I'm not all that fond of dogs myself, but it seems like they need a bit of defending. A few months ago, New Scientist reported that a large dog like a German shepherd has twice the carbon footprint of a Toyota Land Cruiser that's driven 6,000 miles each year—mainly because of the dog's meat-heavy diet. (Cats, by the way, have a carbon footprint comparable to a Volkswagen Golf.) And that's not to mention all the havoc they wreak on local wildlife. So I was all set to be swayed by the eco-case against dogs until I ran into this Richard Layman post on how dog-walkers make urban parks and neighborhoods much safer:

I am not a dog person myself, but I am deeply appreciative of well-managed dog parks because in many urban neighborhoods, dog owners are some of the only regularly walking people in a community--many neighborhoods outside of the inner core of Washington are dominated by automobiles and there is relatively little positive pedestrian activity on often empty sidewalks.

Dog walkers contribute positive activity not just to streets and sidewalks but to parks. It's very easy for a park to devolve into a dangerous place. One technique for people committed to disorder to keep people (especially families and children generally) out of parks is to break a lot of bottles--broken glass keeps a park free of children, making it easier to conduct illicit business and activities.

Dog walkers help rebuild neighborhood groups committed to providing support and focus to neighborhood parks--parks that often are willfully or passively neglected by municipal governments overwhelmed by a variety of responsibilities, and lacking the resources to be able to provide regular maintenance and assistance and supervision.

And if the parks and streets are safer, wouldn't that convince more people to live in those urban neighborhoods (say, instead of the suburbs)? Doesn't that ultimately have a green effect? I don't know how it all tallies up, but surely there are a few marks on the positive side of the dog externality ledger.