San Francisco. The morning is cool and sunny, a typical day in the bay. I strap on my bike helmet and am off, coasting towards the Panhandle. I turn right onto the bike lane, riding low on my ten-speed handlebars. My trusty, rusty Schwinn whizzes past fellow commuters wearing work clothes and game faces. I love this route; it is fast and flat and fun. I turn left onto another bike lane—pedal, pedal, pedal—and am at the office. Lock bike in front. Total commute time: ten minutes. Carbon emission: zero. Total calories (to and fro): about one hundred. 

This was my old commute; I no longer live in SF and now have to take the bus to work. I miss my bike commute. It is the fastest, easiest and most importantly, funnest, way to get around. The only carbon you are emitting is that from your respiration, so minimal the Eucalyptus in the Panhandle easily sucks it up. Biking to work can be so exhilarating and easy that burning calories is an after thought. But they are not negligible. At 200 trips a year, I burned about 20,000 calories, or six pounds. A green, lean biking machine.