Everyone seems to be missing the real problem here, which is that unless the biker and the driver are eating food synthesized from hydrocarbons, they aren't increasing net CO2. Remember that carbon cycle you (should have, if you were paying attention) learned about back in high school?
Carbon cycle - Wikipedia, the free encyclopedia The carbon just goes round & round, powered by the sun. With no fossil fuel burning, it'd stay the same from year to year.
Short term, of course. Ice age cycles are another thing, but they're cycles. Fossil fuel burning is open-ended.