Forest Carbon's Back-End Durability Problem
First, it takes decades before the trees grow enough to capture the maximum of CO2 they are supposed to. So the impact of such programs would (theoretically) be felt in decades while companies book the results of their offsets immediately.
Clement Vouillon • Monitoring carbon offset reforestation projects
For most air pollutants, once the emissions stop, the pollutant will disappear and the impacts will go away. However, this is not the case for CO2, which stays in the atmosphere for centuries.