Are There More Tropical Cyclones Than Before?

How do you measure something when your ruler only goes back 40 years? It’s like trying to measure the height of a tree with your school ruler: you need more ruler!

Climate scientist Aslak Grinsted of the Centre for Ice and Climate at the Niels Bohr Institute at the University of Copenhagen wanted to find a way to determine whether tropical cyclones have been increasing in frequency. Satellite measurements – which is how we keep track of tropical cyclones these days – only go back some 40 years, and prior to that we used observations from ships and aircraft; hardly a systematic method of measuring such a global issue.

So Grinsted looked for instruments that have stood and registered measurements continuously for a longer period of time than 40 years.

Tropical Cyclones Increasing in Frequency
Caption: The study is based on data from monitoring stations along the Eastern Seaboard of the United States, where the daily tide levels have been recorded all the way back to 1923. Rapid changes in sea level show that there has been a tropical storm. The map shows cloud cover and ocean temperatures when Hurricane Katrina hit New Orleans in 2005. Warm colors show ocean temperatures exceeding 28° C which can strengthen hurricanes.

“Tropical cyclones typically form out in the Atlantic Ocean and move towards the U.S. East Coast and the Gulf of Mexico,” Grinsted explains. “I found that there were monitoring stations along the Eastern Seaboard of the United States where they had recorded the daily tide levels all the way back to 1923. I have looked at every time there was a rapid change in sea level and I could see that there was a close correlation between sudden changes in sea level and historical accounts of tropical storms.”

According to this new dataset, there had in fact been an increasing trend in the number of major storm surges since 1923.

Grinsted then got together with colleagues in China and England to look at the global temperatures over the same period to determine whether there was a trend for a higher frequency of cyclones in a warmer climate.

“We simply counted how many extreme cyclones with storm surges there were in warm years compared to cold years and we could see that there was a tendency for more cyclones in warmer years,” explains Grinsted.

“We have calculated that extreme hurricane surges like Katrina are twice as likely in warm years than in cold years. So when the global climate becomes 3 degrees warmer in the future, as predictions show, what happens then?”

Source: Niels Bohr Institute via Eurekalert
Image Source: NASA/GSFC

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