Real nappies offer up to 40% carbon saving according to the Womens' Environmental Network  Estimates vary on how long disposable nappies will take to break down, but the highest estimates are several hundred years.

Your actual environmental benefits will depend on what temperature you wash at, how often you can line-dry rather than tumble-dry, and your choice of detergent etc.

However, with real nappies at least you are in control; you can make your own decisions on how to reduce your impact on the environment.