OK. It isn't much

But maybe it will help a little. If you've visited this site before you may notice a new feature in the right column of the main page – a logo proclaiming this a green website. The good folks at CO2Stats project.promise to

[M]onitor and offsets your blog or web site's carbon dioxide emissions by calculating how much power your visitors are consuming to view it. For each pound of CO2 resulting from your site traffic, The CO2Stats Project purchases carbon offsets that benefit the environment.

Thanks to Environmental Economics for introducing me to the project.