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Cloud computing and virtualization deliver a host of benefits centered around flexibility and reduced costs. There is also a perception that they are more environmentally friendly options. While this may be true to some degree for virtualization, in the case of cloud computing, it is often a misconception.
When an enterprise migrates to a virtualized infrastructure, it most likely consolidates down to fewer machines. Often consolidation is one of the drivers. Sometimes new (and, thus, typically more energy-efficient) hardware must be purchased to accommodate the increased workload per machine. In either case, fewer servers running ultimately means a smaller carbon footprint.
The same cannot be said for cloud computing. In cloud computing, at its simplest, you're basically transferring your computing needs, or part of your needs, to a different location. Sure, the enterprise sending its data to the cloud is reducing its own carbon footprint, but there is no net reduction.