According to a study published by the Lawrence Berkeley National Laboratory titled, "The Energy Efficiency Potential of Cloud-Based Software: A U.S. Case Study" it indicates that cloud-based software may be better for the environment.
Here's a link to the 41-page PDF on cloud efficiency. [crd.lbl.gov
Without being cynical, based upon the supporters of the study, I would have thought it true that a data center would more likely have been more 'gree' that computing based upon coal. That said, we still have to power our tablets, laptops and computers to connect to the cloud.