Ask HN: What's the carbon footprint for browsing the internet?

Is there a way to calculate how much electricity is required for a single network request? Some variables that a calculation might need to consider: the user's local power supply (renewable or not?), the ISP's carbon footprint, the web host's footprint, background advertisement requests, the power demanded by the user's machine, other things I'm not thinking about, etc. Does anybody know of any resource which calculates that? Or have any idea how to calculate it?