).

   Is the word "not" supposed to be there?
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   Moderator Response: The 'not' is correct. You can take seawater and add more salt to it. Therefore seawater is not full or saturated with salts. But good point about language use and that bullet point changed. Thanks. Doug.
8. JosHagelaars at 19:09 PM on 17 July, 2011
   @Doug Mackie 5 I got the message. I just presumed there were dissolution rates involved. Sorry about that, next time I will give it some more thoughts before I pose a quick question. The chemistry is no problem though and I'm very interested in the topic. I had never heard of a spherical cow before, but this cow calculation gave me 4035 years to get rid of 3050 Gt CO2.
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9. Doug Mackie at 20:48 PM on 17 July, 2011
   JosHag @8: Fantastic! Well done. Enrio Fermi was an early pioneer of the spherical cow approach with his famous piano tuners problem.
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10. Steve Brown at 03:49 AM on 18 July, 2011
    The CO2 drawdown from the weathering of the Himalayas is thought to have been the main contributing factor to the cooling trend throughout the Cenozoic and the shift to the Great Ice Age thats been dominating the Earth's climate for the past 2.5 million years.
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11. Steve Brown at 03:57 AM on 18 July, 2011
    ...Sorry, I should have mentioned it's the weathering of the Tibetan Plateau and not just the Himalayas!
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12. Keith Hunter at 20:13 PM on 18 July, 2011
    Patrick 027 is quite correct to point out the importance of silicate weathering. In this post we were trying to focus on the CaCO3 cycle as a simplification. But Patrick is quite correct in pointing out anomalies in which weathering loss off CO2 is balanced by volcanic emissions. However these do not substantially change the picture we have painted. Also, we will deall with this in a future post.
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13. Arkadiusz Semczyszak at 21:38 PM on 18 July, 2011
    I hope that in the next post will discuss the formation of ooliths (= ooids) ...
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14. Patrick 027 at 04:08 AM on 19 July, 2011
    PS the thermodynamic equilibria for various carbonate-silicate reations shown in fig 1 of http://www.pnas.org/content/98/7/3666.full "Initiation of clement surface conditions on the earliest Earth" N. H. Sleep, K. Zahnle, P. S. Neuhoff (not entirely on topic but it is related to chemical weathering) (PS note the temperature plotted as a function of atmospheric CO2 - I'm pretty sure this includes the effects of the faint young Sun.) C&P w/ formatting change, the reactions shown: 1. "leonhardite + albite + CO2 = calcite + paragonite + 4 quartz + 2.5 H2O" 2. "leonhardite + CO2 = calcite + kaolinite + 2 quartz + 1.5 H2O" 3. "clinochlore-14A + 5 calcite + 5 CO2 = 5 dolomite + kaolinite + quartz + 2 H2O" 4. "clinochlore-14A + 5 CO2 = 5 magnesite + kaolinite + quartz + 2 H2O" 5. "daphnite-14A + 5 CO2 = 5 siderite + kaolinite + quartz + 2H2O" 6. "2 albite + CO2 = 3 thermonatrite + kaolinite + 4 quartz " 7. "paragonite + CO2 + 5 H2O = thermonatrite + 3 kaolinite" mineral formulas given in table 1 http://www.pnas.org/content/98/7/3666/T1.expansion.html , and references are identified for thermodynamic data.
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15. Paul W at 18:11 PM on 20 July, 2011
    To extend the discussion about silicates by Patrick 027 to geo-engineering. I'm assuming that calcium and the carbon cycle are the topic here. One of the more natural methods of geo-engineering is to grind calcium silicate containing igneous rocks to a fine powder and disperse them over the oceans to help counter ocean acidification and aid in the draw down of CO2 from the air. The calcium silicate dissolves and the added calcium concentration helps maintain aragonite super-saturation and increase CO2 deposition as CaCO3 and also raises ocean pH. This is one of the two ethical methods of geo-engineering (the other being bio-char addition to farms soil) that have a lower chance of really messing up the planet. I've noticed in trying to discuss these points with environmentalist (I'm a chemist so I like discussing something I'm trained in) I find that all have so far have been convinced that geo-engineering is about sulphur burning and the sky no longer being blue. With so much emotion that geo-engineering is no longer wanted to be talked about. I've noticed that the fuel levy on petrol and diesel is higher than the cost of making bio-char from garden waist. I don't know what the cost of the grinding of igneous rocks is but I thought it worth discussing the option of pushing for liquid fuel to be offset by one of these two methods before sale. Starting to use CO2 sequestration now instead of latter makes some sense to me. Especially if it's not cost prohibitive and we have a tax that could pay for it. Oh by the way I don't think doing this for coal fired power stations is worth doing. I'd rather replace them.
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16. Doug Mackie at 20:40 PM on 20 July, 2011
    Paul W, We are chemists also and it pains us to point out that your idea has not been thought through and is distressingly commonplace amongst chemists. Firstly, as a chemist, you are surely aware that as we noted in post 3, surface seawater is supersaturated in CaCO₃. Secondly, please consider the response (repeated below) we made to a similar comment to post 3 here. The idea was explored with respect to limestone and what is true of adding calcium carbonate also applies to adding calcium silicate. Harvey L.D.D. (2008) Mitigating the atmospheric CO2 increase and ocean acidification by adding limestone powder to upwelling regions. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, C04028, 2008. abstract. The conclusion:

    Geographically optimal application of 4 billion t of CaCO3 a−1 (0.48 Gt C a−1) could induce absorption of atmospheric CO2 at a rate of 600 Mt CO2 a−1 after 50 years, 900 Mt CO2 a−1 after 100 years, and 1050 Mt CO2 a−1 after 200 years.

    That is, a fleet of tankers dumping 4 billion tons of powdered limestone per year could be sucking up 600 million tons of CO2 per year after 50 years. A slight problem is that current emissions are about 30,000 million tons of CO2. Yes, current emissions are 50x the amount that would be being sequestered by 2100 – and your guess is as good as mine for what emissions will be in 2100. Every little bit helps? Perhaps. But mining, crushing and transporting the limestone might be a little carbon intensive. Harvey is not seriously proposing this as a solution. He goes through those calculations too, as he says to show that:

    The calculations presented here serve to illustrate the enormity of the task of even partially reversing the acidification of the oceans that is yet to occur under even the most optimistic scenarios concerning reductions in CO2 emissions. The task is not only enormous but would need to continue for several 100 years. These calculations also underline the fact that in the absence of stringent reductions in CO2 emissions, efforts to reduce adverse impacts on ocean chemistry will be ineffective.

    Harvey goes on to say that if emissions have dropped to zero by 2100:

    then application of limestone at a rate of 4 Gt a_1 (0.48 Gt C a_1) beginning in 2020 serves to restore about 20% of the difference between the minimum pH and preindustrial pH by 2200 and restores about 40% of the difference by 2500, with the same benefits for the degree of supersaturation with respect to calcite.

    Yes quite. If emissions have dropped to zero by 2100.
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17. Paul W at 01:03 AM on 22 July, 2011
    Doug Mackie@16 thank for your thoughts. I agree with you. I would not think of it as a solution but of a nightmare that becomes real at some point. My Ph D topic thirty years ago was hydro treatment of heavy petroleum oils (the clean up step that allows heavy oils to be processed in current refineries). After 6 years I resigned and left that industry not wanting to be part of "baking the planet". I am heartened by the increased numbers opposing the use of fossil fuels particularly the most CO2 intensive alternatives. If I had it my way I'd want coal use to stop ASAP and other fossil fuels to only be used if carbon was put into the ground to match their extraction. Discussion of the needed geo-engineering required to return the planet to below 350 ppm CO2 in air is sobering as it is an extreme task as you rightly point out. I do expect that we will be forced to it (for a very long time) if politics don't change promptly. The discussion of what recovery processes will be needed for the ocean is also sobering. Since aragonite life forms are possibly the most vulnerable, the chemistry of ocean remediation is interesting. There is a slight reduction in the volume of calcium silicate over carbonate needed but it's still profoundly large. James Hansen made a quick reference to grinding igneous rocks in a talk. I've also been considering the work ability of it. You have been focusing on carbonate chemistry but the other part of the calcium carbonate solubility product calcium ions is worth a mention. In my readings so far I have not seen any discussion of Ca2+ concentrations. (I've not yet been looking for it) I wonder what the historical ocean Ca2+ concentrations have been over deep time. I've been curious about the [Ca2+] levels at paleo thermal maximums. I appreciate your work on this topic by the way. OA has been under discussed and is very concerning.
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18. Doug Mackie at 08:50 AM on 22 July, 2011
    Supply of calcium is not an issue. [Ca²⁺] is well known and changes negligibly with depth and is roughly 0.010 mol kg^-1. K'_s varies with both temperature and salinity but for example at 35S and 0^oC K'_s for calcite is about 4.3 x10^-7 and for aragonite is about 6.8 x10^-7. Clearly carbonate dominates the solubility of CaCO₃ in the ocean.
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19. Paul W at 12:53 PM on 22 July, 2011
    Thanks for your extra information. That resolves my question about calcium levels in todays oceans. There is still some un answered questions in my mind. Is the possible change in the pattern of weathering of rocks that occurs with higher CO2 levels in air an allie, enemy or not important? A change in the ratio of calcium silicate and calcium carbonate weathering is a possible positive feedback mechanism. You have focused on carbonate weathering but not silicate weathering. As rain becomes more acid for what ever reason I would expect a change in the ratio of weathering of these two minerals. i would expect more calcium carbonate to dissolve in rain that was more acid. This may not be that simple as land forms also play a role as when the Himalaya's weathered to allow the glacial/interglacial cycle to begin. As silicate precipitation will lead to pH increase which lead to bicarbonate/carbonate level changes it is also a factor. I can't tell if this is significant or not. Is the silicate cycle to some extent connected with the carbonate chemistry in the ocean? The reference (Harvey L.D.D. (2008) Mitigating the atmospheric CO2 increase and ocean acidification by adding limestone powder to upwelling regions.) does point to a limit in the use of ground limestone dissolved in ocean upwelling. It does not deal with dissolution in fresh water rivers or other outfalls which do not face that limit or a preference for silicate over carbonate. As the article points out carbonate is a very inefficient base for changing OA. In aquariums the addition of ground calcium carbonate to raise pH in the short term back fires as the pKa ratios of the dominant buffers have been moved away from 8.3 to 7.6 by the addition and so in the longer term acidification is not remediated. The correct balance of ionic species to give correct pKa (~ 8.3) is needed as well as base to correct aquarium pH not just simple addition of a base to raise pH. (Aquariums become more acid due to nitration and food breakdown products which is different from OA caused by increased CO2 in the air. But the effect of ignoring the existing buffers and there average pKa is interesting) Once a non carbon power source is used a possible method of remediation is less problematic. But as with the aquarium example the correct material will work, other less well designed interventions will just make it worse eventually. Again I'm not wanting to leave room open for fossil fuel use to be justified but to get clearer about possible workable remediation. Our fossil fuel use over the last 150 years has over whelmed the weathering effect that is now much slower. So my interest in speeding up or mimicking weathering comes from that.
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20. Doug Mackie at 14:36 PM on 22 July, 2011
    Paul W, we are glad to see you are paying attention. Many of the things you ask are dealt with in the rest of the series. However one point may help you right now: The thing about future weathering is that the pH of rain just won't change much under realistic future atmospheric pCO₂. As a chemist you will be able to approximate the answer quite easily. You will need to the Henry's law coefficient for CO₂ in freshwater (see next post) and K₁ for H₂CO₃ in freshwater. (Or include K₂ for an exact solution). This change in rainwater pH will make only a slight difference to weathering that will take a long time (geologically long) to make a difference to the ocean carbon speciation. It is certainly possible to rapidly manipulate the chemistry of an aquarium. Then again, if it all goes badly it is possible to empty the aquarium and refill it. We suggest that the time and energy constraints on mining, grinding and dispersing any mineral precludes such an option as a realistic remediation for the ocean.
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21. Paul W at 16:05 PM on 22 July, 2011
    I agree with the lack of pH change from CO2 the other concern was with desperate measures like SO2 injected high into the sky to lower solar insolation. There may be a back fire with the weathering of rocks from the sulphur cycle increasing. Still the amount of sulphur will be small compared to the cooling effect. This comes back to my preferred OA remediation, (after ASAP end to fossil fuel use) charing of the organic waste stream (gardens, farms) and the build up of soil microbes as a result of the extra charcoal in the soil. Since about 60 giga tonnes is turning each year in the carbon cycle that is part of land based life, the task is not small but with out the above hazards. I wonder if we will be using waste heat from bio-char ovens to get warm in the coming generations? There is other buffers not just the ocean that will tend to unload CO2 back into the air after a reduction of the air born CO2 level begins. More reason to try and end fossil fuel use ASAP. Apart from that it's still a Gordian knot. I like to have an end point in a long journey like investigating ocean chemistry. :-)
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22. Doug Mackie at 08:02 AM on 23 July, 2011
    Hi Paul, I see that Doug (comment 20) suggested you calculate the pH of rainwater in order to answer your question about the change in rainwater pH (comment 19). How did you go with this? You are correct - biochar soil remediation may help reduce atmospheric CO₂. However, it is unlikely that the carbon sequestration would be any faster than the 150 years it has taken to put the CO₂ into the atmosphere. This is unlikely to reverse ocean acidification (the topic of these posts) in any remotely useful time frame.
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23. scaddenp at 15:41 PM on 23 July, 2011
    Would anyone who not a NZer spot the referencing in the title of this part? (I assume its a take on "always take the weather"?)
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24. Doug Mackie at 16:27 PM on 23 July, 2011
    Are you referring to this "Australian" band? What kind of dirty creature would stoop to that sort of cultural appropriation? It makes me see red 8-)
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25. Tom Curtis at 16:29 PM on 23 July, 2011
    scaddenp @23, Crowded House was an Australian band, formed in Melbourne, no less. Granted the genius behind it was a Kiwi.
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26. Keith Hunter at 14:49 PM on 27 July, 2011
    Many kiwis consider Crowded House to be a reincarnation of the brilliant Split Enz, a band of obvious Kiwi origins. Anyway, what about Phar lap? And the pavlova?
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27. rjowens at 16:29 PM on 24 August, 2011
    @Paul W and @Doug Mackie Don't forget that weathering rates respond to changes in vegetation type & cover("bioogical enhanced weathering"), and the hydrological cycle - which will change due to increasing global temperatures at higher CO2. These changes will increase weathering rates, CO2 consumption and the supply of alkalinity to the ocean.
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28. Doug Mackie at 14:33 PM on 25 August, 2011
    @rjowens: Please clarify your point. From the post above:

    But, as we shall see in later posts, when atmospheric CO2 is high then more acidic rain causes more weathering and that consumes CO2 to lower the atmospheric CO2. Only problem is that this happens over a geological time scale.

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29. Doug Mackie at 14:33 PM on 25 August, 2011

    second summary post

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