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0 pointsphilipkglass7y ago0 comments

As btilly has mentioned a few times, ocean acidification is a major problem because atmospheric CO2 has risen so quickly. It takes a few thousand years for the atmosphere, the upper portion of the ocean, and the deep ocean to reach equilibrium. There is an especially elevated CO2 concentration and corresponding lowered pH in the upper layers because there hasn't been time to reach equilibrium with the larger volumes of water in the deep ocean. These upper layers of the ocean, sadly, are also where the vast majority of ocean life and photosynthetic activity exist.

Adding alkalinity to the oceans via accelerated silicate weathering can protect sea life against acidification. More alkalinity in the ocean also increases its absorption capacity for CO2, which reduces warming feedback effects caused by radiative forcing from CO2 in the atmosphere. The ocean contains ~60x as much inorganic carbon as the pre-industrial atmosphere; to draw down the content of the present atmosphere to pre-industrial levels, it would require increasing the dissolved inorganic carbon content of the oceans by less than 1%. Converting some of it to solid carbonate minerals is a nice-to-have but not especially critical, because average residence time of dissolved inorganic carbon is about 200,000 years, which will take us well beyond the age of fossil fuels.

https://www.soest.hawaii.edu/oceanography/faculty/zeebe_file...

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DoctorOetker7y ago

I am really grateful for the text you mentioned, but it did not change my opinion on this technology as a carbon removal technology, it only confirmed what I understood. However, it is very well written and I will probably reference others to it whenever I can. The only minor mistake I could detect was a "boron" when clearly carbon was intended. If you have a similar quality text detailing the relevant parameters of adding carbonated rocks to counter the acidification I would be interested.

I was discussing the claimed carbon removal aspect, so I now disregard the acidification aspect...

In my analogy I stand corrected and should have stated that the bottle of water represented the surface seawater, since there is no free fast oceanic mixer. The surface water is equilibrating much faster with atmosphere than the whole water column of the ocean.

You defend the technology by mentioning that the average residence time is about 200.000 years, but that is for all inorganic carbon across the whole depth of the ocean, not just the surface layer! As you say there is not enough mixing, so dissolved inorganic carbon in surface seawater has a much shorter residence time. The long residence time is dominated by the slow movement of deep sea water...

So unless the proposal is augmented with either a huge oceanic mixer, or with dumping the carbonate over the mariana trench (if it dissolves there, it will take a long time before it reaches atmosphere again), lacking these augmentations we are dissolving carbonates in surface seawater, and the carbonate ions can equilibrate back to CO2 so it really is just emissions foisted of as capture...

If it stays in the solid say calcite CaCO3 state as opposed to being dissolved, then it does not affect the alkalinity of the surface water...

philipkglassOP7y ago

The pH shift from adding more alkalinity via silicate weathering shifts the equilibrium to favor more CO2 dissolved as carbonate in the oceans. That is why I do not worry about captured CO2 going back to the atmosphere in less than geological time: the shifted chemical equilibrium will favor more dissolved carbonate.

As a small scale example, consider a beaker of distilled water freely exposed to the atmosphere. It dissolves a small amount of CO2 and becomes slightly acidic from carbonic acid. Now add sodium hydroxide -- it becomes strongly alkaline. Wait again and the pH falls again (though not to its original level), due to dissolving more carbon dioxide, which is deprotonated to form carbonate anions. You can keep adding hydroxide and absorbing more atmospheric CO2 for quite some time, until solubility limits come into play. Even though it is just a solution at equilibrium with the atmosphere, and not a precipitated solid, the sodium carbonate solution will not spontaneously separate back to sodium hydroxide solution in the beaker and CO2 in the surrounding atmosphere. It takes thermodynamic work to reverse the carbonate-heavy equilibrium.

DoctorOetker7y ago

Sure but in your example you are adding sodiumhydroxide.

I agree that after adding say CaCO3 to distilled water containing inorganic carbon (CO2, carbonate ions, ...) the carbon content will have increased after equilibrating with the atmosphere, but not with the claim that the eventual carbon content of the water will be the sum of the original carbon content plus added CaCO3 carbon content... some undisclosed part of the added carbon content will be released as CO2 to atmosphere...

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DoctorOetker7y ago

I was discussing the claimed carbon removal aspect, so I now disregard the acidification aspect...

If it stays in the solid say calcite CaCO3 state as opposed to being dissolved, then it does not affect the alkalinity of the surface water...

philipkglassOP7y ago

DoctorOetker7y ago

Sure but in your example you are adding sodiumhydroxide.

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