as for CO2, short trips in a car are MUCH WORSE than long trips. the engine runs at peak efficiency when (among other things) it's warm. driving short trips, the engine doesn't have a chance to warm up, and the fuel efficiency is not so good.

on my commute, i can take a direct route or one of several indirect routes, including some decent hill climbs. it's still a short commute, but i can turn it into a decent workout.