Somebody just had to go and write a Carbon Footprint plugin for WordPress,
The script searches through every post and every page on your blog, counting the total number of words and images in each post. It then works out how many pages the text would take up assuming a standard of 794 words per page. This was worked out by taking the most widely used fonts, font sizes, the word processor being used and other print options. It also works out how many pages the images would take up, again based on DPI, margins and other factors. The most widely used paper is around 120gsm (grams per square meter) and standard A4 paper is 210x297mm. This means that we can approximate that there is 0.0074844g of carbon per sheet of paper which we multiply by our number of pages to give the total amount carbon saved by the blog vs it being printed out on paper.
Um, yeah, ok I guess. But what about the carbon generated by a) the computer hosting the site, b) the computers of the folks accessing the site, and c) the computer used to write the post. For example, I know I have about 6 computers (including this server) that are on 24/7 sucking up sweet electricity while they gradually kill the planet (mwahahaha).
Simply saying “oh, if you had printed all of your blog posts out it would have used X grams of carbons” is just a silly metric.