John Pescatore (pictured), VP Distinguished Analyst at Gartner, explains that "There have definitely been environmentally caused Internet outages, but in this case denial of service attacks are the leading cause. There are low scale attacks that anyone who can spell LOIC can launch and then there are large scale distributed attacks that take a bit more knowledge of botnets and the like, but DoS attacks are basically like thunderstorms on the Internet: hard to predict when they will hit, but they will and you can build a thunderstorm-proof Internet connection – just the way you can have thunderstorm proof power to your datacenter".
See "Still in Denial About Denial of Service?" - here.
See also - "Yankee Group Prediction: A Denial-of-Service Attack Will Take a 4G Network Down" - here, and "DPI: NSA Scans AT&T, Verizon and CenturyLink Traffic Going to Defense Firms" - here, with a short review of solutions from the DPI players.