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Reading the latest IPCC report, I came across an interesting overview of carbon dioxide removal (CDR) options (5SM-16 onwards). I've never heard of some of them: it includes biochar, ocean alkalinization, enhanced weathering, artificial ocean upwelling. They differ by sequestration potentials, reliability and tradeoffs. For example, you can sequester a considerable amount of carbon by afforestation, but that carbon is vulnerable to forest fires and pest outbreaks, both of which are theorized to become more frequent with climate change. Besides, afforestation can compete with food production for arable land thereby potentially threatening food security. Some of the options are associated with great uncertanties. Which CDR options, on balance, are the most preferable in terms of feasibility and efficiency at long-term carbon sequestration?

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you can sequester a considerable amount of carbon by afforestation

Planting new forest is only capable of capturing about the same amount of carbon dioxide that has been released by burning down forests. This is negligible. All the news headlines are reporting about deforestation in Brazil, but what they won't tell is that releasing this one-time carbon sink creates negligible emissions. The main problems that cause climate change are:

  • Excessive use of fossil fuels
  • Release of methane, from cows and leaks from use of natural gas
  • Production of cement

If you want to solve climate change, you need to tackle these issues. Deforestation is not in this list -- it creates negligible emissions.

It is true that all forest is prone to being burned. If you want to sustainably capture carbon, what you want to do is to use the forest to create biochar and lumber that is used to build houses. (Unfortunately, houses still have a finite lifetime but at the end of their lifetime the scrap wood should be used to create biochar instead of being burned.) However, the problem is that this is slow -- in cold areas forest cycle is 100 years and in warmer areas shorter but still too long to make large-scale carbon capture by forests feasible.

Which CDR options, on balance, are the most preferrable in terms of feasibility and efficiency at long-term carbon sequestration?

My opinion that we should use forests maximally to capture and store carbon, creating biochar and lumber, but the main effort should be to eliminate most of fossil fuel use by renewable electricity, eliminate most of the cattle on this planet, and capture whatever carbon dioxide is produced by cement production and combustion of very little natural gas that is used to create electricity during periods when the sun won't shine and wind won't blow and the nearest area where either sun is shining or wind is blowing is more than 5000 km away so electricity transportation using power lines isn't feasible.

Forests do allow slow carbon capture, but the main effort should be to reduce fossil fuel use very quickly to zero. Forests are a rounding error unless we are able to plant forests in hot deserts like Sahara, being composed of trees that grow very quickly in the warm climate.

Unfortunately, rainforests do have a possibility to slow down climate change. I say "unfortunately" because the possibility to slow down climate change using rainforests includes chopping down the existing ecosystem and creating plantations of very fast-growing trees and using those trees en masse for lumber and biochar. Unlike deserts that haven't demonstrated the possibility to sustain fast-growing trees, rainforests have. The problem with utilizing rainforests in this way is that it destroys the existing ecosystem, and rainforests are home to a huge number of species that would be placed under threat if this was done.

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  • "This is negligible." 3,7 GtCO2e per year (the calculated potential) is quite big compared to most other options Nov 25 '21 at 18:46
  • My information is that deforestation causes between 0.4 and 1.0 GtCO2e per year, whereas for example fossil fuels cause 36.4 GtCO2 per year, methane leakage and methane from cows and permafrost is around 10% that of fossil fuels, and cement is 1.5 GtCO2 per year.
    – juhist
    Nov 26 '21 at 19:02
  • You don't trust the IPCC, do you? Nov 26 '21 at 22:27
  • You don't need to plant forests in the Sahara to stop climate change, you just need to re-forest every single place that humans have cleared for farmland.
    – Mark
    Nov 29 '21 at 21:58
  • @Mark That's not enough. Deforestation is an infinitesimally small component of climate change. Most of climate change is caused by combustion of fossil fuels. To remove the carbon created by fossil fuels, you do really need to plant forests in Sahara. Reforesting every deforested place is nowhere near enough.
    – juhist
    Dec 26 '21 at 12:37

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