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Global warming has coincided with fewer tropical cyclones forming each year around the world compared to the second half of the 19th century, according to a new study.
The average annual number of cyclones dropped by 13% over the course of the 20th century, with steeper declines seen after 1950.
Several studies using climate models have suggested that global warming could reduce the total number of cyclones formed, but there would be a higher proportion of more intense and dangerous systems.
The authors of the new research, published in the journal Nature Climate Change, said their findings are in line with expectations that a warming planet would see fewer cyclones forming overall.
A leading tropical cyclone expert expressed doubts about the study’s findings.
Understanding how climate change is affecting cyclones has proven difficult because the most reliable and complete satellite observations do not begin until the late 1970s. This relatively short timeframe makes it more difficult to separate the effect of global warming from the natural variability of the climate.
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Cyclones – also known as hurricanes or typhoons, depending on where they form – also involve complex local atmospheric conditions that are difficult to model.
Scientists from Australia and the US used climate models and historical observations of atmospheric pressure to calculate the likely number of cyclones from 1850 to 2012.
Declines were found in all seven ocean basins where cyclones form.
Globally, a greater than 23% drop in the number of cyclones formed annually was found after 1950, compared to 13% for the entire 20th century.
The only exception to the biggest decrease in cyclones after 1950 was in the North Atlantic, where the number of cyclones had been increasing in recent decades but, according to the study, was still lower than in the second half of the 19th century.
The study’s lead author, Savin Chand of Federation University Australia, said that as the climate warmed, it likely changed the underlying atmospheric conditions that help form cyclones.
While it may be “good news” that fewer cyclones are forming, Chand said the total number of cyclones is just a measure of the risk to societies.
The study was not done to look for different categories of cyclones, but rather to count any cyclone that would have formed.
Category 1 cyclones usually cause only negligible damage to buildings and crops, while the more destructive Category 5 cyclones, with average winds above 200 km/h, cause billions of dollars in damage and widespread destruction to communities.
Shand said cyclones have intensified in recent decades and are approaching coastal regions. Some studies also suggested that the cyclones were causing more rain and lasting longer after they made landfall.
Co-author Kevin Walsh of the University of Melbourne said the most complete data on cyclones dates back only to the 1970s. Before that, there were some ship records that went back to the 1940s, but they were incomplete.
“They are a very complicated phenomenon, but this study is building confidence in our climate model predictions, showing that they agree with observed trends.”
He said that with warming oceans in the tropics, the upward flow of warm air would be reduced, as would the difference in the speed of winds closer to the surface and higher in the atmosphere; two less favorable factors for creating cyclones.
He said: “It is the really intense cyclones that cause the overwhelming majority of damage, and there are good theoretical reasons to believe that these numbers [of more intense cyclones] will increase in the future”.
Professor Kerry Emanuel, a cyclone expert at the Massachusetts Institute of Technology, said he did not agree with the study’s finding of a trend toward fewer cyclones in general.
He said climate models are still “too crude” to be able to adequately explain tropical cyclones. He also had doubts that the methods used in the new study were accurate enough to provide a reliable picture of the past.
However, he agreed that there was “strong consensus that the intensity of tropical cyclones is expected to increase with global warming.”
“In practice, damage is heavily dominated by intense tropical cyclones – category 3 and higher – while annual counts are heavily dominated by weaker storms,” Emmanuel said. “So trends in the total numbers don’t mean much for social impacts.”