They are not. Each year, more CO2 is emitted than the previous year, and the increase over the previous year is larger than last time.
So, the total CO2 in the atmosphere is larger each year: positive first derivative, slope is up. The total added to the atmosphere per unit time is also larger than last year: positive second derivative. It is possible that the amount by which it is larger than last year's is itself decreasing: negative third derivative. But that is hard to perceive.
So, the total CO2 in the atmosphere is larger each year: positive first derivative, slope is up. The total added to the atmosphere per unit time is also larger than last year: positive second derivative. It is possible that the amount by which it is larger than last year's is itself decreasing: negative third derivative. But that is hard to perceive.