According to a new study, global warming has coincided with fewer tropical cyclones forming each year around the globe compared with the second half of the 19th century.
The average annual number of cyclones fell by 13% across the 20th century, with steeper declines seen after 1950.
Several studies using climate models have suggested global heating could reduce the number of cyclones forming, 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 aligned with expectations that a warming planet would see fewer cyclones forming overall.
One leading expert on tropical cyclones expressed doubts about the study’s conclusions.
Understanding how climate change affects cyclones has proven difficult because the most reliable and complete satellite observations don’t start until the late 1970s. That relatively short timeframe makes it more difficult to seperate the effect of global heating from the natural variability in the climate.
Cyclones – also known as hurricanes or typhoons depending on where they form – also involve complex local atmospheric conditions that are challenging to model.
Scientists from Australia and the US used climate models and historic atmospheric pressure observations to determine the likely number of cyclones from 1850 to 2012.
Declines were found in all seven of the ocean basins where cyclones form.
Globally, a larger drop of 23% in the number of cyclones forming annually was found after 1950, compared to 13% across the entire 20th century.
The only exception to the larger decrease in cyclones after 1950 was in the North Atlantic, where cyclone numbers had been rising in recent decades but, according to the study, were still lower than in the second half of the 19th century.
The study’s lead author, Dr. Savin Chand of Federation University Australia, said that as the climate had warmed, it likely changed underlying atmospheric conditions that help cyclones form.
While it might be “good news” that fewer cyclones were forming, Chand said the total number of storms was only one measure of the risk to societies.
The study was not set up to look for different categories of cyclones but to count any hurricane that would have formed.
Category 1 cyclones generally cause negligible damage to buildings and crops, whereas the most destructive category five hurricanes, with average winds above 200km/h, generate billions of dollars of damage and widespread destruction to communities.
Shand said cyclones had been intensifying recently and moving closer to coastal regions. Some studies also suggested cyclones delivered more rain and lasted longer after landfall.
Co-author Prof Kevin Walsh of the University of Melbourne said the complete data on cyclones only stretched back to the 1970s. Before then, some ship records returned to the 1940s but were incomplete.
“They’re very complicated phenomena, but this study is building confidence in our predictions from climate models by showing they agree with observed trends.”
He said as oceans warmed in the tropics, the flow of warm air upwards would be reduced, as would the difference in the speed of winds closer to the surface and higher in the atmosphere, two factors less favorable for creating cyclones.
He said: “It is the intense cyclones that cause the overwhelming majority of the damage, and there are good theoretical reasons to believe those numbers [of more intense cyclones] will increase in the future.”
Prof Kerry Emanuel, an expert in cyclones at Massachusetts Institute of Technology, said he didn’t agree with the study’s finding of a trend of fewer cyclones overall.
He said climate models were still “way too coarse” to be able to account for tropical cyclones properly. He also had doubts the methods used in the new study were precise enough to give a confident picture of the past.
However, he agreed there was “a strong consensus that tropical cyclone intensity should increase with global warming”.
“In practice, the damage is strongly dominated by intense tropical cyclones – category three and higher – whereas the weaker storms strongly dominate annual counts,” Emmanuel said. “So trends in total numbers do not mean very much for societal impacts.”
