I've read in numerous books about civilizations being identified during excavations through the pottery shards found in the rubble of their buried buildings and homes. That means pottery has lasting power, and once fired, it doesn't return to the earth very well for reuse.
As a result, I try to be very careful in making sure that what I create is worthwhile as either a beautiful decorative object or more likely, as something that is functional, yet beautiful to look at when not in use, too.
Additionally, I only fire my kiln when I have enough work to fill it completely. I recycle all my clay scraps and glazes, too, as well as try to use found objects as my tools rather than buying new ones.
Even my kiln disasters where something cracks, warps or drips are recycled. I have mosaic artists around the country who are happy to accept my pottery shards for their creations.
The way that I think pottery and ceramics is most friendly to the environment is through the durability of it. Rather than throwing away a styrofoam cup or plastic plate, clay dishes can be used over and over for eons.
Handmade pottery also satisfies an internal need for personal contact. By that, I mean that there is nothing that feels as great to drink from as a handmade cup or tumbler where you can feel where the creator's fingers manipulated the clay. Handmade pottery has an allure that makes you want to touch it, hold it and use it...something that I feel a manufactured product cannot hope to achieve.
So the next time you reach for a handmade cup or bowl, consider the thought and care put into its creation that you'll be able to enjoy for many, many years to come.