Two things factor in to the sustainability of a credit card:
- What you do with the credit you have
- What the bank does with the money it makes
Obviously, the first point is totally within your power -- max out the card on carbon offsets!
So, that leaves the second point. According to Green America, there are a number of credit card companies which invest the proceeds of their business (the fees they collect from card-holders and vendors who accept credit cards) in socially and environmentally conscious ways.
These investments are often defined by what they are not:
- Fossil fuel mining, refining, and burning projects and companies
- Predatory lending
Other credit cards (including a few from big banks), called affinity cards, allow you to donate your points to charity, instead of spending them on TVs and plane tickets. However, if you are in the US and typically itemize your tax deductions, you may want to confirm whether or not this type of donation offers any tax benefit.