A recent review of global hurricane data has revealed that the occurrence of powerful cyclones has increased dramatically since 1980, with the number of strong Category-5 storms having more than tripled, with a disturbing trend for these storms to both reach their peak wind speeds farther north and dump ever increasing amounts of rainfall, flooding areas that normally wouldn’t experience weather this extreme. The results of the survey have prompted the authors of the study to call for a reorganization of the Saffir–Simpson scale to include a sixth category, of which currently only divides cyclones into five levels: this measure would not only accommodate the increasing commonality of storms that exceed 250 km/h, but could also save lives with a more accurate method of issuing storm warnings.
The report, titled "Does global warming make tropical cyclones stronger?", was a joint study conducted by Penn State climate scientist Michael Mann, Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, Kerry Emanuel of MIT, and Jim Kossin of NOAA, looked at the effect of global warming on tropical cyclones around the world, in regards to both their wind speed strength and the frequency of their formation. "A significant global increase… can be found in all storms with maximum wind speeds from 175 km/h [108 mph). Storms of 200 km/h [124 mph] and more have doubled in number, and those of 250 km/h [155 mph] and more have tripled."
The authors have also found that these storms are also hitting their stride farther north (and south, in the southern hemisphere), as warm conditions that are favorable to cyclones push closer to the poles. "The average location where the storms are reaching their peak intensity is also slowly migrating poleward and the area where storms occur expands, which changes patterns of storm risk and increases risk in regions that are historically less threatened by these storms," according to the study. As an example, Subtropical Storm Alberto jumped the gun on this year’s Atlantic hurricane season, and although it never reached hurricane status it only weakened into a post-tropical cyclone after reaching northern Michigan.
These results have prompted Mann to call for a reorganization of the Saffir–Simpson scale to include a sixth category that would start when a storm’s winds begin to peak at 190 mph (305 km/h). "The current intensity scale doesn’t capture the fact that a 10 mph increase in sustained wind speeds ups the damage potential by 20 percent," Mann explains. "That’s not a subtle effect. It’s one that we can see."
This new classification could save lives by allowing populations in cyclone-prone regions to better prepare for the storm’s forecast wind strength: as it stands, the wind speeds for Category-5 storms are only measured to 157 mph (252 km/h), with faster wind speeds still being categorized as level five, regardless of how much stronger they may be.