Make It Green!, a German organisation is working with the Arbor Day Foundation in the US to plant trees that will help make blogs carbon neutral. 

The brains at Harvard University have calculated that the average website is responsible for 0.02 grams of carbon dioxide emissions for every visitor. They then calculate that a blog with around 15,000 visitors a month produces about 8 pounds of emissions a year. Now, a tree absorbs around 20 pounds of emissions a year, so even a blog with thousands of visitors a month can be carbon neutral by planting just one tree.

And importantly, Someone checked this claim with the Arbor Day Foundation.