Given the reaction, we see that the photosynthesis process takes $CO_2$, a greenhouse gas, and converts it into $O_2$. Therefore, the cutting down of forests, therefore, limits this process and the amount of $CO_2$ which can be converted into $O_2$. As a result, the cutting down of forests exacerbates the greenhouse effect by allowing less $CO_2$ to be converted into $O_2$.
Work Step by Step
See explanation above.