Cyclone numbers have fallen since start of 20th century, study suggests

Global heating has coincided with fewer tropical cyclones forming each year around the globe compared with the second half of the 19th century, according to a new study.

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 total 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 were in line with expectations that a warming planet would see fewer cyclones forming overall.

One leading expert on tropical cyclones expressed doubts about the conclusions of the study.

Understanding how climate change is affecting cyclones has proven difficult because the most reliable and complete observations from satellites 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.

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Cyclones – known also 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 historical observations of atmospheric pressure to work out the likely number of cyclones from 1850 through 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 the second half of the 19th century.

The study’s lead author, Dr Savin Chand, of Federation University Australia, said as the climate had warmed it had 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 cyclones was only one measure of the risk to societies.

The study was not set up to look for different categories of cyclones, but rather to count any cyclone that would have formed.

Category 1 cyclones generally cause only negligible damage to buildings and crops, whereas the most destructive category 5 cyclones, with average winds above 200km/h, cause billions of dollars of damage and widespread destruction to communities.

Shand said cyclones had been intensifying in recent decades and were moving closer to coastal regions. Some studies had also suggested cyclones were delivering more rain and were lasting for longer after they made landfall.

Co-author Prof Kevin Walsh, of the University of Melbourne, said the most complete data on cyclones only stretched back as far as the 1970s. Before then, there were some ship records that went back to the 1940s, but these were incomplete.

“They’re a 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 favourable for creating cyclones.

He said: “It is the really intense cyclones that cause the overwhelming majority of the damage, and there is 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 to fewer cyclones overall.

He said climate models were still “way too coarse” to be able to properly account for tropical cyclones. 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, damage is strongly dominated by intense tropical cyclones – category 3 and higher – whereas annual counts are strongly dominated be the weaker storms,” Emmanuel said. “So trends in total numbers do not mean very much for societal impacts.”