I'm not sure I want to know the exact percentage, and it's certainly, not going to be an easily accepted number even if it were truly quantifiable.
Suffice it to say, it's non-zero... I can accept that on faith alone.
Even with major restructuring of things, I don't think we can go to a "zero impact" model. Of course, it seems noble to try for it anyhow.
I guess I should be riding that bike to work a little more often. Unfortunately, it's a lot easier to be noble and ride to work when it's warmer than 3 degrees in the morning.
Apparently I'm a fair weather environmentalist.