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A recent Guardian article "Spreading rock dust on fields could remove vast amounts of CO2 from air" has gotten some attention in the nongovernmental and UN organizations working in the Sahel who are desperate to combat the adverse effects of climate change in the region. This article is based on a Nature publication on enhanced rock weathering (ERW).

Let's pretend a large international organization has tasked a newly employed software developer to look into the feasibility of performing ERW in the Sahel. Now this let's say this software developer, who has no geological background whatsoever, is trying to figure out what it would take to run a ERW pilot in the Sahel and decided to investigate Bol, Chad to have a specific place to focus on.

Ideally, this pilot would seek to test two things (i) is ERW feasible to perform in the region of conflict, (ii) does the crushed basalt or silicate, which is added to the soil, increase crop yield?

In order to determine if this pilot is feasible to even consider I think the following should be investigated:

  1. Current farming methods in the region
  2. Feasibility of acquiring and transporting crushed basalt or silicate
  3. Method for applying the basalt or silicate to the soil

What else should I look at?

If you were given this (completely hypothetical) task where would you start?

Is there any giant red flag I am missing which would be enough of a reason not to move forward with a pilot?

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  • $\begingroup$ Coming from StackOverflow I realize this is a HORRIBLE question since it's very broad and not specific. If it gets flagged for removal I completely understand, just wondering if anyone has ideas. Stack Exchange is typically a nice place to turn to :) $\endgroup$ – Eric Jul 9 at 14:04
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    $\begingroup$ Haven't read the Nature paper, it seems to be projection into the year 2050. By then, there may little left in the area that merits bothering. Silicate weathering is a process that takes place on a scale of millions or tens of millions of years, a little boring for current human attention spans :-) 1.) can be looked up, 2.) transportation and infrastructure are terrible CO2 producers 3.) applying basalt ... flood basalts come to mind :-) UAE are levelling the Omani mountains to build islands off their coast. Hypothetical approaches are surely nice ... $\endgroup$ – user20217 Jul 9 at 14:19
  • $\begingroup$ @a_donda that's for the reply! From my understanding, you're suggesting that the amount of CO2 created in transporting the basalt to the fields could likely be greater than the CO2 the basalt would capture (in 30 years per the Nature paper) after it is spread across the field? I guess a back of the envelope calculation could be made to see if this is true, assuming I can figure out where the Basalt would be sourced from (ideally not by destroying a mountain) $\endgroup$ – Eric Jul 9 at 14:27
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    $\begingroup$ @a_donda it's understood that applying ERW will not lower the temperatures in the Sahel. It would be used to capture carbon, which could ideally be sold as a form of carbon credit. Thus providing a bit of income for the region (paying people to spread rock dust on their fields) and ideally increasing the fertility of the soil and therefore increasing crop yields. $\endgroup$ – Eric Jul 9 at 14:34
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    $\begingroup$ There is already a huge project to preserve the Sahel from desertification: the Great Green Wall. A 15 km wide, 7800 km long strip of land, stretching from Senegal to Djibouti, meant to be reforested. For various reasons things are going slower than planned, but efforts are ongoing and this solution seems more realistic than the weathering thing. fao.org/in-action/action-against-desertification/overview/… $\endgroup$ – Jean-Marie Prival Jul 21 at 9:59
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OK I did a paper on mitigating climate change by enhanced weathering of rock on agricultural land for my masters (2014) it's not brilliant, but it does include some useful references if you want to research this further.

I would imagine the main limiting factor in the Sahel region would be that the process needs to be wet. So you're possibly only talking about being able to add material to irrigated areas. There are also the limitations you've already discussed of getting the material on site. In terms of crop yield, I would suggest that it will only increase plant growth in areas which do not already have alkaline geology (and I have no idea what the geology looks like in the Sahel, I'm afraid) and adding too much to soils may well actually decrease site fertility without the addition of something else (manure is the traditional amendment)

As far as I'm aware, enhanced weathering is perfectly technologically feasible, but it does have a limited application - due to the sheer quantity of it we'd need.

Hope this helps.

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    $\begingroup$ And the time frame. I also found: onlinelibrary.wiley.com/doi/full/10.1111/gcb.15089. I think what's missing is (as is usual in economic approches) an approach that looks at the side effects, like nonoculture, release when creating and transporting the stuff and related infastructure, etc. $\endgroup$ – user20217 Jul 10 at 10:45
  • $\begingroup$ Though if it's small scale, and as simple as the farmer gets paid for half a tonne of carbon for every tonne of lime he adds to the farm - it shouldn't be too complicated. $\endgroup$ – Will Jul 10 at 11:36
  • $\begingroup$ Lime ? Not basaltic rock dust ? Which must be produced and transported, still the process takes years must be controlled and will probably lead to monoculture. Yeah, large scale irrigation in the Sahel is scifi. $\endgroup$ – user20217 Jul 10 at 11:48
  • $\begingroup$ Well lime is the standard amendment to increase soil pH, but the technique actually calls for ultramafic rocks (basalt is mafic) mainly olivine I believe (though I'm not a geologist) $\endgroup$ – Will Jul 10 at 13:39
  • $\begingroup$ But with limestone CO2 would be set free. They wont go the other way and in the darkness bind it. $\endgroup$ – user20217 Jul 12 at 18:25
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Note that the sahel is a lost cause for the rainfall/CO2 absorption side of the equation. Heat tends to mineralize soil and reduce its organic content, so the sahel may be too hot. For fertiliser, rock dust is economically significant.

The program should be quantified:

Available budget for the project.

KG mass of rock:

Quarry cost. Transport cost. Truck+Fuel cost. Crushing machine cost. Energy cost. Dissemination labour organization. Training schemes to instruct dust dissemination. Dissemination machines. Rain window period. Maximium rock load for a given precipitation rate, i.e. 10g/m2 per 1cm of rain.

Using planes to lift 100 tonnes of dust per day may be cheaper than using trucks (a 747 can lift 112 metric tonnes).

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    $\begingroup$ While planes might be economically viable I think that would defeat the entire purpose of the project. i.e. sequester CO2 $\endgroup$ – Eric Jul 16 at 7:35
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More information on ERW here:

https://www.nature.com/articles/s41477-018-0108-y.epdf?author_access_token=w1xYBFb3g3BwKnjkBD1cJtRgN0jAjWel9jnR3ZoTv0NSaZpth7Zz94N6_lHXNgths4xErTsML30mZS8Ql0FCuzY9k6ePi5nMsoEvmCXoU5g65l-c0C09tp3iWQMGMdfvbuwlRhm5_snv9tqy8Kc6ig%3D%3D

Farming with crops and rocks to address global climate, food and soil security

Hot climates are good for ERW -- the kinetics of weathering are temperature dependent. But as noted above you need water too.

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