Each year, millions of tons of bentonite-based kitty litter takes up space in landfills and doesn’t decompose. The stuff can absorb up to 10 times its weight in water and swell up to 16 times its original size. More environment-friendly alternatives, such as corn-based litter, haven't really caught on.
Source: Kitty litter helps Wyoming bentonite past drilling slump | U.S. News | US News
On top of that, cats and dogs eat meat-based diets, and we all know that is the most energy intensive diet there is.
Source: The surprisingly large carbon paw print of your beloved pet - Salon.com
Don’t expect pet owners to give any thought to the environment when making a decision to own a “companion animal.” They will continue to cling to their dogs and cats just like some people cling to their guns and Bibles.