[This is a reprint (21 Sep 2017) from Dr. Roy Spencer’s blog (drroyspencer.com) in which he makes the point that two category 4 hurricanes making landfall in the same year is far more likely than the recent eleven year period with no major hurricane landfalls that preceeded the two category 4 hurricanes. Assuming the 177 year return period for no major hurricanes in 9 years estimated by NASA was correct, we agree with Dr. Spencer that the return period for no major hurricanes in 11 years would be 560 years. Ed.]
Weather.com published an article noting that the two Cat 4 hurricane strikes this year (Harvey and Irma) is a new record. Here’s a nice graphic they used showing both storms at landfall.
But the statistics of rare events (like hurricanes) are not very well behaved. Let’s look at this new record, and compared it to the 11+year period of no major hurricane strikes that ended when Harvey struck Texas.
The Probability of Two Cat 4 Strikes in One Year
By my count, we have had 24 Cat 4 or Cat 5 landfalls in the U.S. between 1851 and 2016. This gives a probability (prior to Harvey and Irma) of one Cat4+ strike every 7 years. It also leads to an average return period of two Cat4+ strikes of about 50 years (maybe one of you statiticians out there can correct me if I’m wrong).
So, since the average return period is once every 50 years, we were overdue for two Cat4+ strikes in the same year over the entire 166 period of record. (Again, for rare events, the statistics aren’t very well behaved.)
The Probability of the 11-Year “Drought” in Major Landfalling Hurricane
In 2015, a NASA study was published which calculated how unlikely the (then) 9-year stretch with no major hurricane landfalls was. They came up with a 177 year return period for such an event.
I used that statistic to estimate what eventually happened, which was 11 years with no major hurricane strikes.
I get a return period of 560 years!
Now, which seems more unusual and potentially due to climate change: something that should happen only once every 50 years, or every 560 years?
Maybe global warming causes fewer land-falling major hurricanes.
Roy W. Spencer received his Ph.D. in meteorology at the University of Wisconsin-Madison in 1981. Before becoming a Principal Research Scientist at the University of Alabama in Huntsville in 2001, he was a Senior Scientist for Climate Studies at NASA’s Marshall Space Flight Center, where he and Dr. John Christy received NASA’s Exceptional Scientific Achievement Medal for their global temperature monitoring work with satellites. Dr. Spencer’s work with NASA continues as the U.S. Science Team leader for the Advanced Microwave Scanning Radiometer flying on NASA’s Aqua satellite. He has provided congressional testimony several times on the subject of global warming.
Dr. Spencer’s research has been entirely supported by U.S. government agencies: NASA, NOAA, and DOE. He has never been asked by any oil company to perform any kind of service. Not even Exxon-Mobil.