The environmental impact of emails

This article about charging money for sending emails got me thinking.

How about the concept of a focusing on the carbon equivalent for IT services such as email? I like the idea of green IT. Efficiency across the board is a concept that most people have never thought about. Getting the message across in an efficient way reduces our impact on the environment. Think about a round robin email with a large file attachment. The logically efficient way would be to put the attachment on a server and refer others to it. The costs in terms of carbon can be compared and displayed. One day a carbon allowance will need to be introduced. Everyone, including businesses will be able to buy and sell carbon credits. Why not include emails into this?