USA: Hurricane Florence is a climate change triple threat
By Michael Mann
[...]
First, there is the threat of wind damage. Florence strengthened into a monster category 4 hurricane with 140 mile per hour winds over those very warm western Atlantic waters. Past studies indicate a roughly 7% increase in the peak wind speed of a category 4 storm for each 1C warming of ocean surface temperatures. So the roughly 1.5C warmer-than-normal waters in the subtropical Atlantic where the storm intensified (and keep in mind that “normal” as modernly defined by NOAA as the average during the 1981-2010 period is itself already about 1C warmer than pre-industrial times prior to advent of human-caused greenhouse warming) corresponds to a roughly 11% increase in peak winds. But the destructive potential of a storm goes as the cube of the wind speed. So that 11% increase in wind speed corresponds to a 33% increase in destructive potential. That’s not a subtle effect.
[...]
That brings us to the second, even greater threat: storm surge. Though the storm weakened as it approached the coast, the storm surge was built up over of a period of several days, including the time during which it existed as a category 4 or strong category 3 storm. That means the catastrophic, roughly-10ft storm surge from Cape Hatteras to Myrtle Beach was baked in well in advance of the landfall of the storm.
[...]
Last, but not least, we have the threat of inland flooding. Warmer oceans mean more moisture in the atmosphere. It’s one of the simplest relationships in all of meteorology: for each 1C of warming, there is about 7% more moisture in the air. That means those 1.5C-above-normal ocean temperatures have given the storm about 10% more moisture. All other things being equal, that implies about 10% more rainfall.
But that’s not the whole story. What made Harvey a record flooding event last year and makes Florence such a flooding threat now, is the slow-moving nature of the storm. The slower the storm moves, the more rainfall that accumulates in any one location and the more flooding you get. Such was the case with other devastating storms such as Harvey or 2011’s Hurricane Irene that caused historic flooding in my own state of Pennsylvania.
[...]