Why does carbon accumulate?

the statement is true

"it built up in the atmosphere"

i think thats it, idk, i tried looking for this exact q& a for grad. and idk if i got this question right or sorry if im wrong

not always but somethings can be reproduced in a laboratory.

answer; aerobic conditions; citric acid cycle (krebs cycle) electron transport chain;         anaerobic conditions; fermentation; both glycolysis;