Ernst Georg Beck estimated a CO2 background time-series from 1826 to 1960. In a discussion from Ferdinand Engelbeen, he has a comment ha many of them are unsuited for background CO2-levels. The huge CO2 level around 1940 is physical impossible and contradicts other proxy´s.
Beck collected approximately 100.000 data series from approximately 900 stations. He estimated 20-50 % variations in the period 1826-1855, 2.6-10 % variations from 1855-1865, and 3 % variations from 1865-1960. The computed 3 % variations from mean values confirm the estimated CO2 background level for the period 1850 to 1960 (Beck 2022).
The huge CO2 level was close to 370 ppm in 1940. The CO2 level at Mauna Loa was approximately 370 ppm in 2000. The global sea surface sea temperature in 1940 had a coincidence to global sea surface temperature in 1980. This coincidence supports a possible relation between
global sea temperature variation and a CO2 background level variation.