Think mobile devices are low-power? A study by the Center for Energy-Efficient Telecommunications—a joint effort between AT&T's Bell Labs and the University of Melbourne in Australia—finds that wireless networking infrastructure worldwide accounts for 10 times more power consumption than data centers worldwide. In total, it is responsible for 90 percent of the power usage by cloud infrastructure. And that consumption is growing fast.

The study was in part a rebuttal to a Greenpeace report that focused on the power consumption of data centers. "The energy consumption of wireless access dominates data center consumption by a signiﬁcant margin," the authors of the CEET study wrote. One of the findings of the CEET researchers was that wired networks and data-center based applications could actually reduce overall computing energy consumption by allowing for less powerful client devices.

According to the CEET study, by 2015, wireless "cloud" infrastructure will consume as much as 43 terawatt-hours of electricity worldwide while generating 30 megatons of carbon dioxide. That's the equivalent of 4.9 million automobiles worth of carbon emissions. This projected power consumption is a 460 percent increase from the 9.2 TWh consumed by wireless infrastructure in 2012.

The high end of the growth projected by CEET is based on a scenario where wireless networks reach two billion users worldwide by 2015, generating traffic of 4.3 exabytes a month. A more conservative estimate of 32 TWh used a "low-uptake" scenario for wireless broadband services, with just 1.6 billion users worldwide by 2015 and 2.2 exabytes of traffic. These figures included users who accessed the cloud through in-home and public Wi-Fi and femtocell as well as through carriers' cellular infrastructure.