Seems America had an “emissions” crisis near the turn of the 20th century. Although no one had an answer to the problem it seemed to take care of it self thanks to a new mode of transportation. I first read about this in Levitt and Dubners “Super Freakonomics,” and googling gave me a few articles that seem to predate the book.
http://www.uctc.net/access/30/Access%2030%20-%2002%20-%20Horse%20Power.pdf
Another:
http://www.thefreemanonline.org/columns/our-economic-past-the-great-horse-manure-crisis-of-1894/
http://www.freakonomics.com/2010/09/08/horse-manure-the-gift-that-keeps-on-giving/