Explain why deforestation can be a "double-edged sword" as it cause C02 level in the atmosphere to increase?

This is because when a tree is cut down, the CO2 within it in released. On top of that, because the tree has been cut down, it can no longer use up CO2 from the atmosphere for photosynthesis. In conclusion, deforestation has the effect of releasing more CO2 into the air and also limiting the amount of CO2 being absorbed by trees (because it reduces the number of trees).