Politics: no one wants to hear it. Your coworkers will notice you biking to work at some point. They will ask you why. If one of your reasons is lowering carbon emissions, don't mention it. Doing so will sometimes annoy someone who strongly does not believe in global warming, so it's best to avoid conflict. Just say you do it for fitness and fun, and leave it at that. Don't brag or pretend you are better, or even tell them that they should do it, no one wants to hear it.
Sometimes people will think you're too poor to own a car. Let them.