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It seems there are multiple chemical reactions (from this question and internet in general) where $CO_2$ can be converted to $O_2$. If this is the case, can someone please explain the challenges (manufacturing, setup costs, raw material scarcity etc) we are facing in reducing $CO_2$ in atmosphere? Also, what are the state-of-the-art techniques used to reduce $CO_2$?

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There may be potential in carbon capture using the carbon silicate cycle to our advantage. The carbon silicate cycle occurs on geologic timescales, but "enhanced rock weathering" seeks to speed up the process by finely grinding basalt or silicates, giving them greater surface area and greater exposure to air.

Scientists in the first article found using their model that

Applying a fixed rate of 10 tons of basalt dust per hectare on these sites sequesters 64 gigatons of CO2 over a 75-year period; when extrapolated to all agricultural land, ERW sequesters 217 gigatons of CO2 over the same time interval.

217 gigatons of CO2 is not trivial, although distributing 10 tons of basalt per hectare over all agricultural land is also not trivial. There is discussion on its viability because of cost of implementation, and the fossil fuels that will be used to mine the basalt or whatever type of substrate, and then distribute it.

edit: I feel compelled to add that of course, this is not a remotely viable solution itself. The only solution is a push towards complete decarbonization immediately.

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Carbon capture and storage (CCS) and direct air capture (DAC) are massively energy intensive processes. So a large barrier to deployment is finding the energy to run the capture plants.

Since we emit so much CO2 from our power generation it's better to just turn off that power generation than use any of the resulting energy to then partly undo the process done in the power station. So in a world with any energy generation from fossil fuels, it basically makes no sense to use energy on capturing carbon, we should just turn off that energy generation.

There are some tiny carbon capture plants operating in Iceland since they have an abundance of geothermal energy.

After/if we transition to renewable energy it will still take energy/resources to build renewable energy infrastructure. Energy doesn't suddenly magically become free. So the massive energy bill of trying to capture carbon is still going to be a problem. As ever, the best solution is to stop emitting carbon dioxide. Carbon capture is pushed hard by oil companies for several reasons, and one of those is that it justifies them continuing to profit off destroying the atmosphere for decades more while they watch their delaying tactics play out.

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Also, what are the state-of-the-art techniques used to reduce $\text{CO}_2$?

That the easy part of the question to answer: Drastically reduce human use of fossil fuels, thereby drastically reducing the amount of fossil fuel based $\text{CO}_2$ into the atmosphere. Humanity is partway on the path toward achieving that. Multiple solutions do exist.

As for the rest of the question, there are no other viable solutions. Most proposed alternatives don't scale up well, most are ridiculously expensive, many have lots of nasty side effects, and many turn out to be not carbon neutral due to energy consumption (assuming energy production remains fossil fuel based).

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  • $\begingroup$ Reducing the amount of fossil fuel used globally every year (something we have not achieved yet!) will most likely only reduce the annual increase of $CO_2$ in the atmosphere. Even if all emissions from fossil fuels stopped today, the amount of $CO_2$ in the atmosphere would be reduced only very slowly: the number I have seen is that it would require a full century of a zero-emission scenario to reduce atmospheric $CO_2$ back to pre-industrial levels. $\endgroup$
    – njuffa
    Commented Mar 10 at 2:14

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