Although it is reasonably obvious that digital magazines are much less damaging to the environment than printed consumer magazines (Chris Anderson has a weirdly contrary view), it is quite hard to do serious measurement of the ecological cost of digital publishing. Steve Souders has done some analysis of web sites with Yslow in an attempt to put some cash/calory value on the inefficiencies of the Wikipedia home page. This is highly conjectural and guesstimatory but probably important. O’Reilly are publishing his book.
The technicalities of the Yslow plug-in (its a plugin to a Firefox plugin ) are well beyond me; it checks whether the inspected web page could make fewer HTTP requests, uses a CDN, puts CSS at the top, JSS at the bottom, avoids redirects etc. etc. If you understand all that (which I dont) you will want to plugin Yslow to your already installed Firebug.
This kind of measurement is obviously not the whole picture and Yslow is primarily tackling the problem of why some pages are slower than they should be (slow partly because they waste energy). Its only part of the picture because what happens on the web page in a browser is one thing, how much power the databases consume behind the scenes are another ball game (possibly more important in ecological impact). These issues will concern us as we move towards cloud computing. Apparently Google and Microsoft are anxious about the ecological costs of cloud computing, and are aiming at frugal computing.