As shown in https://news.ycombinator.com/item?id=42439752 my numbers above for the total atmospheric carbon dioxide load and the amount to remove are wrong by orders of magnitude. The total is 3.3 teratonnes, of which we will need to remove 1.1 teratonnes to get back to preindustrial levels, not just 2.3 gigatonnes. I believe this reduces the 2.5km number above (for the range over which diffusion alone would transport the necessary CO2 flux) to some 400m. Assuming I got the rest of the calculation right, which I'm uncertain of.
This also means that the world cement industry is not currently large enough to carry out the necessary direct air capture over a timespan of decades. You need a carbon dioxide capture industry 10 or 20 times larger; at current energy and material prices, this would cost on the order of 6 trillion dollars per year, about 6% of the world GDP of US$105 trillion per year, nominal, World Bank estimate: https://en.wikipedia.org/wiki/List_of_countries_by_GDP_(nomi.... World GDP has been growing about 3.9% per year, so rather than setting us back to the Stone Age, or even to the 18th century, this staggering expense would set us back to about the time Silicon Valley Bank collapsed, Hamas invaded Israel, OpenAI released GPT-4, and Microsoft bought Activision Blizzard.
This clearly demonstrates that it is technically feasible already, just not economically/politically. Rapidly falling energy prices thanks to the transition to super-cheap renewable energy will ease the economic difficulties, though the project may still require international diplomacy.
Also, https://earthscience.stackexchange.com/questions/994/how-lon... says, "The time scale of interhemispheric tropospheric transport is in the order of one year," not a few weeks. I assume that the difference from the Mount Pinatubo number is because the troposphere mixes more slowly than the stratosphere because in general the winds in the troposphere are slower. (However, the jet stream in particular is at the tropopause, where the two meet.)
This also means that the world cement industry is not currently large enough to carry out the necessary direct air capture over a timespan of decades. You need a carbon dioxide capture industry 10 or 20 times larger; at current energy and material prices, this would cost on the order of 6 trillion dollars per year, about 6% of the world GDP of US$105 trillion per year, nominal, World Bank estimate: https://en.wikipedia.org/wiki/List_of_countries_by_GDP_(nomi.... World GDP has been growing about 3.9% per year, so rather than setting us back to the Stone Age, or even to the 18th century, this staggering expense would set us back to about the time Silicon Valley Bank collapsed, Hamas invaded Israel, OpenAI released GPT-4, and Microsoft bought Activision Blizzard.
This clearly demonstrates that it is technically feasible already, just not economically/politically. Rapidly falling energy prices thanks to the transition to super-cheap renewable energy will ease the economic difficulties, though the project may still require international diplomacy.
Also, https://earthscience.stackexchange.com/questions/994/how-lon... says, "The time scale of interhemispheric tropospheric transport is in the order of one year," not a few weeks. I assume that the difference from the Mount Pinatubo number is because the troposphere mixes more slowly than the stratosphere because in general the winds in the troposphere are slower. (However, the jet stream in particular is at the tropopause, where the two meet.)