I had written a post recently on how banks can go Green. Subsequently came across a great site where you can estimate the carbon emissions in your daily life.
Interestingly Nectar has offered its infrastructure in a pilot project wherein customers' carbon credits would be deducted with every fuel purchase at British Petroleum. This of course is a pilot, but a critical step in enabling all of us to measure and and contribute back to the environment.
Given a few years of evolution, banks can perhaps permit individuals to trade in carbon credits once the metrics set in