If annoying users and wasting their time wasn't bad enough, spam emails are also responsible for clogging our atmosphere with carbon dioxide, a gas that shoulders much of the blame for global warming, according to a report commissioned by antivirus vendor 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 Labs. "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 email releases as much carbon dioxide into the atmosphere as 3.1 million cars consuming 2 billion gallons of gasoline.

Spam has long been a target of antivirus 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.

The figures sound bad, but what does it mean? 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. Without these numbers, it's difficult to put the spam energy numbers into context and understand their significance.

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 emails or how this energy could be used for alternative applications. Depending on assumptions like this, 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.