= CO2 =

1ppm global CO2 is how many tonnes of carbon?

The mass of the atmosphere is 5.15e18 kg.  We will assume mole fraction is proportional to volume fraction

|| Gas                || Mole Fraction || At wt   || Mass*e12 kg ||
|| N₂  Nitrogen       || 0.7808        || 28      ||     4054226 ||
|| O₂  Oxygen         || 0.2095        || 32      ||     1087808 ||
|| Ar  Argon          || 0.0093        || 40      ||       48289 ||
|| CO₂ Carbon Dioxide || 0.0004        || 48      ||        2077 ||
|| Dry air atomic wt  || 1.0000        || 28.9576 ||     5129400 ||
||<-4> ||
||                    || 0.9960        || 28.9576 ||     5129400 ||
|| H₂O Water          || 0.0040        || 18      ||       20600 ||
|| Wet air atomic wt  || 1.0000        || 28.9138 ||     5150000 ||

400ppm CO₂ is 2077e12 kg, of which 12/44 is carbon, or 566.45e12 kg.  Hence 1ppm CO₂ is 1.42e12 kg of carbon atoms.

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== Wild Speculation Follows ==

[[ https://www.toraycma.com/products/carbon-fiber/ | Toray ]] [[ https://www.toraycma.com/wp-content/uploads/T1100G-Technical-Data-Sheet-1.pdf.pdf | T1100G ]] carbon fiber has a density of 1.79 (1790 kg/m³), a tensile modulus of 324 GPa, and a tensile strength of 7 GPa.

If 100ppm of excess CO₂ was magically converted into oxygen and 1.42e14 kg of Toray T1100 carbon fiber, that would 80 cubic kilometers of carbon fiber.  

For comparison, high strength steel has a tensile strength of 600 MPa and a density of 8000 kg/m³.  So, T1100G is 11.6 times stronger and 0.224 denser, hence a kilogram of T1100 might replace 50 times its weight of high strength steel.  Annual global steel production is around 2 billion tonnes per year (2e12 kg), so 1.4e14 kg could supply 3500 years of structural needs at current global demand.

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=== Even Wilder Speculation Follows ===

How much energy would it take to extract 1.42e14 kg of carbon from 5.2e14 kg of CO₂?  CO₂ enthalpy of formation is 393.5 kJ/mol, and a mole of CO₂ is 0.044 kg, so at 100% efficiency that would be 8.94 MJ/kg, or 4.65e21 Joules to split 100ppm of CO₂ at 100% efficiency.
If the process was 3% energy efficient, starting with sunlight, that is 1.55e23 Joules.  

The Earth is 70% ocean, and the average surface insolation is around 500 W/m².   The earth's disk averages 6371 km radius, about 1.27e14 square meters total, perhaps 8e13 of that being ocean, capturing perhaps 4e16 watts of sunlight.  If 1% of that ocean surface (which is mostly lifeless now) was covered with 3% energy-efficient "carbon fiber plankton", that would be 4e14 joules/second, 1.26e22 joules per year - converting all the atmospheric CO₂ excess into structural fiber in '''12 years.'''

A long string of speculations with a multiplied probability approaching zero, but still ...

As the old joke goes, "If I had all the money I have spent on drink ... I would spend it ... on drink!".  Chances are, if we could design magic plankton that could make carbon fiber from excess CO₂, we would instead design plankton to make motor fuel, and give everyone on the planet 800 horsepower muscle cars.  And make even more steel.  Humans are perverse.