A geologist translates how much carbon dioxide is emitted by today’s cars into an older measure:

“At one point we had a transportation system that had horses,” said Penn State University geologist Richard Alley. “And the horses made horse ploppies. If our CO2 came out of the car as horse ploppies, it would sort of be a pound per mile. And if you put that on the roads of America, in a year it would average an inch deep. And as I like to say, we would have no more joggers in America. We’ll all be cross-country skiers.”

I have read several accounts by urban historians who have described just how much horse manure was in the streets of major cities in earlier centuries. The smell. The sight. The need for people to clean it up. In comparison to roads in past eras, our streets are clean: free of garbage and waste, typically only for fast-moving vehicles.

But, having a car makes that carbon dioxide emission less visible. The average driver doesn’t really see anything and the waste produced by individual vehicles are dwarfed by large facilities like factories or power plants. Of course, add up all the cars and vehicles in the United States and it is a sizable output. See some of the pictures of Los Angeles on days of major smog.

Thus, the analogy might be useful to remember. Although we wouldn’t stand for horse ploppies on our streets today, our vehicles emit carbon dioxide whether we regularly see it.