The environmental impact of downloading

I downloaded the portableapps.com portable applications suite on a USB stick today. I did the same thing the day before yesterday, and I had mislaid it somewhere in the midst of the plethora of disks.

Which led me to thinking, what is the environmental cost of downloading files? The file size isn't huge, 124MB, but still significant. Is there an approximate carbon footprint for each MB?

This also in the light that broadband internet access is increasingly considered a necessity.