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

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.

Wild Speculation Follows

Toray 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.

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.