The reason that total atmospheric CO2 is increasing is because the rate that it's being added is a lot higher than the rate that it is reabsorbed by the various natural carbon sinks (including plants).

Only about 10% is reabsorbed quickly [1], the rest stays for hundreds or thousands of years. That may not be "trapped" in a pedantic sense, but it's pretty close.

[1] https://climate.mit.edu/ask-mit/how-do-we-know-how-long-carb...