I work for a company that purchases carbon credits. We have done as much as we can to be low-impact but since our primary issue is the CO2 given off in shipping our items, we can only counteract it by purchasing carbon credits.
Some investments are BS I will admit but other times, companies know where the money is going. It isnt just a justification to do bad, it is an investment in future alternatives that will hopefully negate the continued need for carbon credits.
If you dont believe the money is anything more than blood money, well I am sorry. But if you are willing to do your homework, find out where this money is going to. You might be surprised (and I mean that in a positive way.)
If you want to find out what your carbon footprint is, check out
www.drivinggreen.com and see what it tells you about your car. Its what our company used to figure out what carbon we give off when transporting product. Its a useful company and has helped us be a carbon neutral company.