If you want to offset your emissions, you can buy certificates, with the money going to projects that protect forests, capture greenhouse gases and so forth. Some airlines sell these to offset emissions from their flights, and after some scandals in the early days, they started to scrutinize the projects closely, so the offsets can now be bought in good conscience. You can also buy offset certificates in a private marketplace, from companies such as TerraPass in San Francisco that follow strict rules set up by the state of California; some people even give these as holiday gifts. Yet another way: In states that allow you to choose your own electricity supplier, you can often elect to buy green electricity; you pay slightly more, with the money going into a fund that helps finance projects like wind farms.