"...[B]ehind every ... Web page lies its real-world technological counterpart; and all too often the servers that run Web sites and online services aren't nearly as spic-and-span as the pages they bring to life. That's especially true in large cities where Internet companies either use servers owned by other firms or hire such firms to house their servers remotely. These server "farms," also known as data centers, use enormous amounts of energy relative to their size. Exodus Communications' operations in the San Francisco Bay area, for instance, consume as much electricity as 12,000 houses. What's more, server farms can't risk power outages, or the consequent lack of access to the Internet. Therefore, they rely on diesel backup generators, which typically generate more pollution than power plants."
Thanks to Chris Steins