The Earth's surface area is 510,067,420 km^2 (or 510 million km^2 )

In terms of square metres this is 510 x 10^12 m^2.

The weight of air on a square metre of the Earth's surface is 1 atmosphere - by definition.

One atmosphere = 101,325 Pa = 101,325 N/m^2

= 101kN/m^2.

1 ton = 1 tonne (approx.) = 9.81kN = 10kN (approx.)

So, 10.1 tons of air above every square metre of the Earth's surface.

So, total weight of the Earth's atmosphere

= 10tons/m^2 x 510 x 10^12 m^2

= 5100 x 10^12 tons

Or 5,100,000 Gigatons

Of which 400ppm is CO2.

400 / 1 million x 5,100,000 Gigatons

= 2040 Gigatonnes of CO2 already up there in the atmosphere.

This is equivalent to 551 Gigatonnes of Carbon.

Am I right?