Video: Cloud's dirty secret -- energy intensive operations

Three of the largest information technology companies building their business around the cloud -- Amazon, Apple and Microsoft -- are rapidly expanding and relying heavily on so-called "dirty energy" to power computing, according to a report released Tuesday by environmental advocacy group Greenpeace. The use of fossil fuels leaves a larger carbon footprint then renewable sources of energy such as sunlight and wind.

"A few companies, such as Yahoo and Google, have taken meaningful steps to steer their infrastructure investments toward cleaner energy," argues the report, which is titled How Clean Is Your Cloud? It adds, however, that "the replacement of dirty sources of electricity with clean renewable sources is still the crucial missing link in the sector's sustainability efforts."

Apple contested Greenpeace's charges .