CNet writes:

Not only is spam a nuisance and sometimes criminally deceptive, it’s got a carbon footprint.The mere act of people around the world deleting spam and searching for legitimate e-mail falsely labeled as junk creates the annual energy consumption equivalent in the U.S. of 2.4 million homes using electricity and the same greenhouse gas emissions as 3.1 million passenger cars using two billion gallons of gas.

Here is how the different aspects of spam email stack up:

You can download the report (pdf) here.