Makes a good Christmas gift, too:
As of 2008, NC GreenPower also offers carbon offsets to address growing concerns about the impact of greenhouse gases on the environment. The program accepts financial contributions from citizens and businesses to help offset the cost to produce green energy. There is no limit on the number of $4 blocks an individual, organization or business can purchase.
If you're not sure just how much carbon you need to offset, there's a neat calculator on this page.