How much does a new bike set you back?
The Green Lantern Column on Slate tries to answer the question "How far do I have to ride my bike to pay back its carbon footprint?" The questioner wonders how far behind is his carbon footprint to make and ship a brand new bike. According to some calculations, it's 530 pounds of carbon emissions and "Given a "typical U.S. diet," you would have to ride your bike instead of driving for around 400 miles to cover the bike's initial carbon footprint." As the saying goes, your mileage may vary, but a bikable distance for work is usually about 8 - 10 miles. So if you ride once a week, you pretty much break even in less than half a year (20 miles round trip 20 times). Not a bad return on effort. The column goes on to look at other alternative forms of transportation, and the bike still wins.
If you are really concerned about adding new carbon emissions for a new bike, then consider buying a used bicycle. The truth is that many things that require physical effort such as bicycles don't get used after their initial novelty. As a result you can find many great deals on Craigslist or eBay with no new carbon added.