Speeds up the natural cycle of volcanic ash capturing carbon. Would mine Olviline a volcanic substance which naturally reacts with atmospheric carbon to form bicarbonite which when on a seashelf gets absorbed by the Earth's crust which I guess we have an infinite ability to pollute.
Very crude estimate is that it would cost $15.15 trillion and we'd pollute 2+% of the worlds beaches to use olviline to sequester all human carbon emissions since the industrial revolution assuming that there are increasing returns to scale in olviline mining technology.
YC backed startup that creates fuel through "combustion in reverse" using electricity to convert C02 in the atmosphere back into ethanol in water. The YC startup then produces nanofilters which separate the water from the ethanol. If all carbon emissions come from the atmosphere to begin with we'll have no new net carbon emissions.