Inverse of `atmospheric_CO2`: retrieve the year by which a given CO2
concentration is reached.
Usage
aCO2_inverse(aCO2)
Value
year Approximate year (as floating point number) by which target
concentration is reached.
Arguments
aCO2
Target CO2 concentration in ppm.
Details
Does not give a reasonable result for values below 317ppm, corresponding
to the year 1949, as this is where the minimum of the parabola is located
in the second order fit to the data that was used in aCO2.fct.