McAfee has sponsored an ICF International study about the carbon footprint of spam e-mails, you can find the 12-page PDF over here. The report estimates a total of 62 trillion spam e-mails were sent in 2008 and that this resulted in a power consumption of 33 billion kWh, the equivalent of the electricity used in 2.4 million homes, with the same amount of green house gas emissions as 3.1 million passenger cars.
The average business email user is responsible for
131 kg of CO2 per year in email-related emissions
and 22 percent of that figure is spam-related. The
ICF report equates this spam energy to the emissions
that would result if every business email user
burned an extra 3.3 gallons of gasoline annually.
The energy required annually to create, send,
receive, store, and view spam adds up to more
than 33 billion KWh, approximately equivalent
to 4 gigawatts of baseload power generation or
the power provided by four large new coal power
plants. ICF estimates spam-related emissions for
all email users at an annual total of 17 million
metric tons of CO2 or 0.2 percent of the total
global CO2 emissions — a number equivalent to
emissions from approximately 1.5 million in the
United States homes.
The security firm claims spam filtering saves 135 billion kWh of electricity per year and that if everyone used a state-of-the-art spam filter the total energy consumed by spam could be reduced by 25 billion kWh, that's equivalent to taking 2.3 million cars off the road. More details can be found in the report.