Spam is not only clogging up our mailboxes, it's also a significant contributor to global warming, according to a report from McAfee.

"When you look at it from an individual user perspective you're only talking about 0.3 grams of carbon dioxide per spam message," said Dave Marcus, director of security research and communications at McAfee's Avert Lab. "When you extrapolate the math out to the larger numbers, it definitely is significant."

The McAfee report, which was written by consulting company ICF International, said the estimated 62 trillion spam emails that get sent each year consume 33 billion kilowatt hours of electricity, enough to power 2.4 million homes. In addition, spam releases as much carbon dioxide into the atmosphere as 3.1 million cars consuming 2 billion gallons of gasoline.

The report leaves a lot of questions unanswered however. It's not clear ho significant some of these numbers are as McAfee's report didn't provide an estimate for the daily energy usage of a PC or server, or the energy consumed by other applications. The report also failed to detail the methodology and assumptions that ICF used to arrive at these numbers. For example, the report doesn't say what researchers expect computers to be doing if not being used to filter and read spam mails or how this energy could be used for alternative applications. It's possible that computers could be used for tasks that consume more power than applications that fight spam, releasing even greater amounts of carbon dioxide into the atmosphere.

McAfee deferred questions about the methodology and the assumptions that were made to ICF - who could not be reached to comment on this story at the time of writing.

Even so, the crux of McAfee's argument remains unchanged: spam is bad and - all things held equal - it's more efficient to fight spam at the source or mail gateway than at the PC.

"It's just so much less efficient if a user has to clean their own mailbox," Marcus said.

Spam has long been a target of anti-virus vendors but McAfee wanted to reframe discussions of the problem in environmental terms, rather than the annoyance that spam causes users or its links to malware and cybercrime.

"This really gives people a different way of looking at it. Aside from the nuisance factor, it actually has a quantifiable impact on the environment," he said.