Answer
Emphysema does two things based on Fick's Law: it reduces alveolar surface area and thus the rate of diffusion, and it raises the fibrosity/thickness of alveolar walls which reduces diffusion.
Work Step by Step
To understand this see the equation for Fick's Law on p.1004. The two changed noted in the alveolae are directly relevant to that equation and, thus, to the rate of diffusion.