While global warming certainly is happening, the trend of hurricanes since they have been recorded is not fully supportive of this frequency of the storms this season being caused by this warming.
http://en.wikipedia.org/wiki/Global_warming
Also
http://zfacts.com/p/49.html
Note - I am not saying GW is not effecting it. I am just saying the trends are not dispositive of it being the sole cause or really a significant cause.