"All carbon footprints are hard or impossible to pin down accurately, but the internet is a particularly complex case. This isn't just due to the fact that the "net" consists of millions or even billions of machines owned by countless people and companies. There's also another problem: even if we knew exactly how much energy all these devices consumed (which we don't), we still wouldn't know how much of that energy was spent on offline jobs (such as creating documents in Microsoft Office) and how much was spent on online jobs (such as emailing those documents to a friend or colleague). "
"Pulling the plug on old hardware: Life-cycle management explained"