Hindsight 2020: Hurricane Season & Climate Change

The 2020 hurricane season is here and is expected to be a busy one. We’ve already had three tropical cyclones and it’s still June! Forecasts are unanimous in calling for an active season: seven to ten hurricanes, with most of them likely to occur after August 10th. Cold waters across the eastern tropical Pacific ocean (La Niña) and warm waters across much of the Atlantic are the reasons for these ominous predictions. Exactly where the hurricanes will travel is less predictable, so a busy season does not mean Tallahassee will get hit. 

More predictable is the rise in the intensity of the strongest hurricanes as oceans get hotter. Using winds estimated from satellites, we showed (in 2008) a clear upward trend in the strength of the strongest hurricanes worldwide, a trend that is consistent with the theory that hot oceans provide fuel for hurricanes to intensify. Oceans continue to heat up since that research was published and indeed we see continued upward trends in the intensity of the strongest hurricanes with increases in the upper quantile intensities of global tropical cyclones amounting to between 3.5 and 4.5% in the period 2007–2019 relative to the period (1981–2006) [see Figure]. 

Percentile wind speeds in two distinct epochs by major tropical cyclone regions. The epoch years and the number of hurricanes (n) are shown at the top of the left and right columns. The wind speeds corresponding to the 75th, 90th, and 95th percentiles are given in the respective columns below each epoch. The changes are noted by the slope of the line segments colored by percentiles (75th in light gray, 90th in gray, and 95th in black). The vertical scales are the same across the regions. One m/s equals 2.24 m.p.h.

Individual regions show upward intensity trends for at least one upper quantile considered with the North Atlantic and Western North Pacific showing the steepest and most consistent trends across the quantiles. In particular, the 75th percentile fastest wind speed for the set of global hurricanes increased by 4% from 61.7 m/s (138 m.ph.) during the earlier period to 64.3 m/s (144 m.p.h.) during the later period.  And the 90th percentile wind speed increased by 3.6% and the 95th percentile wind speed increased by 4.3% from 72 m/s (161 m.p.h.) during the earlier period to 75.2 m/s (168 m.p.h.) over the later period. All six regions show upward intensity trends for at least one upper quantile with the North Atlantic and Western North Pacific regions showing the steepest and also the most consistent upward trends. 

On the time scale of a single season other factors besides ocean heat play a role so it is not likely we will see a storm as intense as Hurricane Dorian from last year or even Hurricane Michael from a few years ago. The bottom line is to pay attention to the weather forecasts and have a plan if one does approach. This advice is particularly pertinent given the ongoing pandemic since evacuations could be significantly hindered if shelter space is limited and physical distancing rules might break down where rescue operations are needed.

For reference here is a list of names (from the World Meteorological Organization) for tropical cyclones as they develop this season: Arthur, Bertha Cristobal, Dolly, Edouard, Fay, Gonzalo, Hanna, Isaias, Josephine, Kyle, Laura, Marco, Nana, Omar, Paulette, Rene, Sally, Teddy, Vicky, and Wilford.

Dr. James Elsner is the Earl & Sofia Shaw Professor and the Department Chair of Geography.

The feature image is from the New York Post.

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