there have already been hurricanes strong enough to qualify as Category 6s. They'd define those as having sustained winds over 175 or 180 mph. A couple told me they'd measured close to 200 mph on a few occasions.
The Saffir-Simpson hurricane category scale is based on wind speed: A Category 1 hurricane has sustained winds from 74 to 95 mph, Category 2 has sustained winds from 96 to 110 mph, Category 3 has sustained winds from 111 to 130 mph, Category 4 has sustained winds from 131 to 155, and a Category 5 storm has sustained winds greater than 155 mph.
The categories run in roughly 20 mph increments, so a Cat 6 would be greater than 175 or 180 mph.
"Remember, for each 10 mph increase of wind speed," says atmosphere scientist Greg Holland, "there's about 10 times more damage, and 20 times more financial loss."
In other words, the increase is not "linear" but "exponential."
The article points out that there is alink between gloal warming and hurricane strength, but overstates it slightly. As the RealClimate folks pointed out, it's impossible to ascribe cause for any single event (ie, Katrina) to global warming. However, there definitely is a relationship - in a nutshell,
while we cannot draw firm conclusions about one single hurricane, we can draw some conclusions about hurricanes more generally. In particular, the available scientific evidence indicates that it is likely that global warming will make - and possibly already is making - those hurricanes that form more destructive than they otherwise would have been.
The key connection is that between sea surface temperatures (we abbreviate this as SST) and the power of hurricanes. Without going into technical details about the dynamics and thermodynamics involved in tropical storms and hurricanes (an excellent discussion of this can be found here), the basic connection between the two is actually fairly simple: warm water, and the instability in the lower atmosphere that is created by it, is the energy source of hurricanes.
The RealClimate folks also point out that Al Gore's global warming movie An Inconvenient Truth treated the topic of hurricanes and global warming with the proper restraint.